Electronic devices advanced from heavy, bulky origins to smart, mobile appliances. The commercial landscape of today&’s electronics industry is dominated by microelectronics, best reflected by ultrahigh density integrated circuits on rigid silicon. A new trend in electronics evolves from accompanying appliances to an imperceptible form, wearable as glasses, textiles and medical prostheses, directly adherent to the skin, or inner organs like the heart and the brain, establishing a seamless link between living beings and electronic devices. Flexibility, compliance, weight, and softness will be key metrics in next generation electronic appliances. Scientists currently explore the potential of elastic and soft forms of electronics, but also of robots and energy harvesters. The last few years have seen an explosion of such soft matter based demonstrators, so we are currently at the verge of witnessing the demonstration of truly complex bionic systems, eventually similar to the “machine-human” in the science fiction movie Metropolis or the sentient android Data in Star Trek. In the presentation, a few areas of this new branch of soft matter science will be highlighted.

There are numerous motivations to explore stretching polymer semiconductor films including: improving electronic properties, to gain insight into structure-property relationships, and for the development of flexible and stretchable electronics. However, the ability to stretch polymer semiconductor films is not necessarily a given, and the films often have brittle behavior. In this talk, we consider the molecular and morphological dependence of polymer films on mechanical behavior and how processing methods can have a large impact on film ductility. In particular, we will consider the role of: molecular structure by considering several categorically unique polymers; the local morphology varied through solution processing; and the employment of multi-component polymer blends. We will describe processing approaches that improve film ductility in typically brittle high performance donor-acceptor polymer semiconductor systems while maintaining high charge mobility. Once ductile films are produced, large physical strains are applied to the films and the change in morphology is compared to charge transport characteristics when applied in a transistor configuration. This comparison provides several insights into charge transport limitations in polymer semiconductors. This work shows that ductility and charge mobility are not necessarily negatively correlated and that material systems can be developed with optimized mechanical and electrical properties. Finally, the implications this has for the development of stretchable electronics will be discussed.

To improve the accuracy of biometric information sensing, it is ideal to put the sensors in direct contact with the target objects with minimal discomfort of wearing. For this reason, researchers are actively fabricating electronic components on flexible and/or elastic substrates. In this work, we have succeeded in making adhesive gel capable of fabricating fine patterns by photo irradiation. Furthermore, we applied the gel to realize sheet sensors that can perform biometric measurement just by applying the gel to the dynamically-moving body. This sensor is made with two different processes. On top of electronic circuits fabricated on ultrathin polymeric film, the adhesive gel is patterned only to cover the electrodes that interfaces with the living body. These sheet sensors can detect bioelectric signals, such as strained physical quantities and electrical activities of the heart, by directly applying to surfaces of human skin or the heart of a rat. Adhesive gel prevents the sheet sensor from slipping or falling off the surface during the dynamic movement, and allows stable and long-term measurement. The stretchable and/or plaster-like sheet sensor will evoke the technology in collecting biological information from an active living body.

Symmetry breaking in living systems is often achieved by coupling of chemical reactions and selective growth to achieve shape change. In many systems, however, it is becoming more recognized that the underlying materials themselves can cause the symmetry breaking. It is important to understand such mechanisms as they could help us direct growth, shape change, and be a source of morphogenesis and adaptability in artificial systems. Materials exhibiting dynamic shape-change behaviour, have shown much promise for engineering multi- functionality. They are useful for developing novel artificial muscles, adaptable structures, and for bringing insights into the processes or morphogenesis. We show examples of engineering the symmetry breaking and dynamics for multiple structures and processes, on multiple lengthscales - from nanometers to centimeters. We demonstrate the formation of Janus and other asymmetric particles, which form as a result of coupling of chemical reactions to non-linear mechanical properties of materials[1,2]. We also demonstrate the opposite effects - how mechanical deformations and molecular interactions can help one simplify chemical syntheses[3].Further, we also demonstrate that even without reactions, the material properties and geometry alone could cause symmetry breaking. By bending a spherical cap and a cone shell, we characterize the instabilities and show novel behaviors, both static and dynamic. Upon inversion of the magnetic spherical cap, for example, using high speed video, we have captured an intermediate asymmetric quasi-stable state. The results are reproduced faithfully by a finite element model analysis where we only put in the material properties and the remote forces exerted on the cap by a magnetic field[4]. Equilibrium deformations also show symmetry breaking. We have focused on another simple shape - a conical shell. Upon deformation one can achieve in a controlled way symmetry breaking with 2-, 3-, 4- and 5- sided polygonal shapes. We explore the energetics of these transitions, the underlying materials properties which control them, and the dimensionless scaling that could help us predict them for various cones.
References
[1] Ding T, Baumberg J, Smoukov SK, Harnessing Nonlinear Rubber Swelling for Bulk Synthesis of Anisotropic Hybrid Nanoparticles with Tunable Metal-Polymer Ratios, J. Mater. Chem. C, 2, 8745-8749 (2014) DOI: 10.1039/c4tc01660b
[2] Wang Y, Ding T, Baumberg J, Smoukov SK, Symmetry Breaking Polymerization: One-Pot Synthesis of Plasmonic Hybrid Janus Nanoparticles, Nanoscale (2015) DOI: 10.1039/c5nr01999k
[3] Marshall, JE, Gallagher S, Terentjev EM, Smoukov SK, Anisotropic Colloidal Micromuscles from Liquid Crystal Elastomers, J. Am. Chem. Soc., 136 (1), 474-479 (2014), DOI: 10.1021/ja410930g
[4] Loukaides E, Seffen KA, Smoukov SK, Magnetic Actuation and Transition Shapes of a Bistable Spherical Cap, Intl. J. Smart & Nano Mater. (2015) DOI: 10.1080/19475411.2014.997322

With the advent of self-healing (SH) materials a new vision of material science had been accomplished, together with the exploitation of specific chemical and physical principles^[1]. Thus e.g. fast crosslinking and crack-repair after damage has been optimized via encapsulation and embedding of reactive components; ^[2] the application of supramolecular bonds^[3] has enabled multiple self-healing taking into account the relevant timescales of self-healing^[4]. The current presentation addresses principles of self-healing polymers taking place at the site of damage specifically via stress-induced chemical reactions, in particular "click"-based chemistries to induce self-healing responses via force-induced healing. By introducing supramolecular healing principles mechanically stronger materials are obtained, displaying multiple healing-cycles together with eg. graphene-based nanofillers^[5] bearing attached catalytic systems.
Acknowledgements. Financial support from the DFG (project Bi 1337-1/8) within the framework of SPP 1568 (Design and Generic Principles of Self-Healing Polymers) and the EU-project IASS (within EU-FP7) are gratefully acknowledged.
[1] a) W. H. Binder, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2013, p. 425. b) Special Issue on "Self-healing polymers", Polymer July 2015.
[2] a)M. Gragert, M. Schunack, W. H. Binder, Macromol. Rapid Commun. 2011, 32, 419-425; b)M. Schunack, M. Gragert, D. Döhler, P. Michael, W. H. Binder, Macromol. Chem. Phys. 2012, 213, 205-214; c)L. Guadagno, M. Raimondo, C. Naddeo, P. Longo, A. Mariconda, W. Binder, H. , Smart Mater. Struct. 2014, 23, 045001; d)D. Döhler, P. Zare, W. H. Binder, Polymer Chemistry 2014, 5, 992-1000.
[3] a) F. Herbst, D. Döhler, P. Michael, W. H. Binder, Macromol. Rapid Commun. 2013, 34, 203-220. b) S. Chen; M. Mahmood; M. Beiner; W. H. Binder. Self-Healing materials from V- and H-shaped supramolecular architectures Angew. Chem. Int. Ed.2015, DOI: 10.1002/anie.201504136R1.
[4] J. Akbarzadeh, S. Puchegger, A. Stojanovic, H. O. K. Kirchner, W. H. Binder, S. Bernstorff, P. Zioupos, H. Peterlik, Bioinspired, Biomimetic and Nanobiomaterials 2014, 3 (3), 123-130
[5] a) A. Shaygan-Nia, S. Rana, D. Döhler, X. Noirfalise, A. Belfiore, W. H. Binder, Chem. Commun.2014, 50, 15374-15377. b) A. S. Nia, S. Rana, ,D. Döhler, F. Jirsa, A. Meister, L. Guadagno, E. Koslowski, M. Bron, W. H. Binder, Chemistry A European Journal, 2015 (in press).

In this talk, I will present our work on developing stretchable electronic polymers, understanding the design principles and the use of these polymers for stretchable devices, such as transistors and sensors.

Polydimethylsiloxane (PDMS) elastomers are widely used in both industry and research; for example, they are contained in personal care products, applied as sealants, and used as materials for microfluidic devices and stretchable electronics. PDMS elastomers are typically formed by crosslinking entangled linear polymers; such conventional elastomers are intrinsically stiffer than a threshold value set by the density of entanglements that act as effective crosslinks. Making PDMS elastomers softer would allow their deformation with less energy, enabling uses that require them to easily comply with the shape of objects they contact, broadening potential applications. To make the elastomer softer, the density of crosslinks must be lowered; this goal can be easily achieved by swelling the elastomer with solvent. However, the solvent may leach out; moreover, such a PDMS gel is adhesive, which is unacceptable for applications requiring the separation of PDMS from another surface. Therefore, silicone gels intrinsically cannot be soft and nonsticky. The stickiness is lower for elastomers without solvents; however, conventional ‘dry&’ PDMS elastomers cannot have shear moduli lower than 200 kPa, the threshold set by entanglements. A multiple-step, complex chemical synthesis circumvents this threshold by avoiding the entanglements, but results in elastomers with uncontrollable storage and loss modulus. It remains a challenge to develop soft, solvent-free PDMS elastomers with controllable viscoelastic properties through a simple approach.
Here we report soft PDMS elastomers fabricated by crosslinking bottlebrush rather than linear polymers. The bottlebrush architecture prevents the formation of entanglements, enabling soft, yet solvent-free PDMS elastomers with precisely controllable elastic moduli ranging from ~1 to ~100 kPa, much softer than typical PDMS elastomers. We find that the elastic moduli are in excellent agreement with theoretical predictions based on classical rubber elasticity: The modulus is linearly proportional to the density of crosslinking chains. Remarkably, in addition to prescribed stiffness, the bottlebrush structure enables independent control over the loss modulus. We measure the difference in adhesiveness between the soft PDMS elastomers and commercial silicone products of similar stiffness. We find that the soft PDMS elastomers are far less adhesive due to their significantly smaller amount of uncrosslinked, free molecules as quantified by Soxhlet extraction. Importantly, the fabrication of soft PDMS elastomers is a one-step process, as easy as that for commercial silicone elastomer kits.

Electronics of tomorrow will be flexible and will form a seamless link between soft, living beings and the digital world. The unique possibility to adjust the shape of devices, offered by this alternative formulation of electronics, provides vast advantages over conventional rigid components, particularly in medicine and consumer electronics. There is already a remarkable number of available flexible devices starting from interconnects, sensing elements towards complex platforms consisting of communication and diagnostic components [1-4].
We developed flexible [5,6], printable [7,8], stretchable [9,10] and even imperceptible [11] large area passive electronic components with the specific focus on magnetosensitive elements, which were completely missing in the family of flexible electronics, e.g. for smart skin applications. On the other hand, we realized self-assembled compact tubular microchannels based on strain engineering [12] with integrated passive sensory elements [13-15] and communication antenna devices [16] for on-chip and bio-medical applications, e.g. smart implants.
Combining these two research directions carried out at different length scales into a single truly interdisciplinary topic opens up the novel field of smart biomimetics. In this respect, we demonstrated mechanically and electrically active compact biomimetic microelectronics, which can serve as a base for the realization of novel regenerative neuronal cuff implants with unmatched functionalities. Biomimetic microelectronics can mechanically adapt to and impact the environment possessing the possibility to assess, adopt and communicate the environmental changes and even stimulate the environment electrically.
In the talk, these recent developments will be covered.
[1] J. A. Rogers et al., Science 327, 1603 (2010).
[2] J. A. Rogers et al., Nature 477, 45 (2011).
[3] S. Wagner et al., MRS Bull. 37, 207 (2012).
[4] M. Kaltenbrunner et al., Nature 499, 458 (2013).
[5] G. Lin, D. Makarov et al., Lab Chip 14, 4050 (2014).
[6] M. Melzer, D. Makarov et al., Adv. Mater. 27, 1274 (2015).
[7] D. Karnaushenko, D. Makarov et al., Adv. Mater. 27, 880 (2015).
[8] D. Karnaushenko, D. Makarov et al., Adv. Mater. 24, 4518 (2012).
[9] M. Melzer, D. Makarov et al., Adv. Mater. 27, 1333 (2015).
[10] M. Melzer, D. Makarov et al., Nano Lett. 11, 2522 (2011).
[11] M. Melzer, D. Makarov et al., Nat. Commun. 6, 6080 (2015).
[12] O. G. Schmidt et al., Nature 410, 168 (2001).
[13] I. Mönch, D. Makarov et al., ACS Nano 5, 7436 (2011).
[14] C. Müller, D. Makarov et al., Appl. Phys. Lett. 100, 022409 (2012).
[15] E. J. Smith, D. Makarov et al., Lab Chip 12, 1917 (2012).
[16] D. D. Karnaushenko, D. Makarov et al., Nature Asia Materials 7, e188 (2015).

The possibility of stretchable gas barrier nanocoating was studied with an all-polymer multilayer
using layer-by-layer assembly. Electrostatically-bound polyethyleninmine (PEI)/polyacrylic acid
(PAA) and hydrogen bonding-based polyethylene oxide (PEO)/PAA layers were incorporated
into four interbonding layers in which PAA serves as a bridging molecule. Assembly pH had a
direct effect on the film&’s growth and structure. With all layers deposited from pH 3 aqueous
solutions, a densely packed multilayer thin film was formed with relatively high gas barrier,
achieving an oxygen transmission rate (OTR) 15 times lower than the 1 mm thick polyurethane
(PU) rubber substrate. At 10% strain, the film becomes more oriented and densified (reducing
free volume), resulting in a significant improvement in OTR (28 times lower than uncoated PU
rubber). When stretched between 10 and 50%, an OTR that is 7 to 8 times lower than the
substrate was maintained. This unique, stretchable gas barrier coating is a promising opportunity
for producing more lightweight inflatable, elastomeric objects (e.g., tires, pumps, gaskets, etc.),
many of which are used in energy generation and medical devices.

Emerging neural prostheses that employ implanted active electronics and interface to the nervous system with large numbers of electrical stimulation and recording microelectrodes require new approaches to encapsulation. The small size and fragility of the target neural tissue precludes the use of conventional can-based hermetic enclosures. Besides the need to protect implanted devices in vivo for many years, the encapsulation of these devices is challenging because of significant surface topography, such as wire bonds between integrated circuits and patterned metallization, and the presence of significant d.c. and pulsed voltages that could exceed >5 V. In the present work we have investigated a hybrid approach to encapsulation of these devices based on thin films of plasma enhanced chemical vapor deposition (PECVD) amorphous silicon carbide (a-SiC) dielectrics and silicone elastomers. The stability of a-SiC has been established by long-term (>300 day) accelerated soak tests in 87^oC saline using FT-IR measurements of film thickness and by steady-state leakage current measurements under d.c. bias (±5 V). Thin coatings of addition-cured silicone elastomers exhibited low leakage currents (10^-10 A/cm², ±5 V) but are well-known to hydrate in aqueous media. The hybrid encapsulation investigated, comprises a thin film (200-800 nm) of a-SiC with a cast overlayer of silicone elastomer. We investigated plasma-based surface treatment of the a-SiC to promote covalent bonding of methacylate-terminated adhesion promoters (e.g. g-methacryloxypropyltrimethoxysilane (γ-MPS) to the a-SiC surface and subsequent covalent bonding of the elastomer via the γ-MPS functionalized surface. Surface chemical composition at each step in the encapsulation process was investigated via water contact angle measurements, x-ray photoelectron spectroscopy (XPS), and attenuated total reflectance (ATR) infrared spectroscopy. The as-deposited a-SiC had a Si:C ratio of ~1:1 with very little oxygen present (<1 a/o). Oxygen plasma treatment resulted in the formation of a thin silicon oxide layer at the a-SiC surface which was subsequently treated in acid to promote hydroxyl functionality. Treatment with γ-MPS , by either dip or vapor phase coating, significantly increased the water contact angle from ~20^o (acid treated) to 30-60^o depending on the coating method. Depending on the type of substrate, silicone elastomer was applied by dispensing or spin coating and cured (150^oC) to complete the encapsulation. The results of leakage current measurements and accelerated in vitro soak testing of the hybrid encapsulation are reported.

Active devices and circuits in flexible and stretchable opto-electronics need permeation barrier coatings for their long-term protection. These coatings also must seal surface profiles and encapsulate accidental particles. We have developed a flexible barrier material that is deposited in a single process by plasma-enhanced chemical vapor deposition (PECVD) from mixtures of hexamethyl disiloxane (HMDSO) -a small-molecule silicone- and oxygen. Varying the PECVD conditions produces a range of materials. In particular, at a fixed HMDSO/O2 flow ratio, varying just the RF power and gas pressure produces layers that in combination provide a permeation barrier that both conformably coats and encapsulates: (i) High RF power and low gas pressure produce ultra-hermetic layers that coat conformably and are in compressive stress when deposited on a rigid substrate; (ii) Low RF power and high gas pressure produce layers that bridge gaps and are under tensile stress. A film suitably composed of these sub-layers and deposited in a single run meets all required barrier properties. The film stress must stay within a band whose boundaries are set by critical tensile and compressive values. When appropriately employed, deposition conditions (i) and (ii) prevent fracture or delamination during both, deposition of the film and its long-term use. We conducted extensive experiments to analyze coverage of test particles. Definitive permeation barrier tests were conducted on organic light-emitting diodes (OLEDs) whose surface had been contaminated with glass beads prior to barrier deposition. These tests, conducted at 85% relative humidity and 850C, demonstrate that the beads can be encapsulated, and the OLEDs protected reliably, with barrier layer thicknesses of less than the beads&’ diameter.

For the realization of stretchable conductors, a mechanical consideration of each material and understanding the mechanism for the change in electrical properties is important. We used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a current conducting materials and developed two types of stretchable devices which can endure large stretch. First, substrate on film type conductor is developed, and polyimide (PI) is used as a mechanical-property (Poisson&’s ratio and elastic modulus)-matching substrate. The sample successfully stretched up to 60 % strain without any generation of defects (buckles or cracks). Interestingly, the electrical resistivity decreases up to 80 % by stretching, and the PEDOT-rich core growth by the mechanical strain is the mechanism of such resistivity decrease. Secondly, for a stretchable conductor which is mechanically compatible to human-skin and huge stretch, we fabricated soft materials based PEDOT:PSS conductor: PEDOT:PSS-PAAm organogel. Because PEDOT:PSS dispersed uniformly in the cross-linked polymeric network, a reliable electrical conductor with stretching up to 300% strain is achieved. Furthermore, using ethylene glycol as a solvent, no electrochemical reaction occurred and long term electrical stability is obtained. It can be widely applicable to the electrical interconnect for wearable and attachable devices which covers any arbitrary curved surface or three-dimensional structure required large stretchability.

Flexible and wearable electronics are emerging in the consumer market today with the exhibiting smart phones/watches and also patents foreshadowing the future of consumer electronics. The ability to conform and shape conductors while retaining the electronic integrity of a device is imperative for long-term stability. Current technologies employ metals as conductors, which have several limitations including rising cost, high density, and limited amount of flex due to the rigidness. Herein, poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate) was patterned on textiles using several techniques including inkjet printing, screen printing, and a sponge roller. The resulting patterned lines showed exceptional electronic characteristics as conductive wires including sheet resistances as low as 1.8 ohms/#9744; and a current carrying capacity of 10 amps/mm². The charge-carrier mobility was 8.26 cm² V^-1 s^-1 with a charge carrier concentration of 1.82*10²² /cm³ determined by Hall Effect measurements. There are a few advantages of using conjugated polymers as conductors in textiles: 1) low toxicities when in contact with skin; 2) ability to retain the ‘feel&’ of fabrics; 3) capability to stay ‘invisible&’ under metal detectors. We have also demonstrated incorporating the conductive textile for cardiorespiratory sensors with high-resolution of the heartbeat that can be potentially used to detect various cardiovascular diseases. The potential applications for high surface area, highly conductive all-organic fabrics include but are not limited to sensors, thermocouples, thermoelectrics, antennae, wearable electronics and displays such as organic light emitting diodes (OLEDs), radio frequency identification tags (RFIDs), electromagnetic shielding, high surface area electrodes for capacitors and/or batteries, and the applications described as carrying power for the replacement of wire in circuits as well as resistive heating.

Conjugated polyelectrolytes (CPEs) combine the remarkable properties of conjugated polymers and polyelectrolytes, imparting onto these materials intriguing exciton and charge transfer characteristics as well as solubility in highly polar solvents. Their charged nature imbues oppositely-charged CPEs with the ability to spontaneously form partially-neutralized assemblies in solution and the solid state. With the aim of creating soft, artificial light-harvesting antennae, we have for the first time used complexation of oppositely-charged CPEs to directionally funnel electronic excitation energy both in the solid state and in solution using properly-chosen energy donor and acceptor CPEs. We find that these CPE assemblies produce phase-separated solutions with dilute and concentrated (complex coacervate) phases, whose thermodynamic characteristics differ substantially from complexes of non-conjugated polyelectrolytes. Using a combination of dynamic light scattering, small-angle X-ray scattering and photoluminescence spectroscopy, we show that the emission characteristics, the partitioning between the different phases, and the complex size distribution can be tuned by varying the extent of charge density matching between the donor and acceptor species. Moreover, we show that these materials are extremely sensitive to ionic strength, leading to the possibility of controlling the composite emission properties by tuning the ionosphere. Surprisingly, we have found that the concentrated CPE phase can be straight-forwardly deposited as a thin film coating using conventional spin-coating techniques prior to water evaporation. Once deposited, the complex exhibits remarkable resistance to dissolution in both organic and polar solvents.

Organic electronic devices have attracted particular attention in the last decades and established applications in organic light-emitting diodes have been realized. Besides low-temperature processes, low-cost and ease of processing, another significant advantage of organic devices are their mechanical flexibility and ductility.¹ A flexible or stretchable architecture of organic devices can be plastered on skin, heart or brain tissue to monitor e.g. pressure or body movements. As a result, organic electronic devices have already been used to realize artificial electronic skin or wearable sensors.^{2, 3} In recent years we have seen the rise of research on organic bioelectronics, where cells and tissues are demonstrated to directly interface with electronic devices via ionic signal communication. A combination of flexible/stretchable architectures with organic bioelectronics could lead to a mechanical compliance device which will be a promising candidate for future e-skin or e-health applications. However, device development on such materials presents several challenges. For flexible and stretchable substrates are not compatible with conventional photolithography techniques, which renders the fabrication of micro-scale devices difficult. In addition, all the processes involved in the flexible/stretchable bioelectronics device development should be environmentally friendly and the materials used should not contain any toxic ingredients since they will interface directly with cells and human body.
We realized micron-scale electrode arrays and patterned conducting polymer poly-(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS) channels on poly(dimethylsiloxane) (PDMS) substrates. Stretchable as well as bio-compatible organic bioelectronics devices were finally developed. Systematical studies were conducted on the composition of the conducting polymer, water immersion, channel thickness and PDMS pre-stretching to reveal their effect on strechablity and device performance. Long-time film stability, frequency response and device fatigue is also discussed. According to these results, a reasonable device engineering guideline is finally proposed. The optimized device can be stretched up to nearly 100% with reversible I-V curves. Further studies show that these devices can work efficiently to interface with biological systems and living tissues or as tactile sensors.
1. Rogers, J. A.; Someya, T.; Huang, Y., Science 2010,327 (5973), 1603-1607.
2. Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C.; Hellstrom, S. L.; Lee, J. A.; Fox, C. H.; Bao, Z., Nature nanotechnology 2011,6 (12), 788-792.
3. Savagatrup, S.; Chan, E.; Renteria Garcia,; S. M.; Printz, A. D.; Zaretski, A. V.; O'Connor, T. F.; Rodriquez, D.; Valle, E.; Lipomi, D. J., Advanced Functional Materials 2015,25 (3), 427-436.

To accomplish human wearable and attachable electronics successfully, it is essentially required a stretchable electronic conductor which interconnects each device. There have been studies suggested designing of metallic conductor&’s structure with a wavy- or buckled-shape that have extra region to be stretched. Despite of their high electric conductivity, stretchability is limited to the conductor&’s design. In addition, a mismatch of mechanical properties with human body lead to severe delamination at the interface. Here, we developed a soft-material based electronic conductive gel with highly stretchability. By incorporating poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conductive polymer with a hydrated polymer, polyacrylamide (PAAm), electrically conductive gels are successfully fabricated. Through a direct gelation of the PEDOT:PSS solution with PAAm, the PEDOT:PSS uniformly distributed gels was obtained. In addition, for a liquid constituent of the gels, we adopt an ethylene glycol (EG). By replacing water with EG as the liquid constituent, we produced organogels which are electrically conductive without electrochemical reactions, and barely dried in ambience due to its high boiling point. Unlike conventional hydrogels, an electrochemically driven current was prevented during electrical voltage was applied up to ± 5 V. By forming closely packed percolation path inside the gel pores, the percolation path is significantly improved compared to randomly distributed conductive PEDOT:PSS chains. Remarkably, the electrical percolation is stably maintained even stretched above 300%, and the resistance response to strain is invariant up to 50% strain. As stretchable electronic conductors, we demonstrated stretchable LED arrays which are electrically interconnected with the PEDOT:PSS-PAAm organogels. When DC voltage is applied on the arrays, the LED is well operative during extension up to 300% strain without electrochemical reactions.

Block copolymer self-assembly is a powerful approach to control the microstructure of polymeric materials. Here, we demonstrate the development of conjugated block copolymers for use as the active layer in organic photovoltaic devices and flexible cathodes for energy storage. All-conjugated block copolymers with donor and acceptor blocks linked covalently can be used in the active layer of solution processed photovoltaics. These materials self-assemble to form nanostructured, bicontinuous donor and acceptor domains, and the microstructure of solution-processed films is comprehensively characterized by grazing-incidence X-ray scattering. We show that the photovoltaic performance of these block copolymer devices depends on the characteristic domain sizes, crystalline orientation, and the linking group between donor and acceptor groups, and our work suggests that enhancing the segregation between donor and acceptor domains can further increase efficiencies. Block copolymers with one conjugated polymer block and an ion-conductive block can be applied as binders in battery cathodes. Blends of V₂O₅ as a lithium intercalation material and 5-10 wt % poly(3-hexylthiophene)-block-poly(ethyleneoxide) (P3HT-b-PEO) block copolymer form a flexible, volumetrically stable, carbon free hybrid battery cathode. X-ray measurements show that the PEO block intercalates between the V₂O₅ layers. Introduction of the block copolymer significantly enhances mechanical flexibility and toughness without significant loss in electrochemical properties.

Engineered materials that integrate advances in polymer chemistry, nanotechnology, and biological sciences have the potential to create powerful medical therapies. Our group aims to engineer tissue regenerative therapies using water-containing polymer networks called hydrogels that can regulate cell behavior. Specifically, we have developed photocrosslinkable hybrid hydrogels that combine natural biomolecules with nanoparticles to regulate the chemical, biological, mechanical and electrical properties of gels. These functional scaffolds induce the differentiation of stem cells to desired cell types and direct the formation of vascularized heart or bone tissues. Since tissue function is highly dependent on architecture, we have also used microfabrication methods, such as microfluidics, photolithography, bioprinting, and molding, to regulate the architecture of these materials. We have employed these strategies to generate miniaturized tissues. To create tissue complexity, we have also developed directed assembly techniques to compile small tissue modules into larger constructs. It is anticipated that such approaches will lead to the development of next-generation regenerative therapeutics and biomedical devices.

Soft actuators play an important role in engineering because they allow complex locomotion without extensive system control.^[1] These machines are made almost entirely from elastomers and therefore have a comparably low weight. Unlike classical “hard” actuators, they take advantage of the material elasticity to fulfill their tasks. Instead of triggering rigid, mechanical components (i.e. spring or servomotor), soft actuators use fluid expansion, magnetism or electricity to deform their soft structures. Therefore, no lubrication or cooling systems are needed. This recent development has led to a new class of machines, so called soft robots.
Soft actuators can be used for basic tasks such as liquid conveyance. The soft nature of these pumps imitates organs, for instance a human heart. As a consequence, soft pumps are of great interest to the medical implant market that is estimated to account for 200 billion dollars per year.^[2] Soft actuators are usually produced by replica molding of silicones. The corresponding parts then have to be glued together in order to obtain the final actuator. This gluing induces weak spots, which can be critical for long term or stress intense applications.
We present a novel manufacturing technique based on lost wax casting to produce soft actuators in a single piece.^[3] This is beneficial for the design of long lasting soft actuators (e.g. pumps). Avoiding possible tearing spots furthermore allows the use of other power sources. We were therefore able to design and drive soft pumps by the combustion of hydrocarbons instead of pressurized fluids or electricity. The use of hydrocarbons as an energy carrier offers not only high specific energy-densities for fast actuation (i.e. pump rates of 150 beats per minute) but also reduces the weight of the energy carrier (i.e. no heavy battery packs needed). Our pumps were able to power up to 30&’000 combustion cycles at a constant combustion power rating of 500 watts. Water pump rates of up to 14 L/h were measured with developed pump pressure of up to 1.3 bar. Thus, the manufacturing technique discloses promising prospects for long-lasting soft pumps. We further demonstrate successful incorporation of contrast agent into the silicone without significantly changing material properties.^[4] This enables real-time analysis by medical sonography to illustrate the flow inside an operating pump.
[1] A. Verl, A. Albu-Schäffer, O. Brock, A. Raatz, Soft Robotics: Transferring Theory to Application, Springer, 2015.
[2] Medical Implants Market - Growth, Global Share, Industry Overview, Analysis, Trends Opportunities and Forecast 2012 - 2020 Allied Market Research 2012.
[3] M. Loepfe, C. M. Schumacher, W. J. Stark, Industrial & Engineering Chemistry Research2014, 53, 12519.
[4] M. Loepfe, C. M. Schumacher, C. H. Burri, W. J. Stark, Adv. Funct. Mater.2015, 25, 2129.

To advance 3D-printing techniques towards automated manufacturing of multi-material functional devices, appropriate inks and printing procedures must be developed. Integration of sensory, conducting and structural materials on a micron scale represents a particular challenge. Direct write 3D-printing enables a large variety of inks to be deposited in a single procedure, and thus provides an intriguing route to overcoming these challenges. Here, we have developed a direct-write 3D-printing methodology and accompanying inks to fabricate an advanced Heart-on-a-Chip micro-tissue device in a single printing procedure. The core of the device, consisting of a four-material thin film composite less than 50 microns thickness, integrates micron-scale structural cues for guiding cardiac tissue development with embedded soft strain gauges and electrical interconnects, to enable direct and continuous readout of tissue contractility. Device fabrication was enabled by development of elastomeric sensory and insulator inks coupled with machine automation and optical profiling techniques. This approach allowed the dimensions and stiffness of the device to be engineered such that a reliable electrical resistance change, proportional to the stress generated by the tissue, was recorded upon tissue contraction. We demonstrate the value of the fabricated device by recording cardiac tissue contractility and drug-dose response in a controlled cell incubator environment. The device constitutes a rare example of an advanced organ-on-chip micro-tissue system that in addition to accurately recapitulating the structure and function of living tissue in vitro also provide a direct readout of micro-tissue function and response to external stimuli. Such devices promise to accelerate and lower the cost for pharmaceutical and biomedical research, which for decades have relied on physiologically inadequate cell cultures and expensive and low-throughput animal studies. The presented work illustrates how direct-write 3D printing can serve as a versatile one-tool platform for automated fabrication of advanced bio-medical micro-devices.

Hydrogels and ionogels are ionic conductors that combine adequate conductivity and perfect transparency. They maintain conductivity and transparency under giant stretches, readily beyond an areal strain of 1000%. A gel consists of a covalent network of polymers and a solvent (e.g., water or an ionic liquid). The network makes the gel a soft and elastic solid, and the solvent makes the gel a fast ionic conductor. Gels can be as soft as tissues, and as tough as elastomers. Although most hydrogels dry out in open air, hydrogels containing humectants retain water in environment of low humidity, and ionogels are non-volatile even in vacuum. We show that the gels readily function as stretchable conductors in artificial muscles, skins, and axons. We further show that the ionic conductors can replace indium tin oxide to make light-emitting devices low cost and rugged.

The goal of targeted therapeutics and molecular diagnostics is to accumulate drugs or probes at the site of disease in higher quantities relative to other locations in the body. To achieve this, there is tremendous interest in the development of nanomaterials capable of acting as carriers or reservoirs of therapeutics and diagnostics in vivo.^[1] Generally, nanoscale particles are favored for this task as they can be large enough to function as carriers of multiple copies of a given small molecule, can display multiple targeting functionalities, and can be small enough to be safely injected into the blood stream. The general goal is that particles will either target passively via the enhanced permeability and retention (EPR) effect, actively by incorporation of targeting groups, or by a combination of both.^[2] Nanoparticle targeting strategies have largely relied on the use of surface conjugated ligands designed to bind overexpressed cell-membrane receptors associated with a given cell-type.^[3] We envisioned a targeting strategy that would lead to an active accumulation of nanoparticles by virtue of a supramolecular assembly event specific to tumor tissue, occurring in response to a specific signal. The most desirable approach to stimuli-induced targeting would be to utilize an endogenous signal, specific to the diseased tissue itself, capable of actively targeting materials introduced via intravenous (IV) injection. We present the development of nanoparticles capable of assembling in vivo in response to selective, endogenous, biomolecular signals. For this purpose, we utilize enzymes as stimuli, rather than other recognition events, because they are uniquely capable of propagating a signal via catalytic amplification. We will describe the preparation of highly functionalized polymer scaffolds utilizing ring opening metathesis polymerization, their development as in vivo probes and their utility as a multimodal imaging platform and as drug carriers capable of targeting tissue via a new mechanism.
References:
1 J. A. Hubbell, A. Chilkoti, Science, 337, 303-305.
2. a) Y. Matsumura, H. Maeda, Cancer Res 1986, 46, 6387-6392; b) D. Peer, J. M. Karp, S. Hong, O. C. Farokhzad, R. Margalit, R. Langer, Nat. Nanotechnol. 2007, 2, 751-760.
3. a) W. Arap, R. Pasqualini, E. Ruoslllahti, Science 1998, 279, 377-380; b) D. Pan, J. L. Turner, K. L. Wooley, Chem. Commun. 2003, 2400-2401; c) A. R. Hilgenbrink, P. S. Low, J. Pharm. Sci. 2005, 94, 2135-2146.

Stimuli-responsive hydrogels are a promising class of polymeric materials for a range of technological applications, such as electronic, biomedical and electrochemical devices. To modify the properties of hydrogels and endow them designed multifunctionality, thus meeting increasingly complex technological requirements, hybrid hydrogels with delicately controlled chemical composition and micro/nano- structures are needed to be developed. Here we will show two representative examples on rational design and synthesis of hybrid hydrogels with function-enriched properties. The first example is the synthesis of thermally responsive and conductive hybrid hydrogels by in situ formation of continuous network of conductive polymer hydrogels crosslinked by phytic acid in poly(N-isopropylacrylamide) (PNIPAM) matrix. The interpenetrating binary network structure enables the hybrid hydrogel with attractive synergistic characteristics: high electrical conductivity, high thermo-responsive sensitivity and greatly enhanced mechanical properties. The second example is the hybrid hydrogel which incorporates a hydrophilic polymer polyethyleneimine (PEI) into PNIPAM. PEI provides structural modification and tunes the water content in PNIPAM hydrogel, as well as modifying the interaction between hydrogel matrix and charged drugs, resulting in tunable drug release at body temperature. Our works demonstrate that the chemical properties and architecture of the filling phase in the hydrogel matrix and design of hybrid hydrogel structure play an important role in determining the performance of the resulting hybrid material. The multifunctional hybrid hydrogels we developed hold promise in applications in stimuli-responsive electronic devices and controlled drug delivery.

Interest in elastomers with elongations exceeding 1000% has been generated by medical applications for deliverable, highly-deformable devices ranging from intraocular lenses and flexible duct stents to in-vivo microfluidic diagnostics and drug delivery. Similarly, stretchable and conformable material platforms for sensors and electronics that can recover from cannulation and interconnect processes are desirable. Common elastomeric materials are exemplified by natural rubber with elongations commonly reported in the range of 500-800%, and by muscular hydrostats with elongations reported in the range of 100-200%. Synthetic elastomers typically have elongations of less than 800%. We report polysiloxane elastomers with elongations exceeding 5000%. This development has been enabled by a living polymerization that results in heterobifunctional macromonomers of intermediate molecular weight, which in a second distinct step-growth polymerization, are converted to elastomers with high molecular weight. While the homogeneous polymer systems exhibit high elongations, maximum elongations are observed in nanocomposites. With no apparent crosslinking, elastomeric behavior is observed at temperatures higher than both the T_g and T_m of the polymer, suggesting that topological features rather than covalent bonding or domain formation are operative. The exceptional elastic deformation and recovery of these polymers challenges conventional physical models for elastomers.

The preparation of melt blends containing chitosan is one of the great challenges for expanding the application possibilities of this biopolymer. Here we describe the use of poly(vinyl alcohol) (PVA) as a compatibilizing agent in a ternary blend with chitosan and Poly(lactic acid) (PLA). The blends were prepared by a two step process in which in the first stage, a PVA/chitosan blend is prepared by casting, thus chitosan, which is hardly melted, is dispersed in PVA witch is a thermoplastic component. In a second stage, the thermoplastic PVA/chitosan miscible blends was incorporated in PLA matrix by melting mixing to produce a two phase compatible blend. Melting processing occurred in a twin-screw extruder at temperature profile of 130,130, 135, 140, 145°C and 80 rpm. The blends were characterized by Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), dynamical-mechanical analysis (DMA) and scanning electron microscopy (SEM). FTIR analysis confirmed that PVA/chitosan was good interaction and two significant changes in IR specter was observed; the decrease in the intensity of absorption bands at 3355-3300 cm^-1 referent to the -NH and -OH of the chitosan and PVA and the shifting of absorption band at 1750 cm^-1 referred of ester group of PVA. DSC analysis revealed that the presence of PVA/Chitosan phase in PLA results in changes in the crystallization upon heating. The morphology of the blends evaluated by SEM revealed that PVA/chitosan melted and appears as a dispersed phase in the PLA matrix. A reduction in mechanical properties as well as the darkening of the dispersed phase during drying, suggest some degradation during processing, due probably to the presence of residual solvents (acetic acid) and impurities, which are under investigation actually. However, the positive results in chitosan fusion and dispersing of this processing method is innovative and represents a major advance in the study of polymer blends from natural polymers processed by melt processing and have potential allowing dispersion of polymers with high polarity in another polyesters matrixes by melting process. The authors acknowledge FAPESP, CNPq and CAPES for financial support.

During the healing process in wounds centimeters below the surface of the skin, it is desirable to be able to externally actuate the release of non-steroidal anti-inflammatory drugs (NSAIDs) to prevent fibrotic tissue formation. The Layer-by-Layer (LbL) self-assembly process lends itself to conformally coating drug-containing films onto resorbable bandages to be implanted at the wound site. For deep wounds, triggering localized drug release from LbL films by near-infrared (NIR) irradiation is biomedically relevant; the 700-1100 nm range of NIR electromagnetic radiation is relatively biologically transparent and has a penetration depth in biological tissue on the order of centimeters. To produce films with minimal passive release of NSAIDs to the local wound site, these NSAIDs have been covalently linked to negatively charged biodegradable polyelectrolytes that were then self-assembled with polycations into LbL films on the order of microns thick. The effect of introducing an NIR laser stimulus on the drug and/or pro-drug release rate in vitro from LbL films with vs. without NIR-absorbing nanomaterials was investigated. In future experiments, this system will be tuned, and alternative materials or approaches will be explored. Moving forward with this work, a more generalized toolset will be developed for the remotely controlled localized release of therapeutics in the body.

One of the challenges in the design of the flexible conductive films is to achieve high conductivity and desired mechanical properties simultaneously. Because there are trade-offs between these properties such that the improvement of one negatively impacts the other for conventional materials. Recently, the incorporation of nanoparticles and polymers has attracted considerable attention in the fabrication of flexible composites that exhibit advantageous electrical and mechanical performances. In this work, aramid nanofibers (ANFs) are chosen as the host polymer matrix for their exceptional mechanical performances, excellent chemical and thermal stability^1-3 and gold nanoparticles (Au NPs) for their high electrical conductivity and multiple surface functionalities⁴. We prepared the porous polymer matrix from ANFs dispersions derived from the well-known ultrastrong macrofibers Kevlar^TM. ANF films are initially made by spin-coating and later transferred into a free-standing composite films by controlled delamination. Infiltration them with Au NPs by vacuum filtration followed by supercritical drying or oven drying results in unique polymer-metal composites analogous to amorphous metals. Besides, this Au-ANFs composite films presented the high thermal stability of outperform other polymer-based composites and outstanding flexibility. After thermal annealing at 350 °C, the oven dried Au-ANF film containing 32.8 vol. % Au exhibited electrical conductivity of 4.31×10⁴ S/cm, which represent the highest value among all the reported Au-based nanocomposites, combining with tensile strength of 241 MPa, Young&’s modulus of 10.8 GPa and toughness of 3.66 MJ#8729;m^-3. We anticipate that this research might provide a new perspective for fabricating flexible, high performance and human friendly materials which can be used in smart sensor, wearable electronics and other devices.
Reference
(1) Yue, C. Y.; Sui, G. X.; Looi, H. C. Compos. Sci. Technol.2000, 60 (3), 421-427.
(2) Day, R. J.; Hewson, K. D.; Lovell, P. A. Compos. Sci. Technol.2002, 62 (2), 153-166.
(3) O&’Connor, I.; Hayden, H.; Coleman, J. N.; Gun&’ko, Y. K. Small2009, 5 (4), 466-469.
(4) Kim, Y.; Zhu, J.; Yeom, B.; Di Prima, M.; Su, X.; Kim, J.-G.; Yoo, S. J.; Uher, C.; Kotov, N. a. Nature 2013, 500 (7460), 59-63.

With increasing interest in the fields of soft robotics and wearable medical technologies, there exists a need for a human-to-computer interface to discretely integrate into complex environments. Previous work by Sun et al created an “ionic skin” based on electrolyte swollen polyacrylamide, which acts as electrodes in a transparent capacitive touch sensor [1]. In this work we propose a novel pressure/touch sensor which exhibits generative properties due to displacement-induced ionic charge separation in gel electrolyes. A ‘piezoionic&’ effect is hypothesized to originate from a difference in mobilities between positive and negative ions, causing a localized ionic charge gradient upon application of pressure. The gradient is detected as a voltage or current by using thin stainless steel electrodes placed at the sides or at regular intervals along one surface of the gel. The voltage generated is a result of the local concentation gradient produced by the deformation or perhaps is the result of the electro-kinetic effect [2, 3]. Ionic polymer gels based on PVDF/HFP co-polymer were synthesized in situ to incorporate various salts including 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), sodium trifluoromethanesulfonate (CF₃SO₃Na), and lithium perchlorate (LiClO₄). The samples were subjected to dynamic mechanical analysis (DMA) and potentiostatic/galvanostatic measurements simultaneously to determine the transduction coefficient. The open circuit voltages generated are approximately 10 mV at10kPa on a 1cm x 1cm x 200µm gel sample. Results suggest a piezoionic coefficient (g₃₃) that is approximately 3middot;10^-3 V#8729;m/N. When normalized by thickness of the film (30 mu;m in this case), this is similar to or larger than the piezoelectric coefficient found in PVDF. Ionic gels offer a number of advantages including flexibility, biocompatibility, and ease of fabrication. These are a promising class of materials for flexible sensor applications [4]. The non-aqueous, solid-state ionic gels presented in this work provide a better stability and resistance to evaporation than its aqueous, hydrogel based counterpart reported previously [2]. The next step in this work is to demonstrate the sensing mechanism using electrodes that are invisible to the naked eye, and to distinguish touch, stretch and bending motions.
[1] J. -Y. Sun, C. Keplinger, G. M. Whitesides, and Z. Suo, “Ionic skin,” Adv. Mater. 2014.
[2] M. S. Sarwar, Y. Dobashi, E. F. S. Glitz et al, Transparent and conformal 'piezoionic' touch sensor. Proc. SPIE 9430, 2015.
[3] A. Fiumefreddo and M. Utz, “Bulk Streaming Potential in Poly(Acrylic Acid)/ Poly(Acrylamide) Hydrogels,” Macromolecules, Vol. 43, No. 13, January 2010, pp. 5814- 5819
[4] J.-Y. Sun, X. Zhao, W. R. K. Illeperuma, O. Chaudhuri et al, “Highly stretchable and tough hydrogels.,” Nature, vol. 489, no. 7414, pp. 133-6, Sep. 2012.

Advanced thermoplastic materials such as polyetheretherketone (PEEK) are used increasingly for medical applications, also carbon-fiber-reinforced composites which has enhanced mechanical properties, high thermal and chemical stability and ease of processing is presently being studied for manufacturing medical instruments, implants for maxillofacial surgery and for dental applications, thus it is of importance to get adhesive bonding for these high performance polymer materials.
Bonding of PEEK composites is a critical step in the manufacture of medical devices. How to increase the adhesion properties will be discussed in this abstract, surface preparation and modification is adopted to enhance the bonding strength, the lap shear strength of different epoxies will be discussed, also an optimized bonding process will be proposed and a unique vacuum bonding technique with spacer will be demonstrated.
Polymer bonding is essentially a superficial phenomenon depending as it does upon interactions between the epoxy and the surface of the substrate. The surface preparation of PEEK joint to be bonded is therefore of the greatest importance and surface roughness is a key issue based on our study, PEEK bonding with surface roughness of 1um,5um,20um was studied and the lap shear strength test results will be discussed. Also the annealing could be used to release the surface defects and future enhance the bonding properties.
Bonding samples with different epoxies were prepared, Duralco 4538 with a mix ratio of 1.2 was chosen for the surface roughness and annealing condition study. These tests were mainly to check the thermal stability and thermal expansion compatibility of the epoxies and PEEK composites. Similar thermal expansion coefficients are desired for the epoxy to prevent interfacial failure when heated. Therefore a thermal cycle test was carried out. The samples went through the thermal cycle three times, their appearance, especially bonding consistency, was checked after each cycle.
There are several key issues about PEEK composites bonding: Surface should be clean of all grease, oil, dirt, etc; Entrapped air in the epoxy mixture should be removed by either letting it stand for several minutes or vacuum degassing; The bond lines should be between 0.005#42892;#42892; - 0.010 #42892;#42892; (0.127 mm to 0.254 mm). To achieve such a thin layer, applying forces by clamping devices is useful, and vacuum bonding with spacer will also enhance the bonding properties since the uniform thickness epoxy layer will be generated and less defects could be formed after the annealing. In addition, a smooth surface finish such as smaller than 5 micron roughness can make sure the bonding layer is of uniform thickness.
Properties of PEEK bonding with different epoxies and PEEK surface roughness modification is discussed here to ensure good bonding properties of biomedical applications, finally a lap shear strength higher than 10MPa is observed for the optimized bonding process.

In this study, we report on conductive and biocompatible silk nano-films for the electrical stimulation and measurement of electrical signals. Conventionally, microelectrode arrays or transistor biosensors have been developed to stimulate cells or measure electrical signals. However, there is a technical issue as regards controlling the location of cells on the electrodes. This is because the cells randomly adhere to the surface, which hampers experimental reproducibility and accuracy. We thus approach this issue by developing a mobile, conductive film-based interface (nano-film) that enables us to manipulate and electrically activate specific cells.
To prepare such functional nano-film, we improved the film properties of transparency, electrical conductivity, mechanical stiffness, and biocompatibility. To achieve these four properties, we fabricated the nano-film by using a combination of silk fibroin gel and PEDOT:PSS. Silk fibroin was prepared by dissolving silk fabric in LiBr solution followed by dialysis in water. To fabricate the nano-film, 0.1-20.0 mg/mL of silk fibroin aqueous solution was mixed with 3% PEDOT:PSS, and spin-coated on SiO₂ substrates. The film thickness was controllable in the 40 to 200 nm range. The substrate coated with nano-film was immersed in methanol for gelation. Finally, micropatterned nano-films were formed with a controllable size and shape with a photolithographic technique.
The silk fibroin matrix revealed optical transparency in both the visible and ultraviolet regions, enabling us to add-on any type of microscope. The FTIR spectra had peaks at 1535 and 1630 cm^-1 assigned to the amino-1 band, which indicated the film contained β-sheet crystalline fractions. The gelled silk fibroin proteins improved the mechanical properties of the film to 100 MPa. Interestingly, the fabricated nano-film realized 500 times higher conductivity (at a maximum of 1 mS) than pristine PEDOT:PSS film. This result implies that the silk fibroin molecules help the rearrangement of the PEDOT:PSS chains and enhance charge transfer in inter-chains or inter-particles.
Since the silk fibroin and PEDOT:PSS show high biocompatibility, suspended cells tend to migrate and proliferate at the film surface. We micropatterned the film to modulate the behavior of specific cells, and activate them while retaining their adhesive property. We further utilized these films to individually measure and apply cellular action potentials to a target cell by incorporating them in capillary electrodes. By applying the voltage to cell-laden nano-films, the cells expressing P/Q-type calcium channels (Ca_v2.1) were selectively activated under a homogeneous condition. These properties could provide a prototype biocompatible electrode for cells and tissues, therefore ensuring non-cytotoxicity and long-term implantation. We believe that the nano-films can be used for both in vitro electrophysiological analysis and various biomedical applications.

Elastomer films that can achieve high electromechanical strain at relatively low electric fields are of great interest for a variety of applications including artificial muscles, deformable optics, and dielectric elastomer generators. Our approach to lowering the drive voltage required to achieve high strain is to introduce high dielectric constant nanoparticles into an elastomer matrix, with the goal of intensifying the local electric field in the elastomer in regions immediately surrounding the nanoparticles (referred to as the interphase). This effect is driven by the large mismatch in the dielectric constant (ε_r) between the nanoparticles (ε_r > 100) and the elastomer (ε_r ~ 2-5). In principle, very little of the electric field will be concentrated within the nanoparticles themselves, whereas the field in the interphase will be significantly increased, leading to enhanced strain. This should lead to a cumulative effect whereby increased nanoparticle loading results in increased strain.
To test this hypothesis, we recently used microextrusion and hot pressing to prepare a series of composite films containing increasing concentrations of barium strontium titanate (BST) nanoparticles dispersed in SEBS (a block polystyrene-co-butadiene) elastomer. The axial strain (s_z : compression in the direction of the applied field) was measured for each composite. Indeed, the BST:SEBS composites all showed significantly higher s_z at a given electric field than undoped SEBS control films. A nearly three-fold increase in strain (s_z = 2.8%) is observed for 15 vol% BST: SEBS films relative to undoped SEBS (s_z = 1.0%) when measured at E_max = 140 V/mu;m. To better understand the electromechanical properties of the BST:SEBS composites, we imaged their internal structures using a dual beam focused ion beam (FIB) / scanning electron microscope (SEM). Individual BST nanoparticles ranged from 70 to 100 nm in diameter. At low loading rates (5 or 10 vol% BST), the composites contained a mixture of dispersed individual particles and small aggregates (d < 1000 nm), whereas at higher BST concentrations much larger aggregates were formed with a noticeably lower concentration of individually dispersed particles. Composites containing 25 vol% BST showed some aggregates in excess of 10 mu;m in diameter. The correlation between composite morphology (as a function of vol% loading of BST nanoparticles) and resultant electromechanical properties will be discussed. A final topic of discussion will center on the effects of BST nanoparticle loading on the bulk electrical breakdown properties of the composites.

Commercial off the shelf (COTS) electronics generally are not specifically designed to perform in extremely transient high impact scenarios. This research focused on the development of a silver-decorated carbon black-based polymeric nanocomposite with properties such as high conductivity, flexibility and shock absorbing. Polymeric rubber materials are generally very flexible and shock absorbing, however, most polymeric materials are electrical insulators. The dispersion of the silver-decorated carbon black in to the polymeric matrix could significantly improve the electrical conductivity. The processing and fabrication of Ag-CB (silver-carbon black)/Epoxy (thermosetting epoxy polymer) and Ag-CB/TPU (thermoplastic polyurethane) will be reported. Both Ag-CB/Epoxy and Ag-CB/TPU mixtures with solvents showed the shear-thinning behavior, which was an important characteristic for direct printing of traces and additive manufacturing. The mechanical properties of the nanocomposites were measured using dynamic mechanical analysis (DMA) over a wide range of temperatures. The morphology of the nanocomposite was investigated by the TEM, showing that Ag-coated carbon blacks or silver nanoparticles were well-connected to form the network, leading to the extremely good electrical conductivity. These nanocomposite materials were also successfully used to print flexible circuits using 3D-printing machine. The electrical resistance changes for the Ag-CB/Epoxy on PDMS, Ag-CB/TPU on PDMS and Ag-CB/TPU on PET under strain were studied and the results will be discussed. The highly conductive polymer nanocomposites show promise as an alternative solution for electronic materials under high impact scenarios.

Water-soluble polymers and their hydrogels have been investigated as effective carriers for drugs. When a drug crystallizes under a strong influence of a polymer (polymer-directed crystallization), the possible physical and chemical interactions between drug and polymer could lead to novel functional crystals, resulting in possible improvement of bioavailability, processability and stability. Drug release, which has been engineered by the traditional crystal engineering such as polymorph, size, and salt form control, can be designed in a whole new windows by this polymer-directed crystallization. Herein, a novel in-situ gelation system was discovered based on drowning-out crystallization of atorvastatin (a cholesterol-lowering medication) in presence of polyacrylic acid. The crystallization of atorvastatin was significantly inhibited by polyacrylic acid, and after crystallization the whole mixture became hydrogels. This hydrogel formation was sensitive to temperature, concentration, and the type of solvents. The hydrogel system uses amorphous particles of atorvastatin as crosslink points between polymeric chains. The hydrogel system could become sol by physical aging at 80 °C. The size of drug particles were a few tens of microns. In a gel state, the elastic modulus (Gprime;) was higher than the viscous modulus (GPrime;) ,and the elastic modulus (Gprime;) was more than 1 kPa. This novel hydrogel system could be a useful drug delivery system for improved bioavailability and stability.

Flexible electronic skins (e-skins) with high tactile sensitivities and multimodal functionalities are of great interest in various applications such as wearable electronics, prosthetics, and humanoid robotics. By mimicking a sensory system in human fingertip skin, many research groups have recently demonstrated highly sensitive and flexible electronic skins with the ability to simultaneously perceive and differentiate multiple spatio-temporal tactile stimuli such as static and dynamic pressure, temperature, and vibration. However, most of electronic skins have difficulties in demonstrating multifunctional tactile sensing in a single device. It is known that the fingerprint patterns and interlocked epidermal-dermal microridges in human fingertips have critical roles in amplifying and transferring the tactile signals to various mechanoreceptors, enabling the spatio-temporal perception of various static and dynamic tactile signals. Herein, we fabricate a novel electronic skin with fingerprint-like patterns and interlocked microstructures, which can enhance the sensing of static and dynamic mechano-thermal signals. Our flexible and microstructured skins can detect and discriminate multiple spatio-temporal tactile stimuli including static and dynamic pressure, vibration, and temperature with high sensitivities. For proof-of-concept demonstrations, we show the simultaneous monitoring of pulse pressure of artery vessels and temperature-induced pulse pressure variation. In addition, dynamic touch sensing ability is employed for precise detection of acoustic sounds, and discrimination of various surface textures. Our interlocked microstructured e-skins may find applications in robotic skins, wearable sensors, and medical diagnostic devices.

Poly(vinylidene difluoride) (PVDF) is a semi-crystalline polymer, whose molecular structure has the repeated monomer unit (-CH₂-CF₂-)_n. The most common polymorph of PVDF is an α-phase, which has a monoclinic unit cell with TGT#286;(T=Trans, G=Gauche+, #286;=Gauche-). β-phase polymorph is less stable than α-phase, while showing a highly polarized dipole moment and a piezoelectric property. There can be other conformational polymorphs in PVDF, the γ, δ and ε phases, but the α and β phases are by far the most general and important ones.
MWCNTs were firstly functionalized by acid treatment to enhance the interfacial compatibility between PVDF and MWCNTs. After the functionalization process, MWCNTs were easily dispersed in the PVDF-dimethyl formamide(DMF) solution. They were poured in the distilled water, and the precipitation was dried in a vacuum oven to remove the residual DMF and water.
Non-isothermal crystallization analysis of a PVDF and PVDF-MWCNTs nanocomposites provides some important information about the crystalline structure development of these materials. From the non-isothermal crystallization exotherm, we developed the Avrami equation in Seo's method to get the Avramic exponent, n, and the polarized optical microscopy showed the recrystallization behavior at a consistent cooling rate. The crystal structure of nanocomposites could be studied by wide angle X-ray diffraction (WAXD). The morphological observations were executed by scanning electron microscopy(SEM) and transmission electron microcopy(TEM).
In this study, we found that the addition of MWCNT could affect the nonisothermal crystallization kinetics, but the interaction between PVDF and MWCNTs was not strong enough to induce the phase transformation from α phase to β phase. Therefore, strong external stimulus such as mechanical drawing and/or electric poling should be provided to achive the highly β-formed PVDF, which can be applied for electronic devices.

Over the years, polar polyvinylidene fluoride (PVDF) and its copolymers have been successfully integrated in many applications due to their easy availability, superior dimensional stability, better melt processability and last but not the least, good thermal stability up to 120°C with moderate piezoelectric coefficients (~30 pC/N). Recently, polypropylene (PP) like nonpolar polymers has emerged as a new addition to piezoelectric polymer family in which electrically charged internal walls of micro-voids act as small electret transducer. These transducing micro-voids are known to exhibit high piezoelectric d₃₃ coefficient. While PP&’s piezoelectric coefficient (~120-600 pC/N) has been reported higher than that of PVDF, their application in high temperature transduction has been limited by poor thermal stability above 50°C. Therefore, to achieve high thermal stability coupled with required piezoelectricity, surface modification of PP was proposed. Many researchers suggested the use of thermally more stable Fluorocarbon polymers such as Fluorinated Ethylene Propylene (FEP) and Polytetrafluoroethylene (PTFE) for their superior thermal stability; however, difficulty in melt processing and charge instability at high temperatures confined their practical implementation. In this regard, we prepared a modified PVDF-nanoclay-CaCO₃ composite that combines the intrinsic polar nature of β-phase with cellular electret arising from stretch induced voids in PVDF. The effect of nanoclay addition on polar phase content as well as the influence of CaCO₃ on cellular structure in the composite film was investigated. The film was prepared by melt extrusion with a twin-screw extruder with 30-40 wt% of CaCO₃ and 2 wt% of organically modified nanoclay followed by polar phase characterization and stretching at final-to-initial length ratios of 3.5 to 4.5 at 80-100°C. After inflating the voids to optimum shape, the film was annealed followed by corona discharged in order to create electric dipoles using our homebuilt setup. Characterization with FTIR, XRD and DSC indicated 87% β-phase after stretching. The presence of lens shaped voids within the stretched film was confirmed by Scanning electron microscopy (SEM) images. Porosity analysis from both SEM and Ethanol Intrusion technique showed that the void lengths varied between 20-85 mu;m with heights between 3-23 mu;m. The d₃₃ coefficient reached maximum for stretching ratio of 4.2 at 90°C with 67% of void having heights in between 3.6-4.5 mu;m. The results along with future works can hopefully be implemented in sensors applied in verifying structural integrity operating in the temperature range of 30-90°C.

Physiological and external dynamic forces present in the body can provide stimuli for releasing drugs from mechanoresponsive systems. Because of these many processes (i.e., breathing, swelling, stretching), we have designed a strain-dependent layered composite with a hydrophilic drug core and superhydrophobic coating. The outer layers are composed of a biocompatible coating fabricated by electrospraying a mixture of poly(glycerol monostearate-co-ε-caprolactone) and poly(ε-caprolactone) to increase surface roughness, thereby enhancing superhydrophobicity (advancing contact angle = 170°). The hydrophilic cellulose/polyester core tolerates loading of release agents (dye, proteins (FITC-BSA, TNF-α), and chemotherapeutics) using various solvents (water, dimethyl formide, methanol, chloroform). Microcomputed tomography data demonstrates wetting under tension as the weaker superhydrophobic coating cracks, disrupting the otherwise stable air barrier for water infiltration and subsequent release. Analysis of crack patterns under tension reveal crack formation at a critical strain (30%) followed by increasing total crack area and mean area per crack. The graded release of hydrophilic (cisplatin) and hydrophobic (7-ethyl-10-hydroxycamptothecin) chemotherapeutics from this mechanoresponsive system has been applied in vitro against an esophageal cancer cell line (OE33). Our mechanoresponsive system provides an active and intuitive method of controlling drug release, accounting for the dynamics of the human body.

The flexible interlayer dielectrics (ILDs) are inevitable in flexible organic light-emitting diodes (OLEDs). Flexible ILDs require various properties such as a low dielectric constant, low leakage current, thermal and chemical stability, mechanical strength and flexibility for optimal device performance. Recent reports suggested the use of various materials such as porous silica-based materials, fluorinated amorphous carbon and benzoxazine-based polymers. However, several drawbacks still remain in terms of their thermal, mechanical and chemical properties. Herein we report the novel hybrid film, which consist of hollow SiO₂ spheres and polyimide (PI), shows a low dielectric constant of 1.83 and excellent thermal stability up to 500°C. After the bending test for 20,000 cycles, the hybrid film exhibits no degradation in its dielectric constant or leakage current. These results indicate that the hybrid film made up of hollow SiO₂ spheres and polyimide (PI) is useful as a flexible insulator with a low dielectric constant and high thermal stability for flexible OLEDs. Fabrication process and material properties will be discussed in details.

Ionic polymer-metal composites (IPMC) are versatile smart materials with the potential to be used as artificial muscles in soft robotics and medicine due to their human-like actuating and sensing ability. To convert an ionic polymer such as Nafion into an IPMC, one must coat two nanocomposite electrodes on either side of the membrane. For adding the electrodes, several methods exist, for instance electroless plating, physical vapor deposition, or heat-pressing techniques [1]. During electroless plating the polymer is impregnated with the metal ions, which are reduced afterward to produce metal particles inside the polymer. This process is well established and yields well working composites. However, it also involves hazardous chemicals and does not provide the possibility to pattern metal deposition in a controlled and accessible way
As a patterned metal deposition provides an interesting route to study the influence of electrode position and geometry on actuator bending, we investigated the micro- and nano-patterning of the ionic polymer Nafion by direct e-beam writing. Along with increasing the potential interface area between electrodes and polymer and thus the active area, this approach opens up the possibility to create defined trenches with adjustable geometry and adaptable positioning in the polymer. These trenches can then be filled with metal by e.g. physical vapor deposition. Thus, position, geometry, and depth of the metal inside the polymer can be controlled. The penetration of the electrode into the membrane should at the same time enhance the contact area between membrane and metal, which in turn should increase the performance of the IPMC. For the present study, we examined patterning depth and possible irradiation damage in dependence on dosage and spot size for linear and circular patterns. Furthermore, we demonstrate the stability of the patterning also during prolonged immersion in ionic solution, which is necessary for a functional ionic polymer.
[1] Bahramzadeh and Shahinpoor 2014, Soft Robotics 1, 38-51

Recently, the ferroelectric materials with large electrocaloric effect (ECE)have simulated condsiderable interests due to their potential applications in cooling devices requiring high efficiency and environmentally friendliness. Due to the easy processing, light weight, low cost, broad operation temperature range and high refrigeration advantages, polymeric films of the relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) emerged as one of the most promising electrocaloric (EC) materials for cooling applications. However, relatively high electric field is required to induce large ECE in (PVDF-TrFE-CFE), which largely limits its practical application in cooling devices operated at voltage<200 V. In this work, for the first time, we demonstrated that a small loading of reduced graphene oxide can significantly enhanced the ECE of PVDF-TrFE-CFE based composites, particaularly under relatively low electric fields. A entropy change DS of ~33Jkg^-1K^-1and adiabatic temperature change DT of ~7#8451;over a wide temperature range near temperature are deduced for composites with a loading of 3.0wt.% under a low electric field of 80MV/m, which are nearly double those of the pure terpolymer. By the employment of reduced graphene oxide, large ECE over a wide operational temperature range near room temperature can be realized under a relatively low electric field, providing a means to overcome one of the key limitations of PVDF-TrFE-CFE for practical applications.

Magneto-active elastomers (MAEs) consist of hard-magnetic particles embedded in an elastomer matrix. As compliant actuators they provide a material for possible use in non-invasive medical devices. In electronics, they provide flexible, variable/graded permeability materials amenable to additive manufacturing. Their actuation behavior is driven by the interaction of the external field with the particle and the particles with the matrix material. Futhermore, as magnetic volume content increases within the elastomer, the magnitude of the magnetization response is expected to increase accordingly. Previous works detailing the evolution of microstructure with magnetic particle concentration have also suggested that an increase in magnetic material concentration affects the development of particle alignment within the matrix. Chainlike structures formed at lower volume fractions transitioned to less well-aligned clusters of particles at higher concentrations. Given the directional nature of magnetization, chains of oriented hard-magnetic particles would exhibit magnetization responses that differ substantially from that of randomly oriented particles. Consequently, the transition from highly oriented to less well aligned textures would result in changes in measured magnetization behavior as well.
This research examined the relationship between the volume content of magnetic particles within the elastomer matrix, the degree of alignment of the particles&’ magnetization, and the resulting bulk magnetization of the particle-elastomer mixture once cured. The expected effects of changes in the degree of alignment across particles were measured by characterizing the change in remanent magnetization per unit magnetic volume of a range of MAE samples. Previous experimental works have examined soft- but not hard-magnetic MAE composite behavior and have not linked results to particle arrangements.
MAE specimens at five particle concentrations from 10% v/v to 35% v/v, were fabricated from 325 mesh M-type barium hexaferrite particles mixed with Sylgard 184 elastomer. The mixture was cast, and then cured in the presence of a µ_oH = 1T field. After curing in the field, sample magnetizations were tested using a Microsense EZ7 vibrating sample magnetometer, measuring magnetic hysteresis over µ_oH = ±2T. The coercive field H_C, saturation magnetization M_S and remanent magnetization M_R were determined. The coercive field was approximately µ_oH = 0.4T for all samples. The minimum M_s= 0.405T occurred at 10% v/v then increased to a maximum of M_s = 0.614T at 20% before decreasing to M_s =.516T at 35%. Remanent magnetization increased from M_R= 0.312 T at 10% v/v to a maximum of M_R = 0.518T at 20% before decreasing to M_R=.416T at 35%. The data show that regions of enhanced M_R and M_S occur between 10% and 25% v/v. Estimation of the orientation distribution function of the particles&’ magnetizations&’ suggest increased alignment of particles within this same concentration region.

Dielectric elastomer actuators (DEA) are one of the most commonly studied types of Electroactive Polymers (EAPs) due to their fast response and mechanical robustness. However, high voltage requirements versus low actuation ratios have hindered their potential to become widely used as robotic actuators.
In this study, multilayer DEA composites with different geometric configurations were fabricated to improve actuation ratios at lower voltages. A multi-walled carbon nanotube - polydimethylsiloxane (MWCNT/PDMS) composite was used to fabricate mechanically compliant, conductive (thickness < 50mu;m) parallel plates and electrode connections for the DEA actuators. Active surface area, layer thickness, and conductive dopant content were varied to study the effects of these parameters on actuation ratio as a function of applied voltage. Results suggest that geometric configuration could be used as a means of decreasing the high voltage requirements associated with DEAs.

The processing of polymeric materials is gaining importance, especially for the fabrication of flexible and stretchable medical micro-fabricated devices such as wearable electronics. In contrast to the standard inorganic materials, the adhesion between different polymer materials is usually less understood and therefore less controlled. In this paper we study the adhesion between polyimide and polydimethylsiloxane (PDMS) since these materials are frequently used in stretchable devices where polyimide isolated interconnects are embedded into a stretchable PDMS matrix, and whereby delamination of the polyimide-PDMS interface results in an early failure of these devices.
Silicon wafers coated with a 5 µm thick layer of polyimide were treated with different surface modification techniques such as chemical adhesion promotors, oxygen plasma and an Ar⁺ sputter etch. After surface modification the wafers were molded with a 1 mm thick layer of PDMS. The adhesion of the PDMS to polyimide was tested by Instron 90° tensile peel tester and by using a Nordson DAGE wedge shear tester, which “pushes” the PDMS layer off the underlying polyimide layer rather than “pulling” it off.
The best adhesion was obtained when argon ion plasma at 300W with 50sccm of argon was applied on a polyimide coated wafer for 10 seconds before molding of the PDMS. Atomic Force Microscopy (AFM) analysis showed how the surface morphology of the polyimide changes and becomes rougher with increasing sputter etch depth of the polyimide. Apparently, sputtering of polyimide layer creates “ridges” on its surface that provides a good mechanical interlocking surface for the non-polar PDMS layer. Shear tests showed a 14 times better adhesion of PDMS to the sputtered polyimide as compared to non-sputtered polyimide. In fact, the adhesion was so good that it was impossible to manually peel off the PDMS layer from the polyimide surface. On the other hand, treating the polyimide with an adhesion promoter and/or oxygen plasma did not show a significant improvement in adhesion.

A large variety of novel electronic applications, such as flexible/stretchable displays, photovoltaics, batteries, and sensors, require highly and omnidirectionally stretchable electrodes. However, most research activities have mainly focused on improving the stretchability in only one direction so far. We present a versatile conducting layer made of metal nanowire (NW) array that are not only highly stretchable but also capable of being stretched in all directions. A PDMS template with 2 dimensioned patterns was used to fabricate the biaxially aligned metal NW arrays. The metal NW array on elastic substrate exhibited high stretchability up to 200% under a uniaxial strain and 120% under a biaxial one, without any severe degradation in its electrical conductance. Furthermore, the metal NW array film also showed “Omnidirectionality”, which implied the high stretchability under randomly chosen stress direction. A sequential stress test revealed high robustness of the stretchable electrode. A stretchable sensor for electronic skin was demonstrated to confirm the application of our approach to soft electronics.

We introduced an electronics-compatible route, on the basis of physical phenomena that occur during the injection of an alcohol drop onto a water surface, for the self-assembly of a single-layer network structure (SLNS) of nanorods (NRs). The SLNS is a desirable framework in using one-dimensional (1D) nanomaterial for high performance and flexible sensors because the SLNS is similar to two-dimensional thin film, which can be tailored by a photo-lithography. Also, the SLNS has large surface to volume ratio; thus, all nanomaterials consisting of the SLNS are involved in sensing external stimulus. This kind of useful SLNS can quickly be fabricated by our route in a just one-step. To show feasibility of our route, we experimentally assembled a SLNS with zinc oxide (ZnO) NR, which is famous as a UV-light sensing material, and its electronic, optic, and morphological properties were investigated. ZnO NRs were synthesized through a solvothermal process in methanol solution of Zn(CH₃COO)₂middot;2H₂O mixed with KOH. A mono-dispersed ethanol solution of ZnO NRs was dropped onto water using the trapping phenomena of the alcohol/water interface on the water. The ZnO NR SLNS was then transferred onto SiO₂/p-Si wafer and polyethylene terephthalate (PET) film as a flexible substrate.
Furthermore, since the n-type semiconductor ZnO SLNS inherently has many junctions between the nanomaterials, it can show very sensitive behavior against an external strain. Finally, we suggested an algorithm and fabricated a device for simultaneously sensing UV light and strain. The device&’s rising time was approximately 12 s, and the decay time was approximately 4 s and the on-off ratio was also observed to be approximately 10³ at 0.14 mW cm^-2 and linearly increased up to 1.0 mW cm^-2 for various UV intensities.
Keywords: ZnO nanorod, UV sensor, Low temperature process

Breath analysis represents a promising diagnosis method for non-invasive, real-time disease detection and monitoring in health care.¹ Specific gases in human breath, so called breath markers, can indicate certain diseases, e.g. acetone can be used to distinguish between healthy (500 ppb²) and diabetic (1800 ppb³) subjects. Chemo-resistive gas sensors based on metal oxides exhibit high sensitivity, however, these sensors are operated at a relatively high temperature, leading to increased power consumption and reduced lifetime. Conductive polymers such as polyaniline (PAni) have chemo-resistive properties already at room temperature, but are difficult to optimize with respect to sensitivity and selectivity, and have not proven suitable for the measurement of low concentrations of acetone or ethanol. It has been hypothesized that hybrid nanocomposites of these two classes of materials may help to eliminate their particular drawbacks due to synergetic effects.⁴
Here, semiconducting nanoparticles (10 - 15 nm, XRD) of different metal oxides were directly deposited by flame spray pyrolysis on interdigitated electrode substrates forming a nanostructured, highly porous film. Polyaniline is added within this porous metal oxide (MOx) nanoparticle film by in-situ polymerization. This forms around each NP while maintaining the porous film morphology, which is confirmed by scanning electron microscopy. These composite sensors possess a low electrical resistance as is necessary for room temperature gas sensing. The resulting porous structure was confirmed by scanning electron-beam microscopy. The optimal MOx to PAni ratio of the nanocomposite regarding its analyte sensitivity and response and recovery time was investigated. The resulting porous nanocomposite gas sensor was capable of measuring breath relevant acetone and ethanol concentrations at room temperature with a significantly increased sensitivity compared to PAni alone. Most notable are the low drift, high signal-to-noise ratio and reversibility and fast response and recovery times of 30 s and 120 s, respectively. These polymer nanocomposite sensors are a promising solution for heat sensitive, flexible electronics which demand the detection of gaseous analytes at low concentrations.
1. Risby TH, Solga S. Appl. Phys. B-Lasers O. 2006, 85, 421-426.
2. Turner C, Scaron;panecaron;l P, Smith D. Phys. Meas. 2006, 27, 321-337.
3. Deng C, Zhang J, Yu X, Zhang W, Zhang X. J. Chromatogr. B. 2004, 810, 269-275.
4. Nicolas-Debarnot D, Poncin-Epaillard F. Anal. Chim. Acta. 2003, 475, 1-15.

There has been constant interest in applying conducting oxide materials in future optoelectronic device. Transparent conducting oxide (TCO) electrode material shows a resistivity under 1_x10^-3 #8486;.cm with a high optical band gap over 3.5 eV. Typically, indium-thin-oxide (ITO), a representatively widespread TCO, has a crucial disadvantage to flexibility because of its brittle feature. To solve this problem, transparent composite electrode (TCE) composed of TCO/metal/TCO structure is a promising alternative for the next generation devices.
In this study, highly flexible TCEs were applied to fabricate amorphous indium-gallium-zinc-oxide thin film transistors (a-IGZO TFTs). For the comparison of structural differences, three types of electrodes such as Ag, Ag/indium-zinc-oxide (IZO) bi-layer, and IZO/Ag/IZO multi-layer, were prepared as a function of Ag thickness on glass substrates. The results indicate that IZO/Ag/IZO multi-layer composite electrode is the superior due to anti-reflection of symmetrically grown IZO layer on both sides of Ag film and a high conductivity by an inserted thin Ag film. For the suitability of flexible optoelectronic devices, bending test of the optimized IZO/Ag/IZO film on a polyimide (PI) substrate was performed at a radius of curvature of 2 mm and no substantial change in sheet resistance was found after 10,000 cycles of bending. Furthermore, the electrical performance of a-IGZO TFTs with IZO/Ag/IZO TCEs was compared to that of a-IGZO TFTs with the IZO electrodes on thermally oxidized p⁺⁺ Si and PI substrates.

Flexible electronics depend on the use of polymeric substrates containing electric circuits.¹ They are prepared frequently by printing of metallic nanoparticle inks. This is especially attractive for industrial use due to its simplicity and versatility as it is based on three steps: (1) nanoparticle and ink preparation, (2) printing and (3) sintering.² This is why rapid, single-step alternatives at ambient conditions are still sought. Recently, thin, flexible and conductive silver nanoparticle films on PMMA-coated substrates have been prepared by flame aerosol deposition³ within minutes. It is, nevertheless, essential to better understand the deposition and percolation of these silver nanoparticles in order to tune the process. These two aspects are the main focus of this research. More specifically, the polymer-coating was varied by changing its chemistry and molecular weight that strongly influence the percolation rate of the silver nanoparticles and their interaction with the substrate. Similarly, the temperature during deposition was investigated by systematically varying the substrate height above the burner. It strongly influences the nanoparticle film composition and morphology. These results guide the implementation of flexible electronics containing aerosol-deposited silver nanoparticle electrodes since they easily can be incorporated into multi-layered and multi-functional polymer nanocomposite films⁴.
1. Sekine T, Ikeda H, Kosakai A, Fukuda K, Kumaki D, Tokito S. Improvement of mechanical durability on organic TFT with printed electrodes prepared from nanoparticle ink. Appl. Surf. Sci.294, 20-23. (2014)
2. Hu PA, O'Neil W, Hu Q. Synthesis of 10 nm Ag nanoparticle polymer composite pastes for low temperature production of high conductivity films. Appl. Surf. Sci.257(3), 680-685. (2010)
3. Blattmann CO, Sotiriou GA, Pratsinis SE. Rapid synthesis of flexible conductive polymer nanocomposite films. Nanotechnology. 26(12), 125601. (2015)
4. Sotiriou GA, Blattmann CO, Pratsinis SE. Flexible, multifunctional, magnetically actuated nanocomposite films. Adv. Funct. Mater.23(1), 34-41. (2013)

Research on new sources of energy conversion and storage is an issue that currently is an important need for the development of society. The synthesis of new materials for the production of modern devices enabling solutions for the world&’s energy problem has been the subject of intense research. This interest is not only related to the shortage of energy, but also new energy sources with low or even an absence of environmental impact. Considering the concerns mentioned, supercapacitors are promising devices. We report supercapacitors fabricated with the layer-by-layer (LBL) technique using two polymers, namely poly(o-methoxyaniline) (POMA) and poly(3-thiophene acetic acid) (PTAA). The electrochemical performances of POMA/PTAA supercapacitors were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The results were compared with POMA casting film. An important difference is observed for the POMA/PTAA LBL comparing with POMA casting film. In conducting polymers, the oxidation (reduction) leads to the intercalation of counter ions (deintercalation) to compensate for the generated charge in the material. When a self-doping effect occurs the ion transport is partially (or totally) inhibited and, as consequence, an increase in efficiency of the electrochemical process can be expected. In the present case, there is an increase of the anodic current/mass up to 3.5 times. There is a second important observation comparing POMA casting and POMA/PTAA LBL films. For the thinnest films, the current/mass are equal, whereas thick films increase this parameter, also increasing the LBL film present I/m values 3.5 times higher than casting one. As a consequence, the specific capacitance values calculated from the voltammetric curves present the same kind of behavior, although the values calculated for the casting film remains constant near 60 Fg^-1, the LBL films almost linearly increases from 50 Fg^-1 up to 140 Fg^-1. These results had main characteristics of increasing the redox process efficiency as well as an increase in the specific capacitance as the number of bilayers increase. Modeling the impedance data using a transmission line model, it was observed that polymer resistance is negligible. This behavior is equivalent to propose the existence of a short circuit between the bilayers or, different, that each bilayer acts if it is in direct contact with the electrical contact. A possible explanation for this fact is the inhibition of the ionic charge intercalation promoted by the interaction between the carboxylate group in the PTAA unit and the amine one in the POMA layer. Acknowledgments: FAPESP (Grant# 2011/10897-2, 2013/07296-2), CAPES, and CNPq.

Multi-functional nanowires having good electrical and mechanical properties promise many applications in flexible and/or transparent electrodes. For these applications, there exists clear trade-offs between electrical conductivity on one hand, and transparency and/or mechanical flexibility on the other. Such electrodes are usually fabricated from random networks of nanowires and suffer from electrical and mechanical hysteresis. In particular, it has been shown that continuous meso-scale wires can overcome the transport hysteresis currently faced by percolation-governed electrical conductivity. The main approach hence to enable precise tuning of the properties along these trade-offs is to fabricate geometric metal patterns made by photo-lithography and lift-off. This approach is however not cost efficient for applications extending beyond the meter scale.
We fabricate continuous metal wires with sub-micron diameter and continuous length by a modified wire drawing process. Owing to their good mechanical and electrical properties, they can be suitable for applications requiring transparent and/or flexible electrodes. Our process starts from a commercially available 25 micron diameter palladium wire encapsulated in a silica capillary. The composite wire is heated by a CO₂ laser, and then drawn at high speeds of up to 0.3 m/s to form a centimeter-long silica-coated palladium fiber with diameter down to 100 nm and centimeter scale length. We call this process nano draw casting (nDC). During the process, the metal is locally melted under local peak temperatures exceeding 1700 Celsius and, due to capillary forces, wets the cylindrical silica shell as it is being drawn. The grain structure of the wire depends on the process parameter such as the cooling rate determined by the laser configuration, shell outer diameter and drawing speed. During the laser-assisted drawing process, the metal forms bamboo-like grain structure, in which individual crystals span the majority of the cross section of the wires. SEM, EBSD and TEM confirm this chained structure, with a fine nanocrystalline shell near the boundary. Matching the melting temperature to silica drives the choice of metal, and using the same technique we fabricated copper and nickel nanowires.
The nanowire cores can be obtained after hydrofluoric acid etching. The as-pulled palladium wires with 2 micron-diameters exhibit a high conductivity of 4.9 S/m and a fracture strain of 2.9%. The wires can be easily transferred to elastomeric substrates. Moreover, by pre-staining the substrate before transfer, the wires form in-plane wavy buckles that allow the accommodation of higher strains up to 900% without change in their electrical conductivity. Owing to their good mechanical properties, we observe higher modes of buckling such as loop formation, which help reaching the high strains without degrading the electrical properties.

Recently, flexible and wearable electronic devices and their associated technologies have significantly increased in demand, thereby attracting much interest in development of flexible and wearable energy storage systems. However, flexible and stretchable energy storage system must address design issues such as the selection of suitable flexible, stretchable active material, as well as supporting substrate and current collector. Textile based supercapacitors have the ability to maintain electrochemical performance under mechanical strain and have high power density due to fast chargeminus;discharge rates, which are characteristic of electrostatic double-layer capacitance. One disadvantage of supercapacitors, however, is their low energy density compared to other energy storage systems such as the Li-ion battery. To compensate for the low energy density, many researchers have deposited nanostructured pseudocapacitor materials such as metal oxides MnO₂ and RuO₂, as well as conductive polymers that can enhance the capacity by 200minus;300%. However, the metal oxide nanostructures result in large volume changes during chargeminus;discharge cycles that cause delamination of the active materials and hence a decrease in electrochemical reliability.
To prevent the delamination of nanostructured pseudocapacitor materials, Yu et al¹ reported use of a thin layer of conductive polymer PEDOT:PSS(200 F/g) coated on top of the MnO₂ pseudocapacitor. Our aim is to use an alternative material for polymer coating that can serve as a conductive and adhesive layer while also enhancing capacitance. Polypyrrole is favorable due to its high energy capacity(620 F/g)², chemical stability, electrical conductivity (50minus;100 S/cm), and thermal stability. The high conductivity of polypyrrole can result in enhanced power density when coated on top of MnO₂ loaded CNT textile, and high power and energy densities can be expected. In addition, polypyrrole can prevent delamination of active materials during chargeminus;discharge cycles.
In this study, polypyrrole was coated on top of MnO₂ nanoparticles that are deposited on CNT textile supercapacitor to prevent delamination of MnO₂ nanoparticles. An increase of 38% in electrochemical energy capacity to 461 F/g was observed, while cyclic reliability also improved, as 93.8% of energy capacity was retained over 10,000 cycles. An in-situ electrochemical and mechanical study revealed that polypyrroleminus;MnO₂ coated CNT textile supercapacitor can retain 98.5% of its initial energy capacity upon application of 21% tensile strain and showed no observable energy storage capacity change upon application of 13% bending strain. After imposing cyclic bending of 750,000 cycles, the capacitance was retained to 96.3%.

The rapid progression of stretchable electronics, in particular the integration of a variety of materials onto stretchable substrates, has resulted in the emergence of several functional devices such as stretchable and foldable circuits, as well as transparent and epidermal electronics. The usage of stretchable and flexible materials is also well established in optics where they are extensively used to create mechanically tunable devices such as tunable lenses, stretchable optical waveguides, and electronic oculus.
One of the most prevalent uses of elastomeric materials in tunable optical devices are gratings, which act as tunable diffractive elements. Diffraction gratings are used in a range of applications such as tunable distributed feedback lasers (DFB lasers), monochromators, spectrometers, and strain sensors. The fabrication of these diffraction gratings relies on micro-imprinting, which requires high aspect ratio structures due to the low refractive index contrast between air (refractive index of 1.0) and the elastomer (generally in the range of 1.3#8209;1.7 for visible light), and is also limited in resolution. Another method reported to achieve mechanically tunable gratings is by wrinkling metal thin films on elastomeric substrates, which only allows sinusoidal gratings.
In this work, we propose to introduce oxide thin films, patterned as dot gratings, into an elastomeric material, namely polydimethylsiloxane (PDMS). By incorporating titanium dioxide (TiO₂) as the grating material, we exploit the high refractive index contrast between TiO₂ (~2.7) and PDMS (~1.4), which allows for the grating to have a compact footprint (125 µm²) and low film thickness (100 nm). We demonstrate the functionality as tunable grating structures at two wavelengths with operation at strain levels of up to 15%. Furthermore, we focus on the nano mechanics in strained TiO₂ diffractive elements backed by finite element modeling, in situ strained secondary electron imaging and analytical results.

In recent decades, artificial olfactory devices, known as electronic noses (e-noses), have been developed for disease detection and diagnosis by evaluation of exhaled volatile organic compounds (VOCs), produced from multiple metabolic processes. These devices consist of a cross-reactive sensor array, capable of interacting with multiple vapor analytes, a signal transduction mechanism, and response pattern recognition software. Our working hypothesis is that an increase in production of specific VOCs reflecting compromised metabolic processes found in patients suffering with a range of clinically significant anxiety symptoms could be detected with an e-nose. Such a fingerprint approach may also unveil possible biological pathways relevant to anxiety psychopathology.
This paper will present the design and fabrication of our barcoded resin-based (BCR) sensor array and its application for the detection and identification of VOCs. BCRs were prepared from a library of six alkylated styrene monomers, combined in a binary fashion resulting in 63 (2n-1) polymers. The same sub-library was resynthesized by including one of our 10 fluorinated monomer, resulting in 10 additional sub-libraries and a total of 700 copolymers. Upon interaction with an analyte, the vibrational signatures of the polymer array change, resulting in slight but detectable spectral variations for each BCR. The collective response (or analyte-specific patterns) was then be quantified using multivariate data analysis. This platform improves upon existing technologies as it dramatically increases sensitivity and information content using vibrational spectroscopy of a large library of sensory elements (encoded polymers)
Our current goal is to optimize a sensor array of BCRs for detecting clinically significant anxiety and stress VOCs with highest disease specificity in exhaled breath. We also plan to optimize (i.e. train) this device for the recognition of analytes of interest that will enhance our ability to render the most accurate anxiety diagnosis.

We present the results of depolarized light scattering of colloidal silver nanoprisms and their anisotropic properties upon orientation in a polyvinyl alcohol film via stretching of the polymer. We examine two sizes of silver nanoprisms with edge-to-thickness ratios of 2:1 and 5:1 and studied the nature of the anisotropic light scattering processes as well as the linear dichroism of the nanocomposites. The light anisotropy reaches a minimum value of 0.5 in the region from 350-400 nm and a value of 0.6 that extends to 700 nm. Our linear dichroism results confirm the distinction between in-plane and out-of-plane plasmonic resonance modes and show that it is possible to orient the nanoprisms by stretching nanoprism/polyvinyl alcohol nanocomposite films.

This report proposes modulating the work function of single-wall carbon nanotube (SWCNT) electrodes to form an Ohmic contact with active layer by adding aluminum-doped zinc-oxide nanoparticles (AZO NPs). The SWCNT films show potential for use in a variety of device applications as transparent electrodes. However, the low device performance of metal-oxide thin-film transistor (TFT) with the SWCNT electrodes was caused by the high contact resistance originating from the junction between the electrodes and active layer. Furthermore, the contact between the SWCNT electrodes and active layer was occurred as a Schottky barrier, resulting from the Fermi level mismatch due to a high work function of the SWCNTs. For these reasons, the electrical device performance of the solution-processed metal-oxide TFT was degraded.
In order to solve the problem, we fabricated the SWCNT-AZO NP hybrid electrodes and applied this structure to fabricated solution-processed flexible In₂O₃ TFT for reducing the work function and the contact resistance. Indium-oxide (In₂O₃) as an active layer was deposited on polyimide (PI) film substrate by spin-coating. Then the SWCNT-AZO NP hybrid electrodes were deposited on In₂O₃ layer using spin-coating and spray coating. These electrodes have exhibited the diversity of work function of SWCNTs with different densities of the AZO NPs by changing the coating time. Finally, ionic liquid-blended polyvinylphenol (IL-blended PVP) dielectric layer was deposited by spin-coating and Ag nanowires (NWs) as a top gate electrode was deposited by spray coating. Results indicate that the SWCNT-AZO NP hybrid electrodes considerably reduced the electrical contact resistance between the electrodes and active layer because the work function of the SWCNT-AZO NP hybrid electrodes decreased from 4.96 to 4.57 eV. Moreover, the device performance of the solution-processed flexible In₂O₃ TFT with the SWCNT-AZO NP hybrid electrodes showed a mobility of 5.42 cm²/V#8729;s and this mobility decreased by only 2% after a 1000 repeated bending stress at a radius of curvature of 3 mm.

Cell-seeded polymeric hydrogel scaffold is a potential method to deliver cells at damaged or diseased tissue locations to treat them. However, for orthopedic applications, this approach has not been very successful due to its failure to simultaneously provide load-bearing capability and facilitate cell migration via diffusion through the scaffold. To address this issue, this study investigates an inherently new concept, a two-phase hydrogel scaffold. The first-phase is cell-pre-seeded spheres and provide cell-friendly environment, while the continuous second-phase degrades relatively slowly and continues to mechanically support the construct. Tailorability of degradation rate, mechanical properties and cell-proliferation factors of the individual phases as well as in situ photopolymerizability makes two-phase hydrogels very promising for therapeutic applications.
The purpose of this study was to fabricate cell-seeded two-phase hydrogel scaffolds and assess the overall mechanical properties of the composite construct at different first-phase volume fractions. Two different two-phase hydrogel scaffolds were fabricated. In both scaffolds, poly(ethylene glycol) (PEG) hydrogel micro-spheres were used as first-phase. Either PEG hydrogels (higher wt. % compared to the first-phase) or alginate-polyacrylamide (PAAM) double network (DN) hydrogels were used as the second-phases. The mesenchymal stem cells (MSC)-seeded PEG micro-spheres were made with microfluidic flow focusing devices via photo polymerization. In vitro study showed high cell viability and chondrogenic activity after encapsulation into micro-spheres. For mechanical testing, cell-less PEG micro-spheres were packed in the continuous second-phase hydrogels at various volume ratios. Uniform-width, dog-bone and cylindrical shaped samples were prepared for mechanical testing. Samples made of second-phase materials, PEG hydrogels and the alginate-PAAM DN hydrogels were also tested for comparison with their two-phase counterparts. All the samples were tested in monotonic compression and tension. In addition to these, bulk first phase PEG samples were tested to obtain their mechanical properties. The test results showed that if the elastic modulus of first-phase is higher than the second-phase, the two-phase structure has higher elastic modulus and lower failure stress and strain compared to the second-phase material. On the contrary, the lower elastic modulus of the first-phase material compared to the second-phase material leads to lower elastic modulus; fracture stress and strain for the two-phase samples compared the second-phase material. In both cases, lowering the volume fraction of the first-phase made the two-phase system behave more similarly to the second-phase material.

We investigated the growth of nano- and micro-sized, doughnut-shaped, polymers of Polypyrrole (Ppy) modified onto a gold electrode by a simple and low-cost electrochemical polymerization method. The important thing is this Ppy&’s doughnut size could be adjusted by controlling the type of power source used.
Here we present a simple, yet versatile supercapacitor device using conducting polymer, Ppy. To achieve this, we created a various size of Ppy doughnut on the electrode by applying simple direct current (DC) amperometry and alternating current (AC) Electrochemical Impedance Spectroscopy (EIS). In DC amperometry, the change in current density over time was measured in the monomer pyrrole solution. The AC impedance spectroscopy and the magnitude and time variable values (frequency) were also measured in the monomer pyrrole solution. Finally, a larger size of Ppy doughnuts were produced at AC potential compared to Ppy doughnut made with DC potential. The device that Ppy deposited by AC potential also have high specific capacitance (540 F/g), accumulated charge (386 µC), power density (6.131 mW/cm²) compared with DC potential.
This is important to note that the maximum specific capacitance achieved in case of Ppy in acidic electrolyte is ~400 F/g. the stability of the AC polymerized supercapacitor for charge/ discharge cycles at 98 % work efficiency supports the applicability of the AC polymerization process for supercapacitor electrodes.

The primary goal of the field concerned with organic semiconductors is to produce devices with performance approaching that of silicon electronics, but with the deformability of conventional plastics. However, an inherent competition between mechanical compliance and charge transport has long been observed in these materials, and achieving the extreme (or even moderate) deformability implied by the word “plastic” concurrently with high charge transport may be elusive. This competition arises because the properties needed for high carrier mobilities-e.g., rigid chains in π-conjugated polymers and high degrees of crystallinity in the solid state-are antithetical to mechanical compliance. On the device scale, this competition leads to low-mobility, yet mechanically robust devices, or high-mobility devices that fail catastrophically (e.g., cracking, cohesive failure, and delamination) under strain. There are, however, some observations that contradict the notion of the mutual exclusivity of electronic and mechanical performance. These observations suggest that this problem may not be a fundamental trade-off, but rather an inconvenience that may be negotiated by a logical selection of materials and processing conditions. For example, the selection of the poly(3-alkylthiophene) with a critical side-chain length-poly(3-heptylthiophene) (n = 7)-married the high mechanical compliance of poly(3-octylthiophene) (n = 8) with the high electronic performance (as manifested in photovoltaic efficiency) of poly(3-hexylthiophene) (n = 6). This paper explores classes of materials in an effort to better understand the relationship between mechanical compliance and charge transport. The hypothesis that charge transport and mechanical compliance are mutually exclusive is tested. The principal conclusions are that to reduce the compliance-transport competition, the following design rules are identified: (1) decreasing the T_g of the polymer, with one possible route being the lengthening of alkyl side-chains to a critical length, n, (2) promotion of disorder in aggregation, (3) mixing highly compliant materials with high-mobility materials, and (4) the introduction of additives that behave as plasticizers. The aim of this paper is to provide a thorough understanding of the molecular determinants of mechanical compliance and charge transport, what morphological factors are critical for each, and how to decouple their mutual incompatibility, therefore allowing for rational design of materials for applications requiring large-area, low-cost, printable devices that are ultra-flexible or stretchable, such as organic photovoltaics (OPVs) or wearable, conformable, or implantable sensors.

Softness, flexibility, compliance and weight will turn out to be key metrics for future electronic appliances and power supplies[1,2]. Imperceptible electronics[3] integrates nanometer thin film active components on sub-2-mu;m polymer foils and creates devices unmatched in mechanical flexibility, stretchability and weight. Organometallic halide perovskites are capable of delivering very high power-per-weight when fabricated on ultrathin substrates, an important metric for wearable and ultraportable electronics, for remote sensing or for space applications.
Here we demonstrate methods to fabricate perovskite solar cells on 1.4mu;m thick PET substrates with 12% stabilized power conversion efficiency and a record-high solar cell power-per-weight of 26W/g. The solar cells are less than 2mu;m in total thickness and can be bent into radii smaller than 50mu;m. Sandwiched onto a pre-stretched elastomer our solar foils turn into soft, stretchable energy sources that endure 100% tensile strain. Our devices are fabricated from solution in ambient air at temperatures below 120°C to ensure process compatibility with ultrathin polymer foil substrates. Their unique mechanical properties are achieved with an all ITO/FTO free device architecture that does not require titanium oxide interlayers and avoids high sintering temperatures typically employed for rigid devices on glass substrates. Our low cost power sources operate in ambient atmosphere, conform to arbitrary shapes and provide electrical energy wherever high power-per-weight is critical, as in next generation ultra light portables, wearables, small-scale autonomous robots, unmanned areal vehicles and space exploration. The fabrication techniques and materials engineering approaches discussed here culminate in efficient, ultrathin, light, compliant and durable solar foils with high power-per-weight, mechanical flexibility, and prolonged outdoor operational stability.
The authors acknowledge funding from the FWF Wittgenstein award and the ERC advanced investigators grant “Soft Map”.
[1] D. Lipomi et al., Energy Environ. Sci. 4, 3314 (2011)
[2] M. Kaltenbrunner et al., Nature Commun. 3, 770 (2012)
[3] M. Kaltenbrunner et al., Nature 499, 458 (2013)

This talk will present a novel class of polymeric materials we developed recently: nanostructured electronic gels that are hierarchically porous, and structurally tunable in size, shape, composition, porosity and chemical interfaces. Given advantageous features such as intrinsic 3D nanostructured conducting framework, greatly improved electronic conductivity and excellent electrochemical activity to store and transport ions, they have been demonstrated useful for a number of technological applications in energy, bioelectronics, and environmental devices. Representative examples on energy storage and biosensors devices will be discussed to illustrate ‘structure-derived functions&’ of this special class of polymeric materials.

The serious energy shortage can be a major factor that limits the quality of life. Vast amount of acoustic energy from human talking, traffic noise, music etc, are ubiquitous but regretfully being ignored. Acoustic energy harvesting has not been as popular as other types of energy harvesting, and does not get extensively explored and well utilized, which is not only attributed to its much lower power density but also a lack of effective harvesting technology. Currently, the mechanisms of acoustic energy harvesting are limited to transductions based on piezoelectric effect, electrostatic effect and triboelectric effect. Wide range usage of these techniques is limited by factors such as low efficiency, high structure complexity, and large volumes due to the required resonance cavity that results in extremely low volume-specific-energy.
In this work, a 125 mu;m thickness, rollable, paper-based acoustic energy harvester triboelectric nanogenerator (TENG) has been developed for harvesting sound wave energy, which is capable of delivering a maximum power density of 121 mW/m² and 968 W/m³ under a sound pressure of 117 dB_SPL. The TENG is designed in the contact-separation mode using membranes that have rationally designed holes at one side. The TENG can be implemented onto a commercial cell phone for acoustic energy harvesting from human talking; the electricity generated can be used to charge a capacitor at a rate of 0.144 V/s. Additionally, owing to the superior advantages of a broad working bandwidth, thin structure and flexibility, a self-powered microphone for sound recording with rolled structure is demonstrated for all-sounding recording without an angular dependence. The concept and design presented in this work can be extensively applied to a variety of other circumstances for either energy-harvesting or sensing purposes, e.g. wearable and flexible electronics, military surveillance, jet engine noise reduction, low-cost implantable human ear and wireless technology applications.
References: (* indicate co-first author).
[1] X. Fan*, J. Chen*, J. Yang, P. Bai, Z. Li and Z. L. Wang. ACS Nano 9 (2015), 4236-4243.
[2] J. Chen*, G. Zhu*, J. Yang, Q. Jing, P. Bai, W. Yang, X. Qi, Y. Su and Z. L. Wang. ACS Nano 9 (2015), 105-116.
[3] J. Chen*, G. Zhu*, T. Zhang, Q. Jing and Z. L. Wang. Nat. Commun.5 (2014), 3426.
[4] J. Chen*, G. Zhu*, W. Yang, Q. Jing, P. Bai, Y. Yang, T. C. Hou and Z. L. Wang. Adv. Mater.25 (2013), 6094-6099.
[5] J. Chen*, J. Yang*, Z. Li, X. Fan, Y. Zi, Q. Jing, H. Guo, Z. Wen, K. C. Pradel, S. Niu and Z. L. Wang. ACS Nano 9 (2015), 3324-3331.

Derived from the Greek word meaning ‘to squeeze&’, piezoelectricity refers to the phenomenon where a solid material accumulates an electric charge following the application of pressure and, conversely, creates pressure when subjected to an electric field. First described by the French physicists Jacques and Pierre Curie, piezoelectric materials are widely used in modern electronics in the design of smart materials, and especially in the transformation of mechanical energy into an electronic signal. Common household items such as speakers, earphones and microphones invariably contain piezoelectric components, as do specialized equipment such as sonars, acoustic imaging devices and high-precision microbalances, as well as next-generation technologies such as piezoelectric memory devices and artificial skins and muscles. However, piezoelectric materials typically used in device production do not inherently display piezoelectric capacity, and must be processed through costly and complex fabrication steps prior to use. Many of these materials are ceramics, which further suffer from high brittleness, low cyclic endurances, high processing temperatures and high production costs, and are composed of toxic elements that preclude their use in implantable sensors. Polymer-based piezoelectric materials avoid many of these issues and are an attractive alternative to ceramic-based piezoelectronics - and we have recently developed a particularly effective example. Produced from poly(vinylidene floride) (PVDF) and its copolymer Poly(vinylidene floride)-tri(floraethylene) (PVDF-TrFE), our piezoelectric materials were developed through a technique called iterative size reduction, a low-cost thermal drawing method that allows the fabrication of well-ordered nanofibers in prodigious lengths. Possessing the high piezoelectric capacity (d₃₃ = 60 - 80 pm/V), thermal endurance and chemical resistance of ceramic piezoelectric materials with none of the drawbacks, the PVDF fibers are a strong candidate for the design of next-generation piezoelectronics. Using our polymeric materials, we have already designed a high-sensitivity tapping device and an energy-harvesting mechanism with enough power-generation capacity to run small devices such as pacemakers. In addition, we developed a new generation, transparent, passive matrix artificial skin with 10x10 pixel and a replica of a human palm with 21 pressure nodes and 17 angular motion sensitive points. The devices requires no external power sources and generates the self-requires-energy. Similar to the human skin, the artificial skin can detect temperature changes, shear force, high and low frequency pressure stimulations with different intensities. The artificial palm and electronic skin is promising for developing human-like shape and temperature sensing robots. Besides, when interfaced with neural systems, it could provide real touch feeling for disabled people.

Most device materials offer limited reversible elasticity. Micro-structuring a film with distributed cracks can result in macroscopic elasticity provided the carrier substrate is compliant. This mechanism enables the stretchability of thin gold films deposited and patterned on silicone rubber.
We have replicated the micro-crack structure in a range of stiff films, ranging from plastics to inorganic thin films using standard photolithography and etching processes. We report on the engineered elasticity of such films in freestanding or multilayered structures. The fracture strain scales with the film(s) thickness and micro-crack geometry and density.
We will present our preliminary results on the elastic performance of microstructured resistive, capacitive and semiconducting thin-film devices.

Soft, multifunctional materials are key components for emerging applications in wearable electronics, soft robotics, and human/machine interfaces. These applications demand soft materials that undergo large deformations in potentially non-planar configurations without imposing a large mechanical impedance mismatch with the component or user. Although various functional materials such as soft sensors, actuators, and active materials have been demonstrated, the integration of multiple functionalities into a single material is still an emerging area of research. Importantly, multifunctional materials require a comprehensive approach which simultaneously considers material integration schemes, geometric and scaling considerations, and fabrication techniques to enable efficient material investigation and assembly. Here, we start with soft, readily available films and assemble them through rapid prototyping techniques such as laser machining and adhesive controlled material transfer to efficiently create multifunctional soft materials. Characterization of sensor functionality is examined experimentally by simultaneously measuring mechanical deformation and electrical properties such as resistance and capacitance. Under in-plane, tensile deformation, electrical signals increases monotonically with strain while stretching over twice the original length. In the thru-thickness direction, pressure sensing is examined by creating ‘electronic skins&’ that can detect pressure from 0.2 to 20 kPa. Additionally, if these sensors are deformed beyond their sensing limit and conductivity is lost, the sensor can regain functionality by removing the applied strain. Actuator and on-demand material property control are further demonstrated and characterized through benchtop experiments. This work demonstrates a scalable approach to soft, multifunctional materials while offering versatility in design and functionality for a variety of applications through material selection and construction.

Current research efforts in combining elasticity and multifunctionality have focused primarily on the atomic, molecular, and nanoscale structural elements of these materials. Drawing inspiration from paper art, we present a new framework to engineer composites in which the third dimension is coupled to functionality. Here we look into controlling defects, borrowing concepts from kirigami, a Japanese paper cutting technique. We carry out a systematic study of the mechanical response of assembled nanocomposite sheets patterned with periodic arrays of cuts guiding stress concentration and distribution. We show that finite element analysis can predict the mechanism underlying this strain response and show that stress is delocalized around the cut defects, where unpredictable local failure is prevented and ultimate strain increased to >300%. We show that kirigami can enable the fabrication of highly elastic composites that remain conductive at high strains. Finally, we establish a systematic framework to predictively control mechanical properties by design.

Intrinsically stretchable conductors are a necessary component for the realization of wearable and implantable sensors and electronics since these surfaces have complex topographies that move and expand due to their softness [1]. Stretchable wiring is the foundation of realizing complex electronic devices since they serve as a bridge conventional rigid or flexible electronic components. For the fabrication of devices at high throughput, the ability to print robust and highly conductive elastic conductors constitutes a key challenge in the near future [2].
Herein we report printable stretchable conductors with a conductivity of >100 S/cm under >200% uniaxial strain. The elastic conductor shows conductivity even under 93% biaxial strain, 1800° twisting, and 1000 cycles of 10% strain. The printable elastic conductor ink is made from a blend of silver flakes, fluorine rubber, 4-methyl-2-pentanone, and fluorine surfactant water solution. The key mechanism to realize the high conductivity and stretchability is the self-assembly of a conductive network of silver flakes during the printing process. While the ink dries, the fluorine surfactant water solution triggers the phase separation of silver flakes and fluorine rubbers. As a result, high conductivity at a low volume fraction of silver flakes (23 vol%), which does not harm the softness or stretchability of the rubbers. Such a phenomenon was clearly observed with SEM. The amount of 4-methyl-2-pentanone was systematically changed, which resulted in the different localization of silver flakes and different conductivities. The developed elastic conductor can be printed on textiles and was thus used to realize various wearable electronic devices that include both a touch sensor and EMG measurement system.
The insight gained from systematic study of the mechanical and electrical properties will help to develop new elastic conductor formulations - for example, those based on high aspect ratio conductive fillers other elastomers, or other solvent systems.
[1] D.-H. Kim, et al., Science 333, 838-843 (2011).
[2] T. Sekitani, et al., Nat. Mater.8, 494-499 (2009).

Since the successful realization of two-dimensional (2D) graphene, it has been desired to form a connected, three-dimensional (3D) structure of graphene so as to exploit its extraordinary thermal and electrical properties. We are reporting the formation of free-standing graphene foam (GF) via a novel two-step process, in which a polyurethane (PU) foam is first dip-coated with graphene oxide (GO) and subsequently the dried GO-coated-PU is heated in nitrogen atmosphere at 1000°C. During the pyrolysis of the GO-coated-PU, GO is reduced to GF whereas PU is simultaneously decomposed and released completely as volatiles in a step wise mass-loss mechanism. The GF formed has tunable density, shape and scalability and possesses electrical conductivity as high as 160 S/m. Morphology of the formed GF conforms to that of the pure PU foam as indicated by the scanning electronic micrographs. Mechanical tests of the GF under compressive loads demonstrate that its mechanical behavior is similar to that of other cellular solids such as ceramics. Polydimethylsiloxane (PDMS) was successfully infiltrated inside the GF without effecting it&’s electrical properites. The GF-PDMS composite was tested for it&’s pressure and strain sensing capabilities. It is shown that a 30% compressive strain changes resistance of the GF-PDMS composite to about 800% of it&’s original value. The effect of GF density on it&’s pressure/strain sensivitiy is also studied, and it is found that a lower density GF results in a GF-PDMS composite of better pressure/strain sensitivity. Since density of the formed GF is tunable, therefore, the pressure/strain sensivity of the GF-PDMS composite is also tunable.

Wearable electronics are becoming more demanding due to their facile interaction with human body. These flexible and stretchable devices can be easily mount on clothing or directly attached onto skin. Wearable biosensors have been the subject of extensive research recently. In-situ monitoring of biological substances together with high functionality have long been the main challenges for fabrication of a wearable biosensor. Glucose sensors are account for the majority of the entire biosensor market. The discomfort and complications with conventional finger stick devices for glucose test reveal the critical need for the development of less invasive test methods using human body fluids other than blood.
In this study, we report a stretchable glucose sensor with high sensitivity and selectivity based on Ag/PDMS electrode fabricated by a cost-effective wet-metallization process. Sylgard 184 elastomer with 10:1 ratio of monomer to cross-linking agent was spin-coated on Si wafer. PDMS substrate was subjected to oxygen plasma and immediately silanized in a solution of Aminopropyltrimethoxysilane (APTMS) self-assembled monolayers (SAM). The silanized surface was later activated by immersion in palladium chloride solution. Silver deposition process was carried out on PDMS using an electroless bath containing silver nitrite salt and glucose as reducing agent in room temperature. The silver electrode was then coated by platinum using hydrochloroplatinic bath providing the catalytic surface for glucose sensing. The electrode was later functionalized using an enzyme solution of glucose oxidase (GOx) stabilized with bovine serum albumin (BSA). Immobilization of the enzyme was performed by cross-linking with Glutataldehyde (GTA). Nafion was coated on the sensing area for interference prevention from other electro-active species. Electrochemical characterization of the amperometric glucose sensor was performed in a 3-electrode configuration using Ag/AgCl electrode as reference and Pt wire as counter electrode. Cyclic voltammetry response of the sensor in PBS solution was recorded to select the proper potential for glucose detection. The chronoamperometry tests were then carried out at 0.5 V to obtain the calibration curve. Interference studies were done in presence of ascorbic and uric acid. The sensor showed an acceptable linear range with high selectivity toward glucose.

Responsive polymers and composites that undergo rapid color change in response to specific stimuli can be exploited in simple, low cost environmental and biomedical sensors. This talk describes (1) thermoresponsive liquid crystal devices in soft, skin-like formats for precision thermal characterization of the epidermis, and (2) strain-tunable, 3D plasmonic systems that combine gold nanodisks with low modulus, high elongation elastomers.

Epidermal electronics and soft robotics are harnessing the advantages of the adaptable and compliant soft contacts of soft materials. Soft materials can conformally match the morphology of the contact surfaces, which is the vital point in the implementation of epidermal sensors or soft actuators (S. Xu et al., Soft Microfluidic Assemblies of Sensors, Circuits, and Radios for the Skin, Science, 2014, 344, 70-74). Unfortunately, this conformality can suffer from delamination or air trapping at the interface during contact movement. Here, adhesion of soft material surfaces is the critical parameter. For example, an epidermal sensor on human internal organs and skin, or soft-robotic fingers for grabbing or climbing needs proper adhesion to its targeted contact surface. Mechanical softness of elastomer materials provides a good ensemble with surface adhesion because the conformal contact of the soft materials assists the adhesion on the target surfaces. Alas, when the soft device is thicker, with its inherent adhesion its compliance may not suffice but an adhesive layer is needed to ensure good contact.
Sticky surfaces of soft materials will significantly help to improve adhesion on target surfaces by preventing sliding. Therefore, more reliable immobilization and manipulation of contacted objects can be secured. Physical and chemical adhesion forces of the soft material surfaces can be utilized for this purpose. We have developed a sticky elastomer composite based on PDMS, which has a tape-like adhesive surface after curing. This sticky elastomer composite is stretchable and compliant. The processability of it is compatible with PDMS processes for microfluidic stretchable devices. It can be easily shaped with laminating, spinning and casting before curing. And, it is reusable several times without leaving residues on the adhered surfaces after detaching and its adhesive strength is tunable with different mixing ratios with the additive.
The sticky elastomer composite showed high enough adhesion to secure attachment on human skin and to lift small objects with different surface roughness. Here, soft fingers lifting masses which have different surface morphologies were tested to verify the compatibility of adhesion force on various surface conditions for soft-robotic manipulation application. To show the easy and robust implementation, the sticky elastomer composite is demonstrated with a stretchable sensor patch that can be secured to human skin, using much of our recently developed pamphlet of processing technologies (Z.G. Wu, K. Hjort, S.H. Jeong, Microfluidic Stretchable Radio Frequency Devices, Proceedings of the IEEE, 2015, 99, 1-15). Such sensor patches may be suitable as wireless sensor nodes in epidermal body area networks for fitness and healthcare monitoring.

The development of a new sensor for the continuous detection of bio-analytes is a key point in the progress of the healthcare research in order to improve prevention, diagnostics and patient monitoring. A very appealing solution is the integration of the bio-sensing system in fabrics and clothes, so as to obtain a non-invasive active control of the health of patients or of the sport performance of athletes. In order to be suitable to real world applications, a sensor, together with its readout electronics, has to be light-weigth, handy, and cheap. Organic Electrochemical Transistors (OECT) are an emerging technology that satisfies such demands. An OECT is composed by a stripe of conductive polymer that works as a channel, and by another electrode, usually a metal, that works as a gate. When the device is dipped in an electrolyte solution, the current flowing in the channel can be modulated through the gate voltage exploiting electrochemical reactions. Redox compounds can also act on such processes by changing the current density inside the channel. In recent literature the production of OECT-based sensors integrated in a textile¹ for the detection of adrenaline and ionic content in sweat has been reported. Unfortunately the use of a wire of noble metal as gate electrode increases the cost of the device, and reduces its comfort².
This contribution reports on a strategy for overcoming such drawbacks through the use of an OECT wherein both gate and channel are made by a conducting polymer, poly(ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). PEDOT:PSS was deposited by screen printing on different textiles and the devices stability was evaluated during real use and after submitting them to several washing cycles and measuring their electrical performance³. The cleansing increases the sheet resistance of PEDOT:PSS, which after a few cycles reaches a stable value that does not increase further after the following washes. The potentialities of the here described OECT as a sensor were tested in phosphate buffer saline using different redox active bio-molecules (adrenaline, dopamine and ascorbic acid). All tested analytes react with PEDOT:PSS by extracting charge carriers from transistor channel and leading to a decrease of the drain current that results linear dependent on the logarithm of their concentrations. The OECT sensing capability has been assessed in two different experimental contexts: i) totally dipped in an electrolyte solution, to evaluate their performance in the ideal operation; ii) by sequentially adding few drops of electrolyte solution in the area between the gate and channel in order to simulate the exposure of the fabric to human sweat in real application. This contribution wants to demonstrate that all-polymer OECT devices offer a valid platform for developing wearable sensors.
¹Mattana et al., Org. Electron. 12 (2011) 2033
²Coppedè et al., J. Mater. Chem. B, 2 (2014) 5620
³Gualandi et al., in press

Recent advances in soft electronics have attracted great attention due in large to the potential applications in personalized, bio-integrated healthcare devices. The mechanical mismatch between conventional electronic/optoelectronic devices and soft human tissues/organs causes many challenges, such as the low signal to noise ratio of biosensors because of the incomplete integration of rigid devices with the body, inflammations and excessive immune responses of implanted stiff devices originated from frictions and foreign nature to biotic systems, and the huge discomfort and consequent stress to users in wearing/implanting these devices. Ultraflexible and stretchable electronic and optoelectronic devices utilize the low system modulus and the intrinsic system-level softness to solve these issues. Here, we describe our unique strategies in the synthesis of nanoscale materials, their seamless assembly and integration, and corresponding device designs toward wearable and implantable healthcare devices. Good examples include wearable quantum dot light emitting diodes (QLEDs) potentially used for medical information input/output routes of integrated healthcare sensors and transdermal therapeutic devices as well as the multifunctional implantable electronic stent and minimally invasive surgical tools to solve specific cardiovascular and colorectal diseases respectively. These implantable and wearable bioelectronic systems combine recent breakthroughs in unconventional soft electronics to address unsolved issues in the clinical medicine, which provides new opportunities the personalized healthcare.

Temperature control plays a very important role in homeostasis, and body temperature varies both spatially and temporally in an effort to transfer heat between the living body and the environment via skin and respiratory organs. Accurate measurement of localized temperature changes in soft tissue regardless of large scale motion is important in understanding thermal phenomena of homeostasis and realizing future sophisticated health diagnostics. We demonstrate large-area arrays of conformal temperature sensors which measure millikelvin-scale physiological thermal events from the skin including respiration, blood flow through arteries, and local muscle activity. Positive temperature coefficient (PTC) polymers with nanoscale conductive fillers are used as the temperature sensing material with extraordinary large changes in resistance with temperature. These materials exhibit repeatable, six-order-of-magnitude changes in resistivity over five degrees C near body temperature, enabling sensitivity below 20 milliKelvin. An ultra-thin geometry enables flexibility below radii of 200 um and a high speed response time of less than 100 milliseconds. Device repeatability over 1800 thermal cycles is demonstrated. The sensing temperature can be tuned between 25 to 50 degrees Celsius with 0.15 degree C accuracy which covers all relevant physiological temperatures and enables precise tuning.
Semi-crystalline acrylate copolymers are synthesized with a melt temperature near body temperature and subsequently loaded with conductive carbon nanoparticles. Progression through the melt transition of the polymer matrix induces an increase in volume and expansion of the inter-particle spacing. This expansion causes changes large scale resistivity changes in the composite material. Typical PTC materials suffer from low repeatability due to agglomeration of the conductive filler over thermal cycles, which is mitigated here by using a side-chain crystallizing polymer. Crystallizing side chains increase chain entanglement and prevent particle migration over time. Additionally, modulating the average side chain length between 10 and 18 carbons allows precise temperature tuning between 25 and 50 C. Temperature sensing from the skin requires the combination of sensitivity, fast response time, stability in physiological environments and multi-point measurement, which is difficult to demonstrate with conventional temperature sensors including thermocouples, PN diodes, and TCR metals.

The concept of generating dynamic animations using chemical diffusion from arrays of porous voxels is discussed. Voxels can be printed on a wide range of rigid or flexible substrates to create animations. No batteries or wires are required and voxels are reusable. Using simulations and experiments we show how an animation of a running man can be generated in a stationary medium. Our studies highlight how patterns can be designed in silico and used to create organized spatio-temporal patterns. We anticipate that this concept can be used with a wide range of controlled release particles and capsules to generate dynamic patterns for a range of visual and scientific applications.

Electronic skin (e-skin), mimicking human skin cutaneous function and mechanical property, is developing rapidly driven by vast applications including human-machine interfacing, robotic sensing and bio-medical engineering. Many works have made contributions to the advancement of e-skin to enhance its sensitivity. However, there are two problems: lack of stretchable power supply and incapability of detecting multi stimuli.
We developed a 3D e-skin which can detect different stimuli (including strain, bending and pressure) and harvest mechanical energy. The device is composed of 2 orthogonal functional layers with a 3mm gap between them. Top functional layer uses PDMS as substrate and encapsulation with S-shape PEDOT:PSS electrode in it. Bottom layer has similar structure except the orthogonal direction of S-shape electrode. Then, the bottom PDMS layer is treated by C₄F₈ (C-PDMS). Between these 2 layers, four PDMS pillars are fabricated on the corners to separate them. As PDMS and PEDOT:PSS have light transmittance as high as 80% and can tolerant 30% strain, the device is stretchable and transparent.
Strain, bending and pressure sensing are detected by the change of capacitance and resistance. When a force is applied on the surface of the sensor, the air gap will decrease resulting larger capacitance and reflect the magnitude of the force. S-shape electrode is designed as strain gauge, sensitive to strain along its winding direction. Two orthogonal strain gauges can give the detailed direction of strain through comparing change of two resistances.
Meanwhile, the device can harvest mechanical energy while user operating it. PDMS and C-PDMS serve as triboelectric pair. When the device is compressed, the PDMS will contact C-PDMS. Because of C4F8 treatment, PDMS tends to carry positive charges while C-PDMS tends to carry negative charges. After pressure is released, electrons will transfer between two electrodes as the gap increasing. In this way, the device can charge a battery or a super capacitance through specific circuit.
In the test, 0-10kPa is applied on the sensor to measure pressure. It can be seen that in the low pressure region, the sensor has higher sensitivity due to the change in air gap. After the two layer are fully contacted, the sensitivity will decrease dramatically. Similar results appear in strain test (0-30%) and bending test (0-600). Strain and bending direction(00, 300, 450, 600 and 900) lead to different changes in two strain gauges. Detailed direction can be got through calculation. The peak-peak triboelectric output voltage can be reach to 90V when measured using 100 MOmega; probe at 1Hz.
In summary, we develop a 3D structure e-skin which can detect multi stimuli and harvest energy. Change in the capacitance corresponds to the magnitude of different stimuli and changes in the orthogonal resistances reflect the direction of strain and bending. Based on triboelectrification effect, the device can harvest energy by touched.

Polymer submicron fibers have unique properties including small diameter, light weight, mechanical flexibility, and large surface-to-volume ratio. Because such fibers also have inherent waveguide structures, they are promising as building blocks for small, mechanically flexible optical devices such as waveguides, light sources, sensors, and resonators. In this study, we fabricated mechanically flexible polymer submicron fiber waveguides doped with organic dyes by electrospinning and demonstrate wavelength conversion and single-mode waveguiding. Aligned electrospun submicron fibers composed of poly(DL-lactic acid) (PLA) doped with 7.6×10^minus;2 wt% rhodamine 6G (R6G) were prepared. The mean diameters of four single PLA/R6G fibers, investigated along a 120-mu;m length of each fiber, ranged from 261 to 315 nm. Individual PLA/R6G fibers were clad with Cytop and irradiated by a 532-nm laser beam perpendicular to the fiber axis. The 532-nm light was converted to photoluminescence (PL) from the doped R6G with wavelength >532 nm at the excitation spot and the PL was guided in the waveguides. The intensity of the guided light with wavelength from 590 to 670 nm with increasing propagation length was measured at the end of the fiber. The calculated normalized frequency (V) was 0.706-0.967 assuming that the refractive indices of PLA and Cytop were 1.46 and 1.34, respectively, the diameter of each fiber was 261-315 nm, and the wavelength of the guided light was 590-670 nm. Consequently, the waveguides allowed only a single mode (LP₀₁ mode) to propagate because the calculated V was less than 2.405, which is the cutoff of V for the LP₁₁ mode. We evaluated propagation loss (α) in the waveguides by considering the attenuation of the guided PL intensity with increasing propagation length. For the four fibers, α was <44 dB cm^minus;1 for wavelengths from 590 to 670 nm. To determine the origin of the high α in the waveguides, we examined reabsorption loss by R6G molecules. Transmittance measurements of an R6G solution indicated that the reabsorption loss from 590 to 670 nm was less than 2.5 dB cm^minus;1, and less than 0.8 dB cm^minus;1 at >600 nm. This reabsorption loss is less than one-tenth of α, indicating that the propagation losses in the waveguide are mainly inherent to the PLA fiber itself. We found that α increased linearly with (wavelength)^minus;4, so we estimated the loss at the interface between the fiber core and cladding originating from non-uniformity within the fibers (α_un), and loss caused by excess light scattering in the fibers resulting from density inhomogeneity of PLA (α_sc) from the fitted linear functions. For the four waveguides, α_un and α_sc were 3.1-24 and 11-28 dB cm^minus;1 at 650 nm, respectively.

Functionalities only associated with rigid electronics have been demonstrated on a flexible substrate, indicating that flexible devices have a high potential to operate on par with rigid electronics. Furthermore, flexible electronics offer a distinct advantage by organically conforming to irregular surfaces, this enables a new class of electronics that seamlessly integrates with the human body. Fully stretchable electronics pose a challenge for material science and micro fabrication to create devices with the ability to operate impeccably under various mechanically-stressed states. Here, we introduce a distinctive micro-tectonic effect to enable oxygen-deficient, nano-patterned zinc oxide (ZnO) thin films on an elastomeric substrate to realize large area, stretchable, transparent, and ultra-portable sensors. We harness the unique surface structure to create stretchable gas and ultra-violet light sensors, both of which outperform their rigid counterparts under room temperature conditions. The sensors show a high sensitivity to flammable and toxic gases as well as radiation in the UV-A and B band. We characterise the device performance in undeformed and strained states using customised in situ techniques. We demonstrate full functionality under strain as well as an increased sensitivity through micro-tectonic surfaces by comparison to their rigid counterparts. Additionally we show excellent control over dimensions by embedding nanometre ZnO features in an elastomeric matrix which function as tunable diffraction gratings, capable of sensing displacements with nanometre accuracy.
Related Reference:
1. P. Gutruf et al., Small in press (2015).

The skins and surfaces of many animals and plants have fascinating functions unprecedented by engineering devices and coatings. For example, cephalopods can display on-demand patterns of colors and surface textures to dynamically hide in varying environmental backgrounds. The leaves of many plants have high-aspect-ratio topographical patterns that endows them with superhydrophobicity and self-cleaning functions. In this talk, I will discuss a general strategy to design new engineering surfaces capable of transformative functions, or so-called transformative skins, by harnessing instabilities in soft active materials. I will first present a three-dimensional phase diagram that can quantitatively predict various modes of surface instabilities, including wrinkles, creases, delaminated buckles, period doubles, localized ridges, and folds with length scales ranging from nanomaters to meters. I will then discuss strategies to harness these instabilities on demand to actuate novel active materials such as mechanophores and graphene sheets to achieve bio-inspired functions such as superhydrophobicity, tunable stem-cell alignment, and dynamic camouflage.

The surest strategy to promote organic solar cells (OSCs) from laboratory-scale demonstrations to use in the real world is to exploit the advantages possessed by organics that would be difficult or impossible to replicate in more-efficient competing technologies. Such advantages include low cost and embodied energy, extreme thinness, tunable color, biodegradability, semitransparency, extreme flexibility, and stretchability. These characteristics suggest that portable power for displays, mobile health monitoring devices, and mitigation of climate change triggered by burning of biomass in the developing world are—far from applications dismissible as “niche”—important problems for which organic solar cells may provide the ideal solutions. In an effort to understand and anticipate routes of mechanical and photochemical degradation for all-organic solar cells under realistic operating conditions, we fabricated the first organic solar cells that can be mounted on and conformed to hemispherical surfaces and the human skin, and used them to power wearable devices in the outdoor environment.

Electronic skins with location abilities are commonly based on pixelated sensing array. Array structure can realize high spatial resolution location with higher sensor and wire densities, however, the increasing number of terminals complicate circuits and raise power consumption. Here, we present a high spatial resolution bidimensional unpixelated electronic skin with only four terminals. Meanwhile, this electronic skin is self-powered because of the new location strategy based on the combination of single-electrode contact electrification effect and horizontal electrostatic induction.
Spin-coated silver nanowire (Ag NW) electrodes which are flexible and transparent are fabricated along the four edges of polyethylene terephthalate (PET) film substrate. A thin polydimethylsiloxane (PDMS) film with pyramid structures is covered on the PET film and electrodes as friction surface. When a gentle touch is applied on this flexible and transparent electronic skin, it will cause charges transfer between two friction surfaces. With the movement, for example contact or separation, of the charged friction surfaces, the charge distributions on the four electrodes are affected and currents are formed in load circuits. Because of the different distances between touch point and four electrodes, the voltages of four electrodes will differ from each other. With the analysis of different voltages of opposite electrodes, the location can be confirmed in x and y direction respectively.
Both theoretical analysis and experiments will be discussed. Electrostatic theory and equivalent circuit model of this device reveals the potentials of high-resolution and rapid response. To demonstrate our approach, the electronic skin with sides 7 cm was tested on plant surface and curved surface. The average resolution achieves 1.9 mm after repeated tests both on two kinds of surfaces through statistical analysis and the deviation in the location test on curved surface can be as less as 0.3 mm. The typical voltage ratio of two opposite electrodes markedly changes from 0.49 to 1.98 (voltage of one electrode changes from ~7 V to ~14 V) when touch point moves 5 cm. Moreover, this smart skin is with high sensitivity that the disturbance of a honey bee can be located. Less number of terminals and characters of self-powered, high solution, flexibility and transparency bring this unpixelated electronic skin a bright future on artificial intelligence and portable electronics.
1, M. Hammock et al. Adv. Mater. 25, 5997-6038 (2013)
2, S. Mannsfeld et al. Nature Mater. 9, 859-864 (2010).
3, J. Kim et al. Nature Commun. 5, (2014).
4, M. Shi et al. Adv. Mater., submitted.
5, L. Lin et al. ACS Nano. 7, 8266-8274 (2013)
6, F. Yi et al. Adv. Funct. Mater. 24, 7488-7494 (2014)

Tobacco cells and carbon nanotubes (CNTs) have been recently combined in a new material called “cyberwood” that showed a giant temperature response due to the presence of pectin and Ca²⁺ ions inside the plant cell wall. It has been demonstrated that changes in temperature result in different gelation rates of the pectin, resulting in subsequent change of the number of ions free to move when an electric potential is applied across the material. The presence of CNTs partially penetrating the cell walls stabilizes the response of the material at higher temperatures and creates an intercellular matrix that provides mechanical stability to the material.
Here, we show how new pectin-mediated, temperature sensing, soft materials can be synthetized without the need of growing plant cells. We discuss different material compositions and the role of critical parameters governing their temperature sensitivity. We characterize the new materials&’ microstructure and mechanical properties. The possibility to obtain highly sensitive materials from direct synthesis introduces a new class of soft materials for temperature sensors that can be employed in a large number of applications. We demonstrate some of the potential applications and discuss about the technological challenges in the production of this new materials.

Indium tin oxide (ITO) has been widely used as the electrode material in touch-screen displays and solar cells attributing to its combined high electrical conductivity and optical transparency. Moving forward from wafer based electronics to flexible/stretchable electronics, brittle electronic materials like ITO are significantly hindering the deformability of the integrated systems. To minimize strains in inorganic materials when subjected to stretch, thin metallic and ceramic films can be patterned into serpentine-shaped ribbons. Although polymer-supported metallic serpentines have received extensive studies, it has been a challenging task to fabricate brittle ceramic serpentine ribbons on stretchable substrates. In this talk, we report a low cost, completely dry fabrication process to successfully integrate brittle ITO serpentine ribbons on stretchable substrates. Uniaxial tension tests are performed with in situ electrical resistance measurements which are used as an indicator of the mechanical integrity of the ITO ribbons. Effects of serpentine-substrate adhesion and serpentine geometry are systematically investigated. When the adhesion is weak, stretchability as high as 200% can be achieved. When the adhesion is strong, a new failure mechanism is observed. Design guidelines can be proposed for different adhesion conditions based on this study.

We have developed a polymer composite comprising surface-embedded silver nanowires with high transparency, high surface conductivity, and low surface roughness. The mechanical properties of the transparent composite electrode are determined by the polymer matrix employed, and demonstrated properties include flexibility, shape memory, self-healing, and rubbery deformation. The new electrode has been used to replace ITO/glass in fabricating organic light emitting diodes with significantly enhanced luminous efficiency. We have also demonstrated stretchable transparent Joule heater, touch and pressure sensors, large-strain actuators, and polymer photovoltaic cells, all with high mechanical flexibility. OLEDs comprising two composite electrodes sandwiching an emissive polymer layer, can be stretched by as much as 140% linear strain.

Stretchable and flexible electronics represent the emerging paradigm of electronic functionality under substantial strain. Conforming sensing devices, in which a grid of interconnected sensing elements provides integrated information over a wide surface with complex shapes, are potential examples. Such devices are typically heterogeneous systems with components exhibiting high mechanical mismatch, which comes into play when strain is applied to the whole system. Mechanical and, consistently, electrical reliability is therefore a key point in the co-engineering design process. In particular, reliability of deformable electrical interconnects is still an open issue. These usually consist in thin metal films bonded on compliant polymer substrates. The ability to grant constant and high conductivity upon significant strain strongly depends on film thickness, geometrical design and material selection, which are mutually involved in determining failure mechanisms (namely, fracture of the metal film and metal-polymer delamination).
In this work, the selected strategy for compliant interconnects is that of S-shaped planar meanders of metal coating on polymer. The study is aimed at investigating their deformation and failure mechanisms upon stretching of the polymer substrates up to 40% tensile strain. In particular, interconnect samples consist of 1 µm thick Aluminum conductive coating evaporated on a 10 µm thick Polyimide substrate. Five different geometrical designs of the meanders have been investigated.
Uniaxial tensile tests have been performed with in-situ observation of the deformation mechanisms. Both Scanning Electron Microscopy (SEM) and Confocal Laser Scanning Microscopy (CLSM) have been used in combination with a suitably designed micro-tensile stage.
The main result of the in-situ investigation is that, depending on the design parameters of the meander geometry, the leading failure mechanism can be either (a) shear-based or (b) buckling-based delamination. A relevant role was played by the length of rectilinear arms: specifically, longer arms promote buckling-based delamination induced by the polymer transverse shortening, irrespective of the meander width. According to in-situ topographies by CLSM, the amplitude of buckling ranges from 5 to 30 µm.
Upon increasing stretch, in-situ SEM observations reveal detrimental effects such as local fracture: as confirmed by in-situ CLSM height maps, these phenomena are related to the interfacial failure, since facture occurs at the boundary between delaminated regions and bonded areas.
Besides showing the relation between interconnect design and failure mechanism experienced by the metal film, this study emphasizes the role of metal/polymer adhesion as a primary feature to be enhanced in order to achieve higher stretchability.

Emerging additive manufacturing fabrication methods offer promising alternatives to traditional rigid manufacturing tooling and processing for the nascent field of flexible hybrid electronics (FHE) (Nathan et al., Proc. IEEE 100, 2012). Additive manufacturing techniques allow for rapid design prototyping (Vaezi et al.,Int J Adv Manuf Tech, 2013) and, more recently, incorporation of novel materials into complex device geometries (Kim et al.,ACS Appl. Mater. Interfaces, 2013; Folgar et al., Mater. Lett.,2011) . One specific additive manufacturing technique is aerosol inkjet printing, which can precisely deposit any aerosolizable bulk material onto a variety of surfaces - curved, rigid, flexible, nonuniform, elastic, and hybrid (Adams et al., Electron. Lett, 2015). We have investigated the properties of aerosol inkjet printed conductors, specifically silver micro flake and CNT inks, on PDMS and compared them to more conventional sputtered copper and evaporated silver traces.
There are no specific IPC, ASTM or MIL standard characterization techniques for flexible and stretchable devices (DoD FOA-RQKM-2015-0014), though some initial work has been done to adapt these standards to flexible products (IPC-TM-650, # 2.4.3). We have developed a material-agnostic set of mechanical tests for FHE that are characterized by performance in response to tensile and flexural strains. These metrics apply the critical aspects of industry standards for adhesion (IPC-TM-650, #2.4.1.6) and electrical performance (ASTM F1711-96) of conventional electronics to soft dielectric conductor pairings. The electrical properties and interfacial adhesions of sputtered and evaporated metals on PDMS have been characterized under single and cyclic flexural and tensile strains up to 50% and compared to the performance of aerosol inkjet printed materials. Determination of the relative elasticities and critical strain limits of the various conductors will qualify the suitability of traditional and novel materials and methods to FHE platforms, such as implantable biosensors, conformal RF communication systems, wearable electronic devices, zero-stress chip packaging, and more.

In electrophysiological/chemical measurements, for both in vivo and in vitro, there is a need for electrodes that provide a recording of a high quality electronic signal. The devices presented nowadays are mostly made of planar biocompatible materials and do not provide enough contact with the cells due to the cleft formation between the junctional membrane of the cell and the electrode surface; globally resulting in a higher electrode impedance (Z_electrode) and lower coupling coefficient. Fabrication of reliable repeatable periodic structures of conducting polymers down to tens of nanometer size is then a key in such bioelectronics areas and in biological applications in general since they permit the ionic conduction and at the same time can allow controlling cell guidance based on the topographical features.
In this work we present a reliable and highly reproducible fabrication technique for patterning arrays of pillars, holes, gratings, and grooves of varied sizes, from 2 mu;m down to 45 nm of PEDOT:PSS {poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate)} from silicon or quartz or PDMS master mold. Then we show the mechanical stability of such structures in aqueous medium as well as the effect of the percentage of the cross linker in the conducting polymer solution on their strength. These parameters are studied along with their effect on the conductivity. Finally the electrochemical impedance of the structured conducting polymer is compared to the flat one to show the effect of the surface-to-volume ratio as a key parameter. The obtained structured conducting polymer can be integrated in active devices like OECTs as well as passive electrodes for biological electrical recording like in fluidic systems.

Knowing how to characterize and optimize thermomechanical properties is essential to improve the overall stability and mechanical integrity of organic and stretchable electronics. Semiconducting polymer and small molecule devices in particular are frequently mechanically fragile with very low values of interface adhesion and layer cohesion. In addition, they exhibit extreme sensitivity to environmental species and effective and durable barrier technologies are important for reliable function.
We identify and characterize important thermomechanical properties for a number of material and interface systems relevant to organic and stretchable electronics. This includes active semiconducting polymer and charge separation layers, encapsulation and ultra-barrier layers, transparent electrodes and optical elements. We describe research to characterize the coupled thermo-mechanical and photo-chemical degradation mechanisms that determine reliability and operational lifetimes. Studies include quantitative in-situ characterization techniques to measure the synergistic effect of mechanical stresses, temperature, environmental species and the presence of in-situ simulated solar UV light on inherent thermomechanical properties including interface debonding kinetics and cohesive failure of layers. Implications to optimize materials and processing methods, develop accelerated test methods and provide the fundamental basis for realistic lifetime predictions are described.

In the fast-growing field of stretchable electronics, many applications, such as wearable electronics, may require devices that can withstand unexpected mechanical trauma such as punctures and sudden impacts. We describe stretchable and mechanically durable transistors composed of carbon nanotube conductors and semiconductors and a tough thermoplastic elastomer as the substrate and dielectric. The devices have an average mobility of 0.2 cm²/Vs, and an on/off ratio of 10⁴. After stretching to 100% strain, the on current and mobility decrease to ~65% of their original values. After this initial conditioning step, the electrical characteristics remain constant with strain for at least 1000 stretching cycles. The strain-dependent characteristics are similar in orthogonal stretching directions, indicating that the device can be stretched in any direction with similar impacts on the electrical performance. However, when stretching in orthogonal directions, the threshold voltage shifts in opposite directions, caused by the change in electric field strength with changing channel length. Carbon nanotubes have very high tensile strength (~1 TPa), which contributes to the durability of the network. Furthermore, we implemented a tough and durable thermoplastic elastomer that had a high tear strength. Consequently, the devices could be impacted with a hammer and punctured with a needle while remaining functional and stretchable. Retaining stretchability while punctured with a needle is facilitated by the high tear strength of the thermoplastic elastomer.

Organic solar (OPV) cells are cheap electronics that can replace the widely used high cost silicon-based electronics for electricity generation. They are cheap because of the easy techniques involved in their fabrication processes and they can be produced to cover a large surface area. However, the current low performance of organic electronics has been traced to failure due to interfacial adhesion problems, material processes, and service conditions. Therefore, transportation of charge carriers across the bulk heterojunction system of OPV cells becomes very difficult in the presence of these flaws. In this paper a combined experimental and computational technique is used to study the reliability and physics failure of stretchable OPV cells. Interfacial adhesion energies in the layered structures of OPV cells are measured and compared with theoretical estimated energies. The limit stresses/strains applied on layered OPV cells during service condition are estimated using critical values of the measured interfacial adhesion. The results obtained are, therefore, explained to improve the design of reliable OPV cells.

In materials science, the concept of porosity provides the means to tune and/or introduce novel properties to dense materials. A good illustration is porous thin films, where the electrical, optical and adsorption properties are tuned by controlling porosity.[1,2] The accurate determination of mechanical properties is a critical requirement in understanding structure-property relationships and enabling implementation into technologically relevant devices.[3.4] As of today, nanoindentation is the technique of choice but it is widely accepted that materials densification and substrate effects are two inevitable drawbacks of this method when applied to porous thin-films.[5] We have defined a novel protocol based on ultra-low load, quasi-static nanoindentation to measure mechanical properties of porous thin films as a function of porosity (0-60%) and thickness (150-700nm). We prove that densification is not random but follows known mathematical scaling rules. It is now possible to account for its influence and demonstrate that the substrate doesn&’t play a role until a contact depth to film thickness ratio of about 40% for a 150nm thick film. As a result, Young&’s modulus for porous, thin films at the nanoscale can be precisely determined.
[1] W. Volksen, R.D. Miller, and G. Dubois, Chem. Rev., 2010, 110(1), 56-110.
[2] W.C. Tien, and A.K. Chu, Sol. Energy Mater. Sol. Cells, 2014, 120(PA), 18-22
[3] M.S. Oliver , G. Dubois, M. Sherwood, D.M. Gage and R.H. Dauskardt, Adv. Funct. Mater., 2010. 20(17), 2884-2892.
[4] W. Volksen, K. Lionti, T. Magbitang, G. Dubois, Scripta Mater., 2014, 74, 19-24.
[5] X. Chen, Y. Xiang and J.J. Vlassak, J. Mater. Res., 2006, 21, 715-24.

What do we mean by the' Power of soft'? ie soft, responsive, stretchable materials that can generate physical &/or sensory effects while at the same time providing information that can be transformed into a digital feed that can provide actionable output.(Digital - Physical Fusion). A critique of relevant literature pertaining to wellness, medicine and personalised healthcare monitoring indicated that we would have to be able to differentiate current ideas about the human body and interactions around it in new 'calm' forms of sensing, detection and actuation if we were to create burdenless sensor environments for human benefit.
An obvious starting point then became the search for responsive, stretchable materials and the tools by which to fabricate soft material systems. The initial search took us to rubber, dielectric actuating polymers and onto stimuli responsivehydrogels and variants on piezo polymers that could have the biocompatible and biomechanical properties that we were looking for.
From the initial study of sensing and actuation, we were able to focus on a 'Soft technology platform' using eveidence based knowledge gained using an extrusion writing system(Envisiontec Bioplotter), we were able to develop a methodology that could be applied through innovative design ie cutting, patterning and structuring in 2,3 D. These exploit responsive, malleable and functional polymers that can geberate materials to healthcare benefits through ambient (calm) monitoring and feedback on and around the body.
The basis for the above will be described including a raison d'etre based on the need for monitoring of the human body and its interactive relationship to its immediate surroundings.

The design of neural prosthetics necessitates a fine balance between material stiffness and device geometry; a high-modulus material will cause trauma in the soft tissue chronically, while a softer material requires an increased device volume that disrupts critical cell-to-cell signaling after implantation. With these considerations, an ideal device would be composed of a soft-substrate for mechanical coupling of the device and soft tissue, with a minimal prosthetic volume remaining under the critical size limit for the foreign body response. Previous attempts at this device stack have focused on using softening polymer substrates with a high initial modulus for sufficient mechanical rigidity during implantation that can then soften in vivo to reduce the mechanical mismatch between device and tissue. Unfortunately, the ultimate volume of this type of implant remains constant and does not allow for the reclamation of the lost neural space during implantation. In this study, we investigate the design and fabrication of neural prosthetics with variable chronically-stable geometries based on a two-stage substrate fabrication. First, a hydrolytically-resistant substrate is fabricated and used for the production of passive neural electronics. After photolithographic definition of reclaimable areas, a newly designed resorbable thiol-click network is introduced as a mechanical support during implantation. A difunctional anhydride-linked methacrylate monomer Methacrylic Anhydride (MAH) and a trifunctional thiol monomer Trimethylolpropane tris(3-mercaptopropionate) (TMTMP) are used as a base system from which mechanical properties and rate of hydrolysis are tuned. The increased reactivity of the anhydride linkage showed a rapid and tunable hydrolytic separation of the polymer backbone, giving a degradable system with rapidly labile mass in aqueous media. Swelling and mass loss characteristics are investigated in vitro using a 1X Phosphate Buffered Saline (PBS) solution at a physiologically-relevant temperature of 37 °C. Finally the device morphology before, during and after micro-structure evolution is studied using optical and electron microscopy. Overall, a process flow and device definition paradigm is outlined for the design and fabrication of complex micro-structure into neural prosthetics.

Neuromodulation aims to treat various disorders and diseases related to autonomic function by stimulating, blocking, and recording activity from peripheral nerves. The efficacy and success of neuromodulation heavily depends on the quality of the interface (electrode) used. Unfortunately, interfaces commonly used to-date are not matched to the mechanical characteristics of neural tissue, leading to both biotic and abiotic breakdown of bioelectronics. To address this point of failure, we utilized low cure stress softening polymer substrates, along with micron-level microfabrication processes to engineer “soft bioelectronics”. These soft bioelectronics can be implanted at moduli of more than 2 GPa and are capable of softening two orders of magnitude toward the modulus of tissue when implanted into the body cavity.
Our devices are made of shape memory polymers (SMPs), which are smart polymers that can be preprogrammed to change various mechanical characteristics in response to specific stimuli, such as temperature and humidity. We have designed our SMPs to respond to physiological temperatures to enable softening of the substrate, which reduces the modulus mismatch between peripheral nerves and the device. Devices are constructed with various metals, such as gold, titanium nitride, and nickel. The current device designs allow for up to 16 channels of recording and/or stimulation of nerves with a broad range of diameters (80um to 2mm).
To-date, we have implanted soft bioelectronics into a variety of autonomic nerves, including but not limited to the cervical vagus, hepatic branch of the vagus, and greater splanchnic. Our preliminary experiments in both acute and chronic experimental settings demonstrate the ability of these soft bioelectronics to withstand common modes of failure while enabling new paradigms for investigating neural regulation of autonomic function in chronic settings.
This work was supported by the National Institute of Health (RJB), GlaxoSmithKline (RJB, WEV), National Science Foundation (WEV), and Defense Advanced Research Projects Agency (WEV).

In this talk, I will present the emerging field of electroactive polymer (EAP) microsystems for integrated medical applications, highlighting the EAP actuation mechanisms of soft and hard materials for making passive and active devices: a. electrostriction of a recently developed relaxor ferroelectric polymer, polyvinylidene fluoride-trifluouroethylene-chlorotrifluoroethylene, b. diffusion of ions in the case of an anionic Pluronic-based hydrogel, and c. dielectric elastomer actuation of polydimethylsiloxane. We will present the "grand plan" where the variability of polymers, in addition to their intrinsic properties is of great advantage to medical applications. Biocompatibility, light weight, low-cost processing, and flexibility together with operational similarity to biological muscle and the ability to exhibit property changes much beyond what is achievable with inorganic, makes EAPs attractive for applications in artificial muscles, robotics, intelligent medical devices and prosthetics, and drug delivery systems. In each of the aforementioned actuation modes, EAP devices become more efficient as the scale is reduced, providing motivation for downscaling.
While EAPs have the potential to improve many aspects of human life, we will mainly address the applications of flexible micro motors, cardiac tissue engineering and cardiovascular occlusion and the processing and integration challenges that we encountered in working on these systems. Recent advances in polymer microfabrication (i.e. imprint lithography, laser micromachining, and 3D printing), together with breakthroughs in materials science, and understanding of EAP behavior at these small scales will serve to overcome the technological barriers to full integration with microsystems and usher in a new paradigm of medical microsystems.

Controlling network architecture and chain connectivity is critical to understanding elastic energy storage and improving performance of shape-memory polymers. To study how architecture affects the performance of shape memory polymers, acrylate-terminated poly(caprolactones) (PCLs) were converted into thermoset networks by three different reactions: conventional free radical polymerization, radical-induced coupling with multifunctional thiols, and base-catalyzed Michael Addition with multifunctional thiols. The highly efficient thiol-acrylate coupling reaction ensures that the molecular weight between crosslinks is uniform, resulting in tougher, elastic materials with a high degree of crystallinity and outstanding shape-memory properties with high levels of elastic energy storage. PCL networks will also be described that behave as novel shape actuators that undergo fully reversible, elastic elongation in programmed direction, upon cooling. Unlike two-way shape memory polymers, actuation does not require constant stress, and it is significant—exceeding 15% strain—placing this material in a class of only a few other known materials. Actuation is triggered as configurationally biased poly(caprolactone) chains undergo strain-induced crystallization. The material is very straightforward to prepare: acrylate chain ends of a three-arm poly(caprolactone) precursor are partially polymerized by thiol-ene chemistry to form a network with a fraction of dangling ends. The material is then stretched, and dangling ends are further reacted by a photopolymerization

"Robotics" is a field with broad interest: it combines mechanical engineering, information science, and animal physiology with manufacturing, workforce development, economics, and other areas. The most highly developed classes of robots have been build based on conceptual models provided by the body-plans of animals with skeletons (humans, horses), and have made it possible to carry out tasks that humans and animals could not (for a variety of reasons). We are interested in robots based a different, simpler class of organisms (invertebrates: starfish, worms, octopi). Because these organisms, and the robots having designs stimulated by them, have no skeletons, they provide enormous opportunities in materials and polymer science, rather than primarily in mechanical engineering. This seminar will outline one approach to soft robots, and suggest problems and opportunities in this new field.

Wearable actuators are potentially useful in assisting or enhancing human movement. They can be used in exoskeletons for training, therapy, and assisting functional daily living [1]. Such devices have to be compact, robust, lightweight, and offer a suitable interface between the device and the operator&’s body and also provide easy control strategies. Wearable actuators also require stretchable electronics where the circuit can sustain large deformations without electrical failure or detrimental loss of performance [2].
Here we report several materials that are capable of high strain elasticity and maintain an effective actuator capability. Firstly, we describe a simple way to prepare stretchable polypyrrole (PPy) based actuator materials that can be operated over a wide dynamic strain range and generate useable actuation displacements and pressures [1]. The stretchable actuators were prepared as a laminated composite of polypyrrole and a gold-coated roughened rubber sheet that could be stretched to 30% without significant change in electrical resistance. The corrugated PPy/gold/rubber laminates showed ~1% of actuation strain even when pre-stretched to 24%. The effect of pre-stretch on actuation strain was successfully modelled by incorporating the effect of corrugation and changing Young&’s modulus.
Secondly, we describe tough hydrogel actuators that can be reversibly stretched 2-3 times without failure and can generate actuation strains in excess of 50% using pH switching [3]. Modelling of these materials required a thermodynamic treatment wherein the effect of external stress on the water content of the gel was included.
Finally, we compare the above with recently described spring-like coils of oriented polymer fibers that generate large actuation strains (50%) on modest heating [4]. The high strength coils can be pre-stretched to 50-100% of their initial lengths without failure.
References
1. Zheng, W., Alici, G., Clingan, P.R., Munro, B.J., Spinks, G.M., Steele, J.R. and Wallace, G.G., “Polypyrrole stretchable actuators” J. Polym.Sci.Pt. B: Polym.Phys.2012, 51, 57-63.
2. Kim, D. H.; Ahn, J. H.; Choi, W. M.; Kim, H. S.; Kim, T. H.; Song, J. Z.; Huang, Y. G. Y.; Liu, Z. J.; Lu, C.; Rogers, J. A., Stretchable and foldable silicon integrated circuits. Science 2008, 320, 507-511.
3. Naficy, S. and Spinks, G.M. “Effect of tensile load on the actuation performance of pH-sensitive hydrogels” J. Polym.Sci.Pt. B: Polym.Phys.2014, 53, 218-225.
4. Haines, C.S., Lima, M.D., Li, N., Spinks, G.M., Foroughi, J., Madden, J.D.W., Kim, S.H., Fang, S., de Andrade, M. J., Goktepe, F., Goktepe, O, Mirvakili, S., Naficy, S., Lepro, X., Oh, J., Kozlov, M.E., Kim, S.J., Xu, X., Swedlove, B.J., Wallace, G.G. and Baughman, R.H., “Artificial muscles from fishing line and sewing thread” Science2014, 343, 868-872.

Stretchable electronics have attracted long-lasting attentions for their promising applications in next-generation functional devices, including #64258;exible circuitries, stretchable displays, stretchable sensors, epidermal electronics, and implantable devices. This new class of electronics allows devices to be deformed into complex shapes while maintaining the device performance and reliability. However, a sustainable power source is highly desired to drive those electronic devices, and the implementation of traditional power supply remains a challenge due to inconvenient operations and indispensable wire connections. In this regard, a more efficient way is to integrate a power generator to scavenge the ambient energy, especially for the mechanical energy from stretching motions. Attempts have been made to develop stretchable power generators, but their output performances still need further enhancement for practical applications. Recently, triboelectric nanogenerators (TENGs) have been invented based on triboelectrification and electrostatic induction, which are demonstrated to be a cost-effective and high-efficient approach for harvesting ambient mechanical energy. Various working modes were developed to accommodate energy conversion from different types of mechanical motions, and high output power was successfully demonstrated, with numerous applications such as self-powered electronics and active sensors.
In this work, we developed a new type of stretchable TENG (STENG), which was fabricated by assembling serpentine-patterned electrodes and a wavy-structured Kapton film. Owing to the unique structural design, the STENG could be operated at both compressive and stretching mode. At the traditional compressive mode, a high output power density of 5 W m^-2 was delivered at a load resistance of 44 MOmega;. At the stretching mode, the STENG was capable of withstanding a tensile strain of up to 22% and its output performance was up to 70 times larger than that of the planar TENG in the control experiment. Moreover, the STENG was able to provide reliable output performance on curved surfaces (with curvatures of up to 36 cm^-1). On the basis of this superior feature, the STENG could be conformably attached onto human skins for monitoring the gentle motions of joints, muscles, or even the Adam&’s apple. This research presents an unprecedented advancement in energy harvesting and self-powered sensors, and paves the way for the next-generation stretchable electronics and bio-integrated systems.
Reference:
P. K. Yang, L. Lin, F. Yi, X. Li, K. C. Pradel, Y. Zi, C. I. Wu, J. H. He, Y. Zhang, Z. L. Wang, “A Flexible, Stretchable and Shape#8208;Adaptive Approach for Versatile Energy Conversion and Self#8208;Powered Biomedical Monitoring”, Adv. Mater., 2015, DOI: 10.1002/adma.201500652.

Origami is an ancient art form that can turn two-dimensional sheets into three-dimensional structures. The concept of origami has been applied to solving engineering problems, especially the problems intertwined with space restriction and transformation between multiple structure configurations. In recent years, the application of origami concept at small length scales has been explored by researchers. Origami driven by different stimuli is studied. Capillary forces, Lorentz forces and differential swelling of bilayer structures have been used to achieve origami in millimeter scale. Here we develop a new method to actuate origami in small scale by ultraviolet exposed PDMS/SU8 thin film with different localized swelling ratios.
We mix PDMS and SU8 with 2:1 mass ratio and spincoat the mixture onto glass slides to thickness around 120 microns. The thin film is then UV exposed to different patterns of continuous parallel lines or lines of finite lengths. The polymer thin films after exposure and post-baking are cut into 8mm X 20mm rectangular samples and immersed into Toluene. We observe that the sheet would form a rolled up tube for continuous line patterned sheet, or form a kink of different angles at the patterned region. We measured the folding kink angle as a function of pattern size. Range of folding angles was from 0^#9702; to 150^#9702;. Under certain conditions, the continuous line patterned sheets undergo a folding, opening up, and then re-folding in the opposite direction.
With this knowledge of well controlled folding phenomena, we designed different patterns such that different origami shapes may be achieved when submerging the sheet into Toluene. A few origami shapes were constructed using this method. Using the finite element analysis, the deformation mechanism leading to such folding behavior was investigated. It is shown that differential swelling in different cross-linked regions can be utilized to generate a wide range of different shapes at the micro- and meso scales.

Relatively poor dynamic response of current flexible strain gauges has prevented these sensors from being widely adopted in portable electronics.^[1-2] In this work, we presented a much improved flexible strain gauge where one strip of Au NP monolayer assembled onto the flexible polyethylene terephthalate (PET) film is utilized as the strain gauge's active unit.^[3] Enabled by electron tunneling between adjacent nanoparticles within the Au NP monolayer, the proposed flexible gauge was shown to be able to respond to applied stimuli without detectable hysteresis.^[4] By experimental quantification of the time and frequency domain dependence of electrical resistance of the proposed strain gauge, it has been confirmed that acoustic vibrations in the frequency range of 1 to 20,000 Hz can be reliably detected. Besides being used in measuring musical tone, audible speech and creature vocalization, as demonstrated in this study, this proposed flexible strain gauge, due to its ultrafast dynamic response, can find its array of applications, including the miniaturized vibratory sensors, safe entrance guard management systems and ultrasensitive pressure sensors.
References
[1] Segev-Bar, M.; Haick, H., ACS Nano 2013, 7, 8366.
[2] Yan, C.; Wang, J.; Kang, W.; Cui, M.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S., Adv. Mater. 2014, 26, 2022.
[3] Zhang, C.; Li, J.; Yang, S.; Jiao, W.; Xiao, S.; Zou, M.; Yuan, S.; Xiao, F.; Wang, S.; Qian, L., Nano Res. 2014, 7, 824.
[4] Jiao, W.; Yi, L.; Zhang, C.; Wu, K.; Li, J.; Qian, L.; Wang, S.; Jiang, Y.; Das, B.; Yuan, S., Nanoscale 2014, 6, 13809.

We introduce soft and highly stretchable composite films composed of Gallium-Indium (GaIn) alloy and elastomer. The metal, which is liquid at room temperature, does not induce the same matrix stiffening seen in typical particle-polymer composites. This allows a material to maintain its conformability even at large volume percentages of GaIn. These inclusions, dispersed as microscopic droplets via shear mixing, are initially isolated from each other. However, depending on the polymer and inclusion volume ratio, it is possible to rupture the cells through concentrated surface pressure. In doing so, a continuous network of GaIn can be formed, causing the composite to irreversibly become conductive. This is demonstrated using poly(dimethylsiloxane) (PDMS) as the polymer matrix mixed at a 1:1 volume ratio with GaIn alloy, which resulted in a composite with elastic modulus to 10% strain of E = 0.90-1.27 MPa, average strain to failure of ε_f = 133%, and conductivity (post compression) of σ = 1.05 X 10⁴ S m^-1. Other polymers, particularly those with extremely high strain to failure, are resistant to rupture and show potential as stretchable dielectric materials. Here, Vytaflex 50 (polyurethane rubber; Smooth-On, Inc.) is utilized to produce a composite with a dielectric constant of 49-59 (at 1-200 KHz) and elastic modulus of E = 0.4-0.6 MPa.

This talk will discuss work in our group to pattern and actuate liquid metals for stretchable, reconfigurable, and transient electronics. The metal is an alloy of gallium. These alloys are noted for their low viscosity, low toxicity, and negligible volatility. Despite the large surface tension of the metal, it can be molded into non-spherical 2D and 3D shapes due to the presence of an ultra-thin oxide skin that forms on its surface. The metal can be patterned by injection into microchannels or by direct-write techniques. Because it is a liquid, the metal is extremely soft and flows in response to stress to retain electrical continuity under extreme deformation. By embedding the metal into elastomeric substrates, it is possible to form soft electrodes and optical components, stretchable antennas, and ultra-stretchable wires that maintain metallic conductivity up to ~800% strain. The ability of the oxide to reform instantaneously also allows the metal to self-heal in response to damage. In addition, the ability to remove the oxide electrochemically provides a new means to control the shape of the metal for reconfigurable and transient electronics. We show that the oxide is one of the best surfactants ever reported and can tune the surface tension of the metal over an unprecedented range by using electrochemical reactions at the surface of the metal.

C3nano's highly flexible transparent conductive materials will be discussed. Strategies and technologies for improving the overall optoelectronic properties of transparent conducting films for flexible displays and electronics will be described. The following three advances will be covered:
1. Lowering resistances by chemical fusing
2. Manipulating color in optically clear transparent conductive films (L, a*,b*)
3. Ultra-hard, thin and flexible hard-coats and protective coatings

Stretchable conductors that can maintain high electrical conductivity under tensile strain exceeding 100% are needed for applications in flexible electronics, bioelectronics, soft robotics, and smart textiles. State of the art elastic conductors achieve high electrical conductivity through the use of polymer-metallic nanoparticle composites. Conductivity is maintained during tensile deformation through reorganization of the metal nanoparticles in elastomer matrix. However, many of the existing elastic conductor fabrication methods are extremely challenging to apply onto non-planar substrates and often require a transfer step that potentially degrades the electromechanical properties of the composite. This is particularly important for applications in bioelectronics and smart textiles. Here we report a method to fabricate stretchable conductors based on elastomeric polymer fibers and silver nanoparticles. Silver nanoparticles are assembled both in and around styrene-isoprene-styrene fibers forming conductive networks. The fabrication technique involves solution blow spinning of block copolymer-silver nanoparticle solutions, and subsequent nucleation of a secondary silver nanoparticle network using organometallic precursor solutions. Solution blow spinning is a method for depositing polymer fiber mats from concentrated polymer solutions using only a high-pressure gas source. This method allows for conformal deposition and processing of stretchable conductors on non-planar substrates with simple spray apparatus. In addition to deposition capabilities, we have demonstrated that it is possible to control the electrical conductivity and strain-dependence of electrical properties by adjusting the nanoparticle concentration in blow-spinning solution and concentration of the organometallic precursor solution. The resulting composites have demonstrated electrical conductivities reaching 9000 ± 200 S/cm with only a 73% increase in resistance after 500 cycles under 100% strain. The electrical and electromechanical properties of these dual silver nanoparticle network composites make them promising materials for constructing stretchable circuitry.