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.