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 stretc