This paper identifies and characterizes silicone elastomers that are well-suited for fabricating highly stretchable and tear-resistant devices that require interfacial bonding by plasma or UV ozone treatment. The ability to bond two or more pieces of molded silicone is important for creating microfluidic channels, chambers for pneumatically driven soft robotics, and other soft and stretchable devices. Sylgard-184 is a popular silicone, particularly for microfluidic applications. However, its low elongation at break (∼100% strain) and moderate tear strength (∼3 N/mm) make it unsuitable for emerging, mechanically demanding applications of silicone. In contrast, commercial silicones, such as Dragon Skin, have excellent mechanical properties yet are difficult to plasma-bond, likely because of the presence of silicone oils that soften the network yet migrate to the surface and interfere with plasma bonding. We found that extracting silicone oligomers from these soft networks allows these materials to bond but only when the Shore hardness exceeds a value of 15 A. It is also possible to mix highly stretchable silicones (Dragon Skin and Ecoflex) with Sylgard-184 to create silicones with intermediate mechanical properties; interestingly, these blends also only bond when the hardness exceeds 15 A. Eight different Pt-cured silicones were also screened; again, only those with Shore hardness above 15 A plasma-bond. The most promising silicones from this study are Sylgard-186 and Elastosil-M4130 and M4630, which exhibit a large deformation (>200% elongation at break), high tear strength (>12 N/mm), and strong plasma bonding. To illustrate the utility of these silicones, we created stretchable electrodes by injecting a liquid metal into microchannels created using such silicones, which may find use in soft robotics, electronic skin, and stretchable energy storage devices.

transition. We fabricated the conductive fibers by injecting a liquid metal, gallium, into stretchable hollow fibers formed by melt-spinning a commercial thermoplastic elastomer. The ability to change the core of the fiber from solid (with a modulus of a metal) to a liquid (with a viscosity near water) allows for dramatic changes in mechanical properties as well as the ability to have shape memory effects. Although the mechanism differs, this effect is similar to shape memory polymers (SMPs), [1][2][3][4][5][6][7][8][9][10] which are fascinating materials that can be programmed to store and recover elastic energy in response to external stimuli. SMPs can be deformed at elevated temperature and then cooled to retain their temporary shape. Heating the polymer again to the elevated temperature allows the stored strain to relax back to the original, predeformed shape. The use of metallic cores as a mechanism to retain elastic fibers in a temporary shape has several advantages relative to conventional shape memory polymers: Shape Memory Fibers

Shape memory composites are fascinating materials with the ability to preserve deformed shapes that recover when triggered by certain external stimuli. Although elastomers are not inherently shape memory materials, the inclusion of phase-change materials within the elastomer can impart shape memory properties. When this filler changes the phase from liquid to solid, the effective modulus of the polymer increases significantly, enabling stiffness tuning. Using gallium, a metal with a low melting point (29.8 °C), it is possible to create elastomeric materials with metallic conductivity and shape memory properties. This concept has been used previously in core–shell (gallium-elastomer) fibers and foams, but here, we show that it can also be implemented in elastomeric films containing microchannels. Such microchannels are appealing because it is possible to control the geometry of the filler and create metallically conductive circuits. Stretching the solidified metal fractures the fillers; however, they can heal by body heat to restore conductivity. Such conductive, shape memory sheets with healable conductivity may find applications in stretchable electronics and soft robotics.

Polymer conductors that are solution-processable provide an opportunity to realize low-cost organic electronics. However, coating sequential layers can be hindered by poor surface wetting or dissolution of underlying layers. This has led to the use of transfer printing where solid film inks are transferred from a donor substrate to partially fabricated devices using a stamp. This approach typically requires favorable adhesion differences between the stamp, ink, and receiving substrate. Here, we present a shear-assisted organic printing (SHARP) technique that employs a shear load on a post-less polydimethylsiloxane (PDMS) elastomer stamp to print large-area polymer films that can overcome large unfavorable adhesion differences between the stamp and receiving substrate. We explore the limits of this process by transfer printing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films with varied formulation that tune the adhesive fracture energy. Using this platform, we show that the SHARP process is able to overcome a 10-fold unfavorable adhesion differential without the use of a patterned PDMS stamp, enabling large-area printing. The SHARP approach is then used to print PEDOT:PSS films in the fabrication of high-performance semitransparent organic solar cells.

Corrugated SiCN ceramic substrates fabricated by a facile replication process using nonlithographic PDMS masters were employed for the directed assembly of block copolymer microdomains. During thermal annealing of polystyrene-b-polybutadiene diblock copolymer, the material transport was guided by a wrinkled substrate to form regular modulations in the film thickness. As a consequence of the thickness-dependent morphological behavior of cylinder forming block copolymer, the film surface appears as sequenced patterns of alternative microphase-separated structures. The ordering process is attributed to the formation of inverted terraces which match the substrate topography, so that the resulting surface patterns are free from the surface relief structures within macroscopically large areas. The issues of the film thickness, the substrate surface energy, and the pattern geometry are addressed. Our approach demonstrates an effective synergism of external confinement and internal polymorphism of block copolymers toward complex hierarchically structured patterned surfaces.

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Three-dimensional SiC ceramic microstructures with near-zero shrinkage were fabricated from a simple inorganic polymer mixture by inducing dual photocuring routes to produce highly dense polymer features by stereolithography and subsequent pyrolysis at 600 degrees C.

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