Recent research in advanced materials and mechanics demonstrates the possibility for integrating inorganic semiconductors with soft, elastomeric substrates to yield systems with linear elastic mechanical responses to strains that signifi cantly exceed those associated with fracture limits of the constituent materials (e.g. ∼ 1% for many inorganics). This outcome can provide stretching to strain levels of tens of percent (in extreme cases, more than 100%), for diverse, reversible modes of deformation, including bending, twisting, stretching or compressing. [1][2][3][4][5][6][7] Interest in these outcomes is motivated by needs in fl exible display, [8][9][10] curvilinear imaging devices, [11][12][13] structural health monitors [ 14 ] and, more recently, in bio-integrated systems [15][16][17] for advanced therapeutic or diagnostic functionality in clinical medicine. In these latter applications, considerations related to toxicity and biocompatibility of the materials are also critically important. Some of the most well developed strategies exploit confi gurations in which brittle, rigid materials accommodate in-plane strains through out-of-plane motions, via buckling or twisting modes. [ 18 ] Such ideas can be exploited in all parts of an integrated system, or only in interconnections between active devices. The latter design can accommodate the largest strains, but its effi cacy decreases as the areal coverage of the devices increases. [ 12 ] As a result, important applications such as those in light capture (i.e., photovolatics) and detection (i.e., photodetectors), where high coverages are often desired, can be diffi cult to address. Here we report designs for stretchable systems that exploit elastomeric substrates with surface relief that confi nes strains at the locations of the interconnections, and away from the devices. The results enable areal coverages and levels of stretchability with relatively low interfacial stress between devices and substrates, compared to similar layouts with conventional, fl at substrates. We describe, using a combination of theory and experiment, the essential mechanics, and then demonstrate the ideas in stretchable solar modules that use ultrathin, single junction GaAs solar cells.A representative layout for a structured substrate designed for this purpose appears in Figure 1 , in which the relief consists of isolated, raised regions (i.e. islands) separated by recessed features (i.e. trenches). The casting and curing processes of soft lithography [ 19 ] provide a convenient means to form such relief, with excellent dimensional control, in elastomers such as poly(dimethylsiloxane) (PDMS). The image of Figure 1a provides a cross sectional view for a representative case where square islands with edge lengths ( l island ) of 800 μ m are separated by trenches with widths ( l trench ) and depths ( h trench ) of 156 μ m and 200 μ m, respectively. The thickness of the underlying PDMS (i.e. base) is 200 μ m. This type of structure is attractive for stretchable systems that incorporate no...

Arrays of angled microfibers with a gecko-inspired structure were fabricated from a stiff thermoplastic polymer ͑polypropylene͒ with elastic properties similar to those of ␤-keratin of natural setae. Friction experiments demonstrated that this fibrillar polymer surface exhibits directional adhesion. Sliding of clean glass surfaces against and along the microfiber direction without applying an external normal force produced an apparent shear stress of 0.1 and 4.5 N / cm 2 , respectively. This directional adhesion is interpreted in the context of a nonlinear elastic bending model of an angled beam. Shearing and normal contact experiments yielded further evidence of the anisotropic adhesion of the fibrillar polymer and revealed the occurrence of a pull-off ͑adhesive͒ force at the instant of surface detachment, unlike vertically aligned microfiber arrays of the same material that exhibited a zero pull-off force. The results of this study provide impetus for the design of gecko-inspired adhesives with angled structures that demonstrate directional adhesion against different material surfaces.

Gecko-inspired microfibre arrays with 42 million polypropylene fibres cm K2 (each fibre with elastic modulus 1 GPa, length 20 mm and diameter 0.6 mm) were fabricated and tested under pure shear loading conditions, after removing a preload of less than 0.1 N cm K2 . After sliding to engage fibres, 2 cm 2 patches developed up to 4 N of shear force with an estimated contact region of 0.44 cm 2 . The control unfibrillated surface had no measurable shear force. For comparison, a natural setal patch tested under the same conditions on smooth glass showed approximately seven times greater shear per unit estimated contact region. Similar to gecko fibre arrays, the synthetic patch maintains contact and increases shear force with sliding. The high shear force observed (approx. 210 nN per fibre) suggests that fibres are in side contact, providing a larger true contact area than would be obtained by tip contact. Shear force increased over the course of repeated tests for synthetic patches, suggesting deformation of fibres into more favourable conformations.

Natural gecko toes covered by nanomicro structures can repeatedly adhere to surfaces without collecting dirt. Inspired by geckos, we fabricated a high-aspect-ratio fibrillar adhesive from a stiff polymer and demonstrated self-cleaning of the adhesive during contact with a surface. In contrast to a conventional pressure-sensitive adhesive (PSA), the contaminated synthetic fibrillar adhesive recovered about 33% of the shear adhesion of clean samples after multiple contacts with a clean, dry surface.

A subdermally implantable flexible photovoltatic (IPV) device is proposed for supplying sustainable electric power to in vivo medical implants. Electric properties of the implanted IPV device are characterized in live animal models. Feasibility of this strategy is demonstrated by operating a flexible pacemaker with the subdermal IPV device which generates DC electric power of ≈647 μW under the skin.

The adhesive pads of geckos provide control of normal adhesive force by controlling the applied shear force. This frictional adhesion effect is one of the key principles used for rapid detachment in animals running up vertical surfaces. We developed polypropylene microfibre arrays composed of vertical, 0.3 mm radius fibres with elastic modulus of 1 GPa which show this effect for the first time using a stiff polymer. In the absence of shear forces, these fibres show minimal normal adhesion. However, sliding parallel to the substrate with a spherical probe produces a frictional adhesion effect which is not seen in the flat control. A cantilever model for the fibres and the spherical probe indicates a strong dependence on the initial fibre angle. A novel feature of the microfibre arrays is that adhesion improves with use. Repeated shearing of fibres temporarily increases maximum shear and pull-off forces.

Conventional connectors utilize mechanical, magnetic, or electrostatic interactions to enable highly specific and reversible binding of the components (i.e., mates) for a wide range of applications. As the connectors are miniaturized to small scales, a number of shortcomings, including low binding strength, high engagement/disengagement energies, difficulties with the engagement, fabrication challenges, and the lack of reliability are presented that limit their successful operation. Here, we report unisex, chemical connectors based on hybrid, inorganic/organic nanowire (NW) forests that utilize weak van der Waals bonding that is amplified by the high aspect ratio geometric configuration of the NWs to enable highly specific and versatile binding of the components. Uniquely, NW chemical connectors exhibit high macroscopic shear adhesion strength (approximately 163 N/cm(2)) with minimal binding to non-self-similar surfaces, anisotropic adhesion behavior (shear to normal strength ratio approximately 25), reusability (approximately 27 attach/detach cycles), and efficient binding for both micro- and macroscale dimensions.

Notched islands on a thin elastomeric substrate serve as a platform for dual-junction GaInP/GaAs solar cells with microscale dimensions and ultrathin forms for stretchable photovoltaic modules. These designs allow for a high degree of stretchability and areal coverage, and they provide a natural form of strain-limiting behavior, helping to avoid destructive effects of extreme deformations.