Dayong Chen

Creasing instability of elastomer films

The creasing instability of elastomer films under compression is studied by a combination of experiment and numerical simulation. Experimentally, we attach a stress-free film on a much thicker and stiffer pre-stretched substrate. When the substrate is partially released, the film is uniaxially compressed, leading to formation of an array of creases beyond a critical strain. The profile of the folded surface is extracted using confocal fluorescence microscopy, yielding the depths, spacings, and shapes of creases. Numerically, the onset and development of creases are simulated by introducing appropriately sized defects into a finite-element mesh and allowing the surface of the film to self-contact. The measurements and simulations are found to be in excellent agreement.

Mechanically gated electrical switches by creasing of patterned metal/elastomer bilayer films.

DOI: 10.1002/adma.201400992 monolayer (SAM) assisted stamp transferring technique to place the electrodes on soft PDMS substrates. Under compression, the regions covered with gold form wrinkles at small strains due to the near inextensibility of the electrodes compared to the soft substrate. At higher compression, however, a crease forms in the gap between electrodes, pressing the electrodes into self-contact and lowering the electrical resistivity across the gap by many orders of magnitude. This process is highly reversible due to the elastic nature of creasing, and the switching strain can be tuned by the gap width due to the local amplifi cation of strain by the stiff electrode fi lms. While electromechanical switches that rely on defl ection of slender cantilevers to reversibly form and break contacts in circuit elements are a well-established component of nano-electromechanical systems (NEMS) devices, [ 36–39 ] the current method offers a complementary approach that should facilitate integration with fl exible electronics. The fabrication method and device geometry are illustrated in Figure 1 . We make use of a 1-mm-thick PDMS ‘mounting layer’ that is pre-stretched to length L 0 prior to coating with a thinner (45 μm) layer of softer PDMS. Separately, a gold fi lm (100 nm) is photo-lithographically patterned on a Si wafer pretreated with a release layer (Figure 1 a). After lift-off, the gold electrodes are treated with thiol adhesion promoter to promote subsequent anchoring to the PDMS surface during fi lm transfer (Figure 1 b). Upon subsequent release of the mounting layer to a length L , the PDMS substrate/gold fi lm bilayer is placed under compression, which we characterize by the nominal (far-fi eld) uniaxial compressive strain ε = L 0 / L –1. The resulting multilayer devices are highly fl exible and can be bent tightly (Figure 1 c) without any visible damage to the electrodes. Typical device geometries consist of electrodes of width W = 100 μm separated by a gap of length L g ranging from 10 – 60 μm, as shown by optical microscopy in Figure 1 d. According to the critical wrinkling strain of ε W = 0.25(3 E s / E f ) 2/3 from linear stability analysis, [ 40,41 ] wrinkle formation for a continuous gold fi lm would be expected at a vanishingly small strain of ε = 1.9 × 10 −4 based on respective plane-strain elastic moduli of E f = 98 GPa and E s = 670 kPa for the gold fi lm and PDMS substrate. In practice, wrinkles are found to cover parts of the electrode at a small compressive strain of ε = 0.03 and expand to cover the majority of the electrode fi nger by ε = 0.09 ( Figure 2 a). The progressive wrinkling over this range of strain presumably refl ects the infl uence of the gold fi lm boundaries, as the in-plane dimensions of the electrodes are only several times larger than the wrinkle wavelength. The initial wavelength (λ 0 ) is predicted to be 2 /3 0 f f s 1/3 h E E λ π ( ) ( ) = , or 23 μm for a gold fi lm thickness of h f = 100 nm, which is in reasonable agreement with the measured value of 19 μm. Beyond a strain of ε ∼ 0.1 when wrinkles Flexible electronic devices based on lightweight, bendable, and stretchable polymer substrates offer considerable potential for applications including epidermal electronics, [ 1,2 ] organic transistors, [ 3,4 ] fl exible circuits [ 5,6 ] and displays, [ 7,8 ] electronic eye cameras, [ 9 ] artifi cial skins, [ 10 ] sensors, [ 11,12 ] and actuators. [ 13,14 ]

Controlled formation and disappearance of creases

Soft, elastic materials are capable of large and reversible deformation, readily leading to various modes of instability that are often undesirable, but sometimes useful. For example, when a soft elastic material is compressed, its initially flat surface will suddenly form creases. While creases are commonly observed, and have been exploited to control chemical patterning, enzymatic activity, and adhesion of surfaces, the conditions for the formation and disappearance of creases have so far been poorly controlled. Here we show that a soft elastic bilayer can snap between the flat and creased states repeatedly, with hysteresis. The strains at which the creases form and disappear are highly reproducible, and are tunable over a large range, through variations in the level of pre-compression applied to the substrate and the relative thickness of the film. The introduction of bistable flat and creased states and hysteretic switching is an important step to enable applications of this type of instability.

Photopatternable Biodegradable Aliphatic Polyester with Pendent Benzophenone Groups.

Highly efficient photo-cross-linking reactions enable numerous applications in biomaterials. Here, a photopatternable biodegradable aliphatic polyester with benzophenone pendent groups was synthesized by copper-catalyzed alkyne-azide cycloaddition, affording polyesters that undergo UV-induced cross-linking to yield photopatterned films. Using this material, a self-folding multilayer structure containing polyester/hydrogel bilayer hinges was fabricated. Upon swelling of the hydrogel layer, the construct folds into a triangular tube, which subsequently unfolds due to lipase-catalyzed degradation of the polyester layer. The ability to precisely design such degradation-induced structural changes offers potential for biomaterials and medical applications, such as evolving and responsive 2D and 3D tissue engineering scaffolds.