Design of super-conformable, foldable materials via fractal cuts and lattice kirigami
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Materials challenges in conformable and foldable devices Mechanical instabilities in soft materials, precipitated by dewetting,1 swelling,2 crumpling,3,4 wrinkling,5 buckling,6 and collapsing,7,8 are often gateways to large deformation and shape change in response to relatively small external perturbations. Exploiting this provides new opportunities to design reconfigurable materials with dramatically adjustable physical properties. Recently, spontaneous pattern transformation in periodic porous membranes has been demonstrated, triggered by swelling, drying, polymerization, and mechanical compression together with shape memory effects over multiple length scales.9–19 The resulting photonic15 and phononic20,21 bandgap properties, along with varying auxetic response (i.e., negative Poisson ratio),19,22,23 are significantly altered due to the change of lattice symmetry, pore size, pore shape, or the volume filling fraction. Mechanical instabilities, however, if uncontrolled, can lead to sudden structural changes and unwanted features, such as defects, antiphase boundaries, and fracture. At the same time, there is growing interest in flexible and wearable electronics,24–26 energy-storage devices (e.g., batteries27 and supercapacitors),28–31 optics,32,33 artificial muscles,34,35
actuators,36 tunable color displays,14 and smart windows,37,38 which often involve large area or volume changes (with strain up to 1000%). For example, wearable devices must stretch to wrap around an object (e.g., wrist) or fold into a compact package for later deployment. Therefore, the devices should be lightweight, flexible, and, more importantly, conformable to a non-flat object and bendable to a large curvature without losing mechanical or electrical performance. Conformability is especially important for biomedical devices that detect the temperature of the skin or motion of the heart, lung, or brain.39,40 The ability to withstand repeated large deformations in these devices presents a major challenge in materials science and engineering, since most devices are made of hard materials or are hybrids consisting of hard and soft materials. While hard materials offer durability, fast response time, and overall device performance superior to soft materials, they tend to fail when the applied strain is greater than 3%. The critical strain of fracture is defined as:25 ε c ≈ Γ /( Ea ),
(1)
where Γ is the fracture energy, E is the Young’s modulus, and a is the flaw size, on the order of the film thickness. For a typical hard
Shu Yang, Department of Materials Science and Engineering, University of Pennsylvania, USA; [email protected] In-Suk Choi, High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, South Korea; [email protected] Randall D. Kamien, Department of Physics and Astronomy, University of Pennsylvania, USA; [email protected] DOI: 10.1557/mrs.2016.5
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MRS BULLETIN • VOLUME 41 • FEBRUARY 2016 • www.mrs.org/bulletin
© 2016 Materials Research Society
DESIGN OF SUPER-CONFORMABLE, FOLDABLE MA
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