Materials for stretchable electronics in bioinspired and biointegrated devices
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tion Historically, the vast majority of work in electronic materials has addressed, either directly or indirectly, a development pathway that was established shortly after the invention of the integrated circuit (IC) 50 years ago, in which functional improvements in systems follow from increases in the number densities and switching speeds of transistors, driven mainly by decreases in their individual sizes.1–3 Alternative forms of electronics, configured for driving flat panel displays, emerged roughly 15 years ago to establish a market presence that now represents a significant fraction of overall sales of semiconductor devices.4 Here, the primary metric for scaling is overall area coverage, rather than transistor size or speed; the associated challenges in materials science are much different, but no less interesting, than those in ICs.5–10 Flexible electronics represents a natural extension of this large area, or macroelectronics technology, where motivation derives from unique form factors (e.g., paper-like displays)11–14 and processing options (e.g., roll-toroll) that follow from the use of plastic substrates.15,16 Although commercial applications are only now emerging, most believe that this segment will grow rapidly in coming years. Here, we focus on yet a different, and even newer, class of electronics whose key attribute is that it is capable not only of bending, but also of stretching with reversible, linear elastic responses to large strain (>>1%) deformations.17–41 Such electronics can
be twisted, folded, and conformally wrapped onto arbitrarily curved surfaces, without mechanical fatigue or any significant change in operating characteristics.16–20,25,40 These mechanics lead to powerful engineering design options and modes of integration, including direct, seamless mounting on tissues of the human body in ways that provide unprecedented functionality in surgical devices, monitoring systems, and human-machine interfaces.38–40,42–45 In the following, we summarize an approach to stretchable electronics/optoelectronics that exploits established inorganic semiconductors in strategic geometries and layouts to yield levels of performance that can match similarly designed devices fabricated in the conventional way on rigid, planar semiconductor wafers.20–24,43,45 The focus is mainly on our work, as examples, in stretchable electronics; other articles in this issue, and other published reviews,9,10,25,46,47 provide summaries of alternative and, in some cases, complementary approaches.
Materials and processing The technical scheme for high performance stretchable electronics involves two ideas: (1) the use of semiconductor nanomaterials, in shape-engineered forms, as the active components, and (2) circuit/device layouts that minimize strains in these and other “hard” constituent materials when integrated with “soft” elastomeric substrates. For the first, wafer-scale sources
Dae-Hyeong Kim, Seoul National University, Institute of Chemical Processes, Korea; [email protected] Nanshu Lu, University of Texas at Austin, US
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