Design of Strain‐Limiting Substrate Materials for Stretchable and Flexible Electronics

Abstract

Recently developed classes of electronics for biomedical applications exploit substrates that offer low elastic modulus and high stretchability, to allow intimate, mechanically biocompatible integration with soft biological tissues. A challenge is that such substrates do not generally offer protection of the electronics from high peak strains that can occur upon large-scale deformation, thereby creating a potential for device failure. The results presented here establish a simple route to compliant substrates with strain-limiting mechanics based on approaches that complement those of recently described alternatives. Here, a thin film or mesh of a high modulus material transferred onto a prestrained compliant substrate transforms into wrinkled geometry upon release of the prestrain. The structure formed by this process offers a low elastic modulus at small strain due to the small effective stiffness of the wrinkled film or mesh; it has a high tangent modulus (e.g., >1000 times the elastic modulus) at large strain, as the wrinkles disappear and the film/mesh returns to a flat geometry. This bilinear stress–strain behavior has an extremely sharp transition point, defined by the magnitude of the prestrain. A theoretical model yields analytical expressions for the elastic and tangent moduli and the transition strain of the bilinear stress–strain relation, with quantitative correspondence to finite element analysis and experiments.

Highly Conductive and Fatigue‐Free Flexible Copper Film Electrode Fabricated by a Facile Dry Transfer Technique

“…It can be observed that array of microwavy structure is formed on the surface of Cu film electrode during the dry transfer process, which can effectively overcome the fundamental mismatch in mechanics between the different materials. [39] Simulation analysis and experimental investigations reported by Huang and coworkers [40] have also confirmed that the novel wavy structure can not only flex but also stretch and compress, with a mechanics similar to that of accordion bellows (Figure 10c). Hence, the formation of this special peculiar structure can successfully avoid fracture and electrical failure of the conductive metal film when bending to some extent, thus improving the fatigue-free performance.…”

Soft, conformal bioelectronics for a wireless human-wheelchair interface

“…Three-dimensional finite element analysis using a software (ABAQUS, Dassault Systems, Waltham, MA) was conducted to investigate mechanical behaviors of an electrode upon skin deformations: bending and stretching (Chen et al 2016; Ma et al 2016). The soft electrode used in EOG measurement was composed of three layers including 300 nm-thick Au, 1 µm-thick polyimide (PI; HD MicroSystems, Parlin, NJ), and 5 µm-thick silicone elastomer (Smooth-On, Macungie, PA; details in Supplementary Figure S1).…”

Wrinkling of a stiff thin film bonded to a pre-strained, compliant substrate with finite thickness

“…Comparison of the potential energy in equations (2.2) and (2.10) suggests that wrinkling occurs when U bend > U bend+wrinkle , which gives [22], which neglects the effect of film/substrate bending. Figure 3 shows the critical pre-strain ε c pre versus the thickness ratio H/h for a polyimide film (E f = 2.5 GPa, v f = 0.34) on a polydimethylsiloxane substrate (E s = 1 MPa, v s = 0.5).…”

“…The recent study by Ma et al [22] accounted for (1), while this paper aims to establish an analytic model for both (1) and (2). The resulting critical pre-strain will be useful for many applications such as the strain-limiting design of materials [22] and tuneable optical design of the intensity for diffraction peaks [23].…”

“…The resulting critical pre-strain will be useful for many applications such as the strain-limiting design of materials [22] and tuneable optical design of the intensity for diffraction peaks [23].…”