The next generation of flexible electronics will require highly stretchable and transparent electrodes, many of which consist of a relatively stiff metal network (or carbon materials) and an underlying soft substrate. Typically, such a stiff–soft bilayer suffers from wrinkling or folding when subjected to strains, causing high surface roughness and seriously deteriorated optical transparency.
Recently, Southern University of Science and Technology (SUSTech) researchers, Prof. Chuanfei Guo from the Department of Materials Science and Engineering and Prof. Wei Hong from the Department of Mechanics and Aerospace Engineering, as well as collaborators from the University of Houston and Harvard University, published a paper in Advanced Materials that examined methods of reducing wrinkles in various flexible electronics. Their paper, entitled “Giant Poisson’s effect for wrinkle-free flexible electronics,” found that when the stiff layer has a large enough Poisson’s effect, a stiff-soft bilayer will be free of surface instabilities under various deformation modes including bending, compression (or releasing from a pre-stretched state), or uniaxial stretching.
Wrinkles are widely observed, from the landforms of the earth crust, to human skin, and to flexible electronics. A stiff-soft bilayer wrinkles at a tiny compressive strain, which is almost inevitable in real world applications. Flexible transparent electrodes, a key element in flexible electronics, often consist of stiff-soft bilayers, which tend to wrinkle under mechanical loads. Surfaces wrinkles dramatically increase roughness and reduce optical transmittance, and may also deteriorate interfacial strength. Wrinkles have become a plague to flexible transparent electrodes and related devices.
The research team sought to describe a general design strategy to achieve wrinkle-free stretchable electrodes and devices by introducing a network structure with a large Poisson’s effect into a network–elastomer bilayer so that surface instabilities may be suppressed during bending, stretching, or releasing from prestretch. The underlying mechanism is the large Poisson’s effect of the network that results in a lateral contraction larger than that of the substrate, leading to a biaxial tension state that prevents wrinkling. This strategy is size‐independent. The experimental results show that on the nanoscale, a transparent gold‐nanomesh–poly(dimethylsiloxane) (AuNM–PDMS) bilayer electrode is free of wrinkles when released from a large prestrain of up to 100%, while exhibiting significantly improved stretchability and little change in transparency.
A critical condition for preventing surface wrinkles was derived. At small strains, the critical Poisson’s ratio for preventing surface wrinkles is approximately 2. Such a value is far beyond the theoretical limit for Poisson’s ratios of homogeneous isotropic solids, but it can be achieved by employing some network structures. This mechanism has also been confirmed experimentally—flexible transparent electrodes with network structures exhibiting a giant Poisson’s effect, as well as a stretchable tactile sensor employing the electrodes, were found to be wrinkle-free.
The principle is general and can be extended to other materials and systems: a variety of network structures are found to present a giant Poisson’s effect, and are believed to be wrinkle-free platforms for stretchable transparent electrodes, flexible electronics devices, and other applications such as mechanical structures, optical materials and biomedical materials.
The researchers have also provided a new perspective to design materials and structures that demand instability‐free surfaces.
The paper received funding from the National Natural Science Foundation of China, the “Science Technology and Innovation Committee of Shenzhen Municipality,” and the “Guangdong Innovative and Entrepreneurial Research Team Program.” The authors specifically thank the SUSTech Materials Characterization and Preparation Center for preparing the samples.
Original paper – https://onlinelibrary.wiley.com/doi/full/10.1002/adma.201902955
