Understanding the mechanical “shakedown” of hydrogel ionotronics for realizing their highly functional stability

Abstract

Hydrogel ionotronics (HI) has been extensively functionalized as artificial skins, soft robotics, and ultra-flexible displays over the recent years, and these applications require the involved materials to sustain prolonged cyclic stretching. However, HI devices suffer from a “shakedown” after prolonged cycling, as the stress-stretch curves change cycle by cycle. This mechanical failure often leads to functional instability of the HI devices, and the current literature discussing their structural origin and associated structure-property-function relationship is scarce. To this end, this work first discloses the molecular network response of HI devices for their shakedown behavior using the in situ small-angle X-ray scattering. Briefly, the stretching-induced orientation of the crosslinked network is unable to be fully relaxed during the cyclic stretching, which in turn leads to the partial disentanglement of the entangled network. Considering this mechanism, a time-dependent constitutive model is then proposed to quantitatively describe the shakedown behavior of HI devices. The model proposed herein precisely predicts the shakedown of HI devices and their corresponding cyclic stress-stretch curves, by using the parameters extracted from initial 70 cycles. Essentially, this work improves the in-depth understanding of fundamental issues related to structure–property-function relationships of HI devices, and thus, provides inspiration and guidance for designing highly stable and functional ionotronic devices.