Spatially modulated stiffness on hydrogels for soft and stretchable integrated electronics

One major conundrum that impedes the development and application of emerging soft and stretchable electronics lies in the integration of electronic components with soft substrates for rational combination of various device functionalities into a single wearable state, since the rigid, nondeformable electronics tend to detach from the deformable substrate under mechanical loadings like stretch. Modulating the stiffness of soft materials in a spatially controllable manner provides a promising solution to this rigid-soft coupling challenge, by shielding the local strain of rigid components while maintaining the stretchable properties of the soft substrates. Hydrogels with superb biocompatibility and skin-like mechanical features are ideal candidates for interfacing the human body and electronic functionalities for cutting-edge wearable uses, where there exists a challenge of spatially modulating the stiffness of hydrogels to meet the application demands. Herein, we develop a facile and straightforward method to locally stiffen a hydrogel (with an increased Young's modulus of one order of magnitude) via an additional crosslinking strategy. The locally stiffened site undergoes minimal strain (down to 12%) and the untreated area remains stretchable under external deformations (100% strain), which presents excellent and tunable strain shielding capability to prevent detachment of the electronic components from the substrate under strain levels up to 150%. We further demonstrate a multifunctional health sensing device based on a component-integrated locally stiffened hydrogel and its satisfactory performance in monitoring temperature, UV exposure and EMG signals unveils its brilliant prospects for wearable healthcare applications.