Hydrogels from natural egg white with extraordinary stretchability, direct-writing 3D printability and self-healing for fabrication of electronic sensors and actuators

Electronic sensors mimicking the function of human skin are promising for the next generation of bionic skin. In this paper, we present a facile and novel approach to prepare an electronic sensor using a physically crosslinked protein hydrogel from egg white with the capability of incorporating conductive nanomaterials. The current strategy led to the formation of extraordinarily stretchable hydrogels in which their shear-thinning and self-healing properties further enabled direct ink writing 3D printing at room temperature, providing novel insights into the biosensor fabrication process. Furthermore, we enhanced the mechanical properties of these hydrogels by introducing secondary physical crosslinking to better resemble human skin. For electronic skin demonstration, a directly 3D printed sensor (EW-CNT sensor) was fabricated with embedding of carbon nanotubes and utilized to capture the delicate wrist pulse, distant reflection of index finger flexion, and respiration as well as vigorous finger bending. Notably, the radial augmentation index and stiffness index of the cardiovascular system could be clearly revealed by the EW-CNT sensor recording. Moreover, the EW hydrogel can be used to fabricate a reversible humidity actuator through a porous gradient architecture. The advantages of the current hydrogel platform including low cost, easy handling, and ease of fabrication for scale-up may open new horizons in the field of epidermal sensors and actuators.