A green MXene-based organohydrogel with tunable mechanics and freezing tolerance for wearable strain sensors

Hydrogels have emerged as a promising bioelectronic interfacing material. This review discusses the fundamentals and recent advances in hydrogel bioelectronics. Bioelectronic interfacing with the human body including electrical stimulation and recording of neural activities is the basis of the rapidly growing field of neural science and engineering, diagnostics, therapy, and wearable and implantable devices. Owing to intrinsic dissimilarities between soft, wet, and living biological tissues and rigid, dry, and synthetic electronic systems, the development of more compatible, effective, and stable interfaces between these two different realms has been one of the most daunting challenges in science and technology. Recently, hydrogels have emerged as a promising material candidate for the next-generation bioelectronic interfaces, due to their similarities to biological tissues and versatility in electrical, mechanical, and biofunctional engineering. In this review, we discuss (i) the fundamental mechanisms of tissue–electrode interactions, (ii) hydrogels’ unique advantages in bioelectrical interfacing with the human body, (iii) the recent progress in hydrogel developments for bioelectronics, and (iv) rational guidelines for the design of future hydrogel bioelectronics. Advances in hydrogel bioelectronics will usher unprecedented opportunities toward ever-close integration of biology and electronics, potentially blurring the boundary between humans and machines.

In this paper, we propose the design of a family of hydrogel electrolytes that featuring freezing resistance, flexibility, safety, superior ionic conductivity and long-term stability to realize anti-freezing flexible aqueous batteries. It remains a challenge to render aqueous batteries operating at subzero temperatures properly, not even to mention the maintenance of their flexibility and mechanical robustness. This fundamentally arises from the freezing of hydrogel electrolytes under such low temperature, resulting in performance deterioration and elasticity loss. Here we propose an intrinsically freeze-resistant flexible zinc manganese-dioxide battery (Zn-MnO 2 -B) comprising a designed anti-freezing hydrogel electrolyte which can preclude the ice crystallization of the hydrogel component and maintain a high ion conductivity even at −20 °C. Benefiting from exceptional freeze resistance, the fabricated anti-freezing Zn-MnO 2 -B (AF-battery) exhibits excellent electrochemical stability and mechanical durability at subzero temperatures. Even at −20 °C, the specific capacity of the AF-battery can retain over 80% with Coulombic efficiencies approaching ∼100%, compared to the thorough performance failure of the Zn-MnO 2 -B with traditional polyacrylamide (PAM) hydrogel electrolyte. More impressively, the flexibility of batteries can also be well maintained even under severe mechanical stresses at subzero temperatures, such as being bent, compressed, hammered or washed in an ice bath. Furthermore, the AF-battery sealed in an ice cube can be integrated in series to power a wristband of an electronic watch, LED lights and a 72 cm 2 electroluminescent panel. It is believed that this work opens new perspectives to develop anti-freezing batteries and would play the role of a model system for developing new hydrogel aqueous electrolytes for flexible batteries in extremely cold environments.