# Epithelium‐Inspired, Ultrahigh‐Toughness, Ultralow‐Hysteresis, and Highly Compressible Polymer Hydrogels as Self‐Powered, Visual, and Underwater Strain Sensors

## TL;DR

Scientists created tough, flexible hydrogels inspired by epithelial tissue that can sense strain, work underwater, and generate their own power.

## Contribution

The novel hydrogel design combines ultrahigh toughness, low hysteresis, and compressibility for advanced strain sensing.

## Key findings

- PLTAV hydrogel achieved 1368% stretchability and 2.64 MJ·m−3 toughness.
- PLTAV-SC hydrogel showed 2021% stretchability and 6.10 MJ·m−3 toughness.
- Hydrogels function as self-powered, visual, and underwater strain sensors.

## Abstract

Developing high‐toughness, low‐hysteresis, and highly compressible polymer hydrogels as wearable strain sensors with superior detection ranges, durability, and signal accuracy is still a grand challenge due to contradictory characteristics. Herein, inspired by epithelial tissue, an epithelium‐like structure hydrogel with cell‐like particles (PLTAV) is synthesized from a water‐in‐oil high internal phase emulsion. During loading‐unloading processes, the hydrophilic cell‐like particles can deform reversibly and be cyclically divided into small‐sized particles and aggregated, thereby dissipating more energy. Therefore, the PLTAV hydrogel with high water content (90.4 wt%) has superior stretchability (1368%), ultrahigh toughness (2.64 MJ·m−3), ultralow hysteresis (4.7%, ε = 300%), and ultrahigh compressibility (99.9%). Subsequently, choline chloride and sorbitol are introduced into the PLTAV hydrogel (PLTAV‐SC). The as‐prepared PLTAV‐SC hydrogel exhibits improved freezing resistance, enhanced stretchability (2021%) and toughness (6.10 MJ·m−3), and retained low hysteresis and ultrahigh compressibility. Benefiting from the cell‐like particles composed of polymers with ionic structural units and the difference in ionic mobilities, these hydrogels can act as self‐powered strain sensors with high sensitivities. Meanwhile, they can also be used as high‐performance visual and resistance‐type strain sensors. Additionally, these sensors can be further constructed as underwater strain sensors for detecting underwater human/animal motion, water flow velocity, and water depth.

Inspired by epithelial tissue, epithelium‐like structure hydrogels are synthesized. The as‐prepared hydrogels exhibit ultrahigh toughness, ultralow hysteresis, and ultrahigh compressibility, which can be utilized as self‐powered and visual strain sensors. Additionally, resistance‐type strain sensors constructed from the as‐prepared hydrogels have excellent sensing performance, which can be further used for underwater strain sensing.