Abstract
Flexible strain sensors are broadly investigated in electronic skins and human-machine interaction due to their light weight, high sensitivity, and wide sensing range. Hydrogels with unique three-dimensional network structures are widely used in flexible strain sensors for their exceptional flexibility and adaptability to mechanical deformation. However, hydrogels often suffer from damage, hardening, and collapse under harsh conditions, such as extreme temperatures and humidity levels, which lead to sensor performance degradation or even failure. In addition, the failure mechanism in extreme environments remains unclear. In this review, the performance degradation and failure mechanism of hydrogel flexible strain sensors under various harsh conditions are examined. Subsequently, strategies towards the environmental tolerance of hydrogel flexible strain sensors are summarized. Finally, the current challenges of hydrogel flexible strain sensors in harsh environments are discussed, along with potential directions for future development and applications.