The increasingly intimate bond connecting soft actuation devices and emerging biomedical applications is triggering the development of novel materials with superb biocompatibility and a sensitive actuation capability that can reliably function as bio-use-oriented actuators in a human-friendly manner. Stimulus-responsive hydrogels are biocompatible with human tissues/organs, have sufficient water content, are similar to extracellular matrices in structure and chemophysical properties, and are responsive to external environmental stimuli, and these materials have recently attracted massive research interest for fabricating bioactuators. The great potential of employing such hydrogels that respond to various stimuli (e.g., pH, temperature, light, electricity, and magnetic fields) for actuation purposes has been revealed by their performances in real-time biosensing systems, targeted drug delivery, artificial muscle reconstruction, and cell microenvironment engineering. In this review, the material selection of hydrogels with multiple stimulusresponsive mechanisms for actuator fabrication is first introduced, followed by a detailed introduction to and discussion of the most recent progress in emerging biomedical applications of hydrogel-based bioactuators. Final conclusions, existing challenges, and upcoming development prospects are noted in light of the status quo of bioactuators based on stimulus-responsive hydrogels.

Hydrogel‐based electronics have found widespread applications in soft sensing and health monitoring because of their remarkable biocompatibility and mechanical features similar to human skin. However, they are subjected to potential challenges like structural failure, functional degradation, and device delamination in practical applications, especially facing extreme environmental conditions (e.g., abnormal temperature and humidity). To address these, ionically conductive organohydrogel‐based soft electronics are developed, which can perform at subzero and elevated temperatures (thermal compatibility) as well as at dehydrated and hydrated environments (hydration compatibility) for extended applications. More specifically, gelatin/poly(acrylic acid–N‐hydrosuccinimide ester) (PAA–NHS ester)‐based ionic‐conductive organohydrogel is synthesized. By introducing a glycerol–water binary solvent system, the gel can maintain mechanical softness in a wide temperature range (from −80 to 60 °C). Besides, excellent conductivity is achieved under various conditions by soaking the gel into lithium chloride anhydrous (LiCl) solution. Strong adhesion with skin, even under water, can be realized by covalent bonds between NHS ester from gel and amino groups from human skin. The excellent performances of LiCl‐loaded PAA‐based organohydrogel (L‐PAA‐OH)‐based electronics are further demonstrated under freezing and high temperatures as well as underwater conditions, unveiling their promising prospects in wearable health monitoring in various conditions.

Detection of biomarkers associated with body conditions provides in‐depth healthcare information and benefits to disease management, where the key challenge is to develop a minimally invasive platform with the ability to directly detect multiple biomarkers in body fluid. Dermal tattoo biosensor holds the potential to simultaneously detect multiple health‐related biomarkers in skin interstitial fluid because of the features of minimal invasion, easy operation, and equipment‐free result reading. Herein, a colorimetric dermal tattoo biosensor fabricated by a four‐area segmented microneedle patch is developed for multiplexed detection of health‐related biomarkers. The biosensor exhibits color changes in response to the change of biomarker concentration (i.e., pH, glucose, uric acid, and temperature), which can be directly read by naked eyes or captured by a camera for semi‐quantitative measurement. It is demonstrated that the colorimetric dermal tattoo biosensor can simultaneously detect multiple biomarkers in vitro, ex vivo, and in vivo, and monitor the changes of the biomarker concentration for at least 4 days, showing its great potential for long‐term health monitoring.

No abstract