Recent advances in 4D printing hydrogel for biological interfaces

4D printed hydrogels are 3D printed objects whose properties and functions are programmable. In the definition of 4D printing, the fourth dimension arises from the ability of printed structures to change their shape and/or function over time when exposed to given conditions environmental stimuli, during their post-press life. Stimulation-responsive hydrogels produced by the emerging 4D bioprinting technology are currently considered as encouraging tools for various biomedical applications due to their exciting properties such as stretchability, biocompatibility, ultra-flexibility, and printability. Using 3D printing technology, customized functional structures with controllable geometry and trigger ability can be autonomously printed onto desired biological interfaces without considering microfabrication techniques. In this review, by studying the progress in the field of hydrogels for biointerfaces, we summarized the techniques of 4D printing gels, the classification of bioinks, the design strategies of actuators. In addition, we also introduced the applications of 4D hydrogels in tissue repair, vascular grafts, drug delivery, and wearable sensors. Comprehensive insights into the constraints, critical requirements for 4D bioprinting including the biocompatibility of materials, precise designs for meticulous transformations, and individual variability in biological interfaces.