Recent progress of in situ formed gels for biomedical applications

With the rapid progress of biomedical technology, hydrogels that can be prepared under bio-friendly conditions are urgently needed. In situ gelling systems have been extensively investigated with the aim of being applied for minimally invasive drug delivery or injectable tissue engineering. In a premixed state of an aqueous solution, the system contains drugs or cells and other excipients. Chemical or physical triggering processes produce a hydrogel in situ. During the solution-to-gel transition process, all of the ingredients in the system form a matrix, where the drugs can be slowly released or within which cells/stem cells can grow in a specifically controlled manner. Basically, the triggering process and transition should not damage the incorporated elements, including pharmaceuticals, and cells, including stem cells. In addition, once it is formed, a hydrogel should provide a compatible microenvironment for the drugs and cells. Finally the hydrogel should be eliminated from the site after its role as a scaffold or depot is complete. In this review, in situ gelling systems were classified into chemical reaction driven gelation and physicochemical association driven gelation. The triggering mechanism involved in each process and the characteristics of each system are comparatively discussed. In addition, our perspectives on the in situ gelling systems are offered as signposts for the future advancement of this field.