Water soluble polymers can be tailored to exhibit sol-get transition behaviors in response to external stimuli such as temperature and pH. Amphiphilic block copolymers and peptide oligomers with specific sequences can go through self-assembly to form a nanostructured hydrogel under physiological. conditions. A wide range of sol-get transition hydrogels can be synthesized by introducing physically crosslinkable junctions such as hydrophobic groups, pH-sensitive moieties, and stereocomplexed crystallizable domains in the polymer structure. Furthermore, the in situ forming and injectable hydrogels can also be functionalized with bioactive molecules such as heparin, cell adhesive peptides, and hyaluronic acid, to provide sustained release of therapeutic proteins or support the growth and function of cells. Various sot-get transition, biodegradable, and biocompatible hydrogels show great potentials as injectable materials in drug delivery and tissue engineering. (C) 2009 Elsevier Ltd. All rights reserved.