This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 2193–2221 2193 Cite this: Chem. Soc. Rev., 2012, 41, 2193–2221 Injectable and biodegradable hydrogels: gelation, biodegradation and biomedical applications Yulin Li,* Joa˜o Rodrigues and Helena Toma´s* Received 29th July 2011 DOI: 10.1039/c1cs15203c Injectable hydrogels with biodegradability have in situ formability which in vitro/in vivo allows an effective and homogeneous encapsulation of drugs/cells, and convenient in vivo surgical operation in a minimally invasive way, causing smaller scar size and less pain for patients. Therefore, they have found a variety of biomedical applications, such as drug delivery, cell encapsulation, and tissue engineering. This critical review systematically summarizes the recent progresses on biodegradable and injectable hydrogels fabricated from natural polymers (chitosan, hyaluronic acid, alginates, gelatin, heparin, chondroitin sulfate, etc.) and biodegradable synthetic polymers (polypeptides, polyesters, polyphosphazenes, etc.). The review includes the novel naturally based hydrogels with high potential for biomedical applications developed in the past five years which integrate the excellent biocompatibility of natural polymers/synthetic polypeptides with structural controllability via chemical modification. The gelation and biodegradation which are two key factors to affect the cell fate or drug delivery are highlighted. A brief outlook on the future of injectable and biodegradable hydrogels is also presented (326 references). 1. Introduction Hydrogels are a class of materials which present a three dimensional (3D) structure, consist of hydrophilic polymers and retain a significant amount of water. 1 Due to their good CQM–Centro de Quı´mica da Madeira, MMRG, Universidade da Madeira, Campus da Penteada 9020-105 Funchal, Portugal. E-mail: yulinli@uma.pt, lenat@uma.pt; Fax: +351-291705149/249; Tel: +351-291705150 Yulin Li (left), Joa˜o Rodrigues (middle), Helena Toma´s (right) Yulin Li obtained his PhD in Polymer Chemistry and Physics from the Institute of Chemistry, Chinese Academy of Sciences (2005), China. He started his first postdoctoral work at Lille University of Science and Technology, France. In 2006, he moved to Biomaterials Center, National Institute for Materials Science, Japan, and then to Department of Applied Chemistry, Kyushu University, Japan, as a postdoctoral researcher. He is currently a Science 2008 FCT Assistant Researcher at Centro de Quı´mica da Madeira, University of Madeira. His current research interests are fabrication of smart hydrogels for bio- medical applications. Joa ˜o Rodrigues received his PhD degree (1999) in Chemistry/ Inorganic Chemistry from the University of Lisbon (Portugal). He was appointed Assistant Professor at University of Madeira in 1999, teaching general chemistry, inorganic, organometallic chemistry, biomimetics, nanochemistry and nano- materials. He is a leader of the Molecular Materials Research Group (MMRG) since 2004 and a Scientific Director of the Centro de Quı´mica da Madeira since 2006. His research interests are related with the design, synthesis and characterization of dendrimers, molecular wires, nanoparticles and polymers for optoelectronic and biomedical applications. Helena Toma ´s got her PhD degree from the University of Lisbon (Portugal) in 1996, in the domain of Technological Chemistry/ Biomaterials. Currently, she is an Associate Professor at the University of Madeira (Portugal) and a researcher at the Centro de Quı´mica da Madeira. Her current research work is mainly devoted to the development of nanomaterials and hydrogels for biomedical applications, especially for gene/drug delivery and gene silencing. In particular, she has been working in the development of dendrimer based materials and in their application for the control of mesenchymal stem cells differentiation. Chem Soc Rev Dynamic Article Links www.rsc.org/csr CRITICAL REVIEW Downloaded by Universidade da Madeira (UMA) on 28 February 2012 Published on 24 November 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15203C View Online / Journal Homepage / Table of Contents for this issue