Gellan gum is an anionic linear microbial polysaccharide. It has been extensively used in foods and cosmetics industries due to its special gelling property and rheological behavior of its aqueous solution since the early stage of its discovery. Recently, gellan gum as a natural polymer and its hydrogel show a wide range of application perspective in drug delivery and tissue engineering. Gellan gum is nontoxic, biocompatible, biodegradable and the resulting hydrogel is transparent and stable. The mechanical properties of gellan gum hydrogel are similar to those of normal human tissues under certain conditions. These advantages provide gellan gum versatile characteristics as a source of biomaterials. However, gellan gum-based hydrogels have intrinsic disadvantages such as lack of toughness and tissue tolerance as tissue engineering materials. These defects restrict their use in biomedical field. In order to solve these problems, quite a lot of work on chemical and physical modification on gellan gum has been carried out. Modified gellan gum reveals a much more promising perspective in the development of biomedical materials. In this paper, we summarize the gelation mechanism of gellan gum and various modification methods, and highlight the application of gellan gum and modified gellan gum based hydrogels in biomedical field. We also point out a number of main challenges for gellan gum-based tissue engineering materials.

Contents
1 Introduction
2 Structure and properties of gellan gum
2.1 Chemical structure and conformation
2.2 Rheological property of aqueous gellan gum solution
2.3 Gelation mechnism of gellan gum hydrogels
3 Chemical/physical modifications of gellan gum and their effect
3.1 Derivatization
3.2 Hydrophobization
3.3 Crosslinking
3.4 Graft copolymerization
3.5 Dual interpenetrating network
3.6 Compound modification
4 Applications of gellan gum and its modified materials in biomedicine
4.1 Drug controlled release
4.2 Tissue engineering
4.3 Other biological applications
5 Outlook