Communications Innovative Approach for Producing Injectable, Biodegradable Materials Using Chitooligosaccharides and Green Chemistry Luciano F. Boesel, † Rui L. Reis, ‡,§ and Julio San Roma ´n* ,§ Department of Biomaterials, Institute of Science and Technology of Polymers, Spanish National Research Council (CSIC), 28006, Madrid, Spain, 3B’s Research GroupsBiomaterials, Biodegradables and Biomimetics, Department of Polymer Engineering, University of Minho, Campus of Gualtar, 4710-057, Braga, Portugal, and IBBsInstitute for Biotechnology and Bioengineering, PT Government Associated Laboratory, Braga, Portugal Received November 18, 2008; Revised Manuscript Received January 19, 2009 Although there are a number of injectable biomaterials currently under development, they present some drawbacks such as being based on synthetic polymers, needing toxic or aggressive synthesis procedures or using raw materials with low availability and/or high production costs. Having this in mind, a novel injectable biomaterial using chitooligosaccharides as starting materials was developed. This system uses a widely available and cheap polymer from marine biomass (chitosan), which can be turned into an injectable material by water-based and ecologically friendly reactions. Chitooligosaccharides were functionalized with methacrylic groups, to allow in situ cross- linking. The degree of substitution, as determined by 1 H NMR, varied between 5 and 50%. The system was characterized in terms of kinetics of gel formation, rheology, degradation behavior and in vitro cytotoxicity. The gelation time could be easily tailored between 1.5 and 60 min by changing the conditions of the methacrylation reaction, and the final gel presented rheological properties typical of strong gels, that is, shear stresses in the kPa range. The cross-linked gel was degradable and nontoxic, presenting indeed an interesting cytokinetic effect. Injectable materials based on chitooligosaccharides are, therefore, an innovative system combining adequate biological performance, ease of preparation, and an ecologically friendly concept of production. Introduction Injectable systems are an important class of materials employed in the biomedical field. Besides the long-term use of acrylic polymers or calcium-phosphate ceramics as bone ce- ments, recently injectable materials made of biodegradable polymers have drawn considerable attention. The potential applications of injectable biodegradable systems (IBS) are mainly as tissue engineering scaffolds or controlled drug delivery systems. Injectable materials eliminate the need of complicated surgery for implantation and are able to take the exact shape of the defect to be regenerated, thus eliminating the need for precise prefabrication of the scaffold. Moreover, they may be easily mixed with cells, growth factors, or drugs just before implanta- tion, making the handling of such agents much easier. Depend- ing on the composition and hardening mechanism of the injectable material, both hard and soft tissues may be targeted. In general, in situ cross-linking systems lead to stiffer and stronger materials; thermoresponsive hydrogels or self-assembly systems, on the other hand, are weaker, although they raise less cytotoxicity issues due to the lack of solvents or initiators. 1,2 In recent years, the attention has moved from the traditional, nondegradable injectable materials such as poly(isopropylacry- lamide) (PNIPA) or Pluronics to the more appealing degradable counterparts. 3-5 The latter includes di-, tri-, or multiblock copolymers of poly(lactic-co-glycolic acid) (PLGA) and poly- (ethylene glycol) (PEG) 6,7 and cross-linkable systems of acry- lated poly(propylene fumarate) (PPF) 8,9 or poly(anhydrides), 10,11 among other synthetic polymers. Although these and other polymers have fulfilled all the requirements for being used as * To whom correspondence should be addressed. E-mail: jsroman@ ictp.csic.es. † CSIC. ‡ University of Minho. § IBB. March 2009 Published by the American Chemical Society Volume 10, Number 3  Copyright 2009 by the American Chemical Society 10.1021/bm801332u CCC: $40.75  2009 American Chemical Society Published on Web 01/30/2009