Chitosan fibrous scaffolds for cartilage tissue engineering

The biocompatibility of chitosan and its similarity with glycosaminoglycans make it attractive for cartilage engineering despite its limited cell adhesion properties. Structural and chemical characteristics of chitosan scaffolds may be improved for cartilage engineering application. We planned to evaluate chitosan meshes produced by a novel technique and the effect of chitosan structure on mesenchymal stem cells (MSCs) chondrogenesis. Another objective was to improve cell adhesion and chondrogenesis on chitosan by modifying the chemical composition of the scaffold (reacetylation, collagen II, or hyaluronic acid (HA) coating). A replica molding technique was developed to produce chitosan meshes of different fiber-width. A polyglycolic acid (PGA) mesh served as a reference. Constructs were analyzed at two and 21 days after seeding chondrocytes with confocal microscopy, scanning electron microscopy, histology, and quantitative analysis (weights, DNA, glycosaminoglycans, collagen II). Chondrocytes maintained their phenotypic appearance and a high viability but attached preferentially to PGA. Matrix production per chondrocyte was superior on chitosan. Chitosan meshes and sponges were analyzed after seeding and culture of MSCs under chondrogenic condition for 21 days. The cellularity was similar between groups but matrix production was greater on meshes. Chitosan and reacetylated-chitosan scaffolds were coated with collagen II or HA. Scaffolds were characterized prior to seeding MSCs. Chitosan meshes were then coated with collagen at two densities. PGA served as a reference. Constructs were evaluated after seeding or culture of MSCs for 21 days in chondrogenic medium. MSCs adhered less to reacetylated-chitosan despite collagen coating. HA did not affect cell adhesion. The cell attachment on chitosan correlated with collagen density. The cell number and matrix production were improved after culture in collagen coated meshes. The differences between PGA and chitosan are likely to result from the chemical composition. Chondrogenesis is superior on chitosan meshes compared to sponges. Collagen II coating is an efficient way to overcome poor cell adhesion on chitosan. These findings encourage the use of chitosan meshes coated with collagen II and confirm the importance of biomimetic scaffolds for tissue engineering. The decreased cell adhesion on reacetylated chitosan and the poor mechanical stability of PGA limit their use for tissue engineering.

Design of unique composites based on aromatic thermosetting copolyesters

Aromatic thermosetting copolyester (ATSP) has promise in high-temperature applications. It can be employed as a bulk polymer, as a coating and as a matrix for carbon fiber composites (ATSP/C composites). This work focuses on the applications of high performance ATSP/C composites. The morphology of the ATSP matrix in the presence of carbon fiber was studied. The effect of liquid crystalline character of starting oligomers used to prepare ATSP on the final crystal structure of the ATSP/C composite was evaluated. Matrices obtained by crosslinking of both liquid crystalline oligomers (ATSP2) and non-liquid crystalline oligomers (ATSP1) tend to crystallize in presence of carbon fibers. The crystallite size of ATSP2 is 4 times that of ATSP1. Composites made from ATSP2 yield tougher matrices compared to those made from ATSP1. Thus toughened matrices could be achieved without incorporating any additives by just changing the morphology of the final polymer. The flammability characteristics of ATSP were also studied. The limiting oxygen index (LOI) of bulk ATSP was found to be 40% whereas that of ATSP/C composites is estimated to be 85%. Thus, ATSP shows potential to be used as a flame resistant material, and also as an aerospace reentry shield. Mechanical properties of the ATSP/C composite were characterized. ATSP was observed to bond strongly with reinforcing carbon fibers. The tensile strength, modulus and shear modulus were comparable to those of conventionally used high temperature epoxy resins. ATSP shows a unique capability for healing of interlaminar cracks on application of heat and pressure, via the Interchain Transesterification Reaction (ITR). ITR can also be used for reduction in void volume and healing of microcracks. Thus, ATSP resin systems provide a unique intrinsic repair mechanism compared to any other thermosetting systems in use today. Preliminary studies on measurement of residual stresses for ATSP/C composites indicate that the stresses induced are much lower than that in epoxy/C composites. Thermal fatigue testing suggests that ATSP shows better resistance to microcracking compared to epoxy resins.