Fabrication And Properties Of Hydrogel For Cartilage Tissue Engineering Scaffold

In cartilage tissue engineering, scaffold can provide suitable environment and support for chondrocyte growth and tissue formation. Hydrogel has been widely studied for cartilage tissue engineering scaffold because it mimics the natural extracellular matrix of cartilage. Aliphatic polycarbonate has emerged as a new class of polymer with good biocompatibility and mechanical properties. Hyaluronic acid (HA) has unique bioactivity, which can interact with chondrocytes through surface receptors to mediate cell proliferation and differentiation. Considering the advantage of polycarbonate and HA, a series of novle hydrogels based on oligo(2,2-dimethyltrimethylene carbonate)(ODTC)-poly(ethylene glycol)(PEG)-oligo(2,2-dimethyltrimethylene carbonate)(ODTC) triblock copolymer (DPD) and HA were fabricated, and the mechanical properties, degradation, and protein delivery properties of hydrogels were evaluated in details. The chondroctyes was encapsulated and cultured in the obtained hydrogels, aand cell adhesion, proliferation and extra cellular matrix secretion was investigated. The main work in this thesis was as follows:In chapter1, the background and development status of cartilage tissue engineering was reviewed, with emphases on the research status of hydrogels for articular cartilage tissue engineering.In chapter2, a series of biodegradable hydrogels based on DPD triblock copolymer with varied length of hydrophilic PEG segment and hydrophobic ODTC segment were prepared by photopolymerization. Mechanical, degradation and cell adhesion properties of hydrogels were examined, which can be tuned by altering the lengths of the hydrophilic/hydrophobic segment. The hydrogels exhibited different pattern of mechanical properties by altering the lengths of ODTC and PEG. For the hydrogels with same length of ODTC segment, an increase of the toughness and decrease of elastic modulus was observed with the increase of the length of PEG segment; for the hydrogel with same length of PEG segment, both elastic modulus and toughness of the hydrogels increased due to the energy dissipation of the physical aggration formed by hydrophobic interaction of the ODTC segment. The degradation rate of hydrogels increased with the increase of the composition of the ODTC component. In monolayer culture, the number of chondrocytes attached to the hydrogel increased along with the increase of the length of ODTC segment. The results of three dimentional photo-encapsulation of chondrocytes indicated the biodegradable hydrogel has comparable cytotoxicity with the widely recognized PEG hydrogels and this hydrogels have great potential as scaffold in cartilage tissue engineering.In chapter3, hydrogels were prepared by copolymerization of precursors based on DPD and HA. Three-dimentional culture of chondrocytes in the hydrogel demonstrated that the obtained hydrogels are biocompatible. The effects of the content of DPD and HA in hydrogel on the mechanical properties, protein delivery properties and chondrocytes proliferation were studied. The swelling ratio, crosslink density and mesh size can be tuned by altering the content of DPD or HA. The mechanical properties were enhanced with the increase of DPD composition, the proliferation of chondrocytes can be improved in hydrogel with HA component. The permeability of water, nutrition and drug plays the key role in cartilage tissue engineering. Bovine serum albumin (BSA) was selected as the model protein to investigate the protein release properties of the hydrogel, and the effective diffusion coefficient decreased from1.50×10-7to0.27×10-7cm/s with the increase of crosslink density of hydrogels.In chapter4, high-strength double-network (DN) hydrogels were fabricated using HA as the first network and PEG as the second network, which showed more regular structure than the corresponding single-network (SN) hydrogels. The fracture stress of the optimized DN hydrogel was up to45.2±4.4MPa, which was2260and40times more than that of HA and DPD SN hydrogels, respectively. DN technique was carried out to overcome the inferior mechanical properties of traditional hydrogels, and hydrogels with comparable mechanical strength to natural cartilage tissue were prepared.In chapter5, novel DN hydrogels which are suitable for chondrocytes encapsulation were fabricated with DPD and HA. Two kinds of DN hydrogels were synthesized by changing the crosslinking squence:HD hydrogels were prepared using HA and DPD as the first and second network respectively; DH hydrogels were prepared using DPD and HA as the first and second network respectively. Using phosphate buffer solution as the solvent, the fracture stress of the optimized DH and cell-laden DH hydrogels were8.38±0.67MPa and6.28±1.26MPa, respectively, which match with that of the natural cartilage tissue. The cartilage-specific extracellular matrix accumulated in the DH hydrogels after a long-time culture. Histochemical and immunofluorescence analysis further proved the accumulation of collagen type II in the DN hydrogels. The results indicated the potential of the novel DN hydrogels as cartilage tissue engineering scaffold.