Development of Smart Hydrogels with Tunable Properties for Cartilage Tissue Engineering

Abstract

Cartilage is avascular with poor regenerative capabilities. Therefore, a scaffold providing a cartilage-like microenvironment is needed to facilitate the regeneration of damaged cartilage.Poly(N-isopropylacrylamide) (PNIPAm) hydrogels provide an aqueous, crosslinked, and three-dimensional environment with many desirable physicochemical properties needed for tissue engineering, but have poor mechanical strength and limited biocompatibility. Crosslinking PNIPAm with cellulose or cellulose derivatives can enhance its mechanical properties. Cellulose-based biomaterials are biocompatible and have tunable mechanical properties. The goal of this thesis is to develop a mechanically strong and biocompatible hybrid scaffold using PNIPAm and cellulose. Initially, the effect of synthesis-solvent polarity on the properties of PNIPAm microgels were investigated. Microgels synthesized in dioxane and toluene showed a unique crosslinked network that resulted in cartilage-like stiffness. Due to the toxicity of these conventional solvents, and their potential negative effects for in vivo applications, it is desirable to avoid their use. A novel synthesis route was developed to synthesize fully functional PNIPAm microgels using the green bio-based solvent 2-methyltetrahydrofuran (2-MeTHF). Three different crosslinking densities with varying NIPAm to N,N’-methylenebisacrylamide (MBAA) ratios were tested. The swelling degree increased with increasing crosslinking density, which is an atypical behaviour. This may be due to a variation in the relative abundance of amide and isopropyl groups in the microgel network.Next, attention was shifted to developing hybrid PNIPAm-cellulose microgels with enhanced mechanical properties. Amorphized cellulose (AC) was produced by treating microcrystalline cellulose with 1-butyl-3methylimmidazolium chloride. AC-g-PNIPAm interpenetrating polymer network (IPN) microgels with a high swelling degree and a higher stiffness were synthesized by tuning the AC content and crystallinity. Finally, ATDC5 cells were cultured in these IPN scaffolds and analyzed the expression of three gene expression markers, i.e., Prrx1, Sox9, and Osterix. Microgels containing 10 and 15 % AC with 41.7 % crystallinity exhibit superior performance in terms of cell viability and chondrogenic gene (Sox9) expression. Thus, this study demonstrates that tuning the rheological properties of AC-g-PNIPAm microgels by controlling the cellulose content and crystallinity is an effective strategy to develop mechanically superior scaffolds for cartilage tissue engineering.