Tissue engineering is a multidisciplinary field that aims to repair or regenerate lost or damaged tissues and organs in the body. The foundation of tissue engineering encompasses three fundamental strategies, specifically cellular, biochemical, and scaffold-based approaches. For the repair of certain load-bearing parts of the body, success of a tissue regeneration strategy can be dependent on scaffold adhesion or integration with the surrounding host tissue to prevent dislocation. One such area is the regeneration of the nucleus pulposus (NP) of the intervertebral disc (IVD). Tissue engineering of the NP is regarded as a potential strategy for the treatment of lower back pain, one of the most common medical problems in the world. Several researchers have focused on seeding cells in three-dimensional matrices to achieve formation of a new NP matrix. Studies have also shown that adipose derived stem cells (ASCs) can be differentiated into NP-like cells in vitro and in vivo. While these findings are promising, next generation NP engineering scaffolds must have the ability to form a substantial interface with surrounding disc tissue. This will reduce or eliminate the risk o dislocation in the disc and help to provide adequate transmission of force across the interface between the implant and the tissue. Although scaffold integration with tissue can be achieved using a bioadhesive polymer, the currently proposed materials with high adhesive properties have limited biocompatibility. A need for bioadhesive polymers exists in the area of regenerative medicine. The design of a material that covalently bonds with surrounding extracellular matrix components and provides an environment permissive to the survival and differentiation of encapsulated cells would be a major step forward not just in IVD engineering, but in orthopedic tissue engineering, in general. In this proposal, we detail the development of a novel "smart" hydrogel for ASC encapsulation, partially composed of the thermally sensitive polymer poly(N-isopropylacrylamide) (PNIPAAm). Below its lower critical solution temperature (LCST) at 32C, the polymer forms a miscible solution with water. Above the LCST, it becomes hydrophobic, so the polymer and water separate, forming a compact gel. Therefore, aqueous solutions of PNIPAAm can be implanted non-invasively through a small gauge needle and solidify in situ. The biopolymer chondroitin sulfate (CS), an ECM component of the native IVD tissue, is incorporated into the PNIPAAm matrix to form a semi-synthetic injectable hydrogel with the favorable mechanical characteristics of PNIPAAm and the enzymatic degradability, anti-inflammatory activity, water and nutrient absorption of CS. In addition, CS can be modified with aldehyde groups (CS aldehyde), allowing it to react with amines via Schiff's base reaction, thus rendering the hydrogel bioadhesive upon contact with amines of the extracellular matrix proteins. However, the presence of the reactive aldehyde groups can compromise the viability of encapsulated cells. The novel strategy in this proposal is to circumvent this problem with the inclusion of liposomes designed to deliver ECM components after the polymer has adhered to tissue and reached physiological temperature. The discharge of ECM components will enhance the biocompatibility of the material by marking the assembly of a biomimetic matrix, and also covalently reacting with, or "end-capping", the aldehyde functionalities within the gel that did no participate in bonding with tissue upon contact. This work is based on the hypothesis that the three-component bioadhesive (PNIPAAm, CS aldehyde, and ECM-loaded liposomes) will support long term viability and differentiation of ASCs toward a NP phenotype, making it a feasible three- dimensional culture system for use in IVD tissue engineering.
A need for bioadhesive polymers exists in the area of regenerative medicine, and especially in intervertebral disc tissue engineering. This project focuses on the design of a self-assembling biomimetic hydrogel that covalently bonds with tissue and provides an environment permissive to survival and differentiation of encapsulated adipose-derived stem cells. The development of such a scaffold will have transformative potential not just in intervertebral disc repair, but also in orthopedic tissue engineering in general.
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