Development of a Biomimetic Scaffold for Meniscal Tissue Engineering

Menisci are one of the most commonly injured parts of the knee with a limited healing potential, owing to their inherently poor blood supply. To date, meniscal repair remains a challenge in orthopaedic surgery, with more than 1.4 million meniscus surgeries performed worldwide each year. Loss of function of this critical tissue within the knee cavity leads to increased risk of osteoarthritis (OA). Nonetheless, no established treatments have achieved satisfactory meniscal healing. The bottom line is that none of these procedures initiate a reparative process or regeneration of the injured tissue. This challenge can be addressed using a tissue engineering approach, combining scaffolds with mechanical properties that resemble those of the native meniscus, and cells that are capable of synthesising meniscus-specific matrix components. In comparison with naturally-derived scaffolds, synthetic biodegradable polymers are preferred candidates in this context, owing to their tunable bulk properties. However, polymer implants have had limited success due to their biological inactivity. Surface modification is thus essential to promote cellular activities without altering the structure and mechanical properties of the bulk materials. This project focuses on the development of a biomimetic, surface engineered scaffold for meniscal tissue engineering, aimed at enhancing repair of meniscal injuries. To achieve this, a poly(lactide-co-glycolic acid) (PLGA 75:25) porous scaffold was surface-modified with a tailored coating, the so-called “C6S surface”, to generate a biomimetic C6S scaffold. The surface, which was developed in the first part of this thesis, is comprised of a precursor coating of hyaluronic acid/chitosan multilayers (a bioinert surface) to which major meniscal extracellular matrix (ECM) components, namely collagen I/II (at a 2:3 ratio) and chondroitin-6-sulfate (C6S), were covalently immobilised. Utilising 2D monolayer and 3D scaffold culture systems, the effects of the C6S biomimetic surface on cell adhesion, proliferation, matrix synthesis and relevant marker gene expression of passaged rat and human meniscal cells was next evaluated. In addition, we also explored the enhancing effect of hypoxic culture conditions on the human meniscal cellular response to the biomimetic scaffold. To evaluate its clinical applicability, we further investigated the feasibility of using the surface to induce a chondrogenic response in human bone marrow-derived mesenchymal stem cells (hBM-MSCs) without transforming growth factor (TGF)- supplement. In the final part of this thesis, rat-host tissue responses to the subcutaneously implanted C6S scaffold are described. The results of this thesis indicate that coating of the PLGA porous scaffold with a surface that mimics the native meniscus matrix microenvironment rendered it more biologically active and biocompatible. We show that the C6S surface is permissive for cell adhesion, proliferation and ECM synthesis. More importantly, the biomimetic C6S scaffold is able to facilitate redifferentiation of passaged rat and human meniscal cells, and promote a chondroinductive response of hBM-MSCs, resulting in an engineered construct with matrix resembling that of the native meniscus tissue. Moreover, the C6S biomimetic scaffold elicited only mild inflammatory responses in terms of tissue infiltration and macrophage accumulation in the rat model, an important prerequisite for its use as a cell carrier for clinical applications. These studies also provide new insight into the relative gene expression profiles of major ECM components of knee menisci and the extent of their phenotypic changes during monolayer expansion. In developing a scaffold that combines the advantages of naturally-derived and synthetic biomaterials while overcoming their drawbacks, the outcome of this thesis thus represents a significant stride forward in the field of meniscal tissue engineering