Towards Hydrogel-Capped Metal Implants for Cartilage Repair

Ph.D. Defense

Benjamin Wiley, Ph.D, Advisor

Abstract:There are approximately 900,000 people in the US suffering from damage to the articular cartilage, with the knee being most commonly affected. Articular cartilage lacks a vasculature and has a limited ability to heal. A variety of surgical treatments have been developed to repair cartilage lesions. Current strategies for cartilage repair suffer from high failure rates (25-50% at 10 years), long rehabilitation time (more than 12 months) and decreasing efficacy in patients older than 40-50 years. Thus, A focal joint resurfacing method that is widely available, allows immediate weight bearing, has short recovery times and has low long-term failure rates remains an unmet need.
This thesis explores a strategy to address this need. There are two major criteria within this strategy: 1) develop a material that mimics the properties of cartilage and 2) attach this material to an orthopedic base to enable integration with bone.
I developed the first hydrogel to achieve the strength and modulus of cartilage in both tension and compression properties. This hydrogel also exhibits cartilage-equivalent tensile fatigue at 100,000 cycles. The hydrogel was created by infiltrating a PVA-PAMPS double-network hydrogel into a bacterial cellulose (BC) nanofiber network. The BC fibers provide tensile strength in a manner analogous to collagen in cartilage. The PAMPS provides a fixed negative charge and osmotic restoring force similar to the role of aggrecan in cartilage.
Subsequently, I further improved and developed the hydrogel to reach a strength that exceeds that of cartilage. The high strength was achieved through reinforcement of crystallized PVA with BC. Experimental results show that reinforcement of annealed PVA with BC leads to a 3.2-fold improvement in the tensile strength (from 15.6 to 50.5 MPa) and a 1.7-fold increase in the compressive strength (from 56.7 to 95.4 MPa). The BC-reinforced PVA was also 3 times more wear resistant than cartilage over 1 million cycles and exhibited the same coefficient of friction. These properties make the BC-reinforced BC hydrogel an excellent candidate material for replacement of damaged cartilage.
Current strategies for adhering hydrogel to a surface are 10 times weaker than the shear strength with which cartilage is attached to bone. I sought to mimic this strategy by bonding freeze-dried BC to porous titanium with a hydroxyapatite-forming cement.