Effect of Irradiation on the Strength and Lubricity of PVA-PAA Hydrogels for Cartilage Repair

Introduction: Repairing cartilage is an ongoing challenge due to the limited autonomous healing ability of cartilage, and the lack of long term intervention strategies. Therefore, there is a great need for engineered biomaterials for cartilage replacement therapy to delay further invasive treatments. Hydrogels have been explored for this purpose. Polyvinyl alcohol (PVA) hydrogels are one of the most studied hydrogels due to their biocompatibility, lubricity, and high water content [1]. However, PVA hydrogels are not mechanically strong enough in their unprocessed form to serve as a cartilage substitute [1, 2]. In our previous publications we have shown that the addition of Poly(acrylic acid) (PAA) to the PVA network, and their subsequent thermal annealing, resulted in a tough and highly lubricous hydrogel [2,3]. In this study we have investigated the effect of sterilization on these PVA-PAA hydrogels. Sterilization is essential to prevent postoperative infection which can lead to implant rejection. Radiation sterilization is the method of choice for most medical implants. However, high-energy irradiation can change the physical properties of the hydrogels. The focus of this study is to quantify the physical changes that occur in PVA-PAA hydrogels after undergoing sterilization with varying gamma irradiation doses and in different irradiation media. Materials and Methods: Preparation of PVA-PAA hydrogel: PVA-PAA hydrogels with 25% total polymer concentrations were prepared by dissolving PVA (MW = 115,000g/mol, Scientific Polymer Products) and PAA (MW = 200,000g/mol, Polysciences) in a 19:1 ratio, respectively, in deionized water (DI) at 90°C (Fig 1). The solution was poured into glass molds before subjecting them to a freeze-thaw cycle. The resulting hydrogels were dehydrated in PEG400 in a soak ramp dehydration stage before annealing for 1hr at 160°C. The hydrogels were hydrated to equilibrium in DI water at 40°C. A 25% PVA hydrogel with 0% PAA was prepared as a control following the same procedure. Irradiation doses of 25, 50, 100, or 150kGy of gamma irradiation were applied to the hydrogels in an aqueous 50% PEG400 solution. PVA hydrogels were also irradiated in DI water to 25kGy as a control. Post-irradiation, the hydrogels were equilibrated in DI water before further testing. All reported percentages were calculated relative to the final solution weight. The Equilibrium Water Content (EWC), of the hydrogels was quantified using a Thermogravimetric Analyzer (TGA). Creep characteristics were assessed with custom built creep testers (Cambridge Polymer Group, Boston, MA). Hydrogel samples (diameter = 17mm) were loaded at 100N for 10hours and then relaxed at 10N for 10hours in 40°C DI water. The total creep strain (TCS), the strain after ten hours of loading, was reported. Tensile strength was determined by pulling dogbone cut samples according to ASTMD638 standards at a rate of 20mm/min on a mechanical test frame (Insight, MTS) equipped with a 250N load cell. Relative Coefficient of Friction (rCOF) was measured using a stress controlled rheometer (AR2000, TA Instruments) equipped with a CoCr ring rotating at a constant velocity of 0.1rad/s. The torque, normal force, and angular velocity of the ring were recorded and used to calculate the rCOF with the Kavehpour and McKinley [4] method. Gel Content was determined by dehydrating hydrated samples in a vacuum oven at room temperature, boiling away the soluble components of the hydrogel, and then dehydrating the remaining sample at 90°C. Results: As shown in Table1, irradiation did not significantly weaken the PVA-PAA hydrogel. All irradiated hydrogels showed similar break stress and strain values to the non-irradiated control. The moduli, however, of all irradiated groups were significantly lower than the nonirradiated hydrogels. 100kGy irradiated hydrogels exhibited lowest modulus value (4±2). While the irradiation did not affect the EWC significantly, an increase in rCOF was observed in irradiated hydrogels with no distinguishable correlation to the radiation dose (Table2). Table1. Effect of irradiation dosage on mechanical properties PVAPAA hydrogels. Irradiation Dosage Break Stress, (MPa) Break Strain, (%) Modulus, (MPa) 25kGy 14±1 310±41 8±2 50kGy 12±2 298±47 8±0.3 100kGy 11±0.2 303±58 4±2 150kGy 13±1 285±21 8±0.6 Non-Irradiated 15±1 333±39 13±3 Hydrogels irradiated with 25kGy and 50kGy had the highest (0.3±0.01), and lowest (0.1±0.05) rCOFs, respectively. All irradiated hydrogels, except the 25kGy irradiated hydrogels, had gel contents higher than that of the non-irradiated group. As shown in Table 3, the addition of PAA weakens hydrogels,