SENIOR INSTRUCTOR & INNOVATION & EXTERNAL PARTNERSHIPS CHAIR Rowan University, United States
Introduction: Primary challenges associated with the design and success of polymeric biomedical devices are generally related to the control of the biomaterial in terms of degradability characteristics, sufficient processability characteristics, and required mechanical strength that may be altered during sterilization or manufacturing procedures. Polyvinyl alcohol-based thermoresponsive biomaterials provide a distinct advantage for biomedical applications as their physiochemical properties can be easily modified according to their desired use. In this work, we evaluated the thermal degradation characteristics of a Polyvinyl alcohol(PVA)/Polyethylene glycol (PEG)/Polyvinylpyrrolidone(PVP) hydrogel that undergoes a steam sterilization autoclave cycle at 121˚C to induce fluid-like behavior. FTIR was used to characterize the evolution of the area of the carbonyl region between 1800 cm-1 and 1525 cm-1. The carbonyl area increased at temperatures beyond 121˚C which were used to accelerate the onset of degradation during both thermal oxidation and pyrolysis. The change in the carbonyl region was shown to correlate with respect to both temperature and time of exposure. The carbonyl region increased significantly in the presence of oxygen at temperatures above 150˚C. Despite showing signs of thermal degradation at temperatures exceeding 150˚C, our biomaterial was shown to be stable at 121˚C during thermal degradation testing. Furthermore, bulk property analysis showed the hydrogel’s mechanical and swelling properties were preserved even after being subject to multiple autoclave cycles beyond what would be experienced during a sterilization or clinical procedure.
Materials and
Methods: Hydrogels were prepared using an adapted theta solution method. After manufacturing, 5 mL syringes were filled with the hydrogel for further testing. To assess the kinetics of thermal oxidation, a high temperature oven was set to temperatures of 121°C to mimic that of a typical autoclave cycle and increased temperatures of 150°C and 200°C (±2°C) to accelerate the onset of degradation. Hydrogel samples were then positioned inside the oven once the desired temperature was reached. 5 samples were heated at each outlined temperature at the following respective timepoints: (0, 1, 3, 6, 10, 16, and 24 hours). At each timepoint, samples were removed from the oven and immediately tested for FTIR analysis with n=5 representative spectra for each sample and timepoint. To assess the kinetics of pyrolytic degradation, the testing schematic modeled the same procedure for thermal oxidation, with the only exception being the lack of oxygen, characteristic of pyrolytic degradation. Confined compression testing was performed using gel molded into a cylinder with a 10 mm diameter and 5 mm height. Heated gel was injected into molds placed on a flat glass surface. All tests were conducted on a Shimadzu EZX and 500 N load cell with supporting 10 mm diameter testing arm and custom-made confined compression mold with the same diameter. Tests were performed with a strain rate of 100% strain/min until 35% strain or 500 N. The chord modulus was calculated between 150-200 N. The swelling ratio was also calculated for confined compression samples.
Results, Conclusions, and Discussions: Through FTIR analysis of the area of the carbonyl region, the PVA/PEG/PVP hydrogel showed evidence of thermal degradation that was dependent on both temperature and time. Temperature had the largest effect on the increase of the carbonyl and hydroxyl areas, describing 60.44% (p < 0.0001) and 51.67% (p < 0.0001), respectively, of the variation when oxygen was present during heating. Contrary to thermal oxidation, time had the largest effect on the increase of the carbonyl and hydroxyl areas, describing 52.07% (p < 0.0001) and 42.38% (p < 0.0001), respectively, of the variation during pyrolysis.The addition of oxygen during thermal heating accelerated the rate of degradation of the polymer composite in comparison to pyrolysis. Despite showing signs of thermal degradation at temperatures exceeding 150˚C, our biomaterial was shown to be stable at 121˚C during thermal degradation testing. An inverse relationship was identified for the O-H and C-H regions during thermal oxidation testing as the carbonyl area increased. Contrary to carbonyl area analysis, the O-H and C-H regions decreased rapidly as thermal treatment progressed, reaching a minimum at 24 hours of exposure (O-H and C-H areas (p < 0.001)). This finding suggests that O-H and C-H products are intermediaries in the formation of carbonyl species. Similarly, the decrease in these regions was accelerated in the presence of oxygen at temperatures exceeding 150°C. FTIR and bulk property results assessed due to multiple autoclave cycles provided further evidence that our hydrogel was not experiencing any marketable degradation as values were consistent with control gel. While other researchers have used FTIR to measure the thermal degradation properties of the individual components of our hydrogel, we were able to demonstrate a method of spectral and bulk property analyses to understand the thermal stability of a composite biomaterial intended for biomedical applications. With the establishment of this method to chemically assess the mechanisms of thermal degradation, different formulations of a PVA/PEG/PVP biomaterial can be evaluated to optimize a thermally resistant biomaterial. Depending on the intended function of the biomaterial, considerations should be taken to ensure additional mechanical properties are preserved that are central to the site of implantation.
