Effect of Temperature on Thrombogenicity Testing of Biomaterials in an In Vitro Dynamic Flow Loop System.

To develop and standardize a reliable in vitro dynamic thrombogenicity test protocol, the key test parameters that could impact thrombus formation need to be investigated and understood. In this study, we evaluated the effect of temperature on the thrombogenic responses (thrombus surface coverage, thrombus weight, and platelet count reduction) of various materials using an in vitro blood flow loop test system. Whole blood from live sheep and cow donors was used to assess four materials with varying thrombogenic potentials: negative-control polytetrafluoroethylene (PTFE), positive-control latex, silicone, and high-density polyethylene (HDPE). Blood, heparinized to a donor-specific concentration, was recirculated through a polyvinyl chloride tubing loop containing the test material at room temperature (22-24°C) for 1 hour, or at 37°C for 1 or 2 hours. The flow loop system could effectively differentiate a thrombogenic material (latex) from the other materials for both test temperatures and blood species ( p < 0.05). However, compared with 37°C, testing at room temperature appeared to have slightly better sensitivity in differentiating silicone (intermediate thrombogenic potential) from the relatively thromboresistant materials (PTFE and HDPE, p < 0.05). These data suggest that testing at room temperature may be a viable option for dynamic thrombogenicity assessment of biomaterials and medical devices.

Effects of Thermal Aging on Molar Mass of Ultra-High Molar Mass Polyethylene Fibers.

Ultra-high molar mass polyethylene (UHMMPE) is commonly used for ballistic-resistant body armor applications due to the superior strength of the fibers fabricated from this material combined with its low density. However, polymeric materials are susceptible to thermally induced degradation during storage and use, which can reduce the high strength of these fibers, and, thus, negatively impact their ballistic resistance. The objective of this work is to advance the field of lightweight and soft UHMMPE inserts used in various types of ballistic resistant-body armor via elucidating the mechanisms of chemical degradation and evaluating this chemical degradation, as well as the corresponding physical changes, of the UHMMPE fibers upon thermal aging. This is the first comprehensive study on thermally aged UHMMPE fibers that measures their decrease in the average molar mass via high-temperature size exclusion chromatography (HT-SEC) analysis. The decrease in the molar mass was further supported by the presence of carbon-centered free radicals in the polyethylene that was detected using electron paramagnetic resonance (EPR) spectroscopy. These carbon-centered radicals result from a cascade of thermo-oxidative reactions that ultimately induce C-C ruptures along the backbone of the polymer. Changes in the crystalline morphology of the UHMMPE fibers were also observed through wide-angle X-ray diffraction (WAXS), showing an increase in the amorphous regions, which promotes oxygen diffusion into the material, specifically through these areas. This increase in the amorphous fraction of the highly oriented polyethylene fibers has a synergistic effect with the thermo-oxidative degradation processes and contributes significantly to the decrease in their molar mass.

Effect of Elevated Temperature and Humidity on Fibers Based on 5-amino-2-(p-aminophenyl) benzimidazole (PBIA).

Soft body armor is typically comprised of materials such as aramid. Recently, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and PPTA were introduced to the body armor marketplace. The long-term stability of these copolymer fibers have not been the subject of much research, however they may be sensitive to hydrolysis due to elevated humidity because they are condensation polymers. Efforts to evaluate the impact of environmental conditions on fiber strength is very important for the adoption of these materials in armor systems. Three PBIA-based fibers were selected for the study, and were aged at 25 °C, 75 % RH; 43 °C, 41 % RH; 55 °C, 60 % RH; and 70 °C, 76 % RH for up to 524 d. Molecular spectroscopy, scanning electron microscopy, and single fiber tensile testing were performed to characterize changes in their chemical structure, tensile strength, and failure strain as a function of exposure time to different conditions. The fibers were all found to have some reduction in strength at high humidity conditions, with an approximately 14 % reduction for the copolymers and a 29 % reduction for the homopolymer. Molecular spectroscopy revealed some changes which suggest that hydrolysis of the benzimidazole ring is occurring at these elevated temperatures, possibly explaining the observed change in strength.