Characterizing the free volume of ultrahigh molecular weight polyethylene to predict diffusion coefficients in orthopedic liners

Abstract

Liners used in orthopedic devices are often made from ultrahigh molecular weight polyethylene (UHMWPE). A general predictive capability for transport coefficients of small molecules in UHMWPE does not exist, making it difficult to assess properties associated with leaching or uptake of small molecules. To address this gap, we describe here how a form of the Vrentas-Duda free volume model can be used to predict upper-bound diffusion coefficients (D) of arbitrary molecules within UHMWPE on the basis of their size and shape. Within this framework, the free-volume microstructure of UHMWPE is defined by analysis of a curated set of model diffusants. We determined an upper limit on D for vitamin E, a common antioxidant added to UHMWPE, to be 7.1 × 10 cm s . This means that a liner that contains 0.1 wt % or less Vitamin E and has <120 cm patient contacting surface area would elute <100 µg/day of vitamin E. Additionally, the model predicts that squalene and cholesterol-two pro-oxidizing biological compounds-do not penetrate over 820 µm into UHMWPE liners over the course of 5 years because their D is ≤7.1 × 10 cm s . © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2393-2402, 2018.

Relations Between Dynamic Localization and Solute Diffusion in Polymers

“…Given the expected relation between α and free-volume geometry, it would be reasonable to suppose that α would correlate with a measure of molecular shape, such as the aspect ratio. Prior work has shown a relation between molecular anisotropy and diffusivity. , However, we were unable to identify a relation between the best-fit α values and molecular descriptors, shape-based or otherwise.…”

“…Prior work has shown a relation between molecular anisotropy and diffusivity. 75,101 However, we were unable to identify a relation between the best-fit α values and molecular descriptors, shape-based or otherwise.…”

“…Nonetheless, it does serve as an upper bound for all these systems across all temperatures. In the context of medical devices, such an upper bound is potentially useful for protective estimates of leaching because it provides a worst-case estimate of patient exposure. , Thus, for any novel solute/polymer system, a series of short MD simulations can be used to establish D 0 and u 0 , which then provide a protective estimate of D .…”