Temperature-dependent water transport in a thermoplastic polyurethane membrane for ophthalmic applications

Corneal transparency depends on tightly regulated stromal hydration, which is maintained by the posterior cornea through coupled solute and fluid transport. In endothelial dysfunction, impaired regulation leads to edema and optical failure, motivating engineered posterior corneal substitutes that can provide controlled hydraulic resistance while remaining compatible with physiological exchange. This thesis investigates temperature-dependent solute transport through a solvent-cast thermoplastic polyurethane (TPU) thin-film membrane as a candidate platform for ophthalmic use. Temperature dependent permeability and effective diffusivity were quantified using a two- compartment diffusion cell method with periodic receiver-side sampling and UV-Vis analysis of Rhodamine B as a model tracer solute, enabling extraction of steady state flux and transport coefficients within a solution-diffusion framework. Transport was evaluated across sub physiological to mildly elevated conditions to resolve both clinically relevant temperatures and mechanistic transitions. The results reveal a pronounced two-regime behavior: a low-temperature regime (≈20-30 °C) characterized by weak temperature dependence and low flux, and a higher-temperature regime (≈32-42 °C) in which permeability and effective diffusivity increase sharply, with a clear onset near ~30-32 °C. Gravimetric water uptake and thickness measurements showed negligible swelling across the relevant temperature range, indicating that the enhanced tracer transport was not driven by macroscopic hydration or geometric expansion. Arrhenius analysis supported distinct apparent activation energies between the two regimes and showed that any extrapolated intersection temperature should not be interpreted as a thermodynamic phase transition; instead, the experimentally observed onset near 30-32 °C is the more practical transition marker for design. Overall, the TPU membrane demonstrates switchable, thermally activated tracer transport governed primarily by segmental mobility, supporting its potential as a temperature-responsive posterior corneal barrier concept.