Void volume variations in contact lens polymers

In this study, void size and free volume properties in different contact lens materials have been investigated in their hydrated state using positron annihilation lifetime spectroscopy (PALS).

PALS is used to characterize the void size distributions inside the lens materials. Three different types of contact lenses were used (Balafilcon A, Hilafilcon B and Polymacon).

Measurements on different contact lenses reveal significant differences between the materials, up to ∼ 100% difference in void volume was observed between Hilafilcon B and Balafilcon A, the latter having larger voids. As oxygen diffusion is strongly correlated with the void sizes, the results are in good agreement with the usage recommendations of the specific lens types (daily disposable lenses or 1 month continuous use lenses). The void sizes in monthly lenses (Balafilcon A) were found to decrease 25% under artificial aqueous tear (albumin-water solution) exposure in 4 weeks leading to a significant decrease in the oxygen permeation rate through the contact lens. Yet, the voids were still significantly larger than in disposable or semi-disposable lenses.

We have showed that PALS is a viable method to probe the microstructure of biotechnologically relevant polymers and can be used to quantify the void properties in different types of contact lenses. Usage recommendations correlate well with measured void sizes and the median void size decreases during the incubation of albumin solution as a function of time. We anticipate the use of PALS for any polymer-based intracorneal/intraocular device in which diffusivity plays a crucial role.