Surface topography of submerged soft contact lenses used in this study. a) Lotrafilcon B, b) Balafilcon A, c) Senofilcon A, d) Comfilcon A.
Surface topography of submerged soft contact lenses used in this study. a) Lotrafilcon B, b) Balafilcon A, c) Senofilcon A, d) Comfilcon A.

A team from Georgia Tech have measured the mechanical properties of soft contact lenses under practical conditions using an atomic force microscope (AFM).

With more than 30 million contact lens wearers living in the US, making lenses more comfortable is a growing research issue, and defining their properties is key to understanding their performance. Researchers from Georgia Institute of Technology have measured the complex mechanical properties of commercial soft contact lenses, and found that conventional measurement techniques are no longer fit for purpose.

Contact lenses have to fulfil a number of contradictory functions while remaining optically clear – they need to be flexible enough to make them comfortable, but must also maintain their shape in saline conditions. Current mechanical characterisation of lens materials is based solely on tensile tests, which measure only the averaged elastic modulus of the entire lens. With coatings and wetting agents widely used in the latest multiphase lenses, it is becoming increasingly important to measure the local mechanical properties of these materials.

Led by Vladimir V. Tsukruk, Georgia Tech engineers turned to AFM-based surface force spectroscopy (SFS) to characterise the micromechanical properties of commercial contact lenses at the nanoscale. This technique has been used to study surface topography, friction, and protein absorption in contact lens materials and in eye tissue, but the paper from Tsukruk (Polymer 55 (2014) 6091–6101 [DOI: 10.1016/j.polymer.2014.09.053]) is the first to probe the surface mechanical properties in wet conditions. The team’s technique combined two AFM modes - high frequency (tapping mode) measurements, which provide high resolution maps of topography and mechanical properties, and static (force volume) nanoindentation, which utilises tip sample interactions to accurately calculate mechanical properties.

Small pieces of four commercial lenses were submerged in their original saline solution and probed with sharp (10-30nm) aluminium-coated AFM tips, which had been previously characterised. The surface topography of the outer (convex) surface of the contact lens was measured, alongside indentation mapping experiments that characterised both the coating, a soft thin film, and the supporting stiffer lens substrate at nanoscale resolution. The researchers also looked at the lenses in cross-section and in all cases, found a complex, non-uniform sub-surface structure.

The multiphase nature of today’s soft silicone hydrogel contact lenses means that old measurement techniques are not sufficient. In this paper, Tsukruk’s team have proposed a new experimental protocol, based on AFM characterisation, for these materials.

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