Superficial Modulus, Water-Content, and Mesh-Size at Hydrogel Surfaces

The most distal surfaces of lubricious high water-content aqueous gels may have decreasing concentrations and gradients of macromolecular chains on the surface that emanate outward into the environment. This superficial zone of extended polymer chains has a water-content that approaches 100% over the final few hundred nanometers, and the superficial modulus is the elastic modulus of this superficial surface. Micro-rheology using high-speed microscopy with fluorescent nanospheres enabled measurements of both the storage modulus G ′ and the loss modulus G ″ over a frequency range of 0.4 1/s–50 1/s. 100 µm-thick control samples of polyacrylamide gels with equilibrium water-content of 97.9%, 98.4%, and 98.9% had measured storage moduli of 70.3 ± 26.9 Pa, 53.3 ± 22.4 Pa, and 38.8 ±14 Pa, respectively. Master curves based on meta-data analysis from published measurements of mesh-size, water-content, and elastic modulus were created and used to relate rheological measurements of the superficial modulus to water-content and mesh-size. This interfacial microrheological measurement method was further used to quantify the superficial modulus and water-content of two commercial contact lens materials that have water-gradient gels on their surfaces: delefilcon A ( E = 48 ± 11 Pa, > 99.6% water) and lehfilcon A ( E = 10 ± 8 Pa, > 99.7% water). The micro-rheology method on Gemini gel interfaces facilitates measurements of the most distal superficial zone of aqueous gels and the quantification of the superficial modulus and water-content of these surfaces. Graphic Abstract