The aging process of hydrogel contact lenses studied by 1H NMR and DSC methods

• The aging process of the lens was examined.
• Molecular dynamics of the commercial monthly hydrogel contact lenses was studied.
• Thermal properties of hydrogel contact lenses were characterized.
• A model-free approach with Lipari–Szabo spectral density function was used.

This paper presents the comparative study of molecular dynamics and of the thermal properties of the virgin commercial monthly hydrogel contact lenses Bausch & Lomb Soflens 59 and the lenses worn for 1, 2, 3 and 4 weeks. The lenses were made from the hilafilcon B material, which was a hydrophilic copolymer of poly(2-hydroxyethyl methacrylate) pHEMA and poly(N-vinyl pyrrolidone) PVP, containing 59 wt.% of water. The 1H NMR on-resonance and off-resonance techniques have been carried out to study the molecular dynamics of the water contained in soft contact lenses. The measurements were carried out using a homemade pulse spectrometer operating at the frequency of 30.2 MHz at temperature 303 K. On the basis of the dispersion of the spin–lattice relaxation rates R1ρ = 1/T1ρ in the rotating frame the correlation times τc of the water molecules were estimated. A new method for measuring the motion parameters, based on the value of the equilibrium magnetization in the effective field Be was described. To analyze NMR off-resonance results a model-free approach with Lipari–Szabo spectral density function was used to determine the order parameter S2 and the effective correlation time τe. The thermal properties of hydrogel contact lenses have been characterized using differential scanning calorimetry DSC. It has been shown that the period of wearing contact lenses cannot be extended beyond the time specified by the manufacturer.

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