Comparative study of the hydration of hydrophilic contact lenses by refractive index and gravimetry

To compare the percent hydration by wet weight of hydrogel contact lenses given by the manufacturers with the values obtained using the refractive index and the gravimetry methods.

We used a Valentine refractometer to determine the refractive index of new hydrogel contact lenses at equilibrium swelling in the original solution and container. From the refractive indexes we then obtained the percent hydration of the lenses using a previously prepared graph of refractive index vs percent hydration of saccharose in a phosphate buffer solution (pH 7.0). We used an electronic analytical balance of 0.01 mg resolution to obtain the weights of new lenses when dry and at equilibrium swelling dry, and calculated the percent hydration of the lenses from the difference of the wet and dry weights.

The hydration values of the contact lenses obtained from the index of refraction differed more than those obtained by gravimetry from the values given by the manufacturers. Acuvue lenses were an exception-the difference for them was negligible.

Gravimetry is more accurate, but more cumbersome and time-consuming than the index of refraction for the determination of the hydration of the hydrogel contact lenses. However, when a fast and easy method is required to obtain the approximate hydration of hydrogel contact lenses, the use of the refractive index method can give acceptable results for most applications.