ABSTRACT:In this study, electron spin resonance (ESR) spectroscopy was used for the first time to investigate liquid diffusion into contact lenses. As contact lenses are not paramagnetic substances, they were labeled with nitroxide spin probes to get an ESR spectrum. Thus, it gives a solid spinlabeled ESR spectrum. The shape and intensity of the ESR signals depend on the environment of these spin probes. The spin probe environment began to change from solid to liquid if liquid were dropped into the system. Consequently the ESR spectra began to change with time, too. By following these changes, three distinct steps were found. Their diffusion coefficients were determined to be 6.38 ϫ 10 -8 cm 2 /s for the first step (rapid decay region) and 0.37 ϫ 10 -8 cm 2 /s for the second step (slow decay region), and 2.50 ϫ 10 -8 cm 2 /s for the third and last step (desorption region).

ABSTRACT:: Oxygen permeability is the most important parameter of contact lenses, as lack of oxygen causes corneal edema and threatens the vision of the patient. This study was unique in that it used an electron spin resonance (ESR) technique to determine the oxygen diffusion coefficient (D) of contact lenses. Although there are many methods and techniques for investigating oxygen diffusion into contact lenses, ESR was used for the first time in this study. The ESR technique is based on the scavenging of radicals produced in lenses by oxygen. As a contact lens is not a paramagnetic substance, it cannot give an ESR spectrum. But it does produce an ESR spectrum after ␥ irradiation. When a vacuumirradiated contact lens is exposed to air, the radicals trapped in the lens are transformed into peroxide radicals by the addition of molecular oxygen to the free radicals, and the ESR spectrum begins to change with time. This effect can be used as a tool to measure oxygen uptake in irradiated contact lenses. The oxygen diffusion coefficient of a contact lens was determined from changes in ESR signal intensity varying with time. The diffusion coefficients of oxygen for a contact lens were determined for rapid decay [(1.5 ϩ 0.4) ϫ 10 Ϫ8 cm 2 /s] and slow decay [(1.3 ϩ 0.3) ϫ 10 Ϫ9 cm 2 /s] in this study. These values are in agreement with the D values given in the literature for polymeric materials used for contact lenses.