Quality Control of Silicone Hydrogel Contact Lenses by Impedance Spectroscopy
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Quality Control of Silicone Hydrogel Contact Lenses by Impedance Spectroscopy M. Torres1,2 and K.S.V. Santhanam2 1. Bausch & Lomb, Rochester, NY 14609, 2 Center For Materials Science and Engineering and School of Chemistry and Materials Science, Rochester Institute of Technology, Rochester, NY 14623 ABSTRACT We wish to propose an impedance spectroscopic method for the quality control of contact lenses by measuring the pore resistance. Silicone hydrogels are excellent materials for use as contact lenses and their on eye performance is dependent on salt intrusion characteristics which are related to the pore resistance and water uptake. When the contact lenses are placed on the eye, they are expected to permeate ions and molecules to maintain ocular health. The hydrogel pores control the permeability and can be viewed as a quality control parameter. Two models are considered here: in one, the contact lenses are considered as strong rigid films with no permeability. In another, the hydrogels are having ionic permeability. We designed a silicone hydrogel contact lens attachment holder that is amenable for electrochemical impedance measurements. The electrochemical impedance measurements were carried out in an inert medium of 0.1 M Na2SO4. The impedance measurement experimental parameters used were a) AC potential 10 mV rms b) frequency range 0.1-210 kHz and c) open circuit potential of 0.207 V. The impedance variation with frequency was constructed for a number of hydrogels. The ideally acceptable silicone hydrogel contact lenses showed an impedance change with frequency in a sigmoidal fashion with a characteristic phase angle (acceptable in the range of 70-75o). The hydrogel pore resistances for the acceptable contact lenses are in the range of 4.5-11 kΩ. When the impedance showed a linear decrease or no well defined phase angle, the contact lens is considered acting as an insulator-a test for rejection. A test of the model was done with several acceptable contact lenses in the market. This study revealed interesting aspects of the influence of pulsating electric field on the silicone hydrogels. INTRODUCTION The family of siloxane based hydrogel materials has provided impactful developments in the advancement of contact lenses [1-12]. This class of hydrogels has oxygen transmissibility, along with mechanical and surface properties that provide mechanisms for ion permeation that provide ocular hygiene and comfort. Current quality control features for contact lenses such as lens diameter, front sagittal height, power and image quality, center thickness, surface and edge quality, and edge design are defined in American National Standards Institute (ANSI)[10]. They are generally biphasic materials having both silicone and hydrogel phases (13,14). These interdispersed phases provide high ion permeabilities that are essential for maintaining a high salt concentration in the tear film. A high salt concentration is maintained at a physiological value between the lens and the cornea to prevent thinning of the lens and a hydroph
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