Nanotribological testing of contact lenses
Contact lenses are widely adopted for correcting vision problems and are used by many people. Although they have been on the market for more than fifty years, some people still feel discomfort when wearing contact lenses. This is often related to friction between the contact lens and the eyelid. The investigation of tribological properties of contact lenses is therefore important for their development. The Anton Paar Nano Tribometer is one of the few devices that can test the frictional properties of contact lenses in realistic conditions. This application report demonstrates the methodology of testing of contact lenses and the details of this method.
Introduction The primary materials for soft contact lenses are hydrogels, which are cross-linked hydrophilic polymers filled with 30-70% water . Although such soft materials are well suited for the eye, a significant fraction of people are unable to wear contact lenses because of extensive eye irritation. There is an existing clinical hypothesis that ocular comfort is related to friction .
The tribological testing of contact lens materials is usually performed at very low contact pressures in the range of 1-7 kPa in order to simulate the real contact pressure during eyelid blinking. While maintaining such low pressure is feasible, to achieve the speed of eyelid blink of ~12 cm/s presents a serious experimental challenge [3,4]. Tribological experiments are therefore often done at much lower sliding speeds but in similar lubrication regime. The above mentioned pressure and sliding speed values correspond to the system cornea-eyelid without contact lens. Higher contact pressures (12–18 kPa) were found in Ref.  for the system comprised of cornea-lens-eyelid. In order to test these soft materials at such low contact pressures, a very sensitive instrument are required. The Anton Paar Nano Tribometer NTR³ is one of them since it allows for loads from sub-milinewton range to be applied. Furthermore, the contact geometry between the lens and the static partner as well as the normal load control must be as stable as possible in order to avoid fluctuations of the pressure and allow reliable evaluation of the coefficient of friction (CoF). The contact conditions similar to the real contact geometry and motion between the eyelid and the contact lens can be achieved by using the disc-onlens setup using a linear reciprocating movement under controlled normal load.
This application report presents the methodology for testing of frictional properties of contact lenses using the Nano Tribometer and a glass disc. The results are demonstrated on three types of commercially available contact lenses which were tested in three different liquids
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