Publication
Estudo do Atrito em Lentes de Contacto Gelatinosas
| Summary: | Understanding the biotribology involved between contact lenses and the eye, namely the friction between the eyelid and the surface of the contact lens during the blink movement, is a crucial factor in avoiding irritation, discomfort and other eye infections.The aim of the present dissertation is to investigate the tribological behavior of soft contact lenses under different contact conditions. Since there is no commercially available equipment, a tribometer has been developed for this purpose, which operates as a pendulum with vibration horizontal movement and can be considered a vibratory mass - spring - damper mechanical system. The friction assessment method is based on the evaluation of the free damping vibration movement, of the mass - spring system, induced by a mechanical impulse. The contact with the lenses acts as a supplementary dissipation of energy and the friction is determined by inverse analysis, compared to the analytical equation of movement.Thus, the main objectives of this dissertation were: to verify the principles and applicability of the experimental method in contact lenses under different contact conditions, to validate the results and to study the effect of aging of contact lenses in the friction.Four different commercial contact lenses were tested under lubrificated conditions, being the contact lubrificated with a buffered saline solution, the applied normal load was varied between 21 and 62 mN and the sliding velocities between 100 and 200 mm/s. The friction model, adjusted from the experimental results, includes a Coulomb part more a viscous component.In all cases, a good linearity of the results of the friction force was verified as a function of the normal force, so the first friction law can be applied to determine the value of the coefficient of friction. For the new contact lenses, coefficient of friction values were obtained between 0,0051 and 0,0275, the hydrogel lens presented the highest value in comparison with silicone-hydrogel and hypergel lens.The aging procedure of contact lenses was simulated by a cycling process considering that the main parameter influencing the material surface is the transition between the closed and open eye and the exposure to environmental aggressions, particularly ultraviolet radiation. The values of the coefficient of friction and modulus of elasticity before and after the aging process were compared and was verified the increase of both parameters for all contact lenses. The hydrogel lens was the least affected by aging and the silicone - hydrogel lens based on delefilcon A was the one that showed the least stability of properties.The experimental method was validated by comparing the results with obtained in an alternative linear motion tribometer. There was an equal behavior in hierarchical terms of the coefficient of friction. With this, it was concluded that the experimental method and the inverse analysis technique is an efficient and reliable method in characterizing the friction of materials such as contact lenses. |
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| Main Authors: | Carvalho, Ana Luísa da Silva |
| Subject: | Biotribologia Lentes de contacto gelatinosas Atrito Atrito por vibração Biotribology Soft contact lenses Friction Vibration friction |
| Year: | 2019 |
| Country: | Portugal |
| Document type: | master thesis |
| Access type: | metadata only access |
| Associated institution: | Universidade de Coimbra |
| Language: | Portuguese |
| Origin: | Estudo Geral - Universidade de Coimbra |
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