## 1 Introduction  
The eye is a lubricated, moving system in the human body, in which the tear film, among other functions, provides protection to the cornea and maintains optical smoothness of vision. The process of blinking, in which the upper eyelid slides over the eyeball until it meets the lower eyelid and then returns upwards, over a total duration of 1 to 3 s, allows the tear film to be renewed and occurs, on average, 12 times per minute [1, 2]. From a lubrication point of view, these eye movements are considered to occur under a mainly hydrodynamic lubrication regime. Thus, during most of an intermittent cycle, slip resistance is governed by the viscous shear of the lubricant [3]. The correlation between comfort and friction, and the possibility of predicting the in vivo performance of contact lenses (CLs), based on in vitro experiments, motivated researchers to carry out some studies on the tribological characterization of CLs using commercially available or adapted tribometers. [PERSON] et al. (2011) tested senoflicon A-based CLs submerged in saline solution against a stainless-steel sphere (senoflicon A is a new-generation silicone hydrogel material used to manufacture contact lenses and it is FDA-approved). For loads between 0.5 and 100 mN and sliding speeds between 0.0 and 0.5 cm/s, coefficient of friction (CoF) values were obtained between 0.001 and 0.11. Under these conditions, the first law of friction was verified; that is, there was a proportionality between the friction force and the applied load [4]. Additionally, these authors investigated the dependence of friction on sliding speed and, after adjusting the power law, concluded that the CoF is proportional to V\({}^{0.23}\). The result suggested that there is a strong attraction between senoflicon A and stainless steel, which is attributed to the low water content of CLs and the larger areas resulting from solid-solid contact [4]. Regarding the lubricating solutions and artificial tears available on the market for the contact lens to remain moist and thus reduce the symptoms of eye irritation and dryness [5], [PERSON] and [PERSON] (1995) measured the coefficient of friction of polymacon-based CLs with various ophthalmic solutions. They used a polycarbonate disc as an antagonist material, under contact pressures of 3.5 kPa and obtained a CoF value of 0.640 without lubricant [6], whereas under lubrication, the higher the viscosity of the lubricant, the lower the friction coefficient; that is, the greater the lubrication. This lubrication of the CLs was observed in the mixed lubrication regime; that is, when there was contact between the sliding surfaces and, therefore, the friction coefficient was influenced by both the lubricating properties and the surface properties of the CLs [6]. [PERSON] et al. (2016) investigated how the CoF value of HSCL materials, acquired at low sliding speeds, is affected by the buffer component and organic composition of the lubricant, and by prolonged exposure to teardrop fluid (FTL). The CoF value of several commercially available CLs (etafilcon A, nelficon A and senofilcon A) was characterized using microtribometry against a mucin-coated glass disk, tested under different lubricating solutions, including an FTL containing proteins and lipids [3] To determine friction, normal loads between 0.25 and 4 mN were chosen, corresponding to a contact pressure range of 1 to 7 kPa, and a sliding speed of 0.1 mm/s. They obtained values of CoF between 0.01 and 0.1 [3]. [PERSON] et al. (2015) investigated the effect of the presence of albumin and cholesterol in the lubricating medium on the friction response of two hydrogels used in CL, a hydrogel based on hydroxyethyl methacrylate (HEMA) (HEMA/PVP), and another based on silicone (TRIS/NVP/HEMA). Tribological tests were carried out using a PMMA sphere as a counter-body and water as a lubricant, in addition to solutions with the biomolecules under study. Reciprocal movement tests were carried out with normal forces of 20 mN and sliding speeds of 7 mm/s.