## 3 Results
### Effect of BSA Concentration  
In Figure 6, a typical experimental curve of the response of the system in free vibration with energy dissipation by friction in lubricated contact between a contact lens and glass for a BSA concentration of 2 mg/mL is presented. Additionally, the theoretical curve is represented; it was obtained via a numerical integration of the second-order differential equation (Equation (1)). In all cases studied, an almost perfect comparison of results was verified by comparing the two curves, with RMSE values lower than \(10^{-4}\). The value obtained for the coefficient of friction (COF) for an applied load of 100 mN was 0.09. Figure 7 shows a comparison of the response of the system in free vibration with energy dissipation by friction in lubricated contact between a contact lens and glass for a BSA concentration of 1 and 2 mg/mL. It is possible to observe that the system's response to a concentration of 2 mg/mL presents a much greater loss of amplitude; that is, the  
Figure 5: Sensitivity estimation of the (**a**) coefficient of Friction; (**b**) normal force. energy dissipation due to friction is faster, meaning that there is a greater amount of friction (COF\({}_{\rm 2 mg/mL}\) = 0.09 and COF\({}_{\rm 1 mg/mL}\) = 0.023). This allows us to conclude that the increase in BSA content from 1 to 2 mg of BSA/mL induced a rise in the coefficient of friction of around four times. Figure 6: Evolution of the displacement with time for experimental (orange line) and theoretical curves (blue line) for 2 mg/mL. BSA concentration and 100 mN applied normal load (test 2 of 3, COF = 0.09). Figure 7: Comparison of the evolution of the displacement with time for experimental curves with different BSA concentrations (1 and 2 mg/mL) and 100 mN applied Normal load (COF\({}_{\rm 2 mg/mL}\) = 0.09 and COF\({}_{\rm 1 mg/mL}\) = 0.023). In Figure 8, the results for the coefficient of friction were plotted as a function of the different BSA concentrations (0, 0.13, 0.25, 0.50, 1, and 2 mg/mL. For a small concentration of up to 0.25 mg/mL, the friction remains at low values. However, for concentrations of BSA over 0.5 mg/mL, the protein induced a significant increase in the coefficient of friction. For BSA concentrations between 0 and 0.5 mg/L, there was a minimum value for the coefficient of friction around 0.01. It is also noticeable that for BSA concentrations equal to or less than 0.5 mg/L, the respective standard deviations presented values significantly higher than those recorded for concentrations equal to or greater than 1 mg/L. This behaviour can be justified, since for BSA concentrations lower than 0.5 mg/L, the protein chains can contribute to an increase in the viscosity of the fluid, thus contributing to a separation of the tribopair and, consequently, lower friction. For concentrations greater than 0.5 mg/L, excess protein can give the tribopair a certain rigidity and consequently an increase in friction. This behaviour somewhat mimics the real feelings of a contact lens wearer. Most of the solutions used to preserve and hydrate contact lenses at night also have the function of deproteinizing and breaking down the long protein chains that accumulate on the surface of the lenses during the day and create a feeling of discomfort. In this regard, several authors [16; 17; 18; 19; 20] have studied the compatibility between the material of different contact lenses and the surrounding ocular tissues and fluids, as well as the degradation of contact lenses with the accumulation of secretions and the subsequent damage to the corneal epithelial tissue. [PERSON] et al. [21] observed that proteins had a definitive impact on the friction and wear process. Proteins adsorbed on the UHMWPE surface and increased friction and wear by changing the predominant friction and wear mechanism from abrasive to adhesive. Phospholipids also increased friction, while hyaluronic acid decreased friction by increasing the viscosity of the lubricants.