| The mechanical and adhesion properties of hydrogel contact lenses play a role in a variety of manufacturing processes, lens handling, user comfort, and lens optical performance. Additionally, the hydrogel mechanical properties, adhesion properties, or both are hypothesized to be factors in a variety of lens-related ocular complications. Thus, there is a current need for accurate and reliable mechanical and adhesion characterization techniques for these biomedical devices. This thesis establishes novel experimental protocols and methods to accurately characterize hydrogel contact lens adhesion and mechanical properties in a variety of contexts and length scales. Throughout all aims/experiments, two commercially available hydrogel lenses are characterized for comparison, Narafilcon A/Acuvue TruEye and Etafilcon A/Acuvue2.;The first aim of this thesis focuses on macroscopic hydrogel lens adhesion characterization achieved via the Planar Adhesion Test (PAT). The PAT is a novel experimental technique that utilizes the lens native geometry to characterize both mechanical and adhesion properties. The lens is compressed and subsequently removed from a planar substrate, during which the applied load P, the maximum tensile force or "pull-off" force P*, approach distance w0, and contact radius a are measured. A previously established modified Johnson-Kendall-Roberts (JKR) shell model is employed to calculate the adhesion energy, gamma, between the hydrogel and the substrate, and it is found that gamma = 80 +/- 4.6 mJ/m2 for Acuvue2 lenses and gamma = 95 +/- 6.1 mJ/m2 for TruEye lenses. The effect of lens geometry, or optical diopter d, is explored on both the lens mechanical and adhesion behavior.;The second aim of this thesis focuses on characterizing hydrogel lens adhesion against more physiologically relevant samples; donated human corneas. Similar to the PAT, the lens is brought in contact with the cornea and retracted away, recording the "pull-off" force P* for each hydrogel material. The "pull-off" force is directly related to the hydrogel-corneal surface adhesion properties, and is found that P* = 2.40 +/- 0.20 mN for Acuvue2 lenses and P* = 2.78 +/- 0.19 mN for TruEye lenses on human corneas.;The third and final aim of this thesis focuses on characterizing single cell adhesion behavior against the two hydrogel materials via Single Cell Force Spectroscopy (SCFS). Individual Human Corneal Epithelial (HCE) cells are immobilized on a specially functionalized Atomic Force Microscope (AFM) cantilever to form a single cell probe. The cell is brought into contact with the hydrogel surface, and after a specified time Deltat, retracted away from the hydrogel. Individual "pull-off" forces P* are recorded as a function of time in contact to measure the bond-strengthening time, tau, and together P* and tau characterize the adhesion properties of these hydrogels at the cellular level. It is found that the P* forces for HCE cells are significantly larger for Narafilcon A than the P* forces for Etafilcon A for all hold times Deltat ≥ 5s, and that the bond-strengthening time for Etafilcon A is slightly less than Narafilcon A.;Additionally, this work contains an investigation into the mechanochemistry of native type I collagen via small angle light scattering (SALS), which is reviewed as an addendum at the end of this thesis. |
