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indentation - PROOF COPY[BIO-05-1396 004702JBY O PR A...

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PROOF COPY [BIO-05-1396] 004702JBY PROOF COPY [BIO-05-1396] 004702JBY Xiaoming Chen Alison C. Dunn W. Gregory Sawyer Malisa Sarntinoranont e-mail: [email protected] Department of Mechanical and Aerospace Engineering, University of Florida, 5 Gainesville, FL 32611 A Biphasic Model for Micro-Indentation of a Hydrogel-Based Contact Lens The stiffness and hydraulic permeability of soft contact lenses may influence its clinical performance, e.g., on-eye movement, fitting, and wettability, and may be related to the occurrence of complications; e.g., lesions. It is therefore important to determine these properties in the design of comfortable contact lenses. Micro-indentation provides a nondestructive means of measuring mechanical properties of soft, hydrated contact lenses. However, certain geometrical and material considerations must be taken into account when analyzing output force-displacement F - D data. Rather than solely having a solid response, mechanical behavior of hydrogel contact lenses can be described as the coupled interaction between fluid transport through pores and solid matrix deformation. In addition, indentation of thin membranes 100 m requires special consideration of boundary conditions at lens surfaces and at the indenter contact region. In this study, a biphasic finite element model was developed to simulate the micro-indentation of a hy- drogel contact lens. The model accounts for a curved, thin hydrogel membrane supported on an impermeable mold. A time-varying boundary condition was implemented to model the contact interface between the impermeable spherical indenter and the lens. Paramet- ric studies varying the indentation velocities and hydraulic permeability show F - D curves have a sensitive region outside of which the force response reaches asymptotic limits governed by either the solid matrix (slow indentation velocity, large permeability) or the fluid transport (high indentation velocity, low permeability). Using these results, biphasic properties (Young’s modulus and hydraulic permeability) were estimated by fitting model results to F - D curves obtained at multiple indentation velocities (1.2 and 20 m/s ). Fitting to micro-indentation tests of Etafilcon A resulted in an estimated per- meability range of 1.0 10 -15 to 5.0 10 -15 m 4 /N s and Young’s modulus range of 130 to 170 kPa . DOI: 10.1115/1.2472373 Keywords: hydraulic conductivity, pHEMA-MAA, poroelastic, porous media, exudation, FEM, computational model, biphasic, microindentation Introduction Soft contact lenses are composed of polymeric hydrogels. The porous structure of these hydrogels allows for transport of oxygen, water, and other nutrients through the lens to the cornea of the eye. Numerous contact lens studies have focused on oxygen per- meation through the lens 1,2 . However, the flow of water through the lens, governed by the hydraulic permeability property, may also contribute to important lens functions including on-eye lens movement, comfort, and wettability 2–5 . In addition to fluid considerations, there is a direct correlation between the ocular comfort of contact lenses and the bulk stiffness of contact lenses.
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