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Kyle Schulze, W Gregory Sawyer, Thomas Ettore Angelini; Hydrogel Surface Modulus Measurements Using Buoyancy Force Microscopy (BFM). Invest. Ophthalmol. Vis. Sci. 2014;55(13):4639.
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© ARVO (1962-2015); The Authors (2016-present)
Measurements of elastic modulus at the contact lens surface have been achieved recently through the use of colloidal probe atomic force microscopy (AFM).Understanding of the relationship between lens comfort and the stiffness of the outermost surface of lenses is only now emerging. The high cost of AFM instrumentation and the expertise required to make modulus measurements prohibits broad surface modulus characterization.Here we describe a low-cost and easily executed method for characterizing surface modulus that requires only an inverted fluorescence microscope and polymer beads.
A 100 um thick sheet, made from 1.5% Agar, is cast onto a microscope coverslip and mounted onto the microscope. A drop of water containing 200nm diameter fluorescent polystyrene spheres and 100-150 um diameter polymethylmethacrylate (PMMA) beads is pipetted onto the gel.The PMMA spheres are solvated with a coating of pluronic eliminating interfacial tension between the particle and the hydrogel. The gel surface stiffness is determined by computing the dead-weight normal forces applied by the spheres on the gel. Indentation is inferred by measuring the contact area between the bead and the gel, and by measuring the diameter of the bead.Area is measured by imaging the fluorescent spheres over time, and projecting the time-lapse into one image.The area over which the PMMA sphere excludes the fluorospheres is the contact area.
We infer geometrically the indentation depth from the contact area.Many measurements of indentation depth for beads of varying diameter allow the scaling between force and depth, with which we developed a linear contact mechanics model, similar to Hertz indentation.We find that the elastic modulus of the hydrogel is 17.3 +-5.6 kPa.This modulus agrees very well with the Young’s modulus of bulk Agar, 22.5 +-1 kPa, which we measure on macroscopic samples in a rheometer.
The forces applied in the experiments described span a range of 1 nN to 2 nN, the range usually applied using colloidal probe AFM.This range of forces can be dramatically broadened with the method presented here. By using a wider range of bead sizes, from 55 um to 1mm diameter, forces from 0.1 nN to 50 μN can be applied, spanning 5 orders of magnitude. Materials with the corresponding range of stiffness can therefore be characterized using Buoyancy Force Microscopy (BFM) such as all commercially available contact lenses.
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