Influence of specimen thickness on the nanoindentation of hydrogels: measuring the mechanical properties of soft contact lenses.
Influence of specimen thickness on the nanoindentation of hydrogels: measuring the mechanical properties of soft contact lenses.
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DOI：
10.1016/j.jmbbm.2013.11.023
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发表时间：
2014-07
影响因子：
3.9
通讯作者：
A. Selby;C. Maldonado-Codina;B. Derby
中科院分区：
文献类型：
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作者：
A. Selby;C. Maldonado-Codina;B. Derby
纳米压痕为薄水凝胶样品（如隐形眼镜）的测试提供了一种方便的方法，以直接评估其机械性能。在这里，我们研究了一系列均匀厚度值的聚甲基丙烯酸羟乙酯（pHEMA）试样的力学性能，并表明，与50和100 μm半径的球形压头，表观弹性模量显着增加时，试样厚度小于500 μm，压痕深度<1 μm。这是公知的压痕厚度效应的表现，但发生在比其他材料更大的临界厚度处。一个简单的经验关系确定的表观弹性模量与归一化厚度的变化。从pHEMA样本获得的经验厚度校正函数随后用于校正由许多不同制造商提供的由pHEMA和硅酮聚合物制成的接触镜范围内的厚度变化，具有一系列光学强度和厚度分布。对于所有四种接触透镜类型，可以看出校正函数补偿了表观弹性模量随透镜厚度的变化，而与透镜材料无关。将测得的接触透镜材料的杨氏模量（经厚度校正）与接触透镜制造商所引用的通过常规体机械测试获得的杨氏模量进行比较，以发现良好的一致性。
Nanoindentation offers a convenient method for the testing of thin hydrogel specimens, such as contact lenses, to directly assess their mechanical properties. Here we investigate the mechanical properties of poly(hydroxyethyl methacrylate) (pHEMA) specimens of a range of uniform thickness values and demonstrate that, with 50 and 100 μm radius spherical indenters, a significant increase in apparent elastic modulus is seen when the specimen thickness is smaller than 500 μm at indentation depths <1 μm. This is a manifestation of the well known indentation thickness effect but occurring at larger critical thicknesses than seen with other materials. A simple empirical relation is determined for the variation in apparent elastic modulus with normalised thickness. The empirical thickness correction function obtained from pHEMA specimens was subsequently used to correct for the thickness variation within a range of contact lenses supplied by a number of different manufacturers fabricated from both pHEMA and silicone polymers, with a range of optical strengths and hence thickness profiles. The correction function is seen to compensate for the variation in apparent elastic modulus with lens thickness for all four contact lens types, irrespective of lens material. The measured Young's modulus of the contact lens material, corrected for thickness, was compared with that quoted by the manufacturers of the contact lenses, obtained by conventional bulk mechanical testing, to find good agreement.