KTH Royal Institute of Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE 10044 Stockholm, Sweden.
RISE Research Institutes of Sweden, Division of Bioscience and Materials, Box 5607, SE 114 86 Stockholm, Sweden.
Soft Matter. 2019 Oct 14;15(38):7704-7714. doi: 10.1039/c9sm01113g. Epub 2019 Sep 11.
The mechanical properties of hydrogels are of importance in many applications, including scaffolds and drug delivery vehicles where the release of drugs is controlled by water transport. While the macroscopic mechanical properties of hydrogels have been reported frequently, there are less studies devoted to the equally important nanomechanical response to local load and shear. Scanning probe methods offer the possibility to gain insight on surface nanomechanical properties with high spatial resolution, and thereby provide fundamental insights on local material property variations. In this work, we investigate the local response to load and shear of poly(2-hydroxyethyl methacrylate) hydrogels with two different cross-linking densities submerged in aqueous solution. The response of the hydrogels to purely normal loads, as well as the combined action of load and shear, was found to be complex due to viscoelastic effects. Our results show that the surface stiffness of the hydrogel samples increased with increasing load, while the tip-hydrogel adhesion was strongly affected by the load only when the cross-linking density was low. The combined action of load and shear results in the formation of a temporary sub-micrometer hill in front of the laterally moving tip. As the tip pushes against such hills, a pronounced stick-slip effect is observed for the hydrogel with low cross-linking density. No plastic deformation or permanent wear scar was found under our experimental conditions.
水凝胶的力学性能在许多应用中都很重要，包括支架和药物输送载体，药物的释放可以通过水传输来控制。虽然水凝胶的宏观力学性能经常被报道，但对于同样重要的局部负载和剪切的纳米力学响应的研究较少。扫描探针方法提供了以高空间分辨率获得表面纳米力学特性的可能性，从而为局部材料性能变化提供了基本的见解。在这项工作中，我们研究了两种不同交联密度的聚（2-羟乙基甲基丙烯酸酯）水凝胶在水溶液中的局部负载和剪切响应。由于粘弹性效应，水凝胶对纯法向载荷以及载荷和剪切的组合作用的响应非常复杂。我们的结果表明，水凝胶样品的表面硬度随载荷的增加而增加，而当交联密度较低时，只有在载荷作用下，胶-尖端的附着力才会受到强烈影响。载荷和剪切的共同作用导致在横向移动的尖端前方形成一个临时的亚微米级的小山。当尖端推挤这样的小山时，低交联密度的水凝胶会观察到明显的黏滑效应。在我们的实验条件下，没有发现塑性变形或永久性磨损痕迹。
