Bakal Kalpit J, Pollet Andreas M A O, den Toonder Jaap M J, Wyss Hans M
Microsystems Section, Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.
IDEAS Institute, Zhejiang University, Hangzhou 310058, China.
Gels. 2023 Mar 3;9(3):194. doi: 10.3390/gels9030194.
Hydrogels can exhibit a remarkably complex response to external stimuli and show rich mechanical behavior. Previous studies of the mechanics of hydrogel particles have generally focused on their static, rather than dynamic, response, as traditional methods for measuring single particle response at the microscopic scale cannot readily measure time-dependent mechanics. Here, we study both the static and the time-dependent response of a single batch of polyacrylamide (PAAm) particles by combining direct contact forces, applied by using Capillary Micromechanics, a method where particles are deformed in a tapered capillary, and osmotic forces are applied by a high molecular weight dextran solution. We found higher values of the static compressive and shear elastic moduli for particles exposed to dextran, as compared to water (KDex≈63 kPa vs. Kwater≈36 kPa, and GDex≈16 kPa vs. Gwater≈7 kPa), which we accounted for, theoretically, as being the result of the increased internal polymer concentration. For the dynamic response, we observed surprising behavior, not readily explained by poroelastic theories. The particles exposed to dextran solutions deformed more slowly under applied external forces than did those suspended in water (τDex≈90 s vs. τwater≈15 s). The theoretical expectation was the opposite. However, we could account for this behaviour by considering the diffusion of dextran molecules in the surrounding solution, which we found to dominate the compression dynamics of our hydrogel particles suspended in dextran solutions.
水凝胶对外界刺激可表现出极其复杂的响应，并展现出丰富的力学行为。以往关于水凝胶颗粒力学的研究通常聚焦于其静态响应而非动态响应，因为在微观尺度上测量单个颗粒响应的传统方法难以轻易测量随时间变化的力学特性。在此，我们通过结合直接接触力来研究同一批次聚丙烯酰胺（PAAm）颗粒的静态和随时间变化的响应。直接接触力通过毛细管微力学施加，即让颗粒在锥形毛细管中变形；渗透压则由高分子量葡聚糖溶液施加。我们发现，与暴露于水的颗粒相比，暴露于葡聚糖的颗粒具有更高的静态压缩弹性模量和剪切弹性模量（KDex≈63 kPa，而Kwater≈36 kPa；GDex≈16 kPa，而Gwater≈7 kPa），从理论上看，我们认为这是内部聚合物浓度增加的结果。对于动态响应，我们观察到了令人惊讶的行为，这很难用多孔弹性理论来解释。暴露于葡聚糖溶液的颗粒在外部施加力作用下的变形比悬浮在水中的颗粒更慢（τDex≈90 s，而τwater≈15 s）。理论预期则相反。然而，我们可以通过考虑葡聚糖分子在周围溶液中的扩散来解释这种行为，我们发现这种扩散主导了悬浮在葡聚糖溶液中的水凝胶颗粒的压缩动力学。
