Cells are complex, viscoelastic bodies. Their mechanical properties are defined by the arrangement of semiflexible cytoskeletal fibers, their crosslinking, and the active remodeling of the cytoskeletal network. Atomic force microscopy (AFM) is an often-used technique for the study of cell mechanics, enabling time- and frequency-dependent measurements with nanometer resolution. Cells exhibit time-dependent deformation when stress is applied. In this work, we have investigated the stress relaxation of HeLa cells when subjected to a constant strain. We have varied the applied force (1, 2, 4, and 8 nN) and pause time (1, 10, and 60 s) to check for common assumptions for the use of models of linear viscoelasticity. Then, we have applied three models (standard linear solid, five element Maxwell, power law rheology) to study their suitability to fit the datasets. We show that the five element Maxwell model captures the stress relaxation response the best while still retaining a low number of free variables. This work serves as an introduction and guide when performing stress relaxation experiments on soft matter using AFM.

中文翻译：

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用原子力显微镜测量生物材料力学 - 从应力松弛实验中测定粘弹性细胞特性

细胞是复杂的粘弹性体。它们的机械性能由半柔性细胞骨架纤维的排列、它们的交联和细胞骨架网络的主动重塑来定义。原子力显微镜 (AFM) 是研究细胞力学的常用技术，能够以纳米分辨率进行时间和频率相关的测量。当施加压力时，细胞会表现出随时间变化的变形。在这项工作中，我们研究了 HeLa 细胞在受到恒定应变时的应力松弛。我们改变了施加的力（1、2、4 和 8 nN）和暂停时间（1、10 和 60 秒）以检查使用线性粘弹性模型的常见假设。然后，我们应用了三个模型（标准线性固体、五元麦克斯韦、幂律流变学）来研究它们对数据集的适用性。我们表明，五元素麦克斯韦模型能够最好地捕捉应力松弛反应，同时仍保留少量自由变量。在使用 AFM 对软物质进行应力松弛实验时，这项工作可作为介绍和指南。