Dipartimento di Chimica "G. Ciamician", Universita di Bologna, V. F. Selmi 2, Bologna 40126, Italy.
Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 11635, Greece.
J Phys Chem B. 2021 Oct 7;125(39):10883-10892. doi: 10.1021/acs.jpcb.1c01752. Epub 2021 Sep 21.
Living cells are neither perfectly elastic nor liquid and return a viscoelastic response to external stimuli. Nanoindentation provides force-distance curves, allowing the investigation of cell mechanical properties, and yet, these curves can differ from point to point on the cell surface, revealing its inhomogeneous character. In the present work, we propose a mathematical method to estimate both viscoelastic and noise properties of cells as these are depicted on the values of the scaling exponents of relaxation function and power spectral density, respectively. The method uses as input the time derivative of the response force in a nanoindentation experiment. Generalized moments method and/or rescaled range analysis is used to study the resulting time series depending on their nonstationary or stationary nature. We conducted experiments in living spores. We found that spores in the approaching phase present a viscoelastic behavior with the corresponding scaling exponent in the range 0.25-0.52 and in the retracting phase present a liquid-like behavior with exponents in the range 0.67-0.85. This substantial difference of the scaling exponents in the two phases suggests the formation of biomemory as a response of the spores to the indenting AFM mechanical stimulus. The retracting phase may be described as a process driven by bluish noises, while the approaching one is driven by persistent noise.
活细胞既不是完全弹性的，也不是液体的，而是对外界刺激产生粘弹性响应。纳米压痕提供力-距离曲线，允许研究细胞的机械性能，但这些曲线在细胞表面的不同点上可能会有所不同，揭示了其不均匀的特性。在本工作中，我们提出了一种数学方法来估计细胞的粘弹性和噪声特性，分别通过松弛函数和功率谱密度的标度指数来描述。该方法使用纳米压痕实验中响应力的时间导数作为输入。广义矩方法和/或重标极差分析用于研究根据其非平稳或平稳性质得到的时间序列。我们在活孢子中进行了实验。我们发现，在接近阶段的孢子表现出粘弹性行为，相应的标度指数在 0.25-0.52 范围内，在缩回阶段表现出类似液体的行为，指数在 0.67-0.85 范围内。这两个阶段的标度指数的显著差异表明，孢子对 AFM 机械刺激的反应形成了生物记忆。缩回阶段可以描述为一个由蓝噪声驱动的过程，而接近阶段则由持久噪声驱动。
