Abstract
Atomic force microscopy (AFM) has been widely used to acquire surface topography upon different scanning modes and to quantify mechanical properties of a cell using single-point ramp force mode. However, these traditional measurements need massive force curves originating from multiple points of a cell to exclude the potential errors resulted from limited and factitious selections of testing points, making the measurements time-consuming and highly localized. PeakForce Quantitative NanoMechanics (PF QNM) is a high-speed (faster by 3–4 order of magnitude) and global surface mechanical property mapping method with high spatial resolution, overcoming the drawbacks of traditional ramp mode especially used for a live cell with high heterogeneity. In this protocol, we elaborated how to run PF QNM measurements for live cells and relevant modification may be needed when extending this method to other cell-like soft materials.
Highlights
Mechanical features of cells are significant to cellular behaviors and fate decision in many biological processes such as cell migration, growth, and differentiation responses to mechanical stimuli (Calzado-Martın et al 2016; Geitmann and Ortega 2009; Qi et al 2017; Wang et al 2014; Xu et al 2019)
Global and high-speed mechanical property mapping with high spatial resolution is important for understanding the dynamic behaviors of cells
Localized mechanical properties of a cell or cells can be extracted from corresponding force–distance curves of special sites based on contact mechanics theories (Sneddon 1965)
Summary
Mechanical features of cells are significant to cellular behaviors and fate decision in many biological processes such as cell migration, growth, and differentiation responses to mechanical stimuli (Calzado-Martın et al 2016; Geitmann and Ortega 2009; Qi et al 2017; Wang et al 2014; Xu et al 2019). With better force control than FFV and QI modes, PF QNM mode enables more precise global mechanical mapping with high spatial resolution and speed (see details ‘‘PRINCIPLES’’). PF QNM is based on Bruker patented technology of PF tapping released in 2009 and the most dominant feature of this mode is high-speed surface mechanical mapping. In this mode, Z-axis tip engagement is driven by the sinusoidal oscillation of the PZT with high frequency of 0.25–8 kHz (Fig. 1B), resulting in faster force–distance curve acquisition by 3–4 order of magnitude than that of classical ramp mode. Hertzbased Derjaguin–Muller–Toropov (DMT) model is preferred when adhesion forces between the tip and the specimen are taken into consideration (Derjaguin et al 1975):
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