Atomic Force Microscopy
Our lab uses indentation-type atomic force spectroscopy to determine the mechanical properties of hydrogels and various types of cells and tissues. The heart of the indentation setup is the cantilever – a flexible spring leaf with a defined spring constant, which is usually decorated with a sharp tip or a spherical bead. An indentation measurement commences with the cantilever approaching the sample surface in a piezo-controlled manner. Upon establishing contact with the sample surface, the piezo is moved further until a preset force F or indentation depth δ is reached. Interactions between the indenter and the sample cause the cantilever to deflect. After a defined contact time the cantilever is retracted again. During this process, the cantilever deflection is recorded and plotted over the z-position of the piezo. This plot can then be converted into a force-distance curve, from which — after fitting to appropriate mechanical models — the apparent Young’s modulus E of the sample can be derived. In order to obtain information about viscoelastic properties of gels, cells and tissues, also time-dependent mechanical measurements can be conducted, e.g. by performing stress relaxation or creep compliance tests, or by microrheological measurements, where the cantilever is oscillated in contact with the sample and a phase shift (indicative of a viscoelastic response) between indentation and force response is measured.
[1] Möllmert, S. et al. Zebrafish Spinal Cord Repair Is Accompanied by Transient Tissue Stiffening. Biophys J 118, 448–463 (2020). https://doi.org/10.1016/j.bpj.2019.10.044
[2] Kolb, J. et al. Small leucine-rich proteoglycans inhibit CNS regeneration by modifying the structural and mechanical properties of the lesion environment. Nat Commun 14, 6814 (2023). https://doi.org/10.1038/s41467-023-42339-7
[3] Abuhattum, S. et al. Adipose cells and tissues soften with lipid accumulation while in diabetes adipose tissue stiffens. Sci Rep 12, 10325 (2022). https://doi.org/10.1038/s41598-022-13324-9
[4] Abuhattum, S. et al. An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves. iScience 25, 104016 (2022). https://doi.org/10.1016/j.isci.2022.104016
[5] Abuhattum, S., Kuan, H.-S., Müller, P., Guck, J. & Zaburdaev, V. Unbiased retrieval of frequency-dependent mechanical properties from noisy time-dependent signals. Biophysical Reports 2, 100054 (2022). https://doi.org/10.1016/j.bpr.2022.100054
[6] Hofemeier Abuhattum, Shada, On the viscoelastic behaviour of living matter - experimental and theoretical approaches. PhD thesis, Friedrich-Alexander Universität Erlangen-Nürnberg (2023).
