Atomic Force Microscopy

a) Schematic drawing of an AFM-based indentation setup depicting the probe (cantilever with attached bead of defined radius r), the components detecting the cantilever deflection (laser beam, photodiode) and the sample. b) Schematic drawing of a force-distance curve. The cantilever approaches the sample surface by movement of the z-piezo (1). Upon establishing contact with the sample surface (contact point), the cantilever indents the sample causing cantilever deflection (2). Subsequently, the cantilever is retracted (3,4). The indentation part of the approach curve (2) is used to extract the apparent Young’s modulus E. c) Brightfield image of LNCap cells during an indentation experiment. Scale bar, 100 µm. d) Brightfield image of a mouse retina section during an indentation experiment. Scale bar, 100 µm.

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.