Single-cell force spectroscopy
Single cell force spectroscopy, J.Jonne Helenius, Carl-Philipp Heisenberg, Hermann E. Gaub and Daniel J. Muller J. Cell Science 121 (11), 1785 (2009) 
Soft Matter Keywords
Atomic Force Microscope, Cell Mechanics, Adhesion
A method for quantifying the adhesion between cells having various receptor/ligand pairs is discussed. This technique uses an Atomic Force Microscope to probe the cells in an aqueous environment by either pushing a cell and substrate together or pulling apart.
This paper presents a method for investigating the adhesion force of cells to various substrates (either other cells, functionalized surfaces and the like). The basic idea is to turn an Atomic Force MIcroscope cantilever into a cell force probe by sticking a cell to the cantilever. This is depicted schematically in Figure 1. The AFM tip is functionalized so as to stick to the cell. The AFM tip is depressed onto the cell and then elevated, bringing the cell off of the loading substrate. The AFM tip can now be brought in the vicinity of another body of interest. Once the cell-AFM tip apparatus is optically aligned to the area of the substrate one wishes to study, the AFM tip is lowered until the cell and the substrate make contact (Figure 2). Once in contact, the cell and substrate interact via their surface chemistry. The experiment is arranged such that there are certain receptor/ligand pairs present on the cell and substrate. Contact between cell and substrate is held for up to 20 minutes (longer contact times suffer from stage drift). The force that the cell (and hence, the AFM tip) feels is measured by the deflection of a laser beam off of the cantilever just as in the usual AFM setup. Figure 2 shows a typical force-distance curve for an approach (green) and then retraction (blue).
Forces on the order of 0.1-1nN are typically required to detach adhered cells, whereas forces on the order of 10-100pN are observed for single molecule detachment. The number of molecules involved in the adhesion interaction is roughly dependent on the contact time so that two regimes may be studied, namely cell scale adhesion or single molecule adhesion. It is worthwhile noting that while the single molecule interaction force is accesible to optical tweezers, the cell scale version on the experiment is well beyond. This makes the AFM method much more versatile than the optical tweezer method which uses a trapped, functionalized bead as the experimental handle rather than the AFM cantilever.
The detachment force-distance curve contains all of the information regarding the receptor-ligand adhesion energies. The detachment force curve has two particular features denoted as "jumps" and "tethers" (see figure 2). The jumps refer to the unbinding of receptors whereas the tethers refer to the receptor being detached from their membrane. Unfortunately, the retraction force curve also includes effects due to cell elasticity so that it is sometimes difficult to distil actual molecular binding energies or forces from the convolved experimental data. This method for studying cell adhesion mechanics is the most versatile of the methods used to date because it can measure forces in aqueous environments while exploring forces on the scale of pN to hundreds of nN, making it a perfect match for single molecule to single cell scale studies. Cell adhesion is an important area of biophysical research as it tells us how cells interact mechanically with their environment and how this, in turn, is influenced by the low-level membrane chemistry.