4. Adhesion test method
Several techniques can be applied to measure cell adhesion force[117]. It can be divided into two main categories: population cell and single-cell methods, as shown in Figure 7. Many of them, including the simple washing assay, the spinning disk technique, and flow chambers, are based on the hydrodynamic shear flow removing cells from the surface. The adhesion force can be calculated from the applied flow rate,e, and thus, the adhesion force of the cells can be measured. However, these techniques do not enable single-cell targeting, and cell shape strongly impacts the shear force, making it difficult to calculate the exact adhesion force. Furthermore, only weakly adhered cells can be probed due to the technically limited shear stress magnitude.
In order to directly probe the adhesion of single cells, an AFM probe or micropipette can be used as an option. [118-120]Single-cell adhesion assay techniques can be further classified according to how they work. The micropipette aspiration uses a glass micropipette to suck cells from the surface and calculates adhesion forces by hydrodynamic simulation. AFM is combined with modified probes or nanofluidic channels to study cell adhesion. Fluorescence resonance energy transfer (FRET) sensors can also be used to detect transgenic cell protein fluorescence signals to characterize the mechanical tension of cells and to measure the traction force generated by adherent cells in the substrate.