Fig. 1

Schematic illustration of the concept of scRAFA and its advantage over the existing assays in measuring shear adhesion kinetics. a Without a fluid flow, a cell adheres to a functionalized substrate with a binding force along the normal direction of the substrate. To detach the cell from the substrate, a tensile force is applied to break up the bond between the cell receptor and substrate ligand and. b Conventional single-cell adhesion assays (i.e., optical tweezers, magnetic tweezers, adhesion frequency assays, and AFM), which apply different forces to rupture the adhesive contact, measure the tensile force (F_n, F’_n) along the normal direction of the cell-substrate interface. c With a biofluid (i.e., blood, urine, etc.) flow, the adhesion of a cell experiences multiple stages. The cell will first attach to the endothelium cells (as a substrate) at an inclined angle. Some cells can instantaneously adhere to the substrate during this stage while others will roll along the endothelium cells through transient cell–cell interactions. During the cell rolling, the interacting molecules (i.e., receptors on a cell and ligands on a substrate) can experience the shear forces (F_t, F’_t). An arrow with “T” indicates the torque on the cell generated by the fluidic flow. d In scRAFA, we first trap the cell closer to the functionalized substrate from the liquid medium, which mimics the cell pre-attachment as shown in the first stage of c. For those cells that do not instantaneously adhere to the substrate, we achieve the measurement of their shear adhesion kinetics through analyzing the light-driven out-of-plane rotation of the cells near the substrate