Combined laser tweezers and dielectric field cage for the analysis of receptor-ligand interactions on single cells

A new technique based on the combination of optical and chip-based dielectr ophoretical trapping was developed and employed to manipulate cells and bea ds with micrometer precision. The beads were trapped with optical tweezers (OT) and brought into contact for defined times with cells held in the diel ectrophoretic field cage (DFC). The well-defined ligand-receptor system bio tin-streptavidin was used to study the multiple interaction between biotiny lated live cells and streptavidin-coated beads. The biotin density on the c ell surface was varied down to a few single bonds (3 +/- 2 bonds/mum(2)) to control the valency of the binding. The quantitative relationship between the contact area, ligand density and its diffusion rate in the outer membra ne of the cell could be demonstrated. The increase of the strength of the c ell-bead adhesion was strictly dependent on the increase of individual bond numbers in the contact area. This is in part due to accumulation of ligand s (D similar to (0.5 +/- 0.1) 10(-8) cm(2)/s) in the contact area as seen b y confocal laser scanning microscopy. Individual receptor-ligand rupture fo rces were evaluated and are compatible with values obtained by biomembrane force probe techniques, To summarize, the combination leads to a new powerf ul microsystem for cell handling and pN-force measurements on the single-ce ll level.