We present a technique for measuring the static and kinetic friction forces between single cells and engineered surfaces in microfluidic channels. Frictional forces are defined as the sum of the non-specific attractive forces between two surfaces in sliding contact. The microchannels are fabricated using polydimethylsiloxane (PDMS) and modified using polyethylene glycol (PEG). Rectangular microchannels are designed to apply a modest compression (1030%) to individual cells along one axis. Infusing a cell suspension into this channel at a precisely controlled flow rate enables the application of piconewtons of hydrodynamic drag force to each cell. At certain flow rates, the sample cells are separated into a population trapped by friction and a population where the hydrodynamic drag force overcomes friction. The kinetic friction force is measured as a function of cell compression using the velocity difference between free-flowing cells and the fluid. The upper bound static friction force is measured as a function of cell compression by analyzing the size distributions of trapped and free-flowing cells. Specifically, in order to overcome the coupled uncertainties associated with the normal force and the coefficient of friction, the upper bound static friction is estimated using the frictional force overcome by the largest free-flowing cell in the sampled population. Finally, the frictionreduction property of PEGylated surfaces is evaluated using the ratio of trapped versus free-flowing cells. The kinetic friction force between LCC6/Her2 cells and PEGylated PDMS was measured to range from 45 to 370 pN for cells under compression from 10% to 30%. Similarly, the upper bound static friction forces were measured to range from 50 to 700 pN for the same compression range.
Milovanovic, Lazar, Hongshen Ma