Optical stirring in a droplet cell bioreactor.

In the context of a bioreactor, cells are sensitive to cues from other cells and mechanical stimuli from movement. The ability to provide the latter in a discrete fluidic system presents a significant challenge. From a prior finding that the location of the focus of a laser below particles relative to the beam axis producing a pushing effect in a predominant lateral sense, we advance an approach here that generates a gentle and tunable stirring effect. Computer simulation studies show that we are able to characterize this effect from the parameters that govern the optical forces and the movement of the particles. Experimental results with polystyrene microbeads and red blood cells confirm the notions from the simulations.