Force-free manipulation of microorganisms in a 3D microfluidic channel

Our capabilities of micromanipulation have evolved with advances in contact-free trapping techniques under various disciplines, such as optical, magnetic, and microfluidic traps. In these techniques, a microscale particle is held in place under compression due to electromagnetic or hydrodynamic forces. In this work, we present a trap-free design of a microfluidic "treadmill", realized by a uniform flow along arbitrary directions in a 3D microfluidic device. The treadmill comprises a central chamber and pairs of x- and y-channels at different elevations that enable the 3D flow generation. We constructed such a 3D microfluidic device using laser lithography, which is controlled by an array of independent syringe pumps. To demonstrate the capability of 3D micromanipulation, we used the 3D flow to drive a suspension of microscale polystyrene particles and compared their movements with the desired flow pattern. Additionally, we have extended this force-free manipulation to control a collection of motile bacteria, e.g. bacterial swarms.