(485g) Non-Contact Single-Cell Traps Created by Gentle Fluid Forces

Studying behavior of single cells that normally live in suspension (e.g., blood cells) requires a method of holding cells in place without inducing changes in response to contact. Microfluidic have been very successful for studying single adherent cells under controlled chemical environments, but tools for manipulating single cells in suspension are extremely limited. Optical tweezers (OT) and dielectrophoresis (DEP) create forces that can trap individual cells in suspension; however, trapping is dependent on properties of the cell and medium, and conditions far outside normal physiological limits (laser illumination, electric fields) may impact behavior or viability. We developed a non-contact microfluidic single-cell trap that creates trapping forces comparable to OT and DEP using only gentle fluid flow.

Traps are based on steady streaming flow created when oscillating fluid interacts with any obstacle that requires the fluid to turn. Steady streaming spans length scales from bridge pilings in ocean wave action down to the cellular scale presented here, and it offers rich physics and novel flow patterns for microfluidics. We show that audible-frequency fluid oscillation around a post in a microchannel creates four distinct 3D eddies that each suspend a cell without any surface contact, and we describe all aspects of trap behavior via a few simple dimensionless parameters. A key feature of this approach is that traps are insensitive to differences in cell shape, cell density, and fluid medium. We demonstrate the ease of trapping through videos of trapped bubbles, spheres, rod-like debris, non-spherical motile phytoplankton, macrophages, and monocytes in different fluid media. The approach is remarkably simple to implement and control, in fact, early work used hand-built flow channels and a home stereo amplifier.

We use capture and release of swimming phytoplankton to estimate the trap strength; trapping forces comparable to OT and DEP are easily generated (>30 picoNewtons), while gentle shear conditions in the traps are comparable to arterial blood flow. By using flow to displace trapped spheres under different conditions, we determine a simple scaling relationship that quantitatively describes the trapping force for common cell sizes (5-50 microns). Classic steady streaming theory identifies the oscillation frequency and amplitude, fluid kinematic viscosity, and the obstacle length scale as the sole parameters governing the fluid flow. Our experiments show that the trap force and trap location are governed by the same parameters known to govern the flow, with the addition of the cell size as a geometric length scale.

We find that objects remain trapped under net flows as large as 1 cm/second, which enables medium exchange and chemical treatment of single cells in suspension. Posts can be arrayed with little effect on trapping behavior, providing the potential for high-throughput screening of suspension cells based on dynamic measurements. The combination of strong, tunable trapping forces and gentle trapping environment offers a new alternative for manipulating single cells in microfluidic devices. In particular, it is ideally-suited for studying blood cells, such as monocytes, that respond strongly to surface contact and local shear.