A novel helical micro-valve for embedded micro-fluidic applications

In this paper, we report a pneumatically actuated solenoid micro-valve with simulation, design and characterization. In micro-fluidic applications dealing with soft biological fluids very often, digital micro-valves [also known as Quake valves (Unger et al. in Science 288:113, 2000)] are deployed to start/stop flows in pressurized micro-channels. These valves are designed in a manner so that the actuation is performed by a control micro-channel filled with compressed air seating on the top of another micro-channel filled with fluidic sample. The response time of these valves is so miniscule that they almost close immediately qualifying them to be digital in nature. Still in applications like drug screening where a small amount of intermixing may have a counterintuitive effect on the whole experiment, the valves may underperform. In order to address the problem of leakages/leaching of fluids across a fully pressure-closed digital micro-valve, we have tried to incorporate a design modification with a completely different fabrication process, wherein the closing approach of a Quake valve has been varied from top down to radially inwards across the whole cross-section of the micro-channel. The design of this valve enables its wide applicability to embedded micro-fluidics, which is widely used to mimic/study the flow of body fluids across our vasculature or is highly useful for chip cooling applications. Micro-valving has been seldom explored in the embedded domain, and the current architecture of the micro-valve in a way fills this technology gap. In our design, two concentric hollow tracks, one straight and another helical are replicated in polydimethyl siloxane using preformed 80-micron copper wires that are of circular cross-section. Compressed oxygen is passed through the helical channel to control the fluid flow in the concentrically placed straight micro-channel. The micro-valve controls the fluid flow by applying uniform pressure homogeneously on micro-channel wall so that the wall can collapse and close the channel cross-section (stop condition). The system has been designed and simulated using COMSOL multi-physics platform where a design optimization was carried out in details in an earlier work (Singh et al. in Microsyst Technol. doi:10.1007/s00542-013-1738-7, 2013). In this work, we have evaluated the design experimentally through monitoring the inlet/outlet flow rate in the central micro-channel through microparticle image velocimetry (micro-PIV) analysis on opened/closed valve conditions. A flow behavior of the central micro-channel is simulated using COMSOL multi-physics platform, and the pre/post-valving discharge have been estimated theoretically for various input flow rate conditions at a corresponding air pressure of 5 psi in the helical track. We have further validated this through micro-PIV experiments wherein the valving behavior observed is different to the extent of the transient flow part but quite similar as regards the steady-state part.