Currently there are three competing technologies for automated fluid handling: Digital microfluidics 8, droplet based microfluidics 43 and pneumatic valve based microfluidic systems 29. The technique used for this project is a recent advancement in pneumatic valve based systems, that was introduced for the first time by Szydzik et al. 41.
Microfluidic gates are essentially small strips (50 ?m thickness) of PDMS. They can be either fabricated to be in a normally closed or open state. Their state can be manipulated through pressure in an overlaying chamber. This means the PDMS can be lifted like a gate to let fluid flow through it and be closed afterwards to function as a barrier to said fluid. This functionality can be used to actively pump fluid as a microfluidic pump. “Therefore, the desired inlet is initially opened, and the diaphragm is lifted, increasing the volume of the pump chamber and drawing fluid from the inlet; the inlet is then closed, the desired outlet opened, and the diaphragm is forced closed, distending into the pump chamber and reducing its volume, forcing the contained fluid through the open outlet.” 41. This technology can be used in the context of point of care application by fully automating all fluid handling steps. The two geometries, that are currently in use are v-shaped and straight gates. They can be found in Figure 2.
Figure 2 “a. DIC micro-image showing the straight and v-gate test-manifolds. The microfluidic test-manifolds consisted of five primary fluidic channels; a control (no- valve) channel, and four channels containing lifting-gate valves differing by the width of the valve chamber and channel expansion, including 300 ?m, 400 ?m, 500 ?m and 600 ?m. b. Confocal imaging of S300 and V300 microvalves in partially open state following perfusion with TRITC (1?g/mL). Views show 3-dimensional reconstructions of S300 and V300 valve cases. Note the overall deformation of the valve gates under negative pressure resulting in microchannel clearance. c. Confocal imaging showing top down and lateral (midline) views of S300 and V300 gates in fully open and closed states. Note that opening heights were taken from the lateral midline.” 40
A current research endeavour from 2017 by Szydzik et al. looks into haemocompatibility of different gate geometries. Their results conclude, that v-shaped gates with 600 ?m in diameter have the best properties for haemocompatability. This therefore concludes the use of v-shaped gate geometries for all microfluidic valves in this project, which will have direct contact with blood. 40
The most commonly used microfluidic valves are normally closed, because they show advantages over normally open ones in many respects. Normally open channels are only possible with a high aspect ratio for which complex fabrication is necessary. It also needs bulkier additional driver to maintain fluidic isolation. 40 Normally closed gates on the other hand isolate automatically. They are also easier to fabricate and therefore preferred for most application. 40