TITLE: Microfluidics 6: PDMS Chip, Optical Quality Control and Flow Leakage Test
AUTHORS: Serhat Sevli,  C. Yunus Sahan

[DESCRIPTION]
<div class = "text-blocks"><div class = "text-block"><div class = "justify" style = "text-align:left">Fabrication of microfluidic chips requires high quality and high resolution manufacturing techniques. After fabrication, the quality inspection is crucial and can be performed optically for visual parameters and flow application. If the sizes are well, chips are clean and intake, and flow is laminar without leakage, then everything is OK. This protocol describes the methods for testing of the chips fabricated in our laboratory.</div></div><br/></div>

[STEPS]
?. [Chip Quality Control]
Optical Quality Control of the microfluidic chips are performed by using digital microscope. The visual parameters checked for acceptance of the chip are as listed below. Defects and scratchsStrains and voidsDirt in featuresAir bubbles in featuresPorts and Bonding, out of alignmentNot bond featuresIncorrect sizes of featuresBroken PDMS and/or glassUneven PDMS thicknessAll the parameters are controlled under 4X digital microscope and any abnormalities are reported as photo recordings.
?. [Microchannel Quality Control]
The microchannels inside PDMS chips after bonding are needed to be checked for the parameters which areDefects and scratches, micron sizeStrains and voids, micron sizeDirts inside microchannelsAirbubbles inside microchannelsMultiple channel layers, out of on top alignmentNot bond microchannelsIncorrect sizes of microchanels and microfeaturesThe microchannel quality control is performed using a high quality microscope and CCD camera. The sizes of micron features are measured using software of the CCD camera.
?. [Leakage Flow Tests]
The chip setup is formed by the components given asSyringe pump and 1mL syringeTubings and Fittings suitable for 2mm portsMicrofluidic chip to be testesThe substeps to follow the leakage test are
?. [Leakage Flow Tests]
Fill the syringes with 1mL of distilled water.Distilled water and isopropanol are two most used chemical for all tests performed in microfluidics.The distilled water does not have to be sterilized but it is better to filter the liquid sample.
?. [Leakage Flow Tests]
Connect the proper fittings on syringe to connect tubing and port of the chips. The proper fittings are chosen from https://www.ufluidic.com/collections/fittings-and-connectorsMostly, single port is enough to fill all channels of the chip but when multiple ports to be used for filling all channels, distilled water mixed with food coloring is used. Food coloring is also beneficial for detection of the possible leakage points.
Food coloring is unuseful for very small size channels that is smaller thn 25 microns.
?. [Leakage Flow Tests]
The flow rate arranged are increased step by step starting from 0,1 uL/min. The increasins step values are x10 and given as below. When the leakage occurs, the flow rate is recorded. Under normal conditions, leakage must occur higher than 2 mL/min, but chip design and channel resistance effects the mininum flow rate of leakage starts. 0,1 uL/min1,0 uL/min10 uL/min100 uL/min1mL/min (1000 uL/min)