Brian Miller*, Dr. Helen Bridle, Dr Stewart Smith
School of Engineering, The University of Edinburgh, G1 John Muir Building,, Kings Buildings ,Edinburgh, EH9 3JL
Tel: +44 131 650 7860
Why is this useful?
A common issue that arises when attempting to fill microfluidic channels is that of air bubbles becoming trapped against surfaces such as walls and features within the channel. Many different techniques have been used to minimise/eliminate these including pre-filling with surfactant dosed water combined with soaking in an ultra-sonicated bath1, attempting to fill very shortly after O2 plasma treatment while surfaces are hydrophilic2 and bubble traps integrated into the design3-5.
Presented in this tip is a cheap post-manufacture solution for the reduction/elimination of bubbles when filling devices. This method takes advantage of the 10-fold increase in solubility of CO2 gas when compared to O2 and N2. By pre-filling your device with pure CO2 the trapped gas is dissolved away rapidly in comparison to air.
What do I need?
• CO2 “ Cornelius keg charger” and CO2 canister (Amazon B000NV9CE6)
• DI/RO water
• 0.01” ID, 1/16” OD PEEK tubing (Sigma-Aldritch Z226661)
• PEEK Tubing to Luer Lock Adaptor + Ferrule (LS-T116-100 and LS-T116-300 Mengel Engineering)
• AN-4 to 1/8” Female NPT adaptor (Aeroquip#023-FCM2721)
• Male Luer Lock x 1/8″ NPT male adaptor (Cole Parmer PN: OU-31507-84)
What do I do?
Assemble the metal adaptors onto the keg charger as illustrated in Fig.1. Cut a ~3ft to 3 ½ ft length of PEEK tubing and seal into the luer-lock adaptor. This acts as a flow resistor that will reduce the output pressure from ~60 bars to ~ 1-2 bars. Connect the PEEK adaptor to the Luer lock on the charger (Fig.2).
Place PEEK tubing into a device input (Fig.3). Ours mount directly into the device having used a 1.2mm hole punch to create input ports on the PDMS devices. Depending on your device ports you may need to develop a unique solution suitable for your preferred porting method.
Use your fingers to block off any other inputs and use the trigger of the keg charger to release CO2 into your device. Use your fingers to block off the output ports and depress keg charger trigger to fill the other input channels.
Immediately connect DI/RO water filled tubing to your device and begin filling. Remember to pre-fill the tubing with water so that air trapped in the tubing is eliminated.
Please see the time-lapsed videos showing a bubble completely dissolved (SI.1) in ~10 mins (flow rate ~50ul/min in a 50um high device). Also note that for very large bubbles this technique slowly decreases in efficacy until it appears a saturation point is reached (SI.2) after roughly 30 mins. It is recommended to “massage” any very large bubbles towards the outputs and ideally break them up into smaller pockets of trapped gas. Compare these to a video of a medium sized (1mm radius) bubble of normal air to see little to no reduction at a similar flow rate over 10 mins SI.3)
 D. W. Inglis, N. Herman and G. Vesey, Biomicrofluidics, 2010, 4.
 I. Wong and C.-M. Ho, Microfluidics and Nanofluidics, 2009, 7, 291-306.
 A. M. Skelley and J. Voldman, Lab on a Chip, 2008, 8, 1733-1737.
 W. Zheng, Z. Wang, W. Zhang and X. Jiang, Lab on a Chip, 10, 2906-2910.
 C. Lochovsky, S. Yasotharan and A. Gunther, Lab on a Chip, 12, 595-601.