anderswallin.net

A test of the 8-channel 0-7 kPa microfluidic pressure controller today. We’re looking through a 100x inverted microscope. There’s a 2 um latex bead in a ~40 um wide channel formed between a glass coverslip and a PDMS layer. I’m hitting the keyboard to either increase or decrease the pressure in steps of +/-35 Pa which allows me to just barely keep the bead within the field of view during the 1 min video. The controller has plenty more resolution, down to about 1-2 Pa, so it should be possible to control flows down to <1 um/s. Stay tuned for more of the same later…

I got some new microfluidic chips to play with today (courtesy of the Microfabrication Group at TKK). This must be cutting-edge research, since there’s an article about using laminar flow cells for single molecule experiments in the latest issue of Nature Methods. I’m testing our custom-built pressure controller which controls the inlet and outlet pressures between 0 and 7 kPa with about 2 Pa resolution. There are three inlet channels (~40 um wide) with blue fluid in the top channel, clear fluid in the middle, and red fluid in the bottom channel. They all meet in the middle of the chip and there’s a wider (120 um) outlet channel.

The pressure controller is similar to Fluigent’s (described by Fütterer et al. in Lab on Chip), and I’m gluing the 0.6 mm PTFE tubing to the PDMS chip as described by Hartmann et al. in Lab on Chip.

The video shows a sinusoidally modulated pressure applied to each of the input channels as well as varying the pressures manually between zero and maximum.

This 8-channel pressure-gauge card is a step towards proper control of fluid flow in microfluidic devices. The transducers (0-1 psi) are around 30 eur each and made by Honeywell. The mV-level signal from a Wheatstone bridge in the transducer is amplified by an instrumentation amplifier (INA111) to around 0-10 V for input to a 16-bit AD-converter.

I’ve been playing around with a microfluidic channel, to be used with optical tweezers experiments later. There’s clear fluid coming in from the left in the wide (ca 30um) channel, and I’ve colored the fluid from the top red and the fluid coming in from the bottom blue. The top and bottom channels are narrower, ca 10um.

Here all the channels are on at first, then the red channel is switched on/off two times. Here’s the same thing, but switching the blue channel on/off. Here the clear channel is switched off, and the main channel fills with red/blue fluid. It’s interesting to follow the laminar flow at these very small Reynolds numbers - the fluids effectively don’t mix at all (they do mix by diffusion, but very slowly) and there’s a clear boundary between red and blue. At the end the clear channel is switched on again.I’m using pressurized air to drive the fluid flow, similar to a product from French company Fluigent (nice videos here and here). Their product sells for around the price of a small car, so I’m thinking I can come up with a DIY solution for slightly less. Switching is by solenoid valves that switch either high pressure or ambient pressure to the fluid bottles (2ml Eppendorfs). The pressures required are surprisingly small, here I’m using the smallest pressure my regulator will output, 5 psi, but I have a feeling this is too much… so I’ll need a pressure regulator with fine control between 0 and 5 psi, any ideas?The other option is using gravity to drive fluid flow, 0.5m H2O is around 5 kPa or 0.7 psi which could be OK. The problem is you then have to switch the fluid lines directly. I tried this with solenoid pinch-valves, and the valves create huge pressure transients when switching off - completely flushing the channel with rapid flow. So the gravity driven solution requires valves that open and close very gently.