Microfluidics: a primer

This post is going up a little before I’d like, in that I’ve made a few steps towards a tool for the Foldscope, but the tool isn’t easy to use or refined yet.  A teacher I work with outside of Denver, Colorado that does an amazing job of letting students really taste whats its like to work in science by having students take on a research project for a large chunk of the year with paper research, proposals, meetings, working with mentors and troubleshooting, followed by troubleshooting and occasionally pivoting has a group of students with a little more of a biotech meets engineering lean this year and I want to document where I am at on this project so that they can hopefully blow it out of the water.  Most of the material below is from May/June and I just haven’t had time to pick up and move forward.

 

The Goal: I’m interested in building some cheap microfluidic devices that can either be reused or could be easily built in the field with a few supplies. The most basic device idea is just a simple channel that would be wide enough for red blood cells, diatoms and other cells to be flowed in front of the cell phone mounted to the Foldscope. The hope is that it could greatly reduce the time that is needed to scan a slide to look for cell types of interest as one could flow the cells past the lens and just have the cell phone capturing video, which may allow for cell counting and maybe with a little bit of image processing even identifying cell types. I would prefer the device be able to run without electricity, maybe using capillary action or something similar as the “pump” to move volumes through the channel.

image1

 

How to make a channel? 

  1. Etching? I tried just etching in a glass slide with a quartz crystal I had around except I found that my manual dexterity did not produce nice straight channels nor were the channels optically clear enough were I felt I could be able to resolve images afterwards.
  2. pseudo-Lithography? For me this ended up being the easiest route – pick a material that could be a nice sized channel, smash it between a glass slide and a cover slip with a material that could harden around it and then try to remove the channel material
    1.  Channel Material – Human hair – I figured it was a commodity easy to come by, it happens to be relatively smooth so possible to remove from the fixing material once it has set. For this channel making application I’m lucky that my hair is only sort of curly so I was able to pull on the hair to make it straight across the slide for a good channel
      1. Fixing Material – Wax – I used just a candle I had around to first add two drops of hot wax to either side of the slide which allowed me to hold a piece of hair tight between them until the wax set and kept the hair straight. I then dripped wax on to the center of the slide over the hair and placed a cover slip over it, pressing down gently with a piece of cloth so as not to put fingerprints all over it. I let it set up for 30 minutes and then removed the hair. I didn’t image it on the foldscope because I was being lazy, but you can see the channel below is pretty rough. I tried a few iterations where I would slide the hair back and forth multiple times to make it smooth, but it was never as optically clear as I’d like.  IMG_0367IMG_0371IMG_0372
      2.  Fixing Material – Clear Nail Polish –  Trying to think what material would be inexpensive and may give me a more optically clear and smooth channel I settled upon trying clear nail polish and it worked surprisingly well. The downsides are that it does seem to seal all the way and the channel can collapse overtime and the material would often generate little bubbles during the drying process that would then interfere with the channel. Below you can see I use hair straightened between wax droplets to get a channel and in the microscope image you can see the channel is relatively clear although a little collapsed because the nail polish doesn’t completely harden beneath the glass. I assume the organic solvent can’t escape keeping it slightly fluidIMG_0399 IMG_0402

This is all just rough sketches at this point in time to hopefully get others here excited about not just going out and documenting the microcosmos but thinking of more tools that could go hand in hand with the foldscope to make it even easier to quickly explore the world with.

6 Comments Add yours

  1. Matthew Rossi says:

    Holy Moses, Damon! This is some brilliant work. Aside from optic difficulties, have you observed anything in regard to the functionality of the tube? Does it operate as you’d like for cell separation and such?

  2. damontighe says:

    Hi Matthew-

    I was able to flow a few red blood cells using a micro pipettes to kind of force flow the system , but that left me with no hand to document with !

  3. Manu Prakash says:

    Beautiful work Damon. The idea of a negative mold to make channels is brilliant – and when it’s just human hair; that’s all the more exciting (almost everyone got them). Some notes for your consideration:

    – Materials: Off course the spirit of it should be common materials; so PDMS and some expensive silicones are out. But the classic silicones used in rubber molding will do a good job. I find https://www.smooth-on.com to be an incredibly creative source for finding perfect materials.

    – Wetting (capillarity): For the liquid to self-wick; you want the channel to be made out of hydrophilic materials. So test the wetting of a given material (by putting a droplet of water – if it beads up like a ball – its a hydrophobic material and not great for self wicking channels). That’s why wax is not so good but nail polish is very good.

    – Some traditional microfluidic devices have been reported in the community – I like this one for imaging stem cells over long periods of time:
    https://microcosmos.foldscope.com/?p=13705

    – Finally; we (@Saad found this paper) are testing a interesting thermal technique to make channels using regular printers. I think it would be very valuable for what you just said since we can assume that people have access to a simple printer. I will nudge him to post the paper here.

    Finally; here is some microfluidic action with a traditional chip from @Chew..
    https://microcosmos.foldscope.com/?p=9321

    Wonderful direction.

    cheers
    manu

    cheers
    manu

  4. damontighe says:

    Thanks Manu for digging up those other posts and the notes on testing for hydrophobic materials, very inspiring. The smooth-on site has some really nice materials.

    I wish I had more free time to play with this, but hopefully the CO student group will get interested in this and innovate some cool options

  5. Arjun53 says:

    Hi

    How is the printing method coming along?

    Is it possible to use a 3 d printer to create it?

    Thanks

    1. damontighe says:

      Hi Arjun53,

      I haven’t had time to come back and revisit the cheap microfluidics ideas due to my work schedule. The students in Colorado that I worked with were suppose to post to Microcosmos upon completion of their project, but I’m not seeing their blog up. So here is the paper they wrote on their attempts to build a microfluidic device on the Foldscope platform. Distilled – stretched glass capillaries worked better then nailpolish channels or plastic channel for optical quality. Since high schools have a hard time getting approval to use blood the students used some green algae in the same size range as red blood cells.

      https://drive.google.com/open?id=0BzTWRWC5YiXsZlhnTG9DWEEtdjQ

      a 3D printer has potential to print the channel, but the original goal was to have something that would be cheap to make in way out of the way places, so I haven’t gone down the 3D printing path with this.

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