Re: Microfluidics Chat Thread WAS: [DIYbio] Paper request

On Sun, Dec 28, 2014 at 10:21 AM, <scocioba@gmail.com> wrote:
> Looks like a nice little laser set. I have the cheapo china 40w blue and
> white laser cutter but not sure mirror and lenses are aligned. Still need to
> find some time to set it up.

You want to get a square of liquid crystal, like a mood ring, since it
changes color when the CO2 laser heats it up. MUCH easier than reusing
old thermal receipt paper.
http://www.amazon.com/American-Science-Surplus-Liquid-Crystal/dp/B002V05RWI/ref=sr_1_3?ie=UTF8&qid=1419823528&sr=8-3&keywords=liquid+crystal

>
> What kind of photoresist do you use?

So far nothing, but I got some of the non-film stuff from ebay.

> I saw that blue dry film thing on eBay
> but I'm skeptical.

Ben Krasnow showed making useful stuff with the ebay non-film stuff,
check out his youtube videos for a quick overview.

> I have a UV sterilizer bread-box so exposing it should
> work.

Yeah in that case you'd need the photomask, which you'd have to then
find a decent way to make. Projectors are one way, but the CNC UV
laser seems like it should be decent to get started and the general
idea is super-hackable. There are tons of XY gantry from trash
instructables.

> Getting a hold of SU-8 seems tricky especially with all the
> viscosities available.

Nah, especially since you have a business. The main difference with
SU-8 is that it has better aspect-ratio, or how steep the sidewalls
can end up as a limitation of the photoresist alone (i.e. with a
really nice tight spot and focus and depth-of-field through the entire
film thickness). This might not matter at all with large enough
channels (at a certain point you have to think about channel collapse)
and a lot of the valving techniques actually require or can tolerate
some parabolic/curved profile.

> Would also need to make a wafer spinner or hack a
> microcentrifuge sans rotor from one of those super cheap offers on eBay to
> hold a suction cup....i don't trust myself with making one diy and keeping
> it balanced at high speeds.

I was thinking of building one, I even found some reasonably priced
vacuum bearings, but I haven't had the time and neither the machining
experience. I am just now getting started with CAD use pretty much,
but also a lab I have access to now has a decent professional
spin-coater.

> Also with SU-8 you need a bunch of extra stuff
> to silanize the water so it doesn't stick to the pdms and the etchant and
> whatnot.

Well it's all a matter of how long you want to use the SU-8 mold for,
how carefully you peel it, how long it cured for and the temps of
that. Folks in the 3D printing arena actually use PDMS for
anti-stiction of their 3D printing stages! I've been looking and
asking for a while online about the even less-sticky varieties, to
basically no answer.

From checking some things just now, it seems that some people use PDMS
on the photoresist, then peel and coat the PDMS with anti-stiction,
then pour more PDMS and peel the two PDMS apart along the
anti-stiction layer.
http://diyhpl.us/~nmz787/pdf/Anti-stiction_coating_of_PDMS_moulds_for_rapid_microchannel_fabrication_by_double_replica_moulding.pdf

From checking sigma for fluro-silanes, it seems this stuff is know to
work and is $73 for 10g... not great but not horrible either I guess,
relative to the other prices on other known-to-work fluorosilanes:
http://www.sigmaaldrich.com/catalog/product/aldrich/448931?lang=en&region=US

There is also a chance that the never-wet spray-can stuff you might
find at a wal-mart or hardware store may work... that is just
speculation though.

I'll probably expose the photoresist, cure, add PDMS, cure try
peeling, then expose to plasma for a while to hope to ash any
organics from the non-silica-based-photoresist. Then I'll throw down
some $ for anti-stiction. I'll probably also try using photoresist
itself, though I wouldn't trust that for any reactions... just for
proving the exposure and alignment of the system (and use pretty food
coloring).

> At the same time you only need to make one master and you'll be set
> for many many recasts. Currently in trying to get a decent vacuum in my
> hacked Pyrex bottle to spark an oxygen plasma for treating the pdms and
> glass slide. Know how long the hydrophilic silanol groups last? Any means of
> storing?

I haven't done this myself, but it looks trivial to rig up an oxygen
plasma bonder using a home microwave, vacuum pump, small oxygen
cylinder, a section of 6 inch PVC pipe, gaskets, gas valves, and flat
end pieces.

Have you seen this?:
http://www.youtube.com/watch?v=N-R0_nXpc7I

-Nathan

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