VERY small piping question

[Chris Pruet]

Greetings all, I'm a former engineer turned ophthalmologist and had a VERY small scale fluid dynamics question I thought one of you might be able to help with.

When someone has too high a pressure in their eye, we sometimes place a tube shunt to prevent vision loss. This is a 0.305 mm internal diameter tube that connects to a silicone plate inside the eye socket. The aqueous humor (eye fluid, essentially same dynamics as water) then diffuses across the silicone plate and is resorbed inside the eye socket. The main resistance to flow is not the fluid dynamics in the tube, but rather the body's scarring / encapsulation of the silicone plate.

In some patients, the encapsulation process doesn't work well, and there is excessive flow to the plate, often leading to vision decline due to decreased pressure in the eye.

Something we could do to treat this would be to place a suture/thread inside the tube lumen to block some of the flow of aqueous humor to the plate, transferring the resistance to flow from the poorly encapsulated plate to the tube itself. However, our available suture sizes tend to be too small (0.2mm, doing little to nothing to the flow), or too large (0.3mm, essentially stopping all flow).

The length of tube is generally about 13-15mm. Production of aqueous humor in most people is about 2.0 to 2.5 microliters per minute and is generally inflexible; you could assume this is all going through the tube without any feedback control based upon pressure. A good pressure gradient would be somewhere in the 5-7 mmHg range, and would be titrated to the individual patient based upon the length of suture deposited in the tube. I presently imagine this would maintain laminar flow; I could be wrong.

So, my question is how would you calculate the pressure drop per mm or cm in a 0.305 internal diameter pipe with a longitudinally oriented cylinder in the pipe and what would be the ideal diameter of the internal cylinder?

Thanks all,
Chris Pruet
Dean Clinic / SSM Health
Madison, WI

 

[BigInch]

I don't think this is a problem for D'arcy equation. "Fluid dynamics" of this are controlled by surface tension of the fluid and capillary flows. Not my strong suit.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[SwinnyGG]

Interesting problem.

Have you investigated permeable teflon membranes?

These are used in various applications to control very small flow rates of gasses, or possible to allow gas to escape while containing or preventing egress of fluid.

These membranes are available from their manufacturers in controlled mesh sizes- perhaps you could apply this membrane to the outside of the silicone plate over the tube's opening, and vary mesh size to control fluid flow rate.

Assuming that teflon is OK for a medical implant- I'm not sure about that.

 

[cvg]

I dont see this being solved with hydraulic theory, recommend testing to establish a good emperical relationship

 

[LittleInch]

At those sizes, flows and lengths I don't think you'll find anything to work well. There are too many other things going on in terms of laminar flow, surface tension and lack of empirical data about something so small.

What is the tube made of?

Any pictures? (no gory ones...)

Can you not tie the suture round the pipe and squeeze it? or squash it with a pair of pliers if it's metal?

You would need to set up your own lab to measure these sorts of micro quantities

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dhengr]

Chris Pruet:
“Very small piping question,” what say?? That sounds like a very big and complicated piping question to me.

In high school we took glass tubing, over a flame, and stretched it to get a varying i.d. nozzle. This would actually make two nozzles when cut in the middle. Then if you continued to cut small length increments off the small end, you would slowly increase to i.d., maybe to some ideal dia. to provide your restriction and pressure loss/drop. Could your tubing material be necked down this way. Alternatively, could you block the end dia. of your tube, a fixed dia., but them pierce the end with a varying dia. needle until you got the correct flow rate and/or pressure loss/drop? Thirdly, your sutures in your tube i.d. offer too little adjustability, but if you pierced the side of the tube with a varying dia. needle, and continued to press further, you might have infinite restriction variability. This needle, or some such, stays in place to provide the restriction. Exactly how this might all be worked out is your specialty. I have no idea how to actually run calcs. for the perfect tube dia. or actual flow rate or pressure loss/drop, and the other guys are much smarter piping engineers than I am, but I can imagine you might be able to test this in some way, and sneak up on the perfect hole dia. or restriction for each patient or condition.

 

[Chris Pruet]

Thanks all for the replies.


jgKRI: I think you hit the main problem with the teflon membrane on the head, hard to know if it's bio-compatible. A literature search on teflon meshes shows it's great on paper, probably a bad idea in practice. From prior research in meshes for hernia repairs: "Microporous meshes (for example, ePTFE) are at higher risk of infection because macrophages and neutrophils are unable to enter small pores (< 10 μm). This allows bacteria (< 1 μm) to survive unchallenged within the pores. A similar problem applies to multifilament meshes. The meshes at lowest risk of infection are, therefore, those made with monofilament and containing pores greater than 75 μm. Eradication of infection in such meshes can be achieved without their removal." Great idea though.

cvg: Agreed, the proof is in the pudding. The theory's an interesting place to start though.

LittleInch: Tube is made of silicone. Have plenty of gory pictures burned in my mind. As for clean ones:

http://www.glaucoma-association.com/media/wysiwyg/Surgery/FIgure_4_-_Baerveldt_101-350.jpg

http://sanantonioeyeinstitute.com/wp-content/uploads/2014/01/Ahmed-valve.jpg

We actually tie a suture around the tube initially to allow the plate to encapsulate. If you just leave the tube open and allow fluid to flow freely over the plate without any encapsulation first, you get an unsafely low pressure in the eye immediately after surgery. You can either use a dissolvable suture which breaks open at a known time, or use a black plastic suture you can break open in clinic by shooting a laser at it. The problem with tying the tube for flow control is it's almost universally either completely occluded or completely inconsequential from an eye pressure standpoint.

dhengr: Very interesting approach; essentially block flow using an orifice rather than a plug within the tube. I don't think you could easily reshape the tube within the eye: too much high-dollar real estate nearby. I think a snug-fitting cap with a calibrated orifice may be feasible. I like it, a lot actually.

There are a couple of other devices out there that do similar things to tube shunts but have their calibration based upon the size of the tube rather than the encapsulation of a foot-plate: The Xen is a 6mm length 45um internal diameter tube that drains to the same space as tubes, so maybe this is a good place to start for sizing.

https://www.xengelstent.com/about-xen

The Cypass drains to a different anatomical space and has a HUGE diameter by comparison- 300 um, and a shorter length. Obviously some restriction to flow occurs in this space as compared to the subconjunctival space.

https://www.myalconstore.com/glaucoma-surgery/cypass-micro-stent/

Thanks all for the brainstorming, any other ideas are very welcome.
Chris

References:
Amid PK, Lichtenstein IL, Shulman AG. Biomaterials for tension-free hernioplasties and principles of their applications. Minerva Chir. 1995;50:821–6.

 

[itsmoked]

I don't know.. Seems

problem with tying the tube for flow control is it's almost universally either completely occluded or completely inconsequential

is pretty close to identical with

block flow using an orifice rather than a plug within the tube

Or, are you thinking biological access to one side of the orifice would suffice in keeping it open?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Have you considered a valve like the purported vasectomy valves.

It should be possible to fabricate a valve using nano technology that is remotely adjustable. The valve would need no batteries - only a microscopic antenna.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[3DDave]

I would look to changing the diffusion available area, possibly with a silver plate or other anti-bacterial coating to prevent growth beneath it. If it could flex to uncover more area when the pressure is high and close off when the pressure is low, that would be the best outcome I think.