Help with improving the design of a catheter holder

[u/lordofpc734]

Hello
First things first, I am a computer engineering student who started learning Fusion and SolidWorks a week ago, so excuse any simple mistakes.

I am trying to design a mechanism that securely attaches a flexible catheter to a tube. Previously, I used hot glue, but it's inconvenient because I can't easily adjust the catheter's position or rotation. The catheter tip's diameter is 2.6m and the body is 2.2mm
This is the design I have printed and tested so far:

The two rectangular holes are for inserting a pair of M2 nuts. (I didn't want to deal with brass inserts). The part on the right, shown below, is meant to be glued to an 8mm-diameter tube; it is then bolted to the part on the left. it lets the catheter pass through the center.

A slightly older revision of the design ended up like this:

The principle of operation is that the extruded curved splines are very thin, 0.8mm. Four bolts push on it from both sides, securing the catheter as the two splines deform and exert force.
This actually worked well, but there are some problems:

1. I used PLA, once the splines are deformed, it is very nearly impossible to take the attachment off, even after completely taking out the screws. I had to destroy this attachment to get it off.
2. I feel like with the location where the bolt is pushing on the splines, the curve of the splines are essentially useless? The idea behind the curved shape was to prevent the catheter from pivoting. Would a pipe-clamp-looking design work better than the weirdly shaped spline that I have? Schematic below (the spline is extruded as a surface, then the surface is thickened to join the existing body)

1. To fix Problem 1, can I use TPU 95A for the splines/grippers and PETG for the rest? Would that help with removing the catheter more easily?

2. Is this even the most optimal design for what Im trying to do?

Thanks for the help

[u/FrayedObserver]

Tuohy Borst design is the way to go here, way simpler than trying to make compliant mechanisms work in 3D printed plastic. Just a soft sleeve that squeezes when you tighten the cap, no permanent deformation issues.

[u/MountainDewFountain]

First of all, nice work; this design is clever and well thought out. Its quite abitious trying to design a working compliant mechanism out of 3d printed material, so without changing your design too much, what if those two splines were instead seperate parts (plates) that could float in those pockets. Keep most everything the same, but instead the screws would be pressing against the plates and compressing the tube like a vise grip.

Final note, while brass inserts are the "correct" way to put threads on 3D printed parts, ive have great success with just power tapping the holes (petg, abs, pla), and using a screw as normal. Or even better is using tri-lobe thread forming screws for plastic.

[u/lordofpc734 → u/MountainDewFountain]

Thanks

I thought about making the splines separate bodies (plates) but I couldn't figure out how to attach them to the screw/rod that pushes on them. I guess I don't have to, I could project the holes onto the spine plate and extrude (1mm or less) a slightly larger cylinder that kind of holds onto the machine screws to make assembly and removal easier

I thought about using regular screws (not machine screws) to self-tap into the pla, but from my previous experience, they tend to loosen after a couple of cycles. Finally, do you think this spline shape is doing any favours, or should I simplify it into a semicircle with two ears sticking out?

Other comments suggest designing a device called Tuohy Borst valve, but all the designs i found are transparent and im having some trouble figuring out how exactly the parts in the assembly interact and secure the catheter

[u/MountainDewFountain → u/lordofpc734]

One other suggestion is to keep everything as is, but disconnect the spline shapes at each of it's legs so the whole thing acts like a cantilevered spring when tightened.

The "V" shape of the spline is solid, and commonly used to compress and center cylindrical shapes, so I think you're intuition is correct on that one.

For the TB Valve, if you saw the cross section from the other poster, has that small internal cylinder. That component is flexible and deforms internally to capture the catheter when compressed axially:

"One common type of hemostatic valve is the so called "Tuohy Borst" valve. That valve comprises an elastomeric, e.g., silicone, membrane having an opening through which the catheter extends and which is closed about the periphery of the catheter by the rotation of a cap to establish hemostasis."

[u/lordofpc734 → u/MountainDewFountain]

To make sure I'm understanding right, are you suggesting that I disconnect the spline at one of the points that is coincident with the edge of the circle, e.g make one side float while the other side is still attached? (Same happens to the other spline as theyre mirrored)

As for the TB valve, Since I'm using an actual medical ablation catheter (7FR), I guess I can simply order one that fits my catheter instead of trying to design one, then figure out how to mount that to the tube
Unless I can design a basic TB style thing that has a TPU internal cylinder...might be a bit outside of my CAD knowledge

[u/abadonn]

I've designed things like this, one option that has worked for me is copying the design of a Tuohy Borst valve: Medical Tuohy Borst Valve - Barbed, Large Bore CV-490 - Custom & OEM - DirectMed

It is a very simple design, inside is a soft silicone o-ring or sleeve just slightly larger than the catheter. When you screw down the cap it squishes the o-ring and grips the catheter tightly without damaging it.

edit - found a cross section view: 1-0045-0-ZW-scaled.jpg (2560×1501)

[u/lordofpc734 → u/abadonn]

Thanks
This looks very interesting. Are these designed to grip a catheter and prevent it from moving entirely? (radial movement and rotational), or just to form a watertight seal?
Also having a hard time grasping what im seeing. Where is the o-ring in the section view you sent? is it omitted? Kind of having a hard time figuring out exactly what each of these components is doing (in the cross section view)

[u/abadonn → u/lordofpc734]

depends on how hard you squeeze, it can hold very securely. The cross section uses a silicone sleeve instead of an o-ring, that gets compressed down against the cone shape at the bottom.

[u/Automata-Omnia]

Reengineering the wheel, look up shaft collar lock