In our lab, we are preparing an experiment to insert
3 devices into a human subject during a surgical procedure along 4 mm steel rods
arranged in a plane and spaced 10 mm apart. A fixture clamps each rod into
position and allows for adjustment along the axis of the rod, but when loosened
a rod—and the attached device—could drop. We need a small clamp to grip the end
of all three rods and disallow any rod from falling through the fixture hole.

 

CONSIDERATIONS	DESIGN
Small The clamp must neither occupy too much of the rod’s length nor interfere with wires or other fixtures.	I let the other parts control the size. We need to grip a 4mm diameter rod with 10 mm spacing. The largest reasonably usable standard metric screw size which fits between the rods is M2.5. I created the clamp around these necessary pieces with enough wall thickness for stability.
Simple Tools in the operating room need to be simple to operate and easy to understand when possible.	I designed a screw point to clamp around each rod. Engineering a mechanism with only one screw point and a hinge while still gripping each rod securely would be better, but I will choose the crude option.
No Loose Items A loose screw can be dropped as easily as an implantation rod, so our solution should not add new problems with loose fasteners.	Here, we can use features only 3D printing provides. Captive screws and heat set inserts—essentially another form of captive nut.
Sterilizable Any tool for the operating room will be sterilized.	Sterilization provides some interesting requirements. We can’t access autoclavable filaments like PEEK, but Ethylene Oxide (EtO) sterilization works for 3D printed parts and electronics. EtO works as a gas, and we’ll minimize enclosed spaces by printing our parts with 100% infill.
Easily Cleaned Even if already sterilized, tools should have smooth and easily cleanable surfaces.	3D printed surfaces, especially from FDM, suffer from layer-lines and other marks. These create regions more likely to grow bacteria, so we will try post-processing methods to smooth these surfaces.
Strong Our clamp needs to securely grip the rods.	This introduces an interesting point about materials. If our surgical rods are made of steel, plastic parts won’t “bite” well. A better material would be something like TPU with a rubbery surface to create a strong fiction fit. Ideally these TPU gripping point would be backed by a rigid plastic. PETG bods well to TPU and does not suffer from creep like PLA.

 

In sum, we need a small, simple clamp to grab 3 rods
using TPU elements and trapped screws and inserts, along with some post-process
smoothing if possible. We decided to print 3 versions of the clamp.

A TPU only version to better grip
the rods.

An ABS version with a different color
of ABS representing where another material like TPU could go. Think of this as
a mechanical model.

The original idea, a model with PETG
body and TPU gripping inserts. Totally achievable in concept
but possibly hard to achieve on the machines in the OEDK.

 

DESIGN

Here is the final design, imagining”

• hard plastic body   
• printed TPU inserts
• captive screws
•  heat set inserts
• ·  “screw tubes” to prevent material from getting into the screw
  threads (see below)
• ·     simple, smooth shapes

A cross-section of both parts of the design. Notice
the screw rubes, the screws themselves, the rod spaces, the TPU inserts, and
the spaces for the heat set inserts.

 

3D PRINTING

Two iterations of our original TPU and PETG version. Ultimately,
the dual tool head machine in the OEDK uses too large a nozzle size to produce
fine detail and exacerbates TPU printing issues. Ultimately, we can’t submit
this for grading. The concept works, but we’d need more time to tune the prints
and alter the machine.

 

The printed pure TPU version.

 

The printed ABS only version.

 

POST PROCESS

Here is a look at the heat set inserts being
inserted.

 

As for the other post-processing, we only needed a
little sanding on some faces before trying smoothing techniques.

 

With TPU, we tried thermal surface smoothing, and
while effective, it was unsatisfactory overall. After treatment, the surface
feels much smoother and more fully sealed, easing sterilization concerns about
voids. However, the warping is unmistakable, and any fine detail is lost.

 

The ABS acetone smoothing showed more promise and
would have improved with more experienced users. The surface is noticeably
smoother and, therefore, easier to clean, but when acetone touches the parts
directly, it can slough off exterior layers. Fine detail is
lost, but warping is minimized. With better technique, we would have much
better results.

 

RESULT

 

Here is our result. The TPU thermal surface treatment
is pronounced, haha. It looks like a fishing lure.
Both configurations serve their purpose well, and the ABS version gripes strongly,
though the TPU is superior. The main success is the trapped screws and heat set
inserts, which work fine even after surface treatments and show no signs of loosening.
In the end, we learned a ton with this little project, and we’ll definitely make further versions for use in our experiments.

 

 

COST MODEL

 

For simplicity, the cost model is only
for the PETG and TPU versions out of the 3 we tried to make.

 

Cost Type	Cost	$/#	Source	#	Tot($)
Materials	TPU	0.032	sainsmart.com	0.76 g	0.05
	PETG	0.03	prusa3d.com	5.52 g	0.17
	M2.5 Screws	0.065	McMaster-Carr	8 units	0.52
	Heat Set inserts	0.33	McMaster-Carr	8 units	2.64
Labor	Printing and Processing	36	Indeed.com	8 hrs	288
	Prototyping	43	Indeed.com	5 hrs	215
Overhead	Machine Time	50	Rentflex360.com	1 day	50
	CAD Software	680	Autodesk.com	0.003 yr	2.04
				Total Cost	558.37

 

The results are as expected, with the most
significant cost being labor. The time it takes to design an item for 3D printing
will generally be far more than the actual material cost or use of the machine
itself.

 

CLEANLINESS