NanoNerve: Ball Burst Test
A mechanical testing platform that demonstrates that the dural graft is suitable for FDA approval.
|
ROLE
R&D Engineer CONTRIBUTIONS
SKILLS Verification & validation testing Mechanical testing fixtures |
Unmet Clinical Need
|
What is a Dural Substitute?
When neurosurgeons perform surgery, they have to remove a thin membrane, underneath the skull called the dura mater in order to access the brain. For the procedure, parts of the membrane gets removed and has to be replaced.
The gold standard is to use an autograft, the patient's own tissue. Autografts are preferred because they avoid risk of disease transmission or foreign body reaction. However, that requires a separate procedure, more time in the OR, and is dependent upon the size of the wound and amount of tissue available. If the defect is sizeable, the neurosurgeon will need another resource. Hence, dural substitutes are used in place of autografts since they are more readily available off-the-shelf. They are either derived from synthetic (i.e. polymer) or biologic (i.e. bovine) sources. |
About 10,000 neurosurgeries are performed in the United States every year. Unfortunately, 10.7% of neurosurgeries result in post-operative cerebral spinal fluid (CSF) leakage, costing up to $20,000 in additional procedures.
Dural substitutes have demonstrated a reduction in CSF leakage. A dural substitute should mimic that of the native dura mater, allowing for tissue integration while being mechanically strong.
Dural substitutes have demonstrated a reduction in CSF leakage. A dural substitute should mimic that of the native dura mater, allowing for tissue integration while being mechanically strong.
Verification & Validation Testing
It is required by the FDA to perform testing of physical and mechanical properties of a novel medical device before it can be approved. The goal was to create a testing platform that demonstrates that our dural graft will not burst under normal physiological pressures. There is currently no standardized testing platform, and current testing systems are customized and far too complex. We want a platform that is simple and easy to replicate.
Initial Prototypes
Prototype #1
|
I learned that there was a previous capstone project at the UCSF Pediatric Device Consortium, that created a custom hydraulic pressure test for their device. I decided to try it out because its purpose was very similar to the concept I had in mind.
This setup consisted of a water chamber, in which the material is sandwiched in between rubber gaskets at the top of the setup, and water is pumped upwards from the bottom until material would burst. I quickly found out that because our dural graft is porous prior to implantation, any amount of water quickly pushes through the material and no pressure actually holds. Despite the material being porous, once it's implanted, tissue will grow into the graft over time. |
|
Prototypes #2 and #3
|
At the time of this project, the company was also evaluating entering the hernia mesh market. It was then when my team and I realized that a hernia mesh has similar material, composition, structure and function as a dural substitute. I started to consider testing setups to test burst strength of hernia meshes. They are essentially the same material and composition, and serve practically the same function as dural grafts – holding some organ in to some degree. Hernia meshes hold in organs from falling out, and dural grafts prevent the CSF fluid from leaking out. While performing a literature review on hernia mesh tests, I found that most papers refer to ASTM Test D3787, a test for bursting strength of textiles.
Before having to build a new mechanical testing platform, I decided to do some proof-of-concept testing, and to see if I can build a setup with materials already in the lab. I modified a digital force tester with a ball probe, and played around with some pieces that will be able to keep the graft taut during testing. I found that the ziptie did not create enough tension to hold down the graft, while the shaft collar creates too much tension and tears in the material. Both of them did not allow for ideal results in our burst strength test. |
ASTM D3787: Bursting Strength of Textiles
|
Prototype #4
The next step was to build a testing setup similar to that in the ASTM D3787 protocol. I would have to machine two round plates, with a hole at the centers, and find an appropriately sized ball probe.
Steel plate setup #1
|
Steel plate setup #2
|
Solidworks assembly for steel plate setup
|
Hand drawing of steel plate for the machinists
|
Final Design
The final design has two steel plates and rubber gaskets to hold the dural graft in place. A ball probe will slowly lower down and push on the material at the center until it breaks. The point at which the ball probe bursts through the material is the burst strength value of the dural graft.
Ball Burst Test Setup
|
Ball Probe
A 1-inch wooden ball probe will apply a downward force onto the dural graft. Steel Plates
Two 5-inch round steel plates are used to hold the dural graft in place. Rubber Gaskets
Two rubber gaskets to provide additional friction and help keep the dural graft from moving during the test. Fixtures
Bolts and lock nuts are used to entire setup during testing to ensure that nothing moves. |
I performed a pilot test to test the functionality of the setup, and I got pretty consistent results. The maximum load would increase with thickness size. The next steps would be to optimize this testing setup and compare NanoNerve's dural grafts to existing grafts in the market, such as Durasis and Surgisis, as well as the burst strength of the normal dura mater.
Pre-Loading
Post-Loading
|
Pre-Loading
Post-Loading
|
Lessons Learned
I created a pretty robust mechanical testing system that produces repeatable data. Moving forward, there can be adjustments made with the testing system. For example, the wooden ball probe can be replaced with a steel ball in order to produce less friction on the dural substitute. This would be for the next capstone group to work on. It would've also been nice to compare these values to that of native dura mater and competitive grafts, but there were very few values listed in literature to compare to.