Final Design

Core Features

Even though there were two gripping mechanisms designed, they both shared common core features:surface piercing elements (micro-barbs) and a backbone actuation system. 

Micro-Barbs

The gripping surfaces can be modified to increase the friction between the gripping mechanism and nerve to assist in the clamping of the sciatic nerve. The use of micro-scale piercing elements, known as micro-barbs in this document, to penetrate the proximal stump's surface would help prevent the clamp from slipping on the nerve when pulled by the guidewire.

Due to the size constraints of the project, the piercing elements must be fabricated on a micro-scale, yet still have enough height to pierce into the proximal stump's epineurium. The elements should be placed in an array that can be interfaced with the surfaces of the current clamp designs through adhesive or fasteners. Additionally, they must be rigid and not buckle or break under the amount of compressive stress applied to reliably clamp the nerve.

After the various testing, it was determined that the barb design with a uni-directional piercing head would be employed on the final design of the gripping mechanism. Since the gripping mechanism is only being actuated in one direction, the design of having the piercing element opposing the direction of motion further increased both the frictional force and required force to cause slippage.

Backbone Actuation System

The system consists of a high precision stainless steel tubing (Hyptobue) of length 3 mm, outer diameter 1.07 mm, and inner diameter 0.89 mm sliding along a 20 mm long steel shaft (Backbone) of 0.81 mm diameter via a guidewire - a bendable wire of outer diameter 0.203 mm - pulled by the operator of the device. This allowed for a clamp, attached to the tubing and clamped to the nerve, to be pulled incremental distances without slipping or changing properties.

Gripping Mechanisms

Screw - Clamp

The Screw - Clamp design consists of two parts: a top plate (Blue) and a bottom plate (Yellow). The top plate is outfitted with a pinned extrusion, a micro-barbed lined channel to assist in increasing friction between the nerve and the plate, and a counterbored hole for a screw. The bottom plate consists of a slot for the pinned extrusion to be inserted in, a micro-barbed lined channel, a threaded hole for the screw, and a slot for interfacing with the backbone actuation system.

Using the slotted insert assembly, the two plates are aligned such that the channels are parallel. The nerve is placed in between the two plates, and it is secured by tightening the screw. The larger the number of revolutions the screw is turned, the greater the compressive force on the nerve.

Strap - Down

The Strap-Down assembly is designed as a truncated hemispherical shape with thru-holes lined along the sides of the design. At the center of the hemisphere, there exists a micro-barbed lined channel. The bottom of the design features a slot for interfacing with the backbone actuation system.

The nerve is placed on the channel, and then sutures are threaded through the side thru-holes. These sutures are then looped over the never, acting as straps, securing the nerve.

Performance

To measure the performance of the final designs, a test was created. A moveable-wall apparatus held the gripping mechanisms which attached to harvested rat sciatic nerve samples. As the wall steadily moved, the gripping mechanism elongates the nerve. A fixed force sensor was attached to the other end of the nerve to measure the

tensile force felt by the nerve as it was being elongated. 

Performance of the designs can be represented by plotting the tensile stress felt by the nerve against the percent elongation.Plots of the recorded data were generated where the tensile stress felt by the nerve was plotted against the time of each trial procedure. The elongations were matched to the time they occurred and were

plotted as red, dashed vertical lines. A steady increase in tensile stress as nerve elongation increased is interpreted to be a successful demonstration of the device. 

Example of Poor Demonstration (Slippage)

Before a successful demonstration is shown, it is imperative to understand what an unsuccessful trial looks like. Since slippage from the clamp off of the nerve's surface is a major hurdle to our project's objective, it needs to be visualized. Slippage means that the gripping mechanism is not fully imposing a tensile force on the proximal stump, and as a result, it will be represented by a large, sharp, and sudden decrease in tensile stress as seen in the above plot.

Screw - Clamp

For this test, the amount of compression was greater with a half of a revolution turn on the tightening screw. As seen in the plot, there was no sudden decrease in stress throughout the total nerve elongation of 9.1%. As expected from Hooke's Law, the stress steadily increased as percent elongation also increased. The small dips in tensile stress between lengthening was attributed to stress-relaxation due to the nerve being a visco-elastic material. For the purpose of the device, stress-relaxation was not considered a significant factor due to the very little amount of time it took to lengthen the nerve 10%. Additionally, the device did not excessively compress the nerve.

Strap - Down

As shown by the consistent increase in stress as percent elongation increased, slippage did not occur. These two trials demonstrated that with a compression force of 0.42 N was sufficient for the Strap-Down device to secure a nerve sample of diameter 1.221 mm, respectively. Stress-relaxation did occur as evident by the small dips in stress.