Neural Engineering
Marquis Globokar
Undergraduate Researcher and Wen H. Ko Summer Intern
Case Western Reserve University, United States
Margaux Randolph
Graduate Student
Case Western Reserve University, United States
Jeremy Dunning
Biomedical Engineer
Advanced Platform Technology Center, Louis Stokes Cleveland VA Medical Center, United States
Dustin Tyler
Kent H. Smith II Professor of Biomedical Engineering
Case Western Reserve University, United States
Rachel Jakes (she/her/hers)
PhD Candidate
Case Western Reserve University, United States
By the year 2050, it is estimated that 3.6 million people will be living with limb loss in America [1]. However, peripheral nerve interfaces are enabling the restoration of their sense of touch. The ability to feel is crucial for the emotional and social well-being of people living with limb loss as it enables embodiment and feelings of confidence in social situations [2]. By electrically interfacing with remaining peripheral nerves following amputation, patients can once again perceive sensations on their phantom limbs.
The size of the implanted electrode is directly related to the foreign body response [3]. Therefore, interfascicular implantation of a small electrode into a nerve is ideal for neural interfacing as it is less invasive and more selective than other methods [4]. Cuff and flat interface nerve electrodes (FINEs) are selective but likely unable to selectively stimulate central fascicles. Intraneural and regenerative electrodes are capable of accessing central populations, but implantation is invasive and nerve damage is likely [5]. Microwire arrays are the ideal size for minimizing foreign body response during chronic interfascicular placement. However, leads of such small magnitude tunneled subcutaneously break easily [6]. In animal studies, lead breakage was a primary challenge [7].
The objective of this study was to discover a means of creating a joint between a microwire and a larger wire that is more appropriate for tunneling through the body before exiting the skin. A testing mechanism was created to investigate the strength of the produced joints.
A 0.002” in diameter single-stranded and 0.00405” in diameter seven-stranded wire of stainless steel 316 LVM was used. Both wires are coated in Teflon, so hot tweezers were used to remove the coating. Three joint configurations for connecting microwires to lead wires were designed. The first involved taking a section of the smaller and larger wire and then tying them together in a knot. The second configuration involved using a microscope and tweezers to untwist a section of the seven-stranded wire. Once the center wire (0.00135” in diameter) was found, the other six wires were stripped back and cut off. The third configuration involved inserting a piece of the smaller wire into one end of a stainless-steel tube. That end of the tube was crimped and then secured with a spot welder. The same was done on the other side with the larger wire. Five samples of each condition were fabricated. A tensile tester was created using a strain gauge, microstep driver, microslider, servo motor, pin vise, and an Arduino Uno. The tensile tester measured the break load (grams) and displacement (millimeters) that the samples endured prior to failure. The maximum break loads of each of the five samples of one condition were averaged to get a representative value at which each configuration broke.
The average break loads for the knotted, stripped, and crimped and welded samples were 157.7, 129.2, and 87.3 grams, respectively. The expected break point for the knotted sample was that the smaller wire would break at the knot. For the stripped sample, the 0.00135” center wire would break. For the welded sample, the smaller wire would break. The instances in which the samples broke as expected resulted in break loads of 205.8, 300.1, and 215.1 grams. Some of the welded samples did not achieve a solid joint between the wires and the tube. In these cases, the smaller wire pulled out of the tube after a small amount of force was applied. The stripped wire was the only condition that had a maximum value that surpassed the strength of the 0.002” wire. An analysis of variance (ANOVA) was conducted with the few samples that were tested and showed no significance between the configurations. However, it is noteworthy that the 0.00135” wire of the stripped sample endured a break load greater than the 0.002” wire. An advantage with the stripped sample is that it is already connected to a multi-stranded wire that is stronger than the 0.002” wire.
Other variables influenced the resulting break loads. Microbends during fabrication could have compromised the sample’s strength. For some samples, the wire slipped out of the holding rig or broke at the pin vise. Additionally, the tensile tester took mass readings every millimeter which produced sharp and choppy graphs. More testing needs to be conducted and modifications made to fabrication and testing procedures to verify the reliability of the collected data.
Overall, the stripped configuration could be a viable option for subcutaneous routing. Pairing it with interfascicular implantation would be a step closer towards making sensory restoration a reality. While this study dealt with wires, micrometers in magnitude, the big positive impact of these electrodes on restoring the sense of touch for those living with limb loss would enable them to feel the world again through their prosthetics.
2023 Wen H. Ko Summer Internship Program for funding and Case Western Reserve University for facilities