Poster Z20 - Macro-Sieve Electrode Stimulation Frequency and Rat Detection Threshold

Introduction: Upper extremity amputation affects approximately 41,000 persons in the United States (Frontera et al, 2020). Sensory feedback is critical for the complex functions required of upper limb prostheses. Current market models have concerns with tactile and proprioceptive sensitivity. One type of prosthesis that is being pioneered to address this concern is neural prostheses: a type of electrode array that is implanted within the nerve to record and/or stimulate electrical activity (Thompson et al, 2016). Another major concern for peripheral nervous system (PNS) neural prosthesis electrodes is the longevity in the electrode-tissue interface due to nerve retraction from the interface point (Thompson et al, 2016). The macro-sieve electrode (MSE) is an interface intended to be an intermediary between the injured PNS and external sensors to allow for more sensitive prosthetic control. (Chandra et al, 2021). The MSE’s regenerative design where the nerve grows through multiple channels is promising for maintaining a strong and lasting connection between the electrode and nerve. The MSE is able to achieve further sensitivity through selective recruitment of axon clusters using current steering.

Materials and

Methods: The MSE consists of three concentric rings, the central zone, a polyimide ring, and an outer printed circuit board. The central, active, zone consists of 9 transit zones through which the severed axon regenerates. Contained within this zone there is also a circular hub with radiating spokes, which combined hold eight platinum-iridium leads. Within the circular hub, there are four core leads and within the radiating spokes there are four peripheral leads. There are silicon guidance conduits fastened to each side of the polyimide ring, which regenerating axons are guided through to the active zones.

Rats began behavior training in Skinner boxes with auditory and light combination. Once complete, the rats’ sciatic nerves were severed, and then implanted with the Macro-Sieve Electrode. Following surgical recovery, training resumed. The second step of behavior training combined auditory and light stimulation with electric stimulation. The next progression was electric stimulation only, and then the definitive task with only electrical stimulation. For the definitive task, the rats performed a yes/no discrimination task based on multi-channel stimulation that delivered simultaneous and equal currents to the eight MSE channels. The rats were tasked to perform sets of three 20-minute trials at different amplitudes for each selected frequency.

Data was collected using an RZ5D processor, MED-PC, and Matlab. Based on the collected data, psychometric curves were calculated to determine the stimulus threshold and describe the relationship between stimulus amplitude function and forced choice. The calculations were done using the psignifit4 toolbox, integrated with the Bayesian inference method.

Results, Conclusions, and Discussions: Results and

Discussion:
Based on our findings, changing the stimulation frequency signal affects the sensitivity of what the animals feel. The rats’ responses reach a plateau at every measured frequency, but the amplitude at which that occurs depends on the frequency being tested. Additionally, the amplitude at which the rats reached the determined level of acceptable accuracy differed based on frequency.

Conclusion:
The project determined that different combinations of MSE stimulation pulse frequencies resulted in varied detection thresholds. This continues to develop the foundation that MSE can be used as a sensory feedback interface, which could clinically benefit patients who have upper limb amputations to have more life-like prostheses and increase long-term prosthesis adoption and use time. Future direction includes utilizing different channel combinations, to determine their effect on detection threshold.

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