Augmented Transcutaneous Stimulation Using an Injectable Electrode

Abstract

Minimally invasive neuromodulation technologies seek to marry the neural selectivity of implantable devices with the low-cost and non-invasive nature of transcutaneous electrical stimulation (TES). The Injectrode® is a needle-delivered electrode that is injected onto neural structures under image guidance. Power is then transcutaneously delivered to the Injectrode using surface electrodes. The Injectrode serves as a low-impedance conduit to guide current to the deep on-target nerve, reducing activation thresholds by an order of magnitude compared to using only surface stimulation electrodes. To minimize off-target recruitment of cutaneous fibers, the energy transfer efficiency from the surface electrodes to the Injectrode must be optimized. TES energy is transferred to the Injectrode through both capacitive and resistive mechanisms. Electrostatic finite element models generally used in TES research consider only the resistive means of energy transfer by defining tissue conductivities. Here, we present an electroquasistatic model, taking into consideration both the conductivity and permittivity of tissue, to understand transcutaneous power delivery to the Injectrode. The model was validated with measurements taken from (n=4) swine cadavers. We used the validated model to investigate system and anatomic parameters that influence the coupling efficiency of the Injectrode energy delivery system. Our work suggests the relevance of electroquasistatic models to account for capacitive charge transfer mechanisms when studying TES, particularly when high-frequency voltage components are present, such as those used for voltage-controlled pulses and sinusoidal nerve blocks.

Microneurography as a Minimally Invasive Method to Assess Target Engagement During Neuromodulation

“…In this study, we characterized cuff electrodes, longitudinal intrafascicular electrodes (LIFE), an intrafascicular electrode used in pre-clinical studies (Yoshida and Stein, 1999;Nicolai et al, 2020), and microneurography microelectrodes, a clinical microelectrode used routinely in humans (Macefield, 2021), in their ability to measure neural activity in the peripheral nervous system. The cervical vagus, a major autonomic nerve innervating organs in the thorax and abdomen as well as muscles in the throat, and the great auricular nerve (GAN), a sensory nerve innervating the ear and periauricular region, were used as model nerves due to their relevance in existing peripheral neuromodulation therapies -invasive vagus nerve stimulators for epilepsy and depression (Johnson and Wilson, 2018) and non-invasive aVNS (Verma et al, 2021a), respectively.…”

“…Neuromodulation therapies can lack specificity of target engagement (Heusser et al, 2016;De Ferrari et al, 2017). In these therapies, the ability to directly measure neural activation around the stimulation site and understand fiber types activated could be critical to understand mechanism of action and enable electrode contact position adjustments to enable a more efficacious therapy (Verma et al, 2021a). Such measurements of local neural target engagement could be done either acutely in the doctor's office during placement and titration of the therapy or chronically implanted with the therapy device.…”

“…Similar to this existing human use, a microneurography microelectrode may be inserted percutaneously during clinical trials and/or at the deployment of neuromodulation therapies to map sensory innervation (Meier et al, 2018) to determine non-invasive stimulation electrode placement. The temporarily inserted microelectrode can also measure stimulation evoked on-and off-target neural activation and that data can be used in real-time to adjust stimulation electrode position, design, and stimulation parameters (Verma et al, 2021a). To aid in the translation of the microneurography technique to routine clinical use, the technique needs to be easy to perform and should be deployed within minutes.…”

“…Direct and real-time measures of local neural target engagement are critical as surrogate or indirect measures can be confounded by muscle activity (Feucht and Ward, 2021), poor time resolution, etc. (Verma et al, 2021a).…”

“…Inconsistent local neural target engagement and poor isolation of on-and off-target neural substrates is critically limiting neuromodulation therapies (Heusser et al, 2016;De Ferrari et al, 2017;Verma et al, 2021a). Direct measurements of neural activity during the acute deployment of neuromodulation therapies could 1) guide electrode design and placement to chronically improve consistency of local target engagement and 2) prove critical for understanding on-and off-target neural substrates and therapeutic mechanisms of action (Verma et al, 2021a).…”