Augmented Transcutaneous Stimulation Using an Injectable Electrode: A Computational Study

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.

Efficacy of Bone Stimulators in Large-Animal Models and Humans may be Limited by Weak Electric Fields Reaching Fracture

“…Both computational and cadaver models in our study showed that 1) conventional EBGSs deliver lower electric elds to deeper fracture sites (Fig 3A ) and that 2) Injectrode-augmented EBGSs provides more of a bene t ('gain') over conventional EBGSs at deeper fracture sites. These ndings explain why, in a previous study on the Injectrode for noninvasive neural stimulation [17], the Injectrode was shown to improve neural activation by more than an order of magnitude over conventional noninvasive stimulation, while in the current study, the Injectrode is showing a more limited bene t of a 37% increase in electric eld at the fracture site. The current work is studying the tibial bone, where the fracture is at a depth of only a few mm from the stimulation electrodes, while in the previous neural study, the nerve of interest was at a depth of ~2 cm from the stimulation electrodes.…”

“…The cadaver measurements, expected to be re ective of a stage of healing between early-and late-stage due to tissue preparation, resulted in a voltage of 64 mV (SD 45 mV) at the fracture site. The model prediction was within 1 SD of the cadaver measurements, which is a good t considering the known variations in tissue conductivity at low frequencies [17]. The model underpredicted the average measured voltage at the fracture site by 54-66%.…”

“…This prepolymer sets into a solid in the body and forms a conductive interface with the target tissue. In principle, the bene t of this minimally invasive electrode is that it offers increased stimulation strength by targeting the fracture site directly, similar to direct current electrical stimulation (DCES), by routing current from the EBGS TENS electrodes to the fracture site [17]. An Injectrode 'collector' can be added subcutaneously and routed to the bone defect in cases of a deeper fracture (Fig.…”

“…The Injectrode is a minimally invasive method for delivering electrical stimulation that can be injected sterilely under imaging guidance and remains entirely subcutaneous. Energy is wirelessly delivered to the Injectrode using transcutaneous coupling from external surface electrodes [17]. In this manner, there is no open wound, and the technique is hypothesized to be less susceptible to infections while increasing the electric current reaching the fracture site.…”

“…The model overpredicted the average measured voltage at the fracture site by 690%. The error may again be coming from the difference in skin impedance between the model and the cadaver, a sensitive parameter [17]. The cadaver skin impedance could be lower due to the application of Nair™ for an extended duration to remove the thick hair of the sheep.…”