A Multi-Dimensional Analysis of a Novel Approach for Wireless Stimulation

Abstract

The elimination of integrated batteries in biomedical implants holds great promise for improving health outcomes in patients with implantable devices. However, despite extensive research in wireless power transfer, achieving efficient power transfer and effective operational range have remained a hindering challenge within anatomical constraints.

Objective: We hereby demonstrate an intravascular wireless and batteryless microscale stimulator, designed for (1) low power dissipation via intermittent transmission and (2) reduced fixation mechanical burden via deployment to the anterior cardiac vein (ACV, ∼3.8 mm in diameter).

Methods: We introduced a unique coil design circumferentially confined to a 3 mm diameter hollow-cylinder that was driven by a novel transmitter-based control architecture with improved power efficiency.

Results: We examined wireless capacity using heterogenous bovine tissue, demonstrating >5 V stimulation threshold with up to 20 mm transmitter-receiver displacement and 20° of misalignment. Feasibility for human use was validated using Finite Element Method (FEM) simulation of the cardiac cycle, guided by pacer phantom-integrated Magnetic Resonance Images (MRI).

Conclusion: This system design thus enabled sufficient wireless power transfer in the face of extensive stimulator miniaturization.

Significance: Our successful feasibility studies demonstrated the capacity for minimally invasive deployment and low-risk fixation.

Fig. 1:. Circuit and Coil Design

(A) Transmitter and receiver circuit design. Highlighted in the green is the control circuitry that determines pacing rate, rhythm, and level of power input. Highlighted in grey is the receiver circuitry for power conversion. Highlighted in red are the transmitter and receiver coils as positioned in the circuit as well as their structure. (B) Anatomical position of the subcutaneous transmitter in blue and intravascular receiver in yellow, depicted on a sagittal section of a human MRI.

Fig. 2:. In Vitro Experimental Setup.

Photo of experimental setup for in vitro analysis of power transfer efficiency over various distances and misalignment levels. A segment of bovine tissue containing a mixture of muscle, adipose, and bone was placed in between the two coils to mimic the environment of the body.

Fig. 3:. MRI-Phantom Study Data Processing.

(A) Coronal MRI slices and corresponding “pacer” phantom placements across the three representative slices; (B) The location of the red landmarks was calculated by taking the average of their position to create a 2-D version of the object. The centroid and normal vector were then computed based on the averaged red marks that represented the four corners.

Fig. 4:. In Vitro Experiments.

(A) A schematic of the experimental configuration illustrates the transmitter and receiver position across a segment of bovine tissue. (B) Output voltage decreased as the displacement between the transmitter and receiver, δ, increased from 0 to 30 mm at δ = 20 mm and δ = 30 mm. (C) Output voltage fluctuated in response to x-axis vertical misalignment, α, ranging from 0 to 45 degrees at δ = 20 mm and δ = 30 mm. (D) Output voltage fluctuated in response to y-axis vertical misalignment, α, ranging from 0 to 45 degrees at δ = 20 mm and δ = 30 mm. (E) Output voltage decreased in response to x-axis horizontal misalignment, α, ranging from 0 to 45 degrees at δ = 20 mm and δ = 30 mm. (F) Output voltage decreased in response to y-axis horizontal misalignment, α, ranging from 0 to 45 degrees at δ = 20 mm and δ = 30 mm. (G) The effect of B-field (red arrows) capture is shown with a planar circular receiver coil in the presence of absence of misalignment. (H) The effect of B-field (red arrows) capture is shown with a half-cylindrical receiver coil in the presence of absence of misalignment. (B-F) Output voltage was compared against the average pacing amplitude of 0.80 V (dotted line at the upper boundary of the red zones) and maximum potential pacing amplitude of the market-released leadless pacemaker at 5 V (dotted line at the upper boundary of the green zones).

Fig. 5:. Cardiac Cycle Simulation Experiments.

(A) Each box represents one timeframe in the cardiac cycle in (i)-(ix). Cardiac contraction and consequential pacer positional and angular motion are shown in the schematic on the left inset. Stimulation of the interaction between the transmitter and receiver coil is demonstrated by the magnetic field shown on the right inset. (x) demonstrates a zoomed-in view of the events in box (ix). (B) Computed power transfer efficiency (PTE) over the nine captured frames of the cardiac cycle. (C) Computed maximum potential voltage threshold over the nine captured frames of the cardiac cycle given an average input power of 1.26 mW. (D) Electric field simulation of Frame 5 of the cardiac cycle at which time point maximum coupling was observed. (E) Various tissue conductivities and densities, which, in combination with the Electric field, determine SAR.