⚡ electrical engineering

Electric Field Attenuation Techniques for Inductive Wireless Charging of Medical Implants

This paper presents a framework for safely extending mid-range wireless charging of medical implants by demonstrating that combining high-permittivity dielectric shielding, distributed resonant tuning capacitors, and multi-coil array topologies can attenuate electric field emissions from 1416 V/m to 82 V/m—meeting strict safety regulations—without compromising magnetic field strength or power transfer efficiency.

Original authors: Sam Boeckx, Pieterjan Polfliet, Lieven De Strycker, Liesbet Van der Perre

Published 2026-07-01

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sam Boeckx, Pieterjan Polfliet, Lieven De Strycker, Liesbet Van der Perre

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a medical implant inside a patient, like a pacemaker or a cochlear implant. To keep it running, doctors need to charge it wirelessly through the skin. The goal is to make this charger comfortable enough to use while the patient is sleeping or sitting on the couch, meaning the charger can be up to 10 cm (about 4 inches) away from the body.

However, there's a problem. When you try to send power through the air over that distance, the charger creates two invisible "fields":

The Magnetic Field (H-field): This is the "good guy." It's like a gentle, invisible hand that grabs the power and pulls it into the implant. We need this to be strong.
The Electric Field (E-field): This is the "troublemaker." It's like static electricity or a spark. If it gets too strong, it can irritate or stimulate nerves in the skin, which is painful or dangerous.

The paper focuses on a very strict safety rule (from Canada) that says this "spark" (the electric field) cannot be stronger than 83 volts per meter.

The researchers built a prototype charger. When they tested it, the "spark" was 1,416 V/m. That's nearly 17 times too strong! It was like trying to charge a phone with a lightning bolt.

To fix this, the team tried three different "shielding" tricks to calm down the spark without weakening the helpful magnetic hand. Here is how they did it, using simple analogies:

1. The "Sponge" Shield (High-Permittivity Material)

The Problem: The electric field was shooting out of the charger like a hose spraying water everywhere.
The Fix: They placed a special layer of material (Barium Titanate) between the charger and the person.
The Analogy: Imagine the electric field is a stream of water. The special material acts like a super-absorbent sponge. It soaks up the water (the electric energy) and spreads it out, so it doesn't spray violently onto the person.
The Result: This sponge absorbed a lot of the energy, dropping the "spark" from 1,416 down to 496. It was much better, but still too strong.

2. The "Team Effort" (Distributed Capacitors)

The Problem: The charger was using one giant capacitor (a battery-like component) to tune the system. This created a huge voltage spike in one spot, like a single person trying to lift a heavy piano alone.
The Fix: Instead of one giant component, they split the job into 16 smaller capacitors spread out along the wire coils.
The Analogy: Imagine you need to lift a heavy piano. Instead of one person straining to lift it all at once (creating a huge risk of dropping it), you get 16 people to share the load. Each person only lifts a tiny bit. By spreading the work out, the "pressure" (voltage) at any single point drops dramatically.
The Result: This was very effective. The "spark" dropped from 1,416 all the way down to 231. However, it was still a bit too high, and tuning 16 tiny components was a bit of a headache for the engineers.

3. The "Grid" Layout (Changing the Shape)

The Problem: The charger was made of two big coils. Big coils create a wide, messy electric field, like a large bonfire.
The Fix: They broke the charger into a 2x2 grid of four smaller coils.
The Analogy: Instead of one big bonfire, imagine four small, contained campfires. If you stand near one, you feel the heat, but the fire doesn't spread as wildly. Also, by arranging them carefully, they could make the magnetic fields cancel out the "messy" parts of the electric field in certain spots.
The Result: This helped a bit, dropping the "spark" to around 990, but on its own, it wasn't enough.

The Grand Finale: Combining All Three

The researchers realized that no single trick was enough to meet the strict safety rule. So, they combined all three:

They used the Sponge layer.
They used the 16-person Team (distributed capacitors).
They used the 4-coil Grid layout.

The Final Result:
When they put all three tricks together, the "spark" (electric field) dropped to 82 V/m.

Safety: This is just under the strict limit of 83 V/m. The charger is now safe for human nerves.
Power: The helpful "magnetic hand" remained strong (8 A/m), meaning the implant still gets the power it needs.
Efficiency: The charger works well, though it does use a tiny bit more electricity to run the complex setup.

In Summary:
The paper proves that you can build a safe, long-distance wireless charger for medical implants. You just can't rely on one simple fix. You have to use a "Swiss Army Knife" approach: absorb the energy with a special material, spread the electrical pressure across many small components, and arrange the coils in a smart grid. This allows the charger to work effectively without giving the patient a painful electric shock.

Technical Summary: Electric Field Attenuation Techniques for Inductive Wireless Charging of Medical Implants

Problem Statement
Inductive wireless power transfer (WPT) for implantable medical devices faces a critical regulatory and safety bottleneck: the trade-off between efficient power transfer over mid-range distances (up to 10 cm) and compliance with strict electric field (E-field) exposure limits. While magnetic fields (H-fields) are necessary for power transfer, the associated E-fields can induce nerve stimulation (NS) in human tissue. The Canadian RSS-102 standard, released in December 2023, sets a stringent reference level of 83 V/m to prevent NS, a limit currently the most restrictive in the regulatory landscape. Initial simulations of a comfortable mid-range charger (6.78 MHz, 10 cm distance) designed to deliver 100 mW to an implant revealed a peak E-field of 1416 V/m at a 2 cm distance (the closest human tissue can approach the charger casing). This value exceeds the regulatory limit by approximately 17 times, necessitating effective attenuation strategies that do not compromise the required H-field magnitude (targeting ~8 A/m) or power transfer efficiency (PTE).

Methodology
The research employed Finite Element Analysis (FEA) using Ansys HFSS to model and evaluate three distinct mitigation strategies. The baseline model consisted of a two-coil transmitter array operating at 6.78 MHz (an ISM band frequency), designed to cover a 50 cm × 25 cm area with a 10 cm vertical separation from the receiver. The study systematically applied and combined the following techniques:

High-Permittivity Dielectric Shielding: A layer of material was placed 1 cm above the transmit coils. The study analyzed material properties, specifically seeking a high relative permittivity ( $\epsilon_r$ ) and dielectric loss tangent ( $\tan \delta_e$ ) to absorb and redistribute E-fields, while maintaining low relative permeability ( $\mu_r$ ) and magnetic loss tangent ( $\tan \delta_m$ ) to ensure the H-field remains unaltered. Barium Titanate (BaTiO $_3$ ) was identified as a candidate material with $\epsilon_r \approx 900$ .
Distributed Tuning Capacitors: Instead of a single resonant capacitor, the tuning capacitance was distributed along the coil traces to form multiple sub-coils. This approach aims to lower the voltage swing across individual coil segments and spatially separate high-voltage nodes, thereby reducing local E-field concentration. The study evaluated configurations with 2, 4, 8, and 16 capacitors per coil.
Alternative Coil Topologies: The transmitter geometry was altered from a two-coil layout to a 2×2 array of smaller coils. This strategy seeks to confine the E-field to smaller areas and allows for selective activation or phase shifting of individual coils to manage field distribution and eliminate "dead zones" (areas of low H-field caused by field cancellation).

Key Contributions and Results
The paper quantifies the efficacy of each technique in reducing the E-field magnitude while preserving the H-field:

Dielectric Shielding: Applying a 1 mm layer of BaTiO $_3$ reduced the peak E-field from 1416 V/m to 496 V/m (a ~65% reduction). The H-field remained stable at approximately 8.35 A/m.
Distributed Capacitors: Increasing the number of distributed capacitors demonstrated a diminishing return pattern (resembling a 1/x curve).
- 2 capacitors: Reduced E-field to 842 V/m.
- 4 capacitors: Reduced E-field to 519 V/m.
- 16 capacitors: Reduced E-field to 231 V/m (an ~84% reduction from baseline).
- The H-field remained largely unaffected (~8.57 A/m), though system tuning complexity increased significantly due to mutual inductance between sub-coils.
Coil Topology (2×2 Array): Switching to a four-coil array reduced the E-field to 1032 V/m (~30% reduction). However, this initially created a transmission efficiency dead-zone (TEDZ) in the center due to field cancellation. Introducing phase differences (e.g., 90° shifts) between coils mitigated the TEDZ and further optimized the field, reducing the E-field to 929–989 V/m depending on the phase configuration, with H-field magnitudes ranging from 5.88 to 7.98 A/m.
Combined Approach: The most significant result was achieved by combining all three techniques: a 2×2 coil array, 16 distributed capacitors per coil, and a BaTiO $_3$ shielding layer. This configuration successfully attenuated the peak E-field to 82 V/m, bringing it just below the 83 V/m RSS-102 limit. The H-field was maintained at 8.12 A/m.

Significance and Claims
The authors claim that this research demonstrates a feasible framework for extending the application of wireless charging for medical implants to comfortable mid-range distances (up to 10 cm) without violating strict nerve stimulation safety limits. The study highlights that while individual techniques offer significant reductions, only their combination achieves full regulatory compliance.

The paper notes specific trade-offs:

Efficiency: The combined approach resulted in a slight decrease in Power Transfer Efficiency (PTE), with required transmit power increasing from 5.9 W to 18.8 W to maintain the target H-field.
Complexity: The distributed capacitor method and multi-coil topologies introduce significant challenges in system tuning and driver complexity. The authors acknowledge that increasing the number of coils or capacitors further would make tuning increasingly cumbersome and require more complex switching circuits or additional drivers.

The research concludes that by carefully selecting materials and optimizing circuit topology, it is possible to meet the stringent Canadian RSS-102 standard without compromising the magnetic field strength necessary for effective power transfer to medical implants. Future work suggested by the authors includes studying the effects of shielding thickness and height more broadly, independent of the specific application constraints.

1. The "Sponge" Shield (High-Permittivity Material)

2. The "Team Effort" (Distributed Capacitors)

3. The "Grid" Layout (Changing the Shape)

The Grand Finale: Combining All Three

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