Safe inductive power transmission to millimeter-sized implantable microelectronics devices.

Area of Science:

• Biomedical Engineering
• Electrical Engineering
• Implantable Devices

Background:

• Power transfer efficiency (PTE) and power delivered to the load (PDL) are critical for inductive links powering millimeter-sized implants.
• Existing research often suggests high power carrier frequencies (fp) (100s of MHz) to maximize PTE.

Purpose of the Study:

• To investigate the trade-offs between PTE, PDL, and specific absorption rate (SAR) constraints at various inductive link frequencies.
• To determine optimal operating frequencies for safe and efficient power delivery to millimeter-sized implants.

Main Methods:

• Developed a closed-form power function relating maximum safe power transfer levels to frequency under SAR constraints.
• Optimized three inductive link designs at 50 MHz, 200 MHz, and 400 MHz for a 1 mm³ implant.
• Conducted simulations to evaluate PDL and PTE at different frequencies and transmitter coil sizes.

Main Results:

• Operating at lower frequencies (10s of MHz) allows for higher allowable PDL within SAR constraints compared to higher frequencies.
• Reducing fp from 200 MHz to 50 MHz, with a smaller transmitter coil, increased PDL by ~7.8 times.
• This optimization resulted in a 52% decrease in PTE, highlighting a key design trade-off.

Conclusions:

• Lowering inductive link operating frequency to the 10s of MHz range is a viable strategy for maximizing power delivered to millimeter-sized implants under SAR safety limits.
• The study provides a quantitative relationship between frequency, PDL, PTE, and SAR, aiding in inductive link design for medical implants.