Wireless power supply for implants

Medical implants are dependent on a secure power supply. Ultrasound-based wireless technologies will offer an effective solution in the future.

Electricity can be transmitted through the air. That’s nothing new. Nikola Tesla already made light bulbs light up without tangled wires as early as 1890. Tesla’s research came to a halt back then because the financial backers had invested in copper mines. After that, it took over a hundred years before scientists began to devote serious attention again to wireless power transmission. Today, this now promising industry of the future relies on four technologies: inductive, capacitive, electromagnetic (microwaves, lasers) and mechanical (ultrasound, vibration).

Inductive variants (e.g. in charging cradles for mobile phones and small electronic devices) are the most widespread with efficiency levels of over 90 percent, several hundred watts of charging power, and ranges of typically a few centimeters to a few meters. Capacitive coupling, on the other hand, is of little practical significance.

Electromagnetic power transmission basically works like transmitting radio signals with electromagnetic waves. Although that allows vast transmission distances, it reduces the efficiency level considerably. One of the distant future projects, for example, is power supply from space via laser.

Acoustics for implants

Mechanical power transmission uses sound or vibration waves, which a receiver converts into electricity using piezoelectric or triboelectric effects.

While piezo elements generate a voltage when force is applied, the triboelectric variant only requires contact with another material with a different electron affinity. When the two are separated again, the difference in charge creates a voltage. Pulsed ultrasound sources, for example, ensure the rapid sequence.

Despite low efficiency levels and power levels in the milliwatt range, this method is attracting a great deal of attention, particularly in medical technology. After all, it is the safest and therefore predestined as a power supply, especially for implants. In addition, ultrasonic waves in particular penetrate liquids, body tissue and metal housings more easily than electromagnetic waves.

Ultrasound drives nanogenerators

Last year, researchers from the South Korean Sungkyunkwan University presented ultrasound-driven triboelectric nanogenerators (TENG) for implantable medical devices. According to the scientists, the generator implanted in pig tissue at a depth of five millimeters generates 156 microamps at up to 2.4 volts.

The Belgian research center Imec for nano- and microelectronics, together with the Delft University of Technology, have also developed an ultrasound-driven power supply as part of the ERC-funded project “Intranet of Neurons”. It consumes less than half the electricity of other systems.

The scientists rely on a unique adiabatic drive technology that follows the concept of global charge redistribution (GCR). Adiabatic systems are highly efficient since they do not exchange thermal energy with the environment.

Unlike conventional adiabatic methods, the Imec approach does not require external capacitors to redistribute the charges. This task is performed by the parasitic capacitors of the ultrasonic transducer array.

The 65 nm CMOS chip has a fully integrated 116×116 micrometer driver unit that saves 69 percent of the power consumption of conventional class D amplifiers.

For use in vivo, beam steering up to angles greater than 45 degrees is critical in order to maximize power output and compensate for micromovements and misalignments of the brain (as occur, for example, during operations and breathing). With the introduction of a beam steering controller, the Imec GCR system enables beam steering of up to 53 degrees.