Single transducer combines bioimplantable magnetic and piezoelectric collection

The dual magnetic field and ultrasound induced generator (MUDG) produces a very high effective power of 52.1 mW at a power density of 597 mW/cm³ at a magnetic field of ∼500 μT and 675 mW/cm² ultrasound intensity.

The results show that MUDG can overcome the limitations of traditional wireless power transmission systems for human body applications, providing a new platform with the advantages of ultrahigh power density, small size, biocompatibility, and tolerance to angular deviations. Tests and evaluation were carried out for magnetic field harvesting, ultrasonic energy harvesting, and combined harvesting.

Harvesting magnetic field energy for medical devices

The magnetoelectric device works by a two-step process of converting an AC magnetic field into an electric field (direct magnetoelectric effect) or vice versa. Initially, the input magnetic field induces a stress (S) in the magnetostrictive layers via magnetostriction. This stress is then transferred to the piezoelectric layer via elastic coupling. Then, a mechanical stress (T) appears in the piezoelectric layer due to elastic stiffness, which leads to the development of surface charge density (D) or polarization via the direct piezoelectric effect.

This charge separation generates an output voltage or electric field in the piezoelectric layer. The deformation in the piezoelectric layer generates electric charges from the MUDG device via the direct piezoelectric effect. The use of a disc-shaped piezoelectric layer in a radial mode has the unprecedented advantage of being tolerant to angular deviations, offering a larger surface area that is suitable for ultrasonic conversion.

The mechanism of operation of the MUDG device uses the principle of direct magnetoelectric and piezoelectric effects, occurring at different stages under the influence of a magnetic field and ultrasound, respectively.

At the initial stage, when there is no magnetic field or ultrasound, the MUDG device does not generate any electric charge. However, when a magnetic field is applied, the ME device (which contains a piezoelectric layer) experiences stress and generates an electric charge via the direct magnetoelectric effect, resulting in the generation of electricity.

When additional ultrasonic pressure is applied together with the magnetic field, the ME device experiences even greater stress in the piezoelectric material, which generates an additional electrical charge and consequently a higher output voltage.

When the piezoelectric layers are compressed by the magnetic field and ultrasonic pressure, electrons flow from one electrode to the other through the external circuit, causing the ME device to produce a positive signal. Similarly, when the piezoelectric layer expands or decompresses, electrons flow in the opposite direction through the external circuit, generating a negative signal.

In the absence of ultrasound and solely under the influence of a magnetic field, the ME device produces a relatively low output voltage due to the reduced stress in the piezoelectric layer. Finally, in the absence of ultrasound and a magnetic field, the ME device cannot generate any output signal because no stress is generated in the piezoelectric layer.

The use of ultrasonic energy acquisition in medical implants

To evaluate the output performance of the MUDG devices, they were subjected to ultrasonic pressure using a commercial transducer and the output was measured in water to mimic soft tissue at 5 and 15 mm distances (Fig. 2). A specially designed 3D printed holder was used to hold the MUDG/transducers and prevent any deviation. The ultrasonic powered MUDG devices were evaluated by measuring their output signals over a wide frequency range from 220 to 260 kHz.