Unique technique transfers power to medically-implanted device

Article By : Bill Schweber

Researchers have developed a technique to transfer power to a neural stimulator.

Engineers are used to dealing with diverse “effects” ranging from well-known to lesser-known ones. In the former category, for instance, there’s the piezoelectric effect and the skin effect; in the latter category, there are ones such as the Coanda effect. Whether known or obscure, these physics-based phenomena are often exploited as building blocks for sensors, special materials, unique functions, and more.

Recently, a research team at Rice University conceived, built, and tested what they maintain is the first energy-capture and conversion device driven by external magnetic fields. It’s largely unattenuated by the body-tissue mass and avoids the issues of absorption by the body or differences in impedance at interfaces between air, bone, and tissue associated with the use of RF, ultrasound, light, and even magnetic coil.

Researchers used this power to drive a neural stimulator producing different waveforms and patterns; such stimulation is already used to treat conditions including Parkinson’s disease, depression, pain, and obsessive-compulsive disorders.

The power-transfer arrangement literally merges two unrelated physical phenomena: the magnetostrictive effect—a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization—and the piezoelectric effect. That allows it to convert a magnetic field to an electric field and voltage.

The research team used a material that generates a voltage using mechanical coupling between the magnetostrictive and piezoelectric layers in a thin film rather than an implanted coil. The external varying magnetic field creates strain in the magnetostrictive layer, and that strain, in turn, exerts a force on the piezoelectric layer to generate a voltage. The combined magneto-electronics (ME) can be driven by weak magnetic fields on the order of a few millitesla (Figure 1).

diagram of how magnetostrictive films convert alternating magnetic fields into a voltageFigure 1 A magnetostrictive device is applied on a freely-moving rat for wireless neural stimulation (a). A resonant response curve shows the maximum voltage is produced when the magnetic field frequency matches an acoustic resonance at 171 kHz (b). The device testing setup with a permanent magnet applies a bias field, and an electromagnetic coil applies an alternating magnetic field (c). Source: Rice University

To improve energy-transfer efficiency, researchers also applied a constant-bias field using a permanent magnet or an electromagnet. Since the strain in the magnetostrictive material is a sigmoidal (S-shaped) function of the magnetic field strength, the change in voltage produced by the alternating field is largest when the field oscillates about the midpoint of the sigmoid (Figure 2).

graph showing magnetostrictive film output voltage as a function of bias fieldFigure 2 Magnetostrictive film output voltage as a function of bias field; the peak resonance voltage is significantly increased by a modest magnetic bias field. Source: Rice University

Researchers have tested the combined power source plus stimulator by implanting it into a rodent’s brain to invoke various types of neural stimulation. The entire unit—power subsystem and neural stimulator—is smaller than grain rice and had to be entirely custom built. The highly-detailed and readable paper has been published in a recent issue of Neuron, titled “Magnetoelectric materials for miniature, wireless neural stimulation at therapeutic frequencies,” and it provides an interesting explanation of the challenges posed by alternative power-transfer techniques. While it is behind a paywall, fortunately, the identical pre-print is posted, and this general-interest news writeup from Rice University has some interesting personal perspectives and quotes.

Is this power transfer or energy harvesting? Since the external magnetic field isn’t “just there” and thus available in the first place, I’m not sure if it qualifies as true harvesting. On the other hand, it certainly has some attributes of harvesting, at least in a conceptual sense. Let’s just say it is a hybrid of both transfer and harvesting.

Have you ever used a scheme that could be considered energy harvesting but was really more of a unique energy-transfer technique? What issues did you have to address and overcome?

This article was originally published on EDN.

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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