Prof. Jacob Robinson and a team of researchers at Rice University (Houston, TX) developed a prototype of an implantable neural stimulator — the size of a grain of rice — that needs no batteries. Instead, it is powered wirelessly by means of an externally applied magnetic field.
Tech Briefs: How did this project get started?
Prof. Jacob Robinson: We started creating devices for stimulating and recording brain activity because we wanted to create ways that would allow us to learn how the brain works and create new types of electrically powered biomedical devices that could out-perform traditional pharmaceuticals. We began looking at the types of challenges you have when you try to send signals in and out of the body. We found that the typical kinds of electromagnetic waves we use for communication don’t do well inside the body because they’re absorbed. But magnetic fields do a pretty good job of penetrating the body. However, it’s hard to convert that magnetic field into something useful. We had read about a material called magnetoelectrics that converts magnetic fields into electric fields. This was special because electric fields are what we use to communicate with the nervous system — we can stimulate with electric fields. So, if we had this material that could take magnetic fields, which don’t really interact with anything, turn it into an electric field, then we could have a device that mediates this signal that passes through the body to a signal that interacts with nervous tissue. So, when we read reports of people using magnetoelectric materials, we became interested in figuring out how we could use this phenomenon to create miniature bioelectronic devices.
Tech Briefs: How does a magnetoelectric device convert magnetic fields into electric fields?
Professor Robinson: The device consists of two layers of different materials joined to each other in a single film. The first layer, a magnetostrictive foil of iron, boron, silicon, and carbon, vibrates at a molecular level when it’s placed in a magnetic field. The second layer, a piezoelectric crystal, converts the mechanical stresses generated by the foil directly into voltage. However, the amplitude of the voltage is too small to be useable. We solved that problem by tuning the applied alternating magnetic field to the natural mechanical resonance frequency of the magnetostrictive material, which is in the range of 200 – 300kHz.
Tech Briefs: Do you then apply that signal to the brain?
Professor Robinson: No, that’s a much higher frequency than what is natural for neural activity. For that we needed something in the range of 100 – 500 Hz. To solve that problem, we developed a circuit to modulate the high frequency at the rate we needed. We created two rectifying circuits, one positive-going and one negative-going, that were each connected to different magnetoelectric films. These films vibrated at different frequencies. By switching the applied magnetic field between the two different resonant frequencies we can create the positive and negative pulses at the 100-500 Hz needed for a neural stimulator.
Tech Briefs: What kinds of conditions could be treated with this method?
Professor Robinson: While battery-powered implants are frequently used to treat epilepsy and reduce tremors in patients with Parkinson’s disease, research has shown that neural stimulation could also be useful for directly treating depression, and obsessive-compulsive disorders, and also treating chronic, intractable pain, which often leads to anxiety, depression, and opioid addiction. We believe that our technology can become a viable alternative to the use of drugs for treating these conditions.
Tech Briefs: How do you generate the magnetic field?
Professor Robinson: We have a small battery-operated oscillator that sends a current through a coil of wire.
Tech Briefs: How would a person use this device?
Professor Robinson: Depending upon the condition being treated, the stimulator, which is about the size of a grain of rice, would be implanted in a specific area of the brain. It could also be attached to a particular nerve to treat pain or even delivered through the cardiovascular system. The device that generates the magnetic field could be worn as a band near the implantation site. The stimulator would remain implanted, but the magnetic generator would only need to be present to activate the device when a treatment is needed.
Tech Briefs: Have you tested this on a human?
Professor Robinson: Not yet on a human, but we did try it out on rodents. We placed stimulators beneath the skin of rodents that were free to roam throughout their enclosures. In one experiment we showed that rodents preferred to be in portions of the enclosures where a magnetic field activated the stimulator and provided a small voltage to the reward center of their brains.
Tech Briefs: What are your next steps on this project?
Professor Robinson: We will use our wireless power transfer technology to power CMOS chips that can be placed in different parts of the body and perform more complex procedures. We could even send digital signals with the magnetic field to program these chips to deliver specific types of stimulation that could help regenerate nerves, reduce pain, or even reduce inflammation and arthritis by stimulating the vagus nerve.
An edited version of this interview appeared in the September 2020 issue of Tech Briefs.