A new material has been used to precisely stimulate neurons remotely and to bridge the gap in a sciatic nerve in a rat model that has been severed.
Researchers have long recognized the therapeutic potential of magnetoelectrics materials that can convert magnetic fields into electric fields to stimulate neural tissue non-invasively and aid in the treatment of neurological disorders or nerve injury. The issue, however, is that neurons struggle to respond to the shape and frequency of the electric signal produced by this conversion.
Rice University neuroengineer Jacob Robinson and his team have now designed the first magnetoelectric material that not only solves this issue but performs the magnetic-to-electric conversion 120 times faster than similar materials. The material’s qualities and performance could have a profound impact on neurostimulation treatments, making for significantly less invasive procedures, Robinson said.
Ideal Neurostimulation Material
Instead of implanting a neurostimulation device, tiny amounts of the material could simply be injected at the desired site. Moreover, given magnetoelectrics’ range of applications in computing, sensing, electronics and other fields, the research provides a framework for advanced materials design that could drive innovation more broadly.
“We asked, ‘Can we create a material that can be like dust or is so small that by placing just a sprinkle of it inside the body you’d be able to stimulate the brain or nervous system?’”
said lead author Joshua Chen, a Rice doctoral alumnus.
“With that question in mind, we thought that magnetoelectric materials were ideal candidates for use in neurostimulation. They respond to magnetic fields, which easily penetrate into the body, and convert them into electric fields ⎯ a language our nervous system already uses to relay information.”
Magnetorestrictive Element
The researchers began with a magnetoelectric material comprising a piezoelectric layer of lead zirconium titanate sandwiched between two magnetorestrictive layers of metallic glass alloys, or Metglas, which can be magnetized and demagnetized rapidly.
Credit: Robinson lab/Rice University.
The magnetorestrictive element vibrates when a magnetic field is applied, explained Gauri Bhave, a former researcher in the Robinson laboratory who now works in technology transfer for Baylor College of Medicine.
“This vibration means it basically changes its shape,” Bhave said. “The piezoelectric material is something that, when it changes its shape, creates electricity. So when those two are combined, the conversion that you’re getting is that the magnetic field you’re applying from the outside of the body turns into an electric field.”
However, the electric signals magnetoelectrics generate are too fast and uniform for neurons to detect. The challenge was to engineer a new material that could generate an electric signal that would actually get cells to respond.
Nonlinear Response
The relationship between the electric field and the magnetic field in all other magnetoelectric materials is linear, and the researchers needed a material with a nonlinear relationship.
“We had to think about the kinds of materials we could deposit on this film that would create that nonlinear response,”
Robinson said.
The researchers stacked layers of platinum, hafnium oxide, and zinc oxide on top of the original magnetoelectric sheet. Finding fabrication procedures that were compatible with the materials was one of the obstacles they confronted.
“A lot of work went into making this very thin layer of less than 200 nanometers that gives us the really special properties,”
Robinson said.
Bridging Nerve Gaps
As proof of concept, the researchers used the material to stimulate peripheral nerves in rats and demonstrated the material’s potential for use in neuroprosthetics by showing it could restore function in a severed nerve.
“We can use this metamaterial to bridge the gap in a broken nerve and restore fast electric signal speeds,” Chen said. “Overall, we were able to rationally design a new metamaterial that overcomes many challenges in neurotechnology. And more importantly, this framework for advanced material design can be applied toward other applications like sensing and memory in electronics.”
Robinson, who drew inspiration for the new material from his doctoral work in photonics, said it’s “really exciting” that we can now design devices or systems using materials that have never existed before, rather than being limited to those found in nature.
“Once you discover a new material or class of materials, I think it’s really hard to anticipate all the potential uses for them. We’ve focused on bioelectronics, but I expect there may be many applications beyond this field,”
said Robinson, a professor of electrical and computer engineering and bioengineering.
Abstract
Magnetoelectric materials convert magnetic fields into electric fields. These materials are often used in wireless electronic and biomedical applications. For example, magnetoelectrics could enable the remote stimulation of neural tissue, but the optimal resonance frequencies are typically too high to stimulate neural activity. Here we describe a self-rectifying magnetoelectric metamaterial for a precisely timed neural stimulation. This metamaterial relies on nonlinear charge transport across semiconductor layers that allow the material to generate a steady bias voltage in the presence of an alternating magnetic field. We generate arbitrary pulse sequences with time-averaged voltage biases in excess of 2 V. As a result, we can use magnetoelectric nonlinear metamaterials to wirelessly stimulate peripheral nerves to restore a sensory reflex in an anaesthetized rat model and restore signal propagation in a severed nerve with latencies of less than 5 ms. Overall, these results showing the rational design of magnetoelectric metamaterials support applications in advanced biotechnology and electronics.
Reference:
- Chen, J.C., Bhave, G., Alrashdan, F. et al. Self-rectifying magnetoelectric metamaterials for remote neural stimulation and motor function restoration. Nat. Mater. (2023). doi: 10.1038/s41563-023-01680-4
Top Image: credit Josh Chen/Rice University.