Electrostimulation: The light approach to going wireless
A light-powered implanted electrostimulator can stimulate nerve cells.
Researcher Theresa Rienmüller
Fiona Dunlevy
December 2022
Electrostimulation – or using pulses of energy therapeutically - has been around for a while now. An electrostimulator is typically made up of an electrode positioned at the therapeutic site to be zapped, a power source, and a wire connecting them. This can lead to several problems. Batteries need to be replaced, wires can dislodge. And if an external power source is needed, wires running from inside to outside the body can become infected. A multinational collaboration plans to sidestep all these issues thanks to their light powered implantable electrostimulator.
Light up the pigments
The collaboration, involving academic teams from Austria, the Czech Republic and Croatia, have developed what they call organic electrolytic photocapacitors, or OEPCs for short. An OEPC is created by evaporating a series of organic pigments onto a film. The researchers chose cheap, non-toxic pigments that effectively absorb near-infrared light, at wavelengths known to penetrate human tissue particularly well. These pigments well known from their use in cosmetics, printing inks, outdoor paints and other industrial uses. From an electrical point of view, the basic building block of the OEPC is the junction of two of these pigments, the so-called “p-n” boundary, across which current can flow. “You shine the light on the OEPC, and this leads to accumulation of charge in the n-type semiconducting layer.,” explains Theresa Rienmüller, Institute of Health Care Engineering at TU Graz in Austria and corresponding author of a recent paper about the device (1). That accumulated charge is then used to stimulate cells in close proximity to the OEPC.
You shine the light on the OEPC, and this leads to accumulation of charge”
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All fired up
Collaboration team members at the Medical University of Graz tested the OPECs in dishes of cultured neurons to learn exactly how the device stimulated cells. They studied the electrophysiology of individual cells, showing that a single pulse of red light nudged the neuron into firing repetitive electrical signals, called action potentials. As Rienmüller explains, the charged up OEPC “feels like a change in the membrane potential” for the cell.
Rienmüller and her colleagues in Graz have another trick up their sleeve for improving the OEPCs. “We are from the engineering area,” she says, explaining that they use computer modelling to “try to simulate the function of the device and the effect on cells.” First the group performed a raft of experiments where they stimulated the OEPCs with light and measured the effect on cells. Then they used these experimental results to validate a computer model of how the device affects cells. “We can try out different scenarios,” says Rienmüller. “For example, if we decrease the light pulse, or increase the light strength, or change the size of the electrode, change the frequency and so on.”
We want to use this model and look for improvements in response to stimulation”
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Applications
The OEPC electrostimulator could have multiple applications, in different parts of the body, and Rienmüller has tried out several cell types. For the moment the team is focusing on how the OEPCs could treat traumatic brain injury, an area where electrostimulation is already used. Current devices, however, are charged up via wires left sticking out of the person’s head. This is both uncomfortable and risks infection. The wireless nature of the OEPCs sidesteps these risks.
Consortium colleagues in the medical university of Graz have developed a rat model of traumatic brain injury to test the OEPC electrostimulation in vivo. “We want to use this model and look for improvements in response to stimulation,” says Rienmüller. Ultimately the team will test if OEPC electrostimulation improves motor function and learning. “We are really currently at the experimental stage,” says Rienmüller, noting that the project is not yet close to clinical trials in humans. “We're currently working on biocompatibility tests. So what happens to the devices there for a couple of weeks or months?”
The potential advantages offered by the device are intriguing. Other electrostimulation protocols use pulsed direct current or photo-thermal energy, which can damage cells. The OEPCs, on the other hand, provides highly focused stimulation, since only cells in direct contact with the device will be stimulated. “This is the least harmful ways of stimulating,” says Rienmüller. These advantages, along with the reduced infection risk offered by the wireless stimulation, could be a gamechanger.
We all lead each other in the project.”
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A cross-border team
The OEPC project has grown organically across academic teams and borders. “This is really a huge group of people working from different kinds of disciplines,” says Rienmüller. In Austria, interdisciplinary collaboration is baked into the project, with funding from a government scheme to promote interdisciplinary research. “We have medical doctors, biophysicists, electrophysiologists,” says Rienmüller, and that’s just in Austria. Collaborators in Croatia and the Czech Republic are involved in OEPC development, fabrication and modelling, with different partners expert in chemistry, material science, experimental models, electrophysiology and medicine. “It works quite well,” laughs Rienmüller, “we all lead each other in the project.”
- Light Stimulation of Neurons on Organic Photocapacitors Induces Action Potentials with Millisecond Precision. Adv. Mater. Technol. 2022, 7, 2101159. Schmidt, T., Jakešová, M., Đerek, V. et al.