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Flexible Electrodes Devised for Implanted Devices

Hydrogel electrode

Hydrogel electrode fits on simulated brain tissue (Wyss Institute, Harvard Univ.)

17 June 2021. Biomedical engineers designed flexible electrodes that in lab tests safely connect and deliver electrical current to animal hearts and brains. Researchers from the Wyss Institute for Biologically Inspired Engineering at Harvard University describe the flexible electrodes in today’s issue of the journal Nature Nanotechnology (paid subscription required).

A team from the lab of bioengineering professor David Mooney is developing these flexible electrodes to better connect implanted medical devices sending electrical signals to the heart, brain, and other organs in the body. Today those connections are made with stiff metal or plastics that do not easily bend or shape to the soft features or contours of human organs. Those mismatches also open gaps between electrodes and organs requiring higher voltage signals to make a working connection. In addition, some organs in the body, such as the spinal cord, can become permanently deformed if pressure is continually applied, thus the need for devices that fit the organ’s shape without added pressure.

Mooney’s lab cell and tissue engineering lab studies materials for medical devices, particularly hydrogels, water-based polymer gels. Mooney and colleagues boost the strength of natural hydrogels by adding alginate, a bio-based polymer found in the cell walls of brown seaweed and used in wound dressings. As reported in Science & Enterprise in July 2020, researchers in Mooney’s lab created a tough, stretchable, and resilient alginate hydrogel that the university licensed to a Florida company for sutures used in dental surgery.

Graphene, carbon nanotubes provide conductivity

In this project, a team led by doctoral candidate Christina Tringides is creating an alginate hydrogel for soft, flexible form-fitting electrodes. For this application, the researchers configured an alginate hydrogel into a thin plastic-like film that in lab tests show the hydrogel film can safely wrap around and provide more contact with a simulated animal brain than current implanted electrodes. In addition, tests show the hydrogel remains on the simulated brain for two weeks and maintains its shape during that time.

The team then added conductive materials, graphene flakes and carbon nanoscale tubes, to the hydrogel to create the electrodes. Tests of the hydrogel electrode show it produces pathways for electrical signals, while also bending more than 180 degrees and even tying into knots without losing its conductivity. To make a working electrode device, the team layered the hydrogel with an insulating material called PDMS, a soft rubbery silicone polymer also used in medical devices. Further lab tests show neurons and astrocytes, cells found in the brain, grow on the hydrogel electrodes without damage.

To prove the concept in more real-world tests, the researchers attached the hydrogel electrode to a mouse’s heart, then a rat’s heart and brain, and cow heart. The tests show the hydrogel electrode remains in place on these organs even when bent 180 degrees, while commercial electrodes slip from the surface when bent at 90 degree angles. In surgical tests with live animals, the hydrogel electrodes stimulate nerves and accurately measure electrical activity in mouse hind leg muscles, a mouse heart, and rat brain. The surgical tests included bending the electrode to reach hard-to-reach regions of these organs.

“Our hydrogel-based electrodes beautifully take the shape of whatever tissue they’re placed on,” says Tringides in a Wyss Institute statement, “and open the door to the easy creation of less invasive, personalized medical devices.” The researchers say the hydrogel electrodes can be made with today’s electrophysiology fabrication platforms. The university applied for patents on the technology.

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