An Ohio State University (Columbus) researcher is designing a hydrogel coating to enhance the longevity of electrical stimulation implants. The coating promotes the integration of nerve tissue with implant electrodes, which could improve the implant’s performance due to better connectivity.
The material has been used for microÂsphere drug release and to make hydrogel tissue mimetics, but not as a prosthesis coating, according to Jessica Winter, assistant professor of chemical and biomolecular engineering at Ohio State. Her team has also been working with different hydrogel materials for antiinflammatory applications but not with neurotropins, proteins that promote neuron survival.
“We’re working on trying to make brain mimetics—materials that look and feel like brain to a surrounding tissue that we could use as coatings on neural electrodes,†says Winter. One of the issues surrounding neuroelectrode implants is the body’s immune response following implantation. A glial capsule can surround the device, causing the neurons very close to it to die. Being able to prevent neural death would enhance the function of those devices.
Winter is specifically looking at the field of deep-brain stimulation, because it’s one of the most successful areas in which brain electrodes are used. Simply applying the coatings to current deep-brain stimulation electrodes could improve their performance.
Another option is to apply the coatings to recording electrodes that are used on epileptic patients to measure the origin of seizures. Current electrodes only last for several months to a year, and this new coating could make them last much longer.
The hydrogels used in the coating are crosslinked polymers that are hydrophilic but not water-soluble. “They’re about 90–99% water. The particular polymers that we’re using are PEG [polyethylene glycol] hydroxy acids,†says Winter. “That includes lactic acids, glycolic acid, and caprolactone.â€
PEG provides biocompatibility and has been widely used in the medical industry to enhance the circulation time of drugs and to coat implants. It’s also known to resist protein absorption and can prevent an immune response. The advantage of using lactic acid is that it’s natural, so there are fewer concerns about a negative interaction with the body. The hydroxy acid component was chosen because it is degradable over time and would enable a mechanism to tune drug release.
The researchers regulate the thickness of the coating by drop casting it onto the devices. Its thickness, which is controlled by surface tension, measures several hundred µm. They also patterned the coating to improve its thickness. Although most of the coatings have been 30–40 µm thick, Winter says they can be made thinner if necessary.
The team is trying to improve its release time from three weeks to about eight weeks. Winter would like to switch from using lactic acid to caprolactylic acid, because it degrades at a slower rate, which would extend drug release.
They’re also exploring composites that can consist of hydrogels and other materials such as microspheres and electrospun fibers. The microspheres are made from polylactic coglycolic acid, and the electrospun nanofibers are composed from polycaprolactones, which is another hydroxy acid. The composite materials could help them achieve a more linear release profile.
Winter and her team recently presented their research, which is being funded by Ohio State, at an American Chemical Society meeting in Philadelphia.
—Maria Fontanazza
