Close to half a million people around the world suffer from a serious hearing impairment. In some cases, they can find relief with hearing aids, cochlear and other types of implants. Yet these devices do not help people whose inner ear is damaged or whose cochlear nerve does not function properly.
For these patients to recover their sense of hearing, electrical signals must be sent directly to the auditory brainstem. The neuroprosthetic used for this purpose is called an auditory brainstem implant, or ABI. Yet the outcomes of ABIs are modest with most patients recovering only sound awareness. What's more, clinical ABIs are stiff and cannot conform precisely to the curvature of the auditory brainstem.
To address this problem, Stéphanie Lacour's team at EPFL's Laboratory for Soft BioElectronic Interface (LSBI) worked with researchers from Mass. Eye and Ear and Harvard Medical School to develop a soft electronic interface. The highly elastic implant conforms precisely to the curved surface of the auditory brainstem and can therefore send highly targeted electrical signals. It has been tested successfully on mice – the implant has a surface area only 0.25 mm2 – and has now been produced at a size suitable for human use and in a form that is compatible with current surgical techniques. It will undergo further study in preparation for human trials. The researchers' study was just published in Science Translational Medicine. Dr. Lacour and Dr. Daniel Lee are co-senior authors on this paper.
Inspired by Japanese cutouts
The new implant consists of a conformable array of platinum electrodes encased in silicone. "We focused on platinum because it's already used widely in clinical settings," says Nicolas Vachicouras, a post-doc at EPFL's School of Engineering and the article's lead author.
Unfortunately, platinum is a rigid metal that cannot be distorted without being damaged. The researchers overcame this hurdle by applying the traditional Japanese paper-cutting technique called kirigami, etching a Y-shaped pattern into metalized plastic segments. Then they machined the metal at the micron scale (one micron = one thousandth of a millimeter), using techniques commonly found in the microfabrication of integrated circuits. The result is a very compliant and highly conductive electrode implant.
The EPFL researchers are already eyeing other applications. "The properties of our device would be of value for all sorts of implantable neuroprosthetics," says Stéphanie Lacour, "such as those used to stimulate or record neural activity in the spine, brain or even peripheral nerves."
The EPFL researchers worked closely with Dr. Lee's team at Mass. Eye and Ear. Dr. Lee said:
This collaborative study between researchers in Boston and Geneva represents an important step towards improving the performance of the auditory brainstem implant (ABI), a specialized device that is used for patients who are deaf and are not cochlear implant (CI) candidates. Overall, ABI users have modest outcomes compared to CI users. We believe that the translation of our novel "soft" auditory brainstem implant technology that addresses the biomechanical mismatch between existing stiff implants and the curved surface of the brain will improve performance and reduce side effects."