Scanning electron micrograph of micrometer-sized nanocrystalline diamond before culturing with mouse ganglion cells (left) and after (right).
Scanning electron micrograph of micrometer-sized nanocrystalline diamond before culturing with mouse ganglion cells (left) and after (right).

Diamond-coated micro-sized pillars could sharpen up cochlear implants by acting as a guide for regrowing auditory neurons, according to researchers from Uppsala University in Sweden [Cai et al., Acta Biomaterialia (2015), DOI: 10.1016/j.actbio.2015.11.021].

Cochlear implants are now widely used to treat patients – especially children – with profound hearing loss. The devices make up for the lack of neural stimulation from vibration-receptor hair cells by providing an artificial stimulus. But the ear has some 3400 inner hairs cells, while devices typically have only 12-22 electrodes, so there is some inevitable shortcoming in fine hearing and the ability to resolve speech or music. Many patients also lack excitable neurons altogether.

Now, Mikael Karlsson, Hao Li, and Helge Rask-Andersen, along with co-workers, have found that human inner-ear ganglion neurites attach preferentially to micro-textured nanocrystalline diamond deposited on silicon pillars. The 5 x 5 micron nail-head-shaped pillars, spaced 4-9 microns apart, were fabricated using sputtering, photolithography, and plasma etching techniques. Samples of human and mouse inner ear ganglion tissue were then placed on the textured surface and cultured in growth medium for two weeks.

Ganglion cells appear to adhere readily to the micro-textured nanocrystalline diamond surface, even without the usually required extracellular matrix coating. Moreover, auditory neurons grow preferentially wherever there is micro-textured nanocrystalline diamond, forming a fine network of regenerated axons. But when the growing axons come to the edge of the textured nanocrystalline diamond, they halt and do not migrate further.

Since the axons grow in an ordered manner along the nanocrystalline diamond pillars, the researchers believe the approach could be used for neural guidance and to create new neural materials. Together with its antibacterial and electrical properties, textured nanocrystalline diamond could make an ideal electrode for cochlear implants, providing electrical stimulation signals of nerve cells and facilitating the regeneration of new neurons.

In theory, several or small groups of nanocrystalline diamond pillars could make up individual electrodes, vastly increasing the number of stimulation points in an implant and improving the resolution of sound though a cochlear implant.

“After an organized network of neurites is achieved, it becomes possible to stimulate the neurons selectively,” explains Li.

But, the researchers caution, there are still many obstacles to resolve first, such as the stiffness of diamond and the ability to connect it up to external platinum wires.

The researchers are now working on micro multi-electrode array chips based on textured nanocrystalline diamond, says Karlsson, which will be tested in vitro and animal studies.