Human neurons growing on a boron-doped diamond substrate stained to make the various parts of the neurons visible. Blue shows the cell nuclei, green shows tubulin (i.e. where the dendrites are), and red indicates glial fibrillary acidic protein (GFAP). Scale bar = 25 µm.
Human neurons growing on a boron-doped diamond substrate stained to make the various parts of the neurons visible. Blue shows the cell nuclei, green shows tubulin (i.e. where the dendrites are), and red indicates glial fibrillary acidic protein (GFAP). Scale bar = 25 µm.

Diamonds may – or may not – be a girls’ best friend, but they are proving to be the ideal material for devices interfacing with the brain. Over the last decade, the chemical non-reactivity, stability, and lack of immunogenicity of diamond have marked it out as an ideal candidate for neural implants. Now researchers from the UK and Ireland have confirmed diamond’s credentials and devised a protocol for culturing neurons from stem cells on its surface [Nistor et al., Biomaterials 61 (2015) 139, http://dx.doi.org/10.1016/j.biomaterials.2015.04.050].

“Until now, the medical community have not really considered using diamond for implants,” explains Paul W. May of the University of Bristol, who worked with colleagues at Trinity College, Dublin and the University of Exeter on the study. “However, the last two decades has seen the emergence of chemical vapor deposition (CVD)… so diamond can now be considered an inexpensive engineering material.”

Although diamond’s extreme stiffness rules out use as an implant in moving parts of the body, its bio-inertness and ability to conduct electrically when doped are attractive for brain and nerve implants. Diamond is so bio-inert that the body does not recognize it is a foreign body, explains May, minimizing rejection and significantly reducing the build up of scar tissue around the implant. But what, the researchers wondered, happens when diamond is doped with boron to make it conductive? The team compared growth and survival of human neurons on undoped and boron-doped diamond and found no difference.

“Boron in its normal state is considered toxic, but a crucial finding from our studies is that when trapped inside diamond it does not affect or kill any cells attached to the surface,” May told Materials Today. “Boron-doped diamond is safe and nontoxic.”

The researchers found that surface microstructure does make a difference to neuron growth and proliferation, however. While all diamond surfaces can potentially sustain long-term survival of human neuron and glial cells, surfaces with large crystals support few cells. Polycrystalline surfaces, by contrast, with crystallites 10-100 nm in size, appear to be preferred by proliferating cells.

“The reasons are not clear,” admits May, “but it may be something to do with the surface needing to be slightly rough (but not too rough) in order for cells to grip on and adhere.”

The final piece of the jigsaw is to grow neurons on diamond surfaces. Instead of culturing human neurons directly on diamond, the team devised a protocol to culture stem cells and then convert them into neurons later, as required. Electrical signals could then passed between the conducting diamond substrate and the neurons.

“We still have a lot more fundamental studies of the neuron/diamond interface to perform,” says May. “[But] the long term possibilities for this work are exciting.  Long-lifetime diamond bio-implants may offer treatments for Parkinson’s, Alzheimer’s, stroke or even epilepsy.”