Fig.1. Immunofluorescence image of neuron cells differentiated from stem cells for more than 150 days growing on a CVD diamond surface. The cells are stained blue to show the nuclei, green to show the dendrites, and red for glial cells.
Fig.1. Immunofluorescence image of neuron cells differentiated from stem cells for more than 150 days growing on a CVD diamond surface. The cells are stained blue to show the nuclei, green to show the dendrites, and red for glial cells.

When many of us think of diamond, they think of gemstones and the glitz and glamour of celebrities. However, the last two decades has seen the emergence of a new technology, diamond chemical vapor deposition, which allows thin films/coatings of diamond to be deposited onto a range of substrates, from drill bits to engine components to semiconductor devices. Diamond can now be considered to be an inexpensive engineering material with a multitude of uses. Many of these applications are biomedical, but until now the medical community have not really considered using diamond. Now a number of studies have demonstrated the ability to culture living human neurons, or stem cells, or other cells such as blood cells, on a diamond surface, and that these cells happily thrive with no ill effects. Moreover, research has shown that electrical signals can be passed to and from the conducting diamond substrate to neurons attached to its surface, allowing the possibility of using diamond as an interface between living nerve cells and inorganic electronics. As a result, neurosurgeons and bioimplant designers are gradually becoming aware that diamond is now a realistic and exciting choice for these devices.

Diamond is hard and inflexible, so is not suitable as an implant in areas of the body that flex a lot, e.g. muscles. But it can be used in the brain, central nervous system or to contact or wrap around nerves. Here, diamond has two major advantages. The first is that it is bioinert - the body does not seem to recognize diamond as a foreign body and so does not reject it. There is little or no inflammation, and little build-up of glial scar tissue around the implant. In conventional implants, scar tissue gradually builds up between the implant and the nerve cell until it becomes so thick the electrical signals can no longer pass across it. At this point the implant stops working, and has to be replaced, requiring another invasive surgery. With a diamond-based implant, research indicates that scarring is minimal, so implants could survive for much longer, perhaps decades or even outlive the patient. 

The second feature that makes diamond so attractive is that its electrical conductivity can be altered by simply adding a trace of boron during the deposition process. When this is done, the diamond exhibits near metallic conductivity, allowing it to behave like a metal wire. Boron in its normal state is normally considered toxic, but when boron is trapped inside the diamond crystal lattice it does not affect or kill cells attached to the surface. Thus B-doped diamond is safe and non-toxic.

Fig.2. Scanning electron micrograph of a nano-rough CVD diamond film that forms an ideal bioinert substrate for cell growth.
Fig.2. Scanning electron micrograph of a nano-rough CVD diamond film that forms an ideal bioinert substrate for cell growth.

Although the ultimate aim is to develop diamond-based bioimplants, the first steps involve studying how individual cells interact with the diamond surface and finding ways of safely passing signals into neurons, and recording signals from them, without killing or damaging cells. To do this, it is easier to put the cells onto diamond-coated plates, culture them, and see how they survive, propagate and die relative to cells grown on conventional biomaterials, such as glass or plastic. Researchers at the University of Bristol and University College London have recently shown that neurons can be grown and even thrive on an inert diamond surface. It is even possible to culture human stem cells on a diamond surface, and later on convert them into neurons using a special chemical treatment. This offers the possibility of making patterned plates of living neurons, which would act, in effect, like a 2D neural net. Applying electrical stimuli to this net and watching how they propagate would allow neuroscientists to gain insight into how similar signals propagate in a real 3D brain.

The long term possibilities for this work are exciting. Long-lifetime diamond bioimplants may offer treatments for Parkinson's, Alzheimer's, stroke or even epilepsy. It can also be envisaged that nerve signals from the central nervous system could be intercepted by a diamond implant placed above, say, a break in the spine, and passed wirelessly to an external computer that interprets the signals and uses them to control a pair of robotic limbs or powered exoskelton. This could be a potential treatment for paralysis, or even motor neurone disease.  Indeed, there are several projects worldwide in which conducting diamond is already being investigated as the interface between electronics and the human body. The EU has funded a project called NeuroCare, which aims to implant a diamond electrode behind the retina of a blind patient in order to transfer the electrical impulses from the rods and cones to the optic nerve. The University of Melbourne is working on the 'bionic eye' project, in which silicon-based electronics are housed in a hermetically sealed bioinert diamond capsule that is inserted into the eye.

There are other implications for such implanted biosensors, such as brain-computer interfaces (BCIs), that could allow direct thought-control of computers, cars, drones, etc. BCIs are being actively researched at the moment, especially by the military, where the signals are detected by a sensor net or ‘hat’ worn over the patient's head. The advantage of an embedded diamond sensor is that it would be permanent. Hollywood sci-fi movies such as Johnny Mnemonic and Robocop imagined we might one day be able to plug our brains into a computer using something like a USB port! While this may be a bit fanciful, for better or worse, implantable diamond sensors may bring that sort of technology one step closer.

References

  1. P.A. Nistor, P.W. May, F. Tamagnini, A.D. Randall, and M.A. Caldwell, Long-term Culture of Pluripotent Stem Cell Derived Human Neurons on Diamond – a Substrate for Neurodegeneration Research and Therapy, Biomater. 61 (2015) 139-149.
  2. P.W. May, E.M. Regan, A. Taylor, J. Uney, A.D. Dick and J. McGeehan, Spatially controlling neuronal adhesion on CVD diamond" Diamond Relat. Mater. 23 (2012) 100-104.
  3. A. Thalhammer, R.J. Edgington, L.A. Cingolani, R. Schoepfer, R.B. Jackman, The use of nanodiamond monolayer coatings to promote the formation of functional neuronal networks, Biomaterials 31 (2010) 2097-2104.
  4. A. Ahnood, M.C. Escudie, R. Cicione, C.D. Abeyrathne, K. Ganesan, K.E. Fox, et al., Ultrananocrystalline diamond-CMOS device integration route for high acuity retinal prostheses, Biomed. Microdevices 17 (2015) 50.
  5. C.G. Specht, O.A. Williams, R.B. Jackman, R. Schoepfer, Ordered growth of neurons on diamond, Biomaterials 25 (2004) 4073-4078.