(Featured Photo: SLM printed titanium glowing in a plasma microwave)
Currently, the best 3D printed bone implants are made from metal, predominantly titanium alloys. However, they are not the ideal medical material given that there is nothing naturally occurring inside the body comprised of metal. Titanium alloys have been invariably the material of choice due to their acknowledged bio-inertness which means that the surface oxide layer protects the metallic component of the implant from ever coming into contact with the surrounding bone.
What is RMIT doing about it?
RMIT and our Centre for Additive Manufacturing are working to develop mechanisms to improve the way in which the implants can interact with the body. We have a significant program in developing patient-specific implants that can be designed and printed specifically to fit the patient in which it is designed for (See more on Stryker funds research in 3D printed bone cancer implants).
Using a custom algorithm developed by scanning the patient themselves, an implant can be built to fit the individual patient . Ideally, a personalized implant would fit better than one bought off the shelf. Although the fit is better, the solution does not ameliorate the material mismatch. One solution we are investigating is the use of diamond, a type of carbon which is a material that is readily found in most things including the human body.
Diamond as a biomaterial?
As you can imagine, diamond does not immediately spring to mind as an implant material. The diamond itself is a hard-brittle material however combined with other materials we find that it has many useful applications in the biomedical space. For example, diamond has been used as the electrode material in the Australian Bionic Eye project [2-4].
The rationale was that the charge injection of nitrogen incorporated ultra nanocrystalline diamond was equivalent to the industry standard, platinum, with the additional benefit that we could produce conductivity and insulation in the same material (conductivity only changing when doped) and thus form a microelectrode array. However, limitations existed when moving from a planar substrate to a 3D structured alternative.
Figure 1. Diamond electrode system developed to stimulate the retina using electrical signals. Reproduced with permission from Hadjinicolaou et al. 
How can you use diamond in medical AM?
On joining RMIT, I set out to investigate new ways that we could combine additive manufacturing processes well established for metals and polymers with diamond. The first attempt used the low hanging fruit of nanodiamond and polycaprolactone, a low melting point medical-grade polymer that allowed for melt extrusion. Here we were able to show that embedded nanodiamonds can improve wound healing but also allowed us to make new materials that can be visualized under the skin (since diamond as a fluorescence) [5, 6].
Second, we went back to the metal technology. Taking all the lessons learned through our personalized implant projects, we decided to investigate whether or not we could take an SLM printed 3D titanium scaffold and coat it with a polycrystalline diamond. To do so, we took the metallic substrate and used chemical vapor deposition to coat the substrate in diamond. The advantage of this technique is that the mechanical properties of the implant are dictated by the underlying metal with the diamond coating a thin film that masks the metallic implant from any surrounding bone. All indications from this technique suggest that the materials are well accepted by the body and in fact provides an anti-fouling coating .
Figure 2: From clockwise: Diamond-coated SLM titanium; Nanodiamond-polycaprolactone; Confocal imaging showing fluorescence of nanodiamond particles in polymer; LMD co-printing of diamond and titanium and Diamond coated 3D titanium cubes.
We don’t fully understand why the diamond is biocompatible whilst also preventing bacterial adhesion but we continue to look into it. More recently, we have changed our approach to diamond implants and are investigating the use of laser metal deposition to co-print micro-diamond and micro-titanium as a single hybrid material. We have recently shown that we can print materials with up to 50% diamond with the material retaining its fluorescence as well as its biocompatibility . This is really exciting and we are really working here in uncharted territory. We are discovering new things about this material and we cannot wait to tell you more about it.
 D. Shidid, M. Leary, P. Choong, M. Brandt, Just-in-time Design and Additive Manufacture of Patient-specific Medical Implants, Physics Procedia 83 (2016) 4-14.
 K. Ganesan, D.J. Garrett, A. Ahnood, M.N. Shivdasani, W. Tong, A.M. Turnley, K. Fox, H. Meffin, S. Prawer, An all-diamond, hermetic electrical feedthrough array for a retinal prosthesis, Biomaterials 35(3) (2014) 908-915.
 D.J. Garrett, K. Ganesan, A. Stacey, K. Fox, H. Meffin, S. Prawer, Ultra-nanocrystalline diamond electrodes: optimization towards neural stimulation applications, Journal of neural engineering 9(1) (2012) 016002.
 A.E. Hadjinicolaou, R.T. Leung, D.J. Garrett, K. Ganesan, K. Fox, D.A. Nayagam, M.N. Shivdasani, H. Meffin, M.R. Ibbotson, S. Prawer, Electrical stimulation of retinal ganglion cells with diamond and the development of an all diamond retinal prosthesis, Biomaterials 33(24) (2012) 5812-5820.
 K. Fox, P.A. Tran, D.W.M. Lau, T. Ohshima, A.D. Greentree, B.C. Gibson, Nanodiamond-polycaprolactone composite: A new material for tissue engineering with sub-dermal imaging capabilities, Materials Letters 185 (2016) 185-188.
 S. Houshyar, G.S. Kumar, A. Rifai, N. Tran, R. Nayak, R.A. Shanks, R. Padhye, K. Fox, A. Bhattacharyya, Nanodiamond/poly-ε-caprolactone nanofibrous scaffold for wound management, Materials Science and Engineering: C 100 (2019) 378-387.
 A. Rifai, N. Tran, D.W. Lau, A. Elbourne, H. Zhan, A.D. Stacey, E.L.H. Mayes, A. Sarker, E.P. Ivanova, R.J. Crawford, P.A. Tran, B.C. Gibson, A.D. Greentree, E. Pirogova, K. Fox, Polycrystalline Diamond Coating of Additively Manufactured Titanium for Biomedical Applications, ACS Applied Materials & Interfaces 10(10) (2018) 8474-8484.
 K. Fox, N. Mani, A. Rifai, P. Reineck, A. Jones, P.A. Tran, A. Ramezannejad, M. Brandt, B.C. Gibson, A.D. Greentree, N. Tran, 3D-Printed Diamond–Titanium Composite: A Hybrid Material for Implant Engineering, ACS Applied Bio Materials (2019).
About the Author:
Associate Dean Higher Degrees by Research (School of Engineering) at RMIT University
A reformed patent and trademark attorney, I am a biomedical engineer who specializes in medical device development. I hold personal research grants into orthopedic, neural and dental materials research. I am currently employed as an associate professor at the School of Electrical and Biomedical Engineering at RMIT in which I develop novel orthopedic implants using additive manufacturing whilst facilitating the progress of the next generation of hard tissue engineers. I am currently teaching courses in solid mechanics and materials, biomedical engineering design and implant engineering/assistive technology.
Follow me on Twitter @engineeringkate or check me out on Google Scholar (https://scholar.google.com.au/citations?user=WWfgGdMAAAAJ&hl=en)
At the University of Melbourne and Bionic Vision Australia, I developed an integration and collaboration platform between 5 independent joint venture members and have provided the surgical integration necessary to transition an engineered product into a surgically feasible product. I have continued this drive at RMIT with a new diamond orthopedic implant. My skill base is broad and highly relevant to cross-disciplinary research translation.