Cellular Titanium Materials for Medical Implants and Lightweighting of Engineering Materials

What are Cellular Materials?

Cellular Materials are formed by periodic or stochastic (random) variation of solid material and void within a structure. By controlling the distribution, geometry and proportion of the solid and voids, materials can be designed and fabricated to obtain specific engineering properties. The difference between periodic and stochastic structures is highlighted in the two examples shown in Figure 1. Fig. 1a shows a SolidWorks model of a cellular material generated by a cubic (periodic) array of spherical pores. In this example the pore diameter is 660 m and the inter-pore spacing is 600µm, creating a body with 68% porosity where pore overlap leads to each pore having 6 ‘windows’ into its next nearest neighbour pores. The precision of this repetitive array can be assured using state of art 3D printing methods. Fig. 1b shows a stochastic pore structure created in Ti6Al4V alloy from a polyurethane foam template.

Cellular Materials - Dr Karl Dahm

Cubic close packed periodic structures - SolidWorks model

Cubic close packed periodic structures - 3D printed array in Ti6Al4V alloy

Cellular Materials in Medical Implant Manufacture

Increasingly, modern orthopaedic implants are being fabricated using cellular titanium alloy materials, taking advantage of the osteoconductivity of the titanium metal by providing open pores for bone ingrowth. Controlling their cellular structure enables control of key engineering properties, such as elastic modulus (EM) and yield strength (YS), which may be adjusted to suit the specific application. If the EM of the implant material can be adjusted to closely match that of the surrounding bone, the stress-shielding effects often responsible for poor implant fixation will be minimized.

For medical and veterinary implant manufacture, the alloy of choice is Ti6Al4V, a material ideally suited to state-of-art metal AM methods and now the basis of products designed, manufactured and marketed in New Zealand by companies such as Ossis Ltd# and RAM3D%.

Our research has shown that by using variations on a simple cubic pore array we can create designs that mimic the EM and YS of human or animal bone [Ref 1] [Ref 2]. The engineering properties of natural bone vary with form, function and location - often being directional – but we have the design and software tools to instruct 3D printers to fabricate customised structures that emulate the bone behaviour. A summary of our research on pore orientation, presented at MetFoam 2016 in Dresden, is seen in [Ref 3].

Our research has shown that the key to success in designing cellular structures for medical or engineering applications lies at the intersection between modelling and measurement of 3D printed structures. Figure 2 shows how we use Finite Element Analysis models to understand flaw-induced compression failure in structures built upon cellular arrays.

FEA model showing flaw induced compression failure in hexagonal pore arrays.

FEA model showing flaw induced compression failure in hierarchical hexagonal pore arrays.

Leading edge research and innovation carried out by TiDA (now RAM3D) and the Massey University Veterinary School, has led to the successful design, 3D printing and surgical implantation into companion animals. The first of these procedures was the successful implant of a Ti6Al4V jawbone into a Boxer dog with jaw cancer. An image of the 3D printed jawbone showing its array of interconnected pores is seen in Figure 3.

The outcomes of 12 implant procedures are detailed in a 2017 paper in the American Veterinary Medical Association journal [Ref 4].

Figure 3. 3D printed Ti6Al4V jawbone, successfully implanted into a Boxer dog.

Cellular Materials in Engineering

In the context of engineering materials ‘lightweighting’ usually means selecting an alternative lower weight/density alloy eg. substituting Al or Mg or Ti for steel. This is critical in modern engineering design - especially in the motor vehicle and aerospace industries - where reduced weight often correlates with increased fuel efficiency.

For advanced engineering applications we have vastly more materials choice options than for medical implant design, since many of the commonly used steels and other alloys are commercially available as printable metal powders.

New Additive Manufacturing technologies give us dramatically more options. We can now design and create almost any 3-dimensional pore array using metal alloy 3D printing techniques. By controlling pore size, shape and location we can control both strength and stiffness, either uniformly in all directions or selectively along one or two axes.

Where now?

TiTeNZ is ideally positioned to take advantage of the new thinking and materials design choices that arise from our research into cellular materials, enabling new materials innovations in the Medical Implant and Advanced Engineering sectors.

References:

  1. Design and Properties of Cellular Structures for Titanium Alloy Implants. Ian Brown, Geoff Smith, Martin Ryan, Matt Sharp, Warwick Downing, Jonathan Bray. Proceedings of MetFoam 2015, Barcelona, Spain, 31 Aug -3 Sept.
  2. Optimizing Cellular Structures for Titanium Implant Design. Ian Brown, Geoff Smith, Peter McGavin, Matt Sharp, Martin Ryan, Warwick Downing and Jonathan Bray. Refereed proceedings of CellMat 2014, October 22 – 24, Dresden, Germany. CD-ROM.
  3. Effects of Void Array Orientation on Compressive Properties of Cellular Structures. Dahm, K. L., McGavin, P. N. and Brown, I. W. M. (2017). Adv. Eng. Mater., 1700060. doi:10.1002/adem.201700060 (Proceedings of CellMat 2016, Dresden, Germany).
  4. Clinical outcomes of patient-specific porous titanium endoprostheses in dogs with tumors of the mandible, radius, or tibia: 12 cases (2013–2016). Jonathan P. Bray, Andrew Kersley, Warwick Downing, Katherine R. Crosse, Andrew J. Worth, Arthur K. House, Guy Yates, Alastair R. Coomer, and Ian W. M. Brown. Journal of the American Veterinary Medical Association, September 1, 2017, Vol. 251, No. 5, Pages 566-579. (https://doi.org/10.2460/javma.251.5.566)