Automotive technology aids synthetic bone manufacture

Materials World magazine
,
1 Feb 2009
Extruded high-strength hydroxyapatite bone impant showing pore channels with an outer dense shell

Automotives and medical implants might seem worlds apart, but an unusual example of technology transfer has seen the manufacturing process for catalytic converters being applied to synthetic bone scaffolds.

Scientists at the University of Warwick, UK, believe this research could transform the market for synthetic bone substitutes by enabling the development of monolith bioceramic structures that combine strength and porosity for load-bearing applications, such as large fractures and spinal fusions.


The team claims to have achieved a 70-206MPa strength that can compete with metal alternatives, while providing the added advantage of a bioactive scaffold. Unlike passive metal implants that are non-degradable, these structures could enable osteoblast cells to proliferate in its pores and form new bone, eventually replacing the ceramic structure as it degrades.

‘The focus was to devise an ideal scaffold structure for hard tissue engineering,’ says Dr Kajal Mallick at Warwick’s School of Engineering. ‘We tried rapid prototyping technology, but we found that the type of materials we are using did not have the morphology necessary to be technically
feasible. Theoretically, the higher the porosity, the lower the strength.’

The automotive industry, however, does use catalyst monoliths made from aluminium magnesium silicate, which combine porosity with high strength. Researchers at Warwick have therefore been working with an undisclosed Japanese company that specialises in manufacturing catalytic converters to adapt the firm’s technology.

This involves extrusion of bioceramics hydroxyapatite and tri-calcium phosphate through a mould to produce a 3D honeycomb structure with uniform pores throughout. Sintering and sculpting thereafter produces a customised implant.

Mallick explains, ‘The material used in automotives is fired at 1,200-1,300ºC. To adapt from that to a bioceramic, which is fired at 1,100-1,200ºC, we had to make sure we did not lose control over the porosity’.

This involved careful formulation of the material to ensure stability in its green state and during sintering, as well as customising the die technology for extrusion. Longitudinal conduits of between 100µm and three millimetres in diameter, and sidewall pores of about 100µm, have been achieved.

Furthermore, in vitro trials with osteoblast-like MG63 cells have indicated rapid proliferation for hard tissue regeneration.

Changing markets

This research could have a positive impact on the biomedical implant market, says Dr Tony Dagger, Project Leader at Smith & Nephew Research Centre, in York, UK.

Synthetic bone grafts or fillers are applied but titanium, stainless steel and cobalt implants dominate. ‘There is a clear unmet need for synthetic structural bone substitute,’ says Dagger.

‘The brittle nature of these ceramic materials when produced as porous blocks, and the low strength of the degradable polymers they are combined with, [means] they are limited to applications that are not load-bearing and must be stabilised using metal plates, screws, wires, etc.’

He adds, ‘There is a growing recognition that tissues could be regenerated through bioactive materials rather than replaced by biopassive materials, but the evidence required to convince surgeons is somewhat lacking. This research is an innovative approach to improve the properties of materials, reducing the need to remove metal hardware, once healing has been achieved, and the incidence of failure and revision procedures.

Furthermore, compared to stereolithography that have produced similar compressive strengths, this might provide ‘inexpensive and scalable manufacturing’, says Dagger.

The next challenge for the team at Warwick is to conduct in vivo testing. Mallick also hopes to coat the scaffold in stem cells or bone morphogenetic proteins prior to implantation to study how this helps bone regeneration. The team is applying for funding to advance work to a clinical level.

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