Skeleton key

When it comes to lightweight engineering materials, technology is still trying to catch up with nature.

Although bone has a low density which reduces the material's weight, its cellular honeycomb structure is able to withstand huge loads and stresses. Human attempts to mimic this structure — in low-weight metal foams for example — haven't been able to replicate this performance.

The key to the success of bone, explained Andreas Burblies of the

for Manufacturing Engineering and Applied Materials in

, is that not all of the pores are the same size. Saw a femur in half lengthways, for example, and you'll see that the pores in the head of the bone, which has to withstand stresses in several directions, are large, while the shaft of the bone, which mainly withstands stresses along its length only, are smaller.

Burblies and colleagues have used these structural forms to develop a computer simulation system. 'We can now simulate on the computer the sort of internal structure a component needs so it is optimally designed for a specific application,' he explained.

The system breaks the volume of a component into a series of small cubes, and uses finite element analysis modelling to determine the stresses on each part.

'We start with molecular dynamics,' said Burblies, 'which allows us to look at the elastic energy in each part.' This is related to the Young's modulus of the material, which is in turn related to the level of porosity. 'The goal is to minimise the elastic energy.

'Here we begin with a random distribution of pores in the material. If we need a stiffer material, we make the pores smaller; if we need less stiffness, we make them larger. It's an iterative process, taking about 25 cycles, before we reach the optimum configuration with the minimum elastic energy.'

The next phase is to make the components, for which the team has turned to rapid prototyping techniques. Traditionally, metal foams are made almost like a cake, by mixing a metal powder with a foaming agent which is activated by heat.

But this does not allow the control of pore sizes to give different densities, so instead Burblies' team is using selective laser sintering. In this technique, the piece is built up layer by layer by using a laser to melt metal powder where the edges of the pores are to be located.

Before each new layer can be made, the remaining unmelted powder must be removed, but the result is a series of open pores with the correct density at every point in its structure. 'We're also planning to try other rapid prototyping techniques, such as solid printing,' said Burblies.

The techniques are likely to find applications in the automotive sector, where manufacturers will be able to reduce the weight of load-bearing components without sacrificing their strength.

The medical devices industry is also showing interest, said Burblies, to improve the performance of bone implants. These are currently milled from solid titanium and, although their shapes can be made sub-millimetre-perfect, the structure is very different. Being able to match the internal structure as well as the external shape of the natural bone will improve the implants' performance and increase their lifetime.