Building from the inside out

In days gone by if you wanted to build something you used the natural materials around you, stone, wood or clay, for example. You took what you could get, so like the three little pigs you were limited to building your house with straw, sticks or bricks (only one of which was big, bad wolf proof). But suppose you have very specific needs for your building materials. Would it not be better to be able to design your own, to decide what its weight would be, how strong it would be and what shape it would be? That could give the wolf a run for his money. Today you can do exactly that, with some state of the art technology. You can specify the shape of objects you want and the characteristics of the materials and then just print them up.

The bare bones of the process

How can we design new materials that behave the way we want them to? One answer is to look at how nature has solved this problem, and one part of nature in particular. Bones! If you look inside a bone you will see it's not solid. It would be far too heavy if it was. Instead it's an intricate honeycomb, a scaffolding of bone material, which is concentrated in all the places where your bone takes the most strain.

A leg to stand on

For example, your femur, the long bone at the top of your leg, has a massive amount of internal scaffolding around the hip joint because that's the place where your body weight presses down when you're walking, running and jumping. It's where the bone needs to be strongest. In other parts of the femur the scaffolding is less dense, as these parts don't take the same giant load. Nature has found a clever way to make sure that bones get stronger in the right places. You start with a sort of general bone shape and as you learn to walk the pressures you put on the bone cause those parts under the greatest load to create more bone materials. In effect, walking programs the bone to build itself up in the right places.

And the (best) answer is

Now what if you were able to model this type of organic growing behaviour on a computer. You could start with the shape you want with a basic scaffolding inside it, and then work out where the extra strength needs to go. At the same time you can decide how heavy you want the material to be, so there is a pre-defined amount of stuff you can use, it's a question of finding the best way to use it.

You might also want the material to bend in one direction and be rigid in the other; again you can program this requirement in and let the computer solve the problem. You might even want the internal scaffolding shape to look good, what designers would call an aesthetic appeal. You could therefore, for example, require that it's made up of lots of little five pointed stars (or whatever took your fancy).

The process of calculating a best solution is called computer optimisation: how to get the best answer when there are lots of different factors you need to take into consideration. It's one application of artificial intelligence: computers doing clever decision making. (There are a range of cunning techniques to do this. See for example Swarm Intelligence).

Out of the computer and into the laser

Once you have optimised the design of your material in the computer, with the right shape, weight and strength the next step is to make it for real. That's where the process of laser sintering technology comes in. Imagine you wanted to make a ball you had designed in the computer. First the computer slices its virtual version into hundreds of digital layers stacked on top of each other. If you looked through these they would be a series of circles (the cross sections through the ball), which would increase in diameter as you go from the bottom up to the middle, then decrease as you approach the top of the ball. You make the real ball by copying these many layers.

In the laser sintering process a powder, for example a plastic, is spread over a base. This thin layer of plastic is then zapped by a laser, which is tracing out the first of the cross sections, the smallest circle and melting the plastic to form a solid. Once it has done this the next thin layer of power is added on top, and the laser zaps it again, melting a slightly larger circle on top of the first, which sticks to the circle below. Then the next powder layer is added; zap, melt, stick and so on. After many thousands of times doing this you have built your ball, embedded in the dusts, comprising all these layers that the computer calculated. All that remains is to remove the un-melted dust and there you have it, the ball simulated in the computer is now a lovely, real plastic ball sitting in front of you.

Printing things

This laser sintering process, which is in fact a form of high tech 3D printing, can of course be used with much more complicated shapes. For example, the objects you have designed using the optimisation process we looked at above can be made this way. The computer can now not only print your flat drawings but can also print your 3D objects too. The material the object is made of can have exactly the properties you want it to. Laser sintering doesn't just work with plastics, turn the power of your laser up a lot and ZAP you can melt titanium powder, and use this to print metal objects too. This new form of computer based design and 3D printing opens up a whole new range of possibilities, from arty plastic chairs to high strength low weight metal parts for aeroplanes. The limits are now only in the designer's imagination. It's like having a Star Trek replicator. Tell the computer what you want and off it goes and creates it for you. It's also comforting to know that the system is also being investigated as a way to produce custom built replacement bones for patients by using biologically compatible materials, a fitting use in more ways than one for this biologically inspired technology.