SLOWLY but surely the sole of a shoe emerges from a bowl of liquid resin, as Excalibur rose from the enchanted lake. And, just as Excalibur was no ordinary sword, this is no ordinary sole. It is light and flexible, with an intricate internal structure, the better to help it support the wearer’s foot. Paired with its solemate it will underpin a set of trainers from a new range planned by Adidas, a German sportswear firm.

Adidas intends to use the 3D-printed soles to make trainers at two new, highly automated factories in Germany and America, instead of producing them in the low-cost Asian countries to which most trainer production has been outsourced in recent years. The firm will thus be able to bring its shoes to market faster and keep up with fashion trends. At the moment, getting a design to the shops can take months. The new factories, each of which is intended to turn out up to 500,000 pairs of trainers a year, should cut that to a week or less.

3D printing has come a long way, quickly. In February 2011, when The Economist ran a story called “Print me a Stradivarius”, the idea of printing objects still seemed extraordinary. Now, it is well established. Additive manufacturing, as it is known technically, is speeding up prototyping designs and is also being used to make customised and complex items for actual sale. These range from false teeth, via jewellery, to parts for cars and aircraft. 3D printing is not yet ubiquitous. Generally, it remains too slow for mass production, too expensive for some applications and for others produces results not up to the required standard. But, as Adidas’s soles show, these shortcomings are being dealt with. It is not foolish to believe that 3D printing will power the factories of the future. Nor need the technology be restricted to making things out of those industrial stalwarts, metal and plastic. It is also capable of extending manufacturing’s reach into matters biological.

Adding it up

There are many ways to print something in three dimensions, but all have one thing in common: instead of cutting, drilling and milling objects, as a conventional factory does, to remove material and arrive at the required shape, a 3D printer starts with nothing and add stuffs to it. The adding is done according to instructions from a computer program that contains a virtual representation of the object to be made, stored as a series of thin slices. These slices are reproduced as successive layers of material until the final shape is complete.

Typically, the layers are built up by extruding filaments of molten polymer, by inkjet-printing material contained in cartridges or by melting sheets of powder with a laser. Adidas’s soles, however, emerge in a strikingly different way—one that is, according to Joseph DeSimone, the result of chemists rather than engineers thinking about how to make things additively. Dr DeSimone is the boss of Carbon, the firm that produces the printer which makes the soles. He is also a professor of chemistry at the University of North Carolina, Chapel Hill.

One of the earliest adopters of additive manufacturing was the medical industry. For good reason; everybody is different, and so, therefore, should be any prosthetics they might need. As a result, millions of individually sculpted dental implants and hearing-aid shells are now printed, as are a growing number of other devices, such as orthopaedic implants. The big prize, however, is printing living tissue for transplants. Though this idea is still largely experimental, several groups of researchers are already using bioprinters to make cartilage, skin and other tissues.

Bioprinters can work in several ways. The simplest use syringes to extrude a mixture of cells and a printing medium, a method similar to that used by a desktop printer in plastic. Others employ a form of inkjet printing. Some medical researchers are trying a form of 3D printing called laser-induced forward transfer. In this, a thin film is coated on its underside with the material to be printed. Laser-pulses focused onto the film’s upper surface cause spots of that material to detach themselves and land on a substrate below. Sometimes, though, the third dimension needs a helping hand. Certain printers therefore impose the desired shape by printing cells directly onto a pre-prepared scaffold, which dissolves away once the cells have proliferated sufficiently to hold their own shape.

Anthony Atala and his colleagues at the Wake Forest Institute for Regenerative Medicine, in North Carolina, have printed ears, bones and muscles in this way, and have implanted them successfully into animals. The crucial part of the process is ensuring the printed tissue survives and then integrates with the recipient when transplanted. Some types of tissue, such as cartilage, are easy to grow outside the body. Infusing nutrients into the medium they are kept in is sufficient to sustain them, and they tend to take well when transferred to a living organism. More complex structures, though, like hearts, livers and pancreases, require a blood supply to grow beyond being tiny slivers of cells.