A new approach to "printing" living cells could make it easier to arrange them into precise structures without harming them. This could enable future therapies where replacement limbs or organs can be printed to order.

The most advanced form of bioprinting borrows technology from the office. A solution of cells dubbed "bioink" is used in standard inkjet printing heads to make layers of cells on the microscale. But the technique gives a limited degree of precision, says Suwan Jayasinghe at University College London.

"The drops that come out of the inkjet printer needle are generally around twice the needle diameter," he says. "The printers use a 60-micrometre needle, so the droplets are at least 100 µm in diameter."

Those needles can also damage larger cells, Jayasinghe continues. "Some cells, like neonatal cardiomyocites – baby heart cells – can be 100 µm across," he says. Squeezing them through an inkjet needle can make them rupture and die.

Spinning cells

Jayasinghe is developing an alternative approach, called Pressure Assisted Spinning. Three needles nested inside one another separately deliver cells, a viscous polymer and pressurised air. The cells and polymer mix are drawn out and mixed by the pressurised air, explains Jayasinghe.

"Imagine standing next to a motorway with fast moving cars," he says. "The cars create an air pressure that would pull you along."

Because the polymer is viscous, it does not break up into droplets but flows out in a continuous stream of sticky thread like spider silk. Living cells are spaced along the 50-nanometre-wide thread's length. Cells are handled gently because they are delivered by a relatively wide needle – the thread is shaped by air pressure, not mechanical force.

Scanning the needle across a surface can build up a flat sheet of the material (see image, right), doing that over a 3D shape can produce a scaffold of cells ready to grow into any shape, for example, a particular bone or piece of tissue. Jayasinghe thinks sheets of the material might be useful externally as bandages.

Laborious and expensive

However, Vladimir Mironov of the Medical University of Southern California says Jayasinghe's simple solution doesn't tackle the problems hindering all types of cell printing. "The precise placing of different cell types [along the thread] is not possible," he says. "And [manual] cell seeding on a scaffold is laborious and expensive."

As well as inkjet printing, some researchers are experimenting with electrospinning, Mironov points out, a well-understood technology first developed about 100 years ago for making textiles.

In this process, a cell solution flows through an electrically charged hollow needle a few centimetres above an electrically grounded target. The charged solution is drawn towards the target, a little like lightning being drawn towards the Earth, pulling it into a very fine fibre with cells along its length.

But electrospinning also cannot space cells controllably, and has other drawbacks, says Jayasinghe, pointing out that up to 30,000 volts of electricity is needed. The current is low, though, making the chance of serious injury minimal. It is still a hazard, he says, one not present using pressure assisted spinning.

Unlike electrospinning, his method can also handle highly electrically conductive material, such as cell growth medium, he points out.