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A SPRINGY plastic wire that changes shape at the flick of a switch could provide a safer treatment for stroke patients. The wire, which is made from a shape-memory polymer, lets surgeons remove blood clots from the brain without using potentially dangerous clot-busting drugs.

Most strokes are caused by clots clogging cerebral arteries. While the effects vary, the result is often partial paralysis or speech loss.

If patients get to hospital within 3 hours of symptoms appearing, they can be treated with a drug that dissolves the clot. If it works, they stand a good chance of a full recovery. But after three hours the chances of a full recovery drop, and since the drug can cause potentially fatal haemorrhages, the risk outweighs the potential benefits.

Now engineers at the Lawrence Livermore National Laboratory in California think they鈥檝e hit on a new way to remove clots. They say a shape-changing plastic wire could be used to spear and pluck out clots.

The wire can change shape because it鈥檚 made from two types of polyurethane bundled into one. One type is harder than the other. Team member Duncan Maitland says the wire鈥檚 main body is constructed from the softer type, while chunks of the harder type are distributed throughout it.

The wire is first heated to allow both its hard and softer constituents to become rubbery and malleable. The end of the wire is then twisted into a coil and it is cooled. This effectively 鈥渇reezes鈥� the shape of the coil into the polymer.

Next, the wire is reheated, but to a lower temperature than before, so only its softer part can be deformed. The coil is pulled to straighten it out, then cooled down so that it hardens. It can鈥檛 spring back into the coil shape because it鈥檚 too rigid.

To make the end of the wire turn back into a coil, you simply heat it up again. At 60 掳C, the plastic softens enough for it to spring back into a coil.

Maitland fixed the wire to the end of an optical fibre and inserted both into a 0.5-millimetre-wide catheter (see Figure). He then used this to spear clots of pig鈥檚 blood in vitro. Once a clot had been speared, the catheter was withdrawn to leave the shape-memory plastic wire in place, which was then heated by shining infrared light down the fibre鈥攖ransforming the wire into a coil.

鈥淭he wire changed into a coil in a fraction of a second,鈥� says Maitland. When it coils up, the wire grips the clot from behind. The obstruction can then be pulled out by withdrawing the whole bundle. Tests showed the wire could hold a clot securely against a flow of liquid more than 10 times the blood pressure normally found in the brain.

Other medics are impressed with the idea. Peter Rothwell, director of the Oxford Stroke Prevention Research Unit at the University of Oxford, says the device could make a real difference to stroke patients. 鈥淏ecause you wouldn鈥檛 have the risk of haemorrhage, you could probably treat people who come in five to six hours after first symptoms,鈥� he says. 鈥淎 lot come in around the four-hour mark, so that could make a big difference.鈥�