NEW YORK, Apr 20 (NYT Syndicate) - Have you ever felt like a bruised pincushion after a doctor has tried to take a blood sample but couldn't find a vein? Help may be at hand in the shape of a robotic blood taker, say engineers at Imperial College in London.

While you might rather put your trust in the haphazard jabs of a medical student than a bloodsucking machine, the researchers at Imperial insist that their robot is far more reliable than a human. Besides hitting the vein first time, every time -- a track record few human blood takers can match -- the robot won't overshoot the vein. This is a common problem for some patients that can cause painful bruising. To find the vein, the robot uses a primitive sense of touch. Gently prodding the different parts of the arm, it records the force of the probe bouncing off the tissue to determine what lies underneath the skin with an accuracy of one millimeter.

"The force depends upon the elasticity of the skin underneath," says Alex Zivanovic, a mechanical engineer working on the project. Muscle is hard and fat is soft, he explains, while veins have a unique feel to them, "like an underinflated balloon."

Once the location of the veins has been displayed on a screen, the operator can choose the best one and instruct the robot to proceed. As the needle penetrates the skin, the robot uses strain gauges to monitor the force exerted on it by the tissue. The moment the wall of the vein is breached, the robot stops, ensuring that the needle doesn't overshoot.

This approach is particularly flexible, says Tony Firth, an anatomist at Imperial, because it doesn't assume that everybody's veins are all the same size or at the same depth. This makes it useful for finding veins in small children or obese people.

David Keeling, a consultant hematologist at the Oxford Hemophilia Centre, says it can also be extremely difficult to find veins in patients after a lifetime of treatment. "If you are sticking a needle into the same veins you can get inflammation, thrombosis and the vein can become occluded," he says. Essentially, patients run out of veins to use and in some cases permanent catheters must be inserted.

But Keeling wonders how easy it would be to get a small child to keep still -- even when strapped into the device. And Zivanovic admits that many people he has talked to were none too enthusiastic about a robot wielding a needle. Nevertheless, he hopes that drawing people's attention to the robot's safety features, such as its limited field of motion, will persuade the faint-hearted.

The system is semi-autonomous. It suggests the most appropriate area to pierce but leaves it up to the operator to make the final decision. Eventually Zivanovic would like to build a fully autonomous robot that would choose the most suitable vein.

So far, Zivanovic has only tested his device on artificial limbs designed as practice tools for medical students. These contain various layers of gels, tubes and latex membranes, and even artificial blood, which Zivanovic says gushes out into the needle when the vein is penetrated.