WASHINGTON (AP) _ Stairs are about to become less of an obstacle for some of the nation's 2 million wheelchair users.

The Food and Drug Administration on Wednesday approved a wheelchair that literally can go up and down steps _ as well as shift into four-wheel drive for grassy hills and elevate its occupant to standing height.

Called the iBOT Mobility System, the wheelchair uses sensors and gyroscopes to navigate stairs while balancing on two wheels. Doctors have said the technology is potentially revolutionary. But it is so complex that the FDA decided the wheelchair will require a doctor's prescription and special training to drive.

The iBOT costs $29,000, less than some top-of-the-line models for the severely impaired but far more than basic wheelchairs. The maker, Johnson & Johnson subsidiary Independence Technology, is negotiating with Medicare and other insurers but could not say Wednesday if payment to users was likely.

Sales will begin by year's end, a J&J spokesman said.

Wheelchairs have become increasingly sophisticated. Some raise a user a few inches to be able to reach high objects. More agile models specially are designed for zipping around basketball or tennis courts.

In the early 1990s, the FDA approved one model solely for stair-climbing, but it never became popular because it did not provide more routine transport, said Robert DeLuca, the FDA scientist who led the iBOT evaluation.

The iBOT, in contrast, is an all-purpose wheelchair that also climbs stairs, he said.

``We think this is something that can really benefit patients,'' DeLuca said. ``It offers many advantages to anything else we've ever seen.''

Most wheelchairs have two big back wheels and two smaller front wheels. The iBOT has four wheels the same size that rotate up and over one another to go up and down steps.

It does require some user exertion, meaning the iBOT is not an option for all wheelchair users.

People must have the use of at least one arm to operate the iBOT's joystick and other controls. Then they lean forward or backward, directing the chair to climb up or down as the gyroscopes sense and adjust to the person's center of gravity.

Users must hold onto a stair rail to help guide the iBOT, although there is a feature that allows someone else to hold onto the chair's back and assist the more severely disabled on stairs.

So far, it is not built for children or for people who weigh more than 250 pounds.

Dean Kamen, the well-known inventor whose credits include the Segway scooter, created the iBOT and licensed it to Johnson & Johnson. He says he built it not just for the stair-climbing ability but the extra elevation, too _ because wheelchair users had told him they longed to carry on eye-level conversations with people standing nearby, and reach top grocery shelves by themselves.

To prove iBOT works, 18 wheelchair users test-drove it for two weeks. Scientists compared maneuverability in the iBOT versus users' regular wheelchairs in everyday situations and in special road tests.

Twelve patients could navigate stairs alone with the iBOT, while the rest used an assistant. In regular wheelchairs, one patient could literally bump his way down stairs, but no one could go up a single step.

Three people fell out of the iBOT and two fell out of their own wheelchairs during the study _ none on stairs and none was seriously injured _ suggesting the iBOT was as safe as today's technology, the FDA concluded.

But the iBOT is complex enough that the wrong person using it could get hurt or injure bystanders. So, Independence Technology set up an FDA-approved program to strictly control sales.

Doctors and rehabilitation therapists must be licensed to prescribe the iBOT. Then, they would administer tests to potential users to ensure they are physically capable of handling the machine and have the right judgment skills to discern obstacles, such as which hills are too steep to try climbing.

If users fail the test, they cannot buy an iBOT.

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On the Net:

Independence Technology: http://www.independencetechnology.com

FDA: http://www.fda.gov