Replacement arms: mechanical or biological?

Prosthetic limbs are still in their infancy, but there’s a lot of progress being made: Johns Hopkins Applied Physics Laboratory is working with Darpa (who else?), and has a research grant for trying out their mind-controlled modular prosthetic arm on five test subjects over the next couple of years [via SlashDot]:

Phase III testing – human subjects testing – will be used to tweak the system, both improving neural control over the limb and optimizing the algorithms which generate sensory feedback. The Modular Prosthetic Limb (MPL) is the product of years of prototype design – it includes 22 degrees of motion, allows independent control of all five fingers, and weighs the same as a natural human arm (about nine pounds). Patients will control the MPL with a surgically implanted microarray which records action potentials directly from the motor cortex.

Researchers plan to install the first system into a quadriplegic patient; while amputees can be outfitted with traditional prostheses, the MPL will be the first artificial limb that can sidestep spinal cord injury by plugging directly into the brain.

Great news, then, but it’s still a crude kludge compared to the original. Building a new biological limb from the ground up is way beyond our biotech capabilities as they stand… but our own bodies do a pretty good job of it when we’re developing in the womb, and young children can sometime regrow fully functional fingertips lost to accidents. So why can’t we make like salamanders and just sprout replacement limbs? It’s a vexing question, and extremely clever people are working hard to work out the answer. (You’ll have to go read the whole article, because it’s too full of proper science for one or two pulled paragraphs to do it justice.)