18 February 2008—Biomedical engineers are working to develop reliable brain-machine interfaces that will someday let amputees manipulate prosthetic limbs as naturally as they do their native ones. But hacking the nervous system is easier said than done. Today’s state-of-the-art method for connecting to the human nervous system is to deliver electrical pulses near a particular nerve cell to elicit a response, such as a muscle twitch or a sensation. The trouble is that the electrode that delivers the pulse creates a halo of charge that triggers nearby nerve fibers. The effect is similar to that of crosstalk on telecommunications lines. Thus, the brain might misinterpret a jolt from a prosthetic arm intended to indicate that only the index finger is pressed against an object as confirmation that the entire artificial hand has grasped the object.
But researchers at Vanderbilt University, in Nashville, think they’ve found a better way. Late last year, they began clinical tests using a portable solid-state laser that can stimulate nerves more effectively and more precisely than electricity. Using a similar laser aimed at the sciatic nerve of laboratory rats, they caused some part of the animal’s legs to involuntarily twitch with each laser pulse. A slight movement of the beam across the nerve bundle—which causes the narrow beam to shift its focus from one fiber within the nerve to another—can cause the rat to switch from, say, curling its toes to flexing its foot.
Stimulating nerves with lasers, says Anita Mahadevan-Jansen, a professor of biomedical engineering at Vanderbilt and the person who hit upon the idea of using light instead of current, may someday make artificial limbs as dexterous as human arms and might lead to such devices as patches that zap nerves to give relief to chronic pain sufferers. Researchers at Northwestern University, following the Vanderbilt team’s lead, have already shown that optical stimulation works on auditory nerves. They are developing cochlear implants with many more channels than today’s electric versions, capable of detecting many more frequencies.
The work originated from a vexing problem presented to Mahadevan-Jansen by Dr. Peter Konrad, a clinical neurosurgeon at Vanderbilt University Medical Center who is also a professor of biomedical engineering. Konrad asked if she could develop a method for making the centers of critical brain activity light up enough to be detected by a finely tuned sensor. This would dramatically cut down the amount of prep work required before, say, removing a brain tumor. It would eliminate the time-consuming process of touching dozens of spots on a patient’s brain with an electrical probe and making notes on a piece of paper for reference when cutting. ”After thinking about the problem for a while, it struck me that if I could get nerves to light up when stimulated, I might be able to do the reverse as well,” says Mahadevan-Jansen.
She and her colleagues—including Konrad and E. Duco Jansen, a biomedical engineering professor who is also Mahadevan-Jansen’s husband—set about finding the right combination of power and wavelength to stimulate neural activity without damaging the nerve tissue. Their efforts were greatly aided by the fact that Vanderbilt boasts one of the world’s only free-electron lasers, or FELs. Like an ordinary laser, a FEL generates coherent high-power radiation. But because its beam is produced by exciting a stream of freely moving electronsinstead of electrons bound in a particular atomic or molecular arrangement, the FEL can be tuned in order to adjust the beam’s wavelength. ”We tuned the laser to several wavelengths that we had computationally determined might be good candidates and found a couple that worked well,” says Duco Jansen. The wavelengths that worked during an initial experiment on a frog and later tests with lab rats were 3650 and 2120 nanometers, respectively.
Jansen notes that among several strokes of good luck was the discovery that because 2120-nm beams could be produced by a tabletop laser already in the team’s lab—which produces light by exciting a crystal made of yttrium, aluminum, and garnet doped with holmium atoms—they no longer needed to schedule time at the FEL in order to conduct experiments. He added that beams at that wavelength also required less energy and caused less cell damage than those at 3650 nm.
That realization also made the team hopeful about the further miniaturization that would be necessary in order to make portable lasers accessible to hospitals and doctors’ offices. To make the device as compact and inexpensive as possible, the researchers wanted to use a diode laser like the ones used in CD players and laser printers, says Jansen. For human trials, the Vanderbilt researchers are currently working with Aculight Corp., a Bothell, Wash.based maker of laser systems for military applications, to ready a diode laserbased prototype that is roughly the size of a hardcover book.
This prototype laser has been used in the surgical suite at Vanderbilt’s children’s hospital during rhizotomy procedures in which a nerve identified as the cause of debilitating spastic jerking is removed from children with cerebral palsy. Before the nerve is cut, the laser is fired on it, and its response is recorded. After the nerve is extracted, the researchers inspect it to see if the laser has inflicted any damage. Mahadevan-Jansen says that in five rhizotomies they have not discovered any laser-induced nerve damage. The group has already received federal government approval to use the Aculight laser for the next five such procedures.