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  • IR Light Activates Heart, Ear Cells
Mar 2011
SALT LAKE CITY, March 30, 2011 — By exposing inner-ear cells and heart cells to infrared optical signals, scientists have found that the light actually activates the cells so they can send signals to the brain, a discovery that could lead to optical prosthetics for disorders of movement, balance and vision.

"We're going to talk to the brain with optical infrared pulses instead of electrical pulses," which now are used in cochlear implants to provide deaf people with limited hearing, said Richard Rabbitt, a professor of bioengineering at the University of Utah and senior author of the study.

The studies — funded by the National Institutes of Health — also raise the possibility of developing cardiac pacemakers that use optical signals rather than electrical signals to stimulate heart cells. Rabbitt said, however, that because electronic pacemakers work well, "I don't see a market for an optical pacemaker at the present time."

The inner-ear cells of the oyster toadfish, which are well-established models for comparison with human inner ears and sense of balance, were used in the University of Utah research. (Image: Wikimedia Commons)

The scientific significance of the studies is the discovery that optical signals — short pulses of infrared laser light delivered via a thin, glass optical fiber — can activate heart cells as well as inner-ear cells related to balance and hearing.

In addition, the research showed infrared activates the heart cells, called cardiomyocytes, by triggering the movement of calcium ions in and out of mitochondria, the organelles that convert sugar into usable energy. The same process appears to occur when infrared light stimulates inner-ear cells.

Infrared light can be felt as heat, raising the possibility that the heart and ear cells were activated by heat rather than the infrared radiation itself. But Rabbitt and colleagues did "elegant experiments" to show that the cells indeed were activated by the infrared radiation, they said.

The low-power infrared light pulses in the study were generated by a diode — "the same thing that's in a laser pointer, just a different wavelength," Rabbitt said.

The heart cells in the study were newborn rat heart muscle cells, which make the heart pump. The inner-ear cells are hair cells and came from the inner-ear organ that senses motion of the head. The oyster toadfish hair cells used are well-established models for studying human inner ears and the sense of balance.

Inner-ear hair cells "convert the mechanical vibration from sound, gravity or motion into the signal that goes to the brain" via adjacent nerve cells, said Rabbitt.

Using infrared radiation, "we were stimulating the hair cells, and they dumped neurotransmitter onto the neurons that sent signals to the brain," he said.

He believes the inner-ear hair cells are activated by infrared radiation because "they are full of mitochondria, which are a primary target of this wavelength."

Using an elaborate apparatus to study the inner-ear cells of the oyster toadfish, University of Utah bioengineering professor Richard Rabbitt found that infrared light similar to that emitted by a laser pointer, but at longer wavelengths, can make inner-ear "hair cells" send signals to adjacent nerve cells and then to the brain. (Image: Lee Siegel, University of Utah)

The infrared radiation affects the flow of calcium ions in and out of mitochondria – something shown by a companion study on neonatal rat heart cells.

That is important because for "excitable" nerve and muscle cells, "calcium is like the trigger for making these cells contract or release neurotransmitter," said Rabbitt.

The heart cell study found that an infrared pulse lasting a mere one-5000th of a second made mitochondria rapidly suck up calcium ions within a cell, then slowly release them back into the cell – a cycle that makes the cell contract.

"Calcium does that normally," he said. "But it's normally controlled by the cell, not by us. So the infrared radiation gives us a tool to control the cell. In the case of the [inner-ear] neurons, you are controlling signals going to the brain. In the case of the heart, you are pacing contraction."

New Possibilities for Optical versus Electrical Cochlear Implants

Existing cochlear implants convert sound into electrical signals, which typically are transmitted to eight electrodes in the cochlea, a part of the inner ear where sound vibrations are converted to nerve signals to the brain. Eight electrodes can deliver only eight frequencies of sound, Rabbitt said.

"A healthy adult can hear more than 3000 different frequencies. With optical stimulation, there's a possibility of hearing hundreds or thousands of frequencies instead of eight. Perhaps someday an optical cochlear implant will allow deaf people to once again enjoy music and hear all the nuances in sound that a hearing person would enjoy," he said.

Unlike electrical current, which spreads through tissue and cannot be focused to a point, infrared light can be focused, so numerous wavelengths (corresponding to numerous frequencies of sound) could be aimed at different cells in the inner ear.

Nerve cells that send sound signals from the ears to the brain can fire more than 300 times per second, so ideally, a cochlear implant using infrared light would be able to perform as well. In the Utah experiments, the researchers applied laser pulses to hair cells to make adjacent nerve cells fire up to 100 times per second. For a cochlear implant, the nerve cells would be activated within infrared light instead of the hair cells.

Rabbitt cautioned that it may be five to 10 years before the development of cochlear implants that run optically. To be practical, they need a smaller power supply and light source, and they must be power-efficient enough to run on small batteries, as with a hearing aid.

Optical Prosthetics for Movement, Balance and Vision Disorders

Electrical deep-brain stimulation now is used to treat movement disorders such as Parkinson's disease and "essential tremor, which causes rhythmic movement of the limbs so it becomes difficult to walk, function and eat," Rabbitt said.

He is investigating whether optical rather than electrical deep-brain stimulation might increase how long the treatment is effective.

"When we get old, we shuffle and walk carefully, not because our muscles don't work but because we have trouble with balance," he said. "This technology has potential for restoring balance by restoring the signals that the healthy ear sends to the brain about how your body is moving in space."

Optical stimulation also might provide artificial vision in people with retinitis pigmentosa or other loss of retinal cells — the eye cells that detect light and color — but who still have the next level of cells, known as ganglia, Rabbitt said.

"You would wear glasses with a camera [mounted on the frames] and there would be electronics that would convert signals from the camera into pulses of infrared radiation that would be patterned onto the diseased retina that normally does not respond to light but would respond to the pulsed infrared radiation" to create images, he said.

Hearing and vision implants that use optical rather than electrical signals do not have to penetrate the brain or other nerve tissue because infrared light can penetrate "quite a bit of tissue," so devices emitting the light "have potential for excellent biocompatibility," Rabbitt said. "You will be able to implant optical devices and leave them there for life."

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