Researchers from the University of Utah have succeeded in using infrared light to make rat heart cells contract and toadfish inner-ear cells send signals to the brain. This opens up many possibilities for implants using infrared light instead of electrical impulses to stimulate neurons and body functions. From the press release:
The scientists exposed the cells to infrared light in the laboratory. The heart cells in the study were newborn rat heart muscle cells called cardiomyocytes, 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 hair cells came from oyster toadfish, which are well-establish models for comparison with 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, says Rabbitt [Richard Rabbitt, professor of bioengineering and senior author of the heart-cell and inner-ear-cell studies].
Using infrared radiation, "we were stimulating the hair cells, and they dumped neurotransmitter onto the neurons that sent signals to the brain," Rabbitt says.
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."
The infrared radiation affects the flow of calcium ions in and out of mitochondria – something shown by the companion study in 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," says Rabbitt.
The heart cell study found that an infrared pulse lasting a mere one-5,000th 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.
Rabbitt believes the research – including a related study of the cochlea last year – could lead to better cochlear implants that would use optical rather than electrical signals.
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 says.
"A healthy adult can hear more than 3,000 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."
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 were able to apply 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.
Press release: Will We Hear the Light?
Inner-ear study: Infrared photostimulation of the crista ampullaris…
Cardiac study: Intracellular calcium transients evoked by pulsed infrared radiation in neonatal cardiomyocytes…