Consider it a different kind of light therapy. Researchers have for the first time used a laser beam to control a heartbeat, an advance that provides a new tool for studying how hearts form and a possible step toward light-based pacemakers.
Standard pacemakers correct irregular heart rhythms with small electrical signals that spur the organ's muscle cells, called cardiomyocytes, to contract. In 1967, not long after electrical pacemakers came into use, scientists discovered that light could accelerate the cells' beating, but they didn't attempt to control how and when the cells sped up. It wasn't until 2 years ago that a Japan-based team successfully set the pace of a cluster of cardiomyocytes with a near-infrared laser. But no one had used light to pace a whole heart.
Then Michael Jenkins, a biomedical engineer at Case Western Reserve University in Cleveland, Ohio, read about the 1967 discovery. He had been working on techniques for imaging heart tissue and began wondering whether the infrared lasers in his lab could set a heart beating in sync with their beams. So he and his colleagues shone the lasers on the hearts of 2- to 3-day-old quail embryos kept alive outside their eggs. At this stage, the embryos' hearts are about 2 cubic millimeters in volume and are little more than a clump of cells, making them easy to work with, says Jenkins. The team reports  today in Nature Photonics that the embryos' heartbeats synced with the laser's pulses (see video). The researchers could the pulse rate by 50%, from two to three beats per second, and then slow it again.
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The team tested different laser energies to find the safest and most effective one for controlling heart rate. If it delivered about 0.8 Joules per square centimeter (less than the energy needed to power a 60-watt light bulb for 1 second), laser pacemaking didn't appear to damage the hearts. However, if the researchers pushed the laser's energy five times higher, the heart cells cooked. More studies are needed to assess longer-term effects in the seemingly undamaged cells, but the results are promising nevertheless, says Jenkins. "We want to know how congenital heart defects form, and how the heart's rhythms during development affect it later on," he says. "Having a noninvasive way to modify the heart rate would be useful."
"I'm quite happy to see this study—it's the first time anyone has shown you can use light to pace-make a whole heart," says Nicholas Smith, the biophysicist at Osaka University in Japan who led the team that first used lasers to set the pace for cardiomyocytes. "If this work can be expanded, there will be surprising things you can do with light," he says.
Expanding the work will involve answering some questions. As in earlier studies, how the light prods the heart into action isn't clear—scientists suspect that the change in temperature plays a role—and Jenkins doesn't know if the same approach can control the beating of a larger adult heart. If these issues can be addressed and the possibility of cell damage ruled out, Smith says, it may be possible to develop light-based pacemakers for use in clinical applications, such as during surgeries, or as implantable devices.