Femtosecond lasers are used in applications as diverse as optical microscopy and lasik eye surgery. Now scientists at Osaka University in Japan have discovered that femtosecond laser pulses can cause heart muscle cells to contract.An increase in calcium levels within these cells normally signals them to contract. Therefore, as the investigators stimulated the cells with a femtosecond laser, they monitored the intracellular calcium levels using a fluorescent label for calcium and an Olympus microscope in fluorescence mode. They switched the microscope into phase-contrast mode to watch the cells contract.Scientists entrained heart muscle cells to contract and relax in sync with the periodicity of exposure to femtosecond laser pulses. Reprinted with permission of Optics Express. The researchers stimulated both the heart muscle cells and the calcium-indicating fluorophore at 780 nm using a Spectra-Physics Ti:sapphire laser that was mode-locked to deliver 80-fs pulses with an 82-MHz repetition rate. They used a mechanical shutter from Suruga Seiki Co. Ltd. of Shizuoka, Japan, to expose the cells to the laser for 8-ms bursts, applying the bursts at a rate of 1 to 2 Hz. As detailed in the June 9, 2008, issue of Optics Express, the scientists triggered heart muscle cells to contract and relax repeatedly in sync with the period at which they opened the shutter. They demonstrated this effect with both individual cells that were not contracting initially and with groups of cells that already were contracting spontaneously. The results showed that the femtosecond laser could act as a pacemaker. They found a window for this effect from 15 to 30 mW of average laser power, which was measured at the focus with an Ophir photodiode meter. Within this window, calcium levels were normal inside the cells. Below 15 mW, the cells did not contract and calcium levels were low. Above 30 mW, the cells contracted but did not relax, and the calcium levels were unusually high. The pacemaking effect of the femtosecond laser could be used to study irregular heart rhythms known as fibrillations on a cellular level, as well as antifibrillation drugs. Because long-term exposure to pulses from these lasers can damage the heart, this discovery will not have immediate clinical applications.