Order-disorder phase transition of cell membrane induced by THz irradiation

To elucidate the mechanism by which THz radiation non-thermally affects living organisms, the lateral diffusion constants of lipid molecules in the cell membranes of HeLa cells were measured using fluorescence recovery after photobleaching under THz wave irradiation (THz-FRAP) at frequencies of 0.10, 0.29, and 0.48 THz, with power densities ranging from 20 to 89 mW/cm². The potential heating effects of the THz irradiation were eliminated through temperature calibration using an ultrathin thermocouple, allowing for the investigation of the non-thermal effects of THz radiation. Irradiation at 0.10 and 0.29 THz induced an increase in diffusion constants at temperatures lower than the cell growth temperature, indicating fluidity of the cell membrane is enhanced by the THz irradiation. We also performed Laurdan fluorescence imaging for HeLa cells to observe change of membrane dynamics under THz irradiation. The red shifting of the fluorescence emission indicates that THz irradiation induces the order-disorder phase transition of the cell membrane lipids by affecting the dynamics of bound water molecules. Our findings have important implications for the establishment of safety standards for THz radiation and for the potential development of new methods for cell manipulation using THz irradiation in the future.