Fig. 1

Setups for diode laser heat stimulation in vivo and in vitro. a Absorption spectra of water (blue), DMEM F12 medium (red) and DRG neurons suspended in DMEM F12 medium (dark red). DMEM F12 with-/without neurons had an absorption peak near 550 nm due to phenol red, suspended neurons induced a slight parallel upwards shift of absorption spectrum due to scattering. Wavelengths of our two diode lasers near the absorption peak of water (1450 nm) are indicated as dashed lines. Inset shows the almost identical absorption of the slightly different wavelengths used (arrows). Wavelength of a laser used previously is marked with dotted line [19]. b Schematic view of set-up for studies in humans. A pilot laser (635 nm) is used for aiming at the skin surface (see inset); the 1475 nm laser radiation has its focus 930 µm below skin surface. c Schematic view of integrated laser stimulation set-up for cell physiology experiments. The diode laser (DL-1470; “IR Laser”) is coupled into an inverted microscope via a beam splitter and focused to the image plane of the microscope. Mirror galvanometers move the laser beam to excite cells directly without any absorbance by aqueous solution. Aiming is verified by an infrared camera on the microscope (see inset)