Transcranial photobiomodulation (tPBM), a non-invasive and non-thermal brain stimulation therapy, was proposed over 50 years ago. Numerous related devices are also available in the market. tPBM was known as low-level laser (light) therapy at the early stage, but more and more researchers prefer tPBM in recent years because it is more indicative of its scientific principle.15
tPBM refers to applying low irradiance (0.01 to 10 W/cm2) red to near-infrared (NIR) (600 to 1300 nm) light16,17 through the skull directly to brain tissue to achieve neuroprotection, behavioral improvement, and so on. One of the most recognized molecular mechanisms of tPBM is that cytochrome c oxidase (CCO) may dissociate inhibitory nitric oxide (NO) after absorbing photons, thereby enhancing mitochondrial activity and promoting ATP biosynthesis.18 Because brain disorders are closely related to mitochondrial activity, tPBM may have beneficial effects on various brain diseases.19
During the past two decades, a tremendous number of pre-clinical experiments with animals and clinical trials with humans have demonstrated beneficial effects. Clinical trials with humans include, in particular, the treatment of ischemic stroke,20–24 Alzheimer’s disease (AD),25–28 Parkinson’s disease (PD),29–34 traumatic brain injury (TBI),35–40 depression,41–44 aging,45–50 etc.
In these studies, the parameters of the light used are extremely sophisticated and deserve to be deliberated. For wavelength, researchers prefer using 808 nm-light [Figs. 1(a) and 1(b)], which has a strong absorption peak of CCO with an excellent penetration depth.142 Furthermore, there also exist extensive studies that have applied other wavelengths (e.g., 610, 1070, 1267 nm) of light for brain disease treatment for specific biological targets including but not limited to mitochondrial stimulation.51,74,75,143