Review of the First Petit & Laser Live Workshop by Korean Aesthetic Surgery and Laser Society III

Difference between TRT and SRT

There is one important factor that the above study failed to address and that is SRT(stress relaxation time). For example, the central temperature of melanosome takes time to come down to 500℃ from 1000℃. This time is defined as TRT. Irradiating Laser with pulse duration shorter than this time allows the heat to effectively destroy the target. This is the basis of selective photothermolysis. SRT is another simple concept. When tissues expand from exposure to heat and generate shock waves. This is similar to the heat waves that spread hundreds of kilometers following a nuclear bomb explosion. SRT is defined as the time it takes for heat waves to exit the tissues. Sufficient energy given at a shorter pulse duration than SRT allows destruction of target tissues by shock waves with minimal impact on surrounding tissues. This is the biggest difference between picosecond Laser and nanosecond Laser. The SRT of melanosome is defined as 300ps. This is also the reason why the manufacturers of picosecond Lasers are striving toward 450ps and 300ps. Shock waves are not particular to picosecond Laser. Exposure to all Laser energy generates shock waves but whether stress confinement occurs is another matter.

Image 3. <Computational Modeling of Stress Transient and Bubble Evolution in Short-Pulse Laser Irradiated Melanosome Particles>. According to this study, the picosecond Laser has the pulse duration that satisfies melanosome SRT.

If Laser energy destroyed only melanosomes, there would be no negative impact on melanocytes. I assume many doctors may have experienced PIH following Laser toning treatments. In the early days of Laser toning, the recommended output was 2.5J. This, I believe, is now lowered to about 1-1.5J, however, complications still exist. Laser toning inevitably destroys not only melanosomes but melanocytes as well. Currently, I set the nanosecond Laser at 1.5J and the picosecond Laser at 0.8J for Laser toning. The picosecond Laser is more efficacious using almost half the energy.

Picosecond Laser Satisfies Melanosome SRT

Image 4. Effective stress relaxation time in melanosome.

This paper² examined how melanin and melanosome react to picosecond Laser irradiation. The TRT of melanin and melanosome are 400ps and 1μm, respectively. The picosecond Laser model used in the study had the pulse duration of 30-100ps, which is within thermal confinement and sufficiently satisfies TRT. The SRT of melanin and melanosome are 10ps and 300ps, respectively. Pulse duration of 100ps puts melanosome within stress confinement. The picosecond Laser in this study was not able to destroy melanin.