▶ Previous Artlcle: #4-1. Laser in Medicine: Basic Laser Terminology

 

SRT(Stress Relaxation Time)

 

The heat applied to a certain particle (chromophore) results in the thermal expansion of the chromophore, leading to the diffusionof the shock wave to the surrounding tissue, which is called "stress diffusion."

 

If a certain chromophore receives the heat for an extremely short time, the time taken for the stress generated in the chromophore to deliver the shock wave to the surrounding tissue gets insufficient, which is called "stress lock-in".

 

If the stress generated in the chromophore under a state of stress lock-in is very large, the chromophore may be broken, but the shock wave generated in this case does not cause a stress diffusion to the surrounding tissue as its effect is limited only to the inside of the chromophore.

 

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The stress relaxation time means the time taken for the shock wave originating from the center of a stressed chromophore to reach the boundary of the chromophore; and the time of irradiation required to create a stress lock-in condition.

 

For instance, Picosecond Laser can create a stress lock-in condition because it is irradiated for a shorter time than SRT (1ns) of tattoo ink, while Nanosecond Laser generates a phenomenon where the shock wave causes a physical damage to the surrounding tissue, instead of creating a stress lock-in condition, because it is irradiated for longer than 1ns.

 

Thus, Nanosecond Laser satisfies thermal confinement because it has a shorter irradiation time than TRT(20ns~3ms) of tattoo ink but does not satisfy stress confinement because it has a longer irradiation time than SRT (1ns) of tattoo ink. However, Picosecond Laser satisfies both thermal confinement and stress confinement because it has a shorter irradiation time than TRT (20ns~3ms) and STR (1ns) of tattoo ink.

 

- To be continued