Speakers
Description
Introduction:
Thermal heating of the skin using infrared lasers has been a valuable tool in pain research for multiple decades. Recently we have developed a closed-loop thermography-controlled system which allows accurate temperature control during moving laser stimulation. Using this setup the thermal energy transduction is no longer limited to transversally across the skin, but the movement causes a significant longitudinal transduction as well. Thus, the aim of this study was to model and understand how the skin and thus receptors are heated during stimulation and how long temperature increase remains within the tissue.
Method:
Infrared (IR) thermography data from 8 healthy participants (Rujoie et al., 2023) was used in this study. The experimental data contained both open and closed-loop stimulations and two laser stimulation intensities, 42 and 46°. During open-loop the power setting was kept fixed during stimulation (based on trial and error to obtain a target temperature) whereas during closed-loop control the laser power was continuously adjusted based on IR thermography. The stimulation was delivered to the volar forearm and had a length of 100mm and velocity of 10mm/s. A three-layer finite element model was used to investigate the temperature profiles within the tissue during stimulation (Frahm et al., 2010, 2020; Lejeune et al., 2023), tissues were epidermis, dermis and hypodermis. Power and other settings in the model were based on the experimentally used values in our previous study.
Results:
For the 42 and 46°C stimulation intensities, the experimental open-loop average power settings were 8.5 ±0.3 and 13.6±0.5W respectively, and for closed-loop the average power settings (which varied during stimulation) were 11.6±2.4 and 16.4±2.8W respectively.
The average skin temperature for the 42 and 46°C targets were 39.5 ± 0.9°C and 44.1±1.3°C for open-loop respectively, and 42.0±0.5 and 46.0±0.8°C for the closed-loop respectively.
The model showed that when simulated the average temperature during open loop the surface temperature the simulated skin surface temperature matched the temperature targets (~42 and ~46°C respectively), whereas for closed-loop the simulated temperatures were 44.4 and 48.2°C, respectively. For the 46°C stimulation the temperature in deeper tissues (1.35 mm from the surface) reaches more than 51°C.
Furthermore, the model showed that deeper tissues were heated to an even higher temperature than what was observed on the skin surface. This is interesting as this could lead to a higher degree of neural activation that expected, furthermore, the risk of skin damage will increase, and this must be considered for safety aspects. Finally, the energy in the tissue took more than 10 seconds to return to baseline.
Conclusion:
This study shows that heating of the skin is very complex and even using the power settings which the model indicates to be optimal, the experimental temperature was still off target. Conversely, simulating the average power during closed-loop infrared diode-laser stimulation indicated a much higher deeper tissue temperature than desired. This is due to the complexity of tissue thermo-dynamics which will vary during stimulation. Thus, closed-loop control is essential to ensure uniform and less variable temperature profiles.
