A novel temperature-controlled laser system to uniformly activate cutaneous thermal receptors during movable thermal stimulation

Abstract

Objective. Laser stimulators have been widely used in pain studies to selectively activate Aδ and C nociceptors without coactivation of mechanoreceptors. Temperature-controlled laser systems have been implemented with low-temperature variations during stimulations, however, these systems purely enabled stationary stimulation. This study aimed to implement, test and validate a new laser stimulation system that controls skin temperature by continuously adjusting laser output during stimulus movement to allow accurate investigation of tempo-spatial mechanisms in the nociceptive system. Approach. For validation, laser stimuli were delivered to the right forearm of eight healthy subjects using a diode laser. The laser beam was displaced across the skin to deliver a moving thermal stimulation to the skin surface. To test the function and feasibility of the system, different stimulation parameters were investigated involving two control modes (open-loop and closed-loop), three displacement velocities (5, 10 and 12 mm s−1 ), two intensities (high 46 °C and low 42 °C), two stimulus lengths (20 and 100 mm) and two directions (distal and proximal). Main results. During closed-loop control, the stimulation error and variation of stimulation temperatures were significantly smaller than during open-loop control. The standard deviation of stimulation temperatures increased significantly with stimulation intensity and displacement length. Significance. This study showed that more accurate, less variable laser stimulations were delivered to the skin using closed-loop control during a movable stimulus. The more uniform skin temperature during stimuli is likely to ensure a more uniform nociceptor activation.