Human Pain Research Group

Carbon dioxide (CO2) lasers are commonly used in pain stimulation as they emit at a wavelength of 10.6 microns which is absorbed by water in the skin, causing heating. The heat emitted by the laser is absorbed within the epidermis and conducts through the tissue to activate the nociceptors (nerve fibres responsible for pain sensation). Currently, we use a CO2 laser pain stimulator developed within the group for the majority of our pain studies. We are working on the development of a new CO2 laser pain stimulator which will allow variation in the beam diameter and profile, will be operated via a user-friendly computer interface and will be portable for transportation to other laboratories.

Lasers used in pain stimulation generally have Gaussian profile beams which have an intensity peak in the centre. This means that the heat from the laser is not distributed evenly over the skin. Modelling which we have carried out indicates that a super-Gaussian laser beam which has a more uniform intensity distribution will provide more uniform heating of the skin. This may result in a more consistent pain sensation and reduce the likelihood of skin damage. The new laser stimulator will be designed to allow the operator to select either a Gaussian or a super-Gaussian beam profile.

We are also investigating the possibility of delivering the laser through a specialised optical fibre suitable for transmission at this wavelength, and are working on a feedback system incorporating and infrared sensor to maintain the skin at a fixed temperature.

We are developing a novel pain stimulator based on a thulium (Tm) fibre laser, emitting at a wavelength of around 2 microns. At this wavelength, the laser radiation penetrates deeper into the tissue, with the result that the nociceptors may be activated without heating the surface of the skin to such a high temperature. This reduces the risk of skin damage. The wavelength also makes it possible to deliver the stimulus to the skin via an optical fibre so that the pain stimulator can be used in an MRI scanner. In addition, the wavelength of the Tm fibre laser may be tuned, thereby altering the depth to which it penetrates in skin so that different populations of nociceptors can be activated.

We have constructed a prototype Tm fibre laser pain stimulator, which is compact and portable. Studies carried out on volunteers have allowed us to demonstrate that this type of laser is suitable for use in pain research. The laser energies corresponding to sensory and pain thresholds have been found for different beam diameters. In addition, the first fibre laser evoked potentials have been recorded, using electroencephalography (EEG) to measure the electrical activity in the brain resulting from pain elicited by the laser.

Key publication:
Warnaby CE, Coleman DJ, King TA. Photothermal modeling of thulium fibre laser-tissue interactions. Proceedings of Therapeutic Laser Applications and Laser-Tissue Interactions, SPIE 2003; 5142:68-75

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