Measurement of thermal expansion coefficient of melanin for photoacoustic technology

Photoacoustic technology can non-invasively obtain the temperature and pressure of tissues, holding great promise for applications in the laser thermal ablation of pigmented skin diseases. The coefficient of thermal expansion is the primary source of temperature sensitivity in photoacoustic technology. In this paper, a non-contact full-field strain measurement system based on temperature-variable three-dimensional digital image correlation is used to measure the variation of the thermal expansion coefficient of melanin in the retinal pigment epithelium layer of porcine eyes. It is found that the thermal strain of melanin exhibits non-uniformity and nonlinear increase in radial Angle and circular domain. Before the glass-transition temperature (49°C), the average coefficients of thermal expansion for concentric circular regions and different radial directions are 4.14 × 10^-4 K^-1 and 3.82 × 10^-4 K^-1, respectively. Approximating the thermal expansion coefficient of melanin with that of graphite leads to a large error, with a difference of two orders of magnitude.