Local elasticity evaluation of acid-denatured collagen by photoacoustic spectroscopy.

While there are various analytical methods for elasticity evaluation, those with micrometer-order spatial resolution are still under developing. As some of biological tissues such as capillary vessels and cochlea are very small and/or highly heterogeneous, development of analytical techniques with such high spatial resolution has been desired for biological and medical purposes. Especially, the elasticity of capillary vessels (several micrometer in diameter) would be an important indicator to find out early diseases. To measure the local elasticity for such small and/or heterogeneous samples, we have proposed an approach based on a temporal waveform of photoacoustic (PA) signal, i.e., time-domain PA. As the time-domain PA contains both the vibrating frequency and the sound propagation time after the excitation, it provides the information on the local elasticity (from the frequency) at a specific depth (from the propagation time) of samples. In the present study, the signal from collagen sheets were obtained and analyzed as models of blood vessel walls and scaffolds for regenerative medicine. In contrast to previous studies using the agarose gel which showed a single frequency peak, the signal from the collagen sheets was mainly composed of two frequency peaks, assignable to surface and bulk vibration. Further, the bulk vibration was found to sensitively reflect the elasticity of the samples. Since the PA effect can be induced only at the position where the light absorber exists, the analytical method proposed here would allow us to measure the local elasticity and its spatial distribution in blood vessels and other tissues.