Experimental characterization of the distribution of collagen fiber recruitment
The passive biomechanical behavior of blood vessels is generally modeled by a parallel arrangement of elastin and collagen, with collagen recruitment depending on vessel strain. We experimentally determined the collagen recruitment distribution using confocal microscopy. Digital images from sections of rabbit carotid artery under increasing circumferential stretch ratio were acquired. The straightness of the fibers was measured to compute the fraction of recruited fibers for each stretch ratio. The experimental distribution obtained was then used in the model proposed by Zulliger et al. This model is based on a strain energy function (SEF), which provides a constituent-based description of the wall mechanics. Using this model, a fit of the pressure–radius curve was then performed by using the experimental collagen recruitment distribution with the collagen elastic constant as a free fit parameter. The fit was good, but the value of the collagen elastic constant obtained was below values reported in the literature. A second fit of the pressure–radius curve was also performed, whereby the collagen distribution was left free to adapt while the collagen elastic constant was set to a physiological value. The differences between the experimental collagen engagement distribution and the distribution obtained when the collagen elastic constant was fixed were analyzed. Good qualitative agreement was found between the experimental distribution of collagen recruitment and the model. Some experimental limitations and modeling approximations remain. Nevertheless, experimental proof of progressive collagen recruitment was established, which validates the basic hypothesis of the model.