Effect of Preservation Period on the Viscoelastic Material Properties of Soft Tissues With Implications for Liver Transplantation

Abstract

The liver harvested from a donor must be preserved and transported to a suitable recipient immediately for a successful liver transplantation. In this process, the preservation period is the most critical, since it is the longest and most tissue damage occurs during this period due to the reduced blood supply to the harvested liver and the change in its temperature. We investigate the effect of preservation period on the dynamic material properties of bovine liver using a viscoelastic model derived from both impact and ramp and hold experiments. First, we measure the storage and loss moduli of bovine liver as a function of excitation frequency using an impact hammer. Second, its time-dependent relaxation modulus is measured separately through ramp and hold experiments performed by a compression device. Third, a Maxwell solid model that successfully imitates the frequency- and time-dependent dynamic responses of bovine liver is developed to estimate the optimum viscoelastic material coefficients by minimizing the error between the experimental data and the corresponding values generated by the model. Finally, the variation in the viscoelastic material coefficients of bovine liver are investigated as a function of preservation period for the liver samples tested 1 h, 2 h, 4 h, 8 h, 12 h, 24 h, 36 h, and 48 h after harvesting. The results of our experiments performed with three animals show that the liver tissue becomes stiffer and more viscous as it spends more time in the preservation cycle.

Dynamic Material Properties of Human and Animal Livers

“…However, material properties of soft tissues change in time and the results obtained from in vitro measurements can be misleading. The results obtained by Ottensmeyer et al [12] and Ocal et al [11] show that the soft tissues become stiffer and more viscous in time (Fig. 8).…”

“…However, due to the nature of these experiments, the information that can be extracted from each one is different though a conversion from time to frequency domain or vice versa is possible through Laplace transformations. Ocal et al [11] showed that a better fit to viscoelastic tissue model can be achieved if the results of both experiments are taken into account in the analysis (Fig. 7).…”

“…While a part of this variation is due to the differences in measurement devices, techniques, and site, it is also known that tissue properties vary within the same species depending on the age, weight, and gender. It is still an open question how these variations will be [11] integrated into the biomechanical models of liver. Also, most of the existing data has been collected invasively through mechanical measurement devices, which limits their usage in human studies.…”

“…On the other hand, if a robotic arm is used for the same measurements, the problems Fig. 7 The flow-chart of the optimization process followed for estimating the viscoelastic material coefficients of soft organ tissues [11] related to the actuation and position sensing can be solved. More controlled indentations can be performed on tissue surface by a pre-programmed robotic arm and an indenter attached to the arm.…”

Sample, testing and analysis variables affecting liver mechanical properties: A review

“…Moreover, interpreting in-vivo data is challenging due to difficulties in obtaining appropriate alignment between the instrument and tested specimen, the presence of physiological noise and the inability to account for and control the internal condition of the organ [11][12][13]. On the other hand, ex-vivo experiments are preferable when developing new testing devices, protocols and tissue models, enabling easier and more direct testing procedures with better control of boundary conditions, in addition to being less ethically problematic than in-vivo measurements [14][15][16][17][18]. However, although there are many studies and methods published in the literature, a unique set of values of liver mechanical properties is still lacking and reported results are strongly dependent on several factors (many of them are outlined in Fig.…”