  Understanding the rheological properties of soft biological tissue is a key
issue for mechanical systems used in the healthcare field. We propose a simple
empirical model using Fractional Dynamics and Exponential Nonlinearity (FDEN)
to identify the rheological properties of soft biological tissue. The model is
derived from detailed material measurements using samples isolated from porcine
liver. We conducted dynamic viscoelastic and creep tests on liver samples using
a rheometer. The experimental results indicated that biological tissue has
specific properties: i) power law increases in storage elastic modulus and loss
elastic modulus with the same slope; ii) power law gain decrease and constant
phase delay in the frequency domain over two decades; iii) log-log scale
linearity between time and strain relationships under constant force; and iv)
linear and log scale linearity between strain and stress relationships. Our
simple FDEN model uses only three dependent parameters and represents the
specific properties of soft biological tissue.
