Determination of the mechanical properties of brain tissue subjected to low strain rates is beneficial and invaluable to biomedical applications such as the design of bio-compatible neural prosthesis. Published literature on these properties has been attained from experiments using brain tissue that were tested within several hours postmortem. The material properties of brain tissue tested within a time span of several days has never been documented before, yet is invaluable in many fields such as the forensic sciences or cases where only cadaveric tissue is available. Having an understanding of tissue behavior attributed to degradation could have significant implications on how we assess the validity of all published data in the research literature describing brain tissue behavior.
A degradation study on swine cerebrum was performed using an in vitro unconfined compression test protocol at low strain rates to characterize how the tissue mechanical behavior varies with postmortem age and to illustrate the benefit of statistical analysis for exploring the factors that influence the mechanical behavior of degrading brain tissue. The experimental dataset for this study consisted of pig brains that were aggregated into four postmortem age groups (<6 hours, 24 hours, 3 days, and 1 week). The variability in the mechanical behavior of the brain tissue attributed to regional effects and different compressive rates was also considered.
The fractional Zener (FZ) constitutive model, which uses concepts from fractional calculus, was selected to predict the behavior of postmortem swine brain tissue. This model was defined in three-dimensional (3D) form to facilitate its use via the finite element (FE) method. The advantage of using the FZ model to describe brain tissue is the inclusion of a parameter α, which quantifies the viscoelasticity of a material. The relationship between the viscoelasticity of the brain described by α and tissue degradation was examined using statistical analysis. The robustness of the fractional Zener model was illustrated by comparing this model with the experimental data and two integer based models (Zener and hyperviscoelastic). The results showed that the FZ model can (i) be used to describe the viscoelasticity of the brain tissue, (ii) the parameter α should not be used as a metric for quantifying postmortem age, and (iii) the model can be modified to describe tissues subjected to large strains.
The benefits of utilizing a physiologically realistic model to describe silicon-substrate microelectrode arrays coated with hydrogel material were explored using FE simulations. The brain and/or hydrogel coating were described using either the FZ or one of the following two integer based constitutive models: linear elastic and hyperviscoelastic. Varying coating thicknesses were considered for the neural microelectrode to assess its impact on reducing the strains in the cerebral cortex attributed to longitudinal micromotion. Increasing the electrode coating thickness decreased the strain in the brain. The FE study also showed that the maximum electrode displacement and the effect of microelectrode adhesion to the brain on the resulting strain distributions were sensitive to the constitutive model choice.