Viscoelastic Properties of Macaque Neural Tissue at Low Strain Rates

Increased knowledge of the mechanical properties of soft tissue subjected to low strain rates is beneficial to biomedical applications, such as designing bio-compatible implants, developing minimally invasive surgical techniques and surgical simulation devices for training surgeons. Unconfined compression and indentation experiments were conducted to extract macro- and micro-level mechanical properties of Macaque neural tissue. The tissues were placed in physiological saline solution and tested at room temperature within one hour post-sacrifice and three weeks post sacrifice using unconfined compression and indentation experiments. For each test, the temporal lobe was sectioned into 26 mm diameter disks that were subjected to 1%, 2%, 5%, and 10% strain at a loading rate of 5 mm per minute. The impermeable platen used in the unconfined compression test had a diameter equivalent to the tissue sample. The diameter of the flat tip used in the indentation experiment was 5 mm. Both test configurations utilized a ramp-and-hold strain input to capture the features of the tissue attributed to the stress-strain relationship (ramp) and stress-relaxation (hold). Viscoelastic theory was applied to the experimental data for the calculation of the secant and relaxation modulus, which correspond to the ramp and hold portion of the strain input, respectively. The resulting viscoelastic behavior of the Macaque brain at the macro- and micro-scale were compared with (i) bovine and porcine neural tissue, commonly found in the literature and (ii) viscoelastic models for neural tissue, which in the literature are generally applicable only to large deformations.