Mechanical Properties of the Octopus Aorta

The quasi-static mechanical properties of the aorta of Octopus dofleini were investigated using biaxial inflation and uniaxial force-extension tests on vessel segments in vitro. The octopus aorta is a highly compliant and non-linearly elastic structure. The elastic modulus (i.e. the stiffness) measured circumferentially (E_C) and longitudinally (E_L) increased markedly with distension of the vessel wall. E_C was always greater than (E_L), and varied from about 10⁴ to 2×10⁵Nm⁻² between 2 and 5 kPa pressure respectively, the approximate range of resting blood pressure in this species. Increasing vessel wall stiffness is necessary for the aorta to be compliant at low pressure, and at the same time to be protected from ‘blowout’ at high pressure. The non-linear elasticity of the octopus aorta at physiological pressures can be attributed to the properties of the rubber-like elastic fibres which are present in the vessel wall, with little contribution from stiff collagen fibres being required until very high pressures. Dynamic mechanical properties of the aorta were measured by the method of forced oscillations. The dynamic modulus in the circumferential direction increased continuously to almost twice the static value as the frequency was raised from 0.05 to 10 Hz. At the same time, the viscous damping, tan δ, increased from 0.11 to 0.27. The resilience of the octopus aorta was close to 70 % at the revant physiological frequencies. We conclude that this vessel is suitably designed to function as an efficient elastic energy storage component in the octopus circulatory system.