Poynting effect of brain matter in torsion

We investigate experimentally and model theoretically the mechanical behaviour of brain matter in torsion. Using a strain-controlled rheometer, we perform torsion tests on fresh porcine brain samples. We quantify the torque and the normal force required to twist a cylindrical sample at constant twist rate. Data fitting gives a mean value for the shear modulus of mu = 900 +/- 312 Pa and for the second Mooney-Rivlin parameter of c(2) = 297 +/- 189 Pa, indicative of extreme softness. Our results show that brain always displays a positive Poynting effect; in other words, it expands in the direction perpendicular to the plane of twisting. We validate the experiments with finite element simulations and show that when a human head experiences a twisting motion in the horizontal plane, the brain can experience large forces in the axial direction.We thank Badar Rashid for Fig. 1(a–c); David McManus for help with the dissection of pig heads; Xiaolin Li for technical assistance with the rheometer; Christiane Go¨rgen for help with fitting in RStudio; and Giuseppe Saccomandi for insightful discussions on the modelling of torsion. The work has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie grant agreements No. 705532 (Valentina Balbi and Michel Destrade) and No. 642662 (Antonia Trotta and Aisling Nı´ Annaidh)