Figure 1: Mechanical properties of the intact cortex.

(a) Schematic showing the location of the stab injury in the rat brain and the areas of the cortex where elasticity measurements were recorded by AFM. CC indicates corpus callosum, Cg cingulum, AGm medial agranular, AGI lateral agranular and ACC anterior cingulate regions of the cortex. An AFM cantilever indents the brain tissue; the applied force is translated to a bending of the cantilever that is readable through changes in the laser light path and detected by a photodiode. (b) The force–indentation curves obtained by AFM micro-indentation tests followed a spherical Hertzian contact model. For each force–indention curve, the indentation depth and force were divided by their respective maximum values. The black line passing the averaged force values serves as a guide to the eye; error bars are s.d. The inset shows the normalized force–indentation curves on a log–log scale; the curves follow a slope of ∼1.5, consistent with the slope of the Hertz contact model. (c) Spatial mapping of the elastic modulus of a healthy rat brain cortex. The elastic modulus at each pixel is measured by AFM indentation and is represented as a rainbow-coloured palette map with blue denoting softer and red corresponding to stiffer regions. The elasticity map and the brightfield image of the tissue are overlaid. Scale bars, 500 μm. (d,e) The elasticity map is symmetric around the midline; three regions (that is, R₁, R₂ and R₃) with significantly different mechanical properties were identified in each brain hemisphere. The AGm that contains larger amounts of myelinated axons (that is, R₂ and R′₂) was significantly softer than the AGl (R₁ and R′₁) and the ACC (R₃ and R′₃). (e) The average percentage drop in elastic modulus was calculated considering the combined average elasticity of regions R₃ and R′₃ as the baseline. Error bars are s.e.m., **for P=0.003 and ***for P<0.001, Tukey–Kramer post hoc test.