a, Schematic illustration of the behaviour of the body matrix during the EMB3D printing process. (i) When the body matrix is at rest, the fumed silica fillers within the silicone material form a percolated network, giving rise to its equilibrium, at-rest shear yield stress, τ_y,0. (ii) As the nozzle travels through the matrix during EMB3D printing, the matrix is yielded and the percolated filler network is disrupted, decreasing the apparent yield stress of the matrix material, τ_y,t. (iii) Sufficient deformation can completely disrupt the fumed silica microstructure and completely eliminate the yield stress of the matrix material (τ_y,t → 0 Pa). (iv) The fumed silica network does not immediately recover when it returns to a quiescent state. (v) Over time, the network slowly restructures to (vi) its equilibrium microstructure, and τ_y,t → τ_y,0. b, c, Log–log plots of apparent viscosity (b) and corresponding shear stress (c) versus shear rate for various PDMS matrix formulations, which are prepared by blending Sylgard 184 (10:1 ratio of base to hardener) and SE 1700 (4:1 ratio of base to hardener) at various mass fractions. The formulations are listed by the weight ratio of SE 1700 used (0.0,0.33, 0.5, 0.67 and 1.0). Closed and open circles in c represent measurements taken during the flow sweep and flow ramps of the thixotropic loop studies, respectively. The final body matrix, formulated from the 50 wt% SE 1700 blend, shows clear thixotropic behaviour and a substantial decrease in yield stress upon yielding. Blends with higher concentrations of filler particles show diminished thixotropic behaviour, and the yield stress is not eliminated during nozzle translation. Consequently, crevices or air pockets form during printing, with matrix materials possessing higher concentrations of fumed silica.