The preparation process of 3D bioengineered brain tissue. a) Aqueous-based silk sponge hydrated in water, scale bar 1 cm; b) Cutting out 6 mm diameter discs, scale bar 1 cm; c) Magnification of the disc, scale bar 5 mm; d) Cutting out 2 mm diameter central space, scale bar 5 mm; e) Silk donut-shaped scaffold, scale bar 5 mm; f) Silk scaffold filled with collagen gel, scale bar 5 mm; g) 3D bioengineered brain tissue constructs in 24-well tissue culture plate, scale bar 5 mm.