Neural network formation in a deep HCWN system. (a) Concave channel networks were fabricated by exploiting the surface tension of a PDMS prepolymer, as follows: i) Prepolymer PDMS was poured onto a base mold (PDMS). ii) The prepolymer was removed from the base mold by wiping out liquid PDMS, forming a meniscus. iii) For deep HCWN plates, PDMS prepolymer left behind after step ii is removed by suctioning. iv) After curing, the base mold was used for fabrication of a prepolymer PDMS concave channel network. (b,c) SEM images of a cross-section of the concave channel network, showing connections of concave well arrays with concave channels in shallow HCWN plates (b) and deep HCWN plates (c). (d) Schematic view showing the dimensions of shallow and deep HCWN plates. (e) Schematic depiction of formation of a neural network, including satellite spheroids and neurite bundles, in a concave channel network.