a | Cellular identity is spatially patterned by morphogen gradients in vivo. Similar morphogen or growth factor gradients can be generated by microfluidics, and growth factor release can be stabilized using drug-releasing microparticles or nanoparticles. Such approaches can enable spatially patterned in vitro cultures in two dimensions and three dimensions. b | The developing brain comprises different cell types and structures, including blood vessels. Bioprinting and biomaterial-based scaffolds may enable the generation of more complex structures and human neurovascularization in three dimensions. c | Electrical activity during development can be emulated by electrical stimulation using engineered materials, such as carbon nanotubes, graphene and conductive polymers. Ultimately, such techniques may enable a multi-electrode array-like structure embedded in three-dimensional (3D) cultures. BMP, bone morphogenetic protein; SHH, sonic hedgehog.