3D Printing of Air

Historically, three-dimensional (3D) printing involved depositing tangible materials onto a substrate or within a supporting medium to create solid and porous architectures. Here, we unveil 3D printing of air, an intangible and invisible ink having ~10^9 times less viscosity than conventional inks, to spatially pattern bubbles and fabricate freeform air channels in 3D within diverse materials. This study delves into air bubble dynamics, encompassing formation, deformation, spatiotemporal stability in non-spherical configurations, and interaction within yield stress materials. We integrate machine learning algorithms to predict air printability based on material properties, providing a framework for rational material selection. The interplay between yield stress, viscosity, and nozzle diameter facilitates the formation of stable air channels with extremely high aspect ratios (~ 4×10^4). These insights establish a foundation for harnessing air as a printable medium and as a functional ink in applications spanning biology, optics, material science, engineering, and medicine.