3D-printing of selectively porous, freestanding structures via humidity-induced rapid phase change

The emergence of three-dimensional (3D) printing has driven the advancement of fabricating 3D structures for various applications. However, the current printing techniques suffer from long print times and limited versatility. Particularly, for methods using material extrusion, printing of freestanding 3D structures without support remains a challenge. Herein, a new approach is introduced for printing of selectively porous, freestanding structures without support through a rapid liquid-to-solid phase change mechanism enabled by humidity. The mechanism works by rapid diffusion and dissolution of a volatile organic solvent, e.g., tetrahydrofuran (THF), in the humidity, thus leading to a quick solidification of a dissolved polymer precursor, e.g., polycaprolactone (PCL). Various structures including a 3D lattice, a six-petal flower, a honeycomb, a hollow cone, and a freestanding cage were successfully printed. These structures were printed with a 159-μm inner-diameter nozzle at a linear print speed of 25 mm/s, resulting in a volumetric print speed in the range of 0.3–0.4 mm3/s. Correlation of the printing parameters with the resulting structures was systematically investigated. They show surface pores with sizes of tens of micrometers. Their average pore size and density increase with the humidity level. Furthermore, the printed material demonstrated great biocompatibility. It is expected that this novel printing technique can be applied to different inks with various compositions for high-throughput production of complex, multifunctional 3D structures, thus paving a route to many applications.