Abstract
Nature uses hairy and fibrous structures for sensing and structural functions as observed in beetle hairs, mammalian whiskers, ear stereocilia, spider silks, and hagfish slime thread skeins. However, replicating these intricate, thin, high aspect ratio geometries has been a longstanding manufacturing challenge. It is currently not possible to fabricate 3D fibrous structures with microscale diameters and centimeter lengths. Here, we demonstrate 3D printing of fine hair having freeform trajectories and substrate fixation, with fiber diameters as fine as 1.5 µm and continuous length. This is realized via 3D printing by solvent exchange (3DPX), where the nozzle is embedded in a bath of microgranular gel. The yield stress rheology of the bath material allows the free-form placement of the fibers with bioinspired trajectories. The solvent exchange induces rapid radial solidification of the extruded polymer solutions at a rate of 2.33 µm/s, resulting in an elastic plateau modulus increase by four orders of magnitude, which prevents capillarity-induced fiber breakage. Furthermore, 3DPX offers unparalleled versatility in material selection, including the ability to print with commercially available thermoplastic polymers and nanocomposite materials. This study introduces and demonstrates the potential of 3DPX in replicating natural fibrous structures and paving the way for applications in biomimetic functions.