Embedded 3D Printing of Soft Materials and Ultrafine Hair Structures

Embedded 3D printing enables the direct ink writing of ultrasoft materials which, due to gravity, cannot support their own weight in air. For this reason, embedded 3D printing is a platform to replicate nature's complex, branching, and tortuous fibrous structures. However, this recent technology has been limited by a complex interplay of factors such as interfacial tension ratios between the printing materials and the support gel, the viscosity of the ink, and the yield stress of the gel. Because of these complexities, the minimum feature diameter is limited to around 50 µm. Recently, a feature diameter of 8 µm was achieved by tailoring the interfacial tension. This limitation prevents the accurate replication of natural structures, many of which feature dimensions at the micrometer scale, such as long aspect ratio mechanosensing stereocilia and the hairs on the feet of beetles and geckos.<br/>Here, we introduce solvent exchange between the ink and the support gel as a key mechanism to expand the capabilities of embedded 3D printing and allow the fabrication of ultrafine and tortuous structures from a variety of soft materials. This method facilitates the immediate solidification of the extruded polymer solution upon its interaction with the gel's solvent, enabling the printing of ultrafine features as small as 2.1 µm in diameter. Our work exploits the rheological underpinnings that make such precision possible. We elucidate the critical roles played by the gel's yield stress and solvent concentration in achieving this high resolution, thereby offering a comprehensive understanding of the mechanics behind this proposed approach.<br/>Solvent-exchange extends to the versatility of materials available for embedded 3D printing. We demonstrate printing of soft thermoplastic elastomers like styrene-ethylene-butylene-styrene (SEBS) copolymers as well as rigid polymers such as polyvinyl chloride (PVC), and electrically conductive carbon nanotube composites. We applied this method to produce intricate hair arrays, thereby meeting the requirements for bio-mimicking applications. These arrays feature fibers with diameters less than 2 µm and lengths exceeding 1500 µm.<br/>In summary, the solvent exchange method presents a notable advancement in polymer 3D printing. The solvent exchange of direct ink writing embedded in support gels enables the fabrication of biomimetic high ultrafine tortuous structures, overcomes previous resolution limitations, and expands the palette of polymeric materials available for printing. This capability opens new avenues for replicating nature's intricate fibrous morphologies towards the design of new architected composite materials. This work was sponsored by the Defense Advanced Research Project Agency (DARPA), under contract no. N660012124036.