printing is, however, limited by various factors, with printing speed and material versatility being the most decisive. From the standpoint of printing speed, layerwise printing by digital light processing (DLP) offers significant inherent advantage over point-wise printing by most other methods such as fused deposition modeling (FDM) and stereolithography (SLA). [20] Further innovations on DLP such as continuous building in the z-dimension allow achieving a printing speed far exceeding any other methods. [21][22][23] Recent attempt on direct forming 3D object in a volumetric fashion forgoes even the layer-wise printing, but the technique has yet to evolve into the mainstream. [24,25] Generally, DLP employs light curable liquid resins. Upon digital light exposure, the resin cross-links and forms a thermoset polymer that ensures its separation from the surrounding liquid precursors. This rapid liquid-solid separation is the key enabling characteristic for DLP. Although cross-linking ensures such, the resulting thermoset polymers cannot be reprocessed. This prohibits its broader utility in situations that require further processing of the printed materials. In principle, this limitation can be overcome if the DLP processing can be extended to reprocessable thermoplastic polymers. Although light curable thermoplastic polymers are known, meeting the unique DLP requirement for rapid separation between the liquid monomer and the un-crosslinked polymer is not straightforward because of their intrinsic good miscibility. Hereafter, we describe our successful attempt in DLP printing of a thermoplastic polymer and lay out key enabling factors for such a process. We further illustrate that the water dissolvable nature of the resulting polymer is advantageous in making various multifunctional 3D structures. We find that the oxygen naturally present in ambient air can inhibit the light curing process. This effect can be utilized to achieve rapid open-air printing without the complex interfacial engineering typically required for fast DLP printing methods. [21][22][23] We hypothesize that two key factors should be carefully considered for DLP printing of thermoplastic polymers (Figure 1a). Herein, two concurrent and competing processes occur: 1) the polymerization and 2) the diffusion/dissolution of the polymer into the surrounding uncured liquid monomer. Rapid liquidsolid separation is possible only if the first process dominates 3D printing has witnessed a new era in which highly complexed customized products become reality. Realizing its ultimate potential requires simultaneous attainment of both printing speed and product versatility. Among various printing techniques, digital light processing (DLP) stands out in its high speed but is limited to intractable light curable thermosets. Thermoplastic polymers, despite their reprocessibility that allows more options for further manipulation, are restricted to intrinsically slow printing methods such as fused deposition modeling. Extending DLP to thermoplastics is highly desirabl...