Figure 3 . Adhesion strength characterization of DIW printed fully cured two-stage resin structures. (a) Two-stage resin structures printed onto (i ) acrylic, (ii ) wood, (iii ) glass, (iv ) aluminum, and (v ) concrete substrates. (b) Experimental setup showing the tensile grip clamped to the sample on fixed substrate. (c) Quantified adhesion strengths of printed structures on the aforementioned substrates at 0 hour (solid) and 72 hours (patterned).
3.3    Unstructured 3D Printing
To demonstrate the capabilities of our unstructured AM process and two-stage resin, we 3D printed various inverted structures including a bio-inspired “beehive”, conical geometries, and load-bearing arches (Figures 4a-d, Video 1). Using the inverted printing technique, we were able to create conical structures with large overhangs without support materials due to a combination of gravitational forces acting “upwards” relative to the structure as well as rapid photopolymerization to solidify layers. The synergy of these two phenomena enabled us to create large structures with comparably small base layers without sagging. Furthermore, the multi-phase polymerization mechanism of the printed ink ensured that the large structures preserve their shape integrity over time, thereby ensuring the larger structures continue to resist gravitational forces and do not delaminate from the substrate. To further exemplify the demonstrated overhang printing capabilities coupled with progressive substrate adhesion strength, we printed 24 mm tall arches (Video 2). Upon fully cured, the arches could support a one-kilogram weight, as corroborated by adhesion strength characterization previously discussed (Figure 4e). This is of particular interest when considering the application space of such a structure. To accomplish this feat using conventional DIW manufacturing, the thin-walled structure would likely need to be printed with support material [26], then removed from the original print substrate and repositioned in an inverted configuration, thereby compromising the bond strength between the structure and end-use surface. Conversely, the proposed unstructured print technique can satisfy this load bearing task with an as-printed, high-strength structure.