From free radical to radical-free lithographic 3D printing

Stereolithographic 3D printing via free radical polymerization (Fig.1 A) is a versatile manufacturing method due to its high resolution, smooth surface finish and part size scalability compared to other printing techniques. While material formulation variability allows for some flexibility of material properties, most printing processes are restricted by the material properties of radically cured photopolymers. Overcoming major drawbacks such as brittle fracture and high volumetric shrinkage is focus of ongoing research.
By enabling elevated printing temperatures up to 120 °C, slower and more controlled, non-radical polymerization mechanisms become available for stereolithographic 3D printing. For example, less reactive cyclic monomers as they are encountered in cationic and anionic photopolymerization become accessible in 3D printing at elevated temperatures. We have demonstrated printability of pure poly(ether)ester and poly(ether)carbonate networks (Fig.1 B), which exhibit highly desirable characteristics such as biocompatibility, degradability and low shrinkage. Moving from chain growth towards step growth polymerization mechanisms, clever choice of processing temperature even allows printing via polycondensation without bubble-formation from emerging small molecular byproducts. We have shown this for previously inaccessible pure phenolic resins (Fig.1 C).
Another approach to unlock new chemistries and properties for stereolithographic printing is to diversify the light source for light-induced polymerization during the printing process (Figure 1 D). For example, we have recently shown that a simple change in laser intensity during micro stereolithographic manufacturing enables us to produce stable and degradable structures from the same photoresist. Furthermore, wavelength-selective reactions are a promising avenue for stereolithographic 3D printing.