Projection printing of scaffolds with shape recovery capacity and simultaneously improved stiffness and toughness using an ultra-fast-curing poly(propylene fumarate)/hyperbranched additive resin

Projection micro-stereolithography (PμSL) is an effective technique to rapidly fabricate porous scaffolds with complex structures of photo-crosslinkable polymer resins, which normally contain a reactive diluent and a photo-initiator. In previous reports on 3D printing of poly(propylene fumarate) (PPF) scaffolds, diethyl fumarate (DEF) was unavoidably used as a reactive diluent in the PPF resin. Here we present a photo-crosslinkable resin composed of PPF and hyperbranched polyester acrylate (HPA), which has multiple carbon-carbon double bonds to participate in photo-crosslinking as a reactive diluent, and PμSL 3D fabrication of tissue-engineering scaffolds using this resin. The structure and viscoelastic behavior of uncrosslinked PPF/HPA, and the gel fraction, thermal and mechanical properties of photo-crosslinked PPF/HPA samples have been investigated, in comparison with those of PPF/DEF. Blending HPA with PPF was found to decrease the viscosity of PPF and to expedite the photo-crosslinking process, both of which are important in formation of PPF networks. In addition, HPA can effectively enhance the stiffness and toughness of the fabricated network simultaneously. In PμSL 3D fabrication, the cure depth (Cd) experiments proved that the PPF/HPA resin had an extremely low critical energy (Ec) of 2.1 mJ/cm2, leading to ultra-fast curing when exposed to ultraviolet (UV) light. Our rapid prototyping characteristics are evident under the premise of ensuring high precision. For the PPF/HPA resin, the exposure time per layer was only 0.1–2 s, which at least doubled the printing speed compared with the other 3D fabrication techniques. At high printing resolution of 50 µm, an ultra-fast printing speed of 18 cm/h was achieved by using the PPF/HPA resin, while the value for the PPF/DEF resin was only 3.6 cm/h. Benefited from the better crosslinked networks, the fabricated PPF/HPA scaffolds exhibited a lower shrinkage, higher stiffness and toughness, and better recovery capacity than their PPF/DEF counterparts, indicating their 4D printing characteristics.