Naturally Derived Biocompatible Photoinitiators for 3D Printing
Photopolymerization is a technology that fulfills the requirements of instant production. Recently. considerable biocompatible materials have been involved in photopolymerization-based manufacturing. To achieve this, photoinitiator is a crucial component. Commercial photoinitiators, are synthetic and eco-unfriendly, causing potential health concerns due to their migration effect. Therefore, natural photoinitiators have drawn tremendous research interest due to their known bioactivities and nature-derived origin. In this regard, this thesis mainly concentrates on 1) the development of naturally occurring photoinitiators demonstrating excellent photoinitiation abilities, 2) the formulation of the investigated photoinitiators involved conventional commercial resins (i.e., dental resin and epoxy) or 3D resins for shape memory hydrogels. In the present thesis, four natural plant extracts are investigated as photoinitiator candidates, extracted from berries, citrus, pomegranate, and Indigofera, respectively. Moreover, the innovative designed functional materials have been successfully 3D printed. Chapter 1 introduces the recent reported naturally derived photoinitiators. Chapter 2 investigates berries-extracted flavones, morin and quercetin, as photoinitiators. Both morin and quercetin exhibit visible light sensitivity and demonstrate their photoinitiation abilities under the irradiation of visible light. Meanwhile, their photoinitiation ability differences are observed in all kinds of photopolymerizations. Chapter 3 introduces citrus-extracted naringin as a water-soluble natural photoinitiator. Naringin demonstrates its excellent visible light absorption in alkali environments and rapid photoreaction with the addition of a coinitiator upon exposure to visible light. Subsequently, a high-fidelity 3D printing is achieved. Meanwhile, the degree of alkaline manipulates the degree of crosslinking. Moreover, an additional monomer, sodium acrylate is introduced to endow the resulting materials with electro-active behavior. Crosslinked with sodium acrylate, the 3D-printed designed objects can transform their shape in the indicated direction in an applied electric field. Considering the lack of natural water-soluble photoinitiators, the development of naringin offers an environmentally friendly option for light-based 3D printing of waterborne materials. Chapter 4 demonstrates a pomegranate-extracted ellagic acid as a water-soluble photoinitiator. Ellagic acid exhibits its light absorption in violet-cyan light range in alkali environments. Even though this finding is similar to the one of naringin, in identical conditions, ellagic acid can reach its best photoinitiation ability with less amount in photocurable resins. Furthermore, the water content is determined in terms of sufficient shear elasticity, ensuring successful bottom-up 3D printing without deficiency. With the optimized formula, a 3D-printed figurine with obvious facial texture is obtained. In addition, with the help of a bilayer multi-material motif, a programmed water-driven actuator can play a role in a smart switch. The developed ellagic acid-based photocurable formula can therefore be used in the protection of water-sensitive devices in an environmentally friendly manner. Chapter 5 focuses on Indigofera extract, indigo carmine. Indigo carmine exhibits its light sensitivity and photoinitiation ability to violet-green light. This is the only organic salt natural compound among the ones mentioned in the present thesis. Because it's a sodium salt, the excessive introduction of NaOH decelerates the photoinitiation ability of indigo carmine due to the common-ion effect inducing reduced water solubility of indigo carmine. With the optimized formula, a complex surface structure was successfully 3D printed. A water-driven shape memory polymer is thereafter fabricated and exhibits reversible shape transformation in the presence and the absence of aqueous NaCl solution.
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