In an article recently published in the journal Additive Manufacturing, researchers discussed the development of a base and neutral electrolyte-mediated photoinitiator named indigo carmine (IDGCM) for high-fidelity 3D printing.
Study: Indigo Carmine: A Base and Neutral Electrolyte-mediated Photoinitiator for 3D Printing in High Fidelity. Image Credit: ArtzPhoto/Shutterstock.com
In a variety of applications, the advancement of light-based 3D printing has increased and supplanted traditional fabrication methods. The hydrophobic qualities of the existing and recently created photoinitiators for vat photopolymerization/3D printing limit their ability to expand the applications using watery materials.
The investigation of water-compatible photoinitiators has received considerable attention to address the rising need for biomedical applications involving hydrogel. The water-dispersible photoinitiators substantially exhibit efficient photoinitiation abilities, and the solubility modification strategies increased the range of water-compatible photoinitiators; however, these methods can cause the steric effect, which negatively affects the photoinitiation efficiency of the grafted or encapsulated hydrophobic photoinitiators dispersed in the prepolymers.
Water-soluble photoinitiators are preferable when taking into account the aforementioned restriction. Despite their excellent photoinitiation abilities, lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) and 2-hydroxy-4'-(2-hydroxyethoxy)-2methylpropiophenone (Irgacure 2959) are frequently used under ultraviolet (UV) irradiation for photopolymerization, which can have adverse effects on both human health and the environment. Therefore, it is desirable to create aquatic photoinitiators responsive to visible light. Despite the metal salts' excellent water solubility, not enough research has been done on their cytotoxicity.
The investigation and development of visible-light-sensitive, water-soluble, and low-toxic photoinitiators capable of performing quick and high-fidelity 3D printing are in high demand to address the aforementioned issues. IDGCM shows potential interest in pH-mediated photopolymerization based on pH-mediated photophysical features previously reported. However, there is little research on pH-mediated photopolymerization.
About the Study
In this study, the authors discussed the development of a photoinitiating system using indigo carmine, a blue food coloring. Under blue and green light-emitting diodes (LEDs), the indigo carmine-based photoinitiating system accelerated a photochemical reaction and caused photobleaching in the alkaline medium. Under the illumination of low-intensity light sources, the indigo carmine-based photoinitiating system attained an unparalleled photoinitiation ability in terms of the rate of photopolymerization and the final monomer conversion.
The team created a 2D seashell with the ability to flip between 2D and 3D using a 3D printing technique. The results showed that the indigo carmine-based photoinitiating method was capable of overcoming the problems with water solubility and safety and could be used to 3D print biomedical goods. Visible light absorption in the wavelength range 400–700 nm in neutral solution as well as water solubility were indicated by the blue hue and sodium salt form, respectively. Additionally, for cats, rats, and dogs, the safe IDGCM threshold was 8259 mg/kg body weight/day and 250 mg/kg body weight, respectively.
The researchers use real-time Fourier-transform infrared spectroscopy (RT-FTIR) to investigate the photoinitiation capacity of IDGCM under visible light. Ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) spectrophotometers were used to investigate the photochemical mechanism of the IDGCM-based photoinitiating systems in the neutral and alkaline salted solution. The 3D printing settings were illustrated using a planar cat coin model for the more complex structure. The ability to successfully 3D print a topography with excellent shape fidelity was attributed to the dual function of IDGCM as a photoinitiator and photoabsorber. The presence and absence of NaCl aqueous solution induced the reversible 2D-3D transition of a planar sheet created by vat photopolymerization.
The formulation's intrinsic water content in the printed planar seashell was replaced by a 2% aqueous NaCl solution under the influence of osmotic pressure. Reswelling in a 2% aqueous NaCl solution allowed for the successful completion of the reversible 3D-2D conversion. A topography print with exceptional fidelity was successfully created utilizing the suggested recipe and the ideal setting in 3.28 hours.
A gelation period of 17 s was induced by the 0.3 mm thin samples, which was quicker than the thick samples. The first irradiation at 0.3 mm - 1.0 mm with the tgel as low as 0.9 - 2.2 s resulted in a strikingly quick gelation time of the PEGDA 700/water mix in NaOH media.
In contrast to the PEGDA 700/water blend's gelation time under [email protected] nm illumination, under [email protected] nm illumination it was barely affected by thickness in the range of 0.3 mm - 1.0 mm, which was attributed to its quick photobleaching behavior and photochemical process. The slower photobleaching of IDGCM and the ensuing light attenuation through samples caused the [email protected] nm to induce a longer gelation time than the [email protected] nm. It was shown that the quantity of each ingredient in formulations, the length of the PEGDA chain, or various LEDs could all influence the photoinitiation ability of IDGCM. The solubility of IDGCM served as the foundation for the concentration of the examined components.
The photopolymerization efficiency of PEGDA was improved in terms of the photopolymerization rate under the illumination of [email protected] nm due to the higher solubility of IDGCM in the formulations. Additionally, the photopolymerization of PEGDA with a longer chain could enhance the final C=C double bond. The proposed IDGCM-based photoinitiating system performed well with low-intensity green LED and blue LED.
In conclusion, this study used IDGCM/Iod with 0.5%/2% wt.% in the presence of 25 mM NaOH to 3D print PEGDA 700/DI water with 80%/20% wt.% under the direction of investigations of the resin composition and relative concentration.
The authors mentioned that IDGCM's dual function as a photoinitiator and a light absorber could pave the way for quick and accurate 3D printing and the creation of complex things. They also stated that a printed planar seashell with stimuli-responsive functionality through swelling and evaporation permitted reversible switching utilizing the proposed formulation.
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Zhu, D., Peng, X., Xiao, P., Indigo Carmine: A Base and Neutral Electrolyte-mediated Photoinitiator for 3D Printing in High Fidelity. Additive Manufacturing, 103154 (2022). https://www.sciencedirect.com/science/article/abs/pii/S2214860422005437