The creation of a three-dimensional (3D) printed object through a process called additive manufacturing, or more commonly, 3D printing, is an evolving technology that seems to be limitless in its potential applications. When 3D printing an object, aside from having the precise digital data and measurements, the ink to be used on the product plays an important role in determining its structure and function.
Of the most common inks to be used for 3D printing purposes include acrylonitrile butadiene styrene (ABS) plastic, nylon, epoxy resins and powder materials. When used to fabricate biocompatible materials, such as human organs, water-based solutions known as hydrogels are required to deposit cells and other biological materials into the models of the organs.
To allow for the 3D printing of these various inks, a widely used process utilized by 3D printers is known as photopolymerization, in which drops of the liquid plastic material are exposed to an incoming laser beam of ultraviolet (UV) light. As the UV light penetrates the plastic material, it transforms the liquid into a solid that is printed in continuous layers to achieve the final 3D printed object.
During this process, the use of photoinitiators (PIs), organic compounds that undergo a photoreaction following their absorption of the UV light, allow for the important chemical reactions that allow for the physical properties of the 3D object to change. While useful for the production of nonaqueous materials, the use of PIs is limited for materials that require hydrogels in their formation due to the inability of the radical polymerization reaction to occur in the presence of the oxygen in water.
To address this challenge, a group of Researchers from the Hebrew University of Jerusalem’s Center for Nanoscience and Nanotechnology have developed a new type of PI that is based on semiconductor-metal hybrid nanoparticles (HNPs) to 3D print in water-based systems. Cadmium sulfide (CdS) nanorods with gold tips were dispersed in water using polyethylenimine (PEI) as their dispersant, thereby enhancing their photocatalytic potential following the production of reactive oxygen species (ROS).
To fully understand how the CdS-Au HNPs behaved as PIs, the Researchers compared light absorption and polymerization kinetic measurements to CdS rods. Electron-hole recombination of the CdS-Au HNPs allowed for water to react with hydroxide upon light absorption, thereby forming hydroxyl and superoxide radicals.
A second route of radical formation by the CdS-Au HNPs was by the electron’s role in reducing the molecular oxygen, thereby producing either superoxide and/or hydrogen peroxide that is later used during photolysis.
An ideal PI to be used in 3D printing systems should be absorbed strongly at a wavelength range of 385-405 nm, generate free radicals in a rapid and efficient manner and exhibit a sufficient degree of polymerization. As compared to traditional organic photoinitators, the Cd-S Au HNPs used in this experiment were capable of consuming the oxygen as a result of the reduction processes of the excited electrons, which differs greatly from commonly used photoinitiators that consume the energy of the photocatalysis process.
The CdS-Au HNPs act as an ideal photocatalytic photoinitiator as these nanoparticles were confirmed to actively induce photopolymerization reactions in an efficient manner that simultaneously allows for the 3D printing of hydrogel materials. Additionally, the HNPs produced in this study are representative of most HNPs as they are versatile, which is an optimal property in 3D printing inks as they can be customized to tailor a specific printed objects’ requirements for functionality.
These tunable properties include a wide excitation window of the UV visible range of the CdS-Au HNPs and high sensitivity to light. For biomedical purposes, the CdS-Au HNPs have the potential to revolutionize 3D printing that is used in medicine for tissue engineering and specifically designed medical devices.
- “Rapid Three-Dimensional Printing in Water Using Semiconductor Metal Hybrid Nanoparticles as Photoinitiators” A. Pawar, S. Halvini, et al. Nanoletters. (2017). DOI: 10.1021/acs.nanolett.7b01870.