A new approach to 3D printing using thermal initiators can significantly reduce costs and speed up the additive manufacturing process, particularly for water-based applications. Traditional 3D printing methods, often reliant on photoinitiators activated by UV-visible light, can be expensive and time-consuming. This innovative method utilises low-cost thermal initiators, triggering the reaction by converting light into heat, offering a more efficient alternative.
The challenge with current 3D printing technologies
Stereolithography, a widely-used 3D printing technique, cures a photoreactive resin with light, layer by layer, to form a solid object. This process typically requires photoinitiators, chemicals that start the polymerization reaction when exposed to light. However, photoinitiators have a few significant downsides:
- Cost and Availability: Most water-soluble photoinitiators are expensive.
- Water-Incompatibility: A limited selection of these photoinitiators makes it challenging to print in aqueous environments.
- Health and Safety: UV light, frequently used in these systems, poses health risks and is unsuitable for many biological applications.
The novel solution: Thermal initiators
Thermal initiators, widely used in the chemical industry, are plentiful, cost-effective, and far cheaper than photoinitiators. For example, sodium persulfate, a common thermal initiator, costs just a few dollars per kilogram, while photoinitiators can cost hundreds or even thousands per kilogram. Traditionally, thermal initiators are activated by heat, which poses challenges in localising the polymerisation reaction—a key requirement for precise 3D printing.
The newly introduced method enables the use of thermal initiators in 3D printing by applying photothermal converters such as gold nanorods or silver nanoparticles. These nanoparticles absorb light and convert it into concentrated heat, activating the thermal initiators to trigger polymerisation. This approach allows for the precise, localised printing of 3D structures, similar to photoinitiator-based systems but without their limitations.
Key findings
Localized Heating: Light can be converted into localized heat to activate sodium persulfate in water or benzoyl peroxide in non-aqueous solutions using gold nanorods or silver nanoparticles. This process allows for the polymerisation of various monomers, facilitating the creation of intricate 3D structures. Other photothermal converters can also be employed.
Versatility and Safety: The process works efficiently under near-infrared light, which is safer and allows for deeper light penetration. This is especially beneficial for applications in bioprinting and other fields requiring non-toxic and low-temperature processing.
Cost-Effectiveness: Using inexpensive and widely available thermal initiators significantly reduces the cost of materials, making the technology more accessible.
Practical implications
The study demonstrates the concept by successfully producing detailed 3D hydrogel structures and other polymeric objects using standard digital light processing printers. The formulations included various combinations of gold nanorods or silver nanoparticles, along with monomers and stabilisers, to ensure stable dispersion and effective polymerisation.
To achieve stable ink compositions, stabilisers such as xanthan gum were used to maintain the even dispersion of thermal initiators and nanoparticles, a critical factor for consistent 3D printing.
Temperature control was managed by adjusting laser power and speed, allowing precise heating of the ink to the necessary polymerisation temperature, similar to exposure control in traditional stereolithography.
Broader applications and future outlook
This method could potentially be adapted for various 3D printing technologies, including two-photon printing, a subtype of multi-photon lithography. It shows promise for high-resolution printing down to the micron scale. Moreover, by changing the type of nanoparticles, the method can be tailored for different light wavelengths, making it even more versatile. The potential applications include:
- Bioprinting: The safer near-infrared light and compatibility with water-based inks make it ideal for creating biocompatible materials.
- Medical Devices: The lower costs and increased safety could result in more affordable and safer medical devices.
- Composite Materials: This technique opens up opportunities for creating advanced composite materials without the need for expensive photoinitiators.
Conclusion
The study effectively integrates thermal initiators from traditional mass production into the emerging realm of 3D printing, utilising their cost-effectiveness and simplicity through localised photothermal conversion. This breakthrough can potentially lower costs, improve safety, and broaden the applications of 3D printing technologies. It facilitates printing in aqueous environments and the creation of biocompatible materials without the need for expensive and potentially harmful photoinitiators. With ongoing research and development, this innovation is poised to drive advancements across various fields, from bioprinting to developing composite materials.
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Journal reference
Kam, D., Rulf, O., Reisinger, A., Lieberman, R., & Magdassi, S. (2024). 3D printing by stereolithography using thermal initiators. Nature Communications, 15(1), 2285. https://doi.org/10.1038/s41467-024-46532-0
