New 3D Printing Method Uses Highly Entangled Polymers

Researchers have developed a novel approach to three-dimensional (3D) printing they call "CLEAR," which significantly improves the strength and durability of materials by using a combination of light and dark chemical reactions to create densely entangled polymer chains. The authors used their approach to print structures with special features, such as the ability to adhere to wet tissues. Incorporation of polymer chain entanglements as reinforcements within 3D printed materials can significantly enhance their mechanical properties. However, traditional vat photopolymerization-based 3D printing techniques, such as digital light processing (DLP), which uses light to harden layers of a liquid polymer resin, have struggled to achieve the formation of dense entanglements during the photopolymerization process. Techniques like DLP require fast reactions for quick and accurate manufacturing, which prevent the formation of long polymer chains with dense entanglements essential for improving stiffness and toughness. To address this tradeoff, Abhishek Dhand and colleagues introduce a novel technique called Continuous Curing after Light Exposure Aided by Redox initiation, or CLEAR, which uses light-initiated and "dark" polymerization to create highly entangled polymer chains within 3D printed structures. Dhand et al. developed monomers that first undergo a fast photo-initiated polymerization followed by a slower redox-initiated process. The combined approach allows the initial shape of the object to be set by light, as in traditional DLP printing. At the same time, a complementary redox initiation chemical reaction continues to convert unreacted monomers, resulting in improved mechanical performance of the overall material. The approach enables high monomer conversion at room temperature without needing additional exposures to heat, light, or chemicals. According to the findings, CLEAR-printed hydrogels and elastomers exhibit significantly higher extension energies compared to traditional DLP methods, with a fourfold to sevenfold improvement. To demonstrate the advantages of CLEAR, Dhand et al. use the method to print high-resolution and multilateral structures with unique features, including hydrogels with spatially tailored adhesion to wet tissues, highlighting its potential applications in biomedical manufacturing.