Visible Light 3D Printing Encompasses Speed, Precision, and Smart Materials

Newswise — Vat photopolymerization, a light-based 3D printing technique, conventionally relies on UV light to rapidly transform liquid resins into intricate solid objects. However, the use of visible light as an alternative approach has gained significant attention, particularly for its promising applications in tissue engineering and soft robotics. While visible light offers mild reaction conditions, improved energy efficiency, and biocompatibility, slower curing speeds slower curing speeds has posed a significant limitation. In response, researchers have intensified efforts to overcome this limitation, focusing on developing high-speed curing methods that harness visible light irradiation.

A significant constraint of conventional UV-based 3D printing is its incompatibility with smart materials, particularly photoresponsive polymers. These polymers incorporate light-sensitive compounds, or chromophores, in their structure, enabling reversible changes in physical or chemical properties upon light exposure. While photoresponsive polymers have potential applications in actuators, drug delivery systems, and degradable materials, their integration into 3D printing processes poses challenges. The primary issue lies in the spectral overlap between the absorption bands of most chromophores (typically in the UV to violet/blue region) and the emission spectra of UV light sources used in conventional 3D printing. This overlap causes unintended interference, resulting in slower printing rates and reduced feature resolution. Consequently, the 3D printing industry is actively seeking techniques to print photoresponsive polymers efficiently without compromising speed or quality.

The research team from the Korea Research Institute of Chemical Technology (KRICT) led by Dr. Dowon Ahn, Youngchang Yu, and Wonjoo Lee, in collaboration with Prof. Min Sang Kwon from Seoul National University and Prof. Hyun-Jong Paik from Pusan Nation University, has made significant strides in addressing the key challenges of photoresponsive, visible light 3D printing. Their work focused on developing a novel photopolymer resin exhibiting rapid printing rate, high feature resolution, and multifunctionalities.

To address the challenges associated with visible light 3D printing, the research team developed a novel red light (~620nm) curable three-component photoinitiating system (PIS). This innovative system consists of a red light-absorbing photoredox catalyst (PRC), specifically a cyanine derivative, along with two co-initiators: an electron-deficient iodonium salt and an electron-rich borate. The PRC absorbs photons, elevating their electrons to a higher energy state. The co-initiators play a crucial role of accepting and donating electrons from the PRC, respectively, thereby converting both into radicals that trigger rapid polymerization.

For the incorporation of photoresponsive compounds, the researchers utilized hexaarylbiimidazole (HABI)-based crosslinkers due to their dynamic properties. HABIs are well-known photochromic compounds that generate relatively stable lophyl radicals while also being capable of thermally recombining back to their original dimer form. Importantly, their absorption bands do not overlap with the absorption spectrum of the PRC, which extends from UV to violet/blue light. This characteristic enables rapid photoactivation after 3D printing without interference from the curing process. Additionally, HABIs exhibit unique features, such as the ability to irreversibly convert into imidazole compounds in the presence of hydrogen donors such as aromatic alcohols and thiols, which could be leveraged for developing photodegradable polymers.

The adoption of this two wavelength-resolved photosystem — combining violet/blue light-active HABI compounds as photoresponsive units with a red light-absorbing PIS — results in remarkable improvements in printing speed, feature resolution, and multifunctional capabilities, including self-healing and erasability. The printed objects achieve feature sizes of around 20 μm and can be produced at an impressive speed of just eight seconds per layer (equating to a build speed of 22.5 mm/h) under low-intensity red light exposure (3 mW/cm²). These performance metrics are comparable to those of conventional UV-based 3D printing techniques that do not incorporate functional materials.

Moreover, the printed objects exhibit rapid self-healing capabilities; scratches on their surfaces can completely heal within just 10 minutes of visible light irradiation (≤455 nm) at low irradiation intensity (~10 mW/cm²). In addition to self-healing properties, these objects also allow selective photomediated erasing in the presence of thiols, showcasing their potential for applications in photodegradable polymers.

This comprehensive approach not only enhances the efficiency and versatility of visible light 3D printing but also paves the way for future advancements in smart materials and additive manufacturing technologies.

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KRICT is a non-profit research institute funded by the Korean government. Since its foundation in 1976, KRICT has played a leading role to advance national chemical technologies in the fields of chemistry, material science, environmental science, and chemical engineering. Now, KRICT is moving forward to become a globally leading research institute tackling the most challenging issues in the field of Chemistry and Engineering and will continue to fulfill its role in developing chemical technologies that benefit the entire world and and contribute to maintaining a healthy planet. More detailed information on KRICT can be found at https://www.krict.re.kr/eng/

This study was supported by KRICT (KS2341-10 and BSK23-450) and funded by the National Research Foundation of Korea (NRF) through grants provided by the Korean government (MSIT) under grant numbers 2021R1A5AA1030054 and 2022R1A2C2011627. The research was published in Advanced Materials, volume 36 (19) and featured on the frontispiece of the volume.