New approaches for visible light and photoresponsive polymers

Photopolymerization, a light-based 3D printing technique, traditionally uses UV light to quickly transform liquid resins into solid objects. Visible light, a promising alternative, has gained traction in areas such as tissue engineering and soft robotics as it offers milder reaction conditions, higher energy efficiency and biocompatibility. However, its comparatively slow curing has been a significant limitation to date. A research team led by Dr. Dowon Ahn at the Korea Research Institute of Chemical Technology (KRICT) has now made progress that could overcome this hurdle.

The focus is on a novel photoinitiating system (PIS) for red light with a wavelength of around 620 nm. It is based on a photoredox-active catalyst, a cyanine derivative, combined with two co-initiators: an electron-poor iodonium salt and an electron-rich borate. This system enables rapid polymerization by generating radicals that accelerate the curing process. At the same time, it solves the problem of spectral overlap, which makes it difficult to integrate light-sensitive polymers in UV-based processes.

The researchers integrated hexaarylbiimidazole-based crosslinkers (HABIs), which have dynamic properties and do not cause spectral interference with the red light of the PIS. As a result, HABI-based materials can be activated by blue or violet light after printing without compromising print quality. This combination enables functionalities such as self-healing and photochemical quenching, which are relevant for applications in materials science and smart materials.

The new process achieves layer speeds of up to eight seconds and a resolution of 20 μm at low light intensity. Printed objects heal surface damage completely within ten minutes and also offer degradable properties through selective light irradiation. These advances highlight the potential of visible light in 3D printing and open up new perspectives for the additive manufacturing of intelligent materials.