#photopolymerization

Addressing key challenges of photoresponsive, visible light 3D printing

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 have 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.

Researchers propose 3D printing of high-performance elastomers through vat photopolymerization

Acrylate-based ultraviolet (UV)-curable resins are currently used as raw materials to obtain desired performance by adjusting the types and ratios of oligomer and reactive monomers in the resin system. However, due to low degree of free-radical polymerization, the elastomers prepared by vat photopolymerization (VPP) technology show low strength, poor resilience, and unsatisfactory mechanical properties. In a study published in Advanced Materials , the research group led by Prof. Wu Lixin from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences proposed 3D printing of high-performance elastomers through vat photopolymerization. The researchers analyzed the structure-property relationship between molecular weight and mechanical properties, and selected polytetramethylene ether glycols (PTMGs) with different molecular weights (Mn=1000, 2000, 3000 g mol-1, respectively) as reactants to react with isophorone diisocyanate (IPDI). The obtained polyurethane blocked oligomers (PUBs) exhibited high viscosities.

3D printed tires and shoes that self-repair

Instead of throwing away your broken boots or cracked toys, why not let them fix themselves? Researchers at the University of Southern California Viterbi School of Engineering have developed 3D-printed rubber materials that can do just that.

Assistant Professor Qiming Wang works in the world of 3D printed materials, creating new functions for a variety of purposes, from flexible electronics to sound control. Now, working with Viterbi students Kunhao Yu, An Xin, and Haixu Du, and University of Connecticut Assistant Professor Ying Li, they have made a new material that can be manufactured quickly and is able to repair itself if it becomes fractured or punctured. This material could be game-changing for industries like shoes, tires, soft robotics, and even electronics, decreasing manufacturing time while increasing product durability and longevity.

The material is manufactured using a 3D printing method that uses photopolymerization. This process uses light to solidify a liquid resin in a desired shape or geometry. To make it self-healable, they had to dive a little deeper into the chemistry behind the material.

Efficient chain transfer for 3D-printing of tough photopolymers

An ever-growing number of coatings, including varnishes and printing inks, as well as tooth fillings, are cured with light. Yet, homogenous, tailored, polymer networks cannot be produced, and the materials tend to be brittle, which limits the use of photopolymers in applications like 3-D printing, biomedicine, and microelectronics. In the journal Angewandte Chemie, researchers present a method by which methacrylate-based, homogenously crosslinked, tailored, tough polymers can be made – even at high resolution for 3-D printing.

Light-curing is usually a radical chain polymerization. An initiator is split into radicals by light energy. These then attack the monomer, such as the C=C double bond in a vinyl group, which forms a new radical that becomes starting point of a growing polymer network by attacking more monomers and binding to them.

Newer methods to better control radical photopolymerization and the material properties of the products tend to slow the curing process, which is not ideal for 3-D printing. A short irradiation phase is critical for high spatial resolution and economical production times.

Periodically-patterned hydrogels: a model for cooperative deformation

In nature, some organisms use deformations to create three-dimensional movement. One example is the Venus fly trap, which opens and shuts its leaves to catch prey. When open, the leaves are concave, but when closed the leaves are convex. Scientists are interested in mimicking controlled deformations for applications in soft electronics or actuators.

Researchers from Zhejiang University in China, Iowa State University, and Hokkadio University in Japan have used periodically patterned hydrogels to mimic the kind of cooperative deformations seen in nature. These hydrogels spontaneously deform into three-dimensional concave and convex configurations that are guided by nearest-neighbor cooperative effects. Their work appears in Science Advances.

Hydrogels are flexible polymers that absorb water, some of which are high-swelling while others are low- or non-swelling. For this study, Wang, et al. used photolithography to do a two-step photopolymerization process to create a two-dimensional array of alternating hydrogels. They began by placing PAAm, a non-swelling gel, in a certain pattern in which some of the gel is exposed to UV light. Then, a high-swelling gel, P(AAm-co-AMPS), was overlaid on the newly patterned non-swelling gel, which was then exposed to UV light. This created an alternating pattern of two gels that upon swelling, resulted in alternating concave and convex deformations in the high-swelling gel discs.

Photopolymerization-triggered molecular motion for flexible liquid crystal display

With current 2D techniques, one typically irradiates a liquid crystal film that contains added photoresponsive dye molecules, with uniform polarized light. This controls the net liquid crystal alignment via the interaction of the dye dipole and the polarization axis of light. The drawback with these systems is the need for adding strong dyes, which can discolor or degrade optical and stability properties. Thus, a dye-free method is highly desired in the engineering industry.

Currently, only two approaches to dye-free methods have been explored. The first is a two-step alignment method, in which the liquid crystal materials are coated over a very thin dye-containing photoalignment layer and then aligned or fixed by polymerization. While this method has proven very successful in achieving stimuli-responsive 2D aligned liquid crystals and elastomers used in photonics, solar energy harvesting, microfluidics, and soft-robotic devices, it is expensive and time-consuming. The creation of a film with microscopic arrays of microalignment patterns requires precise and dynamic control of the polarized direction of incident light in each pixel, so this method is unsuitable for aligning patterns on the nanoscale over large areas.

The second approach to the development of a dye-free system uses surface topography to overcome the limitations of conventional photoalignment. In this method, the liquid crystals are aligned over a surface topography template through lithography, nanoimprinting, or inkjet techniques among others. While this method allows for 2D micropatterning of molecular alignment, it still requires multi-step processing, making it costly and time-consuming. Due to the surface roughness from the topographic templates, this method proves difficult in the production of thin films.

Metal-silicone microstructures could enable new flexible optical and electrical devices: Laser-based method creates force-sensitive, flexible microstructures that conduct electricity

For the first time, researchers have used a single-step, laser-based method to produce small, precise hybrid microstructures of silver and flexible silicone. This innovative laser processing technology could one day enable smart factories that use one production line to mass-produce customized devices combining soft materials such as engineered tissue with hard materials that add functions such as glucose sensing.

[...]

The metal component of the microstructures renders them electrically conductive while the elastic silicone contributes flexibility. This unique combination of properties makes the structures sensitive to mechanical force and could be useful for making new types of optical and electrical devices.

"These types of microstructures could possibly be used to measure very small movements or changes, such as a slight movement from an insect's body or the subtle expression produced by a human facial muscle," said research team leader Mitsuhiro Terakawa from Keio University, Japan. "This information could be used to create perfect computer-generated versions of these movements."