3D/4D Printing

The flexibility to produce highly complexed yet customizable shapes has made 3D printing an attractive technique for material fabrication. Beyond just a processing tool, incorporating new functions into 3D printed materials has become an interesting direction. In addition, 4D printing, a closely related “cousin” of 3D printing, has recently emerged and is gaining increasing attention from multidisciplinary research communities. On top of the conventional three geometric dimensions for 3D printing, 4D printing involves time as the extra fourth dimension. A somewhat simplified view is that 3D printing produces static 3D objects, whereas 3D objects produced by 4D printing are dynamic and can evolve with time. 4D printing can therefore be viewed as the marriage between 3D printing and dynamic shape morphing materials. The additional dynamic function(s) offered by 4D printing can enable new opportunities for engineering applications beyond 3D printing. This special issue offers a glimpse into some of the latest developments in 3D/4D printing.

Among various 3D printing techniques, light curing based 3D printing, also known as vat photopolymerization, stands out due to the fast printing speed and high printing resolution. In a review contributed by Haiyan Peng’s group, recent breakthroughs in this area were highlighted, with special considerations on photochemistry design including resins, initiators, inhibition, and light sources. In particular, advances in printing efficiency, accuracy, and sustainability were discussed. The authors ended the review by offering their perspective on future directions, which include printing with long-wavelength light, printing of functional materials, and multimaterial printing.

Yanfeng Zhang’s group reported an intriguing example of 3D printing of polymers with phosphorescent characteristics. An organic phosphorescence ink was introduced into a printed ester-containing polymer network with free hydroxyl groups. In the presence of an organic base catalyst, the network can undergo thermally triggered transesterification, which stiffened the material by increasing the cross-linking density. This led to changes in the phosphorescence emission due to the restriction of the thermal motion of chromophores. Consequently, localized control of the transesterification catalyst can introduce patterned material stiffening. This can be visualized via phosphorescence, which makes the system potentially useful for information encryption.

Porous metallic structures are widely used for catalyst applications. Producing designable hierarchical open pores that span orders of magnitude is highly desirable but remain challenging. A collaborative effort by Ruizhe Xing, Renliang Huang, Michael Dickey’s groups offered a promising solution. Their joint effort used an ink consisting of iron and copper particles and methylcellulose as an organic binder. 3D printing followed by thermal treatment yielded porous metallic alloys, which was subsequently coated with a metal organic framework catalyst. They showed that the ability to engineering the porous structure of the catalyst via 3D printing led to remarkable catalyst performance.

3D bioprinting has gained increasing traction, with most efforts focusing on 3D printed biomaterials targeting medical applications. A less common yet highly intriguing direction is 3D printing of living materials containing microorganisms and polymers. This topic was covered by a review authored by Beiheng Wu and Junqiu Liu’s group. They summarized notable advances in microbial-based 3D printing of living materials, with particular focus on printing techniques, types of microorganisms, and polymer bioinks. They further discussed how 3D printing of living materials can lead to solutions for energy and environmental challenges.

Colors are important attributes of products. 3D printing can produce nearly arbitrary shapes, but creating color patterns on geometrically complexed surfaces is challenging especially when printing efficiency is taken into account. Facing this challenge, Tao Xie’s group came up with a dual wavelength 4D color printing approach. The resin chemistry was designed such that a long wavelength light (405 nm) enabled 4D photoprinting. Afterward, exposure to shorter wavelength light (365 nm) activated acid-sensitive dyes to create color patterns. The orthogonality of the chemistry is such that the coloration and the geometric shapes can be independently controlled. The authors demonstrated gray scale printing of different colors by controlling the dose of the shorter wavelength light. However, creating multiple distinct colors on the same printed construct remains infeasible with this approach, a subject that is worthy of future exploration.

The functions of soft actuators and robots are strongly related to both the geometric shapes of the devices and the stimulus-responsive shape-shifting characteristics of the materials. As such, 4D printing is particularly relevant. Zi Liang Wu’s group reviewed recent progresses on this topic. It covered 4D printing of a range of shape-shifting materials including hydrogels, shape memory polymers, liquid crystalline elastomers, and their hybrids. They discussed the actuation performances, their correlation to device locomotion, and the representative applications. At the end, the review compared the advantages and limitations of each material class, and offered a perspective on the challenges and future opportunities in this field.

Liquid crystal elastomers are a typical class of stimuli-responsive shape-shifting materials. − 4D printing offered a flexible platform to prepare liquid crystal elastomer actuators for which the actuation modes can be tailored via the material formulation and printing conditions. The group of Xili Lu, Jianshe Hu, and Hesheng Xia reviewed recent advances on this subject. They systematically overviewed the various 4D printing methodologies that cover the synthesis and actuation performance of the liquid crystal elastomers. The review was ended with their perspective on the opportunities and challenges in this field.

Shape memory polymers make up a unique class of shape-morphing material in that it can be programmed to morph between an arbitrarily defined temporary shape to a permanent shape. , Ultraviolet curable shape memory polymers are idea candidates for 4D printing because it is readily compatible with light curing based 3D printing. Their shape recovery process in the presence of external loading, however, is not fully understood. Chao Yuan’s group developed analytical solutions that can predict shape recovery performance under constrained conditions. Their theoretical model can become useful in guiding the future design of 4D printed shape memory structures.

In summary, 3D/4D printing is a fast growing field with vast potential in material design, device functions, and engineering applications. We note that the papers published in this special issue by no means capture the full picture. In fact, they are far from representing how the field has evolved in the past decade. These examples, however, reflect the diversity of the field and offer some hints on what might be possible for the future.

Views expressed in this editorial are those of the author and not necessarily the views of the ACS.

Published as part of Chem & Bio Engineering special issue “3D/4D Printing”.