Advanced 3D/4D Printing for Functional Materials Innovation

Abstract

Three-dimensional (3D) printing has established itself as an indispensable tool for the rapid fabrication of intricate physical objects, leveraging the precise translation of digital designs into physical materials. Since the commercialization of the first 3D printer in the early 1980s, this field has witnessed substantial expansion, fostering a rich diversity of 3D printing platforms, including fused deposition modeling, stereolithography, digital light processing, and direct-ink-writing. These technologies have empowered the creation of objects with complex geometries, offering a transformative approach to manufacturing. More recently, the advent of 4D printing has brought together the progress in 3D printing and cutting-edge materials. This innovation allows to produce objects that possess the remarkable ability to dynamically evolve, adapt, or self-assemble over time, in response to varying stimuli like alterations in temperature, humidity, or environmental factors. This dynamic dimension adds a profound layer of versatility to 3D printing, unlocking applications in biomaterials and beyond. In this special issue, we present a collection of reviews, perspective, and research articles, that highlight the latest advancements in 3D and 4D printing. This special issue unfolds in two distinctive parts: the initial section delves into the latest advancements in 3D printing, while the subsequent part sheds light on the progress in 4D printing techniques dedicated to the creation of advanced materials. Through these articles, readers will traverse a diverse landscape of innovations, ranging from the design of novel photoinitiator systems to the intricate fabrication of functional materials with applications spanning across fields like biomaterials, soft robotics, cloaking technologies, and illusion devices. This compilation serves as a comprehensive exploration of the ever-evolving landscape of 3D and 4D printing, offering insights into their profound impact on materials science and beyond. While remarkable progress has been made in 3D printing,[1] significant challenges persist, such as expanding the range of printable materials to include composites and biomaterials, while simultaneously improving production speed and feature resolution without inflating costs. Addressing these challenges requires innovative approaches, including the implementation of modern polymerization techniques, such as controlled/living radical polymerization and development of efficient photoinitiator systems to accelerate printing processes.[2] In a review article, Boyer and coworkers provide an overview of the emergence of photocontrolled reversible–deactivation radical polymerization techniques in the field of 3D printing.[2] This approach imparts a “living character” to materials, granting them access to advanced properties. These properties encompass the capability for real-time adjustments of surface and bulk properties, self-healing attributes, and precise control over nanostructuration and mechanical properties. In the pursuit of enhancing the efficiency photoinitiator system, in article 2300571, Blasco and coworkers delved into this endeavor, exploring the utilization of 4 donor–acceptor–donor photoinitiators featuring 4H-pyranylidene and 4-methylcyclohexan-1-one units as donor and acceptor groups, specifically tailored for two-photon laser printing. Furthermore, in the pursuit of optimizing 3D printing processes for novel resins, traditional reliance on empirical methods has proven to be a meticulous and time-consuming procedure. To address this challenge, in article 2300052, Page and colleagues from the University of Texas (Austin) introduced an innovative method based on the modification of Jacob's equation, which is conventionally used in stereolithographic 3D printing. To validate this innovative model, the research team conducted experiments using a green-light liquid-crystal-display 3D printing system. Additionally, the improvement of the printing process can be achieved through the development of novel printing technologies, such as tomographic volumetric printing. In contrast to conventional layer-by-layer 3D printing methods, this technique creates objects by exposure of a rotating volume of photopolymer resin to tomographically-patterned illumination, allowing for high-speed printing. As objects are produced within seconds to minutes, factors like molecular diffusion length scales and temperature gradients become significant in the printing process. Understanding the intricacies of the diffusion process is imperative for the advancement of volumetric printing. In this context, in article 2301054, Weisgraber and coworkers introduced a model capable of simulating reaction, diffusion, and heat generation processes throughout the volumetric printing process, facilitating progress in this promising technology. In the development of new biodegradable resins, in article 2201904, Becker and co-workers introduced a new photocurable resin for the 3D printing of elastomers which can be used for biomedical applications. In this work, the use of ABA triblock poly(propylene fumarate-b-γ-methyl-ε-caprolactone-b-propylene fumarate) copolymers were photocured with a degradable, thiol-based crosslinking system using a continuous liquid interface production (CLIP) digital light processing (DLP) 3D printer. This method allowed them to finely tune the mechanical properties of the 3D printed materials, including strain at failure which varied from 55% to 290%. Furthermore, they achieved controlled hydraulic degradation with negligible effect on pH levels, demonstrating their ability to precisely control the properties of these 3D printed materials based on the stoichiometry of the individual blocks within the ABA copolymer and the alkene-to-thiol crosslinking ratios. These advancements hold great promise for a wide range of applications, from biodegradable materials for tissue engineering to soft robotics and controlled drug delivery systems. The development of functional 3D-printing materials also opened avenues to access hierarchical porosity for various applications. For example, in the pursuit of developing photocuring 3D printing resins for the creation of transparent porous γ-alumina 3D structures, in article 2300123, Magdassi and their team showed an innovative procedure that merges digital light processing 3D printing (DLP) with sol-gel reactions. The authors formulated an aqueous resin composition containing aluminum chloride, propylene oxide, ethanol, and acrylic acid. After the DLP printing process, the printed samples underwent treatment through supercritical drying (SCD) and were subsequently sintered at high temperatures, resulting in the formation of ceramic monolithic porous structures. Through this method, they achieved the production of crystalline γ-Al2O3, with a very high surface area, above 1800 m2 g−1, which is greater than the samples manufactured by conventional mold-based processes. Such materials can be utilized in applications such as ceramic membranes, batteries, heat resistantance, optical devices, and catalysis. The demand for stretchable conductive materials, suitable for use as strain sensors and wearable electronics, has recently gained significant attention for healthcare monitoring. However, these materials pose challenges for 3D printing, and there is a growing need to create resins that can produce materials with enhanced conductivity, mechanical properties, elastic recovery, and durability. To address this issue, in article 2300226, Nelson and his team presented a novel photocurable resin based on imidazolium ionic liquid (specifically, 1-butyl-3-vinylimidazolium bis(trifluoromethane) sulfonamide). These resins were efficiently 3D printed using a stereolithography printer, resulting in the production of highly responsive strain sensors with impressive performance characteristics. For instance, these materials could be printed into a variety of intricate shapes and structures. Moreover, they possessed the remarkable ability to autonomously repair themselves following mechanical damage, thus restoring their original functionality, such as electrical conductivity. In another example related to the development of electrically conductive materials, in article 2300408, Hernandez-Sosa and coworkers presented a two-step process for the fabrication of electrically conductive rigid 3D-printed materials. In the first step, they used DLP to fabricate a 3D-printed nanoporous polymeric material, which served as the substrate. Then, they injected silver nanoparticle ink into the pores using inkjet printing. In a second step, silver nanoparticles were annealed to yield conductive structures with sheet resistance <2 Ω sq−1. 3D printing can play an important role in advancing the field of biomaterials, exemplified by the work of Zolfagharian and their team, who have engineered artificial muscles using soft silicone elastomer materials that can be activated by an electrical current, as presented in article 2300199. These carbon fiber-infused 3D-printed muscles adjust their stiffness when exposed to a low-voltage signal, a feature that has been utilized to create a variable stiffness joint robot. Lastly, the application of 3D printing for the customized production of devices, including microfluidic systems, was demonstrated in article 2300374 by Thiele and their team. In their study, the authors successfully arranged microscopic hydrogel particles with precise spatial control, forming interconnected crosslinked functional hydrogel arrays. This achievement was made possible through the utilization of a 3D-printed droplet-based microfluidic approach. Within the field of 4D printing, this special issue embarks on the exploration of innovative possibilities for the creation of adaptive materials. In article 2300678, Yao and coworkers provide a perspective on the design of sustainable morphing materials through 4D printing. These materials possess the ability to alter their shape and functionality in response to external stimuli, enabling them to perform diverse functions. In this perspective, the authors explore both the design and the end-of-life cycle of such materials. In another review article 2300366, a team led by Dr Gigmes and Guillaneuf from the University of Marseille offers a comprehensive overview of light-induced 3D printing using polyurethanes for the preparation of functional materials for applications in biomedicine and electronics. Their review article summarizes recent advancements in 3D/4D printing techniques, resin formulations, and their applications, with a particular focus on vat photopolymerization. In the context of 4D printing applied in biomaterials, in article 2300200, Pati, Zolfagharian and colleagues review the potential application of 4D printing for the engineering and preparation of vascular tissue. This review elucidates the key techniques employed for the creation of artificial vascular tissue through 3D printing, while the integration of responsive materials via 4D printing enables the construction of dynamic constructs characterized by adaptable properties and reconfigurable architectures. In article 2300727, Blasco and coworkers developed a photocurable responsive resin for the fabrication of multi-responsive bilayered structures exhibiting reversible and complex actuation patterns using DLP as manufacturing technology. To achieve this, the authors incorporated liquid crystal elastomers as active layers and combined with a printable non-responsive elastomer acting as a passive layer. By incorporating various organic dyes absorbing light, the materials were able to respond to different wavelengths of light (459, 622, and 850 nm), which opens the opportunity for the design of complex multi-material structures for soft robotics. Lastly, in article 2202020, Liang and their team unveiled a dynamic metasurface device boasting switchable electromagnetic cloaking and illusion capabilities, all achieved through an innovative 4D printing process. This cutting-edge metasurface design was fabricated by combining fused deposition molding 4D-printing process and liquid metal injection molding. The transformative potential of this device lies in its responsiveness to temperature changes, enabling dynamic shifts in the structural configuration of the metasurface, thereby delivering alterations in its electromagnetic cloaking and illusion functionalities. In conclusion, this special issue provides an overview of the vast potential of 3D and 4D printing in designing and fabricating functional materials that hold the promise to address real-world challenges. We hope this compilation will serve as a source of inspiration, propelling future pursuits in this ever-evolving domain. C.B. is the recipient of an Australian Research Council (ARC) - Australian Laureate Fellowship (Project No. FL220100016) funded by the Australian Government. E.B. acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG) under Germany's Excellence Strategy via the Excellence Cluster 3D Matter Made to Order (EXC-2082 – 390761711, the Federal Ministry of Education and Research (BMBF) and the Ministry of Science Baden-Württemberg. C.K. thanks the support from the Department of Energy, the Basic Energy Sciences DE-SC0022267, and the Beckman Young Investigator program from the Arnold and Mabel Beckman Foundation. The authors declare no conflict of interest.

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