Unleashing the Potential of 3D Printing: Bridging Chemistry and Applications

Three-dimensional (3D) printing, often referred to as additive manufacturing, represents a remarkable technological breakthrough that has fundamentally altered the landscape of how we fabricate physical objects.[1] It offers an efficient and precise method for materializing digital designs into physical items. In contrast to traditional subtractive manufacturing processes, which involve material removal to shape an object, 3D printing meticulously builds items layer by layer, depositing material exactly where it is needed. Since the first commercialization of the stereolithography 3D printer by Dr. Chuck Hull in the early 1980s, the field has witnessed substantial growth, resulting in the development of a variety of 3D printing platforms, including fused deposition modelling, stereolithography (SLS), digital light processing (DLP), direct-ink-writing (DIW), and more.[2] The advancements have successfully translated into significant scientific, technological, and economic impacts on our modern society. What began as a tool for rapid prototyping has evolved into an advanced materials manufacturing platform with applications ranging from biomaterials and soft robotics to construction. More recently, this approach has unleashed an unprecedented field of possibilities, transcending the boundaries of various industries. For instance, in aerospace, 3D printing is commonly employed for the fabrication of intricate and lightweight components, while in healthcare, it enables the rapid production of patient-specific implants and prosthetics. More surprisingly, 3D printing technology has made its mark on the worlds of consumer goods and art, where artists harness its capabilities to create astonishing sculptures and custom-designed creations. In this special issue, we present a selection of captivating examples that highlight the latest advancements in 3D printing. These advancements encompass a wide spectrum of developments, ranging from the creation of novel resins and innovative processes to the fabrication of functional materials with enhanced properties. These advanced printable materials find application in batteries, sensors, and a variety of other cutting-edge technologies. To enhance the utility of 3D printing in bioapplications, there is a pressing need for the development of water-soluble photoinitiators. Addressing this need in article 2300772, Xiao and Lalevée's teams synthesized and characterized two novel carbazole derivatives that could be used as especially efficient water-soluble photoinitiators in combination with co-initiators. Furthermore, multi-component photoinitiating systems based on these carbazole derivatives could be applied for the in-situ preparation of hydrogels containing silver nanoparticles via laser writing procedure with LED@405 nm as light source, and the produced 3D hydrogels exhibited antibacterial activities against Escherichia coli. To fully unlock the potential of 3D printing, there's an urgent need for the implementation of innovative polymerization methods that provide greater control over chemical properties and network formation, but also facilitate the integration of multiple functionalities into printed materials. Most conventional 3D printing techniques rely on polymers produced through conventional free radical or cationic polymerization methods, which inherently limit the range of properties that can be achieved in these materials.[3] A promising avenue for enhancing the versatility of resins lies in the implementation of living polymerization techniques, a field that has seen significant development over the past three decades. In an advancement in the integration of living polymerization into 3D printing, in article 2207637, Zhu and colleagues have introduced cationic reversible addition- fragmentation chain transfer polymerization (RAFT) in a digital light processing (DLP)-based 3D printing system. They successfully demonstrated the implementation of photoinduced cationic RAFT polymerization of vinyl ethers, enabling the rapid fabrication of “living” objects. The “livingness” of the printed objects was demonstrated through precise postmodification of 3D printed objects. In another example, by exploiting the living character of polymers prepared by RAFT polymerization and using recent advancements in photo-RAFT, in article 2206639, Boyer's team has successfully implemented polymerization-induced microphase separation (PIMS) technique using light-mediated 3D printing. This technique allows for the in-situ production of bicontinuous nanostructured materials, which have been exploited for the fabrication of solid polymer electrolytes. By adjusting the composition of 3D printing resins, the authors have effectively fine-tuned the ionic conductivity of these solid polymer electrolytes without significantly compromising their mechanical properties, enabling their successful utilization in structural supercapacitors. In addition to exploiting advanced polymerization techniques, the incorporation of intricate macromolecules, such as peptides, into 3D printing resins has been explored in two distinct examples to fine-tune the mechanical properties of 3D printed materials. In the first study (article 2302794), led by Hawker, Read de Alaniz, the authors showed a robust strategy employing three-arm star-shaped block copolypeptides for 3D-printing biomaterials. By varying the peptide composition, degree of polymerization, and polarity, they were able to modulate hydrogen interactions and 𝛽-sheet formation, consequently exerting a profound influence on the network structure and viscoelastic properties. The study reveals the structure-property relationship, illustrating how to translate molecular features into macroscale viscoelasticity properties for direct-ink-writing (DIW). In a related example involving the use of defined polymers incorporated in photocurable resins, in article 2300844, Blasco's team harnessed their precise sequence with the aim of achieving precise control over network formation. They investigated the incorporation of sequence-defined oligomers containing polymerizable groups into photocurable resins for two-photon 3D printing. Through the examination of three distinct printable peptide-like oligomers with diverse sequences, they demonstrated how the precise positioning of crosslinkable groups had a profound impact on both the printability and the properties of the printed material. In another example combining peptides and the use of natural polymers, in article 2205078, Zhang and coworkers presented a bioresin formulation composed of gelatin methacryloyl, gelatin, methacrylated hyaluronic acid, and skin secretion of Andrias davidianus. This bioresin stands out for its specialized design, tailored specifically for in situ bioprinting applications, which addresses specific medical requirements. Utilizing this bioresin, intricate planar and 3D structures were constructed using extrusion bioprinting techniques. In vitro experiments have yielded promising results, demonstrating the excellent viability of cells when this bioresin is employed. Furthermore, in vivo studies have demonstrated its capability to integrate with biological tissue and promptly seal injuries or wounds due to its excellent adhesion. Besides the advancements in curable materials, progress in creating novel 3D printing materials is inherently linked with the innovation of advanced 3D printing processes. In article 2302405, Chen and colleagues have introduced a new approach for the fabrication of hybrid multi-materials combining vat photopolymerization and DIW processes. In contrast to conventional 3D printing technologies, their approach opens up new avenues, enabling the printing of materials without being constrained by the photocurability and viscosity of the resin. This allows for the creation of heterogeneous structures or multimaterials with intricate geometries and exceptionally high resolution. Traditionally, the optimization of 3D printing conditions in DLP for novel resins has relied on a comprehensive and time-consuming empirical approach. To overcome this challenge, in article 2301987, Killgore's team introduced a machine learning model that aimed at predicting and refining photomasks to enhance precision in digital light processing (DLP) technology, further streamlining the optimization process. In another significant aspect of this special issue, the versatility of 3D printing is harnessed to the design of functional materials tailored for a wide range of applications. By precisely controlling the macrostructures, 3D printing facilitates the development of materials tailored for adsorbing valuable compounds, sensors dedicated to pollutant detection, battery components for energy storage and advanced materials. For example, in article 2300323, Ke and his team have successfully created precisely engineered 3D-printed α-cyclodextrin monoliths featuring nanotubular structures for the specific adsorption of compounds. These monoliths were successfully exploited for selective adsorption of lycopene, an organic pigment present in raw tomato juice, highlighting the potential of templated synthesis of functional 3D-printing materials in phytochemical harvesting. In article 2300771, Studart, Nystrom, Siqueira, and their teams introduced a novel concept of incorporating unicellular algae, specifically diatoms, into 3D-printed hydrogels to create a water quality assessment sensor. These hydrogel-based living materials were printed by a desktop extrusion-based printer with a bio-ink comprising diatoms, cellulose nanocrystals, and alginate. The authors exploited these hydrogels as portable biosensors capable of detecting the presence of sodium chloride, herbicides, and antimicrobial agents in water. The detection relies on monitoring diatom proliferation through the autofluorescence of chlorophyll pigments. In article 2302718, Song and coworkers provided an overview of the use of 3D printing to fabricate precise and programmable structures that can be exploited in energy storage devices. In their review, the authors summarized the different 3D printing processes that can be exploited for the preparation of electrodes (both anodes and cathodes) and solid-state electrolytes for lithium-ion batteries. In article 2305005, Gu, Choi, and colleagues exploited the capability of 3D printing to print complex macroscopic objects for the fabrication of light-weight broadband electromagnetic wave absorbers. By incorporating carbon black and controlling the macrostructure of 3D printed materials (such as beam diameter and unit cell size), the authors were able to successfully prepare a low-density Kelvin-foam-based electromagnetic wave absorber with high mechanical strength and stiffness. Finally, by controlling the formation of nanostructure in 3D printed materials using two-photon lithography, in article 2207779, Meza and Patel have successfully demonstrated the fabrication of fracture-resistant nanoarchitected lightweight 3D printed materials. Using the ability to 3D print different (nano)structures, the authors have demonstrated two unique toughening mechanisms, which enable the development of the next generation of materials with improved mechanical resilience. In summary, this special issue offers a comprehensive glimpse into the immense potential of 3D printing for crafting functional materials. The latest strides in resin development and innovative 3D printing methods are paving the way for fabricating materials with tangible real-world applications. We hope that the insights shared in this collection will serve as a wellspring of inspiration, propelling future endeavours in this rapidly evolving field. 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.