Bridging polymer architecture, printability, and properties by digital light processing of block copolycarbonates

CO2-based aliphatic polycarbonates (aPCs), produced through the alternating copolymerization of epoxides with CO2, present an appealing option for sustainable polymeric materials owing to their renewable feedstock and degradable characteristics. An ongoing challenge in working with aPCs is modifying their mechanical properties to meet specific demands. Herein, we report that monomer ratio and polymer architecture of aPCs impact not only printability by digital light processing (DLP) additive manufacturing, but also dictate the thermomechanical and degradation properties of the printed objects. We found that block copolymers exhibit tailorable thermomechanical properties ranging from soft elastomeric to strong and brittle as the proportion of hard blocks increases, whereas the homopolymer blend failed to print objects and statistical copolymers delaminated or overcured, displaying the weakest mechanical properties. In addition, the hydrolytic degradation of the prints was demonstrated under various conditions, revealing that BCP prints containing a higher proportion of hard blocks had slower degradation and that statistical copolymer prints degraded more slowly than their BCP counterparts. This study underscores that polymer composition and architecture both play key roles in resin printability and bulk properties, offering significant prospects for advancing sustainable materials in additive manufacturing through polymer design.

Manufacturing 3D Biomimetic Tissue: A Strategy Involving the Integration of Electrospun Nanofibers with a 3D-Printed Framework for Enhanced Tissue Regeneration

3D printing and electrospinning are versatile techniques employed to produce 3D structures, such as scaffolds and ultrathin fibers, facilitating the creation of a cellular microenvironment in vitro. These two approaches operate on distinct working principles and utilize different polymeric materials to generate the desired structure. This review provides an extensive overview of these techniques and their potential roles in biomedical applications. Despite their potential role in fabricating complex structures, each technique has its own limitations. Electrospun fibers may have ambiguous geometry, while 3D-printed constructs may exhibit poor resolution with limited mechanical complexity. Consequently, the integration of electrospinning and 3D-printing methods may be explored to maximize the benefits and overcome the individual limitations of these techniques. This review highlights recent advancements in combined techniques for generating structures with controlled porosities on the micro-nano scale, leading to improved mechanical structural integrity. Collectively, these techniques also allow the fabrication of nature-inspired structures, contributing to a paradigm shift in research and technology. Finally, the review concludes by examining the advantages, disadvantages, and future outlooks of existing technologies in addressing challenges and exploring potential opportunities.

Fabrication of Soft Transparent Patient-Specific Vascular Models with Stereolithographic 3D printing and Thiol-Based Photopolymerizable Coatings

An ideal vascular phantom should be anatomically accurate, have mechanical properties as close as possible to the tissue, and be sufficiently transparent for ease of visualization. However, materials that enable the convergence of these characteristics have remained elusive. The fabrication of patient-specific vascular phantoms with high anatomical fidelity, optical transparency, and mechanical properties close to those of vascular tissue is reported. These final properties are achieved by 3D printing patient-specific vascular models with commercial elastomeric acrylic-based resins before coating them with thiol-based photopolymerizable resins. Ternary thiol-ene-acrylate chemistry is found optimal. A PETMP/allyl glycerol ether (AGE)/polyethylene glycol diacrylate (PEGDA) coating with a 30/70% AGE/PEGDA ratio applied on a flexible resin yielded elastic modulus, UTS, and elongation of 3.41 MPa, 1.76 MPa, and 63.2%, respectively, in range with the human aortic wall. The PETMP/AGE/PEGDA coating doubled the optical transmission from 40% to 80%, approaching 88% of the benchmark silicone-based elastomer. Higher transparency correlates with a decrease in surface roughness from 2000 to 90 nm after coating. Coated 3D-printed anatomical replicas are showcased for pre-procedural planning and medical training with good radio-opacity and echogenicity. Thiol-click chemistry coatings, as a surface treatment for elastomeric stereolithographic 3D-printed objects, address inherent limitations of photopolymer-based additive manufacturing.

Additive and Lithographic Manufacturing of Biomedical Scaffold Structures Using a Versatile Thiol-Ene Photocurable Resin

Additive and lithographic manufacturing technologies using photopolymerisation provide a powerful tool for fabricating multiscale structures, which is especially interesting for biomimetic scaffolds and biointerfaces. However, most resins are tailored to one particular fabrication technology, showing drawbacks for versatile use. Hence, we used a resin based on thiol-ene chemistry, leveraging its numerous advantages such as low oxygen inhibition, minimal shrinkage and high monomer conversion. The resin is tailored to applications in additive and lithographic technologies for future biofabrication where fast curing kinetics in the presence of oxygen are required, namely 3D inkjet printing, digital light processing and nanoimprint lithography. These technologies enable us to fabricate scaffolds over a span of six orders of magnitude with a maximum of 10 mm and a minimum of 150 nm in height, including bioinspired porous structures with controlled architecture, hole-patterned plates and micro/submicro patterned surfaces. Such versatile properties, combined with noncytotoxicity, degradability and the commercial availability of all the components render the resin as a prototyping material for tissue engineers.

Technological Advancement and Trend in Selective Bioanalytical Sample Extraction through State of the Art 3-D Printing Techniques Aiming 'Sorbent Customization as per need'

The inherent complexity of biological matrices and presence of several interfering substances in biological samples make them unsuitable for direct analysis. An effective sample preparation technique assists in analyte enrichment, improving selectivity and sensitivity of bioanalytical method. Because of several key benefits of employing 3D printed sorbent in sample extraction, it has recently gained popularity across a variety of industries. Applications for 3D printing in the field of bioanalytical research have grown recently, particularly in the areas of miniaturization, (bio)sensing, sample preparation, and separation sciences. Due to the high expense of the solid phase microextraction cartridge, researcher approaches in-lab production of sorbent material for the extraction of analyte from biological samples. Owing to its distinct advantages such as low costs, automation capabilities, capacity to produce products in a variety of shapes, and reduction of tedious steps of sample preparation, 3D printed sorbents are gaining increased attention in the field of bioanalysis. It is also reported to offer high selectivity and assist in achieving a much lower limit of detection. In this review, we have discussed current advancements in different types of 3D printed sorbents, production methods, and their applications in the field of bioanalytical sample preparation.

Direct conversion of carboxylic acids to free thiols via radical relay acridine photocatalysis enabled by N-O bond cleavage

Carboxylic acids and thiols are basic chemical compounds with diverse utility and widespread reactivity. However, the direct conversion of unprotected acids to thiols is hampered due to a fundamental problem - free thiols are incompatible with the alkyl radicals formed on decarboxylation of carboxylic acids. Herein, we describe a concept for the direct photocatalytic thiolation of unprotected acids allowing unprotected thiols and their derivatives to be obtained. The method is based on the application of a thionocarbonate reagent featuring the N-O bond. The reagent serves both for the rapid trapping of alkyl radicals and for the facile regeneration of the acridine-type photocatalyst.

Photochemically Activated 3D Printing Inks: Current Status, Challenges, and Opportunities

3D printing with light is enabled by the photochemistry underpinning it. Without fine control over the ability to photochemically gate covalent bond formation by the light at a certain wavelength and intensity, advanced photoresists with functions spanning from on-demand degradability, adaptability, rapid printing speeds, and tailored functionality are impossible to design. Herein, recent advances in photoresist design for light-driven 3D printing applications are critically assessed, and an outlook of the outstanding challenges and opportunities is provided. This is achieved by classing the discussed photoresists in chemistries that function photoinitiator-free and those that require a photoinitiator to proceed. Such a taxonomy is based on the efficiency with which photons are able to generate covalent bonds, with each concept featuring distinct advantages and drawbacks.

Highly resilient and fatigue-resistant poly(4-methyl-ε-caprolactone) porous scaffold fabricated via thiol-yne photo-crosslinking/salt-templating for soft tissue regeneration

Elastomeric scaffolds, individually customized to mimic the structural and mechanical properties of natural tissues have been used for tissue regeneration. In this regard, polyester elastic scaffolds with tunable mechanical properties and exceptional biological properties have been reported to provide mechanical support and structural integrity for tissue repair. Herein, poly(4-methyl-ε-caprolactone) (PMCL) was first double-terminated by alkynylation (PMCL-DY) as a liquid precursor at room temperature. Subsequently, three-dimensional porous scaffolds with custom shapes were fabricated from PMCL-DY via thiol-yne photocrosslinking using a practical salt template method. By manipulating the M_n of the precursor, the modulus of compression of the scaffold was easily adjusted. As evidenced by the complete recovery from 90% compression, the rapid recovery rate of >500 mm min^-1, the extremely low energy loss coefficient of <0.1, and the superior fatigue resistance, the PMCL20-DY porous scaffold was confirmed to harbor excellent elastic properties. In addition, the high resilience of the scaffold was confirmed to endow it with a minimally invasive application potential. In vitro testing revealed that the 3D porous scaffold was biocompatible with rat bone marrow stromal cells (BMSCs), inducing BMSCs to differentiate into chondrogenic cells. In addition, the elastic porous scaffold demonstrated good regenerative efficiency in a 12-week rabbit cartilage defect model. Thus, the novel polyester scaffold with adaptable mechanical properties may have extensive applications in soft tissue regeneration.

Chemistry in light-induced 3D printing

In the last few years, 3D printing has evolved from its original niche applications, such as rapid prototyping and hobbyists, towards many applications in industry, research and everyday life. This involved an evolution in terms of equipment, software and, most of all, in materials. Among the different available 3D printing technologies, the light activated ones need particular attention from a chemical point of view, since those are based on photocurable formulations and in situ rapid solidification via photopolymerization. In this article, the chemical aspects beyond the preparation of a formulation for light-induced 3D printing are analyzed and explained, aiming at giving more tools for the development of new photocurable materials that can be used for the fabrication of innovative 3D printable devices.

Graphical abstract

Recent Advances in 3D Printing of Photocurable Polymers: Types, Mechanism, and Tissue Engineering Application

The conversion of liquid resin into solid structures upon exposure to light of a specific wavelength is known as photopolymerization. In recent years, photopolymerization-based 3D printing has gained enormous attention for constructing complex tissue-specific constructs. Due to the economic and environmental benefits of the biopolymers employed, photo-curable 3D printing is considered an alternative method for replacing damaged tissues. However, the lack of suitable bio-based photopolymers, their characterization, effective crosslinking strategies, and optimal printing conditions are hindering the extensive application of 3D printed materials in the global market. This review highlights the present status of various photopolymers, their synthesis, and their optimization parameters for biomedical applications. Moreover, a glimpse of various photopolymerization techniques currently employed for 3D printing is also discussed. Furthermore, various naturally derived nanomaterials reinforced polymerization and their influence on printability and shape fidelity are also reviewed. Finally, the ultimate use of those photopolymerized hydrogel scaffolds in tissue engineering is also discussed. Taken together, it is believed that photopolymerized 3D printing has a great future, whereas conventional 3D printing requires considerable sophistication, and this review can provide readers with a comprehensive approach to developing light-mediated 3D printing for tissue-engineering applications.

Advanced Formulations Based on Poly(ionic liquid) Materials for Additive Manufacturing

Innovation in materials specially formulated for additive manufacturing is of great interest and can generate new opportunities for designing cost-effective smart materials for next-generation devices and engineering applications. Nevertheless, advanced molecular and nanostructured systems are frequently not possible to integrate into 3D printable materials, thus limiting their technological transferability. In some cases, this challenge can be overcome using polymeric macromolecules of ionic nature, such as polymeric ionic liquids (PILs). Due to their tuneability, wide variety in molecular composition, and macromolecular architecture, they show a remarkable ability to stabilize molecular and nanostructured materials. The technology resulting from 3D-printable PIL-based formulations represents an untapped array of potential applications, including optoelectronic, antimicrobial, catalysis, photoactive, conductive, and redox applications.

Additive manufacturing: Frameworks for chemical understanding and advancement in vat photopolymerization

Three-dimensional printing, or additive manufacturing (AM), is a broad term for a wide range of fabrication methods utilizing materials such as small-molecule, polymer, and metal feedstocks. Each method requires different chemical, physical, and engineering needs to be successful. This article will discuss some of the considerations for polymer-based AM methods. Ultimately, we focus on the chemistries of vat photopolymerization, in which light is used to cure a resin from liquid to solid, to provide an example of how chemical advancements have led to increased speed, resolution, and multimaterial printing capabilities not previously possible.

Effects of Kinetic Chain Length on the Degradation of Poly(β-amino ester)-Based Networks and Use in 3D Printing by Projection Microstereolithography

Poly(β-amino ester)-diacrylates (PBAE-dAs) are promising resins for three-dimensional (3D) printing. This study investigated the degradation of two PBAEs with different chemistries and kinetic chain lengths. PBAE-dA monomers were synthesized from benzhydrazide and poly(ethylene glycol) (A6) or butanediol (B6) diacrylate and then photopolymerized with pentaerythritol tetrakis(3-mercaptopropionate), which formed thiol-polyacrylate kinetic chains. This tetrathiol acts as a cross-linker and chain-transfer agent that controls the polyacrylate kinetic chain length. A6 networks exhibited bulk degradation, while B6 networks exhibited surface degradation, which transitioned to a combined surface and bulk degradation. Increasing the tetrathiol concentration shortened the polyacrylate kinetic chain and time-to-reverse gelation but degradation mode was unaffected. Hydrolysis occurred primarily through the β-amino ester. As network hydrophilicity increased, the slower degrading ester in the thiol-polyacrylate chains contributed to degradation. Overall, this work demonstrates control over network degradation rate, mode of degradation, and time-to-reverse gelation in PBAE networks and their application in 3D printing.

A Review of Multi-Material 3D Printing of Functional Materials via Vat Photopolymerization

Additive manufacturing or 3D printing of materials is a prominent process technology which involves the fabrication of materials layer-by-layer or point-by-point in a subsequent manner. With recent advancements in additive manufacturing, the technology has excited a great potential for extension of simple designs to complex multi-material geometries. Vat photopolymerization is a subdivision of additive manufacturing which possesses many attractive features, including excellent printing resolution, high dimensional accuracy, low-cost manufacturing, and the ability to spatially control the material properties. However, the technology is currently limited by design strategies, material chemistries, and equipment limitations. This review aims to provide readers with a comprehensive comparison of different additive manufacturing technologies along with detailed knowledge on advances in multi-material vat photopolymerization technologies. Furthermore, we describe popular material chemistries both from the past and more recently, along with future prospects to address the material-related limitations of vat photopolymerization. Examples of the impressive multi-material capabilities inspired by nature which are applicable today in multiple areas of life are briefly presented in the applications section. Finally, we describe our point of view on the future prospects of 3D printed multi-material structures as well as on the way forward towards promising further advancements in vat photopolymerization.

Synthesis and Characterization of Alkyne-Functionalized Photo-Cross-Linkable Polyesters

An alkyne-functionalized elastomer derived from sebacic acid, 1,3-propanediol, and alkyne-functionalized serinol is synthesized via melt condensation. A low-power UV lamp triggers the cross-linking rapidly via thiol-yne click chemistry. The cross-linking behavior is studied by photorheology and NMR spectroscopy. The resultant elastomer possesses mechanical properties similar to those of human soft tissues and exhibits in vitro degradability and good cytocompatibility.

Advances in Digital Light Processing of Hydrogels

Hydrogels, three-dimensional (3D) networks of hydrophilic polymers formed in water, are a significant type of soft matter used in fundamental and applied sciences. Hydrogels are of particular interest for biomedical applications, owing to their soft elasticity and good biocompatibility. However, the high water content and soft nature of hydrogels often make it difficult to process them into desirable solid forms. The development of 3D printing (3DP) technologies has provided opportunities for the manufacturing of hydrogels, by adopting a freeform fabrication method. Owing to its high printing speed and resolution, vat photopolymerization 3DP has recently attracted considerable interest for hydrogel fabrication, with digital light processing (DLP) becoming a widespread representative technique. Whilst acknowledging that other types of vat photopolymerization 3DP have also been applied for this purpose, we here only focus on DLP and its derivatives. In this review, we first comprehensively outline the most recent advances in both materials and fabrication, including the adaptation of novel hydrogel systems and advances in processing (e.g., volumetric printing and multimaterial integration). Secondly, we summarize the applications of hydrogel DLP, including regenerative medicine, functional microdevices, and soft robotics. To the best of our knowledge, this is the first time that either of these specific review focuses has been adopted in the literature. More importantly, we discuss the major challenges associated with hydrogel DLP and provide our perspectives on future trends. To summarize, this review aims to aid and inspire other researchers investigatng DLP, photocurable hydrogels, and the research fields related to them.

Gradient versus End-Capped Degradable Polymer Sequence Variations Result in Stiff to Elastic Photochemically 3D-Printed Substrates

Additive manufacturing affords the construction of complex scaffolds for tissue engineering, yet the limitation in material choice remains a barrier to clinical translation. Herein, a series of poly(propylene fumarate-co-propylene succinate) were synthesized using both one-pot and sequential ring-opening copolymerization reactions. Continuous liquid interface production-based photochemical 3D printing utilizing thiol-ene chemistry was used to fabricate precise structures with improved build time over the traditional poly(propylene fumarate)/diethyl fumarate 3D printing processes. Significantly, the materials do not exhibit a yield point under tension and Young's modulus of the 3D printed products can be tuned by more than 2 orders of magnitude (0.6-110 MPa) using polymer composition and the degree of polymerization. Printed constructs degrade fully under hydrolytic conditions and degradation rates can be tailored using polymer composition, polymer sequence, and resin formulation.

Smartphone-enabled 3D printing of medicines

3D printing is a manufacturing technique that is transforming numerous industrial sectors, particularly where it is key tool in the development and fabrication of medicinees that are personalised to the individual needs of patients. Most 3D printers are relatively large, require trained operators and must be located in a pharmaceutical setting to manufacture dosage forms. In order to realise fully the potential of point-of-care manufacturing of medicines, portable printers that are easy to operate are required. Here, we report the development of a 3D printer that operates using a mobile smartphone. The printer, operating on stereolithographic principles, uses the light from the smartphone's screen to photopolymerise liquid resins and create solid structures. The shape of the printed dosage form is determined using a custom app on the smartphone. Warfarin-loaded Printlets (3D printed tablets) of various sizes and patient-centred shapes (caplet, triangle, diamond, square, pentagon, torus, and gyroid lattices) were successfully printed to a high resolution and with excellent dimensional precision using different photosensitive resins. The drug was present in an amorphous form, and the Printlets displayed sustained release characterises. The promising proof-of-concept results support the future potential of this compact, user-friendly and interconnected smartphone-based system for point-of-care manufacturing of personalised medications.

Stereolithography 3D printing technology in pharmaceuticals: a review

Three-dimensional printing (3DP) technology is an innovative tool used in manufacturing medical devices, producing alloys, replacing biological tissues, producing customized dosage forms and so on. Stereolithography (SLA), a 3D printing technique, is very rapid and highly accurate and produces finished products of uniform quality. 3D formulations have been optimized with a perfect tool of artificial intelligence learning techniques. Complex designs/shapes can be fabricated through SLA using the photopolymerization principle. Different 3DP technologies are introduced and the most promising of these, SLA, and its commercial applications, are focused on. The high speed and effectiveness of SLA are highlighted. The working principle of SLA, the materials used and applications of the technique in a wide range of different sectors are highlighted in this review. An innovative idea of 3D printing customized pharmaceutical dosage forms is also presented. SLA compromises several advantages over other methods, such as cost effectiveness, controlled integrity of materials and greater speed. The development of SLA has allowed the development of printed pharmaceutical devices. Considering the present trends, it is expected that SLA will be used along with conventional methods of manufacturing of 3D model. This 3D printing technology may be utilized as a novel tool for delivering drugs on demand. This review will be useful for researchers working on 3D printing technologies.

Tough Materials Through Ionic Interactions

This article introduces butyl acrylate-based materials that are toughened with dynamic crosslinkers. These dynamic crosslinkers are salts where both the anion and cation polymerize. The ion pairs between the polymerized anions and cations form dynamic crosslinks that break and reform under deformation. Chemical crosslinker was used to bring shape stability. The extent of dynamic and chemical crosslinking was related to the mechanical and thermal properties of the materials. Furthermore, the dependence of the material properties on different dynamic crosslinkers—tributyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C4ASA) and trihexyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C6ASA)—was studied. The materials’ mechanical and thermal properties were characterized by means of tensile tests, dynamic mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. The dynamic crosslinks strengthened the materials considerably. Chemical crosslinks decreased the elasticity of the materials but did not significantly affect their strength. Comparison of the two ionic crosslinkers revealed that changing the crosslinker from C4ASA to C6ASA results in more elastic, but slightly weaker materials. In conclusion, dynamic crosslinks provide substantial enhancement of mechanical properties of the materials. This is a unique approach that is utilizable for a wide variety of polymer materials.

Chemical and Mechanical Tunability of 3D-Printed Dynamic Covalent Networks Based on Boronate Esters

As the scope of additive manufacturing broadens, interest has developed in 3D-printed objects that are derived from recyclable resins with chemical and mechanical tunability. Dynamic covalent bonds have the potential to not only increase the sustainability of 3D-printed objects, but also serve as reactive sites for postprinting derivatization. In this study, we use boronate esters as a key building block for the development of catalyst-free, 3D-printing resins with the ability to undergo room-temperature exchange at the cross-linking sites. The orthogonality of boronate esters is exploited in fast-curing, oxygen-tolerant thiol-ene resins in which the dynamic character of 3D-printed objects can be modulated by the addition of a static, covalent cross-linker with no room-temperature bond exchange. This allows the mechanical properties of printed parts to be varied between those of a traditional thermoset and a vitrimer. Objects printed with a hybrid dynamic/static resin exhibit a balance of structural stability (residual stress = 18%) and rapid exchange (characteristic relaxation time = 7 s), allowing for interfacial welding and postprinting functionalization. Modulation of the cross-linking density postprinting is enabled by selective hydrolysis of the boronate esters to generate networks with swelling capacities tunable from 1.3 to 3.3.

Fabrication of Microfluidic Devices for Emulsion Formation by Microstereolithography

Droplet microfluidics—the art and science of forming droplets—has been revolutionary for high-throughput screening, directed evolution, single-cell sequencing, and material design. However, traditional fabrication techniques for microfluidic devices suffer from several disadvantages, including multistep processing, expensive facilities, and limited three-dimensional (3D) design flexibility. High-resolution additive manufacturing—and in particular, projection micro-stereolithography (PµSL)—provides a promising path for overcoming these drawbacks. Similar to polydimethylsiloxane-based microfluidics 20 years ago, 3D printing methods, such as PµSL, have provided a path toward a new era of microfluidic device design. PµSL greatly simplifies the device fabrication process, especially the access to truly 3D geometries, is cost-effective, and it enables multimaterial processing. In this review, we discuss both the basics and recent innovations in PµSL; the material basis with emphasis on custom-made photopolymer formulations; multimaterial 3D printing; and, 3D-printed microfluidic devices for emulsion formation as our focus application. Our goal is to support researchers in setting up their own PµSL system to fabricate tailor-made microfluidics.

Additive Manufacture of Dynamic Thiol-ene Networks Incorporating Anhydride-Derived Reversible Thioester Links

A photoprintable dynamic thiol-ene resin was developed based on commercially available anhydride, thiol, and ene monomers. The dynamic chemistry chosen for this study relied on the thermal reversibility of the in situ generated thioester-anhydride links. The resin's rheological and curing properties were optimized to enable 3D printing using the masked stereolithography (MSLA) technique. To achieve a desirable depth of cure of 200 μm, a combination of radical photoinitiator (BAPO) and inhibitor (pyrogallol) were used at a weight ratio of 0.5 to 0.05, resulting in more than 90% thiol-ene conversion within 12 s curing time. In a series of stress relaxation and creep experiments, the dynamic reversible exchange was characterized and yielded rapid exchange rates ranging from minutes to seconds at temperatures of 80-140 °C. Little to no exchange was observed at temperatures below 60 °C. Various 3D geometries were 3D printed, and the printed objects were shown to be reconfigurable above 80 °C and depolymerizable at or above 120 °C. By deactivation of the exchange catalyst (DMAP), the stimuli responsiveness was demonstrated to be erasable, allowing for a significant shift in the actuation threshold. These highly enabling features of the dynamic chemistry open up new possibilities in the field of shape memory and 4D printable functional materials.

Reversible Deactivation Radical Polymerization: From Polymer Network Synthesis to 3D Printing

3D printing has changed the fabrication of advanced materials as it can provide customized and on-demand 3D networks. However, 3D printing of polymer materials with the capacity to be transformed after printing remains a great challenge for engineers, material, and polymer scientists. Radical polymerization has been conventionally used in photopolymerization-based 3D printing, as in the broader context of crosslinked polymer networks. Although this reaction pathway has shown great promise, it offers limited control over chain growth, chain architecture, and thus the final properties of the polymer networks. More fundamentally, radical polymerization produces dead polymer chains incapable of postpolymerization transformations. Alternatively, the application of reversible deactivation radical polymerization (RDRP) to polymer networks allows the tuning of network homogeneity and more importantly, enables the production of advanced materials containing dormant reactivatable species that can be used for subsequent processes in a postsynthetic stage. Consequently, the opportunities that (photoactivated) RDRP-based networks offer have been leveraged through the novel concepts of structurally tailored and engineered macromolecular gels, living additive manufacturing and photoexpandable/transformable-polymer networks. Herein, the advantages of RDRP-based networks over irreversibly formed conventional networks are discussed.

Sustainable Photopolymers in 3D Printing: A Review on Biobased, Biodegradable, and Recyclable Alternatives

The global market for 3D printing materials has grown exponentially in the last decade. Today, photopolymers claim almost half of the material sales worldwide. The lack of sustainable resins, applicable in vat photopolymerization that can compete with commercial materials, however, limits the widespread adoption of this technology. The development of "green" alternatives is of great importance in order to reduce the environmental impact of additive manufacturing. This paper reviews the recent evolutions in the field of sustainable photopolymers for 3D printing. It highlights the synthesis and application of biobased resin components, such as photocurable monomers and oligomers, as well as reinforcing agents derived from natural resources. In addition, the design of biologically degradable and recyclable thermoset products in vat photopolymerization is discussed. Together, those strategies will promote the accurate and waste-free production of a new generation of 3D materials for a sustainable plastics economy in the near future.

Vat photopolymerization 3D printing for advanced drug delivery and medical device applications

Three-dimensional (3D) printing is transforming manufacturing paradigms within healthcare. Vat photopolymerization 3D printing technology combines the benefits of high resolution and favourable printing speed, offering a sophisticated approach to fabricate bespoke medical devices and drug delivery systems. Herein, an overview of the vat polymerization techniques, their unique applications in the fields of drug delivery and medical device fabrication, material examples and the advantages they provide within healthcare, is provided. The challenges and drawbacks presented by this technology are also discussed. It is forecast that the adoption of 3D printing could pave the way for a personalised health system, advancing from traditional treatments pathways towards digital healthcare.

Photochemical Study of a New Bimolecular Photoinitiating System for Vat Photopolymerization 3D Printing Techniques under Visible Light

In this work, we presented a new bimolecular photoinitiating system based on 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives as visible photosensitizers of diphenyliodonium salt. Real-time FTIR and photo-DSC photopolymerization experiments with a cycloaliphatic epoxide and vinyl monomers showed surprisingly good reactivity of the bimolecular photoinitiating systems under UV-A, as well as under visible light sources. Steady-state photolysis, fluorescence experiments, theoretical calculations of molecular orbitals, and electrochemical analysis demonstrated photo-redox behavior as well as the ability to form initiating species via photo-reduction or photo-oxidation pathways, respectively. Therefore, the 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives were also investigated as a type II free-radical photoinitiator with amine. It was confirmed that the 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives, in combination with different types of additives, e.g., amine as a co-initiator or the presence of onium salt, can act as bimolecular photoinitiating systems for cationic, free-radical, and thiol-ene photopolymerization processes by hydrogen abstraction and/or electron transfer reactions stimulated by either near-UV or visible light irradiation. Finally, the 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives were selected for 3D printing rapid prototyping experiments. Test objects were successfully printed using purely cationic photosensitive resin, created on a 3D printer with a visible LED light source.

Additive manufacture of lightly crosslinked semicrystalline thiol-enes for enhanced mechanical performance

Photopolymerizable semicrystalline thermoplastics resulting from thiol-ene polymerizations were formed via fast polymerizations and achieved excellent mechanical properties. These materials have been shown to produce materials desirable for additive manufacturing (3D printing), especially for recyclable printing and investment casting. However, while well-resolved prints were previously achieved with the thiol-ene thermoplastics, the remarkable elongation at break (ϵmax) and toughness (T) attained in bulk were not realized for 3D printed components (ϵmax,bulk ~ 790%, Tbulk ~ 102 MJ m-3 vs. ϵmax,print < 5%, Tprint < 0.5 MJ m-3). In this work, small concentrations (5-10 mol%) of a crosslinker were added to the original thiol-ene resin composition without sacrificing crystallization potential to achieve semicrystalline, covalently crosslinked networks with enhanced mechanical properties. Improvements in ductility and overall toughness were observed for printed crosslinked structures, and substantial mechanical augmentation was further demonstrated with post-manufacture thermal conditioning of printed materials above the melting temperature (Tm). In some instances, this thermal conditioning to reset the crystalline component of the crosslinked prints yielded mechanical properties that were comparable or superior to its bulk counterpart (ϵmax ~ 790%, T ~ 95 MJ m-3). These unique photopolymerizations and their corresponding monomer compositions exhibited concurrent polymerization and crystallization along with mechanical properties that were tunable by changes to the monomer composition, photopolymerization conditions, and post-polymerization conditioning. This is the first example of a 3D printed semicrystalline, crosslinked material with thermally tunable mechanical properties that are superior to many commercially-available resins.

Hierarchical ordering in light-triggered additive manufacturing

Herein, recent developments in the 3D printing of materials with structural hierarchy and their future prospects are reviewed. It is shown that increasing the extent of ordering, is essential to access novel properties and functionalities.

Preparation of Biodegradable Polyethylene Glycol Dimethacrylate Hydrogels via Thiol-ene Chemistry

Through the control of the molecular weight, water content and monomer concentration, polyethylene glycol dimethacrylate (PEGDMA) based hydrogels have been adapted for numerous applications, including as structural scaffolds, drug delivery vehicles and cell carriers. However, due to the low biodegradability rates, the use of PEGDMA in tissue engineering has been limited. Thiol-based monomers have been shown to improve the degradation rates of several PEG-based hydrogels, though their impact on several material properties has not been as well defined. In this work, several mercaptopropianoates, as well as mercaptoacetates, were mixed with PEGDMA and copolymerized. Following an initial polymerization check, it was determined that mercaptoacetate-based thiol monomers did not polymerize in the presence of PEGDMA, whereas mercaptopropionates were more successful. The wettability, and the compressive and tensile strength, in addition to the thermal properties, were determined for successfully copolymerized samples via a combination of differential scanning calorimetry, dynamic mechanical analysis, unconfined compression, and goniometry. Further study determined that dipentaerythritol hexa(3-mercaptopropionate) (DiPETMP) successfully enhanced the biodegradability of PEGDMA.

Real-Time Live-Cell Imaging Technology Enables High-Throughput Screening to Verify in Vitro Biocompatibility of 3D Printed Materials

With growing advances in three-dimensional (3D) printing technology, the availability and diversity of printing materials has rapidly increased over the last years. 3D printing has quickly become a useful tool for biomedical and various laboratory applications, offering a tremendous potential for efficiently fabricating complex devices in a short period of time. However, there still remains a lack of information regarding the impact of printing materials and post-processing techniques on cell behavior. This study introduces real-time live-cell imaging technology as a fast, user-friendly, and high-throughput screening strategy to verify the in vitro biocompatibility of 3D printed materials. Polyacrylate-based photopolymer material was printed using high-resolution 3D printing techniques, post-processed using three different procedures, and then analyzed with respect to its effects on cell viability, apoptosis, and necrosis of adipogenic mesenchymal stem cells (MSCs). When using ethanol for the post-processing procedure and disinfection, no significant effects on MSCs could be detected. For the analyses a novel image-based live-cell analysis system was compared against a biochemical-based standard plate reader assay and traditional flow cytometry. This comparison illustrates the superiority of using image-based detection of in vitro biocompatibility with respect to analysis time, usability, and scientific outcome.

Sustainable near UV-curable acrylates based on natural phenolics for stereolithography 3D printing

Photocured polymers have recently gained tremendous interest for a wide range of applications especially industrial prototyping/additive manufacturing. This work aims to develop natural phenolic-based (meth)acrylates to expand the use of sustainable and mechanically robust 3D printable formulations.

Thiol–yne chemistry for 3D printing: exploiting an off-stoichiometric route for selective functionalization of 3D objects

An alkyne monomer, bis(propargyl) fumarate, is synthesized and mixed to a thiol monomer to produce DLP-3D printable formulations. Using off-stoichiometric formulations it is possible to print functionalizable objects.

Molecularly imprinted polymers by thiol–yne chemistry: making imprinting even easier

Molecularly imprinted polymers (MIPs) are synthetic, bio-mimetic materials with recognition properties on a par with those of antibodies, which feature superior physical and chemical stability.

Directed motion of water droplets on multi-gradient photopolymer surfaces

Rapid and directional movement of water droplets across a photopolymer surface with inscribed wettability and Laplace pressure gradient is demonstrated.

Three-Dimensional Printed Photoluminescent Polymeric Waveguides

In this work, we propose an innovative strategy for obtaining functional objects employing a light-activated three-dimensional (3D) printing process without affecting the materials' printability. In particular, a dye is a necessary ingredient in a formulation for a digital light processing 3D printing method to obtain precise and complex structures. Here, we use a photoluminescent dye specifically synthesized for this purpose that enables the production of 3D printed waveguides and splitters able to guide the luminescence. Moreover, copolymerizing the dye with the polymeric network during the printing process, we are able to maintain the solvatochromic properties of the dye toward different solvents in the printed structures, enabling the development of solvents' polarity sensors.

A 3D Printable Thermal Energy Storage Crystalline Gel Using Mask-Projection Stereolithography

Most of the phase change materials (PCMs) have been limited to use as functional additions or sealed in containers, and extra auxiliary equipment or supporting matrix is needed. The emergence of 3D printing technique has dramatically advanced the developments of materials and simplified production processes. This study focuses on a novel strategy to model thermal energy storage crystalline gels with three-dimensional architecture directly from liquid resin without supporting materials through light-induced polymerization 3D printing technique. A mask-projection stereolithography printer was used to measure the 3D printing test, and the printable characters of crystalline thermal energy storage P(SA-DMAA) gels with different molar ratios were evaluated. For the P(SA-DMMA) gels with a small fraction of SA, the 3D fabrication was realized with higher printing precision both on milli- and micro- meter scales. As a comparison of 3D printed samples, P(SA-DMAA) gels made by other two methods, post-UV curing treatment after 3D printing and UV curing using conventional mold, were prepared. The 3D printed P(SA-DMAA) gels shown high crystallinity. Post-UV curing treatment was beneficial to full curing of 3D printed gels, but did not lead to the further improvement of the crystal structure to get higher crystallinity. The P(SA-DMAA) crystalline gel having the highest energy storage enthalpy was developed, which reached 69.6 J·g-1. Its good thermoregulation property in the temperature range from 25 to 40 °C was proved. The P(SA-DMAA) gels are feasible for practical applications as one kind of 3D printing material with thermal energy storage and thermoregulation functionality.

Making and Breaking Chemical Bonds by Chemiluminescence

Chemiluminescent (CL) reactions are powerful analytical tools and are present in commercially available everyday objects such as glow sticks. Herein, the photons generated by chemiluminescence are exploited to induce covalent bond breakage and formation, using a chemically generated photonic field at ambient temperature through space as energy transducer. Remarkably, the generated photons enable both the cleavage of species generating radicals as well as the execution of [2 + 2] cycloadditions, demonstrating that disparate types of reactions can be triggered. The herein-presented photochemical concept establishes the field of CL-induced photochemistry, which is poised to enable photochemical transformations in situations where physical light sources, such as lamps, LEDs, and lasers cannot be employed, including in biological environments.

3D printing of photopolymers

Recent progress in the photoinitiators and monomers/oligomers of photopolymers for 3D printing is presented in the review.

Polymer structure-property requirements for stereolithographic 3D printing of soft tissue engineering scaffolds

This review highlights the synthesis, properties, and advanced applications of synthetic and natural polymers 3D printed using stereolithography for soft tissue engineering applications. Soft tissue scaffolds are of great interest due to the number of musculoskeletal, cardiovascular, and connective tissue injuries and replacements humans face each year. Accurately replacing or repairing these tissues is challenging due to the variation in size, shape, and strength of different types of soft tissue. With advancing processing techniques such as stereolithography, control of scaffold resolution down to the μm scale is achievable along with the ability to customize each fabricated scaffold to match the targeted replacement tissue. Matching the advanced manufacturing technique to polymer properties as well as maintaining the proper chemical, biological, and mechanical properties for tissue replacement is extremely challenging. This review discusses the design of polymers with tailored structure, architecture, and functionality for stereolithography, while maintaining chemical, biological, and mechanical properties to mimic a broad range of soft tissue types.