Solvent-Free Three-Dimensional Printing of Biodegradable Elastomers Using Liquid Macrophotoinitiators

Carbohydrate-Based Polyester and Amino Acid Polyester Photocrosslinker and Their Resin Formulation for 3D Printing Applications

Fully bio-based polyester was designed and synthesized using the carbohydrate-based diol 2,4:3,5-di-O-methylene-D-mannitol (Manx) and dimethyl ester of 2,3:4,5-di-O-methylene-galactaric acid (Galx). Photocurable resin formulations were prepared by incorporating up to 15 wt% of the carbohydrate polyester into hydroxyl ethyl methacrylate (HEMA) along with polyacrylamide crosslinker derived from L-glutamic acid. Complex 3D structures with good shape fidelity could be 3D printed using these novel polyester resin formulations. The incorporation of the carbohydrate polyester improved the glass transition temperature of the 3D-printed objects. Enzymatic erosion studies conducted using esterase enzyme revealed a higher degradation rate for the 3D-printed films containing the carbohydrate polyester. The hydrolytic degradation analysis conducted in both acidic and basic environments revealed that the 3D-printed polymer network exhibits stability and resilience in acidic conditions, while it undergoes complete degradation in basic conditions. This finding underscores the possibility of tailoring degradation processes under regulated circumstances.

Trends in Photopolymerization 3D Printing for Advanced Drug Delivery Applications

Since its invention in the 1980s, photopolymerization-based 3D printing has attracted significant attention for its capability to fabricate complex microstructures with high precision, by leveraging light patterning to initiate polymerization and cross-linking in liquid resin materials. Such precision makes it particularly suitable for biomedical applications, in particular, advanced and customized drug delivery systems. This review summarizes the latest advancements in photopolymerization 3D printing technology and the development of biocompatible and/or biodegradable materials that have been used or shown potential in the field of drug delivery. The drug loading methods and release characteristics of the 3D printing drug delivery systems are summarized. Importantly, recent trends in the drug delivery applications based on photopolymerization 3D printing, including oral formulations, microneedles, implantable devices, microrobots and recently emerging systems, are analyzed. In the end, the challenges and opportunities in photopolymerization 3D printing for customized drug delivery are discussed.

Influence of heat-assisted vat photopolymerization on the physical and mechanical characteristics of dental 3D printing resins

The effects of heat-assisted vat photopolymerization (HVPP) on the physical and mechanical properties of 3D-printed dental resins, including the morphometric stability of 3D-printed crowns, were investigated. A resin tank was designed to maintain the resin at 30, 40, and 50 ℃ during the 3D printing process. Test specimens were fabricated using a commercial dental resin, with untreated resin serving as the control group. Key properties such as viscosity, curing kinetics, surface microhardness, flexural properties, and dimensional accuracy were evaluated. The viscosity of the resin decreased significantly (P < 0.05) with increasing temperature, thereby enhancing its flow properties. Photo-DSC analysis revealed a 17.58% increase in peak heat flow at 50 ℃, indicating accelerated polymerization. Surface microhardness improved significantly (P < 0.05) with HVPP, though a slight reduction was observed at 50 ℃ compared to that at 30 and 40 ℃. The flexural strength, modulus, and resilience were significantly enhanced (P < 0.05) at higher temperatures, with 50 ℃ yielding the best mechanical properties. However, 3D morphometric analysis showed increased root mean square deviation from the CAD design at elevated temperatures. Our results suggest that HVPP enhances the durability of dental prostheses, although careful optimization of the printing temperature is essential to balance their strength and accuracy.

Synthesis and Solvent Free DLP 3D Printing of Degradable Poly(Allyl Glycidyl Ether Succinate)

Digital light processing (DLP) printing forms solid constructs from fluidic resins by photochemically crosslinking polymeric resins with reactive functional groups. DLP is used widely due to its efficient, high-resolution printing, but its use and translational potential has been limited in some applications as state-of-the-art resins experience unpredictable and anisotropic part shrinkage due to the use of solvent needed to reduce resin viscosity and layer dependent crosslinking. Herein, poly(allyl glycidyl ether succinate) (PAGES), a low viscosity, degradable polyester, was synthesized by ring opening copolymerization and used in combination with degradable thiol crosslinkers to afford a solvent free resin that can be utilized in DLP printing. Varying resin formulations of PAGES polymer are shown to decrease part shrinkage from 14 % to 0.3 %. Photochemically printed parts fabricated from PAGES possess tensile moduli between 0.43 and 6.18 MPa and degradation profiles are shown to vary between 12 and 40 days under accelerated conditions based on degree of polymerization and crosslink ratio.

Biocompatible PVAc-g-PLLA Acrylate Polymers for DLP 3D Printing with Tunable Mechanical Properties

The technological advancement of Additive Manufacturing has enabled the fabrication of various customized artifacts and devices, which has prompted a huge demand for multimaterials that can cater to stringent mechanical, chemical, and other functional property requirements. Photocurable formulations that are widely used for Digital Light Processing (DLP)/Stereolithography (SLA) 3D printing applications are now expected to meet these new challenges of hard and soft or stretchable structural requirements in addition to good resolution in multiple scales. Here we present a biocompatible photocurable resin formulation with tunable mechanical properties that can produce hard or stretchable elastomeric 3D printed materials in a graded manner. Acrylate poly(lactic acid) (PLA) grafted polyvinyl acetate (PVAc) polymer was mixed with hydroxyl ethyl methacrylate (HEMA) and hydroxyl ethyl acrylate (HEA) as reactive diluents (50-70 wt %) in various compositions to form a series of photocurable resin formulations. Depending on the nature of the reactive diluent (HEMA or HEA) and their weight percentage, the mechanical properties of the 3D printed parts could be fine-tuned from hard (Tensile strength 20.6 ± 2 MPa, elongation 2 ± 1%) to soft (Tensile strength 1.1 ± 0.2 MPa, elongation 62 ± 8%) materials. The printed materials displayed remarkable dye absorption (95%), showing stimuli-responsive behavior for dye release (with respect to both pH and enzyme), while also demonstrating high cell viability (>90%) for mouse embryonic (WT-MEF) cells and degradability in PBS solution. These biobased 3D printing resins have the potential for a variety of applications, including tissue engineering, soft robotics, dye absorption, and elastomeric actuators.

Digital Light 3D Printing of Double Thermoplastics with Customizable Mechanical Properties and Versatile Reprocessability

Digital light processing (DLP) is a 3D printing technology offering high resolution and speed. Printable materials are commonly based on multifunctional monomers, resulting in the formation of thermosets that usually cannot be reprocessed or recycled. Some efforts are made in DLP 3D printing of thermoplastic materials. However, these materials exhibit limited and poor mechanical properties. Here, a new strategy is presented for DLP 3D printing of thermoplastics based on a sequential construction of two linear polymers with contrasting (stiff and flexible) mechanical properties. The inks consist of two vinyl monomers, which lead to the stiff linear polymer, and α-lipoic acid, which forms the flexible linear polymer via thermal ring-opening polymerization in a second step. By varying the ratio of stiff and flexible linear polymers, the mechanical properties can be tuned with Young's modulus ranging from 1.1 GPa to 0.7 MPa, while the strain at break increased from 4% to 574%. Furthermore, these printed thermoplastics allow for a variety of reprocessability pathways including self-healing, solvent casting, reprinting, and closed-loop recycling of the flexible polymer, contributing to the development of a sustainable materials economy. Last, the potential of the new material in applications ranging from soft robotics to electronics is demonstrated.

Light from Afield: Fast, High-Resolution, and Layer-Free Deep Vat 3D Printing

Harnessing light for cross-linking of photoresponsive materials has revolutionized the field of 3D printing. A wide variety of techniques leveraging broad-spectrum light shaping have been introduced as a way to achieve fast and high-resolution printing, with applications ranging from simple prototypes to biomimetic engineered tissues for regenerative medicine. Conventional light-based printing techniques use cross-linking of material in a layer-by-layer fashion to produce complex parts. Only recently, new techniques have emerged which deploy multidirection, tomographic, light-sheet or filamented light-based image projections deep into the volume of resin-filled vat for photoinitiation and cross-linking. These Deep Vat printing (DVP) approaches alleviate the need for layer-wise printing and enable unprecedented fabrication speeds (within a few seconds) with high resolution (>10 μm). Here, we elucidate the physics and chemistry of these processes, their commonalities and differences, as well as their emerging applications in biomedical and non-biomedical fields. Importantly, we highlight their limitations, and future scope of research that will improve the scalability and applicability of these DVP techniques in a wide variety of engineering and regenerative medicine applications.

Three-Dimensional Printing of Lignocellulose Structures: Improving Mechanical Properties and Shape Fidelity

Additive manufacturing of nanocellulose (NC) materials is an emergent technological domain that facilitates the fabrication of complex and environment-friendly structures that mitigate greenhouse gas emissions. However, printing high concentrations of NC into intricate structures encounters substantial challenges due to inadequate adhesion between the printed layers attributed to a high cellulose solid content, resulting in low shape fidelity and mechanical properties. Therefore, to address these challenges, this paper reports lignin (LG) blending, a nanofiller, in high-content NC (>25 wt % solid content) paste to improve the layer adhesion of three-dimensional (3D) printed structures. The printed structures are dried in a clean room condition followed by postcuring. The optimized lignocellulose (0.5LG-NC) paste showed high structural shape fidelity, remarkable flexural strength, and moduli of 102.93 ± 0.96 MPa and 9.05 ± 0.07 GPa. Furthermore, the volumetric shrinkage behavior in box-like 3D printed structures with optimized LG-NC paste shows low standard deviations, demonstrating the repeatability of the printed structures. The study can be adapted for high-performance engineering and biomedical applications to manufacture high mechanical strength environment-friendly structures.

Opposite Mechanical Preference of Bone/Nerve Regeneration in 3D-Printed Bioelastomeric Scaffolds/Conduits Consistently Correlated with YAP-Mediated Stem Cell Osteo/Neuro-Genesis

To systematically unveil how substrate stiffness, a critical factor in directing cell fate through mechanotransduction, correlates with tissue regeneration, novel biodegradable and photo-curable poly(trimethylene carbonate) fumarates (PTMCFs) for fabricating elastomeric 2D substrates and 3D bone scaffolds/nerve conduits, are presented. These substrates and structures with adjustable stiffness serve as a unique platform to evaluate how this mechanical cue affects the fate of human umbilical cord mesenchymal stem cells (hMSCs) and hard/soft tissue regeneration in rat femur bone defect and sciatic nerve transection models; whilst, decoupling from topographical and chemical cues. In addition to a positive relationship between substrate stiffness (tensile modulus: 90-990 kPa) and hMSC adhesion, spreading, and proliferation mediated through Yes-associated protein (YAP), opposite mechanical preference is revealed in the osteogenesis and neurogenesis of hMSCs as they are significantly enhanced on the stiff and compliant substrates, respectively. In vivo tissue regeneration demonstrates the same trend: bone regeneration prefers the stiffer scaffolds; while, nerve regeneration prefers the more compliant conduits. Whole-transcriptome analysis further shows that upregulation of Rho GTPase activity and the downstream genes in the compliant group promote nerve repair, providing critical insight into the design strategies of biomaterials for stem cell regulation and hard/soft tissue regeneration through mechanotransduction.

Recent Developments in Synthesis, Properties, Applications and Recycling of Bio-Based Elastomers

In 2021, global plastics production was 390.7 Mt; in 2022, it was 400.3 Mt, showing an increase of 2.4%, and this rising tendency will increase yearly. Of this data, less than 2% correspond to bio-based plastics. Currently, polymers, including elastomers, are non-recyclable and come from non-renewable sources. Additionally, most elastomers are thermosets, making them complex to recycle and reuse. It takes hundreds to thousands of years to decompose or biodegrade, contributing to plastic waste accumulation, nano and microplastic formation, and environmental pollution. Due to this, the synthesis of elastomers from natural and renewable resources has attracted the attention of researchers and industries. In this review paper, new methods and strategies are proposed for the preparation of bio-based elastomers. The main goals are the advances and improvements in the synthesis, properties, and applications of bio-based elastomers from natural and industrial rubbers, polyurethanes, polyesters, and polyethers, and an approach to their circular economy and sustainability. Olefin metathesis is proposed as a novel and sustainable method for the synthesis of bio-based elastomers, which allows for the depolymerization or degradation of rubbers with the use of essential oils, terpenes, fatty acids, and fatty alcohols from natural resources such as chain transfer agents (CTA) or donors of the terminal groups in the main chain, which allow for control of the molecular weights and functional groups, obtaining new compounds, oligomers, and bio-based elastomers with an added value for the application of new polymers and materials. This tendency contributes to the development of bio-based elastomers that can reduce carbon emissions, avoid cross-contamination from fossil fuels, and obtain a greener material with biodegradable and/or compostable behavior.

Possibilities and advantages of additive manufacturing in dry powder formulations for inhalation

In dry powder formulations for inhalation, coarse carrier particles are often used to improve handling, dosing and dispersion of the active pharmaceutical ingredient (API). Carrier particles, mostly alpha-lactose monohydrate crystals, always show a certain size distribution and are never exactly uniform in their geometry. This might be one factor of the rather high invivo variability in fine particle dose from dry powder inhalers. To address the inhomogeneity of carrier particles, additive manufacturing has come to mind. The parametric design of the perfect carrier geometry could further improve the efficiency of dry powder formulations. In this study, a numerical simulation setup using the discrete element method as well as an experimental approach with 3D printed particles were used to determine the loading capacity of a model API onto two different carrier geometries. The difference between the two geometries was reduced solely to their surface's topology to assess the impact of that. The results indicate differences in the loading capacity for the two geometries, depending on the loading process. This study highlights the importance of the carrier geometry for the efficiency of dry powder formulations and thus, strengthens the idea of artificially designed and printed carrier particles.

4D printing of biodegradable elastomers with tailorable thermal response at physiological temperature

4D printing has a great potential for the manufacturing of soft robotics and medical devices. The alliance of digital light processing (DLP) 3D printing and novel shape-memory photopolymers allows for the fabrication of smart 4D-printed medical devices in high resolution and with tailorable functionalities. However, most of the reported 4D-printed materials are nondegradable, which limits their clinical applications. On the other hand, 4D printing of biodegradable shape-memory elastomers is highly challenging, especially when transition points close to physiological temperature and shape fixation under ambient conditions are required. Here, we report the 4D printing of biodegradable shape-memory elastomers with tailorable transition points covering physiological temperature, by using poly(D,L-lactide-co-trimethylene carbonate) methacrylates at various monomer feed ratios. After the programming step, the high-resolution DLP printed stents preserved their folded shape at room temperature, and showed efficient shape recovery at 37 °C. The materials were cytocompatible and readily degradable under physiological conditions. Furthermore, drug-loaded devices with tuneable release kinetics were realized by DLP-printing with resins containing polymers and levofloxacin or nintedanib. This study offers a new perspective for the development of next-generation 4D-printed medical devices.

Digital Light 3D Printed Bioresorbable and NIR-Responsive Devices with Photothermal and Shape-Memory Functions

Digital light processing (DLP) 3D printing is a promising technique for the rapid manufacturing of customized medical devices with high precision. To be successfully translated to a clinical setting, challenges in the development of suitable photopolymerizable materials have yet to be overcome. Besides biocompatibility, it is often desirable for the printed devices to be biodegradable, elastic, and with a therapeutic function. Here, a multifunctional DLP printed material system based on the composite of gold nanorods and polyester copolymer is reported. The material demonstrates robust near-infrared (NIR) responsiveness, allowing rapid and stable photothermal effect leading to the time-dependent cell death. NIR light-triggerable shape transformation is demonstrated, resulting in a facilitated insertion and expansion of DLP printed stent ex vivo. The proposed strategy opens a promising avenue for the design of multifunctional therapeutic devices based on nanoparticle-polymer composites.

Recent Trends in Advanced Photoinitiators for Vat Photopolymerization 3D Printing

3D printing has revolutionized the way of manufacturing with a huge impact on various fields, in particular biomedicine. Vat photopolymerization-based 3D printing techniques such as stereolithography (SLA) and digital light processing (DLP) attracted considerable attention owing to their superior print resolution, relatively high speed, low cost and flexibility in resin material design. As one key element of the SLA/DLP resin, photoinitiators or photoinitiating systems have experienced significant development in recent years, in parallel with the exploration of 3D printing (macro)monomers. The design of new photoinitiating systems can not only offer faster 3D printing speed and enable low-energy visible light fabrication, but also can bring new functions to the 3D printed products and even generate new printing methods in combination with advanced optics. This review evaluates recent trends in the development and application of advanced photoinitiators and photoinitiating systems for vat photopolymerization 3D printing, with a wide range of small molecules, polymers and nanoassemblies involved. Personal perspectives on the current limitations and future directions are eventually provided. This article is protected by copyright. All rights reserved.

3D printed elastomers with Sylgard-184-like mechanical properties and tuneable degradability

The 3D printing of biodegradable elastomers with high mechanical strength is of great interest for personalized medicine, but rather challenging. In this study, we propose a dual-polymer resin formulation for digital light processing of biodegradable elastomers with tailorable mechanical properties comparable to those of Sylgard-184.

Controlling Molecular Aggregation-Induced Emission by Controlled Polymerization

In last twenty years, the significant development of AIE materials has been witnessed. A number of small molecules, polymers and composites with AIE activity have been synthesized, with some of these exhibiting great potential in optoelectronics and biomedical applications. Compared to AIE small molecules, macromolecular systems-especially well-defined AIE polymers-have been studied relatively less. Controlled polymerization methods provide the efficient synthesis of well-defined AIE polymers with varied monomers, tunable chain lengths and narrow dispersity. In particular, the preparation of single-fluorophore polymers through AIE molecule-initiated polymerization enables the systematic investigation of the structure-property relationships of AIE polymeric systems. Here, the main polymerization techniques involved in these polymers are summarized and the key parameters that affect their photophysical properties are analyzed. The author endeavored to collect meaningful information from the descriptions of AIE polymer systems in the literature, to find connections by comparing different representative examples, and hopes eventually to provide a set of general guidelines for AIE polymer design, along with personal perspectives on the direction of future research.