Polyfunctional Acrylic Non-isocyanate Hydroxyurethanes as Photocurable Thermosets for 3D Printing

Non-Isocyanate Urethane Acrylate Derived from Isophorone Diamine: Synthesis, Characterization and Its Application in 3D Printing

In this paper, urethane-based acrylates (UA) were prepared via an environmentally friendly non-isocyanate route. Isophorone diamine (IPDA) reacted with ethylene carbonate (EC), producing carbamate containing amine and hydroxyl groups, which further reacted with neopentyl glycol diacrylate (NPGDA) by aza Michael addition, forming UA. The structures of the obtained intermediates and UA were characterized by 1H NMR and electrospray ionization high-resolution mass spectrometry (ESI-HRMS). The photopolymerization kinetics of UA were investigated by infrared spectroscopy. The composite with obtained UA can be UV cured quickly to form a transparent film with a tensile strength of 21 MPa and elongation at break of 16%. After UV curing, the mono-functional urethane acrylate was copolymerized into the cross-linked network in the form of side chains. The hydroxyl and carbamate bonds on the side chains have high mobility, which make them easy to form stronger dynamic hydrogen bonds during the tensile process, giving the material a higher tensile strength and elongation at break. Therefore, the hydrogen bonding model of a cross-linked network is proposed. The composite with UA can be 3D printed into models.

Color and Chemical Stability of 3D-Printed and Thermoformed Polyurethane-Based Aligners

The significant rise in the use of clear aligners for orthodontic treatment is attributed to their aesthetic appeal, enhancing patient appearance and self-confidence. The aim of this study is to evaluate the aligners' response to common staining agents (coffee, black tea, Coca-Cola, and Red Bull) in color and chemical stability. Polyurethane-based thermoformed and 3D-printed aligners from four brands were exposed to common beverages to assess color change using a VITA Easyshade compact colorimeter after 24 h, 48 h, 72 h, and 7 days, as well as chemical stability using ATR-FTIR spectroscopy. The brand, beverage, and manufacturing method significantly influence color stability. ATR-FTIR analysis revealed compositional differences, with variations in response to beverage exposure affecting the integrity of polymer bonds. Color change analysis showed coffee as the most potent staining agent, particularly affecting Tera Harz TC85 aligners, while ClearCorrect aligners exhibited the least susceptibility. 3D-printed aligners showed a greater color change compared to thermoformed ones. Aligners with a PETG outer layer are more resistant to stains and chemical alterations than those made of polyurethane. Additionally, 3D-printed polyurethane aligners stain more than thermoformed ones. Therefore, PETG-layered aligners are a more reliable choice for maintaining the aesthetic integrity of aligners.

Digital light 4D printing of bioresorbable shape memory elastomers for personalized biomedical implantation

Four-dimensional (4D) printing unlocks new potentials for personalized biomedical implantation, but still with hurdles of lacking suitable materials. Herein, we demonstrate a bioresorbable shape memory elastomer (SME) with high elasticity at both below and above its phase transition temperature (Ttrans). This SME can be digital light 3D printed by co-polymerizing glycerol dodecanoate acrylate prepolymer (pre-PGDA) with acrylic acid monomer to form crosslinked Poly(glycerol dodecanoate acrylate) (PGDA)-Polyacrylic acid (PAA), or PGDA-PAA network. The printed complex, free-standing 3D structures with high-resolution features exhibit shape programming properties at a physiological temperature. By tuning the pre-PGDA weight ratios between 55 wt% and 70 wt%, Ttrans varies between 39.2 and 47.2 ℃ while Young's moduli (E) range 40-170 MPa below Ttrans with fractural strain (εf) of 170 %-200 %. Above Ttrans, E drops to 1-1.82 MPa which is close to those of soft tissue. Strikingly, εf of 130-180 % is still maintained. In vitro biocompatibility test on the material shows > 90 % cell proliferation and great cell attachment. In vivo vascular grafting trials underline the geometrical and mechanical adaptability of these 4D printed constructs in regenerating the aorta tissue. Biodegradation of the implants shows the possibility of their full replacement by natural tissue over time. To highlight its potential for personalized medicine, a patient-specific left atrial appendage (LAA) occluder was printed and implanted endovascularly into an in vitro heart model. STATEMENT OF SIGNIFICANCE: 4D printed shape-memory elastomer (SME) implants particularly designed and manufactured for a patient are greatly sought-after in minimally invasive surgery (MIS). Traditional shape-memory polymers used in these implants often suffer from issues like unsuitable transition temperatures, poor biocompatibility, limited 3D design complexity, and low toughness, making them unsuitable for MIS. Our new SME, with an adjustable transition temperature and enhanced toughness, is both biocompatible and naturally degradable, particularly in cardiovascular contexts. This allows implants, like biomedical scaffolds, to be programmed at room temperature and then adapt to the body's physiological conditions post-implantation. Our studies, including in vivo vascular grafts and in vitro device implantation, highlight the SME's effectiveness in aortic tissue regeneration and its promising applications in MIS.

Thermal and UV Curable Formulations of Poly(propylene glycol)-Poly(hydroxyurethane) Elastomers toward Nozzle-Based 3D Photoprinting

In this work, isocyanate-free formulations for poly(propylene glycol) polyurethane elastomers are studied. These formulations are based on poly(propylene glycol) end-capped by CO2-sourced cyclic carbonate (bisCC PPG) macromonomers able to react with amines leading to poly(hydroxyurethane)s. In order to obtain covalent networks, two curing approaches are studied. First, the direct thermally activated cross-linking of bisCC PPG with a mixture of various aliphatic or aromatic diamines and a triamine is investigated, and in particular the nature of the diamine on the mechanical properties. In the second approach, UV-activated formulations are developed by reacting bisCC PPG with allylamine followed by the addition of a trithiol by photoactivated thiol-ene reaction. The swelling tests show that both systems provide highly cross-linked polymer networks and complementary characterizations highlighted excellent mechanical properties. Thanks to the fast curing and adapted viscosity of the developed photoactive formulation, the latter was found suitable for use as a photoresin for 3D printing as demonstrated by printing a vaginal ring by a nozzle-based photoprinter.

3D printing of ultra-high viscosity resin by a linear scan-based vat photopolymerization system

The current printing mechanism of the bottom-up vat photopolymerization 3D printing technique places a high demand on the fluidity of the UV-curable resin. Viscous high-performance acrylate oligomers are compounded with reactive diluents accordingly to prepare 3D printable UV-curable resins (up to 5000 cps of viscosity), yet original mechanical properties of the oligomers are sacrificed. In this work, an elaborated designed linear scan-based vat photopolymerization system is developed, allowing the adoption of printable UV-curable resins with high viscosity (> 600,000 cps). Briefly, this is realized by the employment of four rollers to create an isolated printing area on the resin tank, which enables the simultaneous curing of the resin and the detachment of cured part from the resin tank. To verify the applicability of this strategy, oligomer dominated UV-curable resin with great mechanical properties, but high viscosity is prepared and applied to the developed system. It is inspiring to find that high stress and strain elastomers and toughened materials could be facilely obtained. This developed vat photopolymerization system is expected to unblock the bottleneck of 3D printed material properties, and to build a better platform for researchers to prepare various materials with diversiform properties developed with 3D printing.

High Tg, Bio-Based Isosorbide Methacrylate Resin Systems for Vat Photopolymerization

The use of isosorbide-derived polymers has garnered significant attention in recent decades as a high-performance, renewable material sourced from biomass. Of particular interest is isosorbide methacrylate, which possesses low viscosity (<500 cps), high thermal properties (T_g ≈ 220 °C), and high modulus (>4 GPa). These characteristics present a promising opportunity to replace BPA-derived methacrylate compounds in various applications. This investigation aims to synthesize and characterize isosorbide-based low-viscosity resin systems for 3D printing. The resin blends are composed of isosorbide methacrylate and two bio-renewable methacrylates, furfuryl methacrylate (FM) and bis-hydroxymethyl-furan methacrylate (BHMF-M), polymerized through a digital light processing (DLP) technique. The addition of the bio-based co-monomers serves to enhance the fracture toughness of the brittle isosorbide methacrylate crosslinked homopolymer (G_Ic = 37 J/m²). The resulting polymers exhibit T_g values greater than 200 °C and G_Ic around 100 J/m². These resin systems hold potential for imparting high bio-based content to polymers used in additive manufacturing for high-performance applications.

Photocurable and elastic polyurethane based on polyether glycol with adjustable hardness for 3D printing customized flatfoot orthosis

Orthopedic insoles is the most commonly used nonsurgical treatment method for the flatfoot. Polyurethane (PU) plays a crucial role in the manufacturing of orthopedic insoles due to its high wear resistance and elastic recovery. However, preparing orthopedic insoles with adjustable hardness, high-accuracy, and matches the plantar morphology is challenging. Herein, a liquid crystal display (LCD) three-dimensional (3D) printer was used to prepare the customized arch-support insoles based on photo-curable and elastic polyurethane acrylate (PUA) composite resins. Two kinds of photo-curable polyurethanes (DL1000-PUA and DL2000-PUA) were successfully synthesized, and a series of fast-photocuring polyurethane acrylate (PUA) composite resins for photo-polymerization 3D printing were developed. The effects of different acrylate monomers on the Shore hardness, viscosity, and mechanical properties of the PUA composite resins were evaluated. The PUA-3-1 composite resin exhibited low viscosity, optimal hardness, and mechanical properties. A deviation analysis was conducted to assess the accuracy of printed insole. Furthermore, the stress conditions of the PUA composite resin and ethylene vinyl acetate (EVA) under the weight load of healthy adults were compared by finite element analysis (FEA) simulation. The results demonstrated that the stress of the PUA composite resin and EVA were 0.152 MPa and 0.285 MPa, and displacement were 0.051 mm and 3.449 mm, respectively. These results indicate that 3D-printed arch-support insole based on photocurable PUA composite resin are high-accuracy, and can reduce plantar pressure and prevent insoles premature deformation, which show great potential in the physiotherapeutic intervention for foot disorders.

Digital light 3D printing of a polymer composite featuring robustness, self-healing, recyclability and tailorable mechanical properties

Producing lightweight structures with high weight-specific strength and stiffness, self-healing abilities, and recyclability, is highly attractive for engineering applications such as aerospace, biomedical devices, and smart robots. Most self-healing polymer systems used to date for mechanical components lack 3D printability and satisfactory load-bearing capacity. Here, we report a new self-healable polymer composite for Digital Light Processing 3D Printing, by combining two monomers with distinct mechanical characteristics. It shows a desirable and superior combination of properties among 3D printable self-healing polymers, with tensile strength and elastic modulus up to 49 MPa and 810 MPa, respectively. Benefiting from dual dynamic bonds between the linear chains, a healing efficiency of above 80% is achieved after heating at a mild temperature of 60 °C without additional solvents. Printed objects are also endowed with multi-materials assembly and recycling capabilities, allowing robotic components to be easily reassembled or recycled after failure. Mechanical properties and deformation behaviour of printed composites and lattices can be tuned significantly to suit various practical applications by altering formulation. Lattice structures with three different architectures were printed and tested in compression: honeycomb, re-entrant, and chiral. They can regain their structural integrity and stiffness after damage, which is of great value for robotic applications. This study extends the performance space of composites, providing a pathway to design printable architected materials with simultaneous mechanical robustness/healability, efficient recoverability, and recyclability.

Sustainable 3D Printing of Recyclable Biocomposite Empowered by Flash Graphene

Sustainability of 3D printing can be reflected in three main aspects: deployment of renewable inks, recycling of printed products, and applications for energy- and materials- savings. In this work, we demonstrated sustainable vat-photopolymerization (VPP)-based 3D printing in a whole life-cycle process by developing a renewable ink made of soybean oil and natural polyphenols and recycling the ink for reprinting or converting printed biocomposite to flash graphene (FG) as reinforcing nanofillers in the biocomposite. We also realized its applications in fabricating lightweight, materials-saving 3D structures, acoustic metamaterials, and disposable microreactors for time-saving and efficiency-improving synthesis of metal-organic framework nanostructures. In addition to enhancing the tensile strength and Young's modulus of the biopolymers by 42% and 232% with only 0.6 wt % FG nanofillers, respectively, FG improved the printability of the ink in forming 3D tubular structures, which are usually very hard to be achieved in transparent resin. Success of this work will inspire further development for sustainability in 3D printing.

Renewable Beta-Elemene Based Cyclic Carbonates for the Preparation of Oligo(hydroxyurethane)s

Conversion of β-elemene into new β-elemene dicarbonates through epoxidation and halide salt-catalyzed CO2 cycloaddition reactions is reported. Step-growth polyaddition of this dicarbonate to five different, commercial diamines was investigated under neat conditions at 150 °C yielding non-isocyanate-based low molecular weight oligo(hydroxyurethane)s with 1.3≤Mn ≤6.3 kDa and 1.3≤Ð≤2.1, and with glass transition temperatures ranging from -59 to 84 °C. The preparation of one selected polyhydroxyurethane material, obtained in the presence of Jeffamine® D-2010 was scaled-up to 43 g. The latter, when combined in a formulation using Irgacure® 2100 and Laromer® LR 9000 allowed the preparation of coatings that were analyzed with several techniques showing the potential of these biobased oligourethanes towards the preparation of commercially relevant materials.

Photocuring Three-Dimensional Printing of Thermoplastic Polymers Enabled by Hydrogen Bonds

The advent of 3D printing has led to a new era of highly customized products. Printing reprocessable thermoplastic polymers is limited to slow printing techniques such as fused deposition modeling. Photocuring 3D printing is a high-speed 3D printing technique suitable for photocurable thermosetting resins because the cross-linked 3D network could achieve rapid solid-liquid separation during printing. However, thermoplastics usually cannot be printed via photocuring 3D printers because rapid solid-liquid separation is hard to be achieved due to the diffusion/dissolution of linear molecular chains in their liquid precursor. Herein, we hypothesize that hydrogen bonds (H-bonds) between monomers may accelerate polymerization and reduce solubility of the polymer in liquid precursors to achieve rapid solid-liquid separation. Using this strategy, a series of UV-curable methacrylic and acrylic monomers was selected as inks to demonstrate the role of H-bonds in photocuring 3D printing. The hypothesis was further verified by using blended inks of N-vinyl-2-pyrrolidinone (NVP) and acrylic acid (AA) via experimental and molecular dynamic simulation. Oil palm occupies the top position of plantation species in southeastern Asian forests. Palm oil (PO) has the lowest price compared with other plant oils. Thus, a PO-based vinyl monomer was selected as the raw material for 3D printing thermoplastic polymers. Various biobased thermoplastics were successfully printed from the PO-based monomer and commercial monomers. The amide structure in the PO monomer formed H-bonds with polar monomers, including NVP and AA, resulting in printed 3D objects with surprising functionalities such as high stretchability and self-healing ability.

Novel Dual-Curing Process for a Stereolithographically Printed Part Triggers a Remarkably Improved Interlayer Adhesion and Excellent Mechanical Properties

Stereolithography (SL) is widely used because of its numerous advantages over other three-dimensional printing (3DP) techniques. However, SL is a layer-by-layer process, where interlayer adhesion between adjacent layers becomes more brittle than the intralayer adhesion, common to all 3DP process. Here, we report a facile method to strengthen the interlayer adhesion for SL. By the addition of monomers with thermally curable functional groups (epoxy or hydroxyl groups), thermal post-curing induces chemical reactions between them in adjacent layers after the photoinitiated printing process. It leads to fully 3D cured objects with enhanced interlayer bonding and substantially improved mechanical properties, but not a significant change in the dimensional stability. This approach can expand the use of 3DP techniques in load-bearing applications that require mechanically robust printed objects with precise dimensions.

A New Kind of Nonconventional Luminogen Based on Aliphatic Polyhydroxyurethane and Its Potential Application in Ink-Free Anticounterfeiting Printing

Organic luminogens have extensive applications due to their unique photophysical properties. In recent years, nonconjugated organic luminogens, in contrast to traditional conjugated luminogens, have gained much attention because of their facile preparation, environmental friendliness, and biocompatibility. In this study, a new kind of nonconventional luminogen based on dynamic covalent cross-linked polyhydroxyurethane is reported for the first time. The new luminogen not only exhibits intrinsic strong fluorescent emission in the solid state but also possesses high mechanical properties along with good shape memory and self-healing properties. In addition, the new luminogens are synthesized from aliphatic polyfunctional cyclic carbonate and amines via a much more straightforward method, avoiding the use of toxic isocyanates. Investigations indicated that the intrinsic luminescence of the resultant luminogens was induced by the cross-linking of polymer chains and could be well tuned by controlling the degree of cross-linking. By taking advantage of the unique characteristics of the resultant polymer luminogens, we further developed a facile method, named "light-mediated ink-free screen printing", for anticounterfeiting paper fabrication. Different from traditional ink-based printing technology, the new method used UV-light instead of expensive security ink to encode anticounterfeiting information on natural cellulose paper. The anticounterfeiting information is stable under various wet conditions, showing promising applications in the fast-growing counterfeiting of pharmaceuticals, packaging, and the food industry.

Fast curing of polyhydroxyurethanes via ring opening polyaddition of low viscosity cyclic carbonates and amines

We investigate the curing of low viscosity di-/tricyclic carbonates and amines for adjustable polyhydroxyurethanes and their application in a double chamber syringe.

3D Printing of a self-healing, high strength, and reprocessable thermoset

A furan-maleimide based 3D printing ink for the fabrication of a self-healing and high strength thermoset with recycling potential.