Rational Design of Acyldiphenylphosphine Oxides as Photoinitiators of Radical Polymerization

Color-Stable Formulations for 3D-Photoprintable Dental Materials

Color stability is crucial for dental materials to ensure they perfectly match a patient's tooth color. This is particularly challenging in photoresist-based additive manufacturing. Although some studies have addressed this issue, the exact causes of discoloration and ways to minimize it remain unclear. In this study, the intrinsic causes of discoloration in materials intended for 3D printing are investigated by examining thin-film samples (1200 µm) of various compositions, which are stored under different conditions. The samples are evaluated by measuring the UV-Vis absorption spectra at regular intervals to monitor changes. The findings reveal that both the composition of the formulations and the storage conditions significantly influence the discoloration behavior. Furthermore, methods have been developed to reduce or completely prevent discoloration. The use of photoinitiators with sterically demanding benzoyl moieties, as well as the addition of stabilizers, effectively decreases the intensity of emerging discoloration. Furthermore, incorporating the oxidizing agent cumene hydroperoxide (CHP) results in materials that maintain color stability.

Novel High-Performance Glyoxylate Derivative-Based Photoinitiators for Free Radical Photopolymerization and 3D Printing with Visible LED

Investigations concerning the glyoxylate moiety as a photocleavable functional group for visible light photoinitiators, particularly in the initiation of free radical photopolymerization remain limited. This study introduces nine innovative carbazole-based ethyl glyoxylate derivatives (CEGs), which are synthesized and found to exhibit excellent photoinitiation abilities as monocomponent photoinitiating systems. Notably, these structures demonstrate robust absorption in the near-UV/visible range, surpassing the commercial photoinitiators. Moreover, the newly developed glyoxylate derivatives show higher acrylate function conversions compared to a benchmark photoinitiator (MBF) in free radical photopolymerization. Elucidation of the photoinitiation mechanism of CEGs is achieved through a comprehensive analysis involving the decarboxylation reaction and electron spin resonance spin trapping. Furthermore, their practical utility is confirmed during direct laser writing and 3D printing processes, enabling the successful fabrication of 3D printed objects. This study introduces pioneering concepts and effective strategies in the molecular design of novel photoinitiators, showcasing their potential for highly advantageous applications in 3D printing.

3D bioprinting of stromal cells-laden artificial cornea based on visible light-crosslinkable bioinks forming multilength networks

3D bioprinting has the potential for the rapid and precise engineering of hydrogel constructs that can mimic the structural and optical complexity of a healthy cornea. However, the use of existing light-activated bioinks for corneal printing is limited by their poor cytocompatibility, use of cytotoxic photoinitiators (PIs), low photo-crosslinking efficiency, and opaque/colored surface of the printed material. Herein, we report a fast-curable, non-cytotoxic, optically transparent bioprinting system using a new water-soluble benzoyl phosphinate-based PI and photocrosslinkable methacrylated hyaluronic acid (HAMA). Compared with commercially available PIs, the newly developed PI, lithium benzoyl (phenyl) phosphinate (BP), demonstrated increased photoinitiation efficiency under visible light and low cytotoxicity. Using a catalytic amount of BP, the HA-based bioinks quickly formed 3D hydrogel constructs under low-energy visible-light irradiation (405 nm, <1 J cm-2). The mechanical properties and printability of photocurable bioinks were further improved by blending low (10 kDa) and high (100 kDa) molecular weight (MW) HAMA by forming multilength networks. For potential applications as corneal scaffolds, stromal cell-laden dome-shaped constructs were fabricated using MW-blended HAMA/BP bioink and a digital light processing printer. The HA-based photocurable bioinks exhibited good cytocompatibility (80%-95%), fast curing kinetics (<5 s), and excellent optical transparency (>90% in the visible range), potentially making them suitable for corneal tissue engineering.

A Photoreactor-Interfaced Mass Spectrometer: An Online Platform to Monitor Photochemical Reactions

An experimental platform is reported that allows for the online characterization of photochemical reactions by coupling a continuous flow photoreactor, equipped with LED light irradiation and a dual-tipped ESI source, directly to a mass spectrometer with electrospray ionization. The capabilities of this platform are demonstrated with two classes of photoreactions: (1) the photopolymerization of methyl methacrylate and (2) photocatalyzed alkyne insertion into a 1,2,3-benzotriazinone. The online technique provides rapid information to inform the underlying photochemical mechanism and evaluate the overall photochemistry.

Development and Digital Light Processing 3D Printing of a Vitrimer Composed of Glycerol 1,3-Diglycerolate Diacrylate and Tetrahydrofurfuryl Methacrylate

The development of biobased reshapable and repairable vitrimers has received extensive attention due to the growing focus on an environmentally friendly society. Therefore, the objective of this research was to synthesize sustainable polymers with an environmentally friendly strategy combining the benefits of renewable resources, UV curing, and vitrimers. Two biobased monomers, glycerol 1,3-diglycerolate diacrylate and tetrahydrofurfuryl methacrylate, were chosen for the preparation of UV-curable resins and tested by real-time photorheometry and RT-FTIR spectroscopy to determine their suitability for digital light processing (DLP) 3D printing. DLP 3D-printed polymer showed shape memory, weldability, and repairability capabilities by triggering the dynamic transesterification process at high temperatures. The vitrimer with a weight ratio of 60:40 of glycerol 1,3-diglycerolate diacrylate and tetrahydrofurfuryl methacrylate showed shape memory properties with a recovery ratio of 100% and a 7-fold improved tensile strength compared to the original sample, confirming efficient weldability and repairability.

High-Performance Photoinitiating Systems for LED-Induced Photopolymerization

Currently, increasing attention has been focused on light-emitting diodes (LEDs)-induced photopolymerization. The common LEDs (e.g., LED at 365 nm and LED at 405 nm) possess narrow emission bands. Due to their light absorption properties, most commercial photoinitiators are sensitive to UV light and cannot be optimally activated under visible LED irradiation. Although many photoinitiators have been designed for LED-induced free radical polymerization and cationic polymerization, there is still the issue of the mating between photoinitiators and LEDs. Therefore, the development of novel photoinitiators, which could be applied under LED irradiation, is significant. Many photoinitiating systems have been reported in the past decade. In this review, some recently developed photoinitiators used in LED-induced photopolymerization, mainly in the past 5 years, are summarized and categorized as Type Ⅰ photoinitiators, Type Ⅱ photoinitiators, and dye-based photoinitiating systems. In addition, their light absorption properties and photoinitiation efficiencies are discussed.

Do germanium-based photoinitiators have the potential to replace the well-established acylphosphine oxides?

In the last few decades, there has been an increasing demand for photoinitiators with growing requirements. Nowadays, photoinitiators need to fulfill several requirements such as a low level of toxicity, biocompatibility, fast polymerization rates, high activities, good photobleaching and much more in order to remain competitive on the market. Accordingly, we compare acylphosphine oxides and acylgermanes, two common classes of photoinitiators, with respect to their various synthetic pathways, toxicity, availability and performance.

Novel Photoinitiators Based on Benzophenone-Triphenylamine Hybrid Structure for LED Photopolymerization

In this study, a new generation of photoinitiator (PI) based on hybrid structures combining benzophenone and triphenylamine is proposed. Remarkably, these photoinitiators (noted monofunctional benzophenone-triphenylamine (MBP-TPA) and trifunctional benzophenone-triphenylamine (TBP-TPA)) are designed and developed for the photopolymerization under light-emitting diodes (LEDs). Benzoyl substituents connected with triphenylamine moiety contribute to the excellent absorption properties which results in both high final conversions and polymerization rates in free radical photopolymerization (FRP). Remarkably, TBP-TPA owning trifunctional benzophenone group exhibits a better Type II PI behavior than well-known 2-isopropylthioxanthone for photopolymerization under LED@365 and 405 nm irradiation. FRP and cationic photopolymerization of TBP-TPA-based systems are applied on 3D printing experiments, and good profiles of the 3D patterns are observed. The high molecular weight of TBP-TPA associated with it trifunctional character can also be very interesting for a better migration stability of PIs that is a huge challenge. The development of this new generation of photoinitiators based on benzophenone hybrid structures is a real breakthrough. It reveals that the novel versatile photoinitiators based on benzophenone-triphenylamine hybrid structures have great potentials for future industrial applications (e.g., 3D printing, composites, etc.).

New Phosphine Oxides as High Performance Near- UV Type I Photoinitiators of Radical Polymerization

Carbazole structures are of high interest in photopolymerization due to their enhanced light absorption properties in the near-UV or even visible ranges. Therefore, type I photoinitiators combining the carbazole chromophore to the well-established phosphine-oxides were proposed and studied in this article. The aim of this article was to propose type I photoinitiators that can be more reactive than benchmark phosphine oxides, which are among the more reactive type I photoinitiators for a UV or near-UV light emitting diodes (LED) irradiation. Two molecules were synthesized and their UV-visible light absorption properties as well as the quantum yields of photolysis and photopolymerization performances were measured. Remarkably, the associated absorption was enhanced in the 350-410 nm range compared to benchmark phosphine oxides, and one compound was found to be more reactive in photopolymerization than the commercial photoinitiator TPO-L for an irradiation at 395 nm.

Evidence for ultrafast formation of tribenzoylgermyl radicals originating from tetraacylgermane photoinitiators

We report strong evidence for the ultrafast formation of tribenzoylgermyl radicals originating from tetraacylgermane photoinitiators on a ps-time scale. In comparison to previous studies, the result can be related to a high initiation efficiency.

Redefining the chemistry of super-macroporous materials: when dendritic molecules meet polymer cryogels

Dendritic cryogels modify the functionality and properties against conventional cryogels and improve the Immunoglobulin G (IgG) adsorption.