In this review, we provide a brief history, progress, and applications, and discuss the remaining challenges of photocontrolled reversible addition-fragmentation chain transfer (RAFT) polymerization (i.e., photoinduced electron/energy transfer-RAFT (PET-RAFT), photoiniferter, and photomediated cationic RAFT polymerization). Among these, visible-light-driven RAFT polymerization has attracted particular attention in recent years due to its benefits, including low energy consumption and the safe reaction procedure. Moreover, the incorporation of visible-light photocatalysis in the polymerization has conferred attractive features, such as spatiotemporal control and oxygen tolerance; however, a clear understanding of the reaction mechanism has not been completely provided. We also present recent research efforts to elucidate the polymerization mechanisms with the aid of quantum chemical calculations combined with experimental evidence. This review offers an insight into the better design of polymerization systems for desired applications and helps realize the full potential of photocontrolled RAFT polymerization in both academic- and industrial-scale applications.

/pdf/photocontrolled-raft-polymerization-past-present-and-future-2nmjzzuv.pdf

Photocontrolled RAFT polymerization: past, present, and future.

Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given.

Tandem cells for unbiased photoelectrochemical water splitting.

Porous Organic Polymers with Defined Morphologies: Synthesis, Assembly, and Emerging Applications

The majority of materials 3D printed using vat photopolymerization techniques are prepared by uncontrolled polymerization methods and cannot be easily modified to introduce additional functionality; these materials can be considered as effectively “dead” materials. Fortunately, a suite of photocontrolled reversible–deactivation radical polymerization (photoRDRP) techniques is recently implemented in 3D printing. In addition to their fast polymerization rate and oxygen tolerance, the high livingness imparted by photoRDRP methods is beginning to disrupt the field of 3D printing by providing access to materials with advanced properties, including on‐demand editing of surface and bulk properties, self‐healing, and control over nanostructuration and mechanical properties. This mini‐review analyzes the development of photoRDRP techniques in the field of photoinduced 3D printing with an emphasis on the advanced and highly tailorable materials possible through these techniques.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/admt.202201054

The Emergence of Reversible–Deactivation Radical Polymerization in 3D Printing

Abstract 4D printing combines 3D printing with nanomaterials to create shape-morphing materials that exhibit stimuli-responsive functionalities. In this study, reversible addition-fragmentation chain transfer polymerization agents grafted onto liquid metal nanoparticles are successfully employed in ultraviolet light-mediated stereolithographic 3D printing and near-infrared light-responsive 4D printing. Spherical liquid metal nanoparticles are directly prepared in 3D-printed resins via a one-pot approach, providing a simple and efficient strategy for fabricating liquid metal-polymer composites. Unlike rigid nanoparticles, the soft and liquid nature of nanoparticles reduces glass transition temperature, tensile stress, and modulus of 3D-printed materials. This approach enables the photothermal-induced 4D printing of composites, as demonstrated by the programmed shape memory of 3D-printed composites rapidly recovering to their original shape in 60 s under light irradiation. This work provides a perspective on the use of liquid metal-polymer composites in 4D printing, showcasing their potential for application in the field of soft robots. 

3D-printed liquid metal polymer composites as NIR-responsive 4D printing soft robot

Although 3D printing allows the macroscopic structure of objects to be easily controlled, controlling the nanostructure of 3D printed materials has rarely been reported. Herein, we report an efficient and versatile process for fabricating 3D printed materials with controlled nanoscale structural features. This approach uses resins containing macromolecular chain transfer agents (macroCTAs) which microphase separate during the photoinduced 3D printing process to form nanostructured materials. By varying the chain length of the macroCTA, we demonstrate a high level of control over the microphase separation behavior, resulting in materials with controllable nanoscale sizes and morphologies. Importantly, the bulk mechanical properties of 3D printed objects are correlated with their morphologies; transitioning from discrete globular to interpenetrating domains results in a marked improvement in mechanical performance, which is ascribed to the increased interfacial interaction between soft and hard domains. Overall, the findings of this work enable the simplified production of materials with tightly controllable nanostructures for broad potential applications. 

/pdf/nano-to-macro-scale-control-of-3d-printed-materials-via-2n1wm8wa.pdf

Nano- to macro-scale control of 3D printed materials via polymerization induced microphase separation

Tungsten trioxide (WO3) is one of the promising semiconductors suitable for photoelectrochemical water oxidation, but its hydroxyl radical (•OH)-induced intrinsic performance degradation remains unclarified. Here, we demonstrate that quenching-treated WO3 with preferred {021} facets shows a highly improved Faradaic efficiency (from 57 to 95%) and its performance stability is more than 36 h relative to that of nontreated WO3 with less than 1-h stability. Using electron paramagnetic resonance (EPR), we find that the •OH could be highly suppressed on the treated WO3 photoanode, while abundant •OH is generated on the nontreated WO3. In situ ultraviolet–visible (UV–Vis) spectroscopy is used to track the presence of surface W–O–O–W intermediates on the treated WO3, suggesting the favorable formation of O–O and thus better oxygen evolution Faradaic efficiency, while the nontreated WO3 favors the formation of •OH, which accumulates on the WO3 surface, thus changing the photoanode/electrolyte interfacial properties and poisoning the oxygen evolution process. This work provides an intrinsic understanding of the degradation of the WO3 photoanode under acidic and neutral conditions. 

Modulating WO3 Crystal Orientation to Suppress Hydroxyl Radicals for Sustainable Solar Water Oxidation

Eutectic gallium indium (EGaIn) is a liquid metal with promising applications due to its favorable thermal and electrical conductivity, low viscosity, and metallic nature. For applications, including imaging, catalysis, and nanomedicine, stable EGaIn particles with submicron diameters are required. However, the low viscosity and high density of EGaIn have typically precluded the formation of stable submicron particles due to rapid EGaIn droplet coalescence. In this work, we show that poly(acrylic acid)-block-poly(N,N'-dimethylacrylamide) copolymers are able to effectively stabilize EGaIn nanodroplets formed upon ultrasonication, where the poly(acrylic acid) block anchors the polymer to the EGaIn surface and the poly(N,N'-dimethylacrylamide) block provides colloidal stability to the particles in solution. Although the high density of EGaIn causes rapid particle settling, the behavior is predictable, which allows the average particle size to be controlled through centrifugation. We demonstrate that stable EGaIn particles with sizes on the order of 50-100 nm and narrow particle size distributions can be easily obtained using this method and further used in photopolymer resins to prepare 3D printed EGaIn-polymer hybrid materials. The predictable sizes and high stability of these EGaIn nanoparticles should allow further applications in soft-electronics, nanomedicine, catalysis, and other nanotechnology.

Diblock Copolymer Stabilized Liquid Metal Nanoparticles: Particle Settling Behavior and Application to 3D Printing.

Ceria-Promoted and Stabilized Copper and Iron Oxides Cooperatively Catalyze No Efficient Degradation

Cartilage regeneration is dependent on cellular-extracellular matrix (ECM) interactions. Natural ECM plays a role in mechanical and chemical cell signaling and promotes stem cell recruitment, differentiation and tissue regeneration in the absence of biological additives, including growth factors and peptides. To date, traditional tissue engineering methods by using natural and synthetic materials have not been able to replicate the physiological structure (biochemical composition and biomechanical properties) of natural cartilage. Techniques facilitating the repair and/or regeneration of articular cartilage pose a significant challenge for orthopedic surgeons. Whereas, little progress has been made in this field. In recent years, with advances in medicine, biochemistry and materials science, to meet the regenerative requirements of the heterogeneous and layered structure of native articular cartilage (AC) tissue, a series of tissue engineering scaffolds based on ECM materials have been developed. These scaffolds mimic the versatility of the native ECM in function, composition and dynamic properties and some of which are designed to improve cartilage regeneration. This review systematically investigates the following: the characteristics of cartilage ECM, repair mechanisms, decellularization method, source of ECM, and various ECM-based cartilage repair methods. In addition, the future development of ECM-based biomaterials is hypothesized.

Preparation and Application of Decellularized ECM-Based Biological Scaffolds for Articular Cartilage Repair: A Review

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