Soft robotics is a growing field of research, focusing on constructing motor-less robots from highly compliant materials, some are similar to those found in living organisms. Soft robotics has a high potential for applications in various fields such as soft grippers, actuators, and biomedical devices. 3D printing of soft robotics presents a novel and promising approach to form objects with complex structures, directly from a digital design. Here, recent developments in the field of materials for 3D printing of soft robotics are summarized, including high-performance flexible and stretchable materials, hydrogels, self-healing materials, and shape memory polymers, as well as fabrication of all-printed robots (multi-material printing, embedded electronics, untethered and autonomous robotics). The current challenges in the fabrication of 3D printed soft robotics, including the materials available and printing abilities, are presented and the recent activities addressing these challenges are also surveyed.

New ink compositions for direct ink writing (DIW) printing of hydrogels, combining superior rheological properties of cellulose nanocrystals (CNCs) and a water-compatible photoinitiator, are presented. Rapid fixation was achieved by photopolymerization induced immediately after the printing of each layer by 365 nm light for 5 s, which overcame the common height limitation in DIW printing of hydrogels, and enabled the fabrication of objects with a high aspect ratio. CNCs imparted a unique rheological behavior, which was expressed by orders of magnitude difference in viscosity between low and high shear rates and in rapid high shear recovery, without compromising ink printability. Compared to the literature, the presented printing compositions enable the use of low photoinitiator concentrations at a very short build time, 6.25 s/mm, and are also curable by 405 nm light, which is favorable for maintaining viability in bioinks.

Hydrogel-polymer hybrids have been widely used for various applications such as biomedical devices and flexible electronics. However, the current technologies constrain the geometries of hydrogel-polymer hybrid to laminates consisting of hydrogel with silicone rubbers. This greatly limits functionality and performance of hydrogel-polymer–based devices and machines. Here, we report a simple yet versatile multimaterial 3D printing approach to fabricate complex hybrid 3D structures consisting of highly stretchable and high–water content acrylamide-PEGDA (AP) hydrogels covalently bonded with diverse UV curable polymers. The hybrid structures are printed on a self-built DLP-based multimaterial 3D printer. We realize covalent bonding between AP hydrogel and other polymers through incomplete polymerization of AP hydrogel initiated by the water-soluble photoinitiator TPO nanoparticles. We demonstrate a few applications taking advantage of this approach. The proposed approach paves a new way to realize multifunctional soft devices and machines by bonding hydrogel with other polymers in 3D forms.

Self-healing hydrogels may mimic the behavior of living tissues, which can autonomously repair minor damages, and therefore have a high potential for application in biomedicine. So far, such hydrogels have been processed only via extrusion-based additive manufacturing technology, limited in freedom of design and resolution. Herein, we present 3D-printed hydrogel with self-healing ability, fabricated using only commercially available materials and a commercial Digital Light Processing printer. These hydrogels are based on a semi-interpenetrated polymeric network, enabling self-repair of the printed objects. The autonomous restoration occurs rapidly, at room temperature, and without any external trigger. After rejoining, the samples can withstand deformation and recovered 72% of their initial strength after 12 hours. The proposed approach enables 3D printing of self-healing hydrogels objects with complex architecture, paving the way for future applications in diverse fields, ranging from soft robotics to energy storage.

Aerogels, the lightest solid material known, are low-density nanoporous solids that have found a wide range of applications such as thermal insulation, scaffolds for tissue engineering, catalysts supports, and micrometeorite collectors. Many types of materials have been used for their preparation, and ceramic/oxide aerogels are by far the most studied and applied family. Here we propose a new comprehensive solution to prepare these materials photochemically and fabricating them in highly complex shapes at all scales, from the macro scale down to the microns scale. The solution to these two challenges is linked, shown in the three photochemical approaches developed, allow unprecedented complexity in shape. The processes are mold irradiation, digital light processing (DLP) 3D printing, and a two-photon printing (TPP) process. The obtained 3D complex silicate objects display low density, high porosity, large surface area, and low thermal conductivity. The fabrication process also enables easy functionalization of the aerogels as inducing in them luminescence or making the printed object superhydrophobic by post printing process. The photochemical approach is ideal for the preparation of components of miniature devices, where low weight is a governing requirement.

Plasmonic thermochromic films are promising for smart window applications. Hereby, we develop a flexible plasmonic thermochromic film towards multifunctionality. The double-layer film consists of a bottom layer of W/Mg co-doped vanadium dioxide (VO2) rods in a polyurethane acrylate matrix and a top layer of hollow silica spheres (HSSs). Based on the finite-difference time-domain (FDTD) method, we demonstrate for the first time, a transverse and a longitudinal mode of VO2 localized surface plasmonic resonance (LSPR) in near- and mid-infrared bands, respectively, and only the transverse mode contributes to the solar energy modulation performance. The film shows a luminous transmittance of 46.2%, a solar energy modulation of 10.8%, and a critical transition temperature of 36.9 °C. The HSSs overcoating enhances the surface hydrophilicity and thermal insulation, which give rise to more favored functionalities for windows.

Hole-Transporting Layers In article number 2103807, Lydia Helena Wong and co-workers use Al-doped CuS as a hole transport layer (HTL) for perovskite solar cells. Here, it has been demonstrated that, due to the interaction between sulfur and lead, better perovskite crystallization takes place at the interface. Because of this improved interface quality, the sulfide-HTL based devices outperform the oxide HTL-based devices in terms of ambient stability.

Semitransparency is an attractive and important property in solar cells since it opens new possibilities in a variety of applications such as tandem cell configuration and building-integrated photovoltaics. Metal halide perovskite has the optimal properties to function as the light harvester in solar cells and can be made as a thin film, while its chemical composition can change its band gap. However, achieving high transparency usually compromises the solar cell's efficiency. Here we report on a unique approach to fabricating semitransparent perovskite solar cells that does not rely on their composition or their thickness. The approach is based on a scalable process, inkjet printing of arrays of transparent pillars, which are composed of inert photopolymerizable liquid compositions and are partly covered by the perovskite. This material can be printed at specific locations and array densities, thus providing a digital control of both the transparency and efficiency of the solar cells. The new semitransparent device structure shows 11.2% efficiency with 24% average transparency without a top metal contact. Further development including deposition of a transparent contact enabled the fabrication of fully semitransparent devices with an efficiency of 10.6% and average transparency of 19%.

Three-dimensional printing (3DP) is considered among the key-technologies for the next industrial revolution, with considerable effects on production processes, economy, and society. In this context, the most relevant part of the market consists of polymeric 3D printing. The 3D printable liquids are composed of various components, among them dyes are usually underrated because they are introduced merely for aesthetical reasons or to enhance the objects' resolution. In recent years, the capability of specific dyes to go beyond conventional use and to confer functional properties to 3D printed objects has become an emerging research area. Modifying elastic moduli upon light irradiation, inducing optical and emitting properties in the matrices or conferring temperature responsivity are just few examples of innovative stimuli-responsive materials that can be produced by combining well-designed dyes with the appropriate 3DP printed matrices. In this Review, we discuss and critically analyze the most relevant recent results achieved in the use of smart dyes in the synthesis of stimuli responsive 3D printed polymers.

Vanadium dioxide (VO2) is a unique active plasmonic material due to its intrinsic metal-insulator transition, remaining less explored. Herein, we pioneer a method to tailor the VO2 surface plasmon by manipulating its atomic defects and establish a universal quantitative understanding based on seven representative defective VO2 systems. Record high tunability is achieved for the localized surface plasmon resonance (LSPR) energy (0.66-1.16 eV) and transition temperature range (40-100 °C). The Drude model and density functional theory reveal that the charge of cations plays a dominant role in the numbers of valence electrons to determine the free electron concentration. We further demonstrate their superior performances in extensive unconventional plasmonic applications including energy-saving smart windows, wearable camouflage devices, and encryption inks. This journal is

Significant advancements in the perovskite solar cells/modules (PSCs/PSMs) toward better operational stability and large area scalability have recently been reported. However, semitransparent (ST), high efficiency, and large area PSMs are still not well explored and require attention to realize their application in building-integrated photovoltaics (BIPV). This work employs multiple synergistic strategies to improve the quality and stability of the ST perovskite film while ensuring high transparency. Europium ions, doped in the perovskite, are found to suppress the generation of detrimental species like elemental Pb and I, resulting in higher atmospheric stability. The effect of the top transparent contact is designed to obtain an average visible transparency (AVT) of >20% for full device and a green colored hue. Lastly, the lower current density due to the thinner ST absorber is enhanced by the application of a down-converting phosphor material which harvests low energy photons and inhibits UV-induced degradation. This multimodal approach renders a power conversion efficiency of 12% under dim light conditions and 9.5% under 1 sun illumination, respectively, on 21 cm2 ST-PSM.

The fabrication of Nd-Nb co-doped SnO2/α-WO3 electrochromic (EC) materials for smart window applications is presented in the present paper. Nb is a good dopant candidate for ECs owing to its ability to introduce active sites on the surface of α-WO3 without causing much lattice strain due to the similar ionic radius of Nb5+ and W6+. These active sites introduce more channels for charge insertion or removal during redox reactions, improving the overall EC performance. However, Nb suffers from prolonged utilization due to the Li+ ions trapped within the ECs. By coupling Nd with Nb, the co-dopants would transfer their excess electrons to SnO2, improving the electronic conductivity and easing the insertion and extraction of Li+ cations from the ECs. The enhanced Nd-Nb co-doped SnO2/α-WO3 exhibited excellent visible light transmission (90% transmittance), high near-infrared (NIR) contrast (60% NIR modulation), rapid switching time (∼1 s), and excellent stability (>65% of NIR modulation was retained after repeated electrochemical cycles). The mechanism of enhanced EC performance was also investigated. The novel combination of Nd-Nb co-doped SnO2/α-WO3 presented in this work demonstrates an excellent candidate material for smart window applications to be used in green buildings.

In an urbanized city, about a third of total electrical consumption is allocated for indoor lighting and air conditioning system in residential and commercial buildings. The majority of the worldwide energy generation comes from burning of non-renewable fossil fuel which is not sustainable in the long run. The use of smart windows technology may catalyze the effort to reduce energy consumption of building and houses. More than 50% of heat entering a building through windows originate from the solar radiation in the near infrared (NIR) region. This candidate smart window material must exhibit dual-band (visible and NIR) modulation that allows selective modulation of NIR heat without affecting visible light transmission. A good electrochromic material in this respect should possess high visible light transmission, high NIR modulation, fast switching between colored and bleached state, and good stability over prolonged usage. In this work, we propose a novel Nd–Mo co-doped SnO2/α-WO3 electrochromic materials (ECs). As compared to the traditional SnO2/α-WO3 ECs, our Nd–Mo co-doped SnO2/α-WO3 ECs exhibits up to 90% visible light transparency (at λ = 600 nm), 62% NIR modulation (at wavelength 1200 nm), high coloration efficiency ($\sim$200 cm2 C−1), fast switching time with only 31% electrochromic performance drop (vs 59% of undoped sample) after up to 1000 reversible cyclic test. The enhanced electrochromic performance comes from the presence of Nd–Mo co-dopants that limit the trapping of Li + ion within α-WO3 framework, reduce the extent of crystallization of α-WO3 layer and enhancement of the electronic conductivity by transferring their excess electron to the conduction band of the SnO2. To the best of the authors' knowledge, the present composition of ECs offers one of the better candidate materials for electrochromic to be used as thermal management layers on smart windows application.

UV-curable particle-free ceramic compositions for stereolithography-based 3D printing technologies present a promising alternative to the commonly used particle-based compositions. So far, such compositions were mainly based on solutions of pre-ceramic polymers which limit their applications to silicon-containing materials. However, the application of particle-free inks for the fabrication of other ceramic materials, in particular dense polycrystalline ones, is very little explored. We present a new and general fabrication approach based on all-solution compositions, by combining sol–gel chemistry and photopolymerization, for obtaining dense 3D ceramic structures by DLP printing. The process is demonstrated here for the fabrication of barium titanate (BaTiO3). By using chelating solvent and monomer, a stable UV-curable solution is obtained. An aging period of 8–14 days was crucial for obtaining dense ceramic objects without any secondary phases. The heat treatment was found to affect the microstructure, density and hardness of the resulting ceramics. The presented process enables obtaining objects free of carbon materials, having a density as high as 98% of the theoretical value, and a hardness of 4.3 GPa.

Semiconductor nanocrystals are promising photocatalysts for a wide range of applications, ranging from alternative fuel generation to biomedical and environmental applications. This stems from their diverse properties, including flexible spectral tunability, stability, and photocatalytic efficiencies. Their functionality depends on the complex influence of multiple parameters, including their composition, dimensions, architecture, surface coating, and environmental conditions. A particularly promising direction for rapid adoption of these nanoparticles as photocatalysts is their ability to act as photoinitiators (PIs) for radical polymerization. Previous studies served to demonstrate the proof of concept for the use of quantum confined semiconductor nanocrystals as photoinitiators, coining the term Quantum PIs, and provided insights for their photocatalytic mechanism of action. However, these early reports suffered from low efficiencies while requiring purging with inert gases, use of additives, and irradiation by high light intensities with very long excitation durations, which limited their potential for real-life applications. The progress in nanocrystal syntheses and surface engineering has opened the way to the introduction of the next generation of Quantum PIs. Herein, we introduce the research area of nanocrystal photocatalysts, review their studies as Quantum PIs for radical polymerization, from suspension polymerization to novel printing, as well as in a new family of polymerization techniques, of reversible deactivation radical polymerization, and provide a forward-looking view for the challenges and prospects of this field.

The extreme miniaturization in NEMS resonators offers the possibility to reach an unprecedented resolution in high-performance mass sensing. These very low limits of detection are related to the combination of two factors: a small resonator mass and a high quality factor. The main drawback of NEMS is represented by the highly complex, multi-steps, and expensive fabrication processes. Several alternatives fabrication processes have been exploited, but they are still limited to MEMS range and very low-quality factor. Here we report the fabrication of rigid NEMS resonators with high-quality factors by a 3D printing approach. After a thermal step, we reach complex geometry printed devices composed of ceramic structures with high Young's modulus and low damping showing performances in line with silicon-based NEMS resonators ones. We demonstrate the possibility of rapid fabrication of NEMS devices that present an effective alternative to semiconducting resonators as highly sensitive mass and force sensors.

Semitransparent solar cells are able to capitalize on land scarcity in urban environments by co-opting windows and glass structures as power generators, thereby expanding the capacity of photovoltaics to meet energy needs. To be successful, devices must be efficient, possess good visual transparency, long-term stability, and low cost. Copper zinc tin sulfide is a promising thin-film material that consists of earth-abundant elements. For optical transparency, the usual molybdenum back contact is replaced with a transparent conducting oxide (TCO). However, due to subsequent high-temperature annealing, the TCO degrades, losing conductivity, or forms a poor interface with CZTS. Lower temperatures mitigate this issue but hinder grain growth in CZTS films. Herein, cadmium substitution and silver and sodium doping are used to aid grain growth and improve film quality at lower annealing temperatures. Thin molybdenum is sputtered on TCO to help improve the interface transition postannealing by conversion to MoS2. Rapid thermal processing is used to minimize high-temperature exposure time to preserve the TCO. With these methods, a semitransparent device with a front illumination efficiency of 2.96% is demonstrated.