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We report a facile but general method to prepare highly water-soluble and biocompatible photoinitiators for digital light processing (DLP)-based 3D printing of high-resolution hydrogel structures. Through a simple and straightforward one-pot procedure, we can synthesize a metal-phenyl(2,4,6-trimethylbenzoyl)phosphinates (M-TMPP)-based photoinitiator with excellent water solubility (up to ∼50 g/L), which is much higher than that of previously reported water-soluble photoinitiators. The M-TMPP aqueous solutions show excellent biocompatibility, which meets the prerequisite for biomedical applications. Moreover, we used M-TMPP to prepare visible light (405 nm)-curable hydrogel precursor solutions for 3D printing hydrogel structures with a high water content (80 wt %), high resolution (∼7 μm), high deformability (more than 80% compression), and complex geometry. The printed hydrogel structures demonstrate great potential in flexible electronic sensors due to the fast mechanical response and high stability under cyclic loadings.

Stretchable strain sensor detects a wide range of strain variation and is therefore a key component in various applications. Unlike traditional ones made of elastomers doped with conductive components or fabricated with liquid conductors, ionically conductive hydrogel-based strain sensors remain conductive under large deformations and are biocompatible. However, dehydration is a challenging issue for the latter. Researchers have developed hydrogel-elastomer-based strain sensors where an elastomer matrix encapsulates a hydrogel circuit to prevent its dehydration. However, the reported multi-step approaches are generally time-consuming. Our group recently reported a multimaterial 3D printing approach that enables fast fabrication of such sensors, yet requires a self-built digital-light-processing-based multimaterial 3D printer. Here, we report a simple projection lithography method to fabricate hydrogel-elastomer-based stretchable strain sensors within 5 minutes. This method only requires a UV projector/lamp with photomasks; the chemicals are commercially available; the protocols for preparing the polymer precursors are friendly to users without chemistry background. Moreover, the manufacturing flexibility allows users to readily pattern the sensor circuit and attach the sensor to a 3D printed soft pneumatic actuator to enable strain sensing on the latter. The proposed approach paves a simple and versatile way to fabricate hydrogel-elastomer-based stretchable strain sensors and flexible electronic devices.

Vertically oriented graphene (VG) is deposited on a wide range of substrates by plasma-enhanced chemical vapor deposition. Moreover, the growth mode of VG is proven to be bottom-up deposition using isotope labeling. The Raman characteristic features of rhodamine 6G (R6G), an aromatic dye, are acquired using VG as a universal fluorescence quencher. The fluorescence quenching mechanism is likely related to the transfer of energy and photon-excited electrons between dye molecules and VG because not only the fluorescence emission spectrum of R6G greatly overlaps with the UV–vis absorption spectrum of VG, but also the resistance of dye-coated VG decreases under light irradiation. In addition, the Raman features of other aromatic dyes, such as eosin Y, rhodamine B, methylene blue, and gallocyanine, are obtained by the same method.

Improving thermal transport between substrate and transistors has become a vital solution to the thermal challenge in nanoelectronics. Recently 2D WTe2 has sparked extensive interest because of heavy atomic mass and low Debye temperature. Here, the thermal transport of supported WTe2 was studied via Raman thermometry with electrical heating. The supported 30 nm WTe2 encased with 70 nm Al2O3 delivered 4.8 W·m−1·K−1 in-plane thermal conductivity along zigzag direction at room temperature, which was almost 1.6 times larger than that along armchair direction (3.0 W·m−1·K−1). Interestingly, the superior and inferior directions for thermal transport are just opposite of those for electrical transport. Hence, a heat manipulation model in WTe2 FET device was proposed. Within the designed configuration, waste heat in WTe2 would be mostly dissipated to metal contacts located along zigzag, relieving the local temperature discrepancy in the channel effectively and preventing degradation or breakdown. Our study provides new insight into thermal transport of anisotropic 2D materials, which might inspire energy-efficient nanodevices in the future.

Thermally responsive shape memory polymers (SMPs) used in 4D printing are often reported to be activated by external heat sources or embedded stiff heaters. However, such heating strategies impede the practical application of 4D printing due to the lack of precise control over heating or the limited ability to accommodate the stretching during shape programming. Herein, we propose a novel 4D printing paradigm by fabricating stretchable heating circuits with fractal motifs via electric-field-driven microscale 3D printing of conductive paste for seamless integration into 3D printed structures with SMP components. By regulating the fractal order and printing/processing parameters, the overall electrical resistance and areal coverage of the circuits can be tuned to produce an efficient and uniform heating performance. Compared with serpentine structures, the resistance of fractal-based circuits remains relatively stable under both uniaxial and biaxial stretching. In practice, steady-state and transient heating modes can be respectively used during the shape programming and actuation phases. We demonstrate that this approach is suitable for 4D printed structures with shape programming by either uniaxial or biaxial stretching. Notably, the biaxial stretchability of fractal-based heating circuits enables the shape change between a planar structure and a 3D one with double curvature. The proposed strategy would offer more freedom in designing 4D printed structures and enable the manipulation of the latter in a controlled and selective manner.

In view of the axial force produced in the working process of double arc helical gear hydraulic pump, the theory of differential equation of curve and curved surface was utilized so that the calculation formula of axial force was obtained and the relationship between the axial force and structure parameters of gears was clarified. In order to balance the axial force, the pressure oil in the high pressure area was introduced into the end face of the plunger to press the plunger against the gear shaft, and the hydrostatic bearing whose type is plunger at the end of the shaft was designed. In order to verify the balance effect of axial force, the leakage owing to end clearance and volume efficiency of gear hydraulic pump before and after the balancing process was analyzed. This paper provides a new analysis idea and balance scheme for the axial force produced in the working process of the double arc helical gear hydraulic pump, which can reduce the leakage owing to end clearance caused by the axial force and improve the volume efficiency of the gear hydraulic pump.

Shape reconfiguration is ubiquitous in nature and widely used in many applications such as soft robotics, metamaterials, energy absorption and tissue engineering. Shape reconfigurable soft actuators, due to their ability to adapt and adjust in complex and unpredictable working environment, have been designed by the use of various delicate structures and active materials. However, soft actuators that exhibit reconfigurable helical deformation have not been proposed; they have the advantage of integrating both bending and twisting actuations in one deformation mode. In this work, we present a thermal-induced shape reconfigurable soft actuator that shows reversible actuations with vastly shape differences under thermal stimulus. It exhibits helical deformation at lower temperature and mainly in-plane bending at relatively higher temperature. The reversible shape transition is controlled by a thermal stimulus that changes the anisotropy of the structure, which consists of shape memory polymer fibers embedded in a homogeneous elastic matrix. A theoretical model is proposed based on the minimum potential energy that incorporates the thermomechanical behavior of the shape memory polymer fibers. Experiments are conducted and the results agree well with the theoretical modeling. Using the theoretical model, we establish design principles for reconfigurable soft actuators whose functional response is programmable given the architecture and external stimulus. A six-handed helical soft actuator, constructed to demonstrate its programmable deformation, is utilized to catch a living fish in water.

The design of a compressible battery with stable electrochemical performance is extremely important in compression-tolerant and flexible electronics. While this remains challenging with the current battery manufacturing method, the field of 3D printing offers the possibility of producing free-standing 3D-printed electrodes with various structural configurations. Through the simple and scalable strategy, various structural configurations can be produced. Herein, we demonstrate a 3D-printed quasi-solid-state Ni-Fe battery (QSS-NFB) that shows excellent compressibility, ultrahigh energy density, and superior long-term cycling durability. Through a rational design and adjustment of chemical components, two electrodes consisting of ultrathin Ni(OH)2 nanosheet array cathode and holey α-Fe2O3 nanorod array anode are achieved with a ultrahigh active material loading over 130 mg cm-3 and excellent compressibility up to 60%. It is noteworthy that the compressible QSS-NFB demonstrated an excellent cycling stability (∼91.3% capacity retentions after 10000 cycles) and ultrahigh energy density (28.1 mWh cm-3 at a power of 10.6 mW cm-3). This work provides a simple method for producing compression-tolerant energy-storage devices, which are expected to have promising applications in next generation stretchable/wearable electronics.

Two-dimensional (2D) semiconductors have been a central focus for next-generation electronics and optoelectronics owing to their great potential to extend the scaling limits in a silicon transistor. However, due to the lack of surface dangling bonds in most 2D semiconductors, such as graphene and transition metal dichalcogenides (TMDs), the direct growth of the high-κ film on these 2D materials via an atomic layer deposition (ALD) technique often produces dielectrics with poor quality, which hinders their integration in the modern semiconductor industry. Here, we comprehensively investigate the ALD growth of the Al2O3 layer on 2D exfoliated black phosphorus (BP). Intriguingly, we found that the 2D BP with "silicon-like" characteristics possesses a native surface oxide layer PxOy after air exposure. The PxOy-induced surface dangling bonds enable the spontaneous integration of the high-quality Al2O3 layer on the BP flake without any pretreatments to functionalize the surface. Additionally, the Al2O3 layer could effectively passivate BP to prevent its degradation in ambient conditions, which addresses the most serious problem of the BP material. Moreover, the Al2O3-encapsulated BP field-effect transistor (FET) exhibits good electrical transport performance, with a high hole mobility of ∼420 cm2 V-1 s-1 and electron mobility of ∼80 cm2 V-1 s-1. Moreover, the high-quality Al2O3 layer can also be integrated into the top-gated BP transistor and inverter. Our findings reveal the silicon-like characteristics of BP for the high-κ ALD dielectric growth technology, which promises the seamless integration of 2D BP in the modern semiconductor industry.

Reverse cyclic lateral testing was undertaken to investigate the seismic behavior of 1/4 scale steel-plate concrete (SC) composite walls. The experimental program involved seven SC wall pier specimens. A new chamber structure is proposed, using steel diaphragms to connect the two steel faceplates to each other and to divide the SC wall pier into two parts. Conventional wall specimens failed mainly by tensile fracture of the concrete at the junction of the wall side and wall base, crushing of the concrete at the toe of the wall, or buckling of the steel faceplate. Tearing of the welded joints at the steel faceplates and steel diaphragm, buckling of steel, steel diaphragms being pulled out, tensile fracture and crushing of the concrete were the main failure modes of the chamber structure walls. A parametric numerical analysis in ABAQUS was developed to investigate the effects of the stiffening rib, steel web amount, material strength, shear-span ratio, and axial compression ratio on the seismic response of SC walls. The chamber structure of the SC wall piers can improve the peak load, ductility, and energy-dissipating capacity. The steel faceplate thickness and stiffening ribs can improve the behavior of SC wall piers.

Since first developed, the conducting materials in wireless communication and electromagnetic interference (EMI) shielding devices have been primarily made of metal-based structures. Here, we present a highly conductive and corrosion-resistant graphene assembled film (GAF) that can be used to fabricate multi-band 5G wireless communication electronics and EMI protection at frequencies ranging from tens of MHz to THz to demonstrate its potential in metal replacement in practical electronics. The GAF based antennas (dipole antenna, ultra-wideband antenna and 5G wireless communication antenna array) are comparable with metal-based antennas in terms of performance and device complexity. The EMI shielding effectiveness of GAF can reach up to 127 dB in the frequency range of 2.6 GHz - 0.32 THz, and a maximum shielding effectiveness per unit thickness is of 6966 dB/mm. Furthermore, the GAF metamaterials exhibit promising frequency selection characteristics and angular stability as flexible frequency selective surfaces that can work at 3.5 GHz and 60 GHz respectively.

Despite the large number of experimental works divulging the use of carbonaceous materials in LIB anodes, there are very few reports modeling the relationships between the characteristics of the electrode active materials and their lithium storage capacity. In this work, it is aimed to model the influence of the size and shape of carbon nano-spheres (carbon dots) on their lithium storage capacity. This study divides lithium storage of carbon nano-spheres into two segregate surface- and bulk-related mechanisms and calculates the capacity as the summation of these two components. Accordingly, a novel model employing a new factor called normalized volume, to accurately add the contribution of the surface lithium storage component to the bulk one, is introduced. The model also considers the size and morphology of the carbonaceous nano-particles simultaneously to estimate the specific capacity of the LIB anodes. The model revealed the fact that the decrease in the size of carbon nano-spheres below 20 nm dramatically results in the enhancement of the lithium storage capacity. The comparison of the estimated values with the experimental data of the literature confirmed the satisfactory consistency of the measured and predicted capacities. More importantly, this model is capable of being utilized as a fundamental tool to predict the specific lithium storage capacity of various carbon nano-structures considering their shape and dimension.

Rhenium disulfide (ReS2) has recently amassed increasing interests as lithium-ion batteries (LIBs) anode material due to its extremely weak interlayer coupling and high Li storage capacity. However, its potential as high-performance sodium-ion batteries (SIBs) has rarely been explored comprehensively. In this work, we demonstrate a novel construct of ReS2 anode consisting of vertical ReS2 nanosheets uniformly grown on metal-organic framework (MOF) derived porous carbonaceous nanocubes (RESNC), is able to achieve high-performance SIBs. Exhibiting high specific surface area (117.40 m2 g−1) and near-microporous (∼2.20 nm) pore size distribution, this unique and conductive nanostructure provides abundant reactive sites which enable rapid and efficient ions/electrons transportation. For the first time, the sodiation/desodiation redox mechanisms of the ReS2-based nanostructure are revealed via ex situ investigations. Our results demonstrated that RESCN nanocomposite is endued with high capacity (365 mA h g−1, 100 mA g−1), high rate capability (up to 2 A g−1) and stable cycling performance (>600 cycles, 2 A g−1). To elaborate on the rate and cycling capabilities, various diffusion pathways of sodium-ions on the ReS2 lattice were unravelled via first-principles/ab initio techniques, with the results indicating favourable diffusion mechanics. Through the joint approach of comprehensive experimental and theoretical investigations, our results suggest that the ReS2/C composites are highly promising as high-performance SIBs anode materials, as well as broadening the current perspectives of materials choices as far as high capacity transition metal disulfides (TMDs) is concerned.

Micro- and nanoscale surface topographies that give rise to superhydrophobic surfaces have been achieved mostly on 2-dimensional planar objects. Increasing interests in superhydrophobic surfaces on consumer products, optics, and biomedical devices demand topographic patterning on free-form nonplanar surfaces via a high throughput manufacturing process such as injection molding. However, successes in high-resolution (submicrometer) injection molding have been limited to flat and planar objects. A challenge associated with achieving submicrometer surface resolution on a 3-dimensional curved object lies in the control of the replication process in a multiscale mold cavity and the nonuniform temperature and pressure distribution over a macroscopically curved mold insert. Here, a two-step simulation approach is employed to investigate the replication of polymer in the macroscopic and submicrometer cavities. Both simulation and experimental data revealed the effects of holding pressure, mold temperature, and ma...

Shape memory alloys (SMAs) have been widely used to fabricate soft actuators by embedding SMA wires into various soft matrices manufactured by conventional moulding methods or novel three-dimensional (3D) printing techniques. However, soft matrices of SMA based actuators are typically fabricated from only one or two different materials. Here, we exploit the great manufacturing flexibility of multimaterial 3D printing to fabricate various bending, twisting and extensional actuators by precisely controlling the spatial arrangements of different printing materials with different stiffnesses. In order to achieve a broad range of deformations, ten different printing materials were characterized and used in the actuators design. In addition, we developed a finite element model to simulate complex deformations of the printed actuators and facilitate the design process. The model incorporates a user defined material subroutine that describes the nonlinear temperature dependant behavior of SMAs. The results show the efficiency and flexibility of multimaterial 3D printing in tailoring the deformed shape of the SMA based soft actuators, which cannot be accomplished using conventional manufacturing methods such as moulding.

The recently developed dual-stage 3D printing reprocessable thermosets (3DPRTs) utilize acrylate functional groups to enable the ultra-violet based 3D printing (Stage I), and employ bond exchange reaction (BERs) to tailor the network structure for mechanical properties enhancement and impart the reshapeability, reparability, and recyclability into traditionally unprocessable thermosets (Stage II). 3DPRT provides a practical solution to address environmental challenges associated with the rapid increase in the consumption of 3D printing materials. However, due to the nascent state of development, fundamental understanding of the chemomechanics during the processing of 3DPRTs is still lacking. In this paper, we present detailed experimental and theoretical studies to understand the effect of thermal treatment condition at Stage II on the evolution of network structure and thermomechanical properties of 3DPRTs. A chemomechanics theory is defined to link the molecular-level BER kinetics to the macroscale thermomechanical properties of 3DPRTs during the thermal treatment. A thermo-viscoelastic multi-branched constitutive model is then established to capture the elastic and glass transition behaviors during the Stage II processing. The developed theory is able to capture the experimental observations on the mechanical properties enhancement and provide theoretical guidance for the network design and the selection of thermal treatment conditions to tailor the final mechanical properties of 3DPRTs.

Conventional 4D printing technologies are realized by combining 3D printing with soft active materials such as shape memory polymers (SMPs) and hydrogels. However, the intrinsic material property limitations make the SMP or hydrogel-based 4D printing unsuitable to fabricate the actuators that need to exhibit fast-response, reversible actuations. Instead, pneumatic actuations have been widely adopted by the soft robotics community to achieve fast-response, reversible actuations, and many efforts have been made to apply the pneumatic actuation to 3D printed structures to realize passive 4D printing with fast-response, reversible actuation. However, the 3D printing of soft actuators/robots heavily relies on the commercially available UV curable elastomers the break strains of which are not sufficient for certain applications which require larger elastic deformation. In this paper, we present two simple approaches to tune the mechanical properties such as stretchability, stiffness, and durability of the commercially available UV curable elastomers by adding: (i) mono-acrylate based linear chain builder; (ii) urethane diacrylate-based crosslinker. Material property characterizations have been performed to investigate the effects of adding the two additives on the stretchability, stiffness, mechanical repeatability as well as viscosity. Demonstrations of fully printed robotic finger, grippers, and highly deformable 3D lattice structure are also presented.

Vitrimers are a new class of thermosetting polymers that can be thermally processed through bond exchange reactions (BERs) without losing network integrity. In engineering applications, the tunability of their thermomechanical properties is highly desirable to meet the requirements of different working conditions. Here, we report a simple composite-based strategy that avoids complex chemistry to prepare vitrimer blends with tunable thermomechanical properties by virtue of the good weldability of base vitrimers. Effects of processing parameters (such as temperature and time) on the properties of recycled vitrimer blends are experimentally investigated. A computational model that accounts for the random distribution of component vitrimer particles is developed to predict the thermomechanical properties of the recycled vitrimer blends with various compositions. Good agreement is achieved between theoretical prediction and experiment. Parametric studies are further conducted by employing the computational model to explore the designability and provide some basic principles to guide the design of recycled vitrimer blends. Reasonable recyclability of the vitrimer blends is verified by multiple generations of recycling experiments.

As a promising energy storage system, lithium–sulfur (Li–S) batteries are attracting increasing attention but still limited by the sluggish reaction kinetics and shuttle effect caused by the dissolution of lithium polysulfides. Herein, a significant improvement of conversion kinetics and areal sulfur loading is achieved using an ordered microchannel graphene scaffold with incorporated catalytic Fe3O4 nanocrystals and porous carbon as a multifunctional sulfur host. The synergy between the polar catalytic Fe3O4 nanocrystals and porous carbon frameworks enables a strong polysulfide anchoring effect and a fast polysulfide conversion rate. Thus, the 3D ternary Fe3O4/porous carbon/graphene aerogel demonstrates an ultrahigh rate performance of 755 mA h g−1 at 3C and a high areal capacity of 6.24 mA h cm−2 at a sulfur loading of 7.7 mg cm−2. Moreover, the promoted reaction kinetics and reliable cyclability are revealed by the visible evolution of polysulfides using in situ X-ray diffraction (XRD), and the enhanced chemical anchoring of polysulfides is disclosed by density functional theory (DFT) calculations. This work provides a promising approach to develop multifunctional ordered porous aerogels with metal oxide nanocrystals for high-performance Li–S batteries, especially those which suffer from low sulfur loading and inferior rate performance.

Graphene aerogels are emerging low density and superelasticity macroscopic porous materials with various applications. However, it still remains a challenge to develop a versatile strategy under ambient conditions for fabricating large-area, high-performance graphene aerogels, which is crucial for their practical applications. Here, we report a novel room-temperature reduction self-assembly (RTRS) strategy to fabricate large-area graphene aerogels under ambient conditions. The strategy is based on using unique hydrazine hydrates as reducing agents to generate stable microbubbles beneficial for the formation of macroporous graphene hydrogels. Interestingly, the resultant hydrogel followed by a simple pre-freeze treatment can be naturally dried into graphene aerogels without noticeable volume shrinkage or structure cracking. Benefiting from the mild conditions, a large-area graphene aerogel with a diameter of up to 27 cm was prepared as an example. The as-formed aerogels exhibit a stable honeycomb-like coarse-pores structure, a low density of 3.6 mg cm-3 and superelasticity (rapidly recoverable from 95% compression) which are suitable for pressure/strain sensors. Moreover, the aerogel exhibits superior particulate matter adsorption efficiency (PM2.5: 93.7%, PM10: 96.2%) and good recycling ability. Importantly, the preparation process is cost-effective and easily scalable without the need for any special drying techniques and heating processes, which provides an ideal platform for mass production of graphene aerogels toward practical applications.

Achievement of optimized lateral and vertical resolution is a key factor to obtaining three-dimensional (3D) structural details fabricated through digital light processing (DLP)-based 3D printing technologies which exploit digitalized ultraviolet (UV) or near-UV light to trigger localized photopolymerization forming solid patterns from liquid polymer resins. Many efforts have been made to optimize printing resolution through improving the optical systems. However, researchers have paid comparatively little attention to understand the influences of polymer formulation on the printing resolution and surface quality. Here, we report an investigation on the effects of in-house formulated (meth)acrylate-based photopolymer constituent types and concentrations on the resolution and quality of structures printed on a bottom-exposure DLP-based 3D printing system. We examined a wide variety of resin formulations to determine optimal formulations that yield best printing resolution and surface quality over a reasonably broad range of mechanical properties. We demonstrated the controlled fabrication of sub-pixel conical and aspherical smooth features, whereby the shape and dimensions could be prescribed with the resin formulation and process parameters. Such features hold promising implications in micro-optic and microfluidic fabrication using the DLP-based 3D printing technique. Additionally, we devised di(meth)acrylate-based resin formulations which could exclusively produce optically-clear layers in contrast to opaque, rough surfaces resulting from commonly-used diacrylate-based resins including those available commercially. Use of this solution minimized the ‘stair-stepping’ effect in components printed in a layer-by-layer manner. We also showed that the maximum lateral and vertical resolution attainable using our present system were 7 μm and 4 μm, respectively, while maintaining uniform width and height. Taken together, the present findings provide a basis for optimized photopolymer resin formulations that retain maximum vertical and lateral resolutions and minimal surface roughness and layering artifacts for a versatile range of mechanical and rheological properties suited to novel applications in 3D printing of smooth free-form solids, micro-optics, and direct fabrication of microfluidic platforms with functional surfaces.

Novel glass-ceramic electrolytes of (100-x) (70Li2S-30P2S5)-xNi2P (x = 0, 1, 2, 3, 4) with high ionic conductivity were proposed with the addition of Ni2P. The 2 mol% Ni2P containing electrolyte (Li7Ni0.2P3.1S11) displays the highest lithium-ion conductivity of 2.22 mS cm−1 at room temperature, which is 1.6 times higher than pristine Li7P3S11. By direct combination with Li2S, the fabricated Li7Ni0.2P3.1S11-Li2S all-solid-state battery exhibits an initial discharge capacity of 429 mAh g−1 at 0.064 mA cm−2 and 454 mAh g−1 at 20th charging/discharging cycle with a high Coulombic efficiency of 99.7%. Keywords: Ni2P doped Li7P3S11, Li2S utilization, Lithium-ion conductivity, All-solid-state lithium‑sulfur batteries

Four-dimensional (4D) printing, which enables 3D printed structures to alter shapes over time, is attracting increasing attention because of its exciting potential in various applications. Among all the 4D printing materials, shape memory polymers (SMPs) have a higher stiffness and faster response rate and therefore are considered as one of the most promising 4D printing materials. However, the current studies of SMP-based 4D printing mainly focused on the deformation behavior and structural design of 4D printed structures. An additional function such as color change is desired for 4D printed structure, which would be potentially beneficial to the applications such as anti-counterfeiting, encryption, and bioinspired camouflage. In this paper, we report an ultraviolet (UV)-curable and thermochromic (UVT) SMP system that enables color-changeable 4D printing. The UVT SMP system is acrylate-based, thus highly UV-curable and compatible with PμSL-based high-resolution 3D printing technique. Thermochromism is imparted by adding the thermochromic microcapsules to the UVT SMP system, which allows the printed structures to reversibly change colors upon heating and cooling. To demonstrate its extraordinary thermochromic and mechanical performance, we use UVT SMP to print QR codes and multilevel anti-counterfeiting patterns which can hide the visible information at room temperature and visualize the information by encrypting, decrypting, and encrypting again steps with the shape-color recovery process. The development of UVT SMP will significantly advance current applications of SMP-based 4D printing, especially for anti-counterfeiting and safe data recording.

Stretchable and flexible electronics conformal to human skin or implanted into biological tissues has attracted considerable interest for emerging applications in health monitoring and medical treatment. Although various stretchable materials and structures have been designed and manufactured, most are limited to two-dimensional (2D) layouts for interconnects and active components. Here, by using projection microstereolithography (PμSL)-based three-dimensional (3D) printing, we introduce a versatile microfabrication process to push the manufacturing limit and achieve previously inaccessible 3D geometries at a high resolution of 2 μm. After coating the printed microstructures with thin Au films, the 3D conductive structures offer exceptional stretchability (∼130%), conformability, and stable electrical conductivity (5% resistance change at 100% tensile strain). This fabrication process can be further applied to directly create complicated 3D interconnect networks of sophisticated active components, as demonstrated with a stretchable capacitive pressure sensor array here. The proposed scheme allows a simple, facile, and scalable manufacturing route for complex, integrated 3D flexible electronic systems.

Soft composite actuators can be fabricated by embedding shape memory alloy (SMA) wires into soft polymer matrices. Shape retention and recovery of these actuators are typically achieved by incorporating shape memory polymer segments into the actuator structure. However, this requires complex manufacturing processes. This work uses multimaterial 3D printing to fabricate composite actuators with variable stiffness capable of shape retention and recovery. The hinges of the bending actuators presented here are printed from a soft elastomeric layer as well as a rigid shape memory polymer (SMP) layer. The SMA wires are embedded eccentrically over the entire length of the printed structure to provide the actuation bending force, while the resistive wires are embedded into the SMP layer of the hinges to change the temperature and the bending stiffness of the actuator hinges via Joule heating. The temperature of the embedded SMA wire and the printed SMP segments is changed sequentially to accomplish a large bending deformation, retention of the deformed shape, and recovery of the original shape, without applying any external mechanical force. The SMP layer thickness was varied to investigate its effect on shape retention and recovery. A nonlinear finite element model was used to predict the deformation of the actuators.

Vitrimers with bond exchange reactions (BERs) are a class of covalent adaptable network (CAN) polymers at the forefront of recent polymer research. They exhibit malleable and self-healable behaviors and combine the advantages of easy processability of thermoplastics and excellent mechanical properties of thermosets. For thermally sensitive vitrimers, a molecular topology melting/frozen transition is triggered when the BERs are activated to rearrange the network architecture. Notable volume expansion and stress relaxation are accompanied, which can be used to identify the BER activation temperature and rate as well as to determine the malleability and interfacial welding kinetics of vitrimers. Existing works on vitrimers reveal the rate-dependent behaviors of the nonequilibrium network during the topology transition. However, it remains unclear what the quantitative relationship with heating rate is, and how it will affect the macroscopic stress relaxation. In this paper, we study the responses of an epoxy-based vitrimer subjected to a change in temperature and mechanical loading during the topology transition. Using thermal expansion tests, the thermal strain evolution is shown to depend on the temperature-changing rate, which reveals the nonequilibrium states with rate-dependent structural relaxation. The influences of structural relaxation on the stress relaxation behaviors are examined in both uniaxial tension and compression modes. Assisted by a theoretical model, the study reveals how to tune the material and thermal-temporal conditions to promote the contribution of BERs during the reprocessing of vitrimers.

Shape-shifting structures have gained growing interest recently and found wide applications in areas such as soft robotics, biomedical devices and self-folding origami, attributed to their ability to construct complicated shapes directly from simple structures. However, an efficient method to design and fabricate programmable 3D shape-shifting structures from 2D polymer films still lacks. In this work, we design programmable shape-shifting 3D structures via the release of internal gradient stress using the frontal photopolymerization (FPP) method. First, the relation between the non-uniformly distributed material and loading parameters, and the geometric and fabrication parameters are established theoretically. The finite element (FE) model is then developed based on the theoretically obtained material and loading parameters. Next, the elastic instability in the shape-shifting behaviors of a cured film is captured through an elastic energy minimization. Furthermore, by using grayscale light patterns, it is shown that we can selectively manipulate the geometric and fabrication parameters to improve the design freedom of various complex 3D structures.

Spatial control of material properties is highly desirable in additive manufacturing of functional structures with complex geometries. Whereas most previous efforts focused on developing new printing or material systems, we propose a new voxel design strategy of constructing digital materials to provide the additively manufactured polymeric structures with spatially customized thermomechanical properties. In our approach, a matrix-inclusion composite layout is adopted in the linearly patterned voxels that perform as building blocks to construct bulk material. Through rational design of voxel size and inclusion content, the printed polymeric digital material displays a tunable storage modulus up to three orders of magnitude and glass transition temperature ranging from 0 °C to 60 °C. By taking advantage of the design freedom, we demonstrate a sequential folding structure with spatially tunable actuation speed, and multi-stable configurations that trap elastic energy in deterministic collapse sequences. Overall, our approach provides an effective and convenient way of spatially customizing material properties for additive manufacturing and offers instructive inspirations to the realm of digital fabrication.

Extraordinary optical, electrical and chemical properties of 2D materials have potential to be useful for quick and sensitive detection of pathological diseases. One important example is malaria disease that can progress rapidly and cause death within days. Therefore, fast, accurate and cost-effective malaria diagnosis available at the point of care is urgently needed to facilitate precise treatment. Here we report rapid and highly sensitive malaria detection with an inexpensive graphene-protected copper surface plasmon resonance biosensor. Using phase sensitive surface plasmon resonance technique and a graphene functionalization protocol for attaching end-tethered DNA probes that were complementary to a malaria specific DNA target, we were able to significantly improve the detection limit of the malarial plasmodium parasite. The phase sensitivity of our graphene-enhanced sensors exceeds by two orders of magnitude the sensitivity of analogous optical biosensors. This enhanced sensitivity could provide means to detect low copy number bacterial infectious agents and to associate dormant bacterial populations with chronic inflammatory diseases using simple label-free optical detection.

Four-dimensional (4D) printing of shape memory polymer (SMP) imparts time responsive properties to 3D structures. Here, we explore 4D printing of a SMP in the submicron length scale, extending its applications to nanophononics. We report a new SMP photoresist based on Vero Clear achieving print features at a resolution of ~300 nm half pitch using two-photon polymerization lithography (TPL). Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. As the nanostructures are flattened, the colours and printed information become invisible. Remarkably, the shape memory effect recovers the original surface morphology of the nanostructures along with its structural colour within seconds of heating above its glass transition temperature. The high-resolution printing and excellent reversibility in both microtopography and optical properties promises a platform for temperature-sensitive labels, information hiding for anti-counterfeiting, and tunable photonic devices., Four-dimensional (4D) printing of shape memory polymer (SMP) imparts time responsive properties to 3D structures. Here, the authors explore 4D printing of a SMP in the submicron length scale, extending its applications to nanophononics.

By 3D printing lattice structure with active materials, the structures can exhibit shape and functional changes under external stimulus. However, the programmable shape changes of the 3D printed lattice structures are limited due to the complex geometries, nonlinear behaviors of the active materials, and the diverse external stimuli. In this work, we propose a design framework combining experiments, theoretical modeling, and finite element simulations for the controllable shape changes of the 3D printed horseshoe under thermal stimulus. The theoretical model is based on a phase evolution model that combines the geometrical nonlinearity and the material nonlinearity. Results show that the shapes with positive or negative Poisson's ratio and bending intermediate shapes can be programmed by tuning the geometrical parameters and the temperature distribution. This work provides a method to aid the design of 3D printed functional lattice structures and have potential applications in soft robotics, biomedicine, and energy absorbing fields.

We report a method to prepare highly stretchable and UV curable hydrogels for high resolution DLP based 3D printing. Hydrogel solutions were prepared by mixing self-developed high-efficiency water-soluble TPO nanoparticles as the photoinitiator with an acrylamide-PEGDA (AP) based hydrogel precursor. The TPO nanoparticles make AP hydrogels UV curable, and thus compatible with the DLP based 3D printing technology for the fabrication of complex hydrogel 3D structures with high-resolution and high-fidelity (up to 7 μm). The AP hydrogel system ensures high stretchability, and the printed hydrogel sample can be stretched by more than 1300%, which is the most stretchable 3D printed hydrogel. The printed stretchable hydrogels show an excellent biocompatibility, which allows us to directly 3D print biostructures and tissues. The great optical clarity of the AP hydrogels offers the possibility of 3D printing contact lenses. More importantly, the AP hydrogels are capable of forming strong interfacial bonding with commercial 3D printing elastomers, which allows us to directly 3D print hydrogel–elastomer hybrid structures such as a flexible electronic board with a conductive hydrogel circuit printed on an elastomer matrix.

Normally-off Gallium Nitride (GaN) transistors with p-GaN gated technology for power applications are studied for the optimization of off-state breakdown performance. The gate-drain distance, gate length, field plate length and its configuration have been studied. Increase gate-drain distance will not only enhance the breakdown performance but also increase the $\mathrm{R}_{\mathrm{on}},\ \mathrm{L}_{\mathrm{GD}}$ between $10 -20\ \mu\mathrm{m}$ is recommended. Extra gate length will lead to the rising of the gate leakage current under high voltage. The overhang length of the field plate is a critical factor for breakdown performance, over placing the length of overhang may result in an additional current leakage path.

The energy dissipation issue has become one of the greatest challenges of the modern electronic industry. Incorporating graphene into the electronic devices has been widely accepted as a promising approach to solve this issue, due to its superior carrier mobility and thermal conductivity. Here, using Raman spectroscopy and infrared thermal microscopy, we identify the energy dissipation behavior of graphene device with different thicknesses. Surprisingly, the monolayer graphene device is demonstrated to have a comparable energy dissipation efficiency per unit volume with that of a few-layer graphene device. This has overturned the traditional understanding that the energy dissipation efficiency will reduce with the decrease of functional materials dimensions. Additionally, the energy dissipation speed of the monolayer graphene device is very fast, promising for devices with high operating frequency. Our finding provides a new insight into the energy dissipation issue of two-dimensional materials devices, which will have a global effect on the development of the electronic industry.

Digital Light Processing (DLP) 3D printing enables the creation of hierarchical complex structures with specific micro- and macroscopic architectures that are impossible to achieve through traditional manufacturing methods. Here, this hierarchy is extended to the mesoscopic length scale for optimized devices that dissipate mechanical energy. A photocurable, thus DLP-printable main-chain liquid crystal elastomer (LCE) resin is reported and used to print a variety of complex, high-resolution energy-dissipative devices. Using compressive mechanical testing, the stress-strain responses of 3D-printed LCE lattice structures are shown to have 12 times greater rate-dependence and up to 27 times greater strain-energy dissipation compared to those printed from a commercially available photocurable elastomer resin. The reported behaviors of these structures provide further insight into the much-overlooked energy-dissipation properties of LCEs and can inspire the development of high-energy-absorbing device applications.

Soft biomaterials have a wide range of applications in many areas. However, one material can only cover a specific range of mechanical performance such as the elastic modulus and stretchability. In order to improve the mechanical performance of soft biomaterials, lattice structures are embedded to reinforce the biomaterials. In this paper, rectangular and triangular lattice structures formed by modified horseshoe microstructures are used because their mechanical properties are tunable and can be tailored precisely to match the desired properties by adjusting four geometrical parameters, the length L, radius R, width w and arc angle θ0. A theoretical design framework for the modified horseshoe lattice structures is developed to predict the dependence of the mechanical behaviors on geometrical parameters. Both experiments and finite element simulations on lattice structures are conducted to validate the theoretical models. Results show that a wide range of design space for the elastic modulus (a few kPa to hundreds of MPa), stretchability (strain up to 180%) and Poisson ratio (ranging from -0.5 to 1.2) can be achieved. Experiments on lattice-hydrogel composites are also conducted to verify the reinforcement effect of lattice structures on the hydrogel. This work provides a theoretical method to predict the mechanical behaviors of the lattice structures and aid the rational design of reinforced biomaterials, which has applications in tissue engineering, drug delivery and intraocular lenses.

Vitrimers have the characteristics of shape-reforming and surface-welding, and have the same excellent mechanical properties as thermosets; so vitrimers hold the promise of a broad alternative to traditional plastics. Since their initial introduction in 2011, vitrimers have been applied to many unique applications such as reworkable composites and liquid crystal elastomer actuators. A series of experiments have investigated the effects of reprocessing conditions (such as temperature, time, and pressure) on recycled materials. However, the effect of particle size on the mechanical properties of recycled materials has not been reported. In this paper, we conducted an experimental study on the recovery of epoxy-acid vitrimers of different particle sizes. Epoxy-acid vitrimer powders with different particle size distributions were prepared and characterized. The effects of particle size on the mechanical properties of regenerated epoxy-acid vitrimers were investigated by dynamic mechanical analysis and uniaxial tensile tests. In addition, other processing parameters such as temperature, time, and pressure are discussed, as well as their interaction with particle size. This study helped to refine the vitrimer reprocessing condition parameter toolbox, providing experimental support for the easy and reliable control of the kinetics of the bond exchange reaction.

This article proposes an approach of 3D printing structures consisting of stretchable hydrogels bonded with UV curable polymers., 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.

To recycle vanadium and chromium from the V–Cr-bearing reducing slag, the thermodynamics of separating V(IV) and Cr(III) at 298 K was summarized in the form of potential–pH diagram and activity–pH diagram. The potential–pH diagrams of V–Mn–H2O and Cr–Mn–H2O systems show that the electrode potential of MnO2/Mn2+ is higher than that of VO2+/VO2+ but lower than that of Cr2O72–/Cr3+, which proves that it is feasible to selectively oxidize low valent vanadium using MnO2. The activity–pH diagrams of V(V)–H2O and Cr(III)–H2O systems show that the precipitation pH of V(V) is far lower than that of Cr(III), and therefore V(V) and Cr(III) can be separated through precipitation method. Based on the thermodynamic analysis, the flowsheet of recovery of vanadium and chromium from the V–Cr-bearing reducing slag is designed.

Electrochemical capacitors, also called supercapacitors (SCs), have been gaining a more significant position as electrochemical energy storage devices in recent years. They are energy storage devices with a considerable power density, a satisfactory energy density and a long-life cycle, suitable for a large number of applications. The further development of these devices relies on providing suitable, low-cost, environmentally friendly, and abundant materials for use as the active materials in the electrodes. Among the current materials used, activated carbons have a superior performance. Their excellent electrochemical performance, high specific surface area, high adsorption, tunable surface chemistry, fast ion/electron transport, abundant functional moieties, low cost, and abundance have made them promising candidates as SC electrodes. These advantages can be enhanced if the activated carbons are prepared from biomass precursors. Recently, scientists have focused on biomass because it is abundant and renewable, low cost, simply processed, and environmentally friendly. The fundamentals of SCs as an electrochemical energy storage device are discussed and biomass from various sources is categorized and introduced. Finally, the activation techniques for these biomass precursors and their use as electrode materials for SCs are discussed.

Nature builds biological materials from limited ingredients, however, with unparalleled mechanical performances compared to artificial materials, by harnessing inherent structures across multi-length-scales. In contrast, synthetic material design overwhelmingly focuses on developing new compounds, and fails to reproduce the mechanical properties of natural counterparts, such as fatigue resistance. Here, a simple yet general strategy to engineer conventional hydrogels with a more than 100-fold increase in fatigue thresholds is reported. This strategy is proven to be universally applicable to various species of hydrogel materials, including polysaccharides (i.e., alginate, cellulose), proteins (i.e., gelatin), synthetic polymers (i.e., poly(vinyl alcohol)s), as well as corresponding polymer composites. These fatigue-resistant hydrogels exhibit a record-high fatigue threshold over most synthetic soft materials, making them low-cost, high-performance, and durable alternatives to soft materials used in those circumstances including robotics, artificial muscles, etc.

4D printing is an emerging fabrication technology that enables 3D printed structures to change configuration over "time" in response to an environmental stimulus. Compared with other soft active materials used for 4D printing, shape-memory polymers (SMPs) have higher stiffness, and are compatible with various 3D printing technologies. Among them, ultraviolet (UV)-curable SMPs are compatible with Digital Light Processing (DLP)-based 3D printing to fabricate SMP-based structures with complex geometry and high-resolution. However, UV-curable SMPs have limitations in terms of mechanical performance, which significantly constrains their application ranges. Here, a mechanically robust and UV-curable SMP system is reported, which is highly deformable, fatigue resistant, and compatible with DLP-based 3D printing, to fabricate high-resolution (up to 2 µm), highly complex 3D structures that exhibit large shape change (up to 1240%) upon heating. More importantly, the developed SMP system exhibits excellent fatigue resistance and can be repeatedly loaded more than 10 000 times. The development of the mechanically robust and UV-curable SMPs significantly improves the mechanical performance of the SMP-based 4D printing structures, which allows them to be applied to engineering applications such as aerospace, smart furniture, and soft robots.

We report a dual-stage photocrosslinked polymer network based on sequential ultraviolet (UV)-triggered radical polymerization and thermally activated etherification, applicable to the fabrication of tailorable and programmable high-resolution structures. The first stage involves photoinitiated polymerization of monomer and crosslinker to obtain an intermediate polymer network. As such, sophisticated two-dimensional (2D) and micro-scale three-dimensional (3D) structures can be made by using UV-based advanced manufacturing technologies. These complex structures can then be readily programmed into other desired, permanent shapes, in the second stage, via thermally triggered etherification which results in a highly crosslinked, robust polymer network. The intermediate network (Stage I) is characterized to have a Young’s modulus ranging from 342 to 1146 MPa and a glass transition temperature from 52 °C to 83 °C, depending on the concentration of crosslinker. The same material attains a glass transition temperature ranging from 67 °C to ~105 °C and a Young’s modulus of up to 1607 MPa after the subsequent heating process (Stage II). Originally 3D printed 2D structures can be further programmed into rigid, permanent 2.5/3D ones. Micropatterns fabricated from intrinsically hydrophilic dual-stage crosslinked photopolymers through soft lithography show superhydrophobicity, and can subsequently be molded with different curvatures for practical applications.

A novel composite consisting of vertical ultrathin MoS2 nanosheet arrays and Fe3O4 quantum dots (QDs) grown on graphite paper (GP) as a high-performance anode material for sodium-ion batteries (SIBs) has been synthesized via a facile two-step hydrothermal method. Owing to the high reversible capacity provided by the MoS2 nanosheets and the superior high rate performance offered by Fe3O4 QDs, superior cycling and rate performances are achieved by Fe3O4@MoS2-GP anodes during the subsequent electrochemical tests, delivering 468 and 231 mA h g−1 at current densities of 100 and 3200 mA g−1, respectively, as well as retaining ∼72.5% of their original capacitance at a current density of 100 mA g−1 after 300 cycles. The excellent electrochemical performance resulted from the interconnected nanosheets of MoS2 providing flexible substrates for the nanoparticle decoration and accommodating the volume changes of uniformly distributed Fe3O4 QDs during the cycling process. Moreover, Fe3O4 QDs primarily act as spacers to stabilize the composite structure, making the active surfaces of MoS2 nanosheets accessible for electrolyte penetration during charge–discharge processes, which maximally utilized electrochemically active MoS2 nanosheets and Fe3O4 QDs for sodium-ion batteries.

Shape memory polymers (SMPs) are a class of active materials that have the capability of fixing a temporary shape and recovering to a permanent shape in response to environmental stimuli. While SMPs have been studied intensively, researchers paid comparatively fewer attentions to SMPs exhibiting anisotropic mechanical and shape memory behaviors by controlling the microscopic architectures. At the same time, the rapidly developed three-dimension (3D) printing technologies enable us to directly print complex 3D structures with multimaterials and provide possibilities to fabricate anisotropic shape memory polymer by precisely controlling the microstructure of fibers. In this paper, we present a new approach for fabricating printed anisotropic shape memory elastomeric composites (p-ASMECs) by taking advantage of 3D printing. In the fabrication process, an elastomer is first printed as matrix and the orientation of fibers is defined by the precisely printed channels. By interrupting the printing process, the printed channels are filled with the crystallizable polymeric fibers that endow the shape memory effect into the composite system. The p-ASMECs exhibit large, controllable anisotropy in both mechanical and shape memory behaviors that can be precisely controlled by geometric parameters of the microstructure such as fiber's volume fraction and position in space. To facilitate design of p-ASEMCs, we also developed a thermomechanical constitutive framework to describe the complex, anisotropic, larger deformation thermomechanical behaviors of p-ASEMCs. The developed constitutive model, used to explain the phenomenon that the lowest stiffness of p-ASEMCs occurs at the fiber orientation angle θ ≈ 55°, successfully predicts the anisotropic shape memory behavior of p-ASMECs, guides the design to enhance the temporary shape fixity, and simulates the shear deformation of the temporary shape after completely releasing the external constraints.

A microlens array has become an important micro-optics device in various applications. Compared with traditional manufacturing approaches, digital light processing (DLP)-based printing enables fabrication of complex three-dimensional (3D) geometries and is a possible manufacturing approach for microlens arrays. However, the nature of 3D printing objects by stacking successive 2D patterns formed by discrete pixels leads to coarse surface roughness and makes DLP-based printing unsuccessful in fabricating optical components. Here, we report an oscillation-assisted DLP-based printing approach for fabrication of microlens arrays. An optically smooth surface (about 1 nm surface roughness) is achieved by mechanical oscillation that eliminates the jagged surface formed by discrete pixels, and a 1-3 s single grayscale ultraviolet (UV) exposure that removes the staircase effect. Moreover, computationally designed grayscale UV patterns allow us to fabricate microlenses with various profiles. The proposed approach paves a way to 3D print optical components with high quality, fast speed, and vast flexibility.