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Graphical Abstract: Graphical Abstract: Flexible surface acoustic wave technology for enhancing transdermal drug delivery

Surface acoustic wave (SAW) devices are generally fabricated on rigid substrates that support the propagation of waves efficiently. Although very challenging, the realisation of SAW devices on bendable and flexible substrates can lead to new generation SAW devices for wearable technologies. In this paper, we report flexible acoustic wave devices based on ZnO thin films coated on various substrates consisting of thin layers of metal (e.g., Ni/Cu/Ni) and/or polymer (e.g., polyethylene terephthalate, PET). We comparatively characterise the fabricated SAW devices and demonstrate their sensing applications for temperature and ultraviolet (UV) light. We also investigate their acoustofluidic capabilities on different substrates. Our results show that the SAW devices fabricated on a polymer layer (e.g. ZnO/PET, ZnO/Ni/Cu/Ni/PET) show enhanced temperature responsivity, and the devices with larger wavelengths are more sensitive to UV exposure. For actuation purposes, the devices fabricated on ZnO/Ni/Cu/Ni layer have the best performance for acoustofluidics, whereas insignificant acoustofluidic effects are observed with the devices fabricated on ZnO/PET layers. We propose that the addition of a metallic layer of Ni/Cu/Ni between ZnO and polymer layers facilitates the actuation capability for the acoustofluidic applications while keeping temperature and UV sensing capabilities, thus enhancing the integration of sensing and acoustofluidic functions.

Droplet impact behavior on a solid surface is critical for many industrial applications such as spray coating, food production, printing, and agriculture. For all of these applications, a common challenge is to modify and control the impact regime and contact time of the droplets. This challenge becomes more critical for non-Newtonian liquids with complex rheology. In this research, we explored the impact dynamics of non-Newtonian liquids (by adding different concentrations of Xanthan into water) on superhydrophobic surfaces. Our experimental results show that by increasing the Xanthan concentration in water, the shapes of the bouncing droplet are dramatically altered, e.g., its shape at the separation moment is changed from a conventional vertical jetting into a “mushroom”-like one. As a result, the contact time of the non-Newtonian droplet could be reduced by up to ∼50%. We compare the impact scenarios of Xanthan liquids with those of glycerol solutions having a similar apparent viscosity, and results show that the differences in the elongation viscosity induce different impact dynamics of the droplets. Finally, we show that by increasing the Weber number for all of the liquids, the contact time is reduced, and the maximum spreading radius is increased.

Glass transition plays a critical role to determine the dynamic behaviors of amorphous shape memory polymers (SMPs). However, the fundamental relationships between shape memory effect (SME) and dynamic glass transition have not been well understood, even though this topic has been studied for decades. In this study, we apply a mean-square displacement function of Adam-Gibbs (AG) domain size model to explore metastable glass transition between normal glass state and rubbery state of amorphous SMPs, based on both mode-coupling theory and mean-field model. A statistic viscosity equation is formulated to study the dynamic glass transition of metastable AG domains in an amorphous SMP. A dynamically spinodal model is also developed to connect dynamic glass transitions to thermomechanical processes, based on statistic viscosity equation and phase transition model. Furthermore, using the spinodal models, multiple shape memory behaviors have been predicted for amorphous SMPs with dual-, triple- and quadruple-SMEs, resulted from their different routes of themomechanical evolutions. Finally, the proposed models are verified using the experimental data reported in literature.

A highly sensitive hydrogen sensor (HS) coated with Pt/WO3 powder with an erbium-doped fiber amplifier (EDFA) is proposed and experimentally demonstrated. The sensing head is constructed by splicing a short Section of tapered small-diameter coreless fiber (TSDCF diameter of 62.5 $\mu \text{m}$ and tapered to 14.5 $\mu \text{m}$ ) between two single-mode fibers (SMFs). The Pt/WO3 powder adheres to the surface of polydimethylsiloxane (PDMS) film coated on the TSDCF structure, which is sensitive to hydrogen. An EDFA is introduced into the sensor system to improve the quality factor ( ${Q}$ -factor) of the output spectrum and thus improve the sensor’s resolution. As the hydrogen concentration varies from 0% to 1.44%, the measured maximum light intensity variation and the sensor’s sensitivity are −32.41 dB and −21.25 dB/%, respectively. The sensor demonstrates good stability with the light intensity fluctuation of < 1.26 dB over a 30-min duration.

Network structures of various polymers have significant effects on their mechanical properties; therefore, numerous studies have investigated the constitutive relationship between symmetrical network structures and their rubber elasticity in polymers. However, few studies have focused on asymmetrical network structures in polymers that undergo bond exchange reactions, self-assembly, or mechanochemical coupling—all of which are induced by transition probabilities of chemical bonding processes. In this study, an extended constraint junction and phantom network model is formulated using the tree-growing theory to establish a constitutive relationship between asymmetrical network structures and their rubber elasticity in polymers. A free-energy equation is further developed to explore working principles of configurational transitions on the dynamic rubber elasticity of symmetrical and asymmetrical network structures. The constitutive relationship between dynamic rubber elasticity and symmetrical and asymmetrical network structures has also been proposed for the gels undergoing mechanochemical and hydromechanical coupling. Finally, the effectiveness of this newly proposed tree-growing model has been verified by comparing with the classical affine network model, finite element analysis, and the experimental results of gels reported in literature.

Thermosetting epoxy polymers can be designed with recycling, remodeling, and repairing properties owing to the special bond exchange reactions (BERs) in their covalent adaptable networks (CANs). However, the mechanisms behind their complex coupling of hydrodynamic diffusion, molecular reactions, and topological network structure changes have not been fully understood. In this paper, a hydro-chemo-mechanical coupling model is developed to describe self-toughening mechanisms of thermosetting epoxy polymers, wherein the CANs cooperatively undergo the combined processes of hydrodynamic diffusion, chemical BERs, and mechanical topology entanglements. Based on free volume theory and rubber elasticity theory, an extended phantom network model is developed to investigate the synergistic effects of hydrodynamics, BERs, and mechanical topology on interfacial welding and self-toughening behaviors of the CANs. A constitutive stress–strain relationship is further developed to understand the self-toughening mechanisms of CANs undergoing the hydro-chemo-mechanical coupling. Finally, the effectiveness of the proposed model is verified using the results obtained from the experiments and finite element analysis.

Effective control of electrical and optoelectronic properties of two-dimensional layered materials, one of the key requirements for applications in advanced optoelectronics with multiple functions, has been hindered by the difficulty of elemental doping, which is commonly utilized in Si technology. In this study, we proposed a new method to synthesize hydrophilic MoS2monolayers through covalently introducing hydroxyl groups during their growth process. These hydroxyl groups exhibit a strong capability of charge trapping, and thus the hydrophilic MoS2monolayers achieve excellent electrical, optical, and memory properties. Optical memory transistors, made from a single component of monolayer hydrophilic MoS2, exhibit not only excellent light-dependent and time-dependent photoelectric performance, but also good photo-responsive memory characteristics with over multi-bit storage and more than 104switching ratios. Atomically thin neuromorphic vision systems (with a concept of proof of 10 × 10 neuromorphic visual image) are manufactured from arrays of hydrophilic MoS2optical memory transistors, showing high quality image sensing and memory functions with a high color resolution. These results proved our new concepts to realize image memorization and simplify the pixel matrix preparation process, which is a significant step toward the development of future artificial visual systems.

Currently simple and conventional structures are unable to meet increasingly critical and diverse applications, as well as establishment of constitutive relationship of complex structure is becoming a major challenge in materials and mechanics disciplines. To overcome these challenges, fractal structures have attracted much attention due to their abilities to establish constitutive relationships of complex structures. In this study, we design a spatial fractal framework by using high elasticity material and 3D printing method. The constitutive relationships of fractal framework are mathematically modeled. Effects of fractal levels, fractal dimension, and spatial pattern of structures on the mechanical behavior of the fractal framework and assembled spatial fractal structures are studied using both finite element analysis and experimental measurements. The constitutive relationships among stress, strain, fractal dimension, fractal levels, and spatial patterns of structures are proposed and discussed. This paper develops a novel method to establish constitutive relationships of complex structures, and provides theoretical and engineering basis for 3D printable spatial fractal structures undergoing auxetic elasticity.

By integrating the Fox–Flory equation and the rubber elasticity principle, a phenomenologically constitutive model was proposed in this study to describe high mechanical recovery strength and a tailorable shape transition behavior of shape memory polymer (SMP) copolymers. The thermodynamics of different monomers in the copolymers were formulated by considering their influences on glass-transition temperatures (Tg) and elastic moduli based on the Fox–Flory and Gordon–Taylor equations. Effects of the Tg, weight fraction, molecular weight, storage modulus, and rubbery modulus of various monomers on thermomechanical and shape recovery behaviors of the SMP copolymers were theoretically investigated and discussed. Working principles of enhanced mechanical strength and a tailorable shape transition behavior of the SMP copolymers have been well-described using this newly proposed model, which offers an effective strategy for designing SMPs with high mechanical strength and a desirable shape memory behavior. Furthermore, molecular dynamics simulations were used to predict the glass-transition temperature from a molecular scale and the experimental results fit well with our modeling results.

Two-dimensional (2D) layered materials, such as MoS2, are greatly attractive for flexible devices due to their unique layered structures, novel physical and electronic properties, and high mechanical strength. However, their limited mechanical strains (<2%) can hardly meet the demands of loading conditions for most flexible and stretchable device applications. In this Article, inspired from Kirigami, the ancient Japanese art of paper cutting, we design and fabricate nanoscale Kirigami architectures of 2D layered MoS2on a soft substrate of polydimethylsiloxane (PDMS) using a top-down fabrication process. Results show that the Kirigami structures significantly improve the reversible stretchability of flexible 2D MoS2electronic devices, which is increased from 0.75% to ∼15%. This increase in flexibility is originated from a combination of multidimensional deformation capabilities from the nanoscale Kirigami architectures consisting of in-plane stretching and out-of-plane deformation. We further discover a new fundamental relationship of electrical conductance and large strain in MoS2Kirigami structures through both experimental work and finite element simulation. Results show that the electrical conductance of the stretchable MoS2Kirigami is closely related to its different stages of structural evolutions under strain: e.g., elastic stretching; then a combination of elastic stretching and out-of-plane buckling; and finally stretching and structural damage. This method provides a new opportunity to fabricate highly flexible and stretchable sensors and actuators using different types of 2D materials.

Creating a robust superhydrophobic surface on the conventional engineering materials at mass production is of great importance for a self-cleaning, anti-icing, nonwetting surface and low flow resistance in industrial applications. Herein, we report a roll-to-roll strategy to create durable and robust superhydrophobic surfaces with designed micro-/nanoscale hierarchical structures on many conventional engineering materials by combining electrical discharge machining and coating of carbon nanoparticles, followed by oil penetration and drying. The treated surface shows good superhydrophobic properties with a static water contact angle of 170 ± 2° and slide angle of 3 ± 1°. The treated surface also exhibits good resilience and maintains the performance after being tested in various harsh conditions, including water flushing for several days, sand abrasion, scratching with sandpapers, and corrosive solution. Significantly, the superhydrophobic surfaces also show a high efficiency of self-cleaning properties even after oil contamination during applications.

Porous CuO nanosheets were prepared on alumina tubes using a facile hydrothermal method, and their morphology, microstructure, and gas-sensing properties were investigated. The monoclinic CuO nanosheets had an average thickness of 62.5 nm and were embedded with numerous holes with diameters ranging from 5 to 17 nm. The porous CuO nanosheets were used to fabricate gas sensors to detect hydrogen sulfide (H2S) operating at room temperature. The sensor showed a good response sensitivity of 1.25 with respond/recovery times of 234 and 76 s, respectively, when tested with the H2S concentrations as low as 10 ppb. It also showed a remarkably high selectivity to the H2S, but only minor responses to other gases such as SO2, NO, NO2, H2, CO, and C2H5OH. The working principle of the porous CuO nanosheet based sensor to detect the H2S was identified to be the phase transition from semiconducting CuO to a metallic conducting CuS.

Co3O4is a good anode candidate for high-performance lithium ion batteries (LIBs), attributed to its high specific capacity. However, due to its poor conductivity and significant volume expansion during electrochemical reactions, electrochemical performance of Co3O4electrode is quite poor for practical applications. Co3O4electrodes with various micro-/nanostructures have been designed and synthesized but their performances are still unsatisfied. In this study, we propose and develop a novel sandwich type of Co3O4nanostructures by adjusting the ratio of ammonium fluoride and urea during hydrothermal reactions. When this sandwich-like Co3O4composite electrode was used as the electrode for LIBs, it retained an outstanding reversible capacity of 857 mAh g−1after 70 cycles at a current density of 500 mA g−1. When the current density was increased to 1000 mA g−1, the reversible capacity was kept at 865 mAh g−1with insignificant changes of its capacity.

In order to detect targets accurately in electromagnetic environment, a decomposition of time reversal operator method for detecting targets with uniform circular array (UCA) is proposed in this paper. In this method, the time reversal operator in UCA is obtained by the transmission functions of targets and antenna array. And then, the eigenvector decomposition of time reversal operator is performed. According to the eigenvectors and the transmission functions of targets and scanning points in the given targets area, the focusing values of all scanning points are calculated, and the direction of scanning points whose focusing values are far greater than others are the direction of targets. Finally, the simulations verify the effectiveness of the proposed method.

An orthogonal frequency division multiplexing (OFDM) is one of the effective techniques used in wireless communications. In OFDM systems, channel impairments due to multipath dispersive wireless channels can cause deep fades in wireless channels. The OFDM receiver also requires an accurate and computationally efficient channel state information when coherent detection is involved. Therefore, it needs a good robust estimation method of the channel in wireless communication for OFDM systems. And one of these channel estimation methods is minimum mean square error (MMSE) channel estimation. MMSE channel estimation one most used method in OFDM systems. In this work we enhanced robustness of MMSE channel estimation by using it in base of quasi-cyclic low density parity check (QC-LDPC) coded OFDM system.

The uniform circular planar array (UCA) has lots of advantages what linear arrays and other surface arrays don't have. However, because of its limitations, such as the high side-lobe level and low zero depth, the application on controlling array space direction by beam forming has been limited. Considering the characteristics of UCA, the receiving UCA's signal model is established in this paper. The influence factors of UCA's beam will be found out on the basis of theoretical derivation. A new method, that can improve the performance of the UCA beam directivity by amplitude weighting, is proposed in this paper. The theoretical derivation results and simulation results improved that the method could have a better ability in side-lobe suppression, and the new method could make the UCA to have a good application prospect.

In this paper, the performance of a new secured coded DS/FFH/TH hybrid spread spectrum system is evaluated. The evaluation is performed using four different spreading codes, which are long sequence, PN sequence, gold sequence, and chaotic sequence. The evaluation is performed under the effect of AWGN, Rayleigh fading, and frequency selective fading channels. The results show that the system presents a superior improvement in the performance especially in the severe fading conditions.

Droplet streaming and nebulization on a shear horizontal surface acoustic wave device made of 36o Y-cut LiTaO3 have been reported. The streaming pattern inside the droplet is sensitive to the droplet shape, the position to the interdigital transducer, and the aperture size. Droplet nebulization was enhanced with the increased aperture size of the driven IDTs and mainly occurred in horizontal direction along two lateral sides of the droplet, which is perpendicular to the designed SAW propagation direction. The atomization duration increases with the droplet size and decreases with the driven power at a given droplet size. The maximum rate of the atomization is ~0.2 μL/s at an applied power of 6.7 W. This provides a simple and effective method of the integration of both bio-liquid sensing and fluid manipulation on a single substrate for lab-on-chip biosensing platform.

Crystals of hexagonal tungsten oxides (hex‐WO3) have been synthesized using hydrothermal method at 150 °C, assisted by the capping reagent of ammonium tartrate (AT). The XRD and EDX results reveal that the lattice distortion exists in all the samples, possibly due to the defects and the intercalation of the residual sodium ions. Different crystal shapes including plate‐like, urchin‐like, and particle structures were obtained by varying concentration of AT and pH values in the precursor solution. Beside the absorption action of the ${\rm NH}_{4}^{ + } $and Na+ions, the capping effect can be reinforced by the hydrogen bonding from the tartrate groups in the crystallization process. The bandgap energies were modulated by the size of the nanostructured hex‐WO3crystals due to quantum confinement effect, which increases from 2.74 to 3.04 eV. Based on the analysis of the photoluminescence and X‐ray photoelectron spectroscopy, the enhancement of the blue emission of the nanocrystals is assigned as a result of a complex of the local intercalation of the residual sodium ions and the oxygen vacancies or defects.

Context-based Adaptive Variable Length Coding (CAVLC) as a new entropy coding algorithm has been introduced into H.264/AVC standard. Through analysing the CAVLC coding algorithm detailedly, the paper proposes an overlapping coverage storage method and a new stream merger method, and gives the specific implementation. This idea improves the structure and performance of the complex module and reduces the implementation complexity. The experimental results show that the proposed entropy encoder is correct, and the highest coding frequency of 81.70MHz can be achieved. Meanwhile all the hardware resource consumption is less than 2% of total hardware resources. The new entropy encoder achieves a better balance in system performance and resource consumption.

Surface acoustic wave (SAW) technology has been extensively explored for wireless communication, sensors, microfluidics, photonics, and quantum information processing. However, due to fabrication issues, the frequencies of SAW devices are typically limited to within a few gigahertz, which severely restricts their applications in 5G communication, precision sensing, photonics, and quantum control. To solve this critical problem, we propose a hybrid strategy that integrates a nanomanufacturing process (i.e., nanolithography) with a LiNbO3/SiO2/SiC heterostructure and successfully achieve a record-breaking frequency of about 44 GHz for SAW devices, in addition to large electromechanical coupling coefficients of up to 15.7%. We perform a theoretical analysis and identify the guided higher order wave modes generated on these slow-on-fast SAW platforms. To demonstrate the superior sensing performance of the proposed ultra-high-frequency SAW platforms, we perform micro-mass sensing and obtain an extremely high sensitivity of approximately 33151.9 MHz·mm2·μg−1, which is about 1011times higher than that of a conventional quartz crystal microbalance (QCM) and about 4000 times higher than that of a conventional SAW device with a frequency of 978 MHz.

Nanocrystalline TiN (or nc-TiN) has been imbedded in amorphous silicon nitride (a-SiNx)matrix to form a nanocomposite thin film (nc-TiN/a-SiNx) via magnetron sputtering deposition on silicon wafer. Two important effects of the Si3N4 sputtering target power on the formation of nc-TiN/a-SiNx have been studied: (1) Aside from forming a-SiNx in the matrix, Si atoms also imbed into TiN to form (Ti,Si)N solid solution crystallites. At low target power, the solid solution is substitutional. With increase of power, the amount of silicon “dissolved” in the TiN crystallite increases, and in the meantime, the interstitial components increase which is manifested in the increase in the TiN lattice parameter. (2) The crystallites have a preferred orientation varying with the deposition target power. As conveniently described by the coefficient of texture, the degree of preferred orientation along [111] direction decreases and finally tails off with increase of power. At the same time, the crystallites orient along [200] and [220] direction and eventually [220] direction dominants.

Antimony selenide (Sb2Se3) is regarded as one of the key alternative absorber materials for conventional thin film solar cells due to its excellent optical and electrical properties. Here, we proposed a Sb2Se3thin film solar cell fabricated using a two-step process magnetron sputtering followed by a post-selenization treatment, which enabled us to optimize the best quality of both the Sb2Se3thin film and the Sb2Se3/CdS heterojunction interface. By tuning the selenization parameters, a Sb2Se3thin film solar cell with high efficiency of 6.06% was achieved, the highest reported power conversion efficiency of sputtered Sb2Se3planar heterojunction solar cells. Moreover, our device presented an outstanding open circuit voltage (VOC) of 494 mV which is superior to those reported Sb2Se3solar cells. State and density of defects showed that proper selenization temperature could effectively passivate deep defects for the films and thus improve the device performance.

Shape memory alloys (SMAs) are amazing materials that, after being severely deformed, can spontaneously return to their original shape upon heating. These materials possess a number of desirable properties, namely, high actuation power or force, large recovery strain, pseudoelasticity (or superelasticity), good chemical resistance and biocompatibility, etc. These unique features have attracted much attention toward the potential applications of SMAs for military, medical, safety, and robotics applications. More recently, thin film SMAs have been recognized as a new type of promising and high performance material for micro-electro-mechanical system (MEMS) and biological applications.

Using a first-principles pseudopotential method, we studied an effective n-type doping strategy through codoping SiAl-XN(X = F, Cl, Br, and I) in aluminum nitride. Results revealed that the donor ionization energy of the SiAl-XNcomplex is much lower than that of the corresponding isolated SiAlimpurity. Theoretically obtained ?(+/0) ionization energies are all near the conduction band minimum (CBM), which is only 1.4 meV below the CBM of the SiAl-FNpair. The low ?(+/0) ionization energy of the SiAl-XNcomplex can be explained by the combined repulsion between the X element (X = F, Cl, Br, and I)- and Si donor-induced levels.