•The self-powered sensor demonstrates ultra-high stretchability (> 300%) and ultra-low detection limit (0.2 mg).•Helix electrohydrodynamic printing is proposed to fabricate the sensor in a low-cost, large-scale and aligned manner.•The sensor can simultaneously measure multiply physical quantities of the own status and extra mechanical stimuli.Hyper-stretchable self-powered sensors with high sensitivity and excellent stability using low-cost, printable, organic nanomaterials are attractive for immense applications. Here we present self-similar piezoelectric nano/microfibers for a hyper-stretchable self-powered sensor that demonstrates high stretchability (> 300%), low detection limit (0.2 mg), and excellent durability (> 1400 times at strain 150%). A proposed helix electrohydrodynamic printing technique (HE-Printing) in combination with in-surface self-organized buckling is used to fabricate aligned self-similar poly[vinylidene fluoride] (PVDF) nano/microfibers with in situ mechanical stretch and electrical poling to produce excellent piezoelectric properties. The hyper-stretchable self-powered sensors have shown repeatable and consistent electrical outputs with detection limit an order of magnitude smaller than the other stretchable sensors. Additionally, such sensors can simultaneously measure the own status and the extra multiply physical quantities, such as lateral pressure, impulse rate and applied strain. The high sensitivity can be further utilized to remotely detect human motion in addition to sensing a piece of paper with 1 mm × 1 mm. Further the fiber-based sensors can avoid the catastrophic collapse or wrinkling of serpentine film-based structure during stretching.Download high-res image (328KB)Download full-size image

Micropatterned ZnO nanorod arrays were fabricated by the mechanoelectrospinning-assisted direct-writing process and the hydrothermal growth process, and utilized as gas sensors that exhibited excellent Ohmic behavior and sensitivity response to oxidizing gas NO2 at low concentrations (1–100 ppm).

Three-dimensional (3D) shape matching finds a wide application in manufacturing industry, such as surface inspection, workpiece localization and reverse engineering. In this paper, a new optimization method is proposed for non-rigid shape matching modeling and solving. The point-tangent distance function is used to construct a nonlinear least-squares model for anisotropic non-rigid shape matching, from which 9 variables with respect to non-rigid matching parameters are computed simultaneously by solving a linear system. In order to strengthen the non-rigid matching robustness to potential outliers, the optimization model is improved by an iteratively reweighted method. The typical characteristic is to weaken the influence of outliers during iterations. Finally, experiments are carried out to evaluate and analyze the proposed method, including matching accuracy, efficiency and robustness. Shape matching is also an important task in computer vision (such as face registration and object recognition), and the proposed method can find its application in this field.

Highlights•The paper presents a large-area stretchable wireless LC strain sensor, based on the concept of self-similar design.•The designed wireless sensors can be stretched up to 40%, as demonstrated by finite element modeling and experiment results.•The wireless strain sensor with self-similar structured coil incorporating variable inductance has been implemented to monitor the strain of artificial skin.•Strain response of the stretchable wireless sensor has been characterized by experiments, and demonstrates high strain responsivity about 33.7 MHz/10%.Stretchable sensors provide a foundation for applications that exceed the scope of conventional device technologies due to their unique capacity to integrate with soft materials and curvilinear surfaces. This article presents the implementation and characterization of a large-area stretchable wireless RF strain sensor, operating at around 760 MHz, based on the concept of self-similar design. It has an electrical LC resonant circuit formed by a self-similar inductor coil and a capacitor to facilitate passive wireless sensor. The inductance of the wireless sensor varies with the elongation of the PDMS substrate, so is the resonance frequency of the sensor that is detected using an external coil linked to a vector network analyzer. Finite element modeling was used in combination with experimental verification to demonstrate that the wireless strain sensor with 300 μm width can be stretched up to 40%. Self-similar structured coil incorporating variable inductance has been implemented to monitor the strain of artificial skin. Strain response of the stretchable wireless sensor has been characterized by experiments, and demonstrates high strain responsivity about 33.7 MHz/10%, which confirms the feasibility of strain sensing for biomedical and wearable applications.

Piezoelectric structures, in forms that allow mere in-surface deformations under large strains, are attractive for bio-integrated systems. Here, mechano-electrospinning (MES) is presented to direct-write straight nanofibers of polyvinylidene fluoride onto a prestrained poly(dimethylsiloxane) (PDMS) substrate, to position and polarize a piezoelectric nanofiber array in one-step. Wrinkled/non-wrinkled buckling modes are found when the substrates are released, and the morphology of the direct-written fiber proved the key to determine the buckling modes, which can be tuned precisely by MES parameters. The non-wrinkled, stretchable piezoelectric devices with a highly synchronized serpentine fiber array exhibit their in-surface deformation and stable piezoelectric performance up the failure strain of PDMS (∼110% in our study), which may be used as stretchable sensors and energy converters/providers.

This paper describes the design and characterization of microfluidic serpentine antennas with reversible stretchability and designed mechanical frequency modulation (FM). The microfluidic antennas are designed based on the Poisson's ratio of the elastomer in which the liquid alloy antenna is embedded, to controllably decrease, stabilize or increase its resonance frequency when being stretched. Finite element modelling was used in combination with experimental verification to investigate the effects of substrate dimensions and antenna aspect ratios on the FM sensitivity to uniaxial stretching. It could be designed within the range of −1.2 to 0.6 GHz per 100% stretch. When the aspect ratio of the serpentine antenna is between 1.0 and 1.5, the resonance frequency is stable under stretching, bending, and twisting. The presented microfluidic serpentine antenna design could be utilized in the field of wireless mobile communication for the design of wearable electronics, with a stable resonance frequency under dynamic applied strain up to 50%.

This paper presents an efficient analytical solution strategy to determine the adhesive stresses in balanced and unbalanced adhesively bonded joints with mixed force loading and/or displacement boundary conditions. The adhesive stresses are expressed in terms of geometrical dimensions and material properties, combined with integration constants obtained numerically. The model is successfully applied for the analysis of various types of joints, including balanced and unbalanced stiffened plate/joint, single-strap joint, and single-lap joint. In all such cases, the linear equation sets are supplied to determine the integration constants in the final stress expressions. The analytical predictions agree well with the finite element results for adhesive stresses. This proposed model can be extended conveniently to predict the mechanical behavior of similar bonded structures such as composite laminates, electronics packaging, and flexible electronics structures.

The electrohydrodynamic (EHD) direct-writing technique can be used to print solid/liquid straight/serpentine nanofibers onto a large-area substrate, in a direct, continuous, and controllable manner. It is a high-efficiency and cost-effective solution-processable technique to satisfy increasing demands of large-area micro/nano-manufacturing. It is ground-breaking to direct-write sub-100 nm fibers on a rigid/flexible substrate using organic materials. A comprehensive review is presented on the research and developments related to the EHD direct-writing technique and print heads. Many developments have been presented to improve the controllability of the electrospun fibers to form high-resolution patterns and devices. EHD direct-writing is characterized by its non-contact, additive and reproducible processing, high resolution, and compatibility with organic materials. It combines dip-pen, inkjet, and electrospinning by providing the feasibility of controllable electrospinning for sub-100 nm nanofabrication, and overcomes the drawbacks of conventional electron-beam lithography, which is relatively slow, complicated and expensive.

This paper presents a mechanoelectrospinning (MES)-assisted surface-tension driven self-organization to provide a possible route towards inexpensive generation of large-scale ordered microarrays in a controllable manner. To control the self-organization driven by surface tension and Plateau–Rayleigh instability, finite length effects are utilized to manipulate the self-organizing processes and adjust the competition between nucleation and free surface instability. We introduce fine ribbon-lattices to determine the boundary conditions of ribbons to make use of the finite length effects. The ribbon-lattices are electrodeposited precisely by MES, borrowing ideas from the “Chinese kite”, by involving the mechanical drawing force and the electric field force. Then the samples are transferred to a moisture-rich environment in which the ribbons absorb water vapour and become liquid lines. Surface instability emerges and leads the liquid lines to controllable self-organization. We uncover the controllable area to manipulate the self-organization behavior. A uniform or hierarchical microarray with a specific position, gap and droplet-size can be generated in a continuously tunable manner. This bottom-up method provides a digital approach for the fabrication of large-scale ordered microarrays and micropatterns.

Automatic localization, aligning the measured points with the design model, is a basic task in free-form surface inspection. The main difficulty of current localization algorithms is how to define effective distance function and localization reliability index. This paper proposes a new method of calculating motion parameters and evaluating localization reliability. First, improved modified coefficient is defined and applied to weighted-iteration distance function, which better approximates the point-to-surface closest distance. It can control the contribution ratios of different measured points by considering the curvature feature and iterative residual. Second, the mapping relationship between localization error and geometric error is analyzed, from which a Lyapunov-test statistic is derived to define a frame-independence index. Then, the determination of localization reliability changes into a supposition examination problem. This can avoid rejecting correct motion parameters, which exists in the traditional judgment of absolute root-mean-square distance. In addition, two test experiments are implemented to demonstrate the proposed localization algorithm.

In industry, the defective point data often make most surface reconstruction methods suffer from inherent problems that some specific aided information is difficult to obtain. To solve the problem, a novel implicit reconstruction method without any such information is proposed. This approach extends morphological operations into 3D space and provides an improved procedure to construct off-set gradient functions for indirect approximation. By this method, the dual relative functions guarantee a minimal crust surrounding the point data. They can generate a smooth and watertight resulting surface, filling holes and merging overlapping samples reasonably. Compared with other existing methods, the proposed method is better suited to handle defective point clouds in a convenient and efficient manner. The feasibility and effectiveness of the method are demonstrated through a series of practical examples.

Inkjet printing, known as digital writing technique, can directly deposit functional materials to form pattern onto substrate. This paper provides an overview of inkjet printing technologies for flexible electronics. Firstly, we highlight materials challenges in implementing flexible devices into practical application, especially for inkjet printing process. Then the micro/nano-patterning technologies of inkjet printing are discussed, including conventional inkjet printing techniques and electrohydrodynamic printing techniques. Thirdly, the related equipments on inkjet printing are shown. Finally, challenges for its future development are also discussed. The main purpose of the work is to condense the basic knowledge and highlight the challenges associated with the burgeoning and exciting field of inkjet printing for flexible electronics.

This paper presents a new approach to predict the loads acting on the shield periphery during excavation. This approach not only represents the dynamic variation of loads, but also can be used to calculate the rectification loads for moving shield backward to the planned alignment. Firstly, the ground reaction curves are adopted to calculate the earth pressure. Particularly, the ground displacement is considered to be induced by two kinds of motion: tunneling-induced ground loss and snake-like motion of shield movement. Then, optimization criteria for load balance, minimum potential energy, shield behavior, and ground settlement have been considered. Finally, basic example is carried out in a straight alignment for the single layer of homogeneous ground. The validity is discussed by comparing the optimization model with numerical simulation, and the results confirm that the proposed approach is reasonable enough to predict the distributed loads acting on the shield periphery.

•Presenting a mechanical model to analyze chip-array on stretched substrate.•Supplying a precise solution to get ERR and adhesive stresses in adherend.•Obtaining geometrical and material effects on the chip-on-substrate structure.•Showing the interaction between the chips on the interfacial peeling.The paper presents a mechanical model for predicting the cohesive failure of a periodic array of integrated circuit (IC) chips adhesively bonded to a stretched substrate. A unit cell of the layered structure consisting of the IC chips, adhesive layer, and substrate is modeled as an assembly of two elastic Timoshenko beams, representing the chip and substrate, connected by an elastic interface, representing the adhesive. Accordingly, the stresses and energy release rate (ERR) in the adhesive layer – responsible for the premature cracking of the adhesive and debonding of the IC chips – are identified with the corresponding quantities computed for the elastic interface. Expressions for the adhesive stresses and ERR are given in terms of geometrical dimensions and material properties, combined with integration constants obtained numerically via the multi-segment analysis method. For comparison, the stresses in the adhesive are also computed based on a finite element model, and the ERR is evaluated using the virtual crack-closure technique (VCCT). The analytical predictions and numerical results match fairly well, considering the effects of key factors, such as the distance between adjacent chips, the chip size, the material properties of adhesive and substrate. The interaction between the chips is shown to have relevant effects on the adhesive stresses. In particular, only the mode II contributes to the ERR which increases with the ratio of the chip size to the distance between the chips and with the compliance of the adhesive and substrate layers.

Micropatterned ZnO nanorod arrays were fabricated by the mechanoelectrospinning-assisted direct-writing process and the hydrothermal growth process, and utilized as gas sensors that exhibited excellent Ohmic behavior and sensitivity response to oxidizing gas NO2 at low concentrations (1–100 ppm).