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This paper presents the multi-directional vibration behaviors of buckled viscoelastic nanoribbons based on the nonlocal theory containing scale effect, where the Lagrange’s motion equation is introduced to construct the nonlocal viscoelastic governing equation. The first-order transverse vibration mode and the first-order longitudinal vibration mode are considered to study the influences of the scale characteristics on the natural frequency of two different vibration modes. The Kelvin–Voigt model is used to characterize the material viscoelastic property, and an analytical method is presented to solve the nonlocal viscoelastic vibration behaviors of the buckled nanoribbon containing scale effect. Results show that the scale effect could decrease the natural frequency of buckled nanoribbon with viscoelastic behavior, as well as the half bandwidth during forced vibration. The presented results may be taken as a useful reference for the fabrications and applications of buckled nanostructures-based sensors & actuators, electronics, energy harvesters, etc.

Considering the potential applications of the buckled structures as nanomechanical mass sensors, this paper presents an analytical method to solve the frequency shifts of the first-order transverse and longitudinal vibration modes when a mass attaches on the surface of a buckled nanoribbon based on the nonlocal elastic theory and the Lagrange's motion equation. The first-order transverse and longitudinal vibration modes of the buckled nanoribbon are introduced. A comparison between the analytic solution and the finite element analysis (FEA) is presented. Then, the effects of the compressive strain, the magnitude and location of attached mass, the nonlocal parameter, and attached mass numbers on the frequency shifts are presented. From example calculations, it is seen that the magnitude of attached mass increases the frequency shifts of the first-order modes, except the first-order transverse vibration mode at the location Z1=0. The frequency shifts for the first-order transverse and longitudinal vibration modes are different, and could be used as important principles in mass sensing. What's more, the compressive strain and the nonlocal parameter play significant roles on the sensing process of the buckled nanoribbon. The results could serve as useful references for the fabrications and applications of buckled structures based nanomechanical mass sensors.

Many forms of stretchable electronic systems incorporate planar, filamentary serpentine structures to realize high levels of stretchability in ways that allow the use of high performance of inorganic functional materials. Recent advances in mechanics-guided, deterministic three-dimensional (3D) assembly provide routes to transform these traditional, two-dimensional (2D) serpentine layouts into 3D architectures, with significantly improved mechanics and potential for applications in energy harvesters, pressure sensors, soft robotics, biomedical devices and other classes of technologies. One challenge is that, by comparison to other geometries, the relatively low bending stiffnesses of the serpentine structures create difficulties in overcoming the interfacial adhesion energy to allow delamination from the underlying elastomeric substrate, as an essential aspect of the assembly process. An additional complication is that many of the functional materials widely used in stretchable electronics have a low strain threshold for failure such that damage can occur during buckling-induced assembly. Therefore, a clear understanding of the mechanics of buckled serpentine structures is essential to their design, fabrication and application. Through theoretical modeling and finite element analysis, we present models for the phase diagram of buckled states and the maximum strain in the globally-buckled serpentine structures. The analysis yield formulae in concise and explicit forms, clearly showing the effect of geometry/material parameters and the prestrain. The results can be used in designing buckled serpentine structures to ensure their compatibility with the mechanics-guided, deterministic 3D assembly and facilitate their applications in stretchable electronics.

Three-dimensional (3D) serpentine mesostructures assembled by mechanics-guided, deterministic 3D assembly have potential applications in energy harvesting, mechanical sensing, and soft robotics. One limitation is that the serpentine structures are required to have sufficient bending stiffness such that they can overcome the adhesion with the underlying substrate to fully buckle into the 3D shape (global buckling). This note introduces the use of cellular substrate in place of conventional homogeneous substrate to reduce the adhesion energy and therefore ease the above limitation. A theoretical model based on energetic analysis suggests that cellular substrates significantly enlarge the design space of global buckling. Numerical examples show that the enlarged design space enables 3D serpentine structures with reduced maximum strains and resonant frequencies, which offers more possibilities for their potential applications.

Flexible, large-area tactile sensors capable of simultaneously measuring in real time both the normal pressure and the tangential shear stress have many important applications, including those in artificial skin for uses in robotics, medicine and rehabilitation. Previously reported sensors exhibit responses to pressure and shear stress that are coupled, mainly due to nonlinearities, in ways that can be difficult to separate. A recently developed sensor based on techniques in mechanics-guided, deterministic three-dimensional (3D) assembly provides a route to decouple these responses such that the pressure and the shear stress can be determined explicitly and linearly from the change in resistance of multiple strain gauges integrated at carefully selected locations on the 3D structure. A clear understanding of the mechanics of this 3D structure-based sensor is essential for its optimization and application. Here analytic models, validated by finite element analysis, are presented for the deformation of such structures under pressure and shear stress. Analytic solutions in concise forms are derived for a buckled wavy ribbon encapsulated in a soft elastomer, which reveal the effect of the geometry and material parameters on the sensitivity, therefore establishing design guidelines for these devices.

To better understand the characterization and secondary organic aerosol (SOA) formation of volatile organic compounds (VOCs) during non-haze and haze days, ambient VOCs were continuously measured by a vehicle-mounted online thermal desorption system coupled with a gas chromatography–mass spectrometry (TD–GC/MS) system in Shanghai, China. The average concentrations of VOCs in haze episodes (193.2 μg m−3) were almost 50% higher than in non-haze periods (130.8 μg m−3). VOC concentrations exhibited a bi-modal pattern in the morning and evening rush hour periods on both non-haze and haze days. The ratios of toluene to benzene (T/B) and m,p-xylene to ethylbenzene (X/E) indicated that VOCs were aged air mass transported from nearby areas. The estimated SOA yields were 12.6 ± 5.3 and 16.7 ± 6.7 μg m−3 for non-haze and haze days, respectively, accounting for 9.6 and 8.7% of the corresponding PM2.5 concentrations, which were slightly underestimated. VOCs–sensitivity (VOCs–S) based on a PM2.5-dependent model was used to investigate the variation between VOCs and PM2.5 concentrations in the morning rush hour. It was found that VOCs were more sensitive to PM2.5 on clean days than during periods of heavy particulate pollution. VOCs–sensitivity was significantly correlated with the ratio of specific PM2.5 to background PM2.5, with a simulated equation of y = 0.84x−0.62 (r 2 = 0.93, p

Sensors that reproduce the complex characteristics of cutaneous receptors in the skin have important potential in the context of artificial systems for controlled interactions with the physical environment. Multimodal responses with high sensitivity and wide dynamic range are essential for many such applications. This report introduces a simple, three-dimensional type of microelectromechanical sensor that incorporates monocrystalline silicon nanomembranes as piezoresistive elements in a configuration that enables separate, simultaneous measurements of multiple mechanical stimuli, such as normal force, shear force, and bending, along with temperature. The technology provides high sensitivity measurements with millisecond response times, as supported by quantitative simulations. The fabrication and assembly processes allow scalable production of interconnected arrays of such devices with capabilities in spatiotemporal mapping. Integration with wireless data recording and transmission electronics allows operation with standard consumer devices.

Development of schemes to form complex three-dimensional (3D) mesostructures in functional materials is a topic of broad interest, thanks to the ubiquitous applications across a diversity of technologies. Recently established schemes in the mechanically guided 3D assembly allow deterministic transformation of two-dimensional structures into sophisticated 3D architectures by controlled compressive buckling resulted from strain release of prestretched elastomer substrates. Existing studies mostly exploited supporting substrates made of homogeneous elastomeric material with uniform thickness, which produces relatively uniform strain field to drive the 3D assembly, thus posing limitations to the geometric diversity of resultant 3D mesostructures. To offer nonuniform strains with desired spatial distributions in the 3D assembly, this paper introduces a versatile set of concepts in the design of engineered substrates with heterogeneous integration of materials of different moduli. Such heterogeneous, deformable substrates can achieve large strain gradients and efficient strain isolation/magnification, which are difficult to realize using the previously reported strategies. Theoretical and experimental studies on the underlying mechanics offer a viable route to the design of heterogeneous, deformable substrates to yield favorable strain fields. A broad collection of 3D mesostructures and associated heterogeneous substrates is fabricated and demonstrated, including examples that resemble windmills, scorpions, and manta rays and those that have application potentials in tunable inductors and vibrational microsystems.

Vibrational resonances of microelectromechanical systems (MEMS) can serve as means for assessing physical properties of ultrathin coatings in sensors and analytical platforms. Most such technologies exist in largely two-dimensional configurations with a limited total number of accessible vibration modes and modal displacements, thereby placing constraints on design options and operational capabilities. This study presents a set of concepts in three-dimensional (3D) microscale platforms with vibrational resonances excited by Lorentz-force actuation for purposes of measuring properties of thin-film coatings. Nanoscale films including photodefinable epoxy, cresol novolak resin, and polymer brush with thicknesses as small as 270 nm serve as the test vehicles for demonstrating the advantages of these 3D MEMS for detection of multiple physical properties, such as modulus and density, within a single polymer sample. The stability and reusability of the structure are demonstrated through multiple measurements of polymer samples using a single platform, and via integration with thermal actuators, the temperature-dependent physical properties of polymer films are assessed. Numerical modeling also suggests the potential for characterization of anisotropic mechanical properties in single or multilayer films. The findings establish unusual opportunities for interrogation of the physical properties of polymers through advanced MEMS design.

Thermoelectric coils for energy harvesting are fabricated in miniature flexible devices., With accelerating trends in miniaturization of semiconductor devices, techniques for energy harvesting become increasingly important, especially in wearable technologies and sensors for the internet of things. Although thermoelectric systems have many attractive attributes in this context, maintaining large temperature differences across the device terminals and achieving low–thermal impedance interfaces to the surrounding environment become increasingly difficult to achieve as the characteristic dimensions decrease. Here, we propose and demonstrate an architectural solution to this problem, where thin-film active materials integrate into compliant, open three-dimensional (3D) forms. This approach not only enables efficient thermal impedance matching but also multiplies the heat flow through the harvester, thereby increasing the efficiencies for power conversion. Interconnected arrays of 3D thermoelectric coils built using microscale ribbons of monocrystalline silicon as the active material demonstrate these concepts. Quantitative measurements and simulations establish the basic operating principles and the key design features. The results suggest a scalable strategy for deploying hard thermoelectric thin-film materials in harvesters that can integrate effectively with soft materials systems, including those of the human body.

Catastrophic failure of materials and structures due to unstable crack growth could be prevented if fracture toughness could be enhanced at will through structural design, but how can this be possible if fracture toughness is a material constant related to energy dissipation in the vicinity of a propagating crack tip. Here we draw inspiration from the deformation behavior of biomolecules in load bearing biological materials, which have been evolved with a large extensibility and a high breaking strength beyond their elastic limit, and introduce an effective biomimetic strategy to enhance fracture toughness of a structure through an intrinsic to extrinsic (ITE) transition. In the ITE transition, toughness starts as an intrinsic parameter at the basic material level, but by designing a protein-like effective stress–strain behavior the toughness at the system level becomes an extrinsic parameter that increases with the system size without bound. This phenomenon is demonstrated through a combination of numerical simulations, analytic modeling and experiments, and leads to a biomimetic strategy which can be broadly adopted to enhance fracture toughness in engineering systems.

Recently, the integration of electronic elements with cellular scaffolds has brought forth the ability to monitor and control tissue function actively by using flexible free-standing two-dimensional (2D) systems. Capabilities for electrically probing complex, physicochemical and biological three-dimensional (3D) microenvironments demand, however, 3D electronic scaffolds with well-controlled geometries and functional-component distributions. This work presents the development of flexible 3D electronic scaffolds with precisely defined dimensions and microelectrode configurations formed using a process that relies on geometric transformation of 2D precursors by compressive buckling. It demonstrates a capability to fabricate these constructs in diverse 3D architectures and/or electrode distributions aimed at achieving an enhanced level of control and regulation of tissue function relatively to that of other approaches. In addition, this work presents the integration of these 3D electronic scaffolds within engineered 3D cardiac tissues, for monitoring of tissue function, controlling tissue contraction through electrical stimulation, and initiating on-demand, local release of drugs, each through well-defined volumetric spaces. These ideas provide opportunities in fields ranging from in vitro drug development to in vivo tissue repair and many others.

Complex, three-dimensional (3D) mesostructures that incorporate advanced, mechanically active materials are of broad, growing interest for their potential use in many emerging systems. The technology implications range from precision-sensing microelectromechanical systems, to tissue scaffolds that exploit the principles of mechanobiology, to mechanical energy harvesters that support broad bandwidth operation. The work presented here introduces strategies in guided assembly and heterogeneous materials integration as routes to complex, 3D microscale mechanical frameworks that incorporatemultiple, independently addressable piezoelectric thin-film actuators for vibratory excitation and precise control. The approach combines transfer printing as a scheme formaterials integrationwith structural buckling as ameans for 2D-to-3D geometric transformation, for designs that range from simple, symmetric layouts to complex, hierarchical configurations, on planar or curvilinear surfaces. Systematic experimental and computational studies reveal the underlying characteristics and capabilities, including selective excitation of targeted vibrational modes for simultaneous measurements of viscosity and density of surrounding fluids. The results serve as the foundations for unusual classes of mechanically active 3D mesostructures with unique functions relevant to biosensing, mechanobiology, energy harvesting, and others.

Recent progress in stretchable forms of inorganic electronic systems has established a route to new classes of devices, with particularly unique capabilities in functional biointerfaces, because of their mechanical and geometrical compatibility with human tissues and organs. A reliable approach to physically and chemically protect the electronic components and interconnects is indispensable for practical applications. Although recent reports describe various options in soft, solid encapsulation, the development of approaches that do not significantly reduce the stretchability remains an area of continued focus. Herein, a generic, soft encapsulation strategy is reported, which is applicable to a wide range of stretchable interconnect designs, including those based on two-dimensional (2D) serpentine configurations, 2D fractal-inspired patterns, and 3D helical configurations. This strategy forms the encapsulation while the system is in a prestrained state, in contrast to the traditional approach that involves the strain-free configuration. A systematic comparison reveals that substantial enhancements (e.g., ≈6.0 times for 2D serpentine, ≈4.0 times for 2D fractal, and ≈2.6 times for 3D helical) in the stretchability can be achieved through use of the proposed strategy. Demonstrated applications in highly stretchable light-emitting diodes systems that can be mounted onto complex curvilinear surfaces illustrate the general capabilities in functional device systems.

The aim of this study was to determine the spatial distribution, potential risks and sources of seven heavy metals in sediments of the Yangtze River Estuary. Analyses of 55 sediment samples revealed that the distributions of metals within the YRE were determined by the combined effects of their sources, hydrodynamic conditions, pH and Eh. According to the geoaccumulation index (Igeo) and sediment quality guidelines, Pb, Cd and Cr were present at low levels of pollution, with Cd posing the largest ecological risk. Positive Factor Matrix (PMF) results indicated that Hg, Zn, As, Pb and Cr mainly originated from natural geological background sources, while Cu originated from anthropogenic activities and atmospheric deposition was the source of Cd. These three sources contributed to 53.0%, 32.8% and 14.2%, respectively of total heavy metal concentrations. These results suggest that reducing the emission of Cd would promote a reduction of potential risks in sediments of the YRE.

Precise, quantitative measurements of the thermal properties of human skin can yield insights into thermoregulatory function, hydration, blood perfusion, wound healing, and other parameters of clinical interest. The need for wired power supply systems and data communication hardware limits, however, practical applicability of existing devices designed for measurements of this type. Here, a set of advanced materials, mechanics designs, integration schemes, and wireless circuits is reported as the basis for wireless, battery-free sensors that softly interface to the skin to enable precise measurements of its temperature and thermal transport properties. Calibration processes connect these parameters to the hydration state of the skin, the dynamics of near-surface flow through blood vessels and implanted catheters, and to recovery processes following trauma. Systematic engineering studies yield quantitative metrics in precision and reliability in real-world conditions. Evaluations on five human subjects demonstrate the capabilities in measurements of skin hydration and injury, including examples of continuous wear and monitoring over a period of 1 week, without disrupting natural daily activities.