Search

ObjectivesThis study investigated the stage-specific and location-specific deposition and characteristics of minerals in human osteoarthritis (OA) cartilages via multiple nano-analytical technologies.MethodsNormal and OA cartilages were serially sectioned for micro-CT, scanning electron microscopy with energy dispersive X-ray spectroscopy, micro-Raman spectroscopy, focused ion beam scanning electron microscopy, high-resolution electron energy loss spectrometry with transmission electron microscopy, nanoindentation and atomic force microscopy to analyse the structural, compositional and mechanical properties of cartilage in OA progression.ResultsWe found that OA progressed by both top-down calcification at the joint surface and bottom-up calcification at the osteochondral interface. The top-down calcification process started with spherical mineral particle formation in the joint surface during early-stage OA (OA-E), followed by fibre formation and densely packed material transformation deep into the cartilage during advanced-stage OA (OA-A). The bottom-up calcification in OA-E started when an excessive layer of calcified tissue formed above the original calcified cartilage, exhibiting a calcified sandwich structure. Over time, the original and upper layers of calcified cartilage fused, which thickened the calcified cartilage region and disrupted the cartilage structure. During OA-E, the calcified cartilage was hypermineralised, containing stiffer carbonated hydroxyapatite (HAp). During OA-A, it was hypomineralised and contained softer HAp. This discrepancy may be attributed to matrix vesicle nucleation during OA-E and carbonate cores during OA-A.ConclusionsThis work refines our current understanding of the mechanism underlying OA progression and provides the foothold for potential therapeutic targeting strategies once the location-specific cartilage calcification features in OA are established.

While osteoarthritis (OA) involves pathological cartilage calcification, its progress is not fully understood. Here, we investigated the temporal-spatial mechanism of mineral transformation pathways in human OA cartilage tissues via multiple imaging tomography at nanoscale resolution. We found that OA progressed by both top-down calcification at the joint surface and bottom-up calcification at the osteochondral transition zone. The top-down calcification process started with the formation of spherical mineral particles in the joint surface during early-stage OA (OA-E), followed by fiber formation and compact material transformation deep into the cartilage during advanced-stage OA (OA-A). The bottom-up calcification in OA-E started when an excessive layer of calcified tissue formed above the original calcified cartilage; there was a 100–200 µm thick layer of noncalcified cartilage tissues between the original and new calcified layers, forming a sandwich structure at the osteochondral transition zone. The abnormal calcified layer was located at the edge of the tidemark that contained acidic components (-COOH) that can chelate Ca2+ and bind with PO43− to form crystals. It is likely that this local mineralization process was assisted by nearby hypertrophic chondrocytes. With OA progression, the original and upper layers of calcified cartilage fused, which thickened the calcified cartilage region and disrupted cartilage. During OA-E, the calcified cartilage was hypermineralized and contained stiffer carbonated hydroxyapatite (Hap); during OA-A, it was hypomineralized and contained softer HAp. Distinct mechanisms, such as nucleation on matrix vesicles in OA-E and carbonate cores in OA-A, may be involved. These findings illustrated that the progress of pathological calcification was location-specific at the joint surface and osteochondral interface in the early and advanced stages of OA. It is very important to understand the mechanism of OA progression and develop better strategies for future OA treatment on targeting cartilage pathological calcification

The load-carrying capacity of the symmetrical and asymmetrical sandwich beams, under a quasi-static central load, is investigated in this paper. Three collapse mechanisms such as face yield, core shear and indentation are considered for symmetrical sandwich beams. Core shear mechanism is taken into account for fully clamped asymmetrical sandwich beams. Continuity equations are established by simple ‘equal area’ method for the postyield behavior of the sandwich beams in face yield and core shear mechanisms at different boundary conditions. In indentation mechanism theoretical model, the effect of the local denting on the large deflection of the sandwich beam is taken into account. Then, finite element simulations are carried out to verify the validity of the proposed analysis, and a good agreement is presented. It is shown that in the core shear mechanism under fully clamped condition, no plateau phase is presented. The effect of the core thickness on the response of the symmetrical beams is discussed in detail. For asymmetry beams in core shear mechanism under fully clamped condition, the effect of the asymmetric factor (strength or thickness) for face-sheets on the load–deflection behavior of the postyield beams can be neglected, if the sum of the strength or thickness of the face sheets is constant.

Graphene aerogels have recently become one of the most attractive carbon metamaterials due to their unique properties such as ultralight weight, large specific surface area, high electrical conductivity and excellent compressibility. However, there still remain some challenges to efficiently fabricate superelastic graphene aerogels through the facile and cost-effective processes and assemble them into a well-designed unity for wider applications. Here we show superelastic grouped graphene/carbon nanotube (CNT) aerogel spheres (GCSs) for intelligent piezoresistive sensors by wet-spinning technique. With the synergistic effect of graphene and CNTs, such grouped GCSs with core-shell structure annealed at 1000 °C show excellent elasticity and fatigue resistance, which are comparable to those of graphene spheres annealed at 2500 °C. The grouped GCSs can fully recover after compression at 95% strain, and still retain 70% of maximum stress with negligible plastic deformation and structural damage even after 1000 cycles at 70% strain. Significantly, the robust, superelastic and conductive GCSs are suitable for pressure sensors, exhibiting extraordinary cycling stability and long working life. Due to intact and tough shells, the small-scale GCSs with high sensitivity can be easily assembled into stretchable piezoresistive sensors with pixel arrays for spatial pressure distribution measurement and object shape recognition.

Graphene-silicon Schottky junction (GSJ) which has the potential for large-scale manufacturing and integration can bring new opportunities to Schottky solar cells for photovoltaic (PV) power conversion. However, the essential power conversion limitation for these devices lies in the small open-circuit voltage ($V_{oc}$), which depends on the Schottky barrier height (SBH). In this study, we introduce an electromechanical method based on the flexoelectric effect to enhance the PV efficiency in GSJ. By atomic force microscope (AFM) tip-based indentation and in situ current measurement, the current-voltage (I-V) responses under flexoelectric strain gradient are obtained. The $V_{oc}$ is observed to increase for up to 20$\%$, leading to an evident improvement of the power conversion efficiency. Our studies suggest that strain gradient may offer unprecedented opportunities for the development of GSJ based flexo-photovoltaic applications., 7 pages, 4 figures. The following article has been accepted by Applied Physics Letters

Designing tunable resonators is of practical importance in active/adaptive sound generation, noise control, vibration isolation and damping. In this paper, we propose to exploit the biasing fields (induced by internal pressure and radial electric voltage) to tune the three-dimensional and small-amplitude free vibration of a thick-walled soft electro-active (SEA) spherical balloon. The incompressible isotropic SEA balloon is characterized by both neo-Hookean and Gent ideal dielectric models. The equations governing small-amplitude vibrations under inhomogeneous biasing fields can be linearized and solved in spherical coordinates using the state–space formalism, which establishes two separate transfer relations correlating the state vectors at the inner surface with those at the outer surface of the SEA balloon. By imposing the mechanical and electric boundary conditions, two separate analytical frequency equations are derived, which characterize two independent classes of vibration for torsional and spheroidal modes, respectively. Numerical examples are finally conducted to validate the theoretical derivation as well as to investigate the effects of both radial electric voltage and internal pressure on the resonant frequency of the SEA balloon. The reported analytical solution is truly and fully three-dimensional, covering from the purely radial breathing mode to torsional mode to any general spheroidal mode, and hence provides a more accurate prediction of the vibration characteristics of tunable resonant devices that incorporate the SEA spherical balloon as the tuning element.

Wearable electronics are demanded in many applications. However, traditional semiconductor manufacturing is based on bulk wafers and it is very challenging to mass-producing flexible electronics. In the last decades, many technologies have been developed to address this challenge such as using flexible materials instead of silicon, using silicon on insulator surface and peeling off the top thin silicon films. Here, we review another promising technology for achieving flexible electronics – controlled spalling, which uses mechanical stress to facilitate peeling off thin film directly off a bulk silicon substrate. This technology is CMOS-compatible (where CMOS is complementary metal-oxide-semiconductor) and can potentially achieve system-level wearable electronics integrating sensors, signal processing, and communicating units.

The dynamic large deflection response of RC beams under low-speed impact loading at their midspan is investigated in this paper. Two simple methods such as extended Hamilton’s principle and equivalent static hypothesis are used to establish the theoretical models for both simply supported and fully clamped RC beams; analytical formulas for the maximum midspan deflection-input impact energy are obtained. The “equal area” method based on the deflection history of beams is only used during these derivations to determine the plastic bending moment and the stress distribution of the structure. Then, finite element simulations are carried out to verify the validity of the proposed predictions. It is shown that the maximum deflections for both simply supported and fully clamped beams are almost proportional with respect to the input impact energy, which agrees well with both simulations and other experimental results. Also, the boundary condition has more effect on the deflection response of the RC beams which is relatively longer.

This paper presents the design, fabrication and characterization of a wind flow sensor using 3-D graphene aerogels as strain sensing elements. The sensor mimics hair-cell mechanical receptors widely found in nature. This work is the first demonstration of using 3-D graphene aerogel for constructing flow sensors. We simulated the graphene aerogel deformation under different wind speeds and predicted the experimental data well with our mechanical model. Moreover, we demonstrated good linearity with good sensitivity of 16.6% resistance change per m/s. Our sensor also shows good directional sensibility. Assuming the detection limit for resistance change is 0.1%, the detection limit can be as small as 6 mm/s. This flow sensor can potentially be used in smart infrastructure monitoring and wind speed measurement.

In this paper the ‘equal area method’ is used to investigate the load carrying capacity of circular sandwich plates (two metallic face-sheets adhered to a metallic foam-core) subjected to transverse quasi-static point central loading, and the plates are simply supported or fully clamped at the edges. During the process of the large deflection, the plate is regarded as a three dimensional body, which deforms into a shallow cone. To validate the analytical solution, a finite element simulation is performed. A parametric study is then carried out to examine the effects of the boundary conditions, core strength and face/core thickness ratio on the structural response. Results show that the analytical model gives a good description on the quasi-static behavior of circular sandwich plates as presented in finite element simulation. It is shown that the existence of an annular plastic hinge at the external boundary weakens the effects of radial stress in the case of fully clamped boundary. After the initial collapse, the face yield is the dominant failure mechanism especially at the stage of large deflection and the initial collapse of the sandwich structure is highly dependent on the face-sheets strength.

Graphene aerogel spheres (GASs), as one of the world’s lightest materials, is promising for constructing high-performance flexible sensors. Here, we demonstrated the capability of this material for sensing temperature, pressure and material elasticity. Our sensor exhibits an excellent temperature sensing property with a temperature coefficient of resistance (TCR) of 2.2 % C−1, 5 times larger than that of commercial resistance thermometers. Due to the 3-D spherical shape, the GAS can detect forces in 360° from all directions. The GAS-based sensors exhibit an enhanced sensitivity of 0.15 kPa-1, response and relaxation times of approximately 100 ms likely due to the unique micro/nanostructures of the GASs. Extraordinary cycling stability and long working life were also demonstrated even after large deformation. Assembling GASs with polydimethylsiloxane (PDMS), a flexible sensor was achieved with a large dynamic range from 4 mN to 2 N covering almost three orders of magnitudes. Moreover, we demonstrated three healthcare applications of GASs-based flexible sensors such as tissue elasticity sensing, finger posture sensing, and kinetic physiological signal monitoring.

A theoretical analysis on electric double layers formed near corners and neighboring branches of a comb electrode with a prescribed electric potential in an ionic conductor was performed using the linearized Poisson-Nernst-Planck theory which is closely related to the linearized Gouy-Chapman theory in static (time-independent) problems. Analytical solutions were obtained for two problems relevant to ionic capacitance applications defined over rectangular domains. The electric potential and ion distributions were calculated which are new, useful, and fundamental to the understanding and design of devices with comb electrodes. The basic behaviors of the distributions are presented and examined. For a positively charged electrode, the distribution of the negative ions was found to be qualitatively similar to that of the electric potential, and the behavior of the positive ions is the opposite. At an electrode corner, the electric double layer is relatively thicker than along a line electrode. When the distan...