The role of microscopic properties on cortical bone strength of femoral neck.

BACKGROUND: Femoral neck fractures are serious consequence of osteoporosis (OP), numbers of people are working on the micro-mechanisms of femoral neck fractures. This study aims to investigate the role and weight of microscopic properties on femoral neck maximum load (Lmax), funding the indicator which effects Lmax most. METHODS: A total of 115 patients were recruited from January 2018 to December 2020. Femoral neck samples were collected during the total hip replacement surgery. Femoral neck Lmax, micro-structure, micro-mechanical properties, micro-chemical composition were all measured and analyzed. Multiple linear regression analyses were performed to identify significant factors that affected the femoral neck Lmax. RESULTS: The Lmax, cortical bone mineral density (cBMD), cortical bone thickness (Ct. Th), elastic modulus, hardness and collagen cross-linking ratio were all significantly decreased, whereas other parameters were significantly increased during the progression of OP (P < 0.05). In micro-mechanical properties, elastic modulus has the strongest correlation with Lmax (P < 0.05). The cBMD has the strongest association with Lmax in micro-structure (P < 0.05). In micro-chemical composition, crystal size has the strongest correlation with Lmax (P < 0.05). Multiple linear regression analysis showed that elastic modulus was most strongly related to Lmax (ß = 0.920, P = 0.000). CONCLUSIONS: Compared with other parameters, elastic modulus has the greatest influence on Lmax. Evaluation of microscopic parameters on femoral neck cortical bone can clarify the effects of microscopic properties on Lmax, providing a theoretical basis for the femoral neck OP and fragility fractures.

Growth of Double-Network Tough Hydrogel Coatings by Surface-Initiated Polymerization.

Hydrogel coatings exhibit versatile applications in biomedicine, flexible electronics, and environmental science. However, current coating methods encounter challenges in simultaneously achieving strong interfacial bonding, robust hydrogel coatings, and the ability to coat substrates with controlled thickness. This paper introduces a novel approach to grow a double-network (DN) tough hydrogel coating on various substrates. The process involves initial substrate modification using a silane coupling agent, followed by the deposition of an initiator layer on its surface. Subsequently, the substrate is immersed in a DN hydrogel precursor, where the coating grows under ultraviolet (UV) illumination. Precise control over the coating thickness is achieved by adjusting the UV illumination duration and the initiator quantity. The experimental measurement of adhesion reveals strong bonding between the DN hydrogel coating and diverse substrates, reaching up to 1012.9 J/m2 between the DN hydrogel coating and a glass substrate. The lubricity performance of the DN hydrogel coating is experimentally characterized, which is dependent on the coating thickness, applied pressure, and sliding velocity. The incorporation of 3D printing technology into the current coating method enables the creation of intricate hydrogel coating patterns on a flat substrate. Moreover, the hydrogel coating's versatility is demonstrated through its effective applications in oil-water separation and antifogging glasses, underscoring its wide-ranging potential. The robust DN hydrogel coating method presented here holds promise for advancing hydrogel applications across diverse fields.

Polyacrylic Acid Hydrogel Coating for Underwater Adhesion: Preparation and Characterization.

Underwater adhesion involves bonding substrates in aqueous environments or wet surfaces, with applications in wound dressing, underwater repairs, and underwater soft robotics. In this study, we investigate the underwater adhesion properties of a polyacrylic acid hydrogel coated substrate. The underwater adhesion is facilitated through hydrogen bonds formed at the interface. Our experimental results, obtained through probe-pull tests, demonstrate that the underwater adhesion is rapid and remains unaffected by contact pressure and pH levels ranging from 2.5 to 7.0. However, it shows a slight increase with a larger adhesion area. Additionally, we simulate the debonding process and observe that the high-stress region originates from the outermost bonding region and propagates towards the center, spanning the thickness of the target substrate. Furthermore, we showcase the potential of using the underwater adhesive hydrogel coating to achieve in-situ underwater bonding between a flexible electronic demonstration device and a hydrogel contact lens. This work highlights the advantages of employing hydrogel coatings in underwater adhesion applications and serves as inspiration for the advancement of underwater adhesive hydrogel coatings capable of interacting with a wide range of substrates through diverse chemical and physical interactions at the interface.

Constrained thickness-shear vibration-based piezoelectric transducers for generating unidirectional-propagation SH0 wave.

Excitation of a pure guided wave with a controllable wavefield is essential in structural health monitoring (SHM). For example, a unidirectional-propagation guided wave can significantly reduce the complexity of signal interpretations by avoiding unwanted reflections. However, few transducers are currently capable of exciting a pure unidirectional-propagation guided wave, which cannot satisfy the emerging demands from the field of SHM. In this work, the thickness-shear vibration characteristics of the piezoelectric PZT wafer bonded on a waveguide are investigated by theoretical modeling and numerical simulations. It is found that there is a phase difference between the electric-excitation signal applied on the PZT wafer and the mechanical response signal of the bottom surface of the viscoelastic adhesive layer that connects the PZT wafer and waveguide. Moreover, such a phase difference can be adjusted by changing the equivalent width of the PZT wafer. Based on this finding, two piezoelectric transducers with different shape configurations are proposed to excite the unidirectional-propagation SH0 wave (the fundamental shear horizontal wave). Finite element simulations and experiments are conducted to verify the performances of the two unidirectional transducers. Results show that the two transducers can excite a pure SH0 wave and enhance the wave energy along a single direction. No time delay is required to excite the proposed transducers. Due to their simple configurations, the developed unidirectional SH0 wave transducers will have great potential applications in SHM.

A Novel Equivalent Method for Computing Mechanical Properties of Random and Ordered Hyperelastic Cellular Materials.

Simulating the mechanical behavior of cellular materials stands as a pivotal step in their practical application. Nonetheless, the substantial multitude of unit cells within these materials necessitates a considerable finite element mesh, thereby leading to elevated computational expenses and requisites for formidable computer configurations. In order to surmount this predicament, a novel and straightforward equivalent calculation method is proposed for the computation of mechanical properties concerning both random and ordered hyper-elastic cellular materials. By amalgamating the classical finite element approach with the distribution attributes of cells, the proposed equivalent calculation method adeptly captures the deformation modes and force-displacement responses exhibited by cell materials under tensile and shear loads, as predicted through direct numerical simulation. This approach reflects the deformation characteristics induced by micro-unit cells, elucidates an equivalent principle bridging cellular materials and equivalent materials, and substantially curtails exhaustive computational burdens. Ultimately, this method furnishes an equivalent computational strategy tailored for the engineering applications of cellular materials.

Effect of hydrogen on super-elastic behavior of NiTi shape memory alloy wires: Experimental observation and diffusional-mechanically coupled constitutive model.

NiTi shape memory alloys (SMAs) are inevitably in contact with hydrogen in specific service environments, which can degrade their mechanical behaviors. In this work, the effect of hydrogen on the super-elasticity of NiTi SMA orthodontic wires is investigated experimentally and theoretically. Firstly, cathodic hydrogen charging was performed for the wires at a current density of 10A/m2 with various charging times (2.5min, 5min, 7.5min and 10min) and charging lengths (20 mm, 40 mm, 60 mm and 80 mm) in 0.5 mol/L H2SO4+2 g/L CH4N2S electrolyte solution at room temperature. Then, ex-situ tension-unloading tests were carried out shortly after the hydrogen charging. The stress-strain responses showed a two-step martensite transformation (MT), i.e., the start stress of MT for the region with hydrogen charging is much larger than that without hydrogen charging. Based on the experimental observations, a diffusional-mechanically coupled constitutive model is constructed. Elastic strain, transformation strain, transformation-induced plasticity (TRIP) and hydrogen swelling deformation are considered. The effect of hydrogen on the thermo-mechanical behavior of NiTi SMA is taken into account by introducing the hydrogen concentration (HC)-dependent critical temperatures of MT and slip resistance of TRIP. The thermodynamic driving forces of MT and TRIP are derived from the constructed Helmholtz free energy and dissipation inequality. The balance equation of hydrogen diffusion is obtained by the chemical potential and Fick's diffusion law. To obtain the overall response of the wire with a heterogeneous HC field, a scale transition rule is proposed. The capability of the proposed model to describe the super-elasticity of NiTi SMA with various hydrogen charging times and charging lengths is validated by comparing the predicted results with the experimental ones.

Torsional whole-life transformation ratchetting under pure-torsional and non-proportional multiaxial cyclic loadings of NiTi SMA at human-body temperature: Experimental observations and life-prediction model.

Torsional whole-life transformation ratchetting is investigated under pure-torsional and non-proportional multiaxial loadings of NiTi SMA micro-tubes at human-body temperature (310 K), where three paths of torsional loadings and five paths of multiaxial ones are considered. It is observed that the evolution of the torsional whole-life transformation ratchetting depends strongly on the loading paths and stress levels, and the fatigue lives of pure-torsional loadings decrease faster than that of uniaxial and multiaxial ones with the increase of peak stress. Based on the experimental investigations, a life-prediction model which depends on the applied stress levels is proposed for NiTi SMA micro-tubes, where the martensite transformation and reorientation of NiTi SMAs are considered. The predicted fatigue lives under uniaxial, torsional and non-proportional multiaxial loadings are mostly located within the triple error band.

In Situ Observation on Rate-Dependent Strain Localization of Thermo-Induced Shape Memory Polyurethane.

In situ monotonic tensile experiments of thermo-induced shape memory polyurethane (SMPU) at different loading rates were carried out by the digital image correlation (DIC) method and infrared camera FLIR®-A655sc in natural convection (NC) and forced convection (FC) conditions, respectively. The multiform strain localization of SMPU was observed by the DIC method, and the influence of thermo-mechanical coupling on the strain localization was analyzed by using the FLIR to measure the temperature field caused by the internal heat generation. The experimental results show that the strain localization mode strongly depends on the strain rate and convection condition, and the strain localization mode can be transformed by changing the convection condition from NC to FC. The competition mechanism between the strain hardening induced by the increasing loading rate and strain softening induced by the internal heat generation is indicated, the transition modes of strain localization are clarified, and the influences of thermo-mechanical coupling on shape memory effect are discussed.

A finite viscoelastic-plastic model for describing the uniaxial ratchetting of soft biological tissues.

In this paper, a phenomenological constitutive model is constructed to describe the uniaxial ratchetting (i.e., the cyclic accumulation of inelastic deformation) of soft biological tissues in the framework of finite viscoelastic-plasticity. The model is derived from a polyconvex elastic free energy function and addresses the anisotropy of cyclic deformation of the tissues by means of structural tensors. Ratchetting is considered by the evolution of internal variables, and its time-dependence is described by introducing a pseudo-potential function. Accordingly, all the evolution equations are formulated from the dissipation inequality. In numerical examples, the uniaxial monotonic stress-strain responses and ratchetting of some soft biological tissues, such as porcine skin, coronary artery layers and human knee ligaments and tendons, are predicted by the proposed model in the range of finite deformation. It is seen that the predicted monotonic stress-strain responses and uniaxial ratchetting obtained at various loading rates and in various loading directions are in good agreement with the corresponding experimental results.

Phase transformation evolution in NiTi shape memory alloy under cyclic nanoindentation loadings at dissimilar rates.

Hysteresis energy decreased significantly as nanocrystalline NiTi shape memory alloy was under triangular cyclic nanoindentation loadings at high rate. Jagged curves evidenced discrete stress relaxations. With a large recovery state of maximum deformation in each cycle, this behavior concluded in several nucleation sites of phase transformation in stressed bulk. Additionally, the higher initial propagation velocity of interface and thermal activation volume, and higher levels of phase transition stress in subsequent cycles explained the monotonic decreasing trend of dissipated energy. In contrast, the dissipated energy showed an opposite increasing trend during triangular cyclic loadings at a low rate and 60 sec holding time after each unloading stage. Due to the isothermal loading rate and the holding time, a major part of the released latent heat was transferred during the cyclic loading resulting in an unchanged phase transition stress. This fact with the reorientation phenomenon explained the monotonic increasing trend of hysteresis energy.

A truncated conical beam model for analysis of the vibration of rat whiskers.

A truncated conical beam model is developed to study the vibration behaviour of a rat whisker. Translational and rotational springs are introduced to better represent the constraint conditions at the base of the whiskers in a living rat. Dimensional analysis shows that the natural frequency of a truncated conical beam with generic spring constraints at its ends is inversely proportional to the square root of the mass density. Under all the combinations of the classical free, pinned, sliding or fixed boundary conditions of a truncated conical beam, it is proved that the natural frequency can be expressed as f = α(rb/L(2))E/ρ and the frequency coefficient α only depends on the ratio of the radii at the two ends of the beam. The natural frequencies of a representative rat whisker are predicted for two typical situations: freely whisking in air and the tip touching an object. Our numerical results show that there exists a window where the natural frequencies of a rat whisker are very sensitive to the change of the rotational constraint at the base. This finding is also confirmed by the numerical results of 18 whiskers with their data available from literature. It can be concluded that the natural frequencies of a rat whisker can be adjusted within a wide range through manipulating the constraints of the follicle on the rat base by a behaving animal.