Evaluation of the mechanical properties and clinical application of nickel–titanium shape memory alloy scaphoid arc nail

To investigate the mechanical and biomechanical properties of nickel–titanium (Ni–Ti) shape memory alloy scaphoid arc nail (NT‐SAN) fixator as well as study the surgical method of treating carpal scaphoid fractures and evaluate its clinical efficacy. (1) Static and dynamic bending tests with embedded axial bending fixture were conducted to study the mechanical properties. (2) To evaluate biomechanical strength and fatigue, 32 scaphoid samples were classified into four groups to perform the fixation rigidity test: intramedullary Kirschner fixation (group A), Kirschner straddle nail fixation (group B), screw nail fixation (group C), and NT‐SAN fixation (group D). Next, 24 scaphoid waist fracture models were classified to conduct fatigue experiments as follows: Kirschner straddle nail fixation (group E), screw nail fixation (group F), and NT‐SAN fixation (group G). (3) The Krimmer score chart was used for clinical evaluations. (1) NT‐SAN showed excellent mechanical performance and a long lifespan. (2) NT‐SAN was fixated with a strong intensity and an anti‐fatigue outcome. (3) Ninety‐eight interviewed patients were satisfied with the therapeutic effects of the arc nail (satisfaction rate: 95.92%). The designed strength and hardness of NT‐SAN corresponded with the anatomical characteristics of the scaphoid, and the designed mechanical properties met the biomechanical requirements of a scaphoid fracture. The fatigue strength can meet the requirements of bone healing after the scaphoid fracture. Clinical trials on NT‐SAN scaphoid fracture treatment have shown that the surgery is simple and the clinical results are satisfactory. The therapeutic level of NT‐SAN is III; thus, it is worth promoting.     

Bundles of Spider Silk,Braided into Sutures,Resist Basic Cyclic Tests: Potential Use for Flexor Tendon Repair

Repair success for injuries to the flexor tendon in the hand is often limited by the in vivo behaviour of the suture used for repair. Common problems associated with the choice of suture material include increased risk of infection, foreign body reactions, and inappropriate mechanical responses, particularly decreases in mechanical properties over time. Improved suture materials are therefore needed. As high-performance materials with excellent tensile strength, spider silk fibres are an extremely promising candidate for use in surgical sutures. However, the mechanical behaviour of sutures comprised of individual silk fibres braided together has not been thoroughly investigated. In the present study, we characterise the maximum tensile strength, stress, strain, elastic modulus, and fatigue response of silk sutures produced using different braiding methods to investigate the influence of braiding on the tensile properties of the sutures. The mechanical properties of conventional surgical sutures are also characterised to assess whether silk offers any advantages over conventional suture materials. The results demonstrate that braiding single spider silk fibres together produces strong sutures with excellent fatigue behaviour; the braided silk sutures exhibited tensile strengths comparable to those of conventional sutures and no loss of strength over 1000 fatigue cycles. In addition, the braiding technique had a significant influence on the tensile properties of the braided silk sutures. These results suggest that braided spider silk could be suitable for use as sutures in flexor tendon repair, providing similar tensile behaviour and improved fatigue properties compared with conventional suture materials.     

Kinetics and mechanics of two-dimensional interactions between T cell receptors and different activating ligands

Adaptive immune responses are driven by interactions between T cell antigen receptors (TCRs) and complexes of peptide antigens (p) bound to Major Histocompatibility Complex proteins (MHC) on the surface of antigen-presenting cells. Many experiments support the hypothesis that T cell response is quantitatively and qualitatively dependent on the so-called strength of TCR/pMHC association. Most available data are correlations between binding parameters measured in solution (three-dimensional) and pMHC activation potency, suggesting that full lymphocyte activation required a minimal lifetime for TCR/pMHC interaction. However, recent reports suggest important discrepancies between the binding properties of ligand-receptor couples measured in solution (three-dimensional) and those measured using surface-bound molecules (two-dimensional). Other reports suggest that bond mechanical strength may be important in addition to kinetic parameters. Here, we used a laminar flow chamber to monitor at the single molecule level the two-dimensional interaction between a recombinant human TCR and eight pMHCs with variable potency. We found that 1), two-dimensional dissociation rates were comparable to three-dimensional parameters previously obtained with the same molecules; 2), no significant correlation was found between association rates and activating potency of pMHCs; 3), bond mechanical strength was partly independent of bond lifetime; and 4), a suitable combination of bond lifetime and bond strength displayed optimal correlation with activation efficiency. These results suggest possible refinements of contemporary models of signal generation by T cell receptors. In conclusion, we reported, for the first time to our knowledge, the two-dimensional binding properties of eight TCR/pMHC couples in a cell-free system with single bond resolution.     

Mechanical behaviour modelling of balloon-expandable stents

Endoprostheses are small struts placed by intravascular way to restore the vascular lumen and flow conditions. The purpose of this work is to provide models for evaluation and characterisation of some mechanical properties of a balloon-expandable stent by using the finite element method. Here we present the results for a metallic tubular peripheral prosthesis: the P308 Palmaz stent. We focus on the mechanisms linked to the structure expansion and its long-term behaviour. Several models are constructed in order to determine the stent shape after dilation and to assess the stress and strain fields in its wall due to this transformation. They inform us about the shortening percentage on expansion, degrees of radial and longitudinal recoil, and weaknesses of the structure. Various methods, differing in their levels of complexity, are then attempted to exhibit the predominant factors responsible for the crushing of a stent under external pressure. Moreover, the sensitivity of this critical pressure to geometric imperfections is studied. Lastly, since this kind of material is implanted for a lifetime, we test the stent with regard to fatigue life. Beyond safety considerations, this type of characterisation provides mechanical properties that are often difficult to obtain by experiments. If it was available for various stents, such information could be used to choose the appropriate prosthesis for specific applications. Moreover, confronted with observations from practitioners, they might lead to a better understanding of the failure or success of a particular design and to work on the product optimisation.     

Compressive fatigue and endurance of juvenile bovine articular cartilage explants

Articular cartilage is an enduring tissue. For most individuals, articular cartilage facilitates a lifetime of pain-free ambulation, supporting millions of loading cycles from activities of daily living. Although early studies into osteoarthritis focused on the role of mechanical fatigue in articular cartilage degeneration, much is still unknown regarding its strength and endurance characteristics. The compressive strength of juvenile, bovine articular cartilage explants was determined to be loading rate-dependent, reaching a maximum strength of 29.5 ± 4.8 MPa at a strain rate of 0.10 %/sec. The fatigue and endurance properties of articular cartilage were characterized utilizing a material testing system, as well as a custom, validated instrument termed the two degrees-of-freedom endurance meter (endurometer). These instruments characterized fatigue in articular cartilage explants at loading levels ranging from 10 to 80 % strength (%S), up to 100,000 cycles. Cartilage explants displayed characteristics of fatigue – fatigue life increased as the loading magnitude decreased. All explants failed within 14,000 cycles at loading levels between 50 and 80 %S. At 10 and 20 %S, all explants endured 100,000 loading cycles. There was no significant difference in equilibrium compressive modulus between run-out explants and unloaded controls, although the pooled modulus increased in response to testing. Histological staining and biochemical assays revealed no material changes in collagen, sulfated glycosaminoglycan, or hydration content between unloaded controls and explants cyclically loaded to run-out. These results suggest articular cartilage may have a putative endurance limit of 20 %S (5.86 MPa), with implications for articular cartilage biomechanics and mechanobiology.     

Structural parameters determining the strength of the porcine vertebral body affected by tumours

Spinal metastatic disease could lead to catastrophic consequences for the patient. However, the structural parameters that explain the weakening of vertebrae affected by tumours are not fully understood. In this study, we developed a specimen-specific finite element model to predict the strength of the porcine vertebra with simulated tumours and used it to find the structural parameters determining the strength. We validated our model with mechanical testing and then we analysed the compressive strength of intact vertebrae and seven defects with different size and shape. The results showed that the minimum bone mineral mass of the cross section and areal defect fraction were the best predictors of the normalized strength. We also found that areal parameters appeared to be better predictors than the volumetric ones. In conclusion, reduction in bone strength for vertebrae weakened by metastatic tumours is mostly associated with decrease in the mechanical properties of the cross section.     

Wing cross veins: an efficient biomechanical strategy to mitigate fatigue failure of insect cuticle

Locust wings are able to sustain millions of cycles of mechanical loading during the lifetime of the insect. Previous studies have shown that cross veins play an important role in delaying crack propagation in the wings. Do cross veins thus also influence the fatigue behaviour of the wings? Since many important fatigue parameters are not experimentally accessible in a small biological sample, here we use the finite element (FE) method to address this question numerically. Our FE model combines a linear elastic material model, a direct cyclic approach and the Paris law and shows results which are in very good agreement with previously reported experimental data. The obtained results of our study show that cross veins indeed enhance the durability of the wings by temporarily stopping cracks. The cross veins further distribute the stress over a larger area and therefore minimize stress concentrations. In addition, our work indicates that locust hind wings have an endurance limit of about 40% of the ultimate tensile strength of the wing material, which is comparable to many engineering materials. The comparison of the results of the computational study with predictions of two most commonly used fatigue failure criteria further indicates that the Goodman criterion can be used to roughly predict the failure of the insect wing. The methodological framework presented in our study could provide a basis for future research on fatigue of insect cuticle and other biological composite structures.     

Rheological properties and mechanical stability of new gel-entrapment systems applied in bioreactors

The mechanical stability of gels applied for entrapment and retention of biocatalysts in bioreactors is of crucial importance for successful scale-up applications. Gel abrasion in agitated reactors will depend on liquid shear, bubble shear, and wall shear, as well as collisions between the gel particles. As a simplified standardized model system, abrasion of gel beads was studied in 1-m-high bubble columns with controlled aeration, and quantified by measuring the loss of gel material into solution. Gel beads were also taken out to measure stress-strain response during controlled compression. More general rheological properties of different gels were studied by applying a variety of regimes of controlled compression of standardized gel cylinders: Gel strength was measured by recording the fracture properties and the Young's modulus. Viscoelastic properties were revealed by recording creep during compression as well as recovery after compression. Oscillation tests up to 1000 cyclic compressions were applied to compare the fatigue of different gels. Results obtained for Ca-alginate gels, gels of chemically modified polyvinyl alcohol with stilbazolium groups (PVA-SbQ) as well as mixed gels of Ca-alginate and PVA-SbQ are compared with previously published data for kappa-carrageenan, agar, and polyethylene glycol (PEG) gels. It is concluded that material fatigue rather than mechanical properties such as stiffness or fracture stress should be considered when selecting a suitable gel material on the basis of abrasion resistance. The very soft and superelastic PVA-SbQ gel showed no significant fatigue in mechanical tests and no abrasion was detected in the standardized model system used. Ca-alginate gels, however, showed severe irreversible changes due to fatigue at oscillating loads and creep at constant load. Due to their similarities with kappa-carrageenan gels in mechanical tests, it is likely that Ca-alginate would also be sensitive to abrasion. Mixed gels of Ca-alginate and PVA-SbQ represent a complex system with intermediate properties, showing significant fatigue and creep, but elastic properties from the PVA-SbQ gel make it less sensitive than the pure Ca-alginate gel.     

Antiviral compounds modulate elasticity,strength and material fatigue of a virus capsid framework

This study investigates whether the biochemical and antiviral effects of organic compounds that bind different sites in the mature human immunodeficiency virus capsid may be related to the modulation of different mechanical properties of the protein lattice from which the capsid is built. Mechanical force was used as a probe to quantify, in atomic force microscopy experiments at physiological pH and ionic strength, ligand-mediated changes in capsid lattice elasticity, breathing, strength against local dislocation by mechanical stress, and resistance to material fatigue. The results indicate that the effects of the tested compounds on assembly or biochemical stability can be linked, from a physics-based perspective, to their interference with the mechanical behavior of the viral capsid framework. The antivirals CAP-1 and CAI-55 increased the intrinsic elasticity and breathing of the capsid protein lattice and may entropically decrease the probability of the capsid protein to assemble into a functionally competent conformation. Antiviral PF74 increased the resistance of the capsid protein lattice to disruption by mechanical stress and material fatigue and may enthalpically strengthen the basal capsid lattice against breakage and disintegration. This study provides proof of concept that the interrogation of the mechanical properties of the nanostructured protein material that makes a virus capsid may provide fundamental insights into the biophysical action of capsid-binding antiviral agents. The implications for drug design by specifically targeting the biomechanics of viruses are discussed.     

Biocompatibility and fatigue properties of polystyrene-polyisobutylene-polystyrene, an emerging thermoplastic elastomeric biomaterial

This paper will discuss the biocompatibility and dynamic fatigue properties of polystyrene-b-polyisobutylene-b-polystyrene thermoplastic elastomer with 30 wt % polystyrene (SIBS30), an emerging FDA-approved biomaterial. SIBS30 is a very soft, transparent biomaterial resembling silicone rubber, with superior mechanical properties. Using the hysteresis method adopted for soft biomaterials, the dynamic fatigue properties of SIBS30 were found to be between those of polyurethane and silicone rubber, with fatigue life twice as long as that of silicone. Under single load testing (SLT, 1.25 MPa), SIBS30 displayed less than half the dynamic creep compared to silicone, both in air and in vitro (37 degrees C, simulated body fluid). Hemolysis and 30- and 180-day implantation studies revealed excellent biocompatibility of the new biomaterial. The results presented in this paper indicate that, in comparison with silicone rubber, SIBS30 has similar biocompatibility and superior dynamic fatigue properties.     

The Influence of Cavity Shape on the Stresses in Composite Dental Restorations: A Finite Element Study

Dental restoration adhering to the cavity exhibits fundamentally different load transfer mechanisms from non-adhering restorations. It is therefore questionable that traditional cavity designs are optimal from a purely mechanical point of view when working with composite materials. Drawing from general engineering experience, it can be hypothesised that smooth, well rounded designs with bevelled margins are superior. A finite element model is used in the present investigation to determine the stress field in four different cavity designs as it develops during the curing of the restoration. The results show that a significant reduction of the stress along the adhesive interface between the tooth and the restoration can be achieved through the use of a rounded cavity shape. They also show that the adoption of bevelled margins leads to a reduction of the stress concentration at this location. These results are confirmed by a set of experimental results published in the literature. It is concluded that adhering restorations will perform better from a mechanical point of view if an appropriate cavity shape is selected.     

The influence of cavity shape on the stresses in composite dental restorations: a finite element study

Dental restoration adhering to the cavity exhibits fundamentally different load transfer mechanisms from non-adhering restorations. It is therefore questionable that traditional cavity designs are optimal from a purely mechanical point of view when working with composite materials. Drawing from general engineering experience, it can be hypothesised that smooth, well rounded designs with bevelled margins are superior. A finite element model is used in the present investigation to determine the stress field in four different cavity designs as it develops during the curing of the restoration. The results show that a significant reduction of the stress along the adhesive interface between the tooth and the restoration can be achieved through the use of a rounded cavity shape. They also show that the adoption of bevelled margins leads to a reduction of the stress concentration at this location. These results are confirmed by a set of experimental results published in the literature. It is concluded that adhering restorations will perform better from a mechanical point of view if an appropriate cavity shape is selected.     

Single Cell Wall Nonlinear Mechanics Revealed by a Multiscale Analysis of AFM Force-Indentation Curves

Individual plant cells are rather complex mechanical objects. Despite the fact that their wall mechanical strength may be weakened by comparison with their original tissue template, they nevertheless retain some generic properties of the mother tissue, namely the viscoelasticity and the shape of their walls, which are driven by their internal hydrostatic turgor pressure. This viscoelastic behavior, which affects the power-law response of these cells when indented by an atomic force cantilever with a pyramidal tip, is also very sensitive to the culture media. To our knowledge, we develop here an original analyzing method, based on a multiscale decomposition of force-indentation curves, that reveals and quantifies for the first time the nonlinearity of the mechanical response of living single plant cells upon mechanical deformation. Further comparing the nonlinear strain responses of these isolated cells in three different media, we reveal an alteration of their linear bending elastic regime in both hyper- and hypotonic conditions.     

Properties of pellets manufactured by wet extrusion/spheronization process using kappa-carrageenan: effect of process parameters

The aim of this study was to systematically evaluate the pelletization process parameters of kappa-carrageenan-containing formulations. The study dealt with the effect of 4 process parameters--screw speed, number of die holes, friction plate speed, and spheronizer temperature--on the pellet properties of shape, size, size distribution, tensile strength, and drug release. These parameters were varied systematically in a 2(4) full factorial design. In addition, 4 drugs--phenacetin, chloramphenicol, dimenhydrinate, and lidocaine hydrochloride--were investigated under constant process conditions. The most spherical pellets were achieved in a high yield by using a large number of die holes and a high spheronizer speed. There was no relevant influence of the investigated process parameters on the size distribution, mechanical stability, and drug release. The poorly soluble drugs, phenacetin and chloramphenicol, resulted in pellets with adequate shape, size, and tensile strength and a fast drug release. The salts of dimenhydrinate and lidocaine affected pellet shape, mechanical stability, and the drug release properties using an aqueous solution of pH 3 as a granulation liquid. In the case of dimenhydrinate, this was attributed to the ionic interactions with kappa-carrageenan, resulting in a stable matrix during dissolution that did not disintegrate. The effect of lidocaine is comparable to the effect of sodium ions, which suppress the gelling of carrageenan, resulting in pellets with fast disintegration and drug release characteristics. The pellet properties are affected by the process parameters and the active pharmaceutical ingredient used.     

Scaling effects in the fatigue strength of bones from different animals

The bones of vertebrates are all made from the same basic material, despite a huge variation in size from one species to another. This introduces a problem: large structures are more prone to fatigue failure (stress fracture) than smaller structures made of the same material. This implies that bones in larger animals cannot withstand as much stress in daily use as bones in smaller animals. In fact, this is not the case, because all bones experience approximately the same stresses and strains in use. This implies a variation in the underlying material: bone material in large animals must have superior fatigue properties to offset the disadvantages of size. This hypothesis is tested here by reference to fatigue data from the literature, taken from a range of animals from cows to mice. Fatigue strength was plotted as a function of stressed volume and modelled mathematically using a Weibull distribution. This shows a general tendency for fatigue strength to reduce as volume increases. But when the volume effect is taken into account, there remains a tendency for bones from smaller animals to have lower fatigue strength. This can be modelled by a simple variation in one of the parameters in the Weibull equation, which defines the intrinsic fatigue strength of the material. When extrapolated to the size of the whole bone for each animal, all bones were found to have the same fatigue strength. This resolves the anomaly and implies a complex system in which the underlying structure of bone varies with animal size in order to cancel out scaling effects.     

On high-cycle fatigue of 316L stents

This paper deals with fatigue life prediction of 316L stainless steel cardiac stents. Stents are biomedical devices used to reopen narrowed vessels. Fatigue life is dominated by the cyclic loading due to the systolic and diastolic pressure and the design against premature mechanical failure is of extreme importance. Here, a life assessment approach based on the Dang Van high cycle fatigue criterion and on finite element analysis is applied to explore the fatigue reliability of 316L stents subjected to multiaxial fatigue loading. A finite element analysis of the stent vessel subjected to cyclic pressure is performed to carry out fluctuating stresses and strain at some critical elements of the stent where cracks or complete fracture may occur. The obtained results show that the loading path of the analysed stent subjected to a pulsatile load pressure is located in the safe region concerning infinite lifetime.     

Phosphocreatine and pH recovery without restoration of mechanical function during prolonged activity of rat gastrocnemius muscle: an in vivo 31P NMR study

Metabolic impairment in skeletal muscle was suggested to be involved in the development of local mechanical fatigue but until now results have dealt with short activity periods whereas little data on exhaustive and prolonged exercises are available. Stimulations of rat leg muscle lasting 45 min were induced by tetanic trains delivered via sciatic nerve at five different rhythms. Energy metabolism of the stimulated gastrocnemius muscle was followed by 31P NMR spectroscopy using surface coil while mechanical function was recorded. Our data showed a decrease in the force level to very low values a few minutes after exercise onset. This mechanical impairment only induced a transient metabolic failure followed by rapid restoration of high phosphocreatine (PCr) values and intracellular pH, without mechanical recovery. In addition, at the end of exercise, the PCr content was proportional to the fatigue level. As these experiments could not have impaired neuromuscular junction, the data would indicate that fatigue was maintained by a mechanism which does not appear to depend directly on muscle cell energy stores.     

Surface shape change during fusion of erythrocyte membranes is sensitive to membrane skeleton agents.

We previously reported that the induction of membrane fusion between pairs of erythrocyte ghosts is accompanied by the formation of a multipore fusion zone that undergoes an area expansion with condition-dependent characteristics. These characteristics allowed us to hypothesize substantial, if not major, involvement of the spectrin-based membrane skeleton in controlling this expansion. It was also found that the fusion zone, which first appears in phase optics as a flat diaphragm, has a lifetime that is also highly condition-dependent. We report here that 2,3-diphosphoglycerate, wheat germ agglutinin, diamide, and N-ethylmaleimide, all known to have binding sites primarily on skeleton components (including spectrin), have condition-dependent effects on specific components of the fusion zone diameter versus time expansion curve and the flat diaphragm lifetime. We also report a pH/ionic strength condition that causes a dramatic stabilization of flat diaphragms in a manner consistent with the known pH/ionic strength dependence of the spectrin calorimetric transition, thus further supporting the hypothesis of spectrin involvement. Our data suggest that the influence of the membrane skeleton on cell fusion is to restrain the rounding up that takes place after membrane fusion and that it may have variable, rather than fixed, mechanical properties. Data show that WGA, a known ligand for sialic acid, and DPG, a known metabolite, influences the flat diaphragm stability and late period expansion rates, raising the possibility that some of these mechanical properties are biologically regulated.     

Tensile loading characteristics of hydrogen stored carbon nanotubes in PEM fuel cell operating conditions using molecular dynamics simulation

Mechanical characteristics of hydrogen stored single walled carbon nanotube (SWCNT) in proton exchange membrane fuel cell (PEMFC) operating conditions are analysed in this work using molecular dynamics simulation method. The investigation of mechanical characteristics of hydrogen stored SWCNT is critical in determining the lifetime and stability of SWCNT-based membranes used in PEMFC. The study provides a comprehensive analysis on the effects of geometry, vacancy defects and PEMFC operating temperature on the mechanical properties of hydrogen stored SWCNT. The findings show that the mechanical strength of the hydrogen stored SWCNT can be enhanced by deploying a bigger armchair SWCNT. Furthermore, increase in operating temperature of PEMFC reduces the mechanical resistance of hydrogen stored SWCNT, which however can be overcome by suitably introducing vacancy defects in the SWCNT geometry. This has provided potential way of increasing the hydrogen storage capacity of SWCNT which is very useful for onboard application of PEMFC. It is anticipated that the findings obtained from this paper will have a paramount importance in the field of hydrogen energy fuel cell technology and further compliment the potential applications of SWCNTs as promising candidates for applications in fuel cells and energy storage devices.     

Effect of Fine-Grain Percent on Soil Strength Properties Improved By Biological Method

In many civil engineering projects, the foundation soils do not provide the required mechanical properties and therefore, there is a need to improve the soil. Compaction, soil reinforcement, soil mixing with natural, or chemical additives are common soil stabilization methods used to improve the soil mechanical properties. The incidence of some environmental problems in traditional improvement techniques has encouraged engineers to explore new methods. Recently in this category, a new technique in geotechnical engineering called biogeotechnology is introduced to improve the mechanical properties of the soil. It is an environmentally friendly approach that uses biological methods to solve geotechnical problems. This technique uses minerals producer microorganisms. This study investigates the possibility of improving soil strength properties with microbial calcite precipitation and the effect of fine-grained percentages in this regard. In order to determine the soil strength properties, consolidated drained direct shear tests have been carried on untreated and treated soil samples. The results showed that this method is applicable to improve all soil samples (from 100% coarse-grained (i.e., sand) to 100% fine-grained (i.e., clay)). However, increasing the strength in the sand is much more enhanced than that for finer soils. It was found that a considerable increase in cohesion of treated soil can be achieved for soil samples with maximum 10% fine content.