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PubMed2005

Contribution of the Nernst potential to stiffness constants: the asymmetrical case.

Inside biological membranes, one of the fundamental functions of active proteins such as pumps is to generate some electrochemical gradient across the membrane and then, to establish a new stationary state. The membrane electric potential generated by activity modifies the stiffness constants of the membrane. A spontaneous curvature appears if the inner and outer Debye lengths are different. The corresponding characteristic radius falls in the range from 0.08 microm to 50 microm. The bending elastic modulus is always increased. This effect is only noticeable in the limit of large Debye length from 0.5 microm to 0.09 microm. For a Nernst potential of 100 mV and a Debye length of 0.2 microm, the bending modulus can reach 40k(B)T.

Biological Transport, Active↗
PubMed1999

Mechanics of the large artery vascular wall.

The mechanics of large artery walls make a large contribution to the transmission toward regional vascular beds of energy generated by the left ventricle. It has been established that abnormal stiffness of the aorta and large conduit arteries is responsible in the short term for left ventricular overload followed by left ventricular hypertrophy. Radial and axial stresses imposed on vessel walls are defined in this review article. Mechanical parameters that can be measured in vivo and in vitro are described and compared; they include compliance, distensibility, elasticity modulus, impedance, and pulse wave velocity.

Animals↗
PubMed1994

On the sufficiency conditions for the stability of bone remodeling equilibrium.

In this technical note a sufficiency condition is established for the stability of a strain-energy-based bone remodeling theory in the special case of a beam loaded by an axial force and a bending moment. In a previous report the same condition was shown to be a necessary condition for stability in the same situation. The remodeling scheme is one characterized by a remodeling stimulus equal to the strain energy density divided by the bulk or apparent density raised to an exponent m as well an elastic modulus proportional to bulk or apparent density raised to an exponent n. In order for a remodeling scheme to be stable for an elastic beam loaded by an axial force and a bending moment, it is established that the condition that m must be greater than n is not only necessary, but also sufficient.

Algorithms↗
PubMedDate not supplied

Bone strength and material properties of the glenoid.

The quality of the glenoid bone is important to a successful total shoulder replacement. Finite element models have been used to model the response of the glenoid bone to an implanted prosthesis. Because very little is known about the bone strength and the material properties at the glenoid, these models were all based on assumptions that the material properties of the glenoid were similar to those of the tibial plateau. The osteopenetrometer was used to assess the topographic strength distribution at the glenoid. Strength at the proximal subchondral level of the glenoid averaged 66.9 MPa. Higher peak values were measured posteriorly, superiorly, and anteriorly to the area of maximum concavity of the glenoid joint surface known as the bare area. One millimeter underneath the subchondral plate, average strength decreased by 25%, and at the 2 mm level strength decreased by 70%. The contribution of the cortical bone to the total glenoid strength was assessed by compression tests of pristine and cancellous-free glenoid specimens. Strength decreased by an average of 31% after the cancellous bone was removed. The material properties of the glenoid cancellous bone were determined by axial compression tests of bone specimens harvested from the central part of the glenoid subchondral area. The elastic modulus varied from approximately 100 MPa at the glenoid bare area to 400 MPa at the superior part of the glenoid. With the elastic constants used a predictor of the mechanical anisotropy, the average anisotropy ratio was 5.2, indicating strong anisotropy. The apparent density was an average 0.35 gr. cm-3, and the Poisson ratio averaged 0.263. According to our findings the anisotropy of the glenoid cancellous bone, details concerning the strength distribution, and the load-bearing function of the cortical shell should be considered in future finite element models of the glenoid.

Adult↗
PubMed1998

Static and dynamic finite element analyses of an idealized structural model of vertebral trabecular bone.

An idealized three-dimensional finite element model of a rodlike trabecular bone structure was developed to study its static and dynamic responses under compressive loading, considering the effects of bone marrow and apparent density. Static analysis of the model predicted hydraulic stiffening of trabecular bone due to the presence of bone marrow. The predicted power equation relating trabecular bone apparent elastic modulus to its apparent density was in good agreement with those of the reported experimental investigations. The ratio of the maximum stress in the trabecular bone tissue to its apparent stress had a high value, decreasing with increasing bone apparent density. Frequency analyses of the model predicted higher natural frequencies for the bone without marrow than those for the bone with marrow. Adding a mass relatively large compared to that of bone rendered a single-degree-of-freedom response. In this case, the resonant frequency was higher for the bone with marrow than that for the bone without marrow. The predicted vibrational measurement of apparent modulus was in good agreement with that of the static measurement, suggesting vibrational testing as a method for nondestructive measurement of trabecular bone elastic moduli.

Bone Density↗
PubMed1994

Biaxial elastic properties of rat arteries in vivo: influence of vascular wall cells on anisotropy.

There is no consensus as to the degree of arterial anisotropy or to its relationship to vascular cell function. Given the relevance of the isotropic assumption in formulating elasticity models, reliable measures of biaxial displacements are needed. In this study, a video motion analyzer (VMA) was used to describe the biaxial in vivo dynamic elasticity of 22 carotid arteries and 5 abdominal aortas in 27 rats. The influence of vascular cell function was also examined by subjecting six rats to a photosensitive drug, chloroaluminum sulfonated phthalocyanine (CASPc), which is focally cytotoxic on activation by laser. Circumferential compliance (Ccirc) was greater than longitudinal compliance (Clong) for all vessels. Compliance pressure curves were nonlinear, and biaxial displacements were in phase. The circumferential elastic modulus was less than the longitudinal modulus at common stresses. CASPc + laser reduced Ccirc but not Clong, thus altering Poisson's ratio. In conclusion, rat arteries are biaxially, nonlinearly elastic and anisotropic in vivo. Vascular cells modulate Poisson's ratio by influencing Ccirc.

Animals↗
PubMed1993

Dynamic intra-vascular pressure harmonic analysis.

The postulate that dynamic intra vascular pressure is a function of positional wall properties has been difficult to verify due to non-linear viscoelastic influences in arteries. It was tested, in rabbit carotid artery in vitro segments using dynamic and static input. A Multifunction Pressure Generator (MPG, Millar Inc.) provide dynamic input of defined swept frequency (2-200 Hz) pressure superimposed on multiple levels of mean intraluminal pressure (IP). Bode plots were analyzed for natural frequency (omega n), gain (dB) and damping (zeta) of the IP/MPG transfer function. An increase in omega n (all zeta > 0.25) was found with increase in mean IP. An increase in segment Elastic modulus (Em) was found with increase in static force input. Relation between Em and omega n results was demonstrative of non-linear elasticity. Results supported the postulate and verified estimating vessel properties by harmonic analysis.

Animals↗
PubMed1983

Flexural rigidity of singlet microtubules estimated from statistical analysis of their contour lengths and end-to-end distances.

Microtubules in solutions, observed under a dark-field microscope, show incessant Brownian movement such as translational, rotational and flexing motion. A large number of microtubules, spontaneously stuck to the under surface of a coverslip, were photographed and the contour lengths and end-to-end distances of their images were measured. From the statistical analysis of the contour lengths and end-to-end distances, a value for the parameter lambda representing the flexibility of singlet microtubules was estimated to be lambda = (6.8 +/- 0.8) . 10(-3) micrometers-1. From the value of lambda, the elastic modulus for bending, epsilon, and Young's modulus, Y, of singlet microtubules were computed to be epsilon = approximately 10(-16) and Y = approximately 10(9) dyne . cm-2, respectively. The microscopic elastic constant, k, of bonding between two tubulin monomers neighboring along the singlet microtubule was computed to be k = congruent to 10(2) dyne . cm-1. A singlet microtubule is an order of magnitude as strong against bending and as weak against stretching as an F-actin filament.

Animals↗
PubMed1998

Transventricular pressure-velocity wave propagation in diastole: adherence to the Moens-Korteweg equation.

In the latter half of the diastolic phase of the cardiac cycle, the left atrium contracts and generates a pressure-velocity wave which enters the left ventricle. The wave moves through the inflow tract of the ventricle, reflects off the apex and heads towards the aortic valve. The time taken for the pressure-velocity wave to propagate through the ventricle, referred to as the A-Ar interval, may be measured using pulsed Doppler echocardiography and occurs in the range 20-80 ms. It has been shown previously that there is a significant negative linear correlation between the A-Ar interval and the passive elastic modulus of the ventricular wall (r = -0.782, p < 0.001). This relationship may be explained by modelling the left ventricle as a folded-over elastic tube through which the pressure-velocity wave is propagated according to the principles of the Moens-Korteweg equation.

Algorithms↗
PubMed2006

Dispersion of multi-walled carbon nanotubes in biodegradable poly(butylene succinate) matrix.

This communication describes the preparation, characterization and properties of biodegradable poly(butylene succinate) (PBS)/multi-walled carbon nanotubes (MWCNTs) nanocomposite. Nanocomposite was prepared by melt-blending in a batch mixer and the amount of MWCNTs loading was 3 wt%. State of dispersion-distribution of the MWCNTs in the PBS matrix was examined by scanning and transmission electron microscopic observations that revealed homogeneous distribution of stacked MWCNTs in PBS matrix. The investigation of the thermomechanical behavior was performed by dynamic mechanical thermal analysis. Results demonstrated substantial enhancement in the mechanical properties of PBS, for example, at room temperature, storage flexural modulus increased from 0.64 GPa for pure PBS to 1.2 GPa for the nanocomposite, an increase of about 88% in the value of the elastic modulus. The tensile modulus and thermal stability of PBS were moderately improved after nanocomposite preparation with 3 wt% of MWCNTs, while electrical conductivity of neat PBS dramatically increased after nanocomposite formation. For example, the in plane conductivity increased from 5.8 x 10(-9) S/cm for neat PBS to 4.4 x 10(-3) for nanocomposite, an increase of 10(6) fold in value of the electrical conductivity.

Biodegradation, Environmental↗
PubMed2004

Nanomechanical properties of facial sutures and sutural mineralization front.

The mechanical properties of craniofacial sutures have rarely been investigated. Three facial sutures-the pre-maxillomaxillary (PMS), the nasofrontal (NFS), and the zygomaticotemporal (ZTS)-and their corresponding sutural mineralization fronts in 8 young New Zealand White rabbits were subjected to nano-indentation with atomic force microscopy as a test of the hypothesis that they have different mechanical properties. The average elastic modulus of the PMS was 1.46 +/- 0.24 MPa (mean +/- SD), significantly higher than both the ZTS (1.20 +/- 0.20) and NFS (1.16 +/- 0.18). The average elastic moduli of sutural mineralization fronts 30 micro m away were significantly higher than their corresponding sutures and had the same distribution pattern: the PMS (2.07 +/- 0.24 MPa) significantly higher than both the ZTS (1.56 +/- 0.29) and NFS (1.71 +/- 0.22). Analysis of these data suggests that facial sutures and their immediately adjacent sutural mineralization fronts have different capacities for mechanical deformation. The elastic properties of sutures and sutural mineralization fronts are potentially useful for improving our understanding of their roles in development.

Analysis of Variance↗
PubMedDate not supplied

[A new approach to arterial rigidity: ultrasonic tissue mode imaging].

Vascular stiffness is a major contributory factor of cardiovascular morbidity and mortality. Tissue Doppler imaging (TDI) could make it possible to evaluate vascular rigidity in a site by the measurement of the arterial wall velocity. The objective of this work is to validate the use of tissue Doppler imaging in the assessment of carotid rigidity. The following parameters were measured with TDI (ATL HDI 5000 and software HDI Lab): maximum velocity and mean acceleration of parietal motion (VMax and AccMax). These measurements were corrected for the arterial diameter and pulse pressure (VMax cor and AccMax cor). These data have been compared to the calculated parameters of elasticity from a mode M echography. Thirty-one subjects aged of 26 to 77 years (41.6 +/- 10.6 years, m +/- ESM), without atheromatous plaque or high blood pressure, have been included. The VMax is correlated very significantly at the parietal velocity calculated with mode M echography (r = 0.77; p = 0.00002). The VMax cor is correlated significantly with the parameters of stiffness following: distensibility coefficient, compliance coefficient, pulse wave velocity, elasticity modulus of Young, coefficient beta. [table: see text] Tissue Doppler imaging allows an easy evaluation of the carotid stiffness correlated with the parameters of elasticity. Therefore it constitutes a method of evaluation of the cardiovascular risk. Further longitudinal studies will be able to assess the involvement of the carotid stiffness as causal agent of the cardiovascular risk.

Adult↗
PubMedDate not supplied

A micro double capillary method for rheologic measurements of lower airway secretions.

We modified the double capillary method of Philipoff et al. [23] to allow rheologic measurements of lower respiratory secretions. The system consists of a precision bore stainless steel capillary (0.58 mm i.d., 5 mm length), a glass capillary (0.92 mm i.d., 15 mm length) and a sample holding tube (1.5 mm i.d., 15 mm length). This series of capillaries is connected to a long polyethylene catheter which permits advancing the system through the inner channel of a fiberoptic bronchoscope, and the placing of the sample holding tube into the lower airways for the aspiration of secretions under direct vision. The required sample volume is 20 microliter. The capillary is then removed from the fiberoptic bronchoscope and placed on a microscope stage; the movement of the meniscus of the sample under an applied vacuum as well as the elastic recoil of the sample after release of the vacuum are measured at body temperature. From this procedure, apparent viscosity (na) and shear elastic modulus (G) are calculated. The accuracy of the method was verified in vitro by using standard viscosity oils and a polymer solution of known elasticity. In five normal conscious sheep, na ranged between 200-3000 poises (shear rate: 0.06-1.0 s-1) and G between 17-83 dyn X cm-2. The micro double capillary method was found adequate for direct rheologic measurements of lower airway secretions.

Animals↗
PubMed1988

On the measurement of shear elastic moduli and viscosities of erythrocyte plasma membranes by transient deformation in high frequency electric fields.

We present a new method to measure the shear elastic moduli and viscosities of erythrocyte membranes which is based on the fixation and transient deformation of cells in a high-frequency electric field. A frequency domain of constant force (arising by Maxwell Wagner polarization) is selected to minimize dissipative effects. The electric force is thus calculated by electrostatic principles by considering the cell as a conducting body in a dielectric fluid and neglecting membrane polarization effects. The elongation A of the cells perpendicular to their rotational axis exhibits a linear regime (A proportional to Maxwell tension or to square of the electric field E2) at small, and a nonlinear regime (A proportional to square root of Maxwell tension or to the electric field E) at large extensions with a cross-over at A approximately 0.5 micron. The nonlinearity leads to amplitude-dependent response times and to differences of the viscoelastic response and relaxation functions. The cells exhibit pronounced yet completely reversible tip formations at large extensions. Absolute values of the shear elastic modulus, mu, and membrane viscosity, eta, are determined by assuming that field-induced stretching of the biconcave cell may be approximately described in terms of a sphere to ellipsoid deformation. The (nonlinear) elongation-vs.-force relationship calculated by the elastic theory of shells agress well with the experimentally observed curves and the values of mu = 6.1 x 10(-6) N/m and eta = 3.4 x 10(-7) Ns/m are in good agreement with the micropipette results of Evans and co-workers. The effect of physical, biochemical, and disease-induced structural changes on the viscoelastic parameters is studied. The variability of mu and eta of a cell population of a healthy donor is +/- 45%, which is mainly due to differences in the cell age. The average mu value of cells of different healthy donors scatters by +/- 18%. Osmotic deflation of the cells leads to a fivefold increase of mu and 10-fold increase of eta at 500 mosm. The shear modulus mu increases with temperature showing that the cytoskeleton does not behave as a network of entropy elastic springs. Elliptic cells of patients suffering from elliptocytosis of the Leach phenotype exhibit a threefold larger value of mu than normal discocytes of control donors. Cross-linking of the spectrin by the divalent S-H agents diamide (1 mM, 15 min incubation) leads to an eightfold increase of mu whereas eta is essentially constant. The effect of diamide is reversed after treatment with S-S bond splitting agents.

Diamide↗
PubMed2004

Mechanics considerations for microporous titanium as an orthopedic implant material.

This article investigates mechanics issues related to potential use of a recently developed porous titanium (Ti) material for load-bearing implants. This material may have advantages over solid Ti of enhancing the bone-implant interface strength by promoting bone and soft tissue ingrowth and of reducing the bone-implant modulus mismatch, which can lead to stress shielding. Experimental data from ultrasound experiments and uniaxial compression testing on microporous Ti are presented. Analytic models to predict its elastic modulus and Poisson's ratio are discussed, including "structural" approaches (Gibson and Ashby's cellular solids) and a "composite material" approach (Mori-Tanaka). Finally, two-dimensional finite element models based on optical micrographs of the material are presented. Simulations were performed for different conditions and levels of approximation. Results demonstrate that simple analytic models provide good estimates of the elastic properties of the porous Ti and that the moduli can be significantly reduced to decrease the mismatch between solid Ti and bone. The finite element simulations show that bone ingrowth will dramatically reduce stress concentrations around the pores.

Biomechanical Phenomena↗
PubMed2003

Nanostructured Ti-based multi-component alloys with potential for biomedical applications.

A group of Ti(60)Cu(14)Ni(12)Sn(4)M(10) (M=Nb, Ta, Mo) alloys was prepared using arc melting and copper mold casting. The as-prepared alloys have a composite microstructure containing a micrometer-sized dendritic beta-Ti(M) phase dispersed in a nanocrystalline matrix. These new alloys exhibit a low Young's modulus in the range of 59-103 GPa, and a high yield strength of 1037-1755 MPa, together with large plastic strains. The combination of high strength and low elastic modulus offers potential advantages in biomedical applications.

Alloys↗
PubMed2001

Relationship among biomechanical, biochemical, and cellular changes associated with osteoarthritis.

Articular cartilage that lines the surface of long bones is a multilayered material. The superficial layer consists of collagen fibrils and chondrocytes that run parallel to the joint surface. In the deeper layers, the collagen fibrils are more randomly arranged and support vertical units termed chondrons containing rows of chondrocytes. In the deepest layers, the collagen fibrils run almost vertically and ultimately insert into the underlying subchondral bone. Osteoarthritis (OA) is a disease that affects articular cartilage and is characterized by enzymatic and mechanical breakdown of the extracellular matrix, leading to cartilage degeneration, exposure of subchondral bone, pain, and limited joint motion. Changes in mechanical properties of articular cartilage associated with OA include decreases in modulus and ultimate tensile strength. These changes parallel the changes observed after enzymatic degradation of either collagen or proteoglycans in cartilage. Results of recent viscoelastic studies on articular cartilage suggest that the elastic modulus of collagen and fibril lengths decrease in OA and are associated with a loss of the superficial zone and a decreased ability of articular cartilage to store elastic energy during locomotion. It is suggested that osteoarthritic changes to cartilage involve enzymatic degradation of matrix components and fibril fragmentation that is promoted by subsequent mechanical loading.

Animals↗