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PubMed2004

Temperature prediction in a finite element model for sliding contact analysis of total hip prosthesis.

A finite-element model for sliding contact in total hip joint prosthesis is presented in this paper. The hip prosthesis studied consists of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup articulating against cobalt-chrome and alumina-ceramic femoral heads. Various aspects of prosthesis operation were analysed using the finite-element model. For example, bulk material and surface stresses were analysed under varying conditions of elastic modulus, friction coefficient, sliding speed, and radial clearance. The resulting variations of temperature were also recorded. The results obtained from the model are useful in understanding the operating conditions and the causes of wear in the hip prosthesis.

Computer Simulation↗

PubMed2001

Relationship of hydraulic impedance and elasticity in the pulmonary artery of maturing newborn pigs.

The current study determined the dynamic stress-strain elastic moduli (E(Y)) and characteristic impedances (Z(0(2-7Hz))) of the main pulmonary artery in open-chest, anesthetized newborn pigs at 2 days, 2 weeks, and 3 months of age. E(Y) and Z(0(2-7Hz)) were compared to those values derived from the Womersley and Moens-Korteweg equations (denoted E(W-MK) and Z(0W-MK), respectively) to test the validity of these equations in describing the elasticity of the intact newborn pulmonary artery. E(Y) was defined as the ratio of stress to strain. The current study hypothesized that: (1) E(Y) and E(W-MK) are numerically similar, and therefore the Womersley and Moens-Korteweg equations accurately describe the viscoelastic properties of the main pulmonary artery of the newborn pig, and (2) that both E(Y) and Z(0) are elevated at birth and undergo a steady decline with maturation. E(Y) was not significantly different from E(W-MK), while Z(0(2-7Hz)) was nearly identical to Z(0W-MK) in all groups. The elastic modulus peaked (P < 0.001) in 2-week-old pigs compared with both younger and older animals, while Z(0(2-7Hz)) decreased with increasing age (48 h = 1237 +/- 251 [SEM] dyn s cm(-5), 2-week = 433 +/- 95 dyn s cm(-5), 3 month = 162 +/- 17 dyn s cm(-5), P < 0.001). These experiments validate the Womersley and Moens-Korteweg equations as accurately describing the elastic properties of the intact newborn pig pulmonary artery. These data also demonstrate that a diminution in Z(0) may occur even with increased wall stiffness, as observed in our 2-week-old pigs.

Age Factors↗

PubMed1988

Entropic elastic processes in protein mechanisms. I. Elastic structure due to an inverse temperature transition and elasticity due to internal chain dynamics.

Numerous physical characterizations clearly demonstrate that the polypentapeptide of elastin (Val1-Pro2-Gly3-Val4-Gly5)n in water undergoes an inverse temperature transition. Increase in order occurs both intermolecularly and intramolecularly on raising the temperature from 20 to 40 degrees C. The physical characterizations used to demonstrate the inverse temperature transition include microscopy, light scattering, circular dichroism, the nuclear Overhauser effect, temperature dependence of composition, nuclear magnetic resonance (NMR) relaxation, dielectric relaxation, and temperature dependence of elastomer length. At fixed extension of the cross-linked polypentapeptide elastomer, the development of elastomeric force is seen to correlate with increase in intramolecular order, that is, with the inverse temperature transition. Reversible thermal denaturation of the ordered polypentapeptide is observed with composition and circular dichroism studies, and thermal denaturation of the crosslinked elastomer is also observed with loss of elastomeric force and elastic modulus. Thus, elastomeric force is lost when the polypeptide chains are randomized due to heating at high temperature. Clearly, elastomeric force is due to nonrandom polypeptide structure. In spite of this, elastomeric force is demonstrated to be dominantly entropic in origin. The source of the entropic elastomeric force is demonstrated to be the result of internal chain dynamics, and the mechanism is called the librational entropy mechanism of elasticity. There is significant application to the finding that elastomeric force develops due to an inverse temperature transition. By changing the hydrophobicity of the polypeptide, the temperature range for the inverse temperature transition can be changed in a predictable way, and the temperature range for the development of elastomeric force follows. Thus, elastomers have been prepared where the development of elastomeric force is shifted over a 40 degrees C temperature range from a midpoint temperature of 30 degrees C for the polypentapeptide to 10 degrees C by increasing hydrophobicity with addition of a single CH2 moiety per pentamer and to 50 degrees C by decreasing hydrophobicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemical Phenomena↗

PubMed1997

Load sharing between the shell and centrum in the lumbar vertebral body.

STUDY DESIGN: A finite element parametric analysis to investigate the relative load carrying roles of the shell and centrum in the lumbar vertebral body. OBJECTIVE: To address the issue of the structural role of the vertebral shell and clarify some of the contradictions raised by previous studies. SUMMARY OF BACKGROUND DATA: A number of experimental and finite element studies have attempted to quantify the relative structural roles of the shell and centrum, but these studies support no consensus on the relative contribution of the shell to vertebral body strength. METHODS: The authors developed finite element models to predict the fraction of the total compressive force acting on the lumbar vertebral body that is carried by the shell. Parametric variations were investigated to determine how the fraction of shell force was affected by changes in shell thickness, shell and centrum modulus, centrum anistropy, and loading conditions. RESULTS: The fraction of compressive force carried by the shell increased from approximately 0 at the endplate to approximately 0.2 at the mid-transverse plane for a typical case. The shell force was highly sensitive to the degree of anisotropy of the trabecular centrum but was relatively insensitive to changes in shell thickness and the ratio of shell-to-centrum elastic modulus. CONCLUSIONS: The conflicting conclusions of previous studies about the structural roles of the vertebral shell and centrum can be explained by differences in their methods. Our findings support the claims that the shell accounts for only approximately 10% of vertebral strength in vivo and that the trabecular centrum is the dominant structural component of the vertebral body.

Biomechanical Phenomena↗

PubMed1984

Adhesivity and rigidity of erythrocyte membrane in relation to wheat germ agglutinin binding.

Binding of the plant lectin wheat germ agglutinin (WGA) to erythrocyte membranes causes membrane rigidification. One of our objectives has been to directly measure the effects of WGA binding on membrane rigidity and to relate rigidification to the kinetics and levels of WGA binding. Our other objective has been to measure the strength of adhesion and mechanics of cell separation for erythrocytes bound together by WGA. The erythrocyte membrane rigidity was measured on single cells by micropipette aspiration. The slope of the suction pressure-length data for entry into the pipette provided the measure of the membrane extensional modulus. Data were collected for cells equilibrated with WGA solutions in the range of concentrations of 0.01-10 micrograms/ml. Erythrocyte-erythrocyte adherence properties were studied by micropipette separation of two-cell aggregates. First, a "test" cell was selected from a WGA solution by aspiration into a small micropipette, then transferred to a separate chamber that contained erythrocytes in WGA-free buffer. Here, a second cell was aspirated with another pipette and maneuvered into close proximity of the test cell surface, and adhesive contact was produced. The flaccid cell was separated from the test cell surface in steps at which the force of attachment was derived from the pipette suction pressure and cell geometry. In addition, we measured the time-dependent binding and release of fluorescently labeled WGA to single erythrocytes with a laser microfluorometry system. The results showed that the stiffening of the erythrocyte membrane and binding of fluorescently labeled WGA to the membrane surface followed the same concentration and time dependencies. The threshold concentration for membrane stiffening was at approximately 0.1 microgram/ml where the time course to reach equilibrium was close to 1 h. The maximal stiffening (almost 30-fold over the normal membrane elastic modulus) occurred in concentrations greater than 2 micrograms/ml where the time to reach equilibrium took less than 1 min. The WGA binding also altered the normal elastic membrane behavior into an inelastic, plastic-like response which indicated that mechanical extension of the membrane caused an increase in cross-linking within the surface plane. Similar to the stiffening effect, we observed that the membrane adhesivity of cells equilibrated with WGA solutions greatly increased with concentration greater than 0.1 microgram/ml.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell Adhesion↗

PubMed2002

Effects of ischemia and myogenic activity on active and passive mechanical properties of rat cerebral arteries.

Passive (papaverine induced) and active (spontaneous pressure induced) biomechanical properties of ischemic and nonischemic rat middle cerebral arteries (MCAs) were studied under pressurized conditions in vitro. Ischemic (1 h of occlusion), contralateral, and sham-operated control MCAs were isolated from male Wistar rats (n = 22) and pressurized using an arteriograph system that allowed control of transmural pressure (TMP) and measurement of lumen diameter and wall thickness. Three mechanical stiffness parameters were computed: overall passive stiffness (beta), pressure-dependent modulus changes (E(inc,p)), and smooth muscle cell (SMC) activity-dependent changes (E(inc,a)). The beta-value for ischemic vessels was increased compared with sham vessels (13.9 +/- 1.7 vs. 9.1 +/- 1.4, P < 0.05), indicating possible short-term remodeling due to ischemia. E(inc,p) increased with pressure in the passive vessels (P < 0.05) but remained relatively constant in the active vessels for all vessel types, indicating that pressure-induced SMC contractile activity (i.e., myogenic reactivity) in cerebral arteries leads to the maintenance of a constant elastic modulus within the autoregulatory pressure range. E(inc,a) increased with pressure for all conditions, signifying that changes in stiffness are influenced by SMC activity and vascular tone.

Animals↗

PubMed1994

Thermal behavior and elastic properties of dimyristoyl phosphatidylcholine bilayers under the effect of pentoxifylline.

We have investigated the effect on dimyristoyl phosphatidylcholine bilayers of pentoxifylline, a derivative of xanthine by using two optical techniques, quasi-elastic light scattering (QLS) and Fourier transform infrared spectroscopy (FT-IR). The results show that in the presence of pentoxifylline, the bilayer phase transition point is lowered and that the elastic modulus is decreased. The FT-IR results indicate strong interactions in the aqueous interface regions of the bilayers. We discussed these results comparatively with those obtained from flavonoid derivatives whose effect was analogous and previously studied.

Dimyristoylphosphatidylcholine↗

PubMed2004

Stress distributions in adhesively cemented ceramic and resin-composite Class II inlay restorations: a 3D-FEA study.

OBJECTIVES: The purpose of this study was to investigate the effect of differences in the resin-cement elastic modulus on stress-transmission to ceramic or resin-based composite inlay-restored Class II MOD cavities during vertical occlusal loading. METHODS: Three finite-element (FE) models of Class II MOD cavity restorations in an upper premolar were produced. Model A represented a glass-ceramic inlay in combination with an adhesive and a high Young's modulus resin-cement. Model B represented the same glass-ceramic inlay in combination with the same adhesive and a low Young's modulus resin-cement. Model C represented a heat-cured resin-composite inlay in combination with the same adhesive and the same low Young's modulus resin cement. Occlusal vertical loading of 400 N was simulated on the FE models of the restored teeth. Ansys FE software was used to compute the local von Mises stresses for each of the models and to compare the observed maximum intensities and distributions. Experimental validation of the FE models was conducted. RESULTS: Complex biomechanical behavior of the restored teeth became apparent, arising from the effects of the axial and lateral components of the constant occlusal vertical loading. In the ceramic-inlay models, the greatest von Mises stress was observed on the lateral walls, vestibular and lingual, of the cavity. Indirect resin-composite inlays performed better in terms of stress dissipation. Glass-ceramic inlays transferred stresses to the dental walls and, depending on its rigidity, to the resin-cement and the adhesive layers. For high cement layer modulus values, the ceramic restorations were not able to redistribute the stresses properly into the cavity. However, stress-redistribution did occur with the resin-composite inlays. SIGNIFICANCE: Application of low modulus luting and restorative materials do partially absorb deformations under loading and limit the stress intensity, transmitted to the remaining tooth structures.

Bicuspid↗

PubMed2006

Collective fluctuations in ordered fluids investigated by two-dimensional electron-electron double resonance spectroscopy.

Two-dimensional electron-electron double resonance (2D-ELDOR) is a technique that is sensitive to the dynamical processes affecting spin labels in complex fluid environments. In ordered fluids, such as membrane vesicles, the 2D-ELDOR experiment is affected by the molecular tumbling in the locally ordered environment. This motion occurs on two different time scales, the faster molecular motion relative to the local director, and the slower collective fluctuations of the director field. In the experimental study of Patyal, Crepeau, and Freed (Biophys. J. 1997, 73, 2201), it was found that the widths of the autopeaks of the 2D-ELDOR spectrum increased as a function of the mixing time. In the present work, a theory is developed for the effects of director fluctuations on the autopeaks in the 2D-ELDOR experiment by employing an analytical solution of the stochastic Liouville equation for which the director field is treated as a multidimensional Gaussian process, as previously developed by Frezzato, Kothe, and Moro (J. Phys. Chem. B 2001, 105, 1281 and J. Phys. Chem. B 2004, 108, 9505). Good agreement is found between theory and experiment, notably the only adjustable parameter is k, the bending elastic modulus of the membrane. The values of k = 11 x 10(-20) J for 1,2-dipalmitoyl-sn-glycero-phosphatidylcholine (DPPC) vesicles and k = 15 x 10(-20) J for DPPC/gramicidin A (5:1) vesicles, both at 45 degrees C, were found from the analysis and agree well with previous related measurements by other physical techniques. This establishes 2D-ELDOR as a useful technique to study the elastic properties of biological membranes.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2002

Influence of type and level of water-soluble additives on drug release and surface and mechanical properties of Surelease films.

Ethylcellulose in combination with water-soluble additives has been used in the development of microporous membrane-coated dosage forms. In the present study, application of three types of water-soluble additives, namely polyethylene glycols (PEG 400, 3350, and 8000), maltodextrins (Maltrin M150, M100, and M040 in the order of lower to higher average polymer size and molecular weight; dextrose equivalence 16.9, 11.1, and 4.8, respectively), and xylitol, as porosity modifiers in the films of a commercially available aqueous ethylcellulose dispersion (Surelease/E-7-7060 plasticized with glyceryl tricaprylate/caprate) was investigated. The effect of type and level of these additives on drug release characteristics and surface and mechanical properties of the polymeric films was studied. Each additive was incorporated at 20 and 30% levels in the polymeric dispersion based on its solids content. Ibuprofen tablets were coated using the polymeric dispersion with and without additive at 3% w/w coat level in a fluid-bed equipment. The coated tablets were evaluated for their drug release rate, coat reflectivity (gloss), Brinell hardness, and elastic modulus. Differential scanning calorimetric analysis of the films was performed to determine the physico-chemical changes in the applied film-coats. The rate of drug release, hence film porosity, was observed to be dependent on the type and level of the additive added. The molecular weight of the additive and its concentration in the polymeric dispersion had significant influence on the rate of drug release, hardness, and elasticity of the film-coats.

Cellulose↗

PubMed1988

Decreased vascular contraction and elastic stiffening after intramural lymphostasis.

Lymphatic obstruction leads to leakage of protein into the vessel wall that is coupled with edema formation and smooth muscle degeneration. To clarify the mechanical effect of these changes, lymphatic trunks, draining the femoral artery, were stained with Evans blue and then ligated in dogs anesthetized with pentobarbital sodium (n = 11). Sham ligation was performed on these lymphatic trunks in the opposite hindlimb (n = 11). Three days later, 7- to 30-mm segments of both femoral arteries were studied in vitro. Intraluminal pressure was cycled between 5 and 250 mmHg at 5 mmHg/s, and external diameter was recorded in both relaxed and activated (norepinephrine, 3 x 10(-5) mol/l) state of smooth muscle. Mechanical parameters were computed for 10-mmHg pressure increments. After lymphatic obstruction, strain energy density and distensibility of the passive wall components as well as isobaric active tangential strain were decreased by a maximum of 27, 59, and 54%, respectively; active tangential stress and elastic modulus, as a function of tangential stretch, was decreased by a maximum of 62%, and increased by 61%, respectively. These results indicate that short-term intramural lymphostasis reduces smooth muscle reactivity and induces vascular stiffening.

Animals↗

PubMed1985

The static elastic properties and chemical composition of the rat aorta in spontaneously occurring and experimentally induced hypertension: the effect of an anti-hypertensive drug.

The static elasticity and scleroprotein content of the aorta have been measured in 24 Okamoto spontaneously hypertensive rats aged 22-25 weeks, and 24 Wistars of the same age in which hypertension had been induced by nephrectomy and treated with a steroid. From the age of 4 weeks half the animals in each group were treated with a diuretic drug. By the age of 15 weeks caudal artery systolic blood pressure was significantly lower than control values in both drug-treated groups and remained so until death. Both types of hypertension were associated with larger diameter, thicker-walled and heavier aortas than those in the drug-treated animals. Vessels from Okamoto animals contained more collagen than those from the Wistars, although the collagen content was unchanged by drug treatment. Neither drug nor strain had any clear-cut affect on elastin content. In spite of these differences in wall thickness and chemical composition, values of the functional stiffness of the aorta measured over a wide range of pressure were similar in all four groups of animals. Using a simple model of the aorta in which elastin and collagen bear stress in parallel we find that the relationship between vessel composition and static incremental elastic modulus (structural stiffness) is similar in both models of hypertension and is not changed by drug treatment in spite of the consequent reduction in blood pressure.

Aging↗

PubMedDate not supplied

Effect of tibial dyschondroplasia on broiler growth and cancellous bone mechanical properties.

The increased incidence of leg abnormalities, particularly tibial dyschondroplasia, in chickens could be related to changes in tibiotarsal cancellous bone properties. To explore this hypothesis, the relationship between lesion occurrence and various tibiotarsal growth parameters, and subchondral bone strength characteristics was investigated. A higher elastic modulus, meaning the cancellous bone was more rigid, was seen for tibiotarsal cancellous bone with lesions. Microfractures in cancellous bone, particularly in the medial growth plate region, may lead to overall bone conformation changes and therefore to lameness.

Aging↗

PubMed1986

Forces developed beneath hydrogel contact lenses due to squeeze pressure.

The hydrodynamic squeeze pressure in the fluid thin film beneath hydrogel contact lenses fitted onto an axisymmetric model eye were measured. These pressures were due to the contact lens relaxing after deformation by an applied force of similar magnitude to the human eyelid force. The distribution of pressure for contact lenses typically fitted to human eyes was negative with respect to atmospheric pressure at the corneal apex and became less negative at the corneo-scleral limbus. The force that the contact lens applied to the cornea was determined by integrating the pressure distribution from the corneal apex to the limbus. This force varied from 6.0 X 10(-4) N to -7.8 X 10(-1) N depending on the thickness, elastic modulus and bearing relationship of the contact lens. An expression was derived to determine the pressure developed beneath the annulus of the hydrogel contact lens overlapping the cornea, in terms of the measured force over the cornea beneath the contact lens and the chord diameters of the contact lens and cornea. It was found that the deformation of hydrogel contact lenses on the model eye did not follow a linear elastic shell theory.

Biophysical Phenomena↗

PubMed2002

Determination of elastic moduli of thin layers of soft material using the atomic force microscope.

We address three problems that limit the use of the atomic force microscope when measuring elastic moduli of soft materials at microscopic scales. The first concerns the use of sharp cantilever tips, which typically induce local strains that far exceed the linear material regime. We show that this problem can be alleviated by using microspheres as probes, and we establish the criteria for their use. The second relates to the common use of the Hertz contact mechanics model, which leads to significant errors when applied to thin samples. We develop novel, simple to use corrections to apply for such cases. Samples that are either bonded or not bonded to a rigid substrate are considered. The third problem concerns the difficulty in establishing when contact occurs on a soft material. We obtain error estimates for the elastic modulus resulting from such uncertainty and discuss the sensitivity of the estimation methods to error in contact point. The theoretical and experimental results are compared to macroscopic measurements on poly(vinyl-alcohol) gels.

Calibration↗

PubMed2005

Acid-induced gelation of enzymatically modified, preheated whey proteins.

Low-pH whey protein gels are formulated using a sequential protocol of heat treatment, enzyme incubation, and cold-set acidification. The heat-induced disulfide and enzyme-catalyzed epsilon-(gamma-glutamyl)lysine linkages, both at neutral pH, produce a polymerized protein solution. The molecular weights of these samples show an exponential increase with protein concentration. The additional enzyme-catalyzed cross-links cause little change in molecular weight from that of heat-treated samples at low protein concentrations, indicating predominant intramolecular cross-linking. Enzyme treatment at higher protein concentration however causes increase in molecular weight, possibly due to formation of intermolecular cross-links. Acidification of the polymerized protein solutions through glucono-delta-lactone acid leads to gel formation at pH 4. The elastic (G') and viscous (G' ') moduli of gels with and without enzyme treatment show similar frequency dependence, indicating comparable microstructures, consistent with all samples exhibiting similar fractal dimensions of approximately 2 obtained independently using rheology and confocal microscopy. A substantial increase in fracture strain and stress of the gel is achieved by enzyme treatment. However, the elastic modulus (G') is only slightly larger after enzyme treatment compared with heat-treated samples. These results indicate that factors responsible for fracture properties may not be apparent in the gel microstructure and linear viscoelastic properties.

Cross-Linking Reagents↗

PubMed2003

Arterial remodeling and stiffness in patients with pseudoxanthoma elasticum.

OBJECTIVE: Proteoglycans organize the extracellular matrix, act as signaling molecules, and are involved in cell migration and proliferation. They may play an important role in arterial geometric and elastic properties. The aim of the present study was to determine large artery phenotype in patients with pseudoxanthoma elasticum (PXE), a genetic disease characterized by proteoglycan accumulation and fragmented elastic fibers in connective tissues. METHODS AND RESULTS: In 27 patients with PXE (25 females and 2 male) and 27 control subjects matched by age, sex, and blood pressure, we noninvasively determined the common carotid and radial artery diameter, intima-media thickness (IMT), and distensibility with high-definition echo-tracking systems and applanation tonometry. Patients with PXE had a significantly higher carotid IMT (611+/-106 versus 520+/-76 microm, P<0.001) independently of body surface area, age, and mean blood pressure. The increase in carotid IMT predominated in older patients with PXE at the time of examination. No significant difference in carotid elastic properties was observed between patients with PXE and control subjects. At the site of the radial artery, distensibility was significantly higher in patients with PXE than in control subjects (11.6+/-11.4 versus 5.9+/-3.4 kPa(-1) x 0.10(-3); P=0.02) and internal diameter was lower (1.66+/-0.51 versus 2.07+/-0.36 mm; P<0.01) without change in intima-media thickness and Young's elastic modulus. CONCLUSIONS: Phenotypic changes of superficial arteries in patients with PXE were represented by a thickening of the carotid artery and a reduced stiffness of the radial artery and predominated in older female patients.

Adult↗

PubMed2004

Temperature-induced gelation of concentrated silicon carbide suspensions.

Due to the steric barrier provided by the adsorption of the dispersant hypermer KD1 (a polyester/polyamine condensation polymer), stable and low-viscosity suspensions of SiC, Y(2)O(3), and Al(2)O(3) powder mixtures could be prepared in methyl ethyl ketone (MEK)/ethanol (E) solvent with solids loading as high as 60 vol%. The solvency of the dispersant in MEK/E decreased dramatically on cooling. Steady shear viscosity and oscillatory measurements were performed as a function of temperature for suspensions with different solids loading. The viscosity and elastic modulus of suspension increased with decreasing temperature and became more sensitive with the increase of solids loading. The suspensions with solids loading higher than 40 vol% could be solidified with decreasing temperature, but gelation temperature and gelation stiffness decreased with decreasing solids loading. The 60 vol% solid-loaded suspension was a stable and free-flowing fluid at 20 degrees C and gradually transformed to a very highly viscous and elastic system upon cooling to about 13 degrees C. Complete solidification occurred when the temperature was decreased to 5 degrees C. The gelation mechanism was mainly based on the collapse of the adsorbed layer as the temperature decreases, which induced incipient flocculation and formed a stiff network. The gelled body was further strengthened by separation of the dispersant from the suspension.

Carbon Compounds, Inorganic↗