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PubMed1998

Comparative assessment of hepatic Glisson's capsule and bovine pericardium in heart valve bioprostheses.

BACKGROUND AND AIM OF THE STUDY: The optimal material for heart valve bioprostheses remains disputable. This investigation was initiated to compare the properties of hepatic Glisson's capsule, clinical experience of which in cardiovascular surgery is minimal, with those of bovine pericardium. METHODS: Hepatic Glisson's capsule was harvested from bull calves and used to create composite pulmonary arterial monocusp grafts and bioprostheses. Comparison of the strength and elastic properties of Glisson's capsule and bovine pericardium, as well as the hydrodynamic characteristics of valves made from these materials, was performed. Late results of operations using these materials were estimated echocardiographically. RESULTS: Although Glisson's capsule tissue is thinner than the bovine pericardium, its elasticity modulus is greater. However, the hydrodynamic characteristics of heart valves made from either tissue are similar. Moreover, valves made from Glisson's capsule have a lower systolic pressure gradient on the prosthesis and a higher effective orifice area. Composite pulmonary arterial xenopericardial grafts with a monocusp of Glisson's capsule were used in 30 patients during tetralogy of Fallot repair. Glisson's capsule was also used for tricuspid valve reconstruction and as a bioprosthesis in six patients with Ebstein's anomaly. At 1-2 years after surgery, the Glisson's capsule tissue remained thin and flexible, with no calcification. CONCLUSIONS: Although the hydrodynamic properties of hepatic Glisson's capsule and the bovine pericardium are similar, the capsule tissue is thinner and has a greater elasticity modulos. Thus, Glisson's capsule may be used for bioprosthesis construction both independently and in combination with bovine pericardium.

Animals↗

PubMed1991

In vivo biophysical characterization of skin physiological differences in races.

The role of race in modulating skin responses has been investigated. Several parameters (skin thickness, transepidermal water loss, water content of the stratum corneum and skin biomechanics) have been measured using noninvasive tools in whites, Hispanics and blacks to assess whether the melanin content could induce changes in skin biophysical properties. Marked differences between races appear in stratum corneum water content and in skin extensibility, recovery and elastic modulus. Measurements done in different sun-exposed sites highlight the effects of solar irradiation on the skin and the role of melanin in preventing skin damage. The study shows that racial differences in skin physiology exist and are mainly related to the protective role of melanin present in races with darker skin. Moreover, differences in skin hydration are not fully explained according to the site and presence of hair.

Black People↗

PubMed2001

Mechanics of the carotid artery wall and baroreflex sensitivity after acute ethanol administration in young healthy volunteers.

The effects of ethanol administered orally (300 mg/kg in 250 ml of water) or intravenously (7.5 mg.min(-1).kg(-1) in 250 ml of saline over 40 min) on common carotid haemodynamics, wall mechanics and baroreflex sensitivity were compared with the effects of the intravenous infusion of 250 ml of saline. Ethanol or saline was administered to 10 healthy volunteers after 30 min of supine rest, and measurements were obtained 40 min (median; range 34-46 min) after administration. After ethanol administration, the plasma alcohol level rose from 0 to 0.3+/-0.07 g/l. Mean arterial blood pressure had risen slightly at 20 min, but was normalized by 40 min, the time at which the haemodynamic study was performed. Heart rate decreased after infusion of either saline or alcohol, but was unchanged after oral ethanol administration. Both oral and intravenous ethanol administration were associated with significant decreases in baroreflex sensitivity, carotid shear stress and blood velocity, compared with resting values, while the mean carotid artery diameter was increased, and blood viscosity and mean blood flow were unchanged. No changes were observed in these parameters after saline administration. Ethanol, administered either intravenously or orally, increased the stiffness of the carotid artery and decreased the pulsatility (systo-diastolic changes) of its diameter. A direct, statistically significant correlation was found between the decrease in shear stress and the decrease in baroreflex heart rate control sensitivity after both modes of alcohol administration, while no such correlation was found between the increase in the Peterson elastic modulus and the decrease in carotid diameter pulsatility on the one hand or the decrease in baroreflex sensitivity on the other. In conclusion, reduced shear stress associated with vasodilatation of the carotid artery wall may contribute to the decrease in baroreflex sensitivity observed after acute ethanol administration.

Administration, Oral↗

PubMed2000

Hydrodynamic and Colloidal Interactions in Concentrated Charge-Stabilized Polymer Dispersions.

Hydrodynamic and colloidal interactions are explored in concentrated, charge-stabilized colloidal dispersions by measuring the dependence of rheology (e.g., low and high-shear viscosity, high-frequency viscosity, and modulus) and self-diffusivity on salt content, particle size, and concentration. Model, sulfonated polystyrene lactices of varying diameter are prepared and investigated by shear rheology, high-frequency torsional resonance, electrophoresis, titration, and dynamic light scattering. The high-frequency and high-shear viscosity both are dominated by hydrodynamic interactions, but are shown not to be identical, due to the microstructure distortion resulting from high shear rates. The short-time self-diffusion is also shown to be insensitive to direct particle interactions, but has a different concentration dependence than the high-frequency viscosity, further illustrating a predicted violation of a generalized Stokes-Einstein relationship for these properties. The apparent colloidal surface charge is extracted from the high-frequency elastic modulus measurements on concentrated dispersions. The surface charge is in good agreement with results from critical coagulation concentration measurements and perturbation theories, but disagrees with electrophoretic mobility experiments. This indicates that the effective surface charge determined by torsional high-frequency measurements is a more reliable predicter of the salt stability of charge-stabilized dispersions, in comparison to zeta-potentials determined from electrophoretic mobilities. Further, we demonstrate by direct comparison that measurements of the apparent plateau modulus by rotational rheometry underestimate the true, high-frequency modulus and provide unreliable estimates for the surface charge. Copyright 2000 Academic Press.

Journal Article↗

PubMed2002

Age-associated changes in arterial elastic properties in children.

UNLABELLED: Age-associated changes in pulsatile components of the arterial system in normal children have not previously been studied. The purpose of the present study was to establish reference ranges for age-associated changes in arterial pulsatile properties in children. Arterial compliance (C(A)) and stiffness of the proximal aorta were estimated from data obtained by cardiac catheterisation in 112 paediatric patients (age range, 6 months to 20 years) who were considered to have normal systemic vascular beds. C(A) was calculated by the area method, using ascending aortic pressure tracings. Aortic wall stiffness was evaluated using the pressure-strain elastic modulus (Ep). C(A) significantly increased with age (C(A) =0.06 5 x age +0.86, r=0.71, P<0.001). However, the value of C(A)/BSA, which represents normalisation of C(A) to body surface area (BSA) (to eliminate factors associated with development of arterial size), significantly decreased with age and declined fastest during the first several years of life, suggesting an increase in arterial wall stiffness with increasing age in children. Ep also significantly increased with age (Ep =9.8 x age +177, r=0.50, P<0.001). CONCLUSION: these findings suggest that both peripheral and proximal arterial wall distensibility in children decline after birth, although total arterial buffering capacity increases. The present results provide the first quantification of in vivo age-associated changes in arterial elastic properties in children, and are potentially useful as a reference for future studies of patients with diseases that involve the vascular system.

Adolescent↗

PubMed1990

Viscoelastic properties of tissue conditioners--stress relaxation test using Maxwell model analogy.

In order to measure the viscoelastic properties of tissue conditioners, a series of stress relaxation tests was carried out using poly ethyl methacrylate polymer powders and liquids composed of butyl phthalyl butyl glycolate/ethyl alcohol mixtures, respectively. The analysis method using the Maxwell model analogy is discussed. The results may be summarized as follows. (i) In this study it was feasible to make the stress relaxation curves for 30 min analogous to the Maxwell model, using a model with a maximum of 5 elements. (ii) The elastic modulus Ei, the coefficient of viscosity eta i, and relaxation time tau i, for each element, and the instantaneous modulus E0, tended to increase with time. These values in the element of the longest relaxation time were most marked in the increase among those in all the elements. Furthermore, in every element the rate of increase of eta i was greater than that of Ei. The method described is considered to be one of the most useful techniques available for the study of the viscoelastic properties of tissue conditioners.

Chemical Phenomena↗

PubMed2003

Respiratory flow in a realistic tracheostenosis model.

The possible mechanism of wheeze generation in tracheostenosis was identified by measuring inspiratory and expiratory flow in a "morphological and distensible" realistic tracheostenosis model. The shape of the model was based on CT (Computed Tomography) images of a patient that had tracheostenosis. A trachea consists of tracheal cartilage rings and smooth muscle. Spatial variation of wall distensibility was achieved in the model by varying the wall thickness based on the elastic modulus measured in pig airways. The spatial variation influenced the flow in the airway and the turbulence production rate decreased faster at smooth muscles. Using the model, we investigated the mechanism of wheeze generation by focusing on the turbulence intensity. The turbulence intensity in expiratory flow was about twice that in inspiratory flow, and larger vortices existed in post-stenosis in expiratory flow, and thus might contribute to wheeze generation.

Animals↗

PubMed1977

Bone cement and antibiotics.

Antibiotics of various kinds were added to the powdered polymer of methylmethacrylate in a concentration of 1 g antibiotic/40 g cement, prior to the addition of the liquid monomer. Specimens were subjected to mechanical testing by loading them in direct compression on a 20-ton Instron Universal Testing Machine, and load deflection curves were recorded. It was found that the addition of antibiotics does have an effect on the mechanical strength of the cement--the loss was small, less than 10% for both the compressive strength and the elastic modulus, provided the antibiotic was in powder form. Liquid antibiotics resulted in a greater loss of the compressive strength of the mixtures, but specimens tended to be less porous. In vitro studies of the antibiotic properties of the mixtures demonstrated potent antibacterial activity in all the antibiotics tested, except chloramphenicol. Possible clinical applications are discussed. Further in vivo studies are needed before widespread clinical use can be advocated.

Anti-Bacterial Agents↗

PubMed1998

Effects of inserting a pressensor film into articular joints on the actual contact mechanics.

Fuji film has been widely used in studies aimed at obtaining the contact mechanics of articular joints. Once sealed for practical use in biological joints, Fuji Pressensor film has a total effective thickness of 0.30 mm, which is comparable to the cartilage thickness in the joints of many small animals. The average effective elastic modulus of Fuji film is approximately 100 MPa in compression, which is larger by a factor of 100-300 compared to that of normal articular cartilage. Therefore, inserting a Pressensor film into an articular joint will change the contact mechanics of the joint. The measurement precision of the Pressensor film has been determined systematically; however, the changes in contact mechanics associated with inserting the film into joints have not been investigated. This study was aimed at quantifying the changes in the contact mechanics associated with inserting sealed Fuji Pressensor film into joints. Spherical and cylindrical articular joint contact mechanics with and without Pressensor film and for varying degrees of surface congruency were analyzed and compared by using finite element models. The Pressensor film was taken as linearly elastic and the cartilage was assumed to be biphasic, composed of a linear elastic solid phase and an inviscid fluid phase. The present analyses showed that measurements of the joint contact pressures with Fuji Pressensor film will change the maximum true contact pressures by 10-26 percent depending on the loading, geometry of the joints, and the mechanical properties of cartilage. Considering this effect plus the measurement precision of the film (approximately 10 percent), the measured joint contact pressures in a joint may contain errors as large as 14-28 percent.

Animals↗

PubMedDate not supplied

Structural determinants of the rigidity of the red cell membrane.

Although the stability and viscoelasticity of the red cell membrane are undoubtedly governed by the membrane's underlying protein skeleton, the mechanism by which this network controls elasticity is uncertain. The structural constraints, that impose end-to-end spacing on the spectrin molecules well below that in free solution, may impart rubbery (entropic) elasticity to the system. However, other enthalpic and entropic contributions due to interactions between spectrin chains or between spectrin and other proteins, the lipid bilayer or the solvent must also prevail. To relate structural features to elasticity, explicit measurements of membrane rigidity are required. The most widely used measurement is that of the membrane shear elastic modulus by micropipette aspiration. Analysis of genetic variants of membrane structure have shown that the density of spectrin is directly correlated with membrane rigidity. Although cross-linking of the skeleton increases rigidity, interruption of the continuity of the network by dissociating spectrin tetramers into dimers does not reduce rigidity as might be expected. On the other hand, external ligands that cause new interactions between integral proteins and the skeletal network do increase rigidity. Moreover, hereditary ovalocytes, which have a deletion of 9 amino acids from band 3 at the first point of entry into the membrane, are extremely rigid. This mutation is associated with decreased translational and rotational mobility of the band 3, and may impair flexural freedom of its cytoplasmic domain. It thus appears that elasticity may be regulated not only by the structure of the spectrin network, but also by its interactions with and freedom of motion relative to the lipid bilayer.

Chemical Phenomena↗

PubMed2005

Flow-induced hardening of endothelial nucleus as an intracellular stress-bearing organelle.

The mechanical contribution of nucleus in adherent cells to bearing intracellular stresses remains unclear. In this paper, the effects of fluid shear stress on morphology and elastic properties of endothelial nuclei were investigated. The morphological observation suggested that the nuclei in the cytoplasm were being vertically compressed under static conditions, whereas they were elongated and more compressed with a fluid shear stress of 2 Pa (20 dyn/cm2) onto the cell. The elongated nuclei remained the shape even after they were isolated from the cells. The micropipette aspiration technique on the isolated nuclei revealed that the elastic modulus of elongated nuclei, 0.62+/-0.15 kPa (n=13, mean+/-SD), was significantly higher than that of control nuclei, 0.42+/-0.12 kPa (n=11), suggesting that the nuclei remodeled their structure due to the shear stress. Based of these results and a transmission electron microscopy, a possibility of the nucleus as an intracellular compression-bearing organelle was proposed, which will impact interpretation of stress distribution in adherent cells.

Animals↗

PubMed2003

A method for in-vivo analysis for regional arterial wall material property alterations with atherosclerosis: preliminary results.

Atherosclerosis is a diffuse arterial disease developing over many years and resulting in a complicated three-dimensional arterial morphology. The arterial wall material properties have been demonstrated to show regional alterations with atheroma development and growth. We present a mechanical analysis of diseased arterial segments reconstructed from intravascular ultrasound images in order to quantitatively identify regional alterations in the elastic constants with atherosclerotic lesions. We employ a finite element and a displacement sensitivity analysis to divide the arterial segment into regions with different material properties and use an optimization algorithm to identify the elastic constants in these regions. The results with regional variations identified with this method correlated qualitatively with the extent and location of atherosclerotic lesions identified by visual inspection of the affected arteries. The optimized elastic modulus in regions affected by early atherosclerotic lesions ranged from 90.9 to 93.0 kPa where as the corresponding magnitudes in normal arterial segments ranged from 97.9 to 101.0 kPa. This method can be potentially employed to identify the extent and location of atherosclerotic lesions in a systematic analysis and may potentially be used for the early detection of lesion growth.

Animals↗

PubMed1999

On the prediction of Young's modulus in calcaneal cancellous bone by ultrasonic bulk and bar velocity measurements.

This study evaluated two different approaches to the prediction of Young's modulus (E) from ultrasonic velocity and density measurements in 23 cubes of cancellous bone from human calcaneae. The first approach used clinically applicable measurements of bulk velocity and bone mineral density (BMD), whilst the second involved bar velocity and apparent density, which are strictly in vitro measurements. Bulk velocities were measured with an immersion technique with 1 MHz transducers using three different transit time markers (first arrival, thresholding, zero-crossing). Bar velocities were measured in the defatted specimens using a contact technique with 37 kHz transducers in air. Volumetric BMD was derived from dual-energy X-ray absorptiometry measurements, and apparent density was measured directly. Compressive mechanical testing was used to determine E. Bulk velocity, bar velocity and E all displayed significant anisotropy, being greatest in the proximo-distal (PD) axis and least in the medio-lateral (ML) axis. Bulk velocity was dependent on the transit time marker used, with velocity differences of up to 20% observed between different markers. Bar velocities were significantly lower than bulk velocities in all directions. Both bulk and bar velocities correlated with E (r2 = 0.26-0.83, r = 0.36-0.81, respectively) with stronger relationships obtained when the data for the three axes were pooled. The predictive ability of bulk velocities determined using different markers was similar. In general, combining velocity and density measurements yielded improved correlations with E. Thus, strong correlations were observed between E and the product of BMD and bulk velocity2 (r2 = 0.58-0.89), and the product of apparent density and bar velocity2 (r2 = 0.58-0.89). These results demonstrate that clinically applicable measurements of bulk velocity and BMD are good predictors of the elastic modulus of calcaneal bone, and that bulk and bar velocity, both alone and when combined with density measurements, have a similar predictive ability for mechanical properties.

Absorptiometry, Photon↗

PubMed2004

Estrogen replacement therapy reverses changes in intramural coronary resistance arteries caused by female sex hormone depletion.

OBJECTIVE: We tested the hypothesis that female sex hormone depletion and estradiol replacement therapy significantly influences the biomechanical properties of intramural coronary resistance arteries. DESIGN: Female rats (n=30) were divided into three groups. In group O, rats were subjected to bilateral ovariectomy. Group HRT was subjected to bilateral ovariectomy and estradiol replacement therapy. Rats in group C served as controls. One month after ovariectomy, intramural coronary arteries (approximately 200 microm in diameter) branching from the left anterior descending coronary were isolated, cannulated and studied by microarteriography. Intraluminal pressure was increased in steps between 0 and 90 mm Hg. The steady state diameter at each step was measured. These measurements were repeated in the presence of U46619, a thromboxane (TX) A2 receptor agonist (at a concentration of 10(-6) M), and bradykinin (BK; at 10(-6) M). Finally, Ca2+-free Krebs-induced passive diameter (PD) was measured in each group. RESULTS: Ovariectomy increased spontaneous myogenic tone of coronary arteries (p<0.05), which was normalized by estrogen replacement. Ovariectomy decreased distensibility observed at low pressure, although passive diameter was not changed. Estrogen replacement decreased wall stress and elastic modulus (p<0.05). The thromboxane A2 agonist induced the largest contraction in the ovariectomized group, whereas bradykinin-induced relaxation was the largest in the estrogen replacement group (p<0.05). CONCLUSION: Estradiol hormone replacement therapy (HRT) may exert a beneficial effect on myocardial perfusion in menopause by opposing the deterioration of biomechanical properties of intramural coronary resistance vessels induced by female sex hormone depletion.

Animals↗

PubMed2006

Harder and stiffer bone osseointegrated to roughened titanium.

Mechanisms underlying the beneficial anchorage of roughened titanium implants have not been identified. We hypothesized that the implant surface roughness alters intrinsic biomechanical properties of bone integrated to titanium. Nano-indentation performed on two- and four-week post-implantation bone specimens of rats revealed that bone integrated to acid-etched titanium was approximately 3 times harder than that integrated to the machined titanium, both at the osseointegration interface and at the inner area of the peri-implant bone. The hardness of the acid-etched surface-associated bone was equivalent to that of untreated cortical bone at week 4, while the bone hardness around the machined surface was equivalent to that of the untreated trabecular bone. The elastic modulus of the integrated bone was 1.5 to 2.5 times greater around the acid-etched surface than around the machined surface. Analysis of the data suggests that the implant surface roughness affects the biomechanical quality of osseo-integrated bone, and that the bone integrated to the acid-etched surface is harder and stiffer than the bone integrated to the machined surface.

Acid Etching, Dental↗

PubMed1996

Biomechanical measurements of the porcine lens capsule.

Biomechanical measurements were carried out on capsular rings prepared from the anterior lens capsule by means of photoablation technique (excimer laser). The outer diameter of the metal ring used to shape the laser output was 3.2 mm and the width (wall thickness) was 100 microns. Thickness of the lens capsule (depth of the tissue ring) was measured with a microscope coupled to an electronic length gauge as the difference in focus between latex spherules placed on the upper and lower surfaces of the capsular rings. For mechanical measurements the capsular rings were mounted around two pins connected to a motorized micropositioner and a force transducer, respectively, and stretched at a constant rate until rupture. Estimates of the precision of the test method were based on differences between measurements of paired eyes from 15 6-month-old pigs. Thickness of the lens capsules was 59 microns (50-66) (median, range). Repeatability of a single measurement was +/- 2 microns for a probability of 95%. Maximum strain as a percentage of the initial length was 78% [68-89] with a repeatability of +/- 6%. Maximum stiffness (elastic modulus) was 23.0 N mm-2 [10.0-31.5] with a repeatability of +/- 4.3 N mm-2, and maximum stress was 4.4 N mm-2 [2.8-7.7] with a repeatability of ./:1.27 N mm-2. There were no significant differences between paired eyes, but a significant variance component between individuals. The experimental set-up described in this paper seems reliable and useful for biomechanical measurements of the lens capsule, in the physiological as well as the surgical function range.

Animals↗

PubMed2006

Evaluation of concrete incorporating bottom ash as a natural aggregates replacement.

A study on the incorporation of coal bottom ash from thermoelectric power stations as a substitute material for natural sand in the production of concrete is here presented. The normally coarse, fused, glassy texture of bottom ash makes it an ideal substitute for natural aggregates. The use of bottom ash in concrete presents several technical challenges: the physical and mineralogical characteristics of the bottom ash; the effect on water demand and the participation on cements hydratation. In the production of the concrete, substitutions in volume were used. Two different ways to employ bottom ash were used to make up the mix proportions: one considering the natural humidity present in the porous particles and the other not considering it, seeking to maintain the same strength. These considerations are fundamental given that the process of bottom ash extraction is carried out through moisture. Mechanical tests by compressive strength were performed and the elastic modulus was determined. An analysis of the influence of bottom ash in the formation of pores was carried out through tests for the water loss by air drying and water uptake by capillary absorption. The results show that the higher the bottom ash contents in the concrete, the worse the performance regarding moisture transport. However, for one bottom ash concrete type, the mechanical properties were maintained.

Adsorption↗

PubMed1995

[Analysis of the breakage process during the measurement of the mechanical strength of guinea pig tympanic membranes].

We analyzed the breakage process of the tympanic membrane in relation to the material mechanics, using the microtesting system which we have reported previously. 111 fresh tympanic membranes from 56 guinea pigs were used. We calculated the stress-strain curve from the load-displacement curve which was obtained in this experiment. In addition, we observed the morphological changes during the breakage process of the tympanic membrane using optical microscope and scanning electron microscope (SEM). We observed multiple notches (small peaks) on the stress-strain curve. The first notch was considered to correspond to the elastic limit of the tympanic membrane. This first notch appeared at the point when the stress was 4.86 MPa and the strain was 0.10. Comparing the stress-strain curve to morphological changes, this notch in the stress-strain curve appeared when the stress suddenly decreased due to a slit on the tympanic membrane. The elastic modulus of the fresh tympanic membrane of guinea pigs was calculated as 5.71 x 10(-2)mN/microns 2. The maximal stress was 18.5 MPa and this value represented the maximal strength of the radial fiber bundles. From these results, we can speculate that from a mechanical point of view, the tympanic membrane of the guinea pig is a combined material consisting of the radial fiber bundles and circular fibers and that at the site of mechanical injury both fibers strengthened their interactions with each other.

Animals↗