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PubMed2004

Model of the viscoelastic behaviour of skin in vivo and study of anisotropy.

BACKGROUND: The single-axis extension test is relatively little used to study the mechanical properties of human skin in vivo. A campaign of tests was carried out with an original, modern machine developed in our laboratory. It can perform extension or compression tests using servo-controlled position or force in different directions. The load can either be of the extension or monotonous compression type, creep or relaxation. The results obtained were used to develop a viscoelastic model. The elastic modulus calculated helps us to determine the main directions of anisotropy on the forearm. METHODS: We use a new in vivo single-axis extension machine (patent no. FR03/09220 application in progress). With it, we can carry out monotonous, creep and relaxation tests on the forearm. An associated finite elements model enables conversion to the intrinsic parameters of the skin under stress and strain from external stress applied in force and displacement. From the tests, we can propose a viscoelastic model and the identification of his parameters. We carried out tests in four directions with respect to the axis of the forearm of 63 people of different ages. The present report is limited to a brief presentation of the experimental set-up used, and a more complete presentation of the viscoelastic model and how it is defined and also the work on the anisotropy in the elastic domain. RESULTS: The viscoelastic model proposed has only four intrinsic parameters: elasticity parameters E(e) and E(ve) and viscosity parameters epsilon(ve) and A. Skin being considered as orthotropic, we were able to determine the average main direction of 63 people, which is of 5.33+/-5.78 around the longitudinal axis of the arm. An average modulus E(1) (ave)=6.57E(5) (Pa) can be found in the direction close to the axis of the arm and E(2) (ave)=1.30E(5) (Pa) in the perpendicular direction and a G(12)=1.32E(5) (Pa) shear modulus. CONCLUSIONS: The parameters obtained with the viscoelastic model are independent of the type of load, the same coefficients enable a correct representation in creep and relaxation tests. The main directions vary from one person to another, Young's modulus in these directions could be an indicator for dermatologists and cosmeticians.

Adult↗

PubMed2003

Loss of stability-a new model for stress relaxation in plant cell walls.

This study addresses the mechanism of wall stress relaxation in growing plant cells. The current viscoelastic model of cell wall relaxation, which dates from the work of Preston, Cleland, Lockhart, and others in the 1960s, has serious shortcomings. It has been shown however that the theory of loss of stability (LOS) can be applied to materials in tension, leading to the conclusion that the relaxation of stresses in the walls of any pressure vessel is rigorously modeled using LOS. We propose that LOS also provides a more appropriate and versatile model of stress relaxation in growing plant cells. We argue that when treated as a manifestation of LOS, the regulation of cell turgor has a rigorous and demonstrable basis in the geometrical and physical properties of the cell wall and the cell's ability to import water. Thus plant cell growth can be regarded as an inherently self-limiting process, tunable by biochemical or structural means. Lastly, despite the current limitations of our model, we apply direct measurement of elastic modulus, wall thickness and cell radius obtained from cylindrical Chara corallina cells to generate an initial calculation of critical pressures in a hypothetical spherical cell with the same material properties.

Cell Wall↗

PubMed1988

Rheological properties of the thoracic aorta in normal and WHHL rabbits.

The tension-strain, stress-strain and stress relaxation curves of longitudinal and circumferential strips of proximal thoracic aortas in normal and WHHL rabbits of different ages were determined using a tensile testing instrument. Wall distensibility of longitudinal and circumferential strips was the greatest in the normal aorta and decreased with advancing age in the atherosclerotic aorta. The wall thickness of the atherosclerotic aorta was positively related to age with a correlation coefficient of 0.66(p less than 0.01). The incremental elastic moduli calculated from the stress-strain curves increased with advancing age in the atherosclerotic aorta. Accordingly, the decreased distensibility of the atherosclerotic wall may be due to the increased wall thickness caused by the intimal thickening as well as to the increase in wall stiffness caused by the increased elastic modulus. The viscoelasticity of the atherosclerotic aorta was larger than that of the normal aorta. This reflects the mechanical effect of atherosclerotic changes that occurred in the thickened intima.

Animals↗

PubMed1995

The influence of tendon Youngs modulus, dimensions and instantaneous moment arms on the efficiency of human movement.

The purpose of the study was to examine the influence of passive tendon work on the gross mechanical efficiency of human whole body movement. Seven male subjects participated in the study. They performed repetitive jumps (like skipping) of three different intensities. Metabolic costs and work rates were recorded to obtain mechanical efficiencies. Net joint moments were calculated from film recordings using inverse dynamics. A general stress-strain relationship for tendons was modelled using a quadratic function, including Youngs elastic modulus of tendon tissue and tendon dimensions. Instantaneous tendon moment arms for the largest leg extensor muscles (m. triceps surae and m. quadriceps femoris) were calculated using joint angle-moment arm transfer functions obtained from the literature (cadaver studies) and the tendon work was calculated from the net joint moments. Gross efficiency values of 0.65-0.69 and efficiency values of 0.77-0.80 at the approximate level of the muscle-tendon complexes were observed. The tendons performed 52-60% of the total work. The enhancement of the muscle-tendon efficiency over the maximal theoretical efficiency of the contractile machinery (0.30) could exclusively be explained by the contribution of the tendon work. A clear negative relationship between repetitive jumping with high mechanical efficiency and running economy at 12 km h-1 was found. Using model calculations the gross efficiency and the muscle-tendon efficiency were shown to be sensitive to tendon Youngs modulus, dimensions and moment arms. The efficiencies were most sensitive to changes in the tendon moment arms. A 10% decrease in tendon moment arms resulted in a 13% increase in the gross efficiency. Optimization or minimisation of the mechanical efficiency by changing the tendon variables 5% was followed by changes in mechanical efficiency of +14% and -10%, respectively.

Adult↗

PubMed2003

Contraction stress of flowable composite materials and their efficacy as stress-relieving layers.

BACKGROUND: The authors compared the polymerization contraction stress produced by flowable resin-based composites with stress values produced by nonflowable composites. They also measured the stress reduction produced by placing a precured layer of flowable composite under a nonflowable composite. METHODS: The authors first tested four flowable and six nonflowable composite materials for contraction stress in a tensiometer. In the second part of the study, they applied a 1.4-millimeter-thick layer of flowable composite or unfilled resin and precured it in the test apparatus to assess the stress relief produced by a low-modulus material during light curing of a subsequent layer of highly filled composite. Flexural moduli of the precured materials were determined via a three-point bending test. RESULTS: The stress values ranged between 6.04 and 9.10 megapascals. The authors found no significant differences in stress between flowable and nonflowable composites. Microfilled composites produced lower contraction stress than did hybrids. The flexural modulus of the flowable composites varied between 4.1 and 8.2 GPa. Regarding the effect of a precured layer of composite on contraction stress, the authors observed significant reductions with only one of the flowable materials and with the unfilled resin. CONCLUSIONS: The flowable composites produced stress levels similar to those of nonflowable materials. Most of the flowable materials tested did not produce significant stress reductions when used under a nonflowable composite. CLINICAL IMPLICATIONS: Using a flowable resin-based composite as a restorative material is not likely to reduce the effects of polymerization stress. When used in a thin layer under a nonflowable composite, the stress reduction depended on the elastic modulus of the lining material.

Composite Resins↗

PubMed1993

Elastic properties of human supraorbital and mandibular bone.

Elastic constants, including the elastic modulus, the shear modulus, and Poisson's ratio, were measured on human craniofacial bone specimens obtained from the supraorbital region and the buccal surfaces of the mandibles of unembalmed cadavers. Constants were determined using an ultrasonic wave technique in three directions relative to the surface of each sample: 1) normal, 2) tangential, and 3) longitudinal. Statistical analysis of these elastic constants indicated that significant differences in the relative proportions of elastic properties existed between the regions. Bone from the mandible along its longitudinal axis was stiffer than bone from the supraorbital region. Directional differences in both locations demonstrated that cranial bone was not elastically isotropic. It is suggested that differences in elastic properties correspond to regional differences in function.

Elasticity↗

PubMed1995

Arterial distensibility and compliance in hypertension.

Under normal circumstances arteries, especially the larger elastic ones, distend during systole, and hence are able to store volume energy during this phase of the cardiac cycle. In this way the increase in blood pressure during systole will be leveled off. The amount of energy that can be stored depends not only on the degree of distension (distensibility), but also on the initial diameter of the vessel--two parameters actually determining the compliance of an artery. In both established and borderline hypertension distensibility and compliance of the larger elastic arteries are reduced, as compared with normotensives, but not of the radial artery, at least in patients with established hypertension. In these patients this artery shows a pronounced increase in intima-media thickness. The elastic modulus of the radial artery is not different in hypertensive and normotensive subjects. In the present state of the art, it cannot be concluded whether the loss of artery wall distensibility is caused solely by the increase in blood pressure or that also structural wall changes are involved. In borderline hypertensive patients distensibility and compliance of the elastic arteries are diminished as compared with normotensive subjects. The loss of distensibility already occurs in the fourth age decade and is not homogeneous along the carotid artery bifurcation. The distensibility is significantly lower at all levels in the carotid artery bulb than in the common carotid artery, the proximal part of the bulb, where the baroreceptors are predominantly located, being most affected. This pattern is similar to that observed in normotensive subjects in the sixth age decade, indicating that the carotid arteries age faster in borderline hypertensive than in normotensive subjects.

Adult↗

PubMed2000

Decreased mechanical stability of neonatal red cell membrane quantified by measurement of the elastic area compressibility modulus.

Previous studies suggest that early loss of membrane might account for the shortened life-span of neonatal red blood cells (RBC). The elastic area compressibility modulus describes the force required to achieve a defined expansion of the membrane and is thus a suitable intrinsic material property to describe mechanical stability of the RBC membrane. We studied RBC in eight cord blood samples (representing fetal blood), ten 5-day old full-term neonates and seven healthy adults. The RBC were suspended in hypotonic buffer solution and investigated using a modification of the micropipet technique by Evans et al. Cord blood RBC (204+/-33 dyn/cm) and neonatal RBC (209+/-11 dyn/cm) showed a 25% lower elastic area compressibility modulus than adult cells (278+/-26 dyn/cm; p < 0.05). Thus, less force was necessary for expansion of neonatal RBC membrane. We conclude that neonatal RBC are more susceptible to mechanical damage of their membrane than adult cells.

Adult↗

PubMed2000

Plasma-sprayed hydroxyapatite (HA) coatings with flame-spheroidized feedstock: microstructure and mechanical properties.

Flame-spheroidized feedstock, with excellent known heat transfer and consistent melting capabilities, were used to produce hydroxyapatite (HA) coatings via plasma spraying. The characteristics and inherent mechanical properties of the coatings have been investigated and were found to have direct and impacting relationship with the feedstock characteristics, processing parameters as well as microstructural deformities. Processing parameters such as particle sizes (SHA: 20-45, 45-75 and 75-125 microm) and spray distances (10, 12 and 14 cm) have been systematically varied in the present study. It was found that the increase of particle sizes and spray distances weakened the mechanical properties (microhardness, modulus, fracture toughness and bond strength) and structural stability of the coatings. The presence of inter- and intralamellar thermal microcracks, voids and porosities with limited true contact between lamellae were also found to degrade the mechanical characteristics of the coatings, especially in coatings produced from large-sized HA particles. An effort was made to correlate the effects of microstructural defects with the resultant mechanical properties and structural integrity of the plasma-sprayed hydroxyapatite (HA) coatings. The effects of different heat treatment temperatures (600, 800 and 900 degrees C) on the mechanical properties of the coatings were also studied. It was found that a heat treatment temperature of 800 degrees C does enhance the microhardness and elastic modulus of the coatings significantly (P < 0.05) whereas a further increment in heat treatment temperature to 900 degrees C did not show any discernable improvements (P > 0.1). The elastic response behaviour and fracture toughness of both the as-sprayed and heat-treated HA coatings using Knoop and Vickers indentations at different loadings have been investigated. Results have shown that the mechanical properties of the coatings have improved significantly despite increasing crack density after heat treatment in air. Coatings produced from the spheroidized feedstock of 20-45 microm (SHA 20-45 microm) sprayed at a stand-off distance of 10 cm were found to possess the most favourable mechanical properties.

Coated Materials, Biocompatible↗

PubMed1985

The use of short carbon fibre reinforced thermoplastic plates for fracture fixation.

Thermoplastic plates of Nylon 6-10 and Polybutylene terephthalate reinforced with 30% short randomly oriented carbon fibres were tested for internal fixation of canine femoral transverse midshaft fractures. The elastic modulus of the plates was one-half that of bone: however, ultimate strength and strain in bending were comparable to bone. The fractures healed with moderate callus formation which was completely remodelled by 8 to 12 wk post surgery. Although a moderate inflammatory reaction to occasional particulate debris was noted, the materials appeared to possess the proper elastic moduli to allow sufficient support for the healing fracture without protecting the remodelling process.

Animals↗

PubMed2002

Female gender increases stiffness of elastic but not of muscular arteries in type I diabetic patients.

The reason for the particularly increased risk for cardiovascular complications in diabetic women is still unclear. We have previously found decreased distensibility of elastic arteries in type I diabetic women, indicating increased cardiac load, not seen in type I diabetic men, which might be one contributing factor. Whether the effect of gender is different in muscular arteries in type I diabetic patients has not been assessed. As estimates of arterial distensibility we measured stiffness (beta) and pressure strain elastic modulus (Ep) in the muscular common femoral artery using echo-tracking sonography in 30 women (mean age 34 years, range 20-61) and 26 men (mean age 38 years, range 22-56) with type I diabetes. The results were compared with those of 89 healthy individuals of corresponding age and gender and with previously published results from elastic arteries in these patients obtained at the same occasion. The internal common femoral diameter was significantly decreased in both diabetic men and women. In sharp contrast to the highly significant decreased distensibility of the elastic abdominal aorta and common carotid artery in the type I diabetic women, the distensibility of the common femoral artery did not clearly differ between patients and controls, neither for women nor for men. Thus, the gender difference in changes of arterial distensibility found in elastic arteries was absent or far less obvious in the femoral artery. In conclusion, female gender seems to affect the mechanical properties of elastic, but not of large muscular arteries in type I diabetic patients. Thus, putative gender differences in arterial changes in type I diabetes are to be sought in elastic rather than muscular arteries.

Adult↗

PubMed1986

The effect of the interface on the bone stresses beneath tibial components.

It was proposed that the stresses in the layer of bone immediately beneath a tibial component are an important determinant of fixation durability. Using finite element analysis, (ANSYS), the stresses were determined as a function of the amount of bone resection, the localization or completeness of implant-bone contact, and the interface material. The model was of two-dimensional sagittal slices consisting of quadrilateral elements (1 mm) with a range of seventeen material properties determined by CT scans. Typical prosthesis designs shifted the center of pressure more centrally rather than posteriorly, and thus increased anterior bone stresses. Resection up to 10 mm could actually decrease bone stresses due to an increase in bone surface area as long as complete coverage was obtained. A cement interface or direct metal on bone produced identical stresses. However a 1 mm compliant interface significantly reduced stresses in regions of high elastic modulus gradient. For rigid interfaces, the contact can be irregular, which leads to areas of over and under-stressing of bone. These conclusions have implications related to implant design.

Arthritis↗

PubMed1988

On the dependence of the elasticity and strength of cancellous bone on apparent density.

This paper presents a statistical analysis of the pooled data from a number of previous experiments concerning the dependence of the Young's moduli and strength of cancellous bone tissue upon apparent density. The results show that both the Young's moduli and the strength are proportional to the square of apparent density of the tissue and are therefore proportional to one another. It is shown that the coefficient of proportionality is different for human and bovine tissue. It is concluded that the suggestion of Wolff (Das Gesetz der Transformation der Knochen, Hirschwald, Berlin, 1892) that compact bone tissue is simply more dense cancellous bone tissue is not an accurate statement when only the mechanical properties of these two tissues are considered. It is noted that estimates for the elastic modulus of the individual trabecula of human cancellous bone vary from 1 to 20 GPa and it is suggested that this question needs further study.

Biomechanical Phenomena↗

PubMed2003

Analytic modeling of breast elastography.

The elastic moduli of tumors change during their pathological evolution. Elastographic imaging has potential for detecting and characterizing cancers by mapping the stiffness distribution in tissues. In this paper a micromechanics-based analytical method was developed to detect the location, size, and elastic modulus of a tumor mass embedded in a symmetric two-dimensional breast tissue. A closed-form solution for the strain elastograms (forward problem) was derived. A computational algorithm for the inverse problem was developed for the detection, localization, and characterization of a heterogeneous mass embedded in a breast tissue. Numerical examples were presented to evaluate the proposed method's performance. The detectability of a tumor mass was estimated with respect to lesion location, size, and modulus contrast ratio. It was shown that the micromechanics theory provides a powerful tool for the diagnosis of breast cancer.

Absorptiometry, Photon↗

PubMed2004

A transversely isotropic, transversely homogeneous microstructural-statistical model of articular cartilage.

Articular cartilage is a multi-phasic, composite, fibre-reinforced material. Therefore, its mechanical properties are determined by the tissue microstructure. The presence of cells (chondrocytes) and collagen fibres within the proteoglycan matrix influences, at a local and a global level, the material symmetries. The volumetric concentration and shape of chondrocytes, and the volumetric concentration and spatial arrangement of collagen fibres have been observed to change as a function of depth in articular cartilage. In particular, collagen fibres are perpendicular to the bone-cartilage interface in the deep zone, their orientation is almost random in the middle zone, and they are parallel to the surface in the superficial zone. The aim of this work is to develop a model of elastic properties of articular cartilage based on its microstructure. In previous work, we addressed this problem based on Piola's notation for fourth-order tensors. Here, mathematical tools initially developed for transversely isotropic composite materials comprised of a statistical orientation of spheroidal inclusions are extended to articular cartilage, while taking into account the dependence of the elastic properties on cartilage depth. The resulting model is transversely isotropic and transversely homogeneous (TITH), the transverse plane being parallel to the bone-cartilage interface and the articular surface. Our results demonstrate that the axial elastic modulus decreases from the deep zone to the articular surface, a result that is in good agreement with experimental findings. Finite element simulations were carried out, in order to explore the TITH model's behaviour in articular cartilage compression tests. The force response, fluid flow and displacement fields obtained with the TITH model were compared with the classical linear elastic, isotropic, homogeneous (IH) model, showing that the IH model is unable to predict the non-uniform behaviour of the tissue. Based on considerations that the mechanical stability of the tissue depends on its topological and microstructural properties, our long-term goal is to clearly understand the stability conditions in topological terms, and the relationship with the growth and remodelling mechanisms in the healthy and diseased tissue.

Cartilage, Articular↗

PubMed2005

Reduced elastic mismatch achieved by interposing vein cuff in expanded polytetrafluoroethylene femoral bypass decreases intimal hyperplasia.

BACKGROUND: Wall shear stress, arterial wall elasticity, and intimal hyperplasia are related. The aim of this study was to investigate the in vitro mechanical properties of ovine femoral arteries, jugular veins, and expanded polytetrafluoroethylene conduits, and to evaluate postoperative intimal hyperplasia. METHODS: Arterial, venous, and ePTFE mechanical properties were studied in a circulating loop at isobaric systemic pressures. Histological studies of intimal hyperplasia in ePTFE-bypassed femoral arteries with and without Miller cuffs were performed at the 40th and 120th day. RESULTS: The incremental elastic modulus of veins was significantly higher than that of femoral arteries (P < 0.05), but significantly lower than that of ePTFE graft conduits (P < 0.05). Intimal hyperplasia was significantly less in Miller-cuff-bypassed arteries both at the 40th and 120th day (P < 0.01). CONCLUSIONS: The Miller cuff acts as a mechanical adapter enhancing wall shear stress and the elastic matching between ePTFE and the native artery, resulting in an early decrease of intimal hyperplasia.

Animals↗

PubMed1994

Arterial mechanics in the fin whale suggest a unique hemodynamic design.

An analysis of the dimensions of the aortic tree and the mechanical properties of arterial wall tissues in the fin whale (Balaenoptera physalus) is presented. The aortic arch is greatly expanded, having an internal radius at an estimated mean blood pressure (13 kPa) that is 2.5 times greater than that of the descending thoracic aorta. At this pressure, the elastic modulus of the arch wall (0.4 MPa) is 30 times less than that of the descending aorta (12 MPa). Consequently, even though some capacitance is provided anteriorly by the relatively compliant innominate and carotid arteries, > 90% of the arterial capacitance resides in the arch. The characteristic pressure wave velocity (C0) and impedance (Z0) were calculated from vessel dimensions and elasticity. A predicted 20-fold increase in Z0 between the arch and thoracic aorta should provide a major reflecting site, effectively uncoupling the arch from the remainder of the arterial tree. The dimensions of the arch relative to the likely pressure wavelengths within it suggest that it acts like a compliant windkessel that greatly reduces the pulsatility of the inflow to the descending aorta, which itself likely acts as a rigid, tapered manifold. It is suggested that the presence of both a highly compliant arch and a relatively rigid descending aorta is an adaptation for diving.

Animals↗

PubMed2004

Quasi-static and dynamic nanoindentation studies on highly crosslinked ultra-high-molecular-weight polyethylene.

In four separate studies, the effect of three methods used to highly crosslink ultra-high-molecular-weight polyethylene (UHMWPE) (gamma irradiation, electron beam irradiation, and peroxide treatment) on the polymer's stiffness was investigated using quasi-static and dynamic nanoindentation. From the quasi-static studies, it was found that (a) the highest value of the elastic modulus, E (2240 +/- 157 MPa) was obtained from specimens prepared from direct compression molded UHMWPE crosslinked using gamma irradiation, with a dosage of 7.5 Mrad; (b) the value of E for specimens formed from direct compression molded UHMWPE was higher (1950 +/- 255 MPa) compared to results obtained from specimens formed from ram extruded UHMWPE (1583 +/- 140 MPa); and (c) the magnitude of the decrease in the value of E as a result of subjecting a highly crosslinked UHMWPE specimen to uniaxial tensile forces depends on the method used for the crosslinking. From the dynamic study, (d) it was found that the extent of the change in both the storage modulus and the loss modulus, as a result of subjecting a highly crosslinked UHMWPE specimen to uniaxial tension forces, depends on the method used for the crosslinking.

Biocompatible Materials↗