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At least 37 records · Page 2Linked to original sources
PubMed1977

Dynamic elasticity of human mitral valve chorade tendinease.

Chordae tendineae are under constant dynamic stress and, therefore, measurements of static properties alone cannot provide a complete analysis of their mechanical response under stress. This study investigated the dynamic viscoelastic properties of human mitral valve chordae tendineae. The tissue was subjected to sinusoidal strain variations over a frequency range of 0.42 to 6.68 Hz. At a fixed strain, the storage modulus, E', was found to be independent of applied frequency but varied inversely with chordal thickness. Values in the order of 10(8) dyn cm-2 (1 dyn=10 muN) were found. E' also increased with strain level. The phase lag, phi, between stressing function and response was found to be small (0.2-0.058 rad) and decreased with frequency. Values of phi at each frequency were found to be independent of chordal size and strain level. This property would enable a smooth and even closure of the valve and provide a more rapid response at elevated heart rates. The loss modulus, E'', was found to be 18 to 50 times smaller than E'. This implied an almost complete recovery on removal of any stress on the tissue. E'', which also decreased with frequency, was found to be smaller for the larger chordae.

Adult↗
PubMed1976

The low frequency dynamic viscoelastic properties of human aortic valve tissue.

Membranous samples of human aortic valve cusps were subjected to sinusoidal fluid pressure variations (frequency range, 0.5-5 Hz) to assess their dynamic viscoelastic properties. The storage (elastic) and loss (viscous) moduli and phase lag between the stressing function and response were found to be independent of the frequencies applied. The respective average values were 1.35 (SE = 0.06) X 10(8) dynes cm-2, 4.14 (SE = 0.28) X 10(6) dynes cm-2, and 0.033 (SE = 0.002) rad. The small phase lag indicates that the tissue would recover almost completely to its original state on removal of any applied stress, and this and the relatively low extensibility should be considered in the design of leaflet-type valve prostheses. The storage modulus of the aortic valve cusps when compared to that of the mitral leaflet shows the mitral leaflet to be almost twice as stiff as the aortic valve cusps. This finding led us to conclude that the vibrations of these two cardiac valves alone cannot contribute in any significant way to the production of the observed lower frequency of the first and the higher frequency of the second heart sounds and that other factors must be considered to explain this finding.

Aged↗
PubMed1989

Tissue conditioners containing poly (butyl methacrylate) powder. I. Viscoelastic properties of homopolymer/plasticizer mixture.

The viscoelastic properties of new experimental tissue conditioners which consist of synthesized poly (butyl methacrylate) PBMA and a liquid of aromatic-ester plasticizer, without alcohol, were investigated to evaluate the effects of the molecular weight of polymer. The master curves of storage modulus G', dynamic viscosity eta' and loss tangent tan delta were constructed from the viscoelastic data at different temperatures. The effects of the molecular weight of polymer on the viscoelastic properties G' and tan delta were observed particularly, in a low frequency range. However, the dynamic viscosity eta' was not very sensitive to the molecular weight. These results suggest that PBMA produces more desirable properties for tissue conditioners, and that it is possible to manufacture new materials without alcohol.

Acrylates↗
PubMed1990

[On mechanical behavior of molecular composite resins reinforced with polyaramides. Molecular motion of Oct-PPTA and Ste-PPTA and thermal properties and dynamic viscoelasticity of Oct-PPTA-PMMA and Ste-PPTA-PMMA].

Molecular composites, composed of polymethylmethacrylate (PMMA) resin as matrix reinforced with polyaramides as a rigid core molecule have been developed to produce a denture base polymer with improved dental material properties. N-substituted polyaramides were prepared via metalation using sodium methylsulfinylcarbanion, followed by the reaction with corresponding octyl bromide and/or stearyl bromide in dimethyl sulfoxide. In these molecular composite resins (called Oct-PPTA-PMMA and Ste-PPTA-PMMA short) compounding 3 wt% of N-octylated-PPTA (Oct-PPTA) and/or N-stearylated-PPTA (Ste-PPTA) to PMMA, their dental material properties were in the order of Oct-PPTA-PMMA greater than Ste-PPTA-PMMA greater than or equal to PMMA. Their polymer properties were analyzed to molecular level, using nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric (TG) analysis and dynamic mechanical thermal analysis (DMTA). The molecular motion of the methyl group of Oct-PPTA proved to be constrained for the rigid main chain by T1 (inversion recovery method) NMR spectra in CDCl3 while that of Ste-PPTA was not affected. The thermal properties of the composites were in the order of Oct-PPTA-PMMA greater than Ste-PPTA-PMMA greater than PMMA by TG analysis, and the dynamic storage modulus values were Oct-PPTA-PMMA greater than Ste-PPTA-PMMA greater than PMMA in the region from rubbery state to viscous flow state by DMTA.

Composite Resins↗
PubMed1990

[The effect of filler content on properties of polyaddition-type silicone impression material].

The effect of quartz filler content on viscosity, dynamic viscoelasticity and tensile stress of polyaddition-type silicone impression materials were investigated. A hydrogen polydimethylsiloxane (H-silicone) and two vinyl polydimethylsiloxane (V-silicone) silicone prepolymers were prepared. Quartz fillers were mixed to them at contents between 0 and 50% by weight. The viscosity of these rubber compounds and mechanical properties of set materials were measured by a cone and plate viscometer, the dynamic shear test and the tension test, respectively. Viscosity eta storage modulus G' and tensile stress sigma increased with the amount of filler increase. G' and sigma were decreased by use of a V-silicone prepolymer which had a high average molecular weight and the material was more flexible. This suggests that the characteristics of the silicone prepolymer greatly affect the mechanical properties of the impression materials. Therefore, the appropriate molecular design for silicone prepolymer and reasonable filler content should be considered to make impression materials that have suitable properties for clinical use.

Dental Impression Materials↗
PubMed1989

[Molecular composite resins reinforced with polyaramides (Part 1). Molecular interaction between N-Octylated-Poly-p-Phenylene Terephthalamide (PPTA) as core molecule and PMMA and Polystyrene (PS) as matrices].

Molecular composite PMMA resin (Oct-PPTA-PMMA) can be reinforced with poly-N-octyl-p-phenylene terephthalamide (Oct-PPTA) as rigid core molecule. Compounding 3 wt% of Oct-PPTA to PMMA increased compressive, diametral and bending strength by 10 to 15%. The molecular interaction between Oct-PPTA as the core molecule and PMMA as a polar matrix and polystyrene (PS) as a non-polar matrix was examined with respect to dynamic viscoelasticity. Compounding 3 wt% of Oct-PPTA (Oct-PPTA-PS) to PS decreased compressive, diametral and bending strength by 15 to 30%. The dynamic storage modulus (G') value of Oct-PPTA-PS is lower than G' of PS in the region from rubbery state to viscous flow state. These results reveal a significant effect of the polar groups on the molecular interaction between the core molecule and matrices in the molecular composites compounding Oct-PPTA as core molecule.

Chemical Phenomena↗
PubMed1989

Viscoelastic properties of the oxygenated sickle erythrocyte membrane.

Although most apparent in permanently misshapen irreversibly sickled erythrocytes (ISC), biochemical and structural alterations are present in the majority of sickle cell membranes. The relationship of membrane rigidity to cell shape and its dependence upon the internal hemoglobin cytosol are not clarified. We therefore examined the frequency dependent viscoelasticity of oxygenated, packed sickle red cell and ghost suspensions and hemoglobin solutions prepared from density gradient separated ISC and reversibly sickled cell (RSC) fractions. Low amplitude, oscillatory shear was applied in a Weissenberg cone and plate viscometer and the resultant viscoelastic signals provided a dynamic viscosity (eta') and elastic storage modulus (G') which varied with frequency of deformation. The viscoelastic response of the cell and ghost suspensions reflected the material properties of the membrane over most of the frequency range tested. Sickle erythrocyte, red ghost, and white ghost suspensions demonstrated greater viscocoelasticity than comparable normal suspensions. The viscoelastic magnitude of ISC was several-fold greater than normal, with little variation of viscoelasticity with frequency. RSC samples which were characterized by normal shape, size, and internal hemoglobin concentration were also significantly harder than normal, although similar in frequency dependence. Red ghosts prepared from ISC manifested 80% of the viscoelasticity of intact ISC despite diminution of the internal hemoglobin concentration by 90%. Under conditions of low amplitude shear, the behavior of the RSC membrane is compatible with a cytoskeleton possessing an increased number of molecular associations. The mechanical stability of the ISC membrane is related to a substantial, intrinsic reorganization of the cytoskeleton.

Adult↗
PubMed1989

Changes in cervical mucus that prevent penetration by spermatozoa.

Two situations that result in the conversion of human mid-cycle cervical mucus from a sperm-receptive to a sperm-hostile form are described here: firstly, the addition of mucospissic agents, and secondly, the presence of antisperm antibodies. Two mucospissic biguanides were studied, chlorhexidine and Vantocil; both were totally spermicidal in the range 1-10 mg ml-1. Treatment of mucus with 1.5 microM to 1.65 mM Vantocil caused a dose-dependent increase in the dynamic storage modulus. The compatibility of the two biguanides with mucus was examined by measuring the rate of entry of diffusion of the [14C]biguanides into mucus. Chlorhexidine entered the mucus up to 0.53 mM, i.e. the highest concentration used, whilst Vantocil only entered at concentrations below 0.53 mM. This limited entry may be caused by the precipitation of mucus at the interface, producing a barrier of reduced permeability. The behaviour of purified mucin on ultracentrifugation was also altered after treatment with chlorhexidine. The s20 (at 2 mg ml-1 purified mucin) increased from 11.2 S to 19.3 S upon addition of 200 microM chlorhexidine. Further indication of structural alteration of biguanide-treated mucin was given by its loss of solubility in 0.22 M-sodium thiocyanate. The application of these biguanides to vaginal contraception is suggested. When antisperm antibodies are present in either the semen or cervical mucus, we suggest that an interaction can occur between galactose residues on the spermatozoa and galactose recognition sites on the antisperm antibody; in addition, binding can also occur between the Fc region of the antibody and cervical mucus. This process could therefore contribute to the binding of spermatozoa to the antibody and the immobilisation of this complex by the cervical mucus that is seen in immunological infertility. It was shown, by Immunobead binding, that immediate exposure of spermatozoa to D-galactose in the presence of chymotrypsin resulted in a considerable decrease or total loss of bound antisperm antibodies in males who had previously had a high titre of antibody. This reduction in the antibody level on the spermatozoa was accompanied by the appearance of the antibody level on the spermatozoa was accompanied by the appearance of the ability of the spermatozoa to penetrate cervical mucus in those couples examined. This pretreatment regimen for the ejaculate is suggested as a form of therapy for infertility related to the presence of antisperm antibodies.

Antibodies↗
PubMed1988

The effect of filament shortening on the mechanical properties of gel-filtered actin.

To address the claim that filaments polymerized from highly purified (gel-filtered) F-actin acquire the elastic properties of a solid attributable to chemical cross-linking, we measured the rheologic spectrum of the dynamic storage modulus, G', and loss modulus, G'' from 5 x 10(-4) to 0.5 Hz for gel-filtered actin alone and in the presence of the actin shortening protein, gelsolin. We confirmed that gel-filtered filamentous actin is a highly elastic material as evidenced by a relatively frequency-independent G', which is consistent with either topologically constrained filaments or a chemically cross-linked gel. Introduction of gel-filtered actin oligomers, however, caused the behavior of gel-filtered actin to become more frequency-dependent and almost identical to that of non-gel-filtered actin, suggesting that the effect of gel filtration on the mechanical behavior of actin is topologic. This conclusion is further supported by the finding that shortening of the actin filaments by the addition of gelsolin at molar ratios to actin of from 1:8000 to 1:500 causes a gradual decrease in elasticity and increase in the amount of flow.

Actin Cytoskeleton↗
PubMed1986

The effect of the 540-kilodalton actin cross-linking protein, actin-binding protein, on the mechanical properties of F-actin.

This study describes the effect of actin-binding protein derived from rabbit lung macrophages on the mechanical properties of F-actin. The dynamic storage modulus, G'(omega), and loss modulus, G"(omega) of F-actin, at concentrations from 1 to 4 mg/ml, in the absence or presence of actin-binding protein at molar ratios to actin of 1:1000 to 1:125, were measured at frequencies ranging from 3 X 10(-3) to 0.5 Hz. Actin-binding protein increased the dynamic moduli of F-actin, but this increase was much greater as either the actin-binding protein/actin ratio or the total protein concentration increased. Moreover, there was a convergence of the values of G' and G" at high frequencies for F-actin which became more prominent upon the addition of actin-binding protein. The value of the modulus obtained by an extrapolation of these data to actin concentrations similar to that found in the cell cortex was close to values which have been obtained by direct measurements. The addition of actin-binding protein to an F-actin solution enabled it to reach an equilibrium strain following the application of a stress, in contrast to pure F-actin. These data allow a more rigorous definition of the "sol" to "gel" transition and suggest that the cross-linking of actin filaments by actin-binding protein leads to the formation of a network structure whose underlying mechanism of mechanical behavior is short range intrafilament bending in contrast to the classical rubber network.

Actins↗
PubMed1985

The elasticity of spectrin-actin gels at high protein concentration.

Human erythrocyte spectrin of high purity was studied alone and mixed with rabbit skeletal actin by dynamic rheometry as a function of protein concentration at pH 7.4 and 24 degrees C. Pure spectrin had a very low storage modulus, G', increasing slightly with increase in protein concentration (approximately 3 dynes/cm at 25 mg/ml). In contrast, unpurified cytoskeletal extracts containing spectrin, actin, and band 4.1 showed a marked concentration dependence for G', increasing to 150 dynes/cm at 20 mg/ml. Mixtures of purified spectrin and skeletal actin at a weight ratio of 4:1 also showed G' markedly dependent on concentration (approximately 150-200 dynes/cm at 20 mg/ml). Maximum elasticity of spectrin-actin gels occurred at a molar ratio of actin monomers to spectrin tetramers of 14:1. We conclude that the reconstituted in vitro spectrin-actin network consists of actin fibers cross-linked by spectrin tetramers at regular intervals. The gel is rapidly reformed after mechanical disruption or thermal collapse, indicating that the polymer fibers are in equilibrium with the constituent monomers.

Acetone↗
PubMed1990

The effect of shear fatigue on bovine articular cartilage.

The objective of this study was to investigate the effects of mechanical fatigue in the form of cyclic shear strain on articular cartilage. Three millimeter diameter full-thickness plugs were cored from the lateral aspect of bovine tibial plateaus. Sinusoidal shear strains of +/- 5, +/- 10, and +/- 15% were applied to the specimens at 100 Hz for 3 h (a total of 108 x 10(4) cycles). The mechanical shear properties of the tissue (loss and storage moduli) were determined as a function of the number of applied strain cycles. A rapid, irreversible decrease of approximately 35% of initial modulus was found to occur in both loss and storage modulus during application of the first 90,000 cycles. Further decay in the moduli was found to occur from 90 x 10(3) to 108 x 10(4) cycles, but was of considerably smaller magnitude than the initial decrease. The moduli remained relatively constant beyond application of 108 x 10(4) cycles. No consistent change in proteoglycan content was found to be associated with the fatigue process when comparing tested specimens with fresh, untested tissue, and with experimental controls. In addition, no structural defects in the mechanically altered tissue were revealed by scanning electron microscopy.

Amino Acids↗
PubMed1990

Elastic energy storage in tendons: mechanical differences related to function and age.

We investigated the possibility that tendons that normally experience relatively high stresses and function as springs during locomotion, such as digital flexors, might develop different mechanical properties from those that experience only relatively low stresses, such as digital extensors. At birth the digital flexor and extensor tendons of pigs have identical mechanical properties, exhibiting higher extensibility and mechanical hysteresis and lower elastic modulus, tensile strength, and elastic energy storage capability than adult tendons. With growth and aging these tendons become much stronger, stiffer, less extensible, and more resilient than at birth. Furthermore, these alterations in elastic properties occur to a significantly greater degree in the high-load-bearing flexors than in the low-stress extensors. At maturity the pig digital flexor tendons have twice the tensile strength and elastic modulus but only half the strain energy dissipation of the corresponding extensor tendons. A morphometric analysis of the digital muscles provides an estimate of maximal in vivo tendon stresses and suggests that the muscle-tendon unit of the digital flexor is designed to function as an elastic energy storage element whereas that of the digital extensor is not. Thus the differences in material properties between mature flexor and extensor tendons are correlated with their physiological functions, i.e., the flexor is much better suited to act as an effective biological spring than is the extensor.

Age Factors↗
PubMed1987

Carbon/graphite fiber reinforced poly(methyl methacrylate): properties under dry and wet conditions.

The flexural properties of poly(methyl methacrylate) (PMMA) reinforced with carbon/graphite (C/G) fibers with three different surface treatments were investigated by transverse bend testing after dry and wet storage. The fibers used were (1) commercially available fibers, (2) cleaned fibers, and (3) cleaned and sized fibers. The coating agents of commercial unidirectional and braided C/G fibers as well as impurities on C/G fibers for medical uses were characterized by means of high-performance liquid chromatography (HPLC). The agar overlay technique was used to assess the cytotoxicity of leachable elements from different fibers and processed composites. Composites with both unidirectional and braided tubular C/G fibers were investigated after storage in water. Fracture stress and flexural modulus decreased when "commercial" fibers were used as reinforcing material. Composites with cleaned and sized fibers gave only minor differences in flexural properties after dry and wet storage. By means of SEM micrographs the adhesion behavior of unsized C/G fibers, epoxy sized fibers, cleaned fibers, and cleaned and sized fibers were assessed. After water storage a substantial part of the cleaned fibers adhered to the matrix material. The adhesion capacity of the other fibers was reduced since the water absorption caused separation of fiber and matrix.

Biocompatible Materials↗
PubMed1988

The effect of cryopreservation on canine menisci: a biochemical, morphologic, and biomechanical evaluation.

This study evaluated the effect of cryopreservation on the structural organization, biosynthetic activity, and material properties of canine menisci. The menisci were cryopreserved by incubating them in a 4% solution of dimethyl sulfoxide (DMSO) in physiologic media and freezing them to -100 degrees C using a controlled rate freezing system. The menisci were then stored for varying periods of time from zero to 12 weeks in liquid nitrogen (-196 degrees C). Following rapid thawing, changes in the histological appearance and biosynthetic activity of the menisci were evaluated as functions of storage time. In addition, the effects of the cryopreservation process on the tensile strength and modulus of the meniscal tissue were assessed. Although cryopreservation and short-term storage did not appear to affect the morphological appearance or biomechanical character of the menisci, biosynthetic activity, as determined by Na2S35SO4 incorporation, was diminished to less than 50% of normal control values immediately following cryopreservation and thawing. Autoradiographic examination of these tissues revealed that only approximately 10% of the meniscal cells were metabolically active, however, indicating that a marked increase in the metabolic activity of individual cells occurs following the freeze-thaw cycle. Total metabolic activity continued to decline with storage time.

Animals↗
PubMed1990

Long-term storage effects on canine osteochondral allografts.

We have studied long-term (to 60 days) effects of 4 degrees C storage in culture media on the histologic, mechanical, and chemical properties of the cartilage from osteochondral shell allografts from the dog. The structural integrity of the cartilage matrix was intact up to 60 days of storage, for the mechanical properties represented by the aggregate modulus and apparent permeability remained normal. These data are supported by normal safranin-O staining as well as normal glycosaminoglycan content and total collagen concentration. However, chondrocyte viability, as assessed by 35SO4 uptake and hematoxylin and eosin preparations, decreased dramatically with time. We believe that the longer storage to 60 days is not indicated, unless conditions can be modified to maintain cell viability.

Animals↗
PubMed1988

Effects of moist-chamber and McCarey-Kaufman medium storage on the metabolic status of the cornea: a 31P-magnetic resonance analysis.

The rate of change in concentration of corneal phosphatic metabolites of cat corneas stored in moist chamber and McCarey-Kaufman (M-K) medium was determined in order to provide a basis for prediction of the corneal metabolic status at a given storage time. Perchloric acid corneal extracts were examined by phosphorus-31 magnetic resonance after storage at 4 degrees C of whole globes under moist-chamber conditions up to 48 h and of excised corneas in M-K medium up to 168 h. A significant decline in the corneal concentrations of ATP and a significant increase in inorganic phosphate occurred for both storage methods; however, depending on the metabolite, the rate of decline or increase was significantly greater for the moist-chamber-stored corneas. The phosphorylated sugars significantly increased and the glycerophosphodiesters significantly decreased in the moist-chamber-stored corneas, whereas both metabolites remained unchanged in the M-K-medium-stored corneas. There was no significant change in the dinucleotides and nucleoside diphospho-sugars during the time course for both storage methods. A threefold greater rate of decline was noted in the tissue energy modulus for the moist-chamber-stored corneas than for the M-K-medium-stored corneas (-0.0465 vs. -0.0121 modulus values/h). M-K medium was significantly more effective in the maintenance of high-energy phosphatic metabolites. The mathematical model for these rate determinations provides a basis for prediction of the corneal metabolic status at a given time in moist-chamber or M-K medium storage.

Adenosine Triphosphate↗
PubMed1985

Mechanical properties of the rat colon: the effect of age, sex and different conditions of storage.

The mechanical properties of the rat colon were studied in old and young Sprague-Dawley rats which were also grouped by sex. Different storage media were used. Rings of colonic tissue were submitted to pulls on an Instron 1026 tensiometer. Gender did not affect the properties of the young rat colon. The rat colon has a tensile strength of around 50 g/mm2 (which places it between the dog and the cat). It increased in strength from proximal to distal, though the rectum was weaker than the colon. The pre-strain of the rat colon was 10% and it was capable of stretching to 200% of its original dimensions. The strength and ability to stretch fell with age, although it initially increased, in the first year of life. Physiological saline at 4 degrees C preserved the burst strength, percentage elongation, hysteresis and Young's modulus between 25 and 100 g stress for up to 1 week. Young's modulus between 125 and 200 g fell progressively with each day of storage. Stress relaxation rose in the first 24 h and thereafter remained constant. Salt appeared to be a good long-term storage medium. Irradiation of the colons before storage did not affect the mechanical properties.

Animals↗