Changes In Viscoelastic Properties Research Articles

Silicon-induced changes in viscoelastic properties of sorghum root cell walls.

Silicon is deposited in the endodermal tissue in sorghum (Sorghum bicolor L. Moench) roots. Its deposition is thought to protect vascular tissues in the stele against invasion by parasites and drying soil via hardening of endodermal cells. We studied the silicon-induced changes in mechanical properties of cell walls to clarify the role of silicon in sorghum root. Sorghum seedlings were grown in nutrient solution with or without silicon. The mechanical properties of cell walls were measured in three separated root zones: basal, apical and subapical. Silicon treatment decreased cell-wall extensibility in the basal zone of isolated stele tissues covered by endodermal inner tangential walls. The silicon-induced hardening of cell walls was also measured with increases in elastic moduli (E) and viscosity coefficients (eta). These results provided new evidence that silicon deposition might protect the stele as a mechanical barrier by hardening the cell walls of stele and endodermal tissues. In contrast to the basal zone, silicon treatment increased cell-wall extensibility in the apical and subapical zones with concomitant decrease in E and eta. Simultaneously, silicon promoted root elongation. When root elongation is promoted by silicon, one of the causal factors maybe the silicon-enhanced extensibility of cell walls in the growing zone.

PRE-CONDITIONING OF HUMAN TENDON-APONEUROSIS, IN VIVO

So-called pre-conditioning of isolated tendon prior to mechanical testing is the procedure of loading and unloading to achieve more repeatable results. However, it remains unknown whether the load-deformation curve of human tendon, in vivo, is altered by such pre-conditioning procedures. PURPOSE To examine the deformation of the human triceps surae tendon-aponeurosis complex in healthy subjects with repeated plantarflexion contractions. METHODS Synchronous real-time ultrasonography for displacement of the tendon-aponeurosis complex of the medial gastrocnemius muscle, electromyography of the gastrocnemius, soleus, and dorsiflexor muscles, and joint angular rotation were obtained during maximal 10-s ramp isometric plantarflexion contractions in 5 healthy male subjects. Displacement obtained during passive range of motion served to correct for displacement attributed to small joint rotation, while EMG and moment data obtained during maximal dorsiflexion efforts served to account for coactivation on tendon force. Five consecutive contractions with ∼30 s rest were performed, and 2 additional contractions were performed after 5 and 10 min. Displacement was analyzed to the least common force for all contractions for each subject. RESULTS (mean ± sem) There was a significant effect of repeated contractions on displacement, P < 0.01. Displacement increased from contraction 1 (5.3 ± 0.9 mm) to contraction 5 (7.2 ± 1.2 mm), P < 0.05. However, displacement 5 min (5.2 ± 1.0 mm) and 10 min (5.2 ± 0.9 mm) after the 5 repeated contractions was similar to that of contraction 1. CONCLUSIONS These data demonstrate that human tendon-aponeurosis deformation, in vivo, was markedly increased (∼40%) with slow isometric contractions. However, the change in viscoelastic properties of the tendon-aponeurosis was abolished within 5 min.

Influence of electron beam irradiation on properties of fluorine-containing poly(aryl ether ketone)s

The influence of electron beam irradiation on the properties of fluorine-containing poly(aryl ether ketone)s (F-PEK), derived from 2,3,4,5,6-pentafluorobenzoic acid, was examined. Irradiation was performed with an electron beam at a dose of 3.63 × 103 Gy s−1 for which the corresponding doses were 29.0, 51.0, and 94.5 MGy. Tensile strength at break increased up to a dose of 29.0 MGy and then decreased very slightly with irradiation. Elongation at break was more susceptible to irradiation and decreased drastically to one tenth at a dose of 29.0 MGy. Young's modulus was enhanced by the irradiation. F-PEKs were changed from elastic materials to strong and brittle materials by irradiation. Relaxation of the viscoelastic property shifted toward higher temperature by irradiation. These tensile and viscoelastic property changes were attributed to the formation of a bulkier and more rigid structure by crosslinking. The fluorine atoms attached to the 1,4-phenylene moiety in F-PEKs were surprisingly susceptible to the irradiation and were completely lost at a dose of 29.0 MGy. The π-electron conjugated aromatic structure was concurrently developed during irradiation. Further, polar functional groups such as carboxyl group and ester group were generated by chain scission and rearrangement. The F-PEKs retained their good transparency and the thermal stability was significantly improved after irradiation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 157–166, 2004

Dynamic and Elongation Rheology of Yeasted Bread Doughs

ABSTRACTThe rheology of yeasted bread doughs is a little‐studied field despite yeast's importance in developing bread structure. A method of thermally inactivating the yeast within mixed bread doughs was developed to overcome the difficulty of yeast fermenting during rheological measurements. Sample stabilization by preshearing of dough samples at a stress amplitude of 1 Pa at 1 Hz for 10 sec improved the reliability of small amplitude oscillatory shear measurements, and resting 20 min within the rheometer was sufficient to produce reliable and consistent observations. Small amplitude oscillatory shear measurements were unable to detect any differences between yeasted and nonyeasted doughs nor any changes in linear viscoelastic properties due to fermentation. However, large strain uniaxial elongation measurements of yeasted doughs revealed a significant progressive decrease in elongational viscosities with fermentation. Size‐exclusion HPLC analysis of yeasted doughs showed an increase in unextractable polymeric dough proteins, which were interpreted as evidence of cross‐linking and therefore a potential improvement in dough properties. The apparent contradictions between uniaxial elongation and SE‐HPLC studies of fermenting yeasted doughs can be attributed to gas bubbles within the dough interrupting the increasingly cross‐linked protein network, resulting in the rheological weakness observed for fermenting yeasted doughs.

Arterial structure and function in end-stage renal disease.

Cardiovascular disease is a major cause of morbidity and mortality in patients with end-stage renal disease (ESRD). Macrovascular disease develops rapidly in ESRD patients and is responsible for the high incidence of left ventricular hypertrophy, ischemic heart disease, cerebrovascular accidents, and peripheral artery diseases. Occlusive lesions due to atheromatous plaques frequently cause these complications; however, atherosclerosis represents only one form of structural response to metabolic and hemodynamic alterations interfering with the “natural” process of aging. The spectrum of arterial alterations in ESRD is broader, including large artery remodeling, changes in viscoelastic properties, and stiffening of arterial walls. Nonatheromatous remodeling principally changes the dampening function of arteries, characterized by stiffening of arterial walls and with deleterious effects on the left ventricle and coronary perfusion. The origin of arterial stiffening in ESRD patients is multifactorial, with extensive arterial calcifications as an important covariate.

Visco-elastic response of human skin and aging

The changes in visco-elastic properties of skin belong to the most conspicuous manifestations of cutaneous aging. In spite of apparent simplicity, the measurement of mechanical parameters of skin in vivo presents both theoretical and practical problems. Reproducibility, standardization, duration of measurement, discomfort for experimental subjects are the main complications. Measurement and analysis of transient deformation response to pressure stress provides theoretically consistent and practically applicable methodology. The transient deformation response of skin was measured in two groups consisting of 15 healthy men and 17 healthy women. The range of age interval was 20 to 58 years. The deformation response was measured as reaction of skin on sudden change of pressure stress between two levels of loading on skin surface. Transient response of human skin consists of sum of two exponential curves. A "rapid" exponential curve has time constant typically of order 10 ms, while "slow" exponential curve has a time constant of order 0.1 to 1s. Both time constants increase with chronological age. Time for drop of deformation on 12.5% of full deformation proved to be a simple and sensitive criterion of skin aging, with strong correlation with chronological age. Main advantage of the method: Measurement is quantitative and reproducible. Procedure is easy to repeat. Its average duration is approx. 2 minutes and it does not represent any discomfort for test subjects.

Assessment of postexercise muscle soreness by electromyography and mechanomyography

Mechanomyography (MMG) and electromyography (EMG) recordings from the first dorsal interosseous muscle of the hand were compared for pre-exercise and immediately after, 24-hour, and 48-hour postexercise muscle soreness. Thirteen healthy male subjects performed progressively increasing number of eccentric contractions from bout 1 (10.34 ± 1.96) to bout 6 (27.46 ± 5.01) (P <.03) with 116% maximum voluntary contraction (MVC) for provocation of postexercise muscle soreness. Increased areas of pain, reduced pressure pain threshold, reduced MVC, and reduced range of motion were present immediately after as compared with pre-exercise (P <.05). During intense eccentric exercise, root mean squared amplitude values of MMG increased progressively from bout 1 to bout 6, but EMG root mean squared amplitude decreased as the muscle fatigued (P <.05). Time course changes of MMG and EMG root mean squared amplitude values during single concentric, isometric, and eccentric contractions at 0%, 25%, 50%, 75%, and 100% MVC weights were measured in relation to postexercise muscle soreness. The EMG root mean squared amplitude values showed insignificant changes for concentric, isometric, and eccentric contractions between pre-exercise, immediately after, 24 hours, and 48 hours. MMG root mean squared amplitude values increased during concentric, isometric, and eccentric contractions at immediately after as compared to pre-exercise, 24 hours, and 48 hours. At immediately after, 24 hours, and 48 hours the maximum EMG root mean squared amplitude values were achieved at lower MVC levels as compared with pre-exercise (P <.05). MMG root mean squared amplitude findings suggest changes in viscoelastic properties resulting in significant mechanical muscle vibrations after intense eccentric exercise. This may suggest a role of stimulation of mechanosensitive nociceptors in relation to postexercise muscle soreness. It is concluded that simultaneous recordings of MMG and EMG may serve as an appropriate means of studying the relationship between electrical and mechanical muscle activity occurring with postexercise muscle soreness. © 2002 by the American Pain Society

Acoustics of blood plasma on solid surfaces

We have quantified surface associated coagulation of human blood plasma with a recently developed methodological system consisting of a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), a method that measures the weight of adsorbed molecules on surfaces as a function of frequency shifts of a quartz crystal. Further, it measures the damping energy (i.e. viscoelasticity) of the adsorbed layer. Four different surfaces where studied: Heparin (Hep) surface as an active inhibitor of clot formation, titanium (Ti) surfaces that are known to activate the intrinsic pathway, polystyrene (PS) surfaces and poly(urethane urea) (PUUR) surfaces. The experiments were initiated by applying citrated human plasma at the sensor surfaces; calcium was then added to initiate coagulation. The Hep surfaces showed no apparent indication of clot formation during one hour of incubation at room temperature. However, on Ti surfaces we observed an early and rapid change in both frequency shift and viscoelastic properties of the coagulating plasma. We inhibited the intrinsic pathway activation by using corn trypsin inhibitor (CTI), which is specific for factor FXIIa in the bulk phase, which prolonged the coagulation times for all non-heparinized surfaces. We have also found a peculiar initial plasma protein interaction phenomenon on Ti surfaces. The described methodology would be very efficient for basic studies of surface associated coagulation and as a screening method for new biomaterials.

Collagen and bone viscoelasticity: a dynamic mechanical analysis.

The purpose of this study was to explore the effects of changes in Type I collagen on the viscoelasticity of bone. Bone coupons were heated at either 100 or 200 degrees C to induce the thermal denaturation of Type I collagen. Half of these specimens were rehydrated after heat treatment; the other half were tested in a dry condition. The degree of denatured collagen (DC%) was analyzed by a selective digestion technique with the use of alpha-chymotrypsin. Isothermal (37 degrees C) and variable temperature tests (scans from 35 to 200 degrees C) were performed with the use of a dynamic mechanical analyzer to evaluate changes in bone viscoelastic properties as a function of collagen damage, specifically, changes in the loss factor (tan delta) and storage modulus (E') were assessed. Significant collagen denaturation occurred only when bone was heated at 200 degrees C irrespective of the hydration condition. Also, DC% did not show a significant effect on tan delta. However, higher values of tan delta were observed in wet samples compared to dry specimens. The temperature-scan tests revealed that the hydration condition, but not DC%, significantly affected the behavior of tan delta. However, E' was not strongly influenced either by DC% or by water content. These results suggest that at a constant frequency the denaturation of collagen triple-helical molecules may have few effects on the viscoelasticity of bone, but moisture may play a prominent role in determining this property.

Hybrid silica-polyimide composite membranes: gas transport properties

The gas permeability, diffusivity, and selectivity properties of gases such as He, O 2, N 2, CH 4, and CO 2 were evaluated for a series of hybrid inorganic–organic composites based on 6FDA-6FpDA and 6FDA-6FpDA-DABA polyimides and various organo-silica structures. The organo-silica domains were introduced into the polymer matrix via sol–gel reactions. The gas transport properties of these hybrid membranes were dependent on the type of alkoxide employed and the final morphology of the hybrid composite. Systems with low degree of cross-linking between the inorganic and polymer components exhibited membrane performance below the upper bound. However, simultaneous improvements in selectivity and permeability were observed in hybrid materials with favorable interactions between inorganic and polymer domains as documented by changes in viscoelastic properties. These improved interactions most likely led to the formation of an interphase consisting of constrained polymer segments partially incorporated into the silicate structure.

Direct visualization of random crystallization and melting in arrays of nanometer-size polymer crystals.

Using tapping mode atomic force microscopy, we visualized in direct space and time resolved the changes in viscoelastic properties during crystallization and melting of polyethyleneoxide in 12 nm spheres of a block copolymer mesophase. All spheres crystallized individually and independently, randomly distributed sphere by sphere. Melting of the confined crystals also proceeded in a stochastic manner. Not all spheres melted at the same temperature, indicating different degrees of order of the individual nanometer-size polymer crystals. The independence of the spheres opens the possibility to manipulate material properties of surfaces at the nanometer scale.

High-speed subfailure stretch of rabbit anterior cruciate ligament: changes in elastic, failure and viscoelastic characteristics

Objective. To study alterations in the ligament mechanical characteristics due to a subfailure stretch delivered at high speed. Design. An in vitro study of rabbit anterior cruciate ligaments. Background. Although ligamentous sprains occur more frequently than the complete failures, only a few biomechanical studies have investigated the effects of such injuries. Purpose of the study was to document changes in elastic, failure and viscoelastic properties of a ligament that had been subjected to a high-speed subfailure stretch. Methods. Thirteen paired fresh adult rabbit femur-anterior cruciate ligament-tibia preparations were used. One ligament of each pair (control) was subjected to two relaxation tests and then stretched until failure. The other ligament (experimental) was subjected sequentially to relaxation test, subfailure stretch (80% of the failure deformation of the control), relaxation test, and then stretched until failure. Load–deformation curve until failure was characterized by nine parameters, which included failure force, deformation and energy absorbed, and deformations measured at various load values, and stiffness. Relaxation curve was parameterized by five relaxation forces measured at 10, 30, 50, 130, and 180 s. Results. Due to subfailure stretch, there were no changes in failure force and stiffness, while deformations increased, and energy absorbed and relaxation forces decreased. Conclusions. The 80% subfailure stretch delivered at high speed increased the deformations in load–deformation tests, and decreased the forces in relaxation tests. Relevance Incomplete injuries of the ligaments are more prevalent than the complete injuries. The incomplete injury is a subfailure stretch delivered at high speed. The present study provides the results of simulating the incomplete injury using an in vitro ligament model.

Bioadhesion of supramolecular structures at supported planar bilayers as studied by the quartz crystal microbalance.

A quartz crystal microbalance (QCM) was used to study the adhesion behavior of supramolecular aggregates at supported planar bilayers (SPBs). The QCM technique is a suitable method to detect the adsorption of biomolecules at the quartz surface owing to its sensitivity for changes in mass and viscoelastic properties. To simulate biomembranes, the quartz plates were coated with highly ordered lipid films. Therefore, a combination of self-assembled monolayers and Langmuir-Blodgett films was used. Firstly, the adsorption of liposomes coupled with the lectin concanavalin A was investigated at glycolipid-containing model membranes. Using different carbohydrates, it was possible to determine specific and nonspecific parts of the interactions. The adhesion occurred owing to specific lectin-carbohydrate interactions (about 20%) and to nonspecific interactions (about 80%). The composition of the liposomes was changed to simulate the structure of a native biomembrane consisting of the glycocalix, the lipid-protein bilayer, and the cytoskeleton. An artificial glycocalix was created by incorporating poly(ethylene glycol) into the liposomes. Liposomes which were intravesicular polymerized with polyacrylamide or polyacrylcholate simulated the cytoskeleton. It was determined that the modified liposomes had significant lower interactions with SPBs. The adsorption was reduced by approximately 80% compared to unmodified liposomes. Secondly, a model was developed for the detection of interactions between simple or mixed bile salt micelles and model membranes. It was found that simple bile salts did not adsorb at model membranes. Binary systems consisting of bile salt and phospholipid induced only small interactions. On the other hand, ternary systems consisting of bile salt, phospholipid, and fatty acid showed strong interactions. A dependence on the chain length of the fatty acid was observed. Thirdly, the interaction between ganglioside-containing model membranes and cholera toxin (beta-subunit) was investigated. Different ganglioside fractions showed varying adsorption in the following sequence: GM1 > GD1a > GD1b > GT1b.

Evaluation of thermal degradation on carbon fiber/cyanate ester composites

A fundamental problem of advanced airplane composite systems is the lack of understanding of the aging process and how it affects the material properties associated with degradation. A cyanate ester resin/carbon fiber composite system was subjected to an accelerated aging environment to investigate the deterioration and degradation process. The change in viscoelastic properties of cured laminates in the range of the glass transition temperature was studied. Thermo-mechanical property changes were evaluated by dynamic mechanical analysis (DMA) through time-temperature equivalence. Master curves were constructed to compare the aging effect between specimens and a model was developed that described the degradation phenomena. During the aging process, a swelling phenomenon of cured laminates was detected as a result of matrix degradation. Collectively, a modeling methodology was developed that quantitatively provides an understanding of the aging process of fiber reinforced composites in isothermal environments.

Dynamic mechanical spectroscopy of plastic crystalline states in n-alkane systems

Dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC) were used to study the plastic crystalline region of several model n-alkane systems and a commercial paraffin wax. Results indicate that DMS provides a valuable complement to existing tools for locating plastic crystalline or rotator states and characterizing the pre-melting region containing these mesophases. DMS measurements demonstrate that the mechanical properties of plastic crystalline states in n-alkanes are difficult to isolate, and that observed viscoelasticity surrounding the mesophases is in great part due to transitions between phases, not the rotator phases themselves. This is indicated from DMS curves that show the dynamic moduli of rotator phases are inhibited from achieving equilibrium values due to the close proximity of successive transitions. Furthermore, study of a system in which stability of a single rotator phase has been extended through strategic blending of n-alkanes shows that dynamic mechanical properties return to near pre-mesophase values when sufficient opportunity following a transition is provided. These results demonstrate that many n-alkane blends can possess a short temperature interval over which the material oscillates between Hookean and viscous behavior, controlling their performance and possibly providing for new applications requiring changes in viscoelastic properties in a narrow temperature span.

Real-time detection of organic compounds in liquid environments using polymer-coated thickness shear mode quartz resonators

The selection of sensitive coatings is a critical task in the design and implementation of chemical sensors using coated thickness shear mode quartz crystal resonators (QCRs) for detection in liquid environments. This design or selection is performed through a study of the sorption process in terms of the partition coefficients of the analytes in the coatings. The partition coefficient, which is controlled by the chemical and physical properties of the coating materials, determines the inherent selectivity and sensitivity toward analyte molecules. The selection of the coatings is logically determined by the interactions between coating and target analyte molecules, but can also be made through a systematic variation of the coating's properties. The determination of the partition coefficients is only accurate if all contributions to the total measured frequency shifts, deltafs, of the coated QCR can be established. While mass loading is often assumed to be the dominant factor used in determining partition coefficients, viscoelastic effects may also contribute to deltafs. Both the effect of viscoelastic properties and the effect of mass loading on the sensor responses are investigated by using a network analyzer and oscillator circuit and by characterizing the total mechanical impedance of the loaded sensor. Different types of coatings including rubbery and glassy polymers are investigated, and the targeted analytes include classes of polar compounds (methanol), nonpolar compounds (toluene, xylenes), and chlorinated hydrocarbons (trichloroethylene, tetrachloroethylene, etc). It is seen that changes in viscoelastic properties due to analyte sorption may be significant enough to place the sensor in the nongravimetric regime. However, for most applications involving the detection of relatively low concentrations of organic compounds and the use of acoustically thin films, changes in the complex shear modulus of the coatings contribute less than 5% of the total shift in the series resonant frequency, depending on the coating. In that case, the measured deltafs and, hence, the calculated approximate classification and selection of the coatings for operation in a complex solution of water/analyte molecules.

Mechanics of the Middle Ear System: Age-Related Changes in Viscoelastic Properties

Morphological studies of age-related changes in the tympanic membrane have suggested a decreased elasticity for increasing age, whereas age-related changes in admittance measured by tympanometry have revealed inconsistent results. In this study we report on the viscoelastic properties of the middle ear system by means of a new method in two different age groups. This method measures hysteresis (μJ), which describes the viscous properties of the system, and compliance (mm³/kPa) reflecting its elasticity. Moreover, P_ec0 (kPa) was recorded, which corresponds to middle ear pressure measured by tympanometry. Recordings were made in a group of 30 older normal subjects (mean age 77 years) and results were compared to previous findings in a group of 39 younger normal subjects (mean age 29). Hysteresis was markedly decreased in the older normal group compared to the younger one (p = 0.057). This may reflect morphological changes previously reported in the tympanic membrane and can be related to theoretical considerations on tympanic membrane rupture pressures. Compliance and P_ec0 were not influenced by variation in age (p = 0.645 and 0.966).

ゴム系複合体の粘弾性と接着強さ

We studied the effects of temperature and rate on adhesive strength of the cord and rubber composite. The peel strength depended strongly on viscoelastic properties of rubber which relates deeply to temperature and the peeling rate. A master curve of the peel strength was also obtained by applying time-temperature superposition principle on the data obtained under several temperatures Observing fracture surface, some types of fracture were found corresponding to the change of viscoelastic properties of rubber compound.The pull-out strength was found to have a similar behavior to the temperature and rate dependencies of the peel strength. Viscoelastic functions could be also applied to the pull-out test.

Changes in viscoelastic properties of rat lung parenchymal strips with maturation.

The lung extracellular matrix changes rapidly with maturation. To further our understanding of the mechanisms underlying lung tissue mechanics, we studied age-related changes in mechanical properties in lung parenchymal strips from baby (10-15 days old), young ( approximately 3 wk old), and adult ( approximately 8 wk old) rats. Subpleural strips were cut and suspended in a fluid-filled organ bath. One end of the strip was attached to a force transducer and the other to a servo-controlled lever arm. Measurements of force (F) and length (L) were recorded during sinusoidal oscillations of various amplitudes and frequencies. Resistance modulus (R) and elastance modulus (E) were estimated by fitting the equation of motion to changes in stress (T) and stretch ratio (lambda). Hysteresivity (eta) was calculated as follows: eta = (R/E)2pif, where f is frequency. Slow-cycling T-lambda curves were measured by applying a constant slow length change. Finally, quasi-static T-lambda curves were measured as stress was increased from 0 to 6 kPa and back to 0 kPa in stepwise increments. Our results showed that lung tissue from immature rats was stiffer and less hysteretic than tissue from more mature animals. In addition, tissue from baby animals behaved in a manner compatible with an increased vulnerability to plastic change.

A rheometer dedicated for the investigation of viscoelastic effects in commercial magnetic fluids

The investigation of viscoelastic properties of commercial ferrofluids based on nanosized magnetite particles in various carrier liquids requires a dedicated rheometer allowing the detection of extremely small changes in viscous and viscoelastic properties. The observation of magnetoviscoelastic behavior in commercial ferrofluids may become particularly important since they exhibit long-term stability allowing reproducibility of the experiments. In addition, they are of relatively simple composition reducing the problems in theoretical description of the observed effects. We will present here the design and main performance data of a rheometer developed for the kind of investigations mentioned above. The quality of the performance will be demonstrated with some new data on rheological properties of a commercial ferrofluid.