Changes In Viscoelastic Properties Research Articles

Sounding out buried nanostructures using subsurface ultrasonic resonance force microscopy

ABSTRACTImaging of nanoscale structures buried in a covering material is an extremely challenging task, but is also considered extremely important in a wide variety of fields. From fundamental research into the way living cells are built up to process control in semiconductor manufacturing would all benefit from the capability to image nanoscale structures through arbitrary covering layers. Combining Atomic Force Microscopy (AFM) with ultrasound has been shown a promising technology to enable such imaging in various configurations. Here we report the development of an alternative method of combining AFM with ultrasound which we call SubSurface Ultrasonic Resonance Force Microscopy (SSURFM) and which is based on a combination of the two most common variants described in literature, which each have their specific strong points: Ultrasonic Force Microscopy (UFM) and Contact Resonance AFM (CR-AFM). We show the excellent performance of this combination on a number of samples designed specifically to mimic relevant conditions for the application as a metrology technique in the semiconductor manufacturing process. We also discuss the physics of the image contrast mechanism which is based on sensing local changes in visco-elastic properties of the sample bygenerating large indentations in the surface.

Physical and Tribological Properties Degradation of Silicone Rubber Using Jatropha Biolubricant

ABSTRACT This article aims to investigate the degradation of physical and tribological properties (friction coefficients and wear resistance) of a dynamic sealing material (silicone rubber [VMQ]) exposed to Jatropha oil (JO), engine mineral oil (EMO), and a blend (B20; 80% EMO–20% JO), separately. JO has demonstrated better lubricating properties than EMO in various mechanical applications; however, the degradation of elastomers by using this oil has not been studied yet, nor have its effect on their tribological properties. The physical degradation was evaluated by conducting static immersion tests (670 h at 25 °C) based on ASTM-D471 and ASTM-D7216 methods. Hence, the changes in mass, volume, tensile and tear strengths, and hardness of VMQ were measured. In addition, creep compliance tests were conducted to determine the changes in viscoelastic properties and the changes in morphology and topography were measured by scanning electron microscopy (SEM) and optical profilometry, respectively. In addition, the compositional changes were determined by Fourier transform infrared (FTIR) analyses to complement the degradation examination. Changes in the friction coefficients were determined by ball-on-disk tests and changes in wear resistance were obtained by accelerated wear tests (microabrasion tests). Finally, VMQ exhibited no significant physical and compositional degradation due to immersion in the three lubricants. However, considerable changes in the friction coefficients and wear resistance were observed; the change in the friction coefficients was minimal using JO. In addition, the coefficients using JO were 50% lower than those for EMO and the changes in wear resistance were lowest after immersion in JO.

Glucocorticoid-Induced Changes in Rat Skeletal Muscle Biomechanical and Viscoelastic Properties: Aspects of Aging

ObjectivesThe purpose of this study was to estimate the state of tension (tone) and the biomechanical and viscoelastic properties of skeletal muscle in aging rats during the administration of different doses of dexamethasone and to find the relationships among the state of muscle atrophy, muscle strength, and the abovementioned muscle properties. MethodsMuscle state of tension, biomechanical (elasticity, dynamic stiffness) and viscoelastic (mechanical stress relaxation time, Deborah number) properties (using MyotonPRO, Myoton Ltd, Tallinn, Estonia), lean body mass (BM), and hind limb grip strength were measured before and after the administration of a 10-day treatment with dexamethasone 100 μg/100 g BM (young and old group) and 50 μg/100 g BM (old group). ResultsMuscle elasticity (logarithmic decrement) was lower in old animals (1.86 ± 0.03) in comparison with young adult rats (1.38 ± 0.04) (P < .01). After the 10-day treatment with dexamethasone 100 μg/100 g BM, young adult rats had 10% lower muscle elasticity (P < .01). The same dose of dexamethasone in old rats increased tone (frequency of natural oscillation) from 29.13 ± 0.51 Hz to 38.50 ± 0.95 Hz (P < .001). There were dose-dependent differences in dynamic stiffness and tone of muscle; changes in elasticity were independent of the dose in old animals. In old rats, the muscle’s viscoelastic properties decreased after dexamethasone administration. Significant correlation was found between changes in muscle logarithmic decrement and stiffness (rs = 0.90; P < .05) in old animals. ConclusionsBiomechanical and viscoelastic properties of skeletal muscle indicate changes in the main function of muscle during glucocorticoid-induced muscle atrophy and are in agreement with changes in hind limb strength. The myometric measurements indicate the direction and magnitude of change in muscle tissue after different doses of dexamethasone administration easily and quickly.

Wheat batter physical properties as influenced by starch/flour types and egg contents

Batter liquid–solid phase interactions can play a critical role in determining the functionality, including adherence, appearance and texture of end use coated product. Batter chemical composition is a key factor affecting batter characteristics. The effect of substituting wheat flour with fractions of rice flour, native corn starch, modified waxy corn starch, tapioca starch and lupine flour ranging from 26 to 56% of the solid phase and egg levels ranging from 6 to 35% of the liquid phase on the rheological properties of batter systems was studied. Results indicated that lupine flour had the greatest water holding capacity (WHC) ranging from 93.9 to 119.3% of various lupine flour percent to egg levels used. Flow behaviour index of batter flour treatments ranged from 0.91 to 1.87; values that were significantly (P < 0.05) greater than a 100% control wheat flour. Consistency coefficient of treatments ranged from 0.01 to 0.37 mPa sn; values that were significantly lower than wheat flour (i.e., 5 mPa sn). Egg level used in this study had minimal effect on treatment rheological properties. The changes in WHC and viscoelastic properties of treatments were attributed to changes in protein and fibre contents and structure as a result of replacing wheat flour with starch/flour. Protein–starch–lipid matrix formation and variation in water absorption kinetics of treatments most probably wrought batter functionality. The increase in pasting properties with the increase in starch/flour contribution suggested an increased contribution of starch in forming a net effect of batter treatments pasting viscosities. Results of this study provide vital information for the potential use of starch types to enhance the rheological properties of coating applications including adhesion and water holding capacity of substituted batter treatment.

The changes of red blood cell viscoelasticity and sports anemia in male 24-hr ultra-marathoners

BackgroundIn endurance sports, stress, dehydration and release of chemical factors have been associated with red blood cell (RBC) alterations of structure and function, which may contribute to sports anemia, a well-observed phenomenon during long-distance running. Until now, the investigation of the changes of viscoelastic properties of RBC membrane, a decisive factor of RBC deformability to avoid hemolysis, is lacking, especially in an Oriental population. Methodsnineteen runners were prospectively recruited into our study. Hematological parameters were analyzed before and immediately after the 2015 Taipei 24H Ultra-Marathon Festival, Taiwan. Video particle tracking microrheology was used to determine viscoelastic properties of each RBC sample by calculating the dynamic elastic modulus G′(f) and the viscous modulus G″(f) at frequency f = 20 Hz. ResultsHaptoglobin, RBC count, hemoglobin, hematocrit, mean cell hemoglobin, plasma free hemoglobin and unsaturated iron-binding capacity values of the recruited runners showed a statistically significant drop in the post-race values. Blood concentration of reticulocyte and ferritin were significantly higher at post-race compared with pre-race. 15 out of the 19 runners had a concurrent change in the elastic and the viscous moduli of their RBCs. Changes in the elastic and the viscous moduli were correlated with changes in the RBC count, hemoglobin and hematocrit. ConclusionViscoelasticity properties, the elastic modulus G′(f) and the viscous modulus G″(f) of RBCs are associated with endurance exercise-induced anemia.

Rheological Measurement of Polymeric Composites before and after UV Degradation

The aim of this paper is the comparison of selected rheological properties of polymeric composites with glass fibers materials before and after exposure in UV box. Rate and depth of degradation induced by the environment were evaluated by Frequency Sweep Test, which monitors changes in viscoelastic properties of polymers with respect to their molecular structure and their behavior in thermoplastic processes. Degradation process resulted in changes of complex dynamic viscosity, storage and loss modulus, changes in molecular weight and its distribution. UV radiation is intense degradation factors affecting the change in the structure and properties of polymers - the polymer matrix gradually degrades by UV radiation, the viscosity of the composites decreases, the COP is moved to lower angular frequencies with increasing molar mass.

Effect of humidity on the interaction of CO2 with alkaline anion exchange membranes probed using the quartz crystal microbalance

The alkaline anion exchange membrane fuel cell (AAEM-FC) is able to deliver a comparable performance to the traditional proton exchange membrane fuel cell (PEM-FC) without the use of precious metal electrocatalysts, making it a more cost-competitive alternative for low-temperature fuel cell applications. However, issues relating to degradation and specifically interaction with CO2 still hinder the technology's commercialisation prospects. With hydration playing a key role in solid polymer electrolyte fuel cell operation, this study examines how membrane hydration affects the AAEM interaction with CO2. The change of membrane conductivity upon exposure to atmospheric CO2 has been compared with the change in viscoelastic properties of a cast thin-film ionomer, both as a function of humidity. The effect of CO2 on the membrane as a function of hydration suggests a link to its solvation and swelling regimes and thus the access of CO2 to the ionic channels within the membrane. The thin-film QCM composite resonator study has suggested that during the solvation (pore opening) regime, there is a linear increase in CO2 uptake as water can further permeate the pore system and the cationic headgroups become increasingly accessible. During the transition to the pore swelling regime, there is a step increase in CO2 uptake as the network is thought to be fully open; as such, subsequent increases in RH do not lead to any significant increase in CO2 uptake.

Characterisation of the viscoelastic properties of avocado puree for process design applications

Avocado is increasingly utilised for the manufacture of products including purees, concentrates, powders and avocado oil. The design of such processes requires understanding of the rheology, particularly during the puree's transformation. This study characterised the viscoelastic properties at 25 °C of fresh avocado puree resulting from various processing operations including mixing, water dilution, and after sonication treatment. The storage ( G ′) and loss ( G ″) moduli of avocado puree demonstrated a solid-like behaviour irrespective of treatment or dilution, contrary to other food matrices. Avocado puree produced with manual-shearing showed greater G ′ and G ″ than when obtained with a food-processor. Extended shearing through the malaxation operation (mixing and kneading at 49 rpm and 45 °C); or water dilution to 72–94% moisture, diminished both moduli, with an exponential decrease observed in the latter. However, sonication for 5 min with high-power-ultrasound (18 kHz and 40 kHz) or megasonics (MS) (2 MHz) (80–90 kJ kg −1 ) increased G ′ and G ″. The net change in viscoelastic properties due to puree low frequency sonication correlated linearly with moisture content. The influence of processing interventions in avocado puree was demonstrated through rheological methods applicable for avocado process optimisation and product development.

The effects of 4 weeks stretching training to the point of pain on flexibility and muscle tendon unit properties.

The purpose of this study wasto compare the benefits and possible problems of 4weeks stretching when taken to the point of pain (POP) and to the point of discomfort (POD). Twenty-six physically active women(20±1.1years)took part in group-based stretching classes of the hamstring muscles, 4 times per week for 4weeks, one group onestretching to POD, the other to POP. Passive stiffness, joint range of motion (ROM), maximal isometric torque and concentric knee flexion torque, were measured before training and 2days after the last training session. Hip flexion ROMincreased by 14.1° (10.1°-18.1°) and 19.8° (15.1°-24.5°) and sit-and-reach by 7.6 (5.2-10.0) cm and 7.5 (5.0-10.0) cm for POD and POP, respectively (Mean and 95% CI; p<0.001 within group; NS between groups), with no evidence of damage in either group. Despite the large increases in flexibility there were no changes in either compliance or viscoelastic properties of the muscle tendon unit (MTU). Hamstrings stretching to POP increased flexibility and had no detrimental effects on muscle function but the benefits were no better than when stretching to POD so there is no justification for recommending painful stretching. The improvements in flexibility over 4weeks of stretching training appear to be largely due to changes in the perception of pain rather than physical properties of the MTU although less flexible individuals benefited more from the training and increased hamstring muscle length.

Influence of synthesis conditions on thermal release of palm oil as liquid core filled in polymeric nanoparticles

After synthesis of poly(styrene-co-maleimide) nanoparticles with 70 mass% encapsulated palm oil, by an imidization reaction of the poly(styrene-co-maleic anhydride) copolymer with different concentrations of ammonium hydroxide, the thermal release properties of palm oil were studied under various temperatures and heating times. The altered reaction conditions lead to progressively higher imide contents, resulting in better thermal stability and retarded release profiles. Based on differential scanning calorimetry, the release of oil is associated with an exotherm reaction, while the glass transition phenomena may be partially masked due to the complex structure of the oil and organic phase. The isothermal scans show different rates of oil release followed by a post-imidization reaction. From dynamic mechanical analysis, the thermal release can be followed by changes in visco-elastic properties and a shift in maximum loss factor. The thermal release profiles after heating for 2 min to 6 h at 125–250 °C were identified from FTIR to Raman spectroscopy, following variations in oil-related and imide-related absorption bands. The amount of exposed oil progressively decreases at higher ammonium hydroxide concentrations, while the oil release is stabilized at temperatures below the glass transition temperature and linear oil release starts at temperatures above the glass transition temperature. The kinetics and mechanism of oil release can be described by the release rate and diffusion exponent of the Korsmeyer–Peppas model. Therefore, it was concluded that the diffusion mechanisms are closely related to the softening properties of the polymer, with linear time-dependent release (Fick diffusion) or more immediate release (anomalous diffusion) stimulated by softening of the polymer matrix.

In vitro oxidative degradation of a spinal posterior dynamic stabilization device.

This study quantified the changes of the frequency-dependant viscoelastic properties of the BDyn (S14 Implants, Pessac, France) spinal posterior dynamic stabilization (PDS) device due to in vitro oxidation. Six polycarbonate urethane (PCU) rings and six silicone cushions were degraded using a 20% hydrogen peroxide/0.1 M cobalt (II) chloride hexahydrate, at 37°C, for 24 days. The viscoelastic properties of the individual components and the components assembled into the BDyn PDS device were determined using Dynamic Mechanical Analysis at frequencies from 0.01 to 30 Hz. Attenuated Total Reflectance Fourier Transform Infra-Red spectra demonstrated chemical structure changes, of the PCU, associated with oxidation while Scanning Electron Microscope images revealed surface pitting. No chemical structure or surface morphology changes were observed for the silicone cushion. The BDyn device storage and loss stiffness ranged between 84.46 N/mm to 99.36 N/mm and 8.13 N/mm to 21.99 N/mm, respectively. The storage and loss stiffness for the components and BDyn device increased logarithmically with respect to frequency. Viscoelastic properties, between normal and degraded components, were significantly different for specific frequencies only. This study demonstrates the importance of analyzing changes of viscoelastic properties of degraded biomaterials and medical devices into which they are incorporated, using a frequency sweep. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1237-1244, 2018.

Membrane shaping by actin and myosin during regulated exocytosis.

The cortical actin network in neurosecretory cells is a dense mesh of actin filaments underlying the plasma membrane. Interaction of actomyosin with vesicular membranes or the plasma membrane is vital for tethering, retention, transport as well as fusion and fission of exo- and endocytic membrane structures. During regulated exocytosis the cortical actin network undergoes dramatic changes in morphology to accommodate vesicle docking, fusion and replenishment. Most of these processes involve plasma membrane Phosphoinositides (PIP) and investigating the interactions between the actin cortex and secretory structures has become a hotbed for research in recent years. Actin remodelling leads to filopodia outgrowth and the creation of new fusion sites in neurosecretory cells and actin, myosin and dynamin actively shape and maintain the fusion pore of secretory vesicles. Changes in viscoelastic properties of the actin cortex can facilitate vesicular transport and lead to docking and priming of vesicle at the plasma membrane. Small GTPase actin mediators control the state of the cortical actin network and influence vesicular access to their docking and fusion sites. These changes potentially affect membrane properties such as tension and fluidity as well as the mobility of embedded proteins and could influence the processes leading to both exo- and endocytosis. Here we discuss the multitudes of actin and membrane interactions that control successive steps underpinning regulated exocytosis.

Rheological properties and microstructure of xylanase containing whole wheat bread dough.

The present research work was undertaken to investigate the effect of xylanase, produced by Penicillium citrinum, on rheological behavior of whole wheat bread dough at large and small deformation respectively. Dough attributes including textural properties (penetration) and structure related characteristics (oscillatory tests) were evaluated. Change in visco-elastic properties of xylanase containing dough was evaluated by oscillatory and creep measurements. The flow experiments were conducted under steady-state condition with shear rate ranging from 0.01 to 100s-1. Frequency sweep experiments were performed between 0.01 and 10Hz. It revealed that in both control and xylanase containing dough formulation, the elastic modulus was higher than viscous modulus in the entire range of frequency. Our results represent the adequacy of fitting of dynamic moduli in Power law model and week gel model. Peleg model as well as six element Kelvin model described well the creep behaviour of control and xylanase-containing dough. Uniaxial extensibility was assessed by Kieffer dough and gluten extensibility rig. Lyophilized powder of untreated and xylanase treated doughs were tested under scanning electron microscope. FTIR spectra of lyophilized powder of untreated and xylanase treated dough were recorded in the range of 600-4000cm-1.

A surface wave elastography technique for measuring tissue viscoelastic properties

A surface wave elastography method is proposed to study the viscoelastic properties of skin by measuring the surface wave speed and attenuation on the skin. Experiments were carried out on porcine skin tissues. The surface wave speed is measured by the change of phase with distance. The wave attenuation is measured by the decay of wave amplitude with distance. The change of viscoelastic properties with temperature was studied at room and body temperatures. The wave speed was 1.83m/s at 22°C but reduced to 1.52m/s at 33°C. The viscoelastic ratio was almost constant from 22°C to 33°C. Fresh and decayed tissues were studied. The wave speed of the decayed tissue increased from 1.83m/s of fresh state to 2.73m/s. The viscoelastic ratio was 0.412/mm at the decayed state compared to 0.215/mm at the fresh state. More tissue samples are needed to study these viscoelastic parameters according to specific applications.

Monitoring the Changes of Material Properties at Bone-Implant Interface during the Healing Process In Vivo: A Viscoelastic Investigation.

The aim of this study was to monitor the changes of viscoelastic properties at bone-implant interface via resonance frequency analysis (RFA) and the Periotest device during the healing process in an experimental rabbit model. Twenty-four dental implants were inserted into the femoral condyles of rabbits. The animals were sacrificed immediately after implant installation or on day 14, 28, or 56 after surgery. Viscoelastic properties at bone-implant interface were evaluated by measuring the implant stability quotient (ISQ) using RFA and by measuring the Periotest values (PTVs) using the Periotest device. The bone/implant specimens were evaluated histopathologically and histomorphometrically to determine the degree of osseointegration (BIC%). The BIC% values at different time points were then compared with the corresponding ISQ values and PTVs. The mean ISQ value increased gradually and reached 81 ± 1.7 on day 56, whereas the mean PTV decreased over time, finally reaching −0.7 ± 0.5 on day 56. Significant correlations were found between ISQ and BIC% (r = 0.701, p < 0.001), PTV and BIC% (r = −0.637, p < 0.05), and ISQ and PTV (r = −0.68, p < 0.05). These results show that there is a positive correlation between implant stability parameters and peri-implant-bone healing, indicating that the RFA and Periotest are useful for measuring changes of viscoelastic properties at bone-implant interface and are reliable for indirectly predicting the degree of osseointegration.

Impact of high dose vitamin C on platelet function.

AIMTo examine the effect of high doses of vitamin C (VitC) on ex vivo human platelets (PLTs).METHODSPlatelet concentrates collected for therapeutic or prophylactic transfusions were exposed to: (1) normal saline (control); (2) 0.3 mmol/L VitC (Lo VitC); or (3) 3 mmol/L VitC (Hi VitC, final concentrations) and stored appropriately. The VitC additive was preservative-free buffered ascorbic acid in water, pH 5.5 to 7.0, adjusted with sodium bicarbonate and sodium hydroxide. The doses of VitC used here correspond to plasma VitC levels reported in recently completed clinical trials. Prior to supplementation, a baseline sample was collected for analysis. PLTs were sampled again on days 2, 5 and 8 and assayed for changes in PLT function by: Thromboelastography (TEG), for changes in viscoelastic properties; aggregometry, for PLT aggregation and adenosine triphosphate (ATP) secretion in response to collagen or adenosine diphosphate (ADP); and flow cytometry, for changes in expression of CD-31, CD41a, CD62p and CD63. In addition, PLT intracellular VitC content was measured using a fluorimetric assay for ascorbic acid and PLT poor plasma was used for plasma coagulation tests [prothrombin time (PT), partial thrombplastin time (PTT), functional fibrinogen] and Lipidomics analysis (UPLC ESI-MS/MS).RESULTSVitC supplementation significantly increased PLTs intracellular ascorbic acid levels from 1.2 mmol/L at baseline to 3.2 mmol/L (Lo VitC) and 15.7 mmol/L (Hi VitC, P < 0.05). VitC supplementation did not significantly change PT and PTT values, or functional fibrinogen levels over the 8 d exposure period (P > 0.05). PLT function assayed by TEG, aggregometry and flow cytometry was not significantly altered by Lo or Hi VitC for up to 5 d. However, PLTs exposed to 3 mmol/L VitC for 8 d demonstrated significantly increased R and K times by TEG and a decrease in the α-angle (P < 0.05). There was also a fall of 20 mm in maximum amplitude associated with the Hi VitC compared to both baseline and day 8 saline controls. Platelet aggregation studies, showed uniform declines in collagen and ADP-induced platelet aggregations over the 8-d study period in all three groups (P > 0.05). Collagen and ADP-induced ATP secretion was also not different between the three groups (P > 0.05). Finally, VitC at the higher dose (3 mmol/L) also induced the release of several eicosanoids including thromboxane B2 and prostaglandin E2, as well as products of arachidonic acid metabolism via the lipoxygenases pathway such as 11-/12-/15-hydroxyicosatetraenoic acid (P < 0.05).CONCLUSIONAlterations in PLT function by exposure to 3 mmol/L VitC for 8 d suggest that caution should be exerted with prolonged use of intravenous high dose VitC.

Viscoelastic Properties of the Chinese Fir (Cunninghamia lanceolata) during Moisture Sorption Processes Determined by Harmonic Tests.

Measured by harmonic tests, the viscoelastic properties of Chinese fir during moisture sorption processes were examined under three relative humidity (RH) modes: RHramp, RHisohume, and RHstep. The stiffness decreased and damping increased as a function of the moisture content (MC), which is presumed to be the effect of plasticization and an unstable state. The increasing damping was associated with the breaking of hydrogen bonds and the formation of free volume within polymer networks. The changes of loss modulus ratio at 1 and 20 Hz, E″1Hz/E″20Hz, proved the changing trend of the unstable state. Higher ramping rates aggravated the unstable state at the RHramp period, and higher constant RH levels provided more recovery of the unstable state at the RHisohume period. Changes of viscoelastic properties were associated with RH (varied or remained constant), and the application of Boltzmann’s superposition principle is a good approach to simulate viscoelasticity development.

A Novel Device for the Evaluation of Hemostatic Function in Critical Care Settings.

Major surgical procedures often result in significant intra- and postoperative bleeding. The ability to identify the cause of the bleeding has the potential to reduce the transfusion of blood products and improve patient care. We present a novel device, the Quantra Hemostasis Analyzer, which has been designed for automated, rapid, near-patient monitoring of hemostasis. The Quantra is based on Sonic Estimation of Elasticity via Resonance Sonorheometry, a proprietary technology that uses ultrasound to measure clot time and clot stiffness from changes in viscoelastic properties of whole blood during coagulation. We present results of internal validation and analytical performance testing of the technology and demonstrate the ability to characterize the key functional components of hemostasis.

Photo-Dimerization Induced Dynamic Viscoelastic Changes in ABA Triblock Copolymer-Based Hydrogels for 3D Cell Culture

Cells sense and respond not only to biochemical signals but also to biophysical signals (e.g., matrix elasticity). In addition to matrix elasticity, recent studies have revealed that viscoelasticity (e.g., stress relaxation) significantly affects cellular functions, such as spreading, proliferation, and differentiation. Herein, we describe a cytocompatible and dynamic hydrogel that is comprised of well-defined ABA triblock copolymers, which enable significant changes of viscoelastic properties solely by UV light irradiation. The A block contains N-isopropylacrylamide and an acrylate monomer with a coumarin side chain, whereas the B block is biocompatible poly(ethylene oxide). The triblock copolymer forms a physically cross-linked hydrogel under physiologically relevant conditions. However, under UV light irradiation, the viscoelasticity of the hydrogel is dynamically modulated due to the photodimerization of coumarin moieties. After UV illumination, rheological properties of the hydrogel are drastically d...

Altered viscoelastic properties of stroke-affected muscles estimated using ultrasound shear waves - Preliminary data.

As a result of a brain injury such as stroke, the skeletal muscles may undergo numerous structural and functional alterations. These abnormal changes are linked to muscle weakness, joint contracture, and abnormal muscle tone and eventually, result in motor impairment. A subset of these alterations affects passive muscle stiffness, i.e., viscoelastic properties. However, in vivo estimation of changes in viscoelastic properties is a challenging task. Here, we used the shear wave velocity, estimated through ultrasound SuperSonic imaging (SSI), as a surrogate for viscoelastic properties. We estimated shear wave group and phase velocities (dispersion), and thus, quantified both elasticity and viscosity of the muscle tissue, respectively in muscles of hemiplegic stroke survivors. In these individuals, we found significantly higher group and phase velocities in the stroke-affected muscles (p<; 05) compared to those of the contralateral non-affected side. We hypothesize that in addition to changes in neural and contractile properties, there are also, changes in elastic and tissue dispersive properties through local mechanisms. An enhanced understanding of post-stroke changes in skeletal muscles will lead to better and targeted interventions for rehabilitation.