Viscoelastic Shear Properties Research Articles

Role of sulfated GAGs in shear mechanical properties of human and porcine cornea

The corneal extracellular matrix is mainly composed of collagen fibers, proteoglycans, and glycosaminoglycans (GAGs). The present work was done to investigate the effect of GAGs on linear viscoelastic shear properties of human and porcine cornea. A clear understanding of structural functions of GAGs could result in the development of new intervention methods for diseased conditions that involve changes to the expression of GAGs/PGs. Here, we used keratanase II enzyme to deplete sulfated GAGs from porcine and human donor corneal disks. After quantifying the GAG content, collagen fiber diameter, and interfibrillar spacings of control and GAG-depleted specimens using the Blyscan assay and transmission electron microscopy, we performed torsional rheometry to determine their shear properties at different levels of axial strain. We found that the GAG content of control human (52.35 ± 3.40 μg/mg dry tissue) and porcine cornea (48.59 ± 7.79 μg/mg dry tissue) significantly reduced following keratanase II enzyme treatment. Moreover, we observed that the diameter of collagen fibers (28.78 ± 2.33 nm) and interfibrillar spacing (45.93 ± 2.33 nm) of human specimens were significantly smaller than the collagen fiber diameter (34.77 ± 21.90 nm) and interfibrillar spacing (54.28 ± 3.99 nm) of porcine corneal samples. Although GAG depletion did not have any significant effect on the collagen fiber diameter, it significantly increased the interfibrillar spacing in both porcine and human samples. Within the range of linear viscoelastic behavior, the shear stiffness of human and porcine corneal samples did not depend on the shear strain but significantly increased with increasing the applied axial strain. The average complex shear modulus was found to be between 1.0 KPa and 6.5 KPa and between 8.5 KPa and 31 KPa for control porcine and human discs, respectively. The GAG removal caused significant reduction of shear stiffness in both human and porcine corneal samples. Based on these findings, we conclude that sulfated GAGs are important in defining shear properties of porcine and human corneas and significant GAG content variation adversely affects corneal shear modulus.

Shear viscoelastic properties of human orbital fat.

The shear viscoelastic behavior of eye's supporting orbital fat is unstudied in humans, yet is important during and after rapid movement. This investigation quantified viscoelastic characteristics of human orbital fat in constitutive form suitable for numerical simulation. Fresh human orbital fat was harvested postmortem from 6 male and 7 female donors of average age 78±13years. Fat samples were trimmed to disks of 20±3.0 (standard deviation) mm average diameter and 2.1±0.2mm thickness. In 8 samples each, the following four testing protocols were performed: strain sweep from 0.0015 to 50% at 1Hz; viscometry at 0.1s-1 shear rate; stress relaxation at physiological temperature; and frequency sweep from 0.159 to 15.9Hz at 0.5% strain to validate the Prony series parameters fitting stress relaxation behavior. Orbital fat exhibited viscoelastic behavior under dynamic shear with a 0.5% linear viscoelastic strain limit. Storage modulus G' averaged 737±310Pa, and loss modulus G″ averaged 197±76Pa. Values were similar for strain and frequency sweep testing. At rupture, shear stress averaged 617±366Pa and rupture strain averaged 200±70%. The long-term relaxation modulus averaged 646±264Pa at 100s. Frequency sweep testing validated the parameters of the Prony series fitted to the experimental stress relaxation data. Human orbital fat is linearly viscoelastic within a range typical of biological materials, and exhibits similar viscoelastic behavior for strain and frequency sweep testing. Stress relaxation data for human orbital fat has been parameterized for constitutive models that can be implemented in finite element analysis.

Marine sulfated polysaccharide affects the characteristics of astaxanthin-loaded oleogels prepared from gelatin emulsions

In this research, gelatin (Gel), κ-carrageenan (Car), and astaxanthin (AST) were utilized to assess the properties of Gel-Car-AST emulsions and emulsion-templated oleogels. Results indicated that Car addition notably decreased particle size and increased potential absolute value of emulsions during storage period. Moreover, Car addition reduced instability indices for Gel-0.1% Car-AST (0.018), Gel-0.5% Car-AST (0.008), and Gel-1.0% Car-AST (0.007) emulsions compared with Gel-AST emulsion (0.306), suggesting that Car addition improved the emulsion's stability. The emulsions' rheological results revealed a presence of weak gel-like behavior primarily characterized by elasticity, indicating that Car addition enhanced the emulsions' ability to withstand shear resistance and viscoelastic properties. The addition of Car into oleogels resulted in reduced oil loss rate, delayed thermal degradation, and improved viscoelasticity, shear resistance, thixotropic recovery properties, high-temperature resistance, and overall stability, especially for Gel-1.0% Car-AST oleogel. Furthermore, Car addition resulted in increased antioxidant capacity, decreased levels of oxidation products, slowed AST degradation, facilitated the rapid releases of free fatty acids (FFA) and AST in the course of intestinal digestion, and enhanced the bioavailability of AST in oleogels. FFA release rate and bioavailability of AST in oleogels were as follows: Gel-AST (33.06%, 55.80%), Gel-0.1% Car-AST (37.32%, 67.38%), Gel-0.5% Car-AST (36.18%, 79.6%), and Gel-1.0% Car-AST (58.22%, 85.42%) respectively. Results demonstrated that Gel-Car-AST oleogels displayed better structural stability, higher gel strength and AST bioavailability. These findings offer a theoretical basis and novel insights for preparing oleogels derived from protein-marine sulfated polysaccharides loading AST through the emulsion-templated method, with potential applications in the food industry.

3D printable gelatin/nisin biomaterial inks for antimicrobial tissue engineering applications.

Modern regenerative medicine approaches can rely on the fabrication of personalised medical devices and implants; however, many of these can fail due to infections, requiring antibiotics and revision surgeries. Given the rise in multidrug resistant bacteria, developing implants with antimicrobial activity without the use of traditional antibiotics is crucial for successful implant integration and improving patient outcomes. 3D printed gelatin-based implants have a broad range of applications in regenerative medicine due to their biocompatibility, ease of modification and degradability. In this paper, we report on the development of gelatin biomaterial inks loaded with the antimicrobial peptide, nisin, for extrusion-based 3D printing to produce scaffolds with controlled porosity, high shape fidelity, and structural stability. Rheological properties were comprehensively studied to develop inks that had shear thinning behaviour and viscoelastic properties to ensure optimal printability and extrudability, and enable precise deposition and structural integrity during 3D printing. The 3D printed scaffolds fabricated from the gelatin/nisin inks demonstrated excellent antimicrobial efficacy (complete kill) against Gram positive bacteria methicillin-resistant Staphylococcus aureus (MRSA). Overall, this ink's high printability and antimicrobial efficacy with the model antimicrobial peptide, nisin, offers the potential to develop customisable regenerative medicine implants that can effectively combat infection without contributing to the development of multidrug resistant bacteria.

DMAR drug loaded in enzymatic poly-l-phenylalanine nanotubes reduce inflammation and cytotoxicity in human synoviocytes

We investigated the release of chloroquine (Clq) as a model DMAR drug encapsulated in enzyme-mediated polyphenylalanine nanotubes (Clq-Nt) using primary cultures of synovial cells from OA inpatients undergoing total knee replacement. The Clq-Nts were also embedded in an injectable vehicle containing hyaluronic acid (Clq-Nt-HA). Clq-Nt and Clq-Nt-HA samples reduced 62 and 37.5 % the concentration of IL-1β, respectively. For TNF-α the reduction was 38 and 26 %, and for IL-6 was 17.1 and 10 % for Clq-Nt and Clq-Nt-HA, respectively. Additionally, Clq-Nt and Clq-Nt-HA allowed cell viability and proliferation of the synoviocytes according to MTT and CaAM/EthD-1 assays after fifteen days exposure. Micrographs of crystal violet assay also assess the adequate morphologies for synoviocytes treated with Clq-loaded in the tubes. The production of intracellular radical oxygen species (ROS), and extracellular H2O2 and nitrites was also investigated with the inpatient synoviocyte cells with and without treatments which showed no significant differences. Additionally, the apparent viscosity with the shear rate and viscoelastic properties in the injectable fluid containing the encapsulated drug Clq-Nt-HA showed no significant variation with control viscous fluid with only HA, and it falls within the synovial fluid viscosity range for arthritis patients.

Thiol‐ene click chemistry for constructing transition layer to enhance the interface interaction between polyacrylonitrile fiber and styrene‐butadiene‐styrene modified asphalt

AbstractBased on the high chemical stability and excellent end‐group functionalization capability of polyhedral oligomeric silsesquioxane (POSS), this study utilized thiol‐functionalized POSS (POSS‐SH) in a click chemistry reaction with styrene‐butadiene‐styrene triblock copolymer (SBS). The objective was to design a functional group forming a chemical bond with the polymer matrix, resulting in the preparation of thiol‐functionalized polyacrylonitrile (PAN) fibers chemically cross‐linked with POSS‐SH/PAN/SBS hybrid material. This hybrid material not only enhanced the compatibility between the components but also facilitated a connection between the fiber and SBS, thereby improving the mechanical properties of the modified asphalt. Fourier transform infrared spectroscopy, field emission scanning electron microscopy (FESEM), and atomic force microscopy analyses confirmed the successful modification of POSS‐SH/PAN fibers. FESEM analysis of the modified asphalt verified that POSS‐SH/PAN fibers contributed to strengthening the interfacial performance between the fiber and asphalt. Cone penetration and dynamic shear rheometer tests indicated that, compared to SBS‐modified asphalt, PAN/SBS and POSS‐SH/PAN/SBS‐modified asphalt exhibited enhanced shear strength, deformation resistance, and viscoelastic properties. Additionally, at 46°C, the storage modulus (G′) of 1.5% POSS‐SH/PAN/SBS‐modified asphalt increased by 11% compared to 1.5% PAN/SBS asphalt, with increases of 13.75% and 10.8% in loss modulus (G″) and complex modulus (G*), respectively.

Development of high internal phase Pickering emulsions stabilized by egg yolk and carboxymethylcellulose complexes to improve β-carotene bioaccessibility for the elderly

The work aimed to develop the multi-protein mixture of egg yolk as natural particles to stabilize high internal phase Pickering emulsions (HIPPEs) to improve the bioaccessibility of β-carotene in the elderly. The results showed that the depletion attraction drove the adsorption of egg yolk protein particles at the oil–water interface and the formation of osmotic droplet clusters due to the attachment of particle-coated droplets in the dispersed phase, leading to kinetic blocking and stable gelation of HIPPEs. Rheological measurements showed that HIPPEs had shear thinning, low shear stress, viscoelastic properties, and structural recovery properties, which facilitated easy consumption for the elderly. The stability of HIPPEs was verified by ionic and centrifugal stability tests, demonstrating their potential for application to complex gastric environments. HIPPEs have been applied to the International Dysphagia Dietary Standardization Initiative (IDDSI) test and simulated in vitro digestion in older adults, demonstrating their safe swallowability and high β-carotene bioaccessibility. Our findings suggest solutions for food practitioners facing the aging problem and provide new insights for preparing age-friendly foods.

Study on rheology of a novel UV-light sensitive trimeric anionic–cationic surfactant/trans-o-methoxycinnamic acid micellar system

Abstract Two novel UV photosensitive micellar systems, trimeric dodecyl anionic–cationic surfactant (TDCC)/trans-OMCA, and trimeric cetyl anionic–cationic surfactant (TCCC)/trans-OMCA, were successfully synthesized by using two different carbon chain length trimeric anionic–cationic surfactants (TACS, including TDCC and TCCC) and the photosensitive additive trans-o-methoxycinnamic acid (trans-OMCA). The incorporation of trans-OMCA resulted in a peak in the zero shear viscosity (η 0) of the system at pH = 6.17–6.61. The flow behaviour of the TACS/OMCA system was well described by the Carreau-Yasuda model. Prior to UV irradiation, the TCCC/trans-OMCA system exhibited pronounced shear thinning, thixotropic, and viscoelastic properties. After UV irradiation at 365 nm, the isomerization of trans-OMCA to cis-OMCA caused the disruption of the network structures, leading to a significant decrease in the thixotropic and viscoelastic properties, resulting in a decrease in viscosity. The viscosity reduction rate of the TCCC/trans-OMCA system reached 99 %. The influence of the hydrophobic carbon chain length on the UV responsiveness was also investigated. The TDCC/trans-OMCA system exhibited an 86 % decrease in η 0 after UV irradiation, highlighting the favorable effect of longer hydrophobic tail chains in improving the UV responsiveness of the micellar system. The UV light kinetics of the TCCC/trans-OMCA solution were studied and a rheological model was developed to accurately describe the viscosity changes. The TCCC was found to predominantly exhibit cationic characteristics at pH = 6.17–6.61. In this pH range, the 2 wt% TCCC/0.12 wt% NaSal system exhibited excellent viscoelasticity, and the addition of trans-OMCA disrupted the network structure resulting in a decrease in viscosity. After UV irradiation, the viscosity of the system increased by 2.5 times, confirming the potential of the TCCC/NaSal/trans-OMCA micellar system as a UV thickener.

Evaluating the viscoelastic shear properties of clear wood via off-axis compression testing and digital-image correlation

Highly anisotropic materials like wood and unidirectional polymer composite structures are sensitive to shear deformations, in particular close to fixed joints. Large wooden structures in buildings and, e.g. wind-turbine blades, are designed to last for decades, and hence are susceptible to unwanted creep deformations. For improved structural design, the shear-creep properties of the material are needed. These are rarely available in the literature, possibly because of technical difficulties to achieve a well-defined shear-stress state in test specimens. For cost-efficient testing, this goal of a pure stress state necessarily needs to be compromised. In the present study, we propose a simple test method based on uniaxial compression on wooden cubes, but is equally applicable for fibre composites. The viscoelastic shear properties of Norway spruce (Picea abies) under off-axis creep compression tests have been characterised in all three directions. The tests are performed in a controlled climate chamber and the creep strains are captured using digital-image correlation.

The importance of shear and extensional rheology and tribology as the design tools for developing food thickeners for dysphagia management

Food texture manipulation and modification is a practical strategy to reduce the risk of aspiration for dysphagia management. Addition of thickening agent to thicken liquid can increase the bolus transit time, thus, potentially leading to safer swallowing. The important bolus characteristics for safe swallowing include viscosity, hardness, cohesiveness, and adhesiveness, which can be related to the rheological and tribological properties of bolus. Therefore, this work highlights the importance of shear and extensional rheology and tribology as the design tools for developing dysphagia food thickeners. A range of commercially available thickening powders (xanthan gum-, guar gum-, and modified starch-based) was studied to create baseline rheological and tribological data. In general, it was observed that the increase in thickener concentration led to increases in shear viscosity, viscoelastic properties, and extensional viscosity, but the impact on tribological behavior was varied depending on the type of thickener and lubrication regime. It was also revealed that, at the same liquid consistency based on IDDSI classification, xanthan gum-based thickener showed the highest low shear viscosity, extensional viscosity, and lubricating capacity than the other thickeners. More specifically, modified starch-based thickener showed the poorest extensional property and could be the least effective thickener for promoting safer swallowing. The proposed design tools may be used beneficially to tailor novel thickeners for dysphagia management based on the obtained baseline data. Nonetheless, in-depth knowledge on the relationship between these properties and in-vivo measurements of swallowing is still required and is key to achieve the most effective therapeutic strategy in the dysphagia treatment.

Rheological characterization of the exopolysaccharide produced by Alteromonas macleodii Mo 169

Rheology modifiers are essential additives in numerous products in a variety of industries. Due to environmental awareness, consumer-oriented industries are interested in novel natural rheological agents that can replace synthetic chemicals. In this study, the chemical composition and rheological properties of a novel exopolysaccharide (EPS) produced by Alteromonas macleodii Mo 169 were investigated. It was mainly composed of uronic acids (50 mol%) and total carbohydrates were 17 % sulfated. The EPS viscosity increased with concentration, and a non-Newtonian shear thinning behavior was found for concentrations above 0.1 wt%. The elastic and viscous moduli indicated a weak gel-like structure above 0.4 wt%. It maintained its shear thinning behavior and viscoelastic properties in the presence of NaCl and CaCl2 for pH range 5–7 and temperatures up to 55 °C. Though the apparent viscosity decreased at pH 3 and 9 and temperatures above 65 °C, the shear thinning behavior was retained. The viscous and viscoelastic properties were recovered after heating (95 °C) and cooling (0 °C), indicating a good thermal stability and recoverability. After high shear force, the solution recovered original rheological properties within few seconds, demonstrating self-healing properties.

Application of Optical and Rheological Techniques in Quality and Storage Assessment of the Newly Developed Colloidal-Suspension Products: Yogurt-Type Bean-Based Beverages.

The objectives of this study were to compare the properties of the yogurt-type bean-based beverages B and BG produced from the nongerminated and germinated beans, respectively, by high-pressure homogenization (HPH) and fermentation with three starter cultures. Optical techniques were used to evaluate the particle size distribution (PSD), color parameters, and instability during storage, while rheological tests were used to evaluate the shear viscosity, flow behavior, and viscoelastic properties. The BG compared to B, irrespective of the starter culture used, showed a higher mean diameter and Span of PSD (d4,3 ≈ 76.8-84.2, Span ≈ 2.24-2.35 for BG vs. d4,3 ≈ 38.2-47.0, Span ≈ 1.90-2.00 for B). The BG vs. B showed lower viscosity (0.47 Pa·s for BG vs. 0.81 Pa·s for B at shear rate 75 s-1) and slightly lower but satisfactory stability (after 21 days at 6 °C, the Turbiscan Stability Index TSI ≈ 1.3-2.0 for BG vs. TSI ≈ 0.6-0.9 for B). Both B and BG were characterized by light-yellow color and showed the characteristics of a viscoelastic fluid. The HPH and germination mainly affected the properties of the tested plant tissue, which has a direct impact on the properties of the final products.

Photoacoustic testing of shear viscoelastic properties of soft tissues using annular beam illumination.

This Letter reports a new, to the best of our knowledge, photoacoustic excitation method for evaluating the shear viscoelastic properties of soft tissues. By illuminating the target surface with an annular pulsed laser beam, circularly converging surface acoustic waves (SAWs) are generated, focused, and detected at the center of the annular beam. The shear elasticity and shear viscosity of the target are extracted from the dispersive phase velocity of the SAWs based on the Kelvin-Voigt model and nonlinear regression fitting. Agar phantoms with different concentrations, and animal liver and fat tissue samples have successfully been characterized. Different from previous methods, the self-focusing of the converging SAWs allows sufficient SNR to be obtained even with low pulsed laser energy density, which makes this approach well compatible with soft tissues under both ex vivo and in vivo testing conditions.

Effect of corneal collagen crosslinking on viscoelastic shear properties of the cornea

The cornea is responsible for most of the refractive power in the eye and acts as a protective layer for internal contents of the eye. The cornea requires mechanical strength for maintaining its precise shape and for withstanding external and internal forces. Corneal collagen crosslinking (CXL) is a treatment option to improve corneal mechanical properties. The primary objective of this study was to characterize CXL effects on viscoelastic shear properties of the porcine cornea as a function of compressive strain. For this purpose, corneal buttons were prepared and divided into three groups: control group (n = 5), pseudo-crosslinked group (n = 5), and crosslinked group (n = 5). A rheometer was used to perform dynamics torsional shear experiments on corneal disks at different levels of compressive strain (0%–40%). Specifically, strain sweep experiments and frequency sweep tests were done in order to determine the range of linear viscoelasticity and frequency dependent shear properties, respectively. It was found that the shear properties of all samples were dependent on the shear strain magnitude, loading frequency, and compressive strain. With increasing the applied shear strain, all samples showed a nonlinear viscoelastic response. Furthermore, the shear modulus of samples increased with increasing the frequency of the applied shear strain and/or increasing the compressive strain. Finally, the CXL treatment significantly increased the shear storage and loss moduli when the compressive strain was varied from 0% to 30% (p < 0.05); larger shear moduli were observed at compressive 40% strain but the difference was not significant (P = 0.12).

Rheological Investigation of Hydroxypropyl Cellulose-Based Filaments for Material Extrusion 3D Printing.

The rheological properties of drug–polymer mixtures have a significant influence on their processability when using transformative techniques, such as hot-melt-extrusion and material-extrusion 3D printing; however, there has been limited data on printable systems. This study investigated the rheological properties of 17 formulations of successful printed tablets for both immediate and controlled release. Hydroxypropyl cellulose was used in various ratios to obtain printable filaments in combination with various drugs (indomethacin or theophylline), polymers and disintegrants. The complex viscosity, shear thinning behavior and viscoelastic properties were affected by the drug load, polymer composite, disintegrant type, temperature and shear rate applied. Larger windows of processing viscosity were revealed. The viscosity of the printable blends could be as low as the range 10–1000 Pa·s at 100 rad/s angular frequency. All formulations showed shear thinning behavior with a broad slope of complex viscosity from −0.28 to −0.74. The addition of 30–60% drug or disintegrant tended to have greater viscosity values. While microcrystalline cellulose was found to be an alternative additive to lower the storage and loss modulus among disintegrants. This rheological data could be useful for the preformulation and further development of material-extrusion 3D-printing medicines.

Measuring viscoelastic parameters in Magnetic Resonance Elastography: a comparison at high and low magnetic field intensity

Magnetic Resonance Elastography (MRE) is a non-invasive imaging technique which involves motion-encoding MRI for the estimation of the shear viscoelastic properties of soft tissues through the study of shear wave propagation. The technique has been found informative for disease diagnosis, as well as for monitoring of the effects of therapies. The development of MRE and its validation have been supported by the use of tissue-mimicking phantoms. In this paper we present our new MRE protocol using a low magnetic field tabletop MRI device at 0.5 T and sinusoidal uniaxial excitation in a geometrical focusing condition. Results obtained for gelatin are compared to those previously obtained using high magnetic field MRE at 11.7 T. A multi-frequency investigation is also provided via a comparison of commonly used rheological models: Maxwell, Springpot, Voigt, Zener, Jeffrey, fractional Voigt and fractional Zener. Complex shear modulus values were comparable when processed from images acquired with the tabletop low field scanner and the high field scanner. This study serves as a validation of the presented tabletop MRE protocol and paves the way for MRE experiments on ex-vivo tissue samples in both normal and pathological conditions.

Percolation Effects in MCNT-filled Polystyrene: Rheological,Optical, Adhesion and Conductive Investigations

This work is devoted to the preparation and characterization of some polystyrene/multiwall carbon nanotubes (PS/MCNT) systems. The dispersion of the reinforcement agent within the PS medium was done via sonication and the resulting nanocomposites containing 0-40 wt% MCNTs were achieved by solution blending procedure. Shear flow and viscoelastic properties were tested by means of rheology, revealing some changes in the sample microstructure. Dispersion curves of the matrix and low filled nanocomposite were registered at variable temperatures. The theoretical refractive index and corresponding dielectric constant at optical frequencies were analyzed as a function of the system composition. Heat transport in the reinforced materials was examined by computer modeling, which enabled calculation of thermal conductivity. Electrical transport features were assessed using a theoretical approach relying on the physical properties of each phase. The surface adhesion of the samples with various materials was determined to check the suitability for applications in technical or bio-related fields.

Axially- and torsionally-polarized radially converging shear wave MRE in an anisotropic phantom made via Embedded Direct Ink Writing

Magnetic Resonance Elastography (MRE) is a non-invasive imaging method to quantitatively map the shear viscoelastic properties of soft tissues. In this study, Embedded Direct Ink Writing is used to fabricate a muscle mimicking anisotropic phantom that may serve as a standard for imaging studies of anisotropic materials. The technique allowed us to obtain a long shelf life silicone-based phantom expressing transverse isotropic mechanical properties. Another goal of the present investigation is to introduce a torsionally-polarized, radially-converging shear wave actuation method for MRE. The implemented design for this novel setup was first validated via its application to isotropic and homogeneous gelatin phantoms. Then, a comparison of the resulting complex wave images from axially- and torsionally-polarized MRE on the developed anisotropic phantom and on a skeletal muscle murine sample is presented, highlighting the value of using multiple actuation and motion encoding polarization directions when studying anisotropic materials.

Viscoelastic and equilibrium shear properties of human meniscus: Relationships with tissue structure and composition

The meniscus is crucial in maintaining the knee function and protecting the joint from secondary pathologies, including osteoarthritis. Although most of the mechanical properties of human menisci have been characterized, to our knowledge, its dynamic shear properties have never been reported. Moreover, little is known about meniscal shear properties in relation to tissue structure and composition. This is crucial to understand mechanisms of meniscal injury, as well as, in regenerative medicine, for the design and development of tissue engineered scaffolds mimicking the native tissue. Hence, the objective of this study was to characterize the dynamic and equilibrium shear properties of human meniscus in relation to its anisotropy and composition.Specimens were prepared from the axial and the circumferential anatomical planes of medial and lateral menisci. Frequency sweeps and stress relaxation tests yielded storage (G′) and loss moduli (G″), and equilibrium shear modulus (G). Correlations of moduli with water, glycosaminoglycans (GAGs), and collagen content were investigated.The meniscus exhibited viscoelastic behavior. Dynamic shear properties were related to tissue composition: negative correlations were found between G′, G″ and G, and meniscal water content; positive correlations were found for G′ and G″ with GAG and collagen (only in circumferential samples). Circumferential samples, with collagen fibers orthogonal to the shear plane, exhibited superior dynamic mechanical properties, with G′ ~70 kPa and G″ ~10 kPa, compared to those of the axial plane ~15 kPa and ~1 kPa, respectively. Fiber orientation did not affect the values of G, which ranged from ~50 to ~100 kPa.

Sensitivity analysis of effective transverse shear viscoelastic and diffusional properties of myelinated white matter

Motivated by the need to interpret the results from a combined use of in vivo brain Magnetic Resonance Elastography (MRE) and Diffusion Tensor Imaging (DTI), we developed a computational framework to study the sensitivity of single-frequency MRE and DTI metrics to white matter microstructure and cell-level mechanical and diffusional properties. White matter was modeled as a triphasic unidirectional composite, consisting of parallel cylindrical inclusions (axons) surrounded by sheaths (myelin), and embedded in a matrix (glial cells plus extracellular matrix). Only 2D mechanics and diffusion in the transverse plane (perpendicular to the axon direction) was considered, and homogenized (effective) properties were derived for a periodic domain containing a single axon. The numerical solutions of the MRE problem were performed with ABAQUS and by employing a sophisticated boundary-conforming grid generation scheme. Based on the linear viscoelastic response to harmonic shear excitation and steady-state diffusion in the transverse plane, a systematic sensitivity analysis of MRE metrics (effective transverse shear storage and loss moduli) and DTI metric (effective radial diffusivity) was performed for a wide range of microstructural and intrinsic (phase-based) physical properties. The microstructural properties considered were fiber volume fraction, and the myelin sheath/axon diameter ratio. The MRE and DTI metrics are very sensitive to the fiber volume fraction, and the intrinsic viscoelastic moduli of the glial phase. The MRE metrics are nonlinear functions of the fiber volume fraction, but the effective diffusion coefficient varies linearly with it. Finally, the transverse metrics of both MRE and DTI are insensitive to the axon diameter in steady state. Our results are consistent with the limited anisotropic MRE and co-registered DTI measurements, mainly in the corpus callosum, available in the literature. We conclude that isotropic MRE and DTI constitutive models are good approximations for myelinated white matter in the transverse plane. The unidirectional composite model presented here is used for the first time to model harmonic shear stress under MRE-relevant frequency on the cell level. This model can be extended to 3D in order to inform the solution of the inverse problem in MRE, establish the biological basis of MRE metrics, and integrate MRE/DTI with other modalities towards increasing the specificity of neuroimaging.