Sensory regulation and mechanical effects of sustained high intensity stretching of the anterior compartment of the thigh

BackgroundBallet dancers, contortionists, gymnasts, or other sportspeople spend long hours performing stretches while training. Although most studies on stretching consider fascia lengthening to be difficult, athletes manage to lengthen their fascia.AimTo assess the relationship between lengthening fascial structures of the anterior compartment of the thigh and the self-reported sensation of discomfort and pain during a sustained and repeated high intensity stretch.MethodsOur analysis was based on the data of 7 high school male elite rugby players who completed 11 sessions of stretching (10-min quasi-static stretch of the rectus femoris and fascia lata, at the maximum intensity tolerated), performed twice per week. The measured outcomes included hip range of motion, the length of the structures of the anterior compartment, subjective pain and tension during the stretch, and the level of surface electromyography activity. Values were compared before and after completion of the 11 sessions.ResultsMyofascial length increased by 1 cm. The necessary force applied increased from 124 to 164 N. However, the maximal tolerated stretching intensity did not change significantly (from 205 to 206 N). The increase in length was principally contributed by the rate of fascial creep upon force application, and not by contractile tissue. Subjective levels of tension were related to the stretching force applied and pain was related to the lengthening.ConclusionSensations can be used to adjust the intensity and duration of stretching. Soft matter physics provides a new interpretation of fascia lengthening and strengthening during a high intensity stretch.     

Cerebral multifrequency MR elastography by remote excitation of intracranial shear waves

The aim of this study was to introduce remote wave excitation for high‐resolution cerebral multifrequency MR elastography (mMRE). mMRE of 25–45‐Hz drive frequencies by head rocker stimulation was compared with mMRE by remote wave excitation based on a thorax mat in 12 healthy volunteers. Maps of the magnitude |G*| and phase φ of the complex shear modulus were reconstructed using multifrequency dual elasto‐visco (MDEV) inversion. After the scan, the subjects and three operators assessed the comfort and convenience of cerebral mMRE using two methods of stimulating the brain. Images were acquired in a coronal view in order to identify anatomical regions along the spinothalamic pathway. In mMRE by remote actuation, all subjects and operators appreciated an increased comfort and simplified procedural set‐up. The resulting strain amplitudes in the brain were sufficiently large to analyze using MDEV inversion, and yielded high‐resolution viscoelasticity maps which revealed specific anatomical details of brain mechanical properties: |G*| was lowest in the pons (0.97 ± 0.08 kPa) and decreased within the corticospinal tract in the caudal–cranial direction from the crus cerebri (1.64 ± 0.26 kPa) to the capsula interna (1.29 ± 0.14 kPa). By avoiding onerous mechanical stimulation of the head, remote excitation of intracranial shear waves can be used to measure viscoelastic parameters of the brain with high spatial resolution. Therewith, the new mMRE method is suitable for neuroradiological examinations in the clinic. Copyright © 2015 John Wiley & Sons, Ltd.     

A New Viscoelasticity Dynamic Fitting Method Applied for Polymeric and Polymer-Based Composite Materials

The accurate analysis of the behaviour of a polymeric composite structure, including the determination of its deformation over time and also the evaluation of its dynamic behaviour under service conditions, demands the characterisation of the viscoelastic properties of the constituent materials. Linear viscoelastic materials should be experimentally characterised under (i) constant static load and/or (ii) harmonic load. In the first load case, the viscoelastic behaviour is characterised through the creep compliance or the relaxation modulus. In the second load case, the viscoelastic behaviour is characterised by the complex modulus, E*, and the loss factor, η. In the present paper, a powerful and simple implementing technique is proposed for the processing and analysis of dynamic mechanical data. The idea is to obtain the dynamic moduli expressions from the Exponential-Power Law Method (EPL) of the creep compliance and the relaxation modulus functions, by applying the Carson and Laplace transform functions and their relationship to the Fourier transform, and the Theorem of Moivre. Reciprocally, once the complex moduli have been obtained from a dynamic test, it becomes advantageous to use mathematical interconversion techniques to obtain the time-domain function of the relaxation modulus, E(t), and the creep compliance, D(t). This paper demonstrates the advantages of the EPL method, namely its simplicity and straightforwardness in performing the desirable interconversion between quasi-static and dynamic behaviour of polymeric and polymer-composite materials. The EPL approximate interconversion scheme to convert the measured creep compliance to relaxation modulus is derived to obtain the complex moduli. Finally, the EPL Method is successfully assessed using experimental data from the literature.     

Hyperbolic Divergence Cleaning in Lattice Boltzmann Magnetohydrodynamics

Magnetohydrodynamics couples the Navier–Stokes and Maxwell’s equations to describe the flow of electrically conducting fluids in magnetic fields. Maxwell’s equations require the divergence of the magnetic field to vanish, but this condition is typically not preserved exactly by numerical algorithms. Solutions can develop artifacts because structural properties of the magnetohydrodynamic equations then fail to hold. Magnetohydrodynamics with hyperbolic divergence cleaning permits a nonzero divergence that evolves under a telegraph equation, designed to both damp the divergence, and propagate it away from any sources, such as poorly resolved regions with large spatial gradients, without significantly increasing the computational cost. We show that existing lattice Boltzmann algorithms for magnetohydrodynamics already incorporate hyperbolic divergence cleaning, though they typically use parameter values for which it reduces to parabolic divergence cleaning under a slowly-varying approximation. We recover hyperbolic divergence cleaning by adjusting the relaxation rate for the trace of the tensor that represents the electric field, and absorb the contribution from the symmetric-traceless part of this tensor using a change of variables. Numerical experiments confirm that the qualitative behaviour changes from parabolic to hyperbolic when the relaxation time for the trace of the electric field tensor is increased.     

Dysphagia dietary guidelines and the rheology of nutritional feeds and barium test feeds

BACKGROUND: Dysphagia can lead to aspiration of oral feeds, thus causing pneumonia. Dysphagia is diagnosed by assessing the ability to swallow barium test feeds (BTF) of different viscosities. Dysphagia diet foods (DDF) are thickened as recommended by the National Dysphagia Diet (NDD) guidelines. To our knowledge, there are no published data evaluating if the viscosity of BTF or commercial DDF meet NDD guidelines. METHODS: A TA1000 rheometer (TA Instruments; New Castle, DE) measured dynamic viscosity of BTF and DDF using creep transformation under controlled stress. Thin DDF studied included Plus Energy Drink (Boost; Novartis/Nestle; Fremont, MI) and Instant Breakfast (Carnation; Wilkes-Barre, PA) and nectar- and honey-thick DDF from Hormel (Hormel Health Labs; Savannah, GA) and Novartis (Novartis/Nestle). The BTF studied were thin, nectar-, and honey-thick Polibar barium suspension or Varibar (E-Z-EM, Inc.; Lake Success, NY). We measured batch-to-batch variability in the viscosity of DDF, with and without shaking, and after 2 h at ambient temperature at a shear rate chosen to match natural swallowing. RESULTS: We observed the following: (1) DDF: the viscosity of honey-thick DDF was consistent with NDD guidelines, but other products were not. All products had minimal change in viscosity over 2 h. Boost thin liquid had > 300% increase in viscosity after shaking. (2) BTF: thin barium had a viscosity consistent with NDD guidelines. The nectar- and honey-thick Polibar BTFs were thixotropic and had unacceptably high viscosity. Varibar BTFs were not thixotropic but were more viscous than the NDD guidelines. CONCLUSIONS: There was a poor relationship between the viscosity of DDF and BTF. The viscosity of BFTs is much greater than the correspondingly named diet foods and the NDD guidelines. This can place patients at significant risk for oral aspiration.     

Nonsteady fracture of transient networks: The case of vitrimer

We have discovered a peculiar form of fracture that occurs in polymer network formed by covalent adaptable bonds. Due to the dynamic feature of the bonds, fracture of this network is rate dependent, and the crack propagates in a highly nonsteady manner. These phenomena cannot be explained by the existing fracture theories, most of which are based on steady-state assumption. To explain these peculiar characteristics, we first revisit the fundamental difference between the transient network and the covalent network in which we highlighted the transient feature of the cracks. We extend the current fracture criterion for crack initiation to a time-evolution scheme that allows one to track the nonsteady propagation of a crack. Through a combined experimental modeling effort, we show that fracture in transient networks is governed by two parameters: the Weissenberg number W0 that defines the history path of crack-driving force and an extension parameter Z that tells how far a crack can grow. We further use our understanding to explain the peculiar experimental observation. To further leverage on this understanding, we show that one can “program” a specimen’s crack extension dynamics by tuning the loading history.

Understanding the conditions that lead to fracture in polymeric materials is an important problem of both industrial and fundamental interests. A common agreement is that fracture of polymer originates from successive chain scission resulting from the application of an excessive stress on the network (1). This consideration leads to the early work of Griffith that predicts the onset of crack propagation based on the competition between two quantities: the crack tip–driving force Gtip that provides the fuel for fracture and the intrinsic fracture toughness G0, which represents the material’s resistance to fracture (2). To propagate a crack, the former needs to reach or exceed the latter. This criterion further leads to deformation-based measurements such as the critical crack opening distance (COD) (3), critical stretch (4), or network damage models based on the chains’ stretch limit (5–7). These models have so far been instrumental in predicting the fracture of covalent polymer networks that are both elastic (2, 8–10) and viscoelastic (11–17). However, materials formed by weaker bonds [(e.g., covalent adaptable bonds (18), ionic interaction (19, 20), or entanglements (21–23)] exhibit a much richer fracture behavior that does not only depend on deformation but also greatly depends on the rate of loading (21, 22, 24). Due to their inherent weakness, these bonds are prone to spontaneous dissociation and reassociation over time under the effects of thermal fluctuations. This leads to a wide spectrum of rate-dependent mechanical response wherein the networks behave like viscous fluid at slow loading rates (relative to the rate of bond exchange), while they exhibit elastic solid-like behavior at fast loading (21). This coupling between deformation and network relaxation makes the prediction of fracture challenging, since the mechanical response becomes both time and rate dependent. In addition, the physical picture of chain scission at its stretch limit is no longer valid as a chain may dissociate in any conformation. This raises questions about the molecular origin of fracture and on its macroscopic manifestation and particularly, the conditions for its nucleation and its speed of propagation. Although some initial efforts were taken to understand the role of loading rate on fracture (22, 25) in transient networks, a systematic study of the physical rules behind crack characteristics, initiation, and propagation is still not established.To address these questions, we have carried out fracture experiments on a vitrimer network formed by disulfide bonds. In the presence of a catalyst, the bond exchange reaction (Fig. 1B) can be triggered by thermal fluctuations at room temperature (26). A full characterization of this network has been performed in our previous study (27). In this paper, we are specifically interested in the stress relaxation experiment, as it unveils the rate of bond exchange reaction (i.e., the bond dynamics). For this, we conducted a series of stress relaxation experiments at different stretch levels, in which the inverse of characteristic relaxation time is interpreted as the average rate of bond exchange at the applied deformation (28). After calibration, we found that bond dissociation is well described by the relation kd=kd0⁡exp(α(λe−1)), where kd0 is the spontaneous rate, and α=58.4 is a sensitivity parameter, as shown by the relaxation results in Fig. 1C and the fitting of relaxation time τR=1/kd in its Inset. This exponential dependency agrees with Eyring’s model (29, 30) based on the transition state theory. To characterize the response of this vitrimer in fracture, we then devised a pure-shear fracture experiment, where a precut sample of width L=35 mm and height H0 (10 mm), which contains a precut of length c0=15 mm, was stretched vertically at constant nominal strain rate λ˙=H˙/H0 (Fig. 2A). In what follows, this variable is normalized by the bond dynamics rate kd0 so that the ratio W0=λ˙/kd0 (denoted as the nominal Weissenberg number) describes the competition between network deformation and reconfiguration.Open in a separate windowFig. 1.(A) Schematic of the transient network, where (B) bond exchange reaction can spontaneously occur at rate kd. (C) Stress relaxation experiment at different strain λe with uniaxial tensile loading at large deformation (λe>1.3).Open in a separate windowFig. 2.(A) A schematic of fracture of the specimen. The crack tip–driving force G measures the elastic energy infused to the crack tip. (B) Measurement of crack extension during the deformation history. (C). Experimental and finite element simulation snapshots of crack profile under different loading rates. (Scale bar, 10 mm.)Our measurements of crack extension (Fig. 2 B and C) reveal three distinct characteristic regimes depending on the magnitude of W0. For a small loading rate (W0=0.095), the induced cut opens continuously and eventually becomes a curved edge. Over the course of the experiment, no observable fracture is recorded. For an intermediate rate (W0=0.19), a peculiar phenomenon occurs where a sharp crack first nucleates from the tip of the cut and quickly propagates with a characteristic trumpet-shaped profile (Fig. 2C). However, the propagation stops at a finite time, after which the crack “dies” and gives rise to a blunted edge. We note that this phenomenon is not observed in covalently crosslinked networks, since a propagating crack usually travels continuously at constant velocity through the specimen (31) under monotonic loading. Finally, for fast loading (W0=0.38), we similarly observe an initial blunting of the cut, quickly followed by crack nucleation. In this case, this newborn crack accelerates and fully ruptures the specimen. We used image processing techniques and measured the crack extension as a function of stretch λ (Fig. 2B), where we see that crack propagation is highly unsteady with varying velocity. The above observations cannot be explained by the current fracture theories for the following reasons. First, the onset of fracture highly depends on loading rate in addition to the level of deformation, therefore, a criterion derived from the elastic theory such as the COD or the critical stretch cannot be used. Second, the life of a propagation crack varies between loading conditions, where it may accelerate, decelerate, or even stop under monotonic loading.To understand these observations, we have recently developed a theoretical framework that allows one to evaluate the energetic fracture criterion and calculate crack velocity based on the stress field and bond dynamics (32). Combining this approach with finite element simulations (details provided in SI Appendix, Supplemental Information S2), we were able to simulate the nonsteady state fracture behavior of the vitrimer and qualitatively match the experimental crack profiles in Fig. 2C. Predominantly, our simulations suggest that crack initiation and propagation are associated with the increase of strain energy density ψ stored in the network, which itself is a function of nominal Weissenberg number W0 and loading history. In addition, the speed of the crack depends on both the magnitude of ψ and the sensitivity of bond dynamics kd upon deformation. In summary, our simulation work has allowed us to identify three kinetic rates that collectively govern fracture in transient networks: the rate of loading, the rate of bond dynamics, and the rate of crack propagation. The way by which these rates compete during fracture is however unclear. This work’s objective is thus to combine theoretical analysis with experiment and extract the physical mechanisms behind these peculiar phenomena.     

Tumour biomechanical response to the vascular disrupting agent ZD6126 in vivo assessed by magnetic resonance elastography

Background:

Magnetic resonance elastography (MRE) is an emerging imaging technique that affords non-invasive quantitative assessment and visualization of tissue mechanical properties in vivo.

Methods:

In this study, MRE was used to quantify (kPa) the absolute value of the complex shear modulus |G*|, elasticity G_d and viscosity G_l of SW620 human colorectal cancer xenografts before and 24 h after treatment with either 200 mg kg⁻¹ of the vascular disrupting agent ZD6126 (N-acetylcolchinol-O-phosphate) or vehicle control, and the data were compared with changes in water diffusivity measured by diffusion-weighted magnetic resonance imaging.

Results:

A heterogeneous distribution of |G*|, G_d and G_l was observed pre-treatment with an intertumoral coefficient of variation of 13% for |G*|. There were no significant changes in the vehicle-treated cohort. In contrast, ZD6126 induced a significant decrease in the tumour-averaged |G*| (P<0.01), G_d (P<0.01) and G_l (P<0.05), and this was associated with histologically confirmed central necrosis. This reduction in tumour viscoelasticity occurred at a time when no significant change in tumour apparent diffusion coefficient (ADC) was observed.

Conclusions:

These data demonstrate that MRE can provide early imaging biomarkers for treatment-induced tumour necrosis.     

Development of a nondestructive compliance test for resilient denture liners

PURPOSE: Resilient denture liners are prescribed for patients who cannot adjust to hard-based dentures because of a thin mucosa or severe alveolar ridge resorption. A nondestructive test to evaluate compliance of new soft liner materials will be useful in clinical trials. The purpose of this study was to evaluate a nondestructive compliance testing technique designed to characterize long-term, silicone-based resilient denture liner materials. MATERIALS AND METHODS: Samples of thicknesses of 1.1, 2.2, 3.3, and 4.4 mm of 2 materials (MPDS-SL [Lai Laboratories, Inc, Burnsville, MN] and Molloplast-B [Buffalo Dental, New York, NY]) were assessed for compliance using a closed-loop servohydraulic testing system, applying a 3 lb force following a squarewave pattern; force and position values were recorded using a storage oscilloscope. The oscilloscope values were analyzed using computer software to determine compliance values. The effect of material thickness was examined by testing wedges of the 2 materials. RESULTS: The testing technique used showed that differing thicknesses had significantly different compliance values (p <.0001). In the materials used to evaluate the technique, MPDS-SL behaved more elastically than did Molloplast-B (p <.0001). Material thicknesses beyond 2.2 mm did not increase compliance, although MPDS-SL had a steeper thickness-compliance curve than Molloplast-B. CONCLUSIONS: The method used to test compliance proved to be sufficiently sensitive to distinguish between 2 materials and between varying thicknesses. The sensitivity and nondestructive nature of this test show its suitability for clinical evaluation of resilient denture liners.     

A new method of measuring arterial dilation and its application

An instrument has been designed and constructed for thein-vivo measurement of instantaneous (phasic) diameter changes of arteries. The continuous record of this blood-vessel dimension, when recorded simultaneously with blood pressure, may be used to estimate the viscoelastic properties of the arterial wall.