Elastic Stretch Research Articles

Stretch-induced structural evolution of poly (vinyl alcohol) at different concentrations of boric acid: An in-situ synchrotron radiation small- and wide- angle X-ray scattering study

Abstract Mechanical and structural evolution of poly (vinyl alcohol) (PVA) induced by uniaxial deformation at different concentrations of boric acid is systematically studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS and WAXS). The PVA films were stretched uniaxially in the aqueous solution at room temperature with concentrations of boric acid at 0.3 wt%, 1 wt% and 3 wt%, respectively. The stretching process can be divided into three stages through WAXS and SAXS results, whose boundary is the end of the crystallinity platform and the onset of the formation of nanofibrils. In the first stage, the films show elastic stretch with the crystallinity keeping constant and the d-spacing of (101) plane (d101) decreasing. The unstable crystal breaks up and transfers into amorphous during the second stage so that the force is gradually removed from the unit cell. The connection between the crystal network breaks up in the third stage when the content of boric acid is not sufficient enough. Moreover, increasing the concentration of boric acid leads to the earlier emergence of nanofibrils due to the lower energy barrier induced by entropy decrease, which also results in a reduction of nanofibrils content by limiting the mobility of molecular chain. The results have been applied to establish relationships between the microstructure and the physical characteristics of PVA to help us tune the processing parameters during the practical production.

An interface-capturing Godunov method for the simulation of compressible solid-fluid problems

In this paper a new three-dimensional Eulerian interface-capturing model is proposed for the simulation of compressible solid-fluid problems. The model is a development of a well-established five equation multi-fluid model to include material strength, by incorporating a new kinematic evolution equation for the elastic stretch tensor, and augmenting the Mie-Grüneisen equation-of-state to include a contribution from elastic strain. Principal to the development of the model is the fact that a great number of solid materials can be described by the particular general equation-of-state framework, which reduces to the Mie-Grüneisen equation-of-state in the limit of zero strength and is thus equally applicable to a wide class of fluids. The constitutive models are founded on hyperelastic theory and are therefore thermodynamically compatible. Only one kinematic equation is required for arbitrary numbers of components by assuming that mixtures are described by a common deviatoric strain tensor. The system of evolution equations can be written in conservation law form, and are therefore suitable for application of Godunov's method; specifically an HLLD Riemann solver is formulated for the calculation of numerical fluxes. Interfaces are sharpened by using the THINC method in conjunction with MUSCL reconstruction. A time operator splitting strategy is used to divide the time integration into an explicit elastic update, which uses the third order TVD Runge-Kutta method, followed by an implicit plastic update. The method is verified using a number of challenging solid-fluid problems, including a high-velocity impact of a viscoplastic strain-hardening cylinder in air. The potential of the model for practical problems is demonstrated through three-dimensional simulation of an explosive buried in solid ground material.

Thermodynamics of force-induced B-DNA melting: Single-strand discreteness matters

Overstretching of B-DNA is currently understood as force-induced melting. Depending on the geometry of the stretching experiment, the force threshold for the overstretching transition is around 65 or 110 pN. Although the mechanisms behind force-induced melting have been correctly described by Rouzina and Bloomfield [Biophys. J. 80, 882 (2001)BIOJAU0006-349510.1016/S0006-3495(01)76067-5], neither force threshold has been exactly calculated by theory. In this work, a detailed analysis of the force-extension curve is presented, based on a description of single-stranded (ss) DNA in terms of the discrete Kratky-Porod model, consistent with (i) the contour length expected from the crystallographically determined monomer distance and (ii) a high value of the elastic stretch modulus arising from covalent bonding. The value estimated for the ss-DNA persistence length, λ=1.0 nm, is at the low end of currently known estimates and reflects the intrinsic stiffness of the partially, or fully stretched state, where electrostatic repulsion effects are expected to be minimal. A detailed analysis of single- and double-stranded DNA free energies provides estimates of the overstretching force thresholds. In the unconstrained geometry, the predicted threshold is 64 pN. In the constrained geometry, after allowing for the entropic penalty of the plectonemic topology of the molten state, the predicted threshold is 111 pN.

Identifying a suitable heat setting temperature to optimize the elastic performances of cotton spandex woven fabric

PurposeThe purpose of this paper is to facilitate the production of cotton spandex woven fabric with some user-friendly properties like wearer comfort, super stretch and elasticity. The findings could contribute to ease spandex production and to optimize its property of elasticity. Stretch or a super stretch property is generally desirable, as it can increase the comfort level of those who wear it. In this experiment, the difficulties which were identified while manufacturing cotton spandex woven fabric resolved after identification.Design/methodology/approachIn this experiment, three types of cotton spandex woven fabrics, with different composition and constructions, were used to find out their elastic properties. Temperature ranging from 160°C to 200°C with the machine speed of 20 to 26 MPM (meter per minute) was applied with an adjusted industrial setting with the facilities of a stenter machine to optimize the properties of cotton spandex woven fabric.FindingsThe findings establish that the temperature treatment closely compacted the elastic portions with cotton fibers, giving stability to the spandex yarn, which as a result, influenced cotton spandex woven fabric’s elastic properties, namely, stretch, growth and recovery. The consequences of temperature on cotton spandex yarns were assessed using a microscope, and the results were subsequently analyzed.Research limitations/implicationsBecause of the poor facilities in testing laboratory, only few tests with microscopic evaluation were conducted to assess the elastic performances of cotton spandex woven fabric.Practical implicationsIt is a practice-based research, and the findings could be beneficial to personnel in the textile industry, who are responsible for the manufacturing of cotton spandex woven fabric.Social implicationsThis research could enhance the wearer’s satisfaction, with some comfort elastic properties, which can have a positive influence over spandex clothing industries.Originality/valueThis research establishes that heat setting had a progressive influence on the production of cotton spandex woven fabric and for the optimization of its elastic performances. This research opens a possible way for scholars to further study in this field.

Fractional Ablative Laser Therapy is an Effective Treatment for Hypertrophic Burn Scars: A Prospective Study of Objective and Subjective Outcomes

Background: Scarring causes severe long-term morbidity in many survivors of burn injury. Fractional laser treatment has been reported to improve burn scars, with increasing clinical use despite a paucity of controlled, prospective clinical studies using objective measures of improvement. We conducted a site-controlled, prospective open-label study to determine objective and subjective changes in mature hypertrophic burn scars treated with a fractional ablative carbon dioxide (CO2) laser. Methods: A multicenter, prospective cohort study was conducted in subjects with hypertrophic burn scars, who received three treatment sessions with an ablative fractionated CO2 laser. Repeated measures employed were: mechanical skin rheology to measure viscoelastic properties; ultrasonic imaging to measure scar thickness; and reflectometry to measure erythema and pigmentation for 6 months after treatment. Stability of scars was documented for three months prior to laser treatment. Subjective measures included health-related quality of life, patient and investigator scar assessment scales, and blinded scoring of digital photographs. Findings: Twenty-nine subjects were enrolled, of whom 26 received at least one fractional CO2 laser treatment and 22 received 3 treatments. Mean age of those completing all three treatments was 28 years. Major, statistically significant objective improvements in elastic stretch (P<0.01), elastic recovery (P<0.01), extensibility (P<0.01), and thickness (P<0.01) were noted. Subjectively, health-related quality of life, appearance and pain/pruritus were significantly improved (P<0.01). There was no significant regression of improvement for at least 6 months after treatment. Interpretation: Fractional ablative laser treatment provides significant, sustained improvement of elasticity, thickness, appearance and symptoms of mature hypertrophic burn scars. Funding: Funding for this study was provided by the Department of Defense, award number FA8650-11-2-6241. Declaration of Interest: Lumenis has donated equipment for patient care that was used in this study. Dr. Anderson has consulted with Lumenis in the past, with the consultancy ending prior to the conceptualization of this study. Lumenis played no role in financial support, design, performance, data analysis, or reporting of this study. Ethical Approval: This study was approved by the Partners Healthcare IRB and listed on www.clinicaltrials.gov with identifier NCT02115646.

In situ, 3D characterization of the deformation mechanics of a superelastic NiTi shape memory alloy single crystal under multiscale constraint

Microstructural elements in NiTi shape memory alloys (SMAs) – precipitates, phase boundaries, inclusions, grain boundaries – can be viewed as sources of multiscale constraint that influence their deformation response. We characterized in situ, and in 3D, the deformation and the evolution of microstructure during a tension test in a superelastic NiTi specimen containing some of these sources of constraint. The method used was far-field high-energy X-ray diffraction microscopy (ff-HEDM), complemented by electron microscopy. We simulated the local stress state in the specimen using a microstructural model informed by the experimental data. Using these combined microstructure, deformation, and stress data, we report three phenomena, and relate them to specific sources of constraint. During initial elastic loading, axial lattice strain in austenite increased monotonically. On partial stress-induced phase transformation to martensite, the stress redistributed to both phases leading to a stress relaxation in austenite. The specimen contained a dense distribution of inclusions, which led to the activation of martensite habit plane variants that produce less than theoretical maximum transformation strain. Large Ni4Ti3 precipitates potentially contributed to the poor transformation response. Under load, proportional gradients in local rotation and elastic stretch developed in the martensite phase, because of the constraint at phase interfaces. This combined ff-HEDM, electron microscopy, microstructural simulation toolbox provides a versatile method to understand the effect of constraint on inelastic deformation in other alloys with hierarchical microstructure.

Ultra-Stretchable Interconnects for High-Density Stretchable Electronics.

The exciting field of stretchable electronics (SE) promises numerous novel applications, particularly in-body and medical diagnostics devices. However, future advanced SE miniature devices will require high-density, extremely stretchable interconnects with micron-scale footprints, which calls for proven standardized (complementary metal-oxide semiconductor (CMOS)-type) process recipes using bulk integrated circuit (IC) microfabrication tools and fine-pitch photolithography patterning. Here, we address this combined challenge of microfabrication with extreme stretchability for high-density SE devices by introducing CMOS-enabled, free-standing, miniaturized interconnect structures that fully exploit their 3D kinematic freedom through an interplay of buckling, torsion, and bending to maximize stretchability. Integration with standard CMOS-type batch processing is assured by utilizing the Flex-to-Rigid (F2R) post-processing technology to make the back-end-of-line interconnect structures free-standing, thus enabling the routine microfabrication of highly-stretchable interconnects. The performance and reproducibility of these free-standing structures is promising: an elastic stretch beyond 2000% and ultimate (plastic) stretch beyond 3000%, with <0.3% resistance change, and >10 million cycles at 1000% stretch with <1% resistance change. This generic technology provides a new route to exciting highly-stretchable miniature devices.

Growth and remodeling with application to abdominal aortic aneurysms

In this paper, we apply a mixture theory of growth and remodeling to study the formation and dilatation of abdominal aortic aneurysms. We adapt the continuum theory of mixtures to formalize the processes of production and removal of constituents from a loaded body. Specifically, we consider a mixture of elastin and collagen fibers which endow the material with anisotropic properties. An evolving recruitment variable defines the intermediate configuration from which the elastic stretch of collagen is measured. General formulations of the equations governing homeostatic state and aneurysm development are provided. In the homeostatic state, the idealized geometry of the aorta is a thick-walled tube subject to constant internal pressure and axial stretch. The formation of an aneurysm induces an increase of mass locally achieved via production of new material that exceeds the removal of old material. The combined effects of loss of elastin, degradation of existing and deposition of new collagen, as well as fiber remodeling results in a continuous enlargement of the aneurysm bulge. The numerical method makes use of a purposely written material subroutine, called UMAT, which is based on the constitutive formulation provided in the paper. Numerical results based on patient-based material parameters are illustrated.

Abstract 84: Modeling Tissue Expansion with Isogeometric Analysis: Skin Growth is Correlated with Increased Latency After Expansion

PURPOSE: Tissue expansion (TE) often has high complication rates, particularly in anatomically critical regions such as the face and the extremities with limited donor sites. Improved understanding of the biomechanics of TE to optimize true skin growth (i.e. non-reversible deformation) vs. elastic skin stretch could serve to minimize undue tissue necrosis and resultant infection. The aim of the present study is to differentiate the levels of skin growth vs. skin stretch at various time points after a single large volume expansion, and to correlate biomechanical growth to histologic and gene transcriptome changes during TE. METHODS: Two 5–6 week old minipigs were each tattooed with 4 grids, with a tissue expander implanted under 2 of these grids and the contralateral side serving as an internal control. Each of the 4 expanders were inflated once with 60cc of saline 1 hour, 24 hours, 3 days, and 7 days prior sacrifice. 3D photographs were obtained immediately before and after each expansion and the day of sacrifice, both in vivo and ex vivo. Isogeometric analysis of reconstructed 3D skin grids was preformed to calculate skin growth and stretch. Briefly, prestrain was calculated by amount of “snap-back” of control patches after being excised. In expanded patches, the “snap-back” after skin excision determines stretch, after controlling for prestrain. After controlling for natural growth, the remaining skin size increase following tissue expander removal determines growth. Epidermal thickness was evaluated via histology for skin grids expanded 1 & 24 hours prior to sacrifice. Histology for skin grids expanded 3 & 7 days prior to sacrifice is pending. RESULTS: Total deformation of the 4 expanded skin patches was similar. Total deformation, however, was attributable to increasing degrees of skin growth with increased latency period after expansion (1.12 for 1 hour, 1.11 for 24 hours, 1.06 for 3 days, 1.34 for 7 days; p<0.0001). Collagen fibers appeared more randomly oriented in apical expander skin compared to control skin where fibers appears to be oriented parallel to the epidermis. There was no significant difference in epidermal thickness 1 hour or 24 hours post expansion compared to controls. CONCLUSIONS: While similar degrees of deformation were seen on all 4 stretched skin grids, skin growth was correlated with increased latency after a single expansion. Our investigation is ongoing regarding histological evaluation of collagen architecture, immunohistochemistry, and gene transcriptome analyses. Furthermore, we are actively investigating growth vs. stretch in animals undergoing serial expansions with the goal of optimizing the common clinical practice of TE.

Ordinary state-based peridynamics for thermoviscoelastic deformation

This study presents the ordinary state-based peridynamic (PD) constitutive relations for viscoelastic deformation under mechanical and thermal loads. The behavior of the viscous material is modeled in terms of Prony series. The constitutive constants are the same as those of the classical history-integral model, and they are also readily available from relaxation tests. The state variables are conjugate to the PD elastic stretch measures; hence, they are consistent with the kinematic assumptions of the elastic deformation. The PD viscoelastic deformation analysis successfully captures the relaxation behavior of the material. The numerical results concern first the verification problems, and subsequently, a double-lap joint with a viscoelastic adhesive where failure nucleates and grows.

Elastic ankle muscle-tendon interactions are adjusted to produce acceleration during walking in humans.

Humans and other cursorial mammals have distal leg muscles with high in-series compliance that aid locomotor economy. This muscle-tendon design is considered sub-optimal for injecting net positive mechanical work. However, humans change speed frequently when walking and any acceleration requires net positive ankle work. The present study unveiled how the muscle-tendon interaction of human ankle plantar flexors are adjusted and integrated with body mechanics to provide net positive work during accelerative walking. We found that for accelerative walking, a greater amount of active plantar flexor fascicle shortening early in the stance phase occurred and was transitioned through series elastic tissue stretch and recoil. Reorientation of the leg during early stance for acceleration allowed the ankle and whole soleus muscle-tendon complex to remain isometric while its fascicles actively shortened, stretching in-series elastic tissues for subsequent recoil and net positive joint work. This muscle-tendon behaviour is fundamentally different from constant-speed walking, where the ankle and soleus muscle-tendon complex undergo a period of negative work to store energy in series elastic tissues before subsequent recoil, minimizing net joint work. Muscles with high in-series compliance can therefore contribute to net positive work for accelerative walking and here we show a mechanism for how in human ankle muscles.

Stabilized mixed three‐field formulation for a generalized incompressible Oldroyd‐B model

SummaryThis paper presents a generalization of the incompressible Oldroyd‐B model based on a thermodynamic framework within which the fluid can be viewed to exist in multiple natural configurations. The response of the fluid is viewed as a combination of an elastic component and a dissipative component. The dissipative component leads to the evolution of the underlying natural configurations, while the response from the natural configuration to the current configuration is considered elastic and therefore non‐dissipative. For an incompressible fluid, it is necessary that both the elastic behavior as well as the dissipative behavior is isochoric. This is achieved by ensuring that the determinant of the stretch tensor associated with the elastic response meets the constraint that its determinant is unity.A new stabilized mixed method is developed for the velocity, pressure and the kinematic tensor fields. Analytical models for fine scale fields are derived via the solution of the fine‐scale equations facilitated by the Variational Multiscale framework that are then variationally embedded in the coarse‐scale variational equations. The resulting method inherits the attributes of the classical SUPG and GLS methods, while a significant new contribution is that the form of the stabilization tensors is explicitly derived. A family of linear and quadratic tetrahedral and hexahedral elements is developed with equal‐order interpolations for the various fields. Numerical tests are presented that validate the incompressibility of the elastic stretch tensor, show optimal L2 convergence for the conformation tensor field, and present stable response for high Weissenberg number flows. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Dynamic response analysis of offshore wind turbine installation suspended by a floating crane

In this paper, a dynamic model for the offshore wind turbine installation is proposed. And this model is coupled by the wind turbine and the floating crane considering 6-DOF floating crane, 5-DOF wind turbine and elastic stretch of the hoisting cable. And when the wind turbine lands on the supporting structure, the displacement constraint is applied at the wind turbine. During the process, the relaxation of the hoisting cable is considered. In addition, the nonlinear hydrostatic force, environmental force, hoisting cable force and mooring force are considered as the external force. The motions of wind turbine and the floating crane are studied. From the numerical analysis, it is found that the release velocity, the release height and wave condition have a great effect on the motion of wind turbine.

Significant Pressure Loss Occurs Under Tourniquets Within Minutes of Application.

Pressure decreases occur after tourniquet application, risking arterial occlusion loss. Our hypothesis was that the decreases could be mathematically described, allowing creation of evidence-based, tourniquet-reassessment- time recommendations. Four tourniquets with width (3.8cm, 3.8cm, 13.7cm, 10.4cm), elasticity (none, none, mixed elastic/nonelastic, elastic), and mechanical advantage differences (windlass, ratchet, inflation, recoil) were applied to 57.5cm-circumference 10% and 20% ballistic gels for 600 seconds and a 57.5cmcircumference thigh and 31.5cm-circumference arm for 300 seconds. Time 0 target completion-pressures were 262mmHg and 362mmHg. Two-phase decay equations fit the pressure-loss curves. Tourniquet type, gel or limb composition, circumference, and completionpressure affected the curves. Curves were clinically significant with the nonelastic Combat Application Tourniquet (C-A-T), nonelastic Ratcheting Medical Tourniquet (RMT), and mixed elastic/nonelastic blood pressure cuff (BPC), and much less with the elastic Stretch Wrap And Tuck-Tourniquet (SWATT). At both completion-pressures, pressure loss was faster on 10% than 20% gel, and even faster and greater on the thigh. The 362mmHg completion-pressure had the most pressure loss. Arm curves were different from thigh but still approached plateau pressure losses (maximal calculated losses at infinity) in similar times. With the 362mmHg completion-pressure, thigh curve plateaus were -68mmHg C-A-T, -62mmHg RMT, -34mmHg BPC, and -13mmHg SWATT. The losses would be within 5mmHg of plateau by 4.67 minutes C-A-T, 6.00 minutes RMT, 4.98 minutes BPC, and 6.40 minutes SWATT and within 1mmHg of plateau by 8.18 minutes C-A-T, 10.52 minutes RMT, 10.07 minutes BPC, and 17.68 minutes SWATT. Timesequenced images did not show visual changes during the completion to 300 or 600 seconds pressure-drop interval. Proper initial tourniquet application does not guarantee maintenance of arterial occlusion. Tourniquet applications should be reassessed for arterial occlusion 5 or 10 minutes after application to be within 5mmHg or 1mmHg of maximal pressure loss. Elastic tourniquets have the least pressure loss.

실버세대의 운동유형에 따른 액티브 티셔츠 디자인 제안

The silver generation have clothing style of optimal daily life comparing than young generation because they do not participate a specific sport event but daily- life exercise. As the human body ages, the figure of the silver generation shows different body shape because upper body changes to curved figure including the belly and waist part. Therefore, clothing characteristics for the silver generation should be considered with proper function, design and textiles to optimize body movement. This study investigated various exercise types according to motion analysis of the silver generation in order to develop the design of the active T-shirts reflecting the structural properties and providing the optimum exercise circumstance. The results to consider design needs are as followed; As the T-shirts design for the flexible exercise which required frequent movement of upper body such as bending and waist twisting during body stretching, a stretch fabric applied to the waist part considering T-shirts allowance and length to make extreme elongation and support for well-fitting appearance of the T-shirts. As the T-shirts design for the instantaneous reactionary exercise, high elastic four-way stretch fabric is applied to the part of arm hole to optimize skeletal and muscle movement for entire body and arm work. As the T-shirts design for the endurance exercise such as climbing, cycling, and walking, the shoulder line of the back part has cutting line allowance to make optimum movement of the upper body but no change of the waist part.

A viscoplastic model for the active component in cardiac muscle.

The HMK model (Hunter et al. in Prog Biophys Mol Biol 69:289-331, 1998) proposes mechanobiological equations for the influence of intracellular calcium concentration [Formula: see text] on the evolution of bound calcium concentration [Formula: see text] and the tropomyosin kinetics parameter z, which model processes in the active component of the tension in cardiac muscle. The inelastic response due to actin-myosin crossbridge kinetics is modeled in the HMK model with a function Q that depends on the history of the rate of total stretch of the muscle fiber. Here, an alternative model is proposed which models the active component of the muscle fiber as a viscoplastic material. In particular, an evolution equation is proposed for the elastic stretch [Formula: see text] in the active component. Specific forms of the constitutive equations are proposed and used to match experimental data. The proposed viscoplastic formulation allows for separate modeling of three processes: the high rate deactivation of crossbridges causing rapid reduction in active tension; the high but lower rate reactivation of crossbridges causing recovery of active tension; and the low rate relaxation effects characterizing the Hill model of muscles.

Dynamic Transmission of Protein Allostery without Structural Change: Spatial Pathways or Global Modes?

We examine the contrast between mechanisms for allosteric signaling that involve structural change, and those that do not, from the perspective of allosteric pathways. In particular we treat in detail the case of fluctuation-allostery by which amplitude modulation of the thermal fluctuations of the elastic normal modes conveys the allosteric signal, and address the question of what an allosteric pathway means in this case. We find that a perturbation theory of thermal elastic solids and nonperturbative approach (by super-coarse-graining elasticity into internal bending modes) have opposite signatures in their structure of correlated pathways. We illustrate the effect from analysis of previous results from GlxR of Corynebacterium glutamicum, an example of the CRP/FNR transcription family of allosteric homodimers. We find that the visibility of both correlated pathways and disconnected sites of correlated motion in this protein suggests that mechanisms of local elastic stretch and bend are recruited for the purpose of creating and controlling allosteric cooperativity.

Computer-animated model of accommodation and presbyopia

To understand, demonstrate, and further research the mechanisms of accommodation and presbyopia. Private practice, Little Silver, New Jersey, USA. Experimental study. The CAMA 2.0 computer-animated model of accommodation and presbyopia was produced in collaboration with an experienced medical animator using Autodesk Maya animation software and Adobe After Effects. The computer-animated model demonstrates the configuration and synchronous movements of all accommodative elements. A new classification of the zonular apparatus based on structure and function is proposed. There are 3 divisions of zonular fibers; that is, anterior, crossing, and posterior. The crossing zonular fibers form a scaffolding to support the lens; the anterior and posterior zonular fibers work reciprocally to achieve focused vision. The model demonstrates the important support function of Weiger ligament. Dynamic movement of the ora serrata demonstrates that the forces of ciliary muscle contraction store energy for disaccommodation in the elastic choroid. The flow of aqueous and vitreous provides strong evidence for our understanding of the hydrodynamic interactions during the accommodative cycle. The interaction may result from the elastic stretch in the choroid transmitted to the vitreous rather than from vitreous pressue. The model supports the concept that presbyopia results from loss of elasticity and increasing ocular rigidity in both the lenticular and extralenticular structures. The computer-animated model demonstrates the structures of accommodation moving in synchrony and might enhance understanding of the mechanisms of accommodation and presbyopia. Dr. Goldberg is a consultant to Acevision, Inc., and Bausch & Lomb.

Stability of active muscle tissue

The notion of material stability is examined in the context of active muscle tissue modeling, where the nonlinear constitutive law is dependent both on the physiologically driven muscle contraction and finite mechanical deformation. First, the governing equations and constitutive laws for a general active-elastic material with a single preferred direction are linearized about a homogeneous underlying configuration with respect to both the active contraction and deformation gradient. In order to obtain mathematical restrictions analogous to those found in elastic materials, stability conditions are derived based on the propagation of homogeneous plane waves with real wave speeds, and the generalized acoustic tensor is obtained. Focusing on 2D motions, and considering a simplified, decoupled transversely isotropic energy function, the restriction on the active acoustic tensor is recast in terms of a generally applicable constitutive law, with specific attention paid to the fiber contribution. The implication of the material stability conditions on material parameters, active contraction, and elastic stretch is investigated for prototype material models of muscle tissue.

A computational model that predicts reverse growth in response to mechanical unloading.

Ventricular growth is widely considered to be an important feature in the adverse progression of heart diseases, whereas reverse ventricular growth (or reverse remodeling) is often considered to be a favorable response to clinical intervention. In recent years, a number of theoretical models have been proposed to model the process of ventricular growth while little has been done to model its reverse. Based on the framework of volumetric strain-driven finite growth with a homeostatic equilibrium range for the elastic myofiber stretch, we propose here a reversible growth model capable of describing both ventricular growth and its reversal. We used this model to construct a semi-analytical solution based on an idealized cylindrical tube model, as well as numerical solutions based on a truncated ellipsoidal model and a human left ventricular model that was reconstructed from magnetic resonance images. We show that our model is able to predict key features in the end-diastolic pressure-volume relationship that were observed experimentally and clinically during ventricular growth and reverse growth. We also show that the residual stress fields generated as a result of differential growth in the cylindrical tube model are similar to those in other nonidentical models utilizing the same geometry.