75 research outputs found
Control of Tension-Compression Asymmetry in Ogden Hyperelasticity with Application to Soft Tissue Modelling
This paper discusses tension-compression asymmetry properties of Ogden
hyperelastic formulations. It is shown that if all negative or all positive
Ogden coefficients are used, tension-compression asymmetry occurs the degree of
which cannot be separately controlled from the degree of non-linearity. A
simple hybrid form is therefore proposed providing separate control over the
tension-compression asymmetry. It is demonstrated how this form relates to a
newly introduced generalised strain tensor class which encompasses both the
tension-compression asymmetric Seth-Hill strain class and the
tension-compression symmetric Ba\v{z}ant strain class. If the control parameter
is set to q=0.5 a tension-compression symmetric form involving Ba\v{z}ant
strains is obtained with the property
{\Psi}({\lambda}_1,{\lambda}_2,{\lambda}_3 )={\Psi}(1/{\lambda}_1
,1/{\lambda}_2 ,1/{\lambda}_3 ). The symmetric form may be desirable for the
definition of ground matrix contributions in soft tissue modelling allowing all
deviation from the symmetry to stem solely from fibrous reinforcement. Such an
application is also presented demonstrating the use of the proposed formulation
in the modelling of the non-linear elastic and transversely isotropic behaviour
of skeletal muscle tissue in compression (the model implementation and fitting
procedure have been made freely available). The presented hyperelastic
formulations may aid researchers in independently controlling the degree of
tension-compression asymmetry from the degree of non-linearity, and in the case
of anisotropic materials may assist in determining the role played by, either
the ground matrix, or the fibrous reinforcing structures, in generating
asymmetry.Comment: 20 page
Additively manufactured polyethylene terephthalate scaffolds for Scapholunate Interosseous Ligament Reconstruction
The regeneration of the ruptured scapholunate interosseous ligament (SLIL)
represents a clinical challenge. Here, we propose the use of a
Bone-Ligament-Bone (BLB) 3D-printed polyethylene terephthalate (PET) scaffold
for achieving mechanical stabilisation of the scaphoid and lunate following
SLIL rupture. The BLB scaffold featured two bone compartments bridged by
aligned fibres (ligament compartment) mimicking the architecture of the native
tissue. The scaffold presented tensile stiffness in the range of 260+/-38 N/mm
and ultimate load of 113+/-13 N, which would support physiological loading. A
finite element analysis, using inverse finite element analysis for material
property identification, showed an adequate fit between simulation and
experimental data. The scaffold was then biofunctionalized using two different
methods: injected with a Gelatin Methacryloyl solution containing human
mesenchymal stem cell spheroids or seeded with tendon-derived stem cells and
placed in a bioreactor to undergo cyclic deformation. The first approach
demonstrated high cell viability, as cells migrated out of the spheroid and
colonised the interstitial space of the scaffold. These cells adopted an
elongated morphology suggesting the internal architecture of the scaffold
exerted topographical guidance. The second method demonstrated the high
resilience of the scaffold to cyclic deformation and the secretion of a
fibroblastic related protein was enhanced by the mechanical stimulation. This
process promoted the expression of relevant proteins, such as Tenomodulin,
indicating mechanical stimulation may enhance cell differentiation and be
useful prior to surgical implantation. In conclusion, the PET scaffold
presented several promising characteristics for the immediate mechanical
stabilisation of disassociated scaphoid and lunate and, in the longer-term, the
regeneration of the ruptured SLIL
Self-Organization of Muscle Cell Structure and Function
The organization of muscle is the product of functional adaptation over several length scales spanning from the sarcomere to the muscle bundle. One possible strategy for solving this multiscale coupling problem is to physically constrain the muscle cells in microenvironments that potentiate the organization of their intracellular space. We hypothesized that boundary conditions in the extracellular space potentiate the organization of cytoskeletal scaffolds for directed sarcomeregenesis. We developed a quantitative model of how the cytoskeleton of neonatal rat ventricular myocytes organizes with respect to geometric cues in the extracellular matrix. Numerical results and in vitro assays to control myocyte shape indicated that distinct cytoskeletal architectures arise from two temporally-ordered, organizational processes: the interaction between actin fibers, premyofibrils and focal adhesions, as well as cooperative alignment and parallel bundling of nascent myofibrils. Our results suggest that a hierarchy of mechanisms regulate the self-organization of the contractile cytoskeleton and that a positive feedback loop is responsible for initiating the break in symmetry, potentiated by extracellular boundary conditions, is required to polarize the contractile cytoskeleton
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Journal of Open Source Software (JOSS) : design and first-year review
This article describes the motivation, design, and progress of the Journal of Open Source Software (JOSS). JOSS is a free and open-access journal that publishes articles describing research software. It has the dual goals of improving the quality of the software submitted and providing a mechanism for research software developers to receive credit. While designed to work within the current merit system of science, JOSS addresses the dearth of rewards for key contributions to science made in the form of software. JOSS publishes articles that encapsulate scholarship contained in the software itself, and its rigorous peer review targets the software components: functionality, documentation, tests, continuous integration, and the license. A JOSS article contains an abstract describing the purpose and functionality of the software, references, and a link to the software archive. The article is the entry point of a JOSS submission, which encompasses the full set of software artifacts. Submission and review proceed in the open, on GitHub. Editors, reviewers, and authors work collaboratively and openly. Unlike other journals, JOSS does not reject articles requiring major revision; while not yet accepted, articles remain visible and under review until the authors make adequate changes (or withdraw, if unable to meet requirements). Once an article is accepted, JOSS gives it a digital object identifier (DOI), deposits its metadata in Crossref, and the article can begin collecting citations on indexers like Google Scholar and other services. Authors retain copyright of their JOSS article, releasing it under a Creative Commons Attribution 4.0 International License. In its first year, starting in May 2016, JOSS published 111 articles, with more than 40 additional articles under review. JOSS is a sponsored project of the nonprofit organization NumFOCUS and is an affiliate of the Open Source Initiative (OSI)
Finite element analysis of the performance of additively manufactured scaffolds for scapholunate ligament reconstruction
Rupture of the scapholunate interosseous ligament can cause the dissociation of scaphoid and lunate bones, resulting in impaired wrist function. Current treatments (e.g., tendon-based surgical reconstruction, screw-based fixation, fusion, or carpectomy) may restore wrist stability, but do not address regeneration of the ruptured ligament, and may result in wrist functional limitations and osteoarthritis. Recently a novel multiphasic bone-ligament-bone scaffold was proposed, which aims to reconstruct the ruptured ligament, and which can be 3D-printed using medical-grade polycaprolactone. This scaffold is composed of a central ligament-scaffold section and features a bone attachment terminal at either end. Since the ligament-scaffold is the primary load bearing structure during physiological wrist motion, its geometry, mechanical properties, and the surgical placement of the scaffold are critical for performance optimisation. This study presents a patient-specific computational biomechanical evaluation of the effect of scaffold length, and positioning of the bone attachment sites. Through segmentation and image processing of medical image data for natural wrist motion, detailed 3D geometries as well as patient-specific physiological wrist motion could be derived. This data formed the input for detailed finite element analysis, enabling computational of scaffold stress and strain distributions, which are key predictors of scaffold structural integrity. The computational analysis demonstrated that longer scaffolds present reduced peak scaffold stresses and a more homogeneous stress state compared to shorter scaffolds. Furthermore, it was found that scaffolds attached at proximal sites experience lower stresses than those attached at distal sites. However, scaffold length, rather than bone terminal location, most strongly influences peak stress. For each scaffold terminal placement configuration, a basic metric was computed indicative of bone fracture risk. This metric was the minimum distance from the bone surface to the internal scaffold bone terminal. Analysis of this minimum bone thickness data confirmed further optimisation of terminal locations is warranted.</p
A structural model of passive skeletal muscle shows two reinforcement processes in resisting deformation
Passive skeletal muscle derives its structural response from the combination of the titin filaments in the muscle fibres, the collagen fibres in the connective tissue and incompressibility due to the high fluid content. Experiments have shown that skeletal muscle tissue presents a highly asymmetrical three-dimensional behaviour when passively loaded in tension or compression, but structural models predicting this are not available. The objective of this paper is to develop a mathematical model to study the internal mechanisms which resist externally applied deformation in skeletal muscle bulk. One cylindrical muscle fibre surrounded by connective tissue was considered. The collagenous fibres of the endomysium and perimysium were grouped and modelled as tension-only oriented wavy helices wrapped around the muscle fibre. The titin filaments are represented as non-linear tension-only springs. The model calculates the force developed by the titin molecules and the collagen network when the muscle fibre undergoes an isochoric along-fibre stretch. The model was evaluated using a range of literature based input parameters and compared to the experimental fibre-direction stress-stretch data available. Results show the fibre direction non-linearity and tension/compression asymmetry are partially captured by this structural model. The titin filament load dominates at low tensile stretches, but for higher stretches the collagen network was responsible for most of the stiffness. The oblique and initially wavy collagen fibres account for the non-linear tensile response since, as the collagen fibres are being recruited, they straighten and re-orient. The main contribution of the model is that it shows that the overall compression/tension response is strongly influenced by a pressure term induced by the radial component of collagen fibre stretch acting on the incompressible muscle fibre. Thus for along-fibre tension or compression the model predicts that the collagen network contributes to overall muscle stiffness through two different mechanisms: (1) a longitudinal force directly opposing tension and (2) a pressure force on the muscle fibres resulting in an indirect longitudinal load. Although the model presented considers only a single muscle fibre and evaluation is limited to along-fibre loading, this is the first model to propose these two internal mechanisms for resisting externally applied deformation of skeletal muscle tissu
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