Modeling optical behavior of birefringent biological tissues for evaluation of quantitative polarized light microscopy

Quantitative polarized light microscopy (qPLM) is a popular tool for the investigation of birefringent architectures in biological tissues. Collagen, the most abundant protein in mammals, is such a birefringent material. Interpretation of results of qPLM in terms of collagen network architecture and anisotropy is challenging, because different collagen networks may yield equal qPLM results. We created a model and used the linear optical behavior of collagen to construct a Jones or Mueller matrix for a histological cartilage section in an optical qPLM train. Histological sections of tendon were used to validate the basic assumption of the model. Results show that information on collagen densities is needed for the interpretation of qPLM results in terms of collagen anisotropy. A parameter that is independent of the optical system and that measures collagen fiber anisotropy is introduced, and its physical interpretation is discussed. With our results, we can quantify which part of different qPLM results is due to differences in collagen densities and which part is due to changes in the collagen network. Because collagen fiber orientation and anisotropy are important for tissue function, these results can improve the biological and medical relevance of qPLM results

Risk factors for developing heel ulcers for bedridden patients: A finite element study

Background: The heel is one of the most common sites of pressure ulcers and the anatomical location with the\u3cbr/\u3ehighest prevalence of deep tissue injury. Several finite element modeling studies investigate heel ulcers for\u3cbr/\u3ebedridden patients. In the current study we have added the implementation of the calf structure to the current\u3cbr/\u3eheel models. We tested the effect of foot posture, mattress stiffness, and a lateral calcaneus displacement to the\u3cbr/\u3econtact pressure and internal maximum shear strain occurring at the heel.\u3cbr/\u3eMethods: A new 3D finite element model is created which includes the heel and calf structure. Sensitivity\u3cbr/\u3eanalyses are performed for the foot orientation relative to the mattress, the Young's modulus of the mattress, and\u3cbr/\u3ea lateral displacement of the calcaneus relative to the other soft tissues in the heel.\u3cbr/\u3eFindings: The models predict that a stiffer mattress results in higher contact pressures and internal maximum\u3cbr/\u3eshear strains at the heel as well as the calf. An abducted foot posture reduces the internal strains in the heel and a\u3cbr/\u3elateral calcaneus displacement increases the internal maximum shear strains. A parameter study with different\u3cbr/\u3emattress-skin friction coefficients showed that a coefficient below 0.4 decreases the maximum internal shear\u3cbr/\u3estrains in all of the used loading conditions.\u3cbr/\u3eInterpretation: In clinical practice, it is advised to avoid internal shearing of the calcaneus of patients, and it\u3cbr/\u3ecould be taken into consideration by medical experts and nurses that a more abducted foot position may reduce\u3cbr/\u3ethe strains in the heel

Image-based analysis of uniaxial ring test for mechanical characterization of soft materials and biological tissues

\u3cp\u3eUniaxial ring test is a widely used mechanical characterization method for a variety of materials, from industrial elastomers to biological materials. Here we show that the combination of local material compression, bending, and stretching during uniaxial ring test results in a geometry-dependent deformation profile that can introduce systematic errors in the extraction of mechanical parameters. We identify the stress and strain regimes under which stretching dominates and develop a simple image-based analysis approach that eliminates these systematic errors. We rigorously test this approach computationally and experimentally, and demonstrate that we can accurately estimate the sample mechanical properties for a wide range of ring geometries. As a proof of concept for its application, we use the approach to analyze explanted rat vascular tissues and find a clear temporal change in the mechanical properties of these explants after graft implantation. The image-based approach can therefore offer a straightforward, versatile, and accurate method for mechanically characterizing new classes of soft and biological materials.\u3c/p\u3

Post natal development of collagen architecture in articular cartilage

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Deformation thresholds for chondrocyte death and the protective effect of the pericellular matrix

In cartilage tissue engineering studies, the stimulatory effect of a constant magnitude of mechanical perturbation declines after the first two weeks of culture. Similarly, it is known that chondrocyte-agarose constructs should not be loaded within the first days after seeding, to prevent considerable cell death, suggesting a mechanical threshold. This study aims to establish a relationship between chondrocyte deformation and death, and to evaluate the protective effect of the pericellular matrix (PCM) that is formed in 3D cultures. Chondrocyte viability was monitored every hour for 24 hours after applying a strain range of 0% to 25% to agarose constructs containing chondrocytes, cultured for 1, 3, 5, 7 or 10 days. At these culture time points, PCM thickness and chondrocyte deformation were assessed by means of histology and assayed for biochemical contents. Inverse finite element simulations were used to evaluate the change of mechanical properties of the chondrocyte and PCM over the 10 day culture duration. Chondrocyte death was demonstrated to be dependent on both the magnitude and duration of straining. The highest cell death was observed at day 1 (43%), reducing over culture duration (15% at day 3, and 2.5% at day 10). Cell deformation at 25% compression decreased significantly over culture duration (aspect ratio of 2.24 ± 0.67 at day 1 and 1.45 ± 0.24 at day 3) and with increased matrix production. Inverse finite element simulations showed an increasing PCM Young’s modulus of 45 KPa at day 3 to 162 KPa at day 10. The current results provide evidence for a mechanical threshold for chondrocyte death and for the protective effect of the PCM. As such, these insights may help in establishing mechanical loading protocols for cartilage tissue engineering studies

There is an individual tolerance to mechanical loading in compression induced deep tissue injury

\u3cp\u3eBackground: Deep tissue injury is a type of pressure ulcer which originates subcutaneously due to sustained mechanical loading. The relationship between mechanical compression and damage development has been extensively studied in 2D. However, recent studies have suggested that damage develops beyond the site of indentation. The objective of this study was to compare mechanical loading conditions to the associated damage in 3D. Methods: An indentation test was performed on the tibialis anterior muscle of rats (n = 39). Changes in the form of oedema and structural damage were monitored with MRI in an extensive region. The internal deformations were evaluated using MRI based 3D finite element models. Findings: Damage propagates away from the loaded region. The 3D analysis indicates that there is a subject specific tolerance to compression induced deep tissue injury. Interpretation: Individual tolerance is an important factor when considering the mechanical loading conditions which induce damage.\u3c/p\u3

Multiscale mechanics of articular cartilage : potentials and challenges of coupling musculoskeletal, joint, and microscale computational models

Articular cartilage experiences significant mechanical loads during daily activities. Healthy cartilage provides the capacity for load bearing and regulates the mechanobiological processes for tissue development, maintenance, and repair. Experimental studies at multiple scales have provided a fundamental understanding of macroscopic mechanical function, evaluation of the micromechanical environment of chondrocytes, and the foundations for mechanobiological response. In addition, computational models of cartilage have offered a concise description of experimental data at many spatial levels under healthy and diseased conditions, and have served to generate hypotheses for the mechanical and biological function. Further, modeling and simulation provides a platform for predictive risk assessment, management of dysfunction, as well as a means to relate multiple spatial scales. Simulation-based investigation of cartilage comes with many challenges including both the computational burden and often insufficient availability of data for model development and validation. This review outlines recent modeling and simulation approaches to understand cartilage function from a mechanical systems perspective, and illustrates pathways to associate mechanics with biological function. Computational representations at single scales are provided from the body down to the microstructure, along with attempts to explore multiscale mechanisms of load sharing that dictate the mechanical environment of the cartilage and chondrocytes. © 2012 Biomedical Engineering Society

Behavior of CMPCs in unidirectional constrained and stress-free 3D hydrogels

Cardiomyocyte progenitor cells (CMPCs) are a candidate cell source for cardiac regenerative therapy. However, like other stem cells, after transplantation in the heart, cell retention and differentiation capacity of the CMPCs are low. Combining cells with biomaterials might overcome this problem. By serving as a (temporal) environment, the biomaterial can retain the cells and provide signals that enhance survival, proliferation and differentiation of the cells. To gain more insight into the effect that the encapsulation of CMPCs in a biomaterial has on their behavior, we cultured CMPCs in unidirectional constrained and stress-free collagen/Matrigel hydrogels. CMPCs cultured in 3D hydrogels stay viable and keep their cardiomyogenic profile independent of the application of strain. Moreover, the increased expression of Nkx2.5, myocardin and cTnT in 3D hydrogels compared to 2D cultures, suggests enhanced cardiomyogenic differentiation capacity of cells in 3D. Furthermore, increased expression of collagen I, collagen III, elastin and fibronectin and of the matrix remodeling enzymes MMP-1, MMP-2, MMP-9, and TIMP-1 and TIMP-2 in the 3D hydrogels is indicative of an enhanced matrix remodeling capacity of CMPCs in a 3D environment, independent of the application of strain. Interestingly, the additional application of static strain to the 3D hydrogels, as imposed by hydrogel constrainment, stabilized CMPC viability and proliferation, resulted in enhanced cardiac marker protein expression and appeared crucial for cellular organization and morphology. More specifically, CMPCs cultured in 3D collagen/Matrigel constrained hydrogels became readily mechanosensitive, had a rod-shaped morphology, and responded to the applied strain by orienting in the direction of the constraint. Overall, our data demonstrate the applicability of CMPCs in a 3D environment since encapsulation of CMPCs may stabilize survival and proliferation, can enhance the differentiation and remodeling capacity of the cells, and could induce cellular re-organization, which all may contribute to an improved efficiency of cardiac stem cell therapy