scholarly journals Planar Biaxial Mechanical Behavior of Bioartificial Tissues Possessing Prescribed Fiber Alignment

2009 ◽  
Vol 131 (8) ◽  
Author(s):  
Choon-Sik Jhun ◽  
Michael C. Evans ◽  
Victor H. Barocas ◽  
Robert T. Tranquillo
Keyword(s):  
Aspect Ratio ◽  
Mechanical Behavior ◽  
Mechanical Response ◽  
Great Influence ◽  
Mechanical Anisotropy ◽  
Fiber Direction ◽  
Modulus Ratio ◽  
Biaxial Testing ◽  
Fiber Alignment ◽  
Aspect Ratios

Though it is widely accepted that fiber alignment has a great influence on the mechanical anisotropy of tissues, a systematic study of the influence of fiber alignment on the macroscopic mechanical behavior by native tissues is precluded due to their predefined microstructure and heterogeneity. Such a study is possible using collagen-based bioartificial tissues that allow for alignment to be prescribed during their fabrication. To generate a systemic variation of strength of fiber alignment, we made cruciform tissue constructs in Teflon molds that had arms of different aspect ratios. We implemented our anisotropic biphasic theory of tissue-equivalent mechanics to simulate the compaction by finite element analysis. Prior to tensile testing, the construct geometry was standardized by cutting test samples with a 1:1 cruciform punch after releasing constructs from the molds. Planar biaxial testing was performed on these samples, after stretching them to their in-mold dimensions to recover in-mold alignment, to observe the macroscopic mechanical response with simultaneous fiber alignment imaging using a polarimetry system. We found that the strength of fiber alignment of the samples prior to release from the molds linearly increased with anisotropy of the mold. In testing after release, modulus ratio (modulus in fiber direction/modulus in normal direction) was greater as the initial strength of fiber alignment increased, that is, as the aspect ratio increased. We also found that the fiber alignment strength and modulus ratio increased in a hyperbolic fashion with stretching for a sample of given aspect ratio.

scholarly journals In Silico Performance of a Recellularized Tissue-Engineered Transcatheter Aortic Valve

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Christopher Noble ◽  
Joshua Choe ◽  
Susheil Uthamaraj ◽  
Milton Deherrera ◽  
Amir Lerman ◽  
...  
Keyword(s):  
Mechanical Behavior ◽  
Heart Valves ◽  
Mechanical Response ◽  
Mechanical Performance ◽  
Fe Analysis ◽  
Model Parameters ◽  
Biaxial Testing ◽  
Principal Stresses

Commercially available heart valves have many limitations, such as a lack of remodeling, risk of calcification, and thromboembolic problems. Many state-of-the-art tissue-engineered heart valves (TEHV) rely on recellularization to allow remodeling and transition to mechanical behavior of native tissues. Current in vitro testing is insufficient in characterizing a soon-to-be living valve due to this change in mechanical response; thus, it is imperative to understand the performance of an in situ valve. However, due to the complex in vivo environment, this is difficult to accomplish. Finite element (FE) analysis has become a standard tool for modeling mechanical behavior of heart valves; yet, research to date has mostly focused on commercial valves. The purpose of this study has been to evaluate the mechanical behavior of a TEHV material before and after 6 months of implantation in a rat subdermis model. This model allows the recellularization and remodeling potential of the material to be assessed via a simple and inexpensive means prior to more complex ovine orthotropic studies. Biaxial testing was utilized to evaluate the mechanical properties, and subsequently, constitutive model parameters were fit to the data to allow mechanical performance to be evaluated via FE analysis of a full cardiac cycle. Maximum principal stresses and strains from the leaflets and commissures were then analyzed. The results of this study demonstrate that the explanted tissues had reduced mechanical strength compared to the implants but were similar to the native tissues. For the FE models, this trend was continued with similar mechanical behavior in explant and native tissue groups and less compliant behavior in implant tissues. Histology demonstrated recellularization and remodeling although remodeled collagen had no clear directionality. In conclusion, we observed successful recellularization and remodeling of the tissue giving confidence to our TEHV material; however, the mechanical response indicates the additional remodeling would likely occur in the aortic/pulmonary position.

Remodeling in Cell-Seeded Fibrin Gels: Changes in Composition, Organization, and Planar Biaxial Mechanical Behavior

2009 ◽  
Author(s):  
Edward A. Sander ◽  
Sandra L. Johnson ◽  
Victor H. Barocas ◽  
Robert T. Tranquillo
Keyword(s):  
Aspect Ratio ◽  
Stress Field ◽  
Mechanical Behavior ◽  
The Other ◽  
Fibrin Gel ◽  
Fiber Alignment ◽  
Mechanical Environment ◽  
Heterogeneous Sample ◽  
Fibrin Gels ◽  
Stress Environment

Engineered tissues are necessary to replace diseased and damaged tissues incapable of healing on their own. One method employed to produce them involves cell entrapment in a fibrin gel constrained by specially designed molds [1]. As the cells compact and remodel the gel, the combination of mold constraints and cell tractions produces fiber alignment similar to native tissues [2]. One potentially important factor in the remodeling outcome is the local mechanical environment that develops during the compaction and remodeling process. It is well established that the global stress environment leads to changes in remodeling in an isotropic sample [3], but we do not know the effect of local variations in stress field in a heterogeneous sample. To begin to assess the local mechanical environment’s role, we examined the remodeling process in cross-shaped Teflon molds (cruciforms). In this experiment, two mold geometries with differing channel widths were examined: a 1:1 aspect ratio in which the both axes possessed 8 mm wide channels, and a 1:0.5 aspect ratio in which one axis had 8 mm wide channels and the other 4 mm wide channels (fig. 1).

scholarly journals Deformation Mechanisms of Very Long Single-Wall Carbon Nanotubes Subject to Compressive Loading

2004 ◽  
Vol 126 (3) ◽  
pp. 245-249 ◽  
Author(s):  
Markus J. Buehler ◽  
Yong Kong ◽  
Huajian Gao
Keyword(s):  
Carbon Nanotubes ◽  
Mechanical Behavior ◽  
Continuum Mechanics ◽  
Mechanical Response ◽  
Compressive Loading ◽  
Single Wall Carbon Nanotubes ◽  
Buckling Mode ◽  
Single Wall Carbon ◽  
Aspect Ratios ◽  
Mechanics Concepts

We report atomistic studies of single-wall carbon nanotubes with very large aspect ratios subject to compressive loading. These long tubes display significantly different mechanical behavior than tubes with smaller aspect ratios. We distinguish three different classes of mechanical response to compressive loading. While the deformation mechanism is characterized by buckling of thin shells in nanotubes with small aspect ratios, it is replaced by a rod-like buckling mode above a critical aspect ratio, analogous to the Euler theory in continuum mechanics. For very large aspect ratios, a nanotube is found to behave like a flexible macromolecule which tends to fold due to vdW interactions between different parts of the carbon nanotube. This suggests a shell-rod-wire transition of the mechanical behavior of carbon nanotubes with increasing aspect ratios. While continuum mechanics concepts can be used to describe the first two types of deformation, statistical methods will be necessary to describe the dynamics of wire-like long tubes.

Collagen Fiber Alignment Does Not Explain Mechanical Anisotropy in Fibroblast Populated Collagen Gels

2007 ◽  
Vol 129 (5) ◽  
pp. 642-650 ◽  
Author(s):  
Stavros Thomopoulos ◽  
Gregory M. Fomovsky ◽  
Preethi L. Chandran ◽  
Jeffrey W. Holmes
Keyword(s):  
Mechanical Behavior ◽  
Network Model ◽  
Continuum Model ◽  
Collagen Fiber ◽  
Collagen Gel ◽  
Mechanical Anisotropy ◽  
Collagen Gels ◽  
Fiber Alignment ◽  
High Degree ◽  
Collagen Fiber Alignment

Many load-bearing soft tissues exhibit mechanical anisotropy. In order to understand the behavior of natural tissues and to create tissue engineered replacements, quantitative relationships must be developed between the tissue structures and their mechanical behavior. We used a novel collagen gel system to test the hypothesis that collagen fiber alignment is the primary mechanism for the mechanical anisotropy we have reported in structurally anisotropic gels. Loading constraints applied during culture were used to control the structural organization of the collagen fibers of fibroblast populated collagen gels. Gels constrained uniaxially during culture developed fiber alignment and a high degree of mechanical anisotropy, while gels constrained biaxially remained isotropic with randomly distributed collagen fibers. We hypothesized that the mechanical anisotropy that developed in these gels was due primarily to collagen fiber orientation. We tested this hypothesis using two mathematical models that incorporated measured collagen fiber orientations: a structural continuum model that assumes affine fiber kinematics and a network model that allows for nonaffine fiber kinematics. Collagen fiber mechanical properties were determined by fitting biaxial mechanical test data from isotropic collagen gels. The fiber properties of each isotropic gel were then used to predict the biaxial mechanical behavior of paired anisotropic gels. Both models accurately described the isotropic collagen gel behavior. However, the structural continuum model dramatically underestimated the level of mechanical anisotropy in aligned collagen gels despite incorporation of measured fiber orientations; when estimated remodeling-induced changes in collagen fiber length were included, the continuum model slightly overestimated mechanical anisotropy. The network model provided the closest match to experimental data from aligned collagen gels, but still did not fully explain the observed mechanics. Two different modeling approaches showed that the level of collagen fiber alignment in our uniaxially constrained gels cannot explain the high degree of mechanical anisotropy observed in these gels. Our modeling results suggest that remodeling-induced redistribution of collagen fiber lengths, nonaffine fiber kinematics, or some combination of these effects must also be considered in order to explain the dramatic mechanical anisotropy observed in this collagen gel model system.

Self-Folding and Unfolding of Carbon Nanotubes

2005 ◽  
Vol 128 (1) ◽  
pp. 3-10 ◽  
Author(s):  
Markus J. Buehler ◽  
Yong Kong ◽  
Huajian Gao ◽  
Yonggang Huang
Keyword(s):  
Carbon Nanotubes ◽  
Critical Temperature ◽  
Aspect Ratio ◽  
Mechanical Response ◽  
Driving Forces ◽  
Compressive Loading ◽  
Single Wall Carbon Nanotubes ◽  
Stable States ◽  
Buckling Mode ◽  
Aspect Ratios

Carbon nanotubes (CNTs) constitute a prominent example of nanomaterials. In most studies on mechanical properties, the effort was concentrated on CNTs with relatively small aspect ratio of length to diameters. In contrast, CNTs with aspect ratios of several hundred can be produced with today’s experimental techniques. We report atomistic-continuum studies of single-wall carbon nanotubes with very large aspect ratios subject to compressive loading. It was recently shown that these long tubes display significantly different mechanical behavior than tubes with smaller aspect ratios (Buehler, M. J., Kong, Y., and Guo, H., 2004, ASME J. Eng. Mater. Technol. 126, pp. 245–249). We distinguish three different classes of mechanical response to compressive loading. While the deformation mechanism is characterized by buckling of thin shells in nanotubes with small aspect ratios, it is replaced by a rodlike buckling mode above a critical aspect ratio, analogous to the Euler theory in continuum mechanics. For very large aspect ratios, a nanotube is found to behave like a wire that can be deformed in a very flexible manner to various shapes. In this paper, we focus on the properties of such wirelike CNTs. Using atomistic simulations carried out over a several-nanoseconds time span, we observe that wirelike CNTs behave similarly to flexible macromolecules. Our modeling reveals that they can form thermodynamically stable self-folded structures, where different parts of the CNTs attract each other through weak van der Waals (vdW) forces. This self-folded CNT represents a novel structure not described in the literature. There exists a critical length for self-folding of CNTs that depends on the elastic properties of the tube. We observe that CNTs fold below a critical temperature and unfold above another critical temperature. Surprisingly, we observe that self-folded CNTs with very large aspect ratios never unfold until they evaporate. The folding-unfolding transition can be explained by entropic driving forces that dominate over the elastic energy at elevated temperature. These mechanisms are reminiscent of the dynamics of biomolecules, such as proteins. The different stable states of CNTs are finally summarized in a schematic phase diagram of CNTs.

Mechanical Response of Human Muscle at Intermediate Strain Rates

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
Xuedong Zhai ◽  
Eric A. Nauman ◽  
Yizhou Nie ◽  
Hangjie Liao ◽  
Roy J. Lycke ◽  
...  
Keyword(s):  
Strain Rate ◽  
Mechanical Behavior ◽  
Compressive Stress ◽  
Mechanical Response ◽  
Human Muscle ◽  
Fiber Direction ◽  
Strain Rates ◽  
Strain Response ◽  
Stress Strain ◽  
Intermediate Strain Rates

We experimentally determined the tensile stress–strain response of human muscle along fiber direction and compressive stress–strain response transverse to fiber direction at intermediate strain rates (100–102/s). A hydraulically driven material testing system with a dynamic testing mode was used to perform the tensile and compressive experiments on human muscle tissue. Experiments at quasi-static strain rates (below 100/s) were also conducted to investigate the strain-rate effects over a wider range. The experimental results show that, at intermediate strain rates, both the human muscle's tensile and compressive stress–strain responses are nonlinear and strain-rate sensitive. Human muscle also exhibits a stiffer and stronger tensile mechanical behavior along fiber direction than its compressive mechanical behavior along the direction transverse to fiber direction. An Ogden model with two material constants was adopted to describe the nonlinear tensile and compressive behaviors of human muscle.

On the Mechanical Behavior of Healthy and Aneurysmal Abdominal Aorta

2011 ◽  
Author(s):  
J. Ferruzzi ◽  
M. S. Enevoldsen ◽  
J. D. Humphrey
Keyword(s):  
Mechanical Behavior ◽  
Abdominal Aorta ◽  
Arterial Wall ◽  
Mechanical Response ◽  
Pathological Condition ◽  
Uniaxial Tensile ◽  
Infrarenal Aorta ◽  
Biaxial Testing ◽  
Biomechanical Behavior ◽  
Uniaxial Tensile Testing

Abdominal aortic aneurysm (AAA) is a pathological condition of the infrarenal aorta characterized by a local dilatation of the arterial wall. The main histopathologic features of an AAA are smooth muscle cell death and loss of elastin. The biomechanical behavior of AAAs has been widely studied to determine the rupture potential according to the principles of material failure. However, most prior approaches are limited by the use of data from uniaxial tensile testing and by the assumption of material isotropy, leading to inaccurate characterization of the 3D multiaxial mechanical response of the aneurysmal tissue. To date, the best data available on the behavior of human abdominal aorta (AA) and AAA to planar biaxial testing are the ones reported by Vande Geest et al. [1,2]. In a recent work [3], we considered a structurally motivated four-fiber family strain energy function (SEF) [4] to capture the biaxial behavior of the human AA and AAA from Vande Geest et al. [1,2]. We showed that this constitutive relation fits human data better than prior models and most importantly it captures the stiffening of the arterial wall related to both aging and aneurysmal development. These changes in mechanical behavior are mirrored by changes in the best-fit values of the parameters, with a progressive decrease of the isotropic part attributed to elastin and a parallel increase in values associated with the families of collagen fibers.

Determination of the finite-strain ellipsoid from deformed porphyroblastic mineral aggregates and preferentially oriented feldspars in a mylonitized metamafic dyke

1989 ◽  
Vol 26 (11) ◽  
pp. 2333-2340
Author(s):  
J. Victor Owen
Keyword(s):  
Aspect Ratio ◽  
Strain Ratio ◽  
Mechanical Anisotropy ◽  
Dislocation Glide ◽  
Recovery Processes ◽  
Strain Ellipsoid ◽  
Aspect Ratios ◽  
Subgrain Rotation ◽  
Intermediate Value ◽  
Mineral Aggregates

Strain in a narrow mylonite zone has been estimated from deformed garnetiferous porphyroblastic aggregates and from preferentially oriented plagioclase porphyroclasts with high aspect ratios. In the undeformed metamafic dyke hosting the mylonite, the mineral aggregates have spheroidal to slightly oblate shapes, and plagioclase is nearly randomly oriented. In the mylonite, the mineral aggregates are prolate ellipsoids, and plagioclase in the aggregates and matrix is symmetrically oriented about the mylonitic planar fabric. Comparison with the radii of spheres of equal volume shows that the ellipsoidal mineral aggregates underwent triaxial strain, with maximum extension of 50–140% parallel to X and with shortening of up to −30 and −45% parallel to Y and Z, respectively. The maximum strain ratio varies between 1.9 and 4.2 (mean of 10 measurements = 3.1). The orientation and aspect ratios of elongate plagioclase grains measured in the X–Z plane indicate an intermediate value (2.7) for the strain ratio. Plagioclase deformation was apparently accommodated by dislocation glide on (010), recovery processes (subgrain rotation), and microcracking. The effects of mechanical anisotropy in plagioclase, however, were subordinate to the strain regime, strain ratio, and initial aspect ratio of grains in determining the final aspect ratio and rest position of these porphyroclasts.Both the deformed garnetiferous aggregates and the plagioclase porphyroclasts record state of strain in the mylonite. This suggests that the preferred orientation of densely packed feldspars of high aspect ratio potentially may be used to estimate strain in tectonites.

scholarly journals Effects of Steel Fiber Percentage and Aspect Ratios on Fresh and Harden Properties of Ultra-High Performance Fiber

2021 ◽  
Vol 2 (3) ◽  
pp. 501-515
Author(s):  
Rajib Kumar Biswas ◽  
Farabi Bin Ahmed ◽  
Md. Ehsanul Haque ◽  
Afra Anam Provasha ◽  
Zahid Hasan ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Aspect Ratio ◽  
High Performance ◽  
Steel Fiber ◽  
Setting Time ◽  
Fiber Reinforced Concrete ◽  
Future Research ◽  
Manufacturing Cost ◽  
Aspect Ratios ◽  
High Performance Fiber

Steel fibers and their aspect ratios are important parameters that have significant influence on the mechanical properties of ultrahigh-performance fiber-reinforced concrete (UHPFRC). Steel fiber dosage also significantly contributes to the initial manufacturing cost of UHPFRC. This study presents a comprehensive literature review of the effects of steel fiber percentages and aspect ratios on the setting time, workability, and mechanical properties of UHPFRC. It was evident that (1) an increase in steel fiber dosage and aspect ratio negatively impacted workability, owing to the interlocking between fibers; (2) compressive strength was positively influenced by the steel fiber dosage and aspect ratio; and (3) a faster loading rate significantly improved the mechanical properties. There were also some shortcomings in the measurement method for setting time. Lastly, this research highlights current issues for future research. The findings of the study are useful for practicing engineers to understand the distinctive characteristics of UHPFRC.

scholarly journals Surface Texturing of Si with Periodically Arrayed Oblique Nanopillars to Achieve Antireflection

Materials ◽  
2021 ◽  
Vol 14 (2) ◽  
pp. 380
Author(s):  
Jun-Hyun Kim ◽  
Sanghyun You ◽  
Chang-Koo Kim
Keyword(s):  
Aspect Ratio ◽  
Plasma Etching ◽  
Surface Texturing ◽  
Light Reflection ◽  
Si Substrate ◽  
Weighted Mean ◽  
Optical Reflectance ◽  
Si Substrates ◽  
Aspect Ratios ◽  
Shadowing Effect

Si surfaces were texturized with periodically arrayed oblique nanopillars using slanted plasma etching, and their optical reflectance was measured. The weighted mean reflectance (Rw) of the nanopillar-arrayed Si substrate decreased monotonically with increasing angles of the nanopillars. This may have resulted from the increase in the aspect ratio of the trenches between the nanopillars at oblique angles due to the shadowing effect. When the aspect ratios of the trenches between the nanopillars at 0° (vertical) and 40° (oblique) were equal, the Rw of the Si substrates arrayed with nanopillars at 40° was lower than that at 0°. This study suggests that surface texturing of Si with oblique nanopillars reduces light reflection compared to using a conventional array of vertical nanopillars.

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