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ABSTRACT: During limb movement and locomotion, animals store elastic energy in the tendons of the feet, legs, and other limbs. In the turkey, much of the force generated by the gastrocnemius muscle during locomotion is stored as elastic energy through deformation of the tendon. During growth and development, the leg tendons in some avians, including turkeys, mineralize and result in an increase in tensile strength and modulus. The purpose of our study was to evaluate the effects of mineralization on elastic energy storage and transmission in turkey tendons. Elastic and viscous stress-strain curves and elastic energy storage behavior were used to compare the behavior of mineralized turkey gastrocnemius tendons and mineralized self-assembled type I collagen fibers. Based on analysis of these two systems, we concluded that a simple mineralized fibrillar collagenous substrate can mimic the behavior of a more complex fibrillar collagenous substrate such as mineralized turkey tendon; however, the exact mechanism of mineralization may be different between the two substrates. Changes in mechanical properties of turkey tendon were consistent with a model in which mineralization appears to increase the effective collagen fibril length by efficiently transferring stress between neighboring collagen fibrils. Mineralization in self-assembled collagen fibers increased elastic energy storage less efficiently as compared with turkey tendon suggesting that the noncollagenous components of mineralizing tissue may act to promote collagen fibril to collagen fibril interactions.
Connective Tissue Research 01/2004; 45(3):131-41. · 1.20 Impact Factor
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ABSTRACT: The development of the musculoskeleton and the ability to locomote requires controlled cell division as well as spatial control over deposition of extracellular matrix. Self-assembly of procollagen and its final processing into collagen fibrils occurs extracellularly. The formation of crosslinked collagen fibers results in the conversion of weak liquid-like embryonic tissues to tough elastic solids that can store energy and do work. Collagen fibers in the form of fascicles are the major structural units found in tendon. The purpose of this paper is to review the literature on collagen self-assembly and tendon development and to relate this information to the development of elastic energy storage in non-mineralizing and mineralizing tendons. Of particular interest is the mechanism by which energy is stored in tendons during locomotion. In vivo, collagen self-assembly occurs by the deposition of thin fibrils in recesses within the cell membrane. These thin fibrils later grow in length and width by lateral fusion of intermediates. In vitro, collagen self-assembly occurs by both linear and lateral growth steps with parallel events seen in vivo; however, in the absence of cellular control and enzymatic cleavage of the propeptides, the growth mechanism is altered, and the fibrils are irregular in cross section. Results of mechanical studies suggest that prior to locomotion the mechanical response of tendon to loading is dominated by the viscous sliding of collagen fibrils. In contrast, after birth when locomotion begins, the mechanical response is dominated by elastic stretching of crosslinked collagen molecules.
Journal of Biomechanics 11/2003; 36(10):1529-53. · 2.43 Impact Factor
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ABSTRACT: We have studied the strain-rate dependency of the viscoelastic mechanical properties of human dermis from young (23-year-old) and old (87-year-old) donors using incremental stress–strain measurements. The elastic spring constant for elastic fibers was found to be strain-rate and age dependent, whereas that for collagen was only age dependent. Fibril lengths were observed to decrease with increased strain rates and age for both elastic and collagen fibers; however, the large decrease in collagen fibril viscosity was hypothesized to be a result of thixotropy that results when neighboring collagen fibrils slide by each other. It is concluded that the elastic spring constant measured for elastic fibers may be higher than previously reported and is consistent with stretching of α-helical segments of elastin into a more extended conformation during the initial part of the elastic stress–strain curve. The decrease in the elastic spring constant with increased age observed is consistent with disruption of the elastic fibers and loss of α-helical structure. The pH dependency of the elastic modulus reported previously for collagen suggests that charge–charge interactions within and between collagen molecules are involved in energy storage during stretching. Elastic energy storage is consistent with the stretching of charged pairs located in flexible regions of the collagen molecule. Shear thinning, or thixotropy of skin, is hypothesized to reflect breakage of bonds that occur between collagen fibrils. It is hypothesized that both collagen and elastin are complex macromolecules that are hybrids of flexible and rigid regions. The flexible regions reversibly store elastic energy during stretching by breakage of secondary bonds. After stretching, the flexible regions become extended and transfer stress to the rigid regions of these molecules. This prevents premature mechanical failure of collagen and elastic fibers in the dermis. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1978–1985, 2002
Journal of Applied Polymer Science 11/2002; 86(8):1978 - 1985. · 1.29 Impact Factor
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ABSTRACT: Background/aims: The purpose of this work is to attempt to determine the elastic spring constant for collagen and elastic fibers (elastin) in skin and to detemine if the values of these elastic constants are similar to those reported for other tissues.Methods: We studied the viscoelastic mechanical properties of human skin and dermis by measuring the incremental stress-strain behavior. Elastic stress-strain curves were used to obtain the elastic spring constant of elastin and collagen while the collagen fibril length was obtained from the slope of viscous stress-strain curves.Results: Our results suggest that the elastic spring constant for elastin is about 4.0 MPa while that for collagen is about 4.4 GPa. The former value is similar to that calculated for ligamentum nuchae while the latter value is about 70% of the value found for tendon and self-assembled type I collagen fibers. The differences between the elastic constants for collagen molecules in tendon and skin is hypothesized to reflect the higher molecular tilt angle and lower D period found in skin compared to tendon as well as a shorter fibril length.Conclusion: The differences in the collagen types present in skin and tendon may influence collagen self-assembly and the resulting viscoelastic properties.
Skin Research and Technology 01/2001; 7(1):18 - 23. · 1.71 Impact Factor
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ABSTRACT: Tissue engineering and the use of nanofibrous biomaterial scaffolds offer a unique perspective for studying cancer development in vitro. Current in vitro models of tumorigenesis are limited by the use of static, two-dimensional (2D) cell culture monolayers that lack the structural architecture necessary for cell–cell interaction and three-dimensional (3D) scaffolds that are too simplistic for studying basic pathological mechanisms. In this study, two nanofibrous biomaterials that mimic the structure of the extracellular matrix, bacterial cellulose and electrospun polycaprolactone (PCL)/collagen I, were investigated as potential 3D scaffolds for an in vitro cancer model. Multiple cancer cell lines were cultured on each scaffold material and monitored for cell viability, proliferation, adhesion, infiltration, and morphology. Both bacterial cellulose and electrospun PCL/collagen I, which have nano-scale structures on the order of 100–500 nm, have been used in many diverse tissue engineering applications. Cancer cell adhesion and growth were limited on bacterial cellulose, while all cellular processes were enhanced on the electrospun scaffolds. This initial analysis has demonstrated the potential of electrospun PCL/collagen I scaffolds toward the development of an improved 3D in vitro cancer model.
Materials Science and Engineering: C. 31(1):37-42.
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ABSTRACT: Collagen type I is a structural protein that provides tensile strength to tendons and ligaments. Type I collagen molecules form collagen fibers, which are viscoelastic and can therefore store energy elastically via molecular elongation and dissipate viscous energy through molecular rearrangement and fibrillar slippage. The ability to store elastic energy is important for the resiliency of tendons and ligaments, which must be able to deform and revert to their initial lengths with changes in load.In an earlier paper by one of the present authors, molecular modeling was used to investigate the role of mineralization upon elastic energy storage in collagen type I. Their collagen model showed a similar trend to their experimental data but with an over-estimation of elastic energy storage. Their simulations were conducted in vacuum and employed a distance-dependent dielectric function. In this study, we performed a re-evaluation of Freeman and Silver's model data incorporating the effects of explicit solvation and water infiltration, in order to determine whether the model data could be improved with a more accurate representation of the solvent and osmotic effects. We observed an average decrease in the model's elastic energy storage of 45.1%±6.9% in closer proximity to Freeman and Silver's experimental data. This suggests that although the distance-dependent dielectric implicit solvation approach was favored for its increased speed and decreased computational requirements, an explicit representation of water may be necessary to more accurately model solvent interactions in this particular system. In this paper, we discuss the collagen model described by Freeman and Silver, the present model building approach, the application of the present model to that of Freeman and Silver, and additional assumptions and limitations.
Journal of Theoretical Biology.
