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.
Collagen fibers are under tension in most extracellular matrices both prior to and during normal loading. This tension not only provides mechanical advantages, but also appears to establish a loading ...basis for the stimulation of mechanochemical transduction processes. The presence of tensile loads applied to collagen fibers also results in physical alignment of the collagen fibrils along the tensile axis. This alignment may influence biological processes such as mineralization.
In this study we report a comparison between elastic and viscous stress-strain curves and mineral contents of self-assembled collagen fibers that were strained to 30% of their original lengths and then mineralized, and self-assembled collagen fibers that were not strained before being mineralized. We concluded that the application of strain changes the organization of the collagenous matrix and alters the calcium phosphate nucleation and/or growth in the matrix. In addition, when the mechanical behavior of collagen fibers is compared with mechanical data from mineralized turkey tendon, the results indicate that collagen fibril-to-fibril interactions present in turkey tendon appear to be more organized compared with self-assembled aligned collagen fibers. We concluded that organized collagen-collagen interactions appear to be an important characteristic required for elastic energy storage in tendon.
We have studied the strain rate dependence of incremental stress-strain curves of self-assembled type I collagen fibers in an effort to understand the molecular phenomena that contribute to the ...macroscopic mechanical behavior of tendons. Results of viscoelastic tests at strain rates between 10% and 1000% per min suggest that the slope of the elastic stress-strain curve is to a first approximation independent of strain rate while the slope of the viscous stress-strain curve increases with increased strain rate. After correction of the slope of the viscous stress-strain curve for the changes in strain rate, it is observed that the apparent viscosity decreases with increased strain rate. It is concluded that the approximate strain rate independence of the elastic spring constant of collagen is consistent with the spring-like behavior of the 12 flexible regions that make up the collagen D-period. These regions are poor in the rigid amino acid residues proline and hydroxyproline. In contrast, the thixotropy of collagen is consistent with the slippage of subfibrillar subunits during tensile deformation. It is hypothesized that at high strain rates subfibrillar subunits appear to "hydroplane" by each other on a layer of loosely bound water.
PurposeTo determine the in vivo elastic modulus of the human cornea using vibrational optical coherence tomography (VOCT). MethodsVibrational analysis coupled with optical coherence tomography (OCT) ...was used to obtain the resonant frequency (RF) and elastic modulus of corneal structural components. VOCT corneal thickness values were measured using OCT images and correlated with corneal thickness determined with Pentacam (Oculus, Wetzlar, Germany). Moduli were obtained at two locations: central cornea (CC) and inferior cornea (IC). Measurements were obtained with and without anesthetic eye drops to assess their effect on the modulus measurements. ResultsVOCT thickness values correlated positively (R2 = 0.97) and linearly (y = 1.039x-16.89) with those of Pentacam. Five RF peaks (1-5) were present, although their presence was variable across eyes. The RF for peaks 1 to 5 in the CC and IC ranged from 73.5 ± 4.9 to 239 ± 3 Hz and 72.1 ± 6.3 to 238 ± 4 Hz, respectively. CC and IC moduli for peaks 1 to 5 ranged from 1.023 ± 0.104 to 6.87 ± 0.33 MPa and 0.98 ± 0.15 to 6.52 ± 0.79 MPa, respectively. Topical anesthesia did not significantly alter the modulus (P > 0.05 for all), except for peak 2 in the CC (P < 0.05). ConclusionsThis pilot study demonstrates the utility of VOCT as an in vivo, noninvasive technology to measure the elastic modulus in human corneas. The structural origin of these moduli is hypothesized based on previous reports, and further analyses are necessary for confirmation. Translational RelevanceThis work presents VOCT as a novel approach to assess the in vivo elastic modulus of the cornea, an indicator of corneal structural integrity and health.
Our team previously identified the presence of five corneal resonant frequency (RF) peaks in healthy volunteers using vibrational optical coherence tomography (VOCT). Prior studies have suggested ...that the ≤100 Hz RF peak represents the cellular element of tissue. The aim of this study was to confirm that this peak reflects the human corneal cellular component using VOCT and histological analysis.
Two human research globes were obtained from the same donor, and VOCT measurements were collected from the full-thickness corneas. A microkeratome was then used to create serial-free corneal caps from each cornea, with VOCT performed on the residual stromal bed after each excision. All lamellar sections from both globes were sent for histological analysis to determine cellularity. Cell counts on the specimens were performed by two independent observers.
The average of the normalized ≤100 Hz peak values before lamellar sectioning was significantly higher than the average of this peak values after the first, second, and third cuts (P = 0.023), which was 33.9% less than before any cuts. The cell count values in the first slice were significantly higher than the average cell count values of the three deeper slices (P < 0.001), and the cell count dropped 84.4% after the first slice was removed.
The findings of this study suggest that the ≤100 Hz corneal peak identified by VOCT corresponds to the cellular component of the cornea.
This work furthers our understanding of the origin of the corneal ≤100 Hz peak identified using VOCT.
Collagen fibers form the basic structural components of extracellular matrix (ECM) of vertebrates that serve to: (1) store elastic energy during muscular deformation, (2) transmit stored energy into ...joint movement, and (3) transfer excess energy from the joint back to the attached muscles for dissipation. They also act as mechanotransducers by transferring stress borne by the musculoskeleton to the attached cells in order to either up - or down - regulate tissue metabolism as a result of changes in mechanical loading. Finally, they prevent premature mechanical failure of tissues by limiting deformation of most ECMs and organs.
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.
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