Bowstring Stretching and Quantitative Imaging of Single Collagen Fibrils via Atomic Force Microscopy

The anisotropic and region-dependent mechanical response of wrap-around tendons under tensile, compressive and combined multiaxial loads

The present work reports on the multiaxial region and orientation-dependent mechanical properties of two porcine wrap-around tendons under tensile, compressive and combined loads based on an extensive study with n=175 samples. The results provide a detailed dataset of the anisotropic tensile and compressive longitudinal properties and document a pronounced tension-compression asymmetry. Motivated by the physiological loading conditions of these tendons, which include transversal compression at bony abutments in addition to longitudinal tension, we systematically investigated the change in axial tension when the tendon is compressed transversally along one or both perpendicular directions. The results reveal that the transversal compression can increase axial tension (proximal-distal direction) in both cases to orders of 30%, yet by a larger amount in the first case (transversal compression in anterior-posterior direction), which seems to be more relevant for wrap-around tendons in-vivo. These quantitative measurements are in line with earlier findings on auxetic properties of tendon tissue, but show for the first time the influence of this property on the stress response of the tendon, and may thus reveal an important functional principle within these essential elements of force transmission in the body. STATEMENT OF SIGNIFICANCE: The work reports for the first time on multiaxial region and orientation-dependent mechanical properties of wrap-around tendons under various loads. The results indicate that differences in the mechanical properties exist between zones that are predominantly in a uniaxial tensile state and those that experience complex load states. The observed counterintuitive increase of the axial tension upon lateral compression points at auxetic properties of the tendon tissue which may be pivotal for the function of the tendon as an element of the musculoskeletal system. It suggests that the tendon's performance in transmitting forces is not diminished but enhanced when the action line is deflected by a bony pulley around which the tendon wraps, representing an important functional principle of tendon tissue.

Effect of out of plane orientation on polarization second harmonic generation of single collagen fibrils

Second harmonic generation (SHG) microscopy has emerged as a powerful technique for visualizing collagen organization within tissues. Amongst the many advantages of SHG is its sensitivity to collagen nanoscale organization, and its presumed sensitivity to the relative out of plane polarity of fibrils. Recent results have shown that circular dichroism SHG (CD-SHG), a technique that has been commonly assumed to reveal the relative out of plane polarity of collagen fibrils, is actually insensitive to changes in fibril polarity. However, results from another research group seem to contradict this conclusion. Both previous results have been based on SHG imaging of collagen fibrils within tissues, therefore, to gain a definitive understanding of the sensitivity of SHG to relative out of plane polarity, the results from individual fibrils are desirable. Here we present polarization resolved SHG microscopy (PSHG) data from individual collagen fibrils oriented out of the image plane by buckling on an elastic substrate. We show through correlation with atomic force microscopy measurements that SHG intensity can be used to estimate the out of plane angle of individual fibrils. We then compare the sensitivity of two PSHG techniques, CD-SHG and polarization-in, polarization-out SHG (PIPO-SHG), to the relative out of plane polarity of individual fibrils. We find that for single fibrils CD-SHG is insensitive to relative out of polarity and we also demonstrate the first direct experimental confirmation that PIPO-SHG reveals the relative out of plane polarity of individual collagen fibrils.

High numerical aperture imaging allows chirality measurement in individual collagen fibrils using polarization second harmonic generation microscopy

Second harmonic generation (SHG) microscopy is a commonly used technique to study the organization of collagen within tissues. However, individual collagen fibrils, which have diameters much smaller than the resolution of most optical systems, have not been extensively investigated. Here we probe the structure of individual collagen fibrils using polarization-resolved SHG (PSHG) microscopy and atomic force microscopy. We find that longitudinally polarized light occurring at the edge of a focal volume of a high numerical aperture microscope objective illuminated with linearly polarized light creates a measurable variation in PSHG signal along the axis orthogonal to an individual collagen fibril. By comparing numerical simulations to experimental data, we are able to estimate parameters related to the structure and chirality of the collagen fibril without tilting the sample out of the image plane, or cutting tissue at different angles, enabling chirality measurements on individual nanostructures to be performed in standard PSHG microscopes. The results presented here are expected to lead to a better understanding of PSHG results from both collagen fibrils and collagenous tissues. Further, the technique presented can be applied to other chiral nanoscale structures such as microtubules, nanowires, and nanoribbons.

Mechanics of isolated individual collagen fibrils

Collagen fibrils are the fundamental structural elements in vertebrate animals and compose a framework that provides mechanical support to load-bearing tissues. Understanding how these fibrils initially form and mechanically function has been the focus of a myriad of detailed investigations over the last few decades. From these studies a great amount of knowledge has been acquired as well as a number of new questions to consider. In this review, we examine the current state of our knowledge of the mechanical properties of extant fibrils. We emphasize on the mechanical response and related deformation of collagen fibrils upon tension, which is the predominant load imposed in most collagen-rich tissues. We also illuminate the gaps in knowledge originating from the intriguing results that the field is still trying to interpret. STATEMENT OF SIGNIFICANCE: : Collagen is the result of millions of years of biological evolution and is a unique family of proteins, the majority of which provide mechanical support to biological tissues. Cells produce collagen molecules that self-assemble into larger structures, known as collagen fibrils. As simple as they appear under an optical microscope, collagen fibrils display a complex ultrastructural architecture tuned to the external forces that are imposed upon them. Even more complex is the way collagen fibrils deform under loading, and the nature of the mechanisms that drive their formation in the first place. Here, we present a cogent synthesis of the state-of-knowledge of collagen fibril mechanics. We focus on the information we have from in vitro experiments on individual, isolated from tissues, collagen fibrils and the knowledge available from in silico tests.

Nanomechanical and Nonlinear Optical Properties of Glycated Dental Collagen

Nonenzymatic glycation is a multistep, slow reaction between reducing sugars and free amino groups of long-lived proteins, which affects the structural and mechanical properties of collagen-rich tissues via accumulation of advanced glycation end products (AGEs). Dental collagen is exposed to glycation as part of the natural aging process. However, in case of chronically high blood glucose, the process can be accelerated, resulting in premature stiffening of dentin, leading to tooth fragility. The molecular mechanisms whereby collagen glycation evokes the loss of mechanical stability in teeth are currently unknown. In this study, we used 2-photon and atomic force microscopies to correlate structural and mechanical changes in dental collagen induced by in vitro glycation. Young tooth samples were demineralized and cut longitudinally into 30-µm sections, then artificially glycated in 0.5 M ribose solution for 10 wk. Two-photon microscopy analysis showed that both the autofluorescence and second harmonic-generated (SHG) signal intensities of glycated samples were significantly greater than those of the controls. Regarding the structural alteration of individual collagen fibers, a remarkable increase could be measured in fiber length of ribose-treated sections. Furthermore, nanoindentation of intertubular dentin regions revealed significantly higher stiffness in the ribose-treated samples, which points at a significant accumulation of AGEs. Thus, collagen glycation occurring during sustained exposure to reducing sugars leads to profound structural and mechanical changes in dentin. Besides the numerous oral complications associated with type 2 diabetes, the premature structural and mechanical deterioration of dentin may also play an important role in dental pathology.

Instrument for tensile testing of individual collagen fibrils with facile sample coupling and uncoupling

Collagen is the major structural protein in human bodies constituting about 30% of the entire protein mass. Through a self-assembly process, triple helical collagen molecules assemble into high aspect-ratio fibers of tens to hundreds of nanometer diameter, known as collagen fibrils (CFs). In the last decade, several methods for tensile testing these CFs emerged. However, these methods are either overly time-consuming or offer low data acquisition bandwidth, rendering dynamic investigation of tensile properties impossible. Here, we describe a novel instrument for tensile testing of individual CFs. CFs are furnished with magnetic beads using a custom magnetic tweezer. Subsequently, CFs are lifted by magnetic force, allowing them to be picked-up by a microgripper structure, which is mounted on a cantilever-based interferometric force probe. A piezo-lever actuator is used to apply tensile displacements and to perform tensile tests of tethered CFs, after alignment. Once the mechanical tests are finished, CFs are removed from the microgripper by application of a magnetic field. Our novel instrument enables tensile tests with at least 25-fold increased throughput compared to tensile testing with an atomic force microscope while achieving force resolution (p-p) of 10 nN at a strain resolution better than 0.1%.

Atomic Force Microscopy Nanoindentation Method on Collagen Fibrils

Atomic Force Microscopy nanoindentation method is a powerful technique that can be used for the nano-mechanical characterization of bio-samples. Significant scientific efforts have been performed during the last two decades to accurately determine the Young’s modulus of collagen fibrils at the nanoscale, as it has been proven that mechanical alterations of collagen are related to various pathological conditions. Different contact mechanics models have been proposed for processing the force–indentation data based on assumptions regarding the shape of the indenter and collagen fibrils and on the elastic or elastic–plastic contact assumption. However, the results reported in the literature do not always agree; for example, the Young’s modulus values for dry collagen fibrils expand from 0.9 to 11.5 GPa. The most significant parameters for the broad range of values are related to the heterogeneous structure of the fibrils, the water content within the fibrils, the data processing errors, and the uncertainties in the calibration of the probe. An extensive discussion regarding the models arising from contact mechanics and the results provided in the literature is presented, while new approaches with respect to future research are proposed.

Nano-elastic modulus of tendon measured directly in living mice

The nano-biomechanical environment of the extracellular matrix is critical for cells to sense and respond to mechanical loading. However, to date, this important characteristic remains poorly understood in living tissue structures. This study reports the experimental measurement of the in vivo nano-elastic modulus of the tendon in a mouse tail model. The experiment was performed on the tail tendon of an 8-week-old C57BL/6 live mouse. Mechanical loading on tail tendons was regulated by changing both voltage and frequency of alternating current stimulation on the erector spinae. The nano-elastic modulus of the tail tendon was measured by atomic force microscope. The nano-elastic modulus showed significant variation (2.19-35.70 MPa) between different locations and up to 39% decrease under muscle contraction, suggesting a complicated biomechanical environment in which cells dwell. In addition, the nano-elastic modulus of the tail tendon measured in live mice was significantly lower than that measured in vitro, suggesting a disagreement of tissue mechanical properties in vivo and in vitro. This information is important for the designs of new extracellular biomaterial that can better mimic the biological environment, and improve clinical outcomes of musculoskeletal tissue degenerations and associated disorders.

Nanomechanical mapping of single collagen fibrils under tension

At the most fundamental level, collagen fibrils are rope-like structures assembled from triple-helical collagen molecules. One key structural characteristic of the fibril is the 67 nm D-band pattern arising from the quarter-stagger packing of the molecules. Our current understanding of the structural changes induced by tensile loading of collagen fibrils comes mostly from atomistic molecular dynamics simulations and tissue level experiments. Tensile testing of individual fibrils is an upcoming field of investigation, and thus far structural analysis has always taken place after the fibrils have been ruptured or strained and subsequently dried. There is therefore a gap in understanding how the structure of collagen fibrils transforms under tension, and how this reorganization affects the functionality of collagen fibrils within tissues. In this study, atomic force microscopy based nanomechanical mapping is introduced to image hydrated collagen fibrils absorbed to an elastic substrate. Upon stretching the substrate between 5 and 30%, we observe a radial stiffening consistent with the fibrils being under tension. This is associated with an increase in D-band length. In addition the indentation modulus contrast associated with the D-band pattern increases linearly with D-band strain. These results provide direct confirmation of, and new information on the axially inhomogeneous structural response of collagen fibrils to applied tension as previously proposed on the basis of X-ray scattering experiments on stretched tissues. Furthermore our approach opens the road for studying the structural impacts of tension on cell-matrix interactions at the molecular level.

Collagen Fibrils: Nature's Highly Tunable Nonlinear Springs

Tissue hydration is well known to influence tissue mechanics and can be tuned via osmotic pressure. Collagen fibrils are nature's nanoscale building blocks to achieve biomechanical function in a broad range of biological tissues and across many species. Intrafibrillar covalent cross-links have long been thought to play a pivotal role in collagen fibril elasticity, but predominantly at large, far from physiological, strains. Performing nanotensile experiments of collagen fibrils at varying hydration levels by adjusting osmotic pressure in situ during atomic force microscopy experiments, we show the power the intrafibrillar noncovalent interactions have for defining collagen fibril tensile elasticity at low fibril strains. Nanomechanical tensile tests reveal that osmotic pressure increases collagen fibril stiffness up to 24-fold in transverse (nanoindentation) and up to 6-fold in the longitudinal direction (tension), compared to physiological saline in a reversible fashion. We attribute the stiffening to the density and strength of weak intermolecular forces tuned by hydration and hence collagen packing density. This reversible mechanism may be employed by cells to alter their mechanical microenvironment in a reversible manner. The mechanism could also be translated to tissue engineering approaches for customizing scaffold mechanics in spatially resolved fashion, and it may help explain local mechanical changes during development of diseases and inflammation.

In tendons, differing physiological requirements lead to functionally distinct nanostructures

The collagen-based tissues of animals are hierarchical structures: even tendon, the simplest collagenous tissue, has seven to eight levels of hierarchy. Tailoring tissue structure to match physiological function can occur at many different levels. We wanted to know if the control of tissue architecture to achieve function extends down to the nanoscale level of the individual, cable-like collagen fibrils. Using tendons from young adult bovine forelimbs, we performed stress-strain experiments on single collagen fibrils extracted from tendons with positional function, and tendons with energy storing function. Collagen fibrils from the two tendon types, which have known differences in intermolecular crosslinking, showed numerous differences in their responses to elongation. Unlike those from positional tendons, fibrils from energy storing tendons showed high strain stiffening and resistance to disruption in both molecular packing and conformation, helping to explain how these high stress tissues withstand millions of loading cycles with little reparative remodeling. Functional differences in load-bearing tissues are accompanied by important differences in nanoscale collagen fibril structure.

Low-level laser therapy (780 nm) combined with collagen sponge scaffold promotes repair of rat cranial critical-size defects and increases TGF-β, FGF-2, OPG/RANK and osteocalcin expression

The aim of this study was to evaluate the effect of collagen sponge scaffold (CSS) implantation associated with low-level laser therapy (LLLT) on repairing bone defects. A single 5-mm cranial defect was surgically created in forty Wistar rats, which then received one of the following four interventions (n = 10 per group): no treatment (G0); bone defect implanted with collagen sponge scaffold (CSS) alone (G1); defect treated with low-level laser therapy (LLLT) (wavelength 780 nm; total energy density 120 J/cm² ; power 50 mW) alone (G2); and CSS associated with LLLT treatment (G3). After surgery, animals in each group were euthanized at 21 days and 30 days (n = 5 per euthanasia time group). Bone formation was monitored by X-ray imaging analysis. Biopsies were collected and processed for histological analysis and immunohistochemical evaluation of transforming growth factor-beta (TGF-β), fibroblast growth factor-2 (FGF-2), osteoprotegerin (OPG) and receptor activator of nuclear factor ƙ (RANK). Osteocalcin (OCN) was detected by immunofluorescence analysis. Compared to the G0 group, defects in the 30-day G3 group exhibited increased bone formation, both by increase in radiopaque areas (P < 0.01) and by histomorphometric analysis (P < 0.001). The histopathological analysis showed a decreased number of inflammatory cells (P < 0.001). The combined CCS + LLLT (G3) treatment also resulted in the most intense immunostaining for OPG, RANK, FGF-2 and TGF-β, and the most intense and diffuse OCN immunofluorescent labelling at 30 days postsurgery (G3 vs. G0 group, P < 0.05). Therefore, the use of CCS associated with LLLT could offer a synergistic advantage in improving the healing of bone fractures.