Collagen fibrils from both positional and energy-storing tendons exhibit increased amounts of denatured collagen when stretched beyond the yield point

Region-dependent properties of lamellae constituents: A microscopic insight into intervertebral disc herniation mechanisms

Joint bending is associated with intervertebral disc (IVD) herniations. To better understand herniation disorders, the mechanics of annulus layers have been studied extensively. However, the properties and potential contributions of independent constituents (i.e., collagen fibres and the intra-lamellar matrix) to these larger-scale responses remain poorly understood but this knowledge could uncover molecular insights into herniation pathways. This study characterized the tensile properties of isolated collagen fibres and the adhesion properties of the intra-lamellar matrix in the posterior and anterior IVD regions. IVDs were extracted from eight porcine cervical spines. Single annulus layers were dissected from the anterior and posterior regions. From each layer, two separate samples were harvested: i) isolated collagen fibre and ii) two adjacent collagen fibres together with the matrix that connects them, totalling 32 samples. Once mounted, isolated fibres were longitudinally stretched while double fibre specimens were displaced with respect to each other. All preloaded specimens were strained at 1 % per second of the initial specimen length until failure occurred. From the stress-strain relationships, the Young's modulus along with stress and strain at yield and ultimate failure were determined. Within-specimen differences were evaluated with paired-tests and non-parametric Wilcoxon tests. In isolated collagen fibres, the Young's modulus and ultimate stress were 45 % and 51 % greater in the posterior region compared to the anterior region (p ≤ 0.047). All properties of intra-lamellar matrix adhesion were similar between regions (p ≥ 0.345). Interestingly, these constituents experienced comparable strains at yield (30 %) and failure (40 %), but the tensile strength of isolated fibres was approximately 5 times greater than the intra-lamellar matrix adhesion. This study demonstrated unique mechanical properties of annulus layer constituents. When incorporated into future models, these data could help discern the sequence of molecular damage leading to IVD herniations.

Piezoelectric nanofilms fabricated by coaxial electrospun polycaprolactone/Barium titanate promote Achilles tendon regeneration by reducing IL-17A/NF-κB-mediated inflammation

Tendon injuries are often exacerbated by persistent inflammation, which hampers tissue regeneration. In this study, we developed a noninvasive, wirelessly controlled, and self-powered piezoelectric nanofilm fabricated by coaxial electrospinning of polycaprolactone (PCL) and tetragonal barium titanate nanoparticles (BTO), and investigated its roles in modulating inflammation and repairing Achilles tendon defects as well as the mechanism in a rat model. In vitro study and in vivo study upon subcutaneous implantation showed that the piezoelectric PCL/BTO nanofilms could inhibit M1 macrophage polarization and reduce the secretion of inflammatory factors. Moreover, when bridging an Achilles tendon defect, the nanofilms could promote tenogenic gene expression including collagen deposition, and collagen remodeling, facilitate functional tendon recovery and significantly reduce tissue inflammation by suppressing M1 macrophage polarization and promoting M2 polarization. Moreover, the piezoelectric stimulation could also enhance tendon regeneration by inhibiting angiogenesis, reducing lipid deposition, and decreasing ectopic ossification. Mechanistically, the piezoelectric nanofilms reduced tissue inflammation mainly via inhibiting the nuclear factor (NF)-κB signaling pathway that is mediated by interleukin (IL)-17A secreted from CD3+ T cells, and thus to reduce proinflammatory factors, such as IL-1β and IL-6, inducible nitric oxide synthase, monocyte chemoattractant protein-1, and tumor necrosis factor-α. These findings indicate the potential of piezoelectric stimulation in immunomodulation, and in promoting tendon regeneration via IL-17A/NF-κB-mediated pathway.

Softening of elastic and viscoelastic properties is independent of overstretch rate in cerebral arteries

Collagenous soft tissues are frequently injured by supraphysiologic mechanical deformation, leading to measurable changes in both extra-cellular matrix (ECM) structure and mechanical properties. While each of these alterations has been well studied following quasi-static deformation, little is known about the influence of high strain rate. Previous investigations of high-rate ECM alterations found tropocollagen denaturation and fibrillar kinking to be rate dependent. Given these observations of rate dependence in microstructure alterations, the present work evaluated if the rate and magnitude of overstretch affect the baseline viscoelastic properties of porcine middle cerebral arteries (MCAs). Changes in tissue response were assessed using a series of harmonic oscillations before and after sub-failure overstretches across a large range of rates and magnitudes. We used collagen-hybridizing peptide (CHP) to evaluate the role of tropocollagen denaturation in mechanical softening. Experiments show that softening is dependent on overstretch magnitude but is independent of overstretch rate. We also note that softening progresses at the same rate for both equilibrium (quasi-static) and non-equilibrium (high-rate) properties. Finally, results suggest that tropocollagen denaturation is not the source of the observed sub-yield softening behavior. This study expands fundamental knowledge on the form-function relationship of constituents in collagen fibrils and clarifies material behavior following sub-failure overstretch across a range of strain rates.

Correlation of collagen damage and failure mechanics in porcine pia-arachnoid complex

Mild traumatic brain injury (mTBI) accounts for approximately 75% of all TBI cases, and the mechanisms are still poorly understood, in part due to limitations of current diagnostic tools. Yet, there is a critical need to detect the presence of mTBI to mitigate risk of further injury. In this study, we explore the potential of collagen hybridizing peptides (CHPs), which selectively bind to damaged collagen, to detect damage in the pia-arachnoid complex (PAC), a major load-transferring interface during head trauma. To generate damage, porcine PAC samples underwent peel tests. Peak force to failure and CHP fluorescence were measured in three regions of the brain at multiple post-mortem times. The peak force of PAC failure was region-specific, with increasing failure forces moving anterior to posterior (frontal: 20.91 ± 38.77 mN; parietal: 64.72 ± 33.31 mN; occipital: 86.68 ± 43.46 mN) and significantly different between frontal and occipital regions (p = 0.034). CHP fluorescence was significantly different between control and peeled PAC samples in mean pixel intensity (MPI; p = 0.031), median pixel intensity (MedPI; p = 0.009), and percent pixels above a defined threshold (PP; p = 0.014). Each of these CHP fluorescence metrics were significantly and positively correlated with peak force at failure (MPI: p = 0.049; MedPI: p = 0.026; PP: p = 0.002). These data suggest CHP is a viable solution to detecting the presence and severity of damage at the brain-skull interface, and may be a useful tool for quantifying damage in vivo.

From Collagen Mimetics to Collagen Hybridization and Back

Facilitated by the unique triple-helical protein structure, fibrous collagens, the principal proteins in animals, demonstrate a dual function of serving as building blocks for tissue scaffolds and as a bioactive material capable of swift renewal in response to environmental changes. While studies of triple-helical collagen mimetic peptides (CMPs) have been instrumental in understanding the molecular forces responsible for the folding and assembly of triple helices, as well as identifying bioactive regions of fibrous collagen molecules, single-strand CMPs that can specifically target and hybridize to denatured collagens (i.e., collagen hybridizing peptides, CHPs) have proven useful in identifying the remodeling activity of collagen-rich tissues related to development, homeostasis, and pathology. Efforts to improve the utility of CHPs have resulted in the development of new skeletal structures, such as dimeric and cyclic CHPs, as well as the incorporation of artificial amino acids, including fluorinated proline and N-substituted glycines (peptoid residues). In particular, dimeric CHPs were used to capture collagen fragments from biological fluid for biomarker study, and the introduction of peptoid-based collagen mimetics has sparked renewed interest in peptidomimetic research because peptoids enable a stable triple-helical structure and the presentation of an extensive array of side chain structures offering a versatile platform for the development of new collagen mimetics. This Account will cover the evolution of our research from CMPs as biomaterials to ongoing efforts in developing triple-helical peptides with practical theranostic potential in targeting denatured and damaged collagens. Our early efforts in functionalizing natural collagen scaffolds via noncovalent modifications led to the discovery of an entirely new use of CMPs. This discovery resulted in the development of CHPs that are now used by many different laboratories for the investigation of pathologies associated with changes in the structures of extracellular matrices including fibrosis, cancer, and mechanical damage to collagen-rich, load-bearing tissues. Here, we delve into the essential design features of CHPs contributing to their collagen binding properties and practical usage and explore the necessity for further mechanistic understanding of not only the binding processes (e.g., binding domain and stoichiometry of the hybridized complex) but also the biology of collagen degradation, from proteolytic digestion of fibrils to cellular processing of collagen fragments. We also discuss the strengths and weaknesses of peptoid-based triple-helical peptides as applied to collagen hybridization touching on thermodynamic and kinetic aspects of triple-helical folding. Finally, we highlight current limitations and future directions in the use of peptoid building blocks to develop bioactive collagen mimetics as new functional biomaterials.

A method for defining tissue injury criteria reveals that ligament deformation thresholds are multimodal

Soft tissue injuries (such as ligament, tendon, and meniscus tears) are the result of extracellular matrix damage from excessive tissue stretching. Deformation thresholds for soft tissues, however, remain largely unknown due to a lack of methods that can measure and compare the spatially heterogeneous damage and deformation that occurs in these materials. Here, we propose a full-field method for defining tissue injury criteria: multimodal strain limits for biological tissues analogous to yield criteria that exist for crystalline materials. Specifically, we developed a method for defining strain thresholds for mechanically-driven fibrillar collagen denaturation in soft tissues, using regional multimodal deformation and damage data. We established this new method using the murine medial collateral ligament (MCL) as our model tissue. Our findings revealed that multiple modes of deformation contribute to collagen denaturation in the murine MCL, contrary to the common assumption that collagen damage is driven only by strain in the direction of fibers. Remarkably, hydrostatic strain (computed here with an assumption of plane strain) was the best predictor of mechanically-driven collagen denaturation in ligament tissue, suggesting crosslink-mediated stress transfer plays a role in molecular damage accumulation. This work demonstrates that collagen denaturation can be driven by multiple modes of deformation and provides a method for defining deformation thresholds, or injury criteria, from spatially heterogeneous data. STATEMENT OF SIGNIFICANCE: Understanding the mechanics of soft tissue injuries is crucial for the development of new technology for injury detection, prevention, and treatment.  Yet, tissue-level deformation thresholds for injury are unknown, due to a lack of methods that combine full-field measurements of multimodal deformation and damage in mechanically loaded soft tissues. Here, we propose a method for defining tissue injury criteria: multimodal strain thresholds for biological tissues. Our findings reveal that multiple modes of deformation contribute to collagen denaturation, contrary to the common assumption that collagen damage is driven by strain in the fiber direction alone. The method will inform the development of new mechanics-based diagnostic imaging, improve computational modeling of injury, and be employed to study the role of tissue composition in injury susceptibility.

A method for defining tissue injury criteria reveals ligament deformation thresholds are multimodal

Soft tissue injuries (such as ligament, tendon, and meniscus tears) are the result of extracellular matrix damage from excessive tissue stretching. Deformation thresholds for soft tissues, however, remain largely unknown due to a lack of methods that can measure and compare the spatially heterogeneous damage and deformation that occurs in these materials. Here, we propose a method for defining tissue injury criteria : multimodal strain limits for biological tissues analogous to yield criteria that exist for crystalline materials. Specifically, we developed a method for defining injury criteria for mechanically-driven fibrillar collagen denaturation in soft tissues, using regional multimodal deformation and damage data. We established this new method using the murine medial collateral ligament (MCL) as our model tissue. Our findings revealed that multiple modes of deformation contribute to collagen denaturation in the murine MCL, contrary to the common assumption that collagen damage is driven by strain in the fiber direction alone. Remarkably, our results indicated that hydrostatic strain, or volumetric expansion, may be the best predictor of mechanically-driven collagen denaturation in ligament tissue, suggesting crosslink-mediated stress transfer plays a role in molecular damage accumulation. This work demonstrates that collagen denaturation can be driven by multiple modes of deformation and provides a method for defining deformation thresholds, or injury criteria, from spatially heterogeneous data.