Simple synthesis of soft, tough, and cytocompatible biohybrid composites.

Collagen is the most abundant component of mammalian extracellular matrices. As such, the development of materials that mimic the biological and mechanical properties of collagenous tissues is an enduring goal of the biomaterials community. Despite the development of molded and 3D printed collagen hydrogel platforms, their use as biomaterials and tissue engineering scaffolds is hindered by either low stiffness and toughness or processing complexity. Here, we demonstrate the development of stiff and tough biohybrid composites by combining collagen with a zwitterionic hydrogel through simple mixing. This combination led to the self-assembly of a nanostructured fibrillar network of collagen that was ionically linked to the surrounding zwitterionic hydrogel matrix, leading to a composite microstructure reminiscent of soft biological tissues. The addition of 5-15 mg mL-1 collagen and the formation of nanostructured fibrils increased the elastic modulus of the composite system by 40% compared to the base zwitterionic matrix. Most notably, the addition of collagen increased the fracture energy nearly 11-fold ([Formula: see text] 180 J m-2) and clearly delayed crack initiation and propagation. These composites exhibit elastic modulus ([Formula: see text] 0.180 MJ) and toughness ([Formula: see text]0.617 MJ m-3) approaching that of biological tissues such as articular cartilage. Maintenance of the fibrillar structure of collagen also greatly enhanced cytocompatibility, improving cell adhesion more than 100-fold with >90% cell viability.

Understanding the Influence of Local Physical Stimuli on Chondrocyte Behavior.

Investigating the mechanobiology of chondrocytes is challenging due to the complex micromechanical environment of cartilage tissue. The innate zonal differences and poroelastic properties of the tissue combined with its heterogeneous composition create spatial- and temporal-dependent cell behavior, which further complicates the investigation. Despite the numerous challenges, understanding the mechanobiology of chondrocytes is crucial for developing strategies for treating cartilage related diseases as chondrocytes are the only cell type within the tissue. The effort to understand chondrocyte behavior under various mechanical stimuli has been ongoing over the last 50 years. Early studies examined global biosynthetic behavior under unidirectional mechanical stimulus. With the technological development in high-speed confocal imaging techniques, recent studies have focused on investigating real-time individual and collective cell responses to multiple / combined modes of mechanical stimuli. Such efforts have led to tremendous advances in understanding the influence of local physical stimuli on chondrocyte behavior. In addition, we highlight the wide variety of experimental techniques, spanning from static to impact loading, and analysis techniques, from biochemical assays to machine learning, that have been utilized to study chondrocyte behavior. Finally, we review the progression of hypotheses about chondrocyte mechanobiology and provide a perspective on the future outlook of chondrocyte mechanobiology.

The role of SLRPs and large aggregating proteoglycans in collagen fibrillogenesis, extracellular matrix assembly, and mechanical function of fibrocartilage.

PURPOSE: Proteoglycans, especially small leucine rich proteoglycans (SLRPs), play major roles in facilitating the development and regulation of collagen fibers and other extracellular matrix components. However, their roles in fibrocartilage have not been widely reviewed. Here, we discuss both SLRP and large aggregating proteoglycan's roles in collagen fibrillogenesis and extracellular matrix assembly in fibrocartilage tissues such as the meniscus, annulus fibrosus (AF), and TMJ disc. We also discuss their expression levels throughout development, aging and degeneration, as well as repair. METHODS: A review of literature discussing proteoglycans and collagen fibrillogenesis in fibrocartilage was conducted and data from these manuscripts were analyzed and grouped to discuss trends throughout the tissue's architectural zones and developmental stage. RESULTS: The spatial collagen architecture of these fibrocartilaginous tissues is reflected in the distribution of proteoglycans expressed, suggesting that each proteoglycan plays an important role in the type of architecture presented and associated mechanical function. CONCLUSION: The unique structure-function relationship of fibrocartilage makes the varied architectures throughout the tissues imperative for their success and understanding the functions of these proteoglycans in developing and maintaining the fiber structure could inform future work in fibrocartilage replacement using tissue engineered constructs.

Boundary mode lubrication of articular cartilage with a biomimetic diblock copolymer.

We report the design of a diblock copolymer with architecture and function inspired by the lubricating glycoprotein lubricin. This diblock copolymer, synthesized by sequential reversible addition-fragmentation chain-transfer polymerization, consists of a cationic cartilage-binding domain and a brush-lubricating domain. It reduces the coefficient of friction of articular cartilage under boundary mode conditions (0.088 ± 0.039) to a level equivalent to that provided by lubricin (0.093 ± 0.011). Additionally, both the EC50 (0.404 mg/mL) and cartilage-binding time constant (7.19 min) of the polymer are comparable to purified human and recombinant lubricin. Like lubricin, the tribological properties of this polymer are dependent on molecular architecture. When the same monomer composition was evaluated either as an AB diblock copolymer or as a random copolymer, the diblock effectively lubricated cartilage under boundary mode conditions whereas the random copolymer did not. Additionally, the individual polymer blocks did not lubricate independently, and lubrication could be competitively inhibited with an excess of binding domain. This diblock copolymer is an example of a synthetic polymer with lubrication properties equal to lubricin under boundary mode conditions, suggesting its potential utility as a therapy for joint pathologies like osteoarthritis.

A Century of Cartilage Tribology Research Is Informing Lubrication Therapies.

Articular cartilage is one of the most unique materials found in nature. This tissue's ability to provide low friction and low wear over decades of constant use is not surpassed, as of yet, by any synthetic materials. Lubrication of the body's joints is essential to mammalian locomotion, but breakdown and degeneration of cartilage is the leading cause of severe disability in the industrialized world. In this paper, we review how theories of cartilage lubrication have evolved over the past decades and connect how theories of cartilage lubrication have been translated to lubrication-based therapies. Here, we call upon these historical perspectives and highlight the open questions in cartilage lubrication research. Additionally, these open questions within the field's understanding of natural lubrication mechanisms reveal strategic directions for lubrication therapy.

Inflammatory and Noninflammatory Synovial Fluids Exhibit New and Distinct Tribological Endotypes.

Inferior synovial lubrication is a hallmark of osteoarthritis (OA), and synovial fluid (SF) lubrication and composition are variable among OA patients. Hyaluronic acid (HA) viscosupplementation is a widely used therapy for improving SF viscoelasticity and lubrication, but it is unclear how the effectiveness of HA viscosupplements varies with arthritic endotype. The objective of this study was to investigate the effects of the HA viscosupplement, Hymovis®, on the lubricating properties of diseased SF from patients with noninflammatory OA and inflammatory arthritis (IA). The composition (cytokine, HA, and lubricin concentrations) of the SF was measured as well as the mechanical properties (rheology, tribology) of the SF alone and in a 1:1 mixture with the HA viscosupplement. Using rotational rheometry, no difference in SF viscosity was detected between disease types, and the addition of HA significantly increased all fluids' viscosities. In noninflammatory OA SF, friction coefficients followed a typical Stribeck pattern, and their magnitude was decreased by the addition of HA. While some of the IA SF also showed typical Stribeck behavior, a subset showed more erratic behavior with highly variable and larger friction coefficients. Interestingly, this aberrant behavior was not eliminated by the addition of HA, and it was associated with low concentrations of lubricin. Aberrant SF exhibited significantly lower effective viscosities compared to noninflammatory OA and IA SF with typical tribological behavior. Collectively, these results suggest that different endotypes of arthritis exist with respect to lubrication, which may impact the effectiveness of HA viscosupplements in reducing friction.

Matrix stiffening promotes a tumor vasculature phenotype.

Tumor microvasculature tends to be malformed, more permeable, and more tortuous than vessels in healthy tissue, effects that have been largely attributed to up-regulated VEGF expression. However, tumor tissue tends to stiffen during solid tumor progression, and tissue stiffness is known to alter cell behaviors including proliferation, migration, and cell-cell adhesion, which are all requisite for angiogenesis. Using in vitro, in vivo, and ex ovo models, we investigated the effects of matrix stiffness on vessel growth and integrity during angiogenesis. Our data indicate that angiogenic outgrowth, invasion, and neovessel branching increase with matrix cross-linking. These effects are caused by increased matrix stiffness independent of matrix density, because increased matrix density results in decreased angiogenesis. Notably, matrix stiffness up-regulates matrix metalloproteinase (MMP) activity, and inhibiting MMPs significantly reduces angiogenic outgrowth in stiffer cross-linked gels. To investigate the functional significance of altered endothelial cell behavior in response to matrix stiffness, we measured endothelial cell barrier function on substrates mimicking the stiffness of healthy and tumor tissue. Our data indicate that barrier function is impaired and the localization of vascular endothelial cadherin is altered as function of matrix stiffness. These results demonstrate that matrix stiffness, separately from matrix density, can alter vascular growth and integrity, mimicking the changes that exist in tumor vasculature. These data suggest that therapeutically targeting tumor stiffness or the endothelial cell response to tumor stiffening may help restore vessel structure, minimize metastasis, and aid in drug delivery.

Stable recombinant production of codon-scrambled lubricin and mucin in human cells.

Widespread therapeutic and commercial interest in recombinant mucin technology has emerged due to the unique ability of mucin glycoproteins to hydrate, protect, and lubricate biological surfaces. However, recombinant production of the large, highly repetitive domains that are characteristic of mucins remains a challenge in biomanufacturing likely due, at least in part, to the inherent instability of DNA repeats in the cellular genome. To overcome this challenge, we exploit codon redundancy to encode desired mucin polypeptides with minimal nucleotide repetition. The codon-scrambling strategy was applied to generate synonymous genes, or "synDNAs," for two mucins of commercial interest: lubricin and mucin 1. Stable, long-term recombinant production in suspension-adapted human 293-F cells was demonstrated for the synonymous lubricin complementary DNA (cDNA), which we refer to as SynLubricin. Under optimal conditions, a 293-F subpopulation produced recombinant SynLubricin at more than 200 mg/L of media and was stable throughout 2 months of continuous culture. Functionality tests confirmed that the recombinant lubricin could effectively inhibit cell adhesion and lubricate cartilage explants. Together, our work provides a viable workflow for cDNA design and stable mucin production in mammalian host production systems.

Dynamics of Synovial Fluid Aggregation under Shear.

The synovial fluid (SF) that lubricates articular joints exhibits complex rheological and tribological properties due to the interactions and behaviors of its various molecular components. Under shear, SF films abruptly thicken by more than 300% and large, dense aggregates form within the fluid. In this study, we used the Surface Force Apparatus to elucidate which SF components are involved in this shear-induced transformation by (i) determining which (if any) of all major SF components replicate the behavior of SF under shear and (ii) observing the effect of removing implicated components from SF by enzymatic digestion. While most previous studies of SF have focused on the tribological roles of lubricin or hyaluronic acid, our results indicate that albumin is a key contributor to the formation of aggregates in SF under shear. Our results also suggest that SF aggregation is associated with efficient surface protection against wear. As our findings are based on experiments involving rigid, nonporous surfaces, they may be used to investigate shear-mediated aggregation mechanisms occurring during the lubrication of artificial joints, ultimately advancing our current vision of implant design.

The Effect of Charge and Mechanical Loading on Antibody Diffusion Through the Articular Surface of Cartilage.

Molecular transport of osteoarthritis (OA) therapeutics within articular cartilage is influenced by many factors, such as solute charge, that have yet to be fully understood. This study characterizes how solute charge influences local diffusion and convective transport of antibodies within the heterogeneous cartilage matrix. Three fluorescently tagged solutes of varying isoelectric point (pI) (4.7-5.9) were tested in either cyclic or passive cartilage loading conditions. In each case, local diffusivities were calculated based on local fluorescence in the cartilage sample, as observed by confocal microscopy. In agreement with past research, local solute diffusivities within the heterogeneous cartilage matrix were highest around 200-275 µm from the articular surface, but 3-4 times lower at the articular surface and in the deeper zones of the tissue. Transport of all 150 kDa solutes was significantly increased by the application of mechanical loading at 1 Hz, but local transport enhancement was not significantly affected by changes in solute isoelectric point. More positively charged solutes (higher pI) had significantly higher local diffusivities 200-275 µm from the tissue surface, but no other differences were observed. This implies that there are certain regions of cartilage that are more sensitive to changes in solute charge than others, which could be useful for future development of OA therapeutics.

Stribeck Curve Analysis of Temporomandibular Joint Condylar Cartilage and Disc.

Temporomandibular joint (TMJ) diseases such as osteoarthritis and disc displacement have no permanent treatment options, but lubrication therapies, used in other joints, could be an effective alternative. However, the healthy TMJ contains fibrocartilage, not hyaline cartilage as is found in other joints. As such, the effect of lubrication therapies in the TMJ is unknown. Additionally, only a few studies have characterized the friction coefficient of the healthy TMJ. Like other cartilaginous tissues, the mandibular condyles and discs are subject to changes in friction coefficient due to fluid pressurization. In addition, the friction coefficients of the inferior joint space of the TMJ are affected by the sliding direction and anatomic location. However, these previous findings have not been able to identify how all three of these parameters (anatomic location, sliding direction, and fluid pressurization) influence changes in friction coefficient. This study used Stribeck curves to identify differences in the friction coefficients of mandibular condyles and discs based on anatomic location, sliding direction, and amount of fluid pressurization (friction mode). Friction coefficients were measured using a cartilage on glass tribometer. Both mandibular condyle and disc friction coefficients were well described by Stribeck curves (R2 range 0.87-0.97; p < 0.0001). These curves changed based on anatomic location (Δµ ∼ 0.05), but very few differences in friction coefficients were observed based on sliding direction. Mandibular condyles had similar boundary mode and elastoviscous mode friction coefficients to the TMJ disc (µmin ∼ 0.009 to 0.19) and both were lower than hyaline cartilage in other joints (e.g., knee, ankle, etc.). The observed differences here indicate that the surface characteristics of each anatomic region cause differences in friction coefficients.

Tissue Engineering Auricular Cartilage Using Late Passage Human Auricular Chondrocytes.

PURPOSE: The significant shortcomings associated with current autologous reconstructive options for auricular deformities have inspired great interest in a tissue engineering solution. A major obstacle in the engineering of human auricular cartilage is the availability of sufficient autologous human chondrocytes. A clinically obtainable amount of auricular cartilage tissue (ie, 1 g) only yields approximately 10 million cells, where 25 times this amount is needed for the fabrication of a full-scale pediatric ear. It is thought that repeated passaging of chondrocytes leads to dedifferentiation and loss of the chondrogenic potential. However, little to no data exist regarding the ideal number of times that human auricular chondrocytes (HAuCs) can be passaged in a manner that maximizes the cellular expansion while minimizing dedifferentiation. METHODS: Human auricular chondrocytes were isolated from discarded otoplasty specimens. The HAuCs were then expanded, and cells from passages 3, 4, and 5 were encapsulated into discs 8 mm in diameter made from type I collagen hydrogels with a cell density of 25 million cells/mL. The constructs were implanted subcutaneously in the dorsa of nude mice and harvested after 1 and 3 months for analysis. RESULTS: Constructs containing passages 3, 4, and 5 chondrocytes all maintained their original cylindrical geometry. After 3 months in vivo, the diameters of the P3, P4, and P5 discs were 69 ± 9%, 67 ± 10%, and 73 ± 15% of their initial diameter, respectively. Regardless of the passage number, all constructs developed a glossy white cartilaginous appearance, similar to native auricular cartilage. Histologic analysis demonstrated development of an organized perichondrium composed of collagen, a rich proteoglycan matrix, cellular lacunae, and a dense elastin fibrin network by Safranin-O and Verhoeff stain. Biochemical analysis confirmed similar amounts of proteoglycan and hydroxyproline content in late passage constructs when compared with native auricular cartilage. CONCLUSIONS: These data indicate that late passage HAuCs (up to passage 5) form elastic cartilage that is histologically, biochemically, and biomechanically similar to native human elastic cartilage and have the potential to be used for auricular cartilage engineering.

Fiber development and matrix production in tissue-engineered menisci using bovine mesenchymal stem cells and fibrochondrocytes.

Mesenchymal stem cells (MSCs) have been investigated with promising results for meniscus healing and tissue engineering. While MSCs are known to contribute to extracellular matrix (ECM) production, less is known about how MSCs produce and align large organized fibers for application to tissue engineering the meniscus. The goal of this study was to investigate the capability of MSCs to produce and organize ECM molecules compared to meniscal fibrochondrocytes (FCCs). Bovine FCCs and MSCs were encapsulated in an anatomically accurate collagen meniscus using monoculture and co-culture of each cell type. Each meniscus was mechanically anchored at the horns to mimic the physiological fixation by the meniscal entheses. Mechanical fixation generates a static mechanical boundary condition previously shown to induce formation of oriented fiber by FCCs. Samples were cultured for 4 weeks and then evaluated for biochemical composition and fiber development. MSCs increased the glycosaminoglycan (GAG) and collagen production in both co-culture and monoculture groups compared to FCC monoculture. Collagen organization was greatest in the FCC monoculture group. While MSCs had increased matrix production, they lacked the fiber organization capabilities of FCCs. This study suggests that GAG production and fiber formation are linked. Co-culture can be used as a means of balancing the synthetic properties of MSCs and the matrix remodeling capabilities of FCCs for tissue engineering applications.

Degenerative changes of the canine cervical spine after discectomy procedures, an in vivo study.

BACKGROUND: Discectomies are a common surgical treatment for disc herniations in the canine spine. However, the effect of these procedures on intervertebral disc tissue is not fully understood. The objective of this study was to assess degenerative changes of cervical spinal segments undergoing discectomy procedures, in vivo. RESULTS: Discectomies led to a 60% drop in disc height and 24% drop in foraminal height. Segments did not fuse but showed osteophyte formation as well as endplate sclerosis. MR imaging revealed terminal degenerative changes with collapse of the disc space and loss of T2 signal intensity. The endplates showed degenerative type II Modic changes. Quantitative MR imaging revealed that over 95% of Nucleus Pulposus tissue was extracted and that the nuclear as well as overall disc hydration significantly decreased. Histology confirmed terminal degenerative changes with loss of NP tissue, loss of Annulus Fibrosus organization and loss of cartilage endplate tissue. The bony endplate displayed sclerotic changes. CONCLUSION: Discectomies lead to terminal degenerative changes. Therefore, these procedures should be indicated with caution specifically when performed for prophylactic purposes.

Cyclic Mechanical Loading Enhances Transport of Antibodies Into Articular Cartilage.

The goal of this study was to characterize antibody penetration through cartilage tissue under mechanical loading. Mechanical stimulation aids in the penetration of some proteins, but this effect has not characterized molecules such as antibodies (>100 kDa), which may hold some clinical value for treating osteoarthritis (OA). For each experiment, fresh articular cartilage plugs were obtained and exposed to fluorescently labeled antibodies while under cyclic mechanical load in unconfined compression for several hours. Penetration of these antibodies was quantified using confocal microscopy, and finite element (FE) simulations were conducted to predict fluid flow patterns within loaded samples. Transport enhancement followed a linear trend with strain amplitude (0.25-5%) and a nonlinear trend with frequency (0.25-2.60 Hz), with maximum enhancement found to be at 5% cyclic strain and 1 Hz, respectively. Regions of highest enhancement of transport within the tissue were associated with the regions of highest interstitial fluid velocity, as predicted from finite-element simulations. Overall, cyclic compression-enhanced antibody transport by twofold to threefold. To our knowledge, this is the first study to test how mechanical stimulation affects the diffusion of antibodies in cartilage and suggest further study into other important factors regarding macromolecular transport.

Multiscale Strain as a Predictor of Impact-Induced Fissuring in Articular Cartilage.

Mechanical damage is central to both initiation and progression of osteoarthritis (OA). However, specific causal links between mechanics and cartilage damage are incompletely understood, which results in an inability to predict failure. The lack of understanding is primarily due to the difficulty in simultaneously resolving the high rates and small length scales relevant to the problem and in correlating such measurements to the resulting fissures. This study leveraged microscopy and high-speed imaging to resolve mechanics on the previously unexamined time and length scales of interest in cartilage damage, and used those mechanics to develop predictive models. The specific objectives of this study were to: first, quantify bulk and local mechanics during impact-induced fissuring; second, develop predictive models of fissuring based on bulk mechanics and local strain; and third, evaluate the accuracy of these models in predicting fissures. To achieve these three objectives, bovine tibial cartilage was impacted using a custom spring-loaded device mounted on an inverted microscope. The occurrence of fissures was modulated by varying impact energy. For the first objective, during impact, deformation was captured at 10,000 frames per second and bulk and local mechanics were analyzed. For the second objective, data from samples impacted with a 1.2 mm diameter rod were fit to logistic regression functions, creating models of fissure probability based on bulk and local mechanics. Finally, for the third objective, data from samples impacted with a 0.8 mm diameter rod were used to test the accuracy of model predictions. This study provides a direct comparison between bulk and local mechanical thresholds for the prediction of fissures in cartilage samples, and demonstrates that local mechanics provide more accurate predictions of local failure than bulk mechanics provide. Bulk mechanics were accurate predictors of fissure for the entire sample cohort, but poor predictors of fissure for individual samples. Local strain fields were highly heterogeneous and significant differences were determined between fissured and intact samples, indicating the presence of damage thresholds. In particular, first principal strain rate and maximum shear strain were the best predictors of local failure, as determined by concordance statistics. These data provide an important step in establishing causal links between local mechanics and cartilage damage; ultimately, data such as these can be used to link macro- and micro-scale mechanics and thereby predict mechanically mediated disease on a subject-specific basis.

The Effect of Antibody Size and Mechanical Loading on Solute Diffusion Through the Articular Surface of Cartilage.

Because of the heterogeneous nature of articular cartilage tissue, penetration of potential therapeutic molecules for osteoarthritis (OA) through the articular surface (AS) is complex, with many factors that affect transport of these solutes within the tissue. Therefore, the goal of this study is to investigate how the size of antibody (Ab) variants, as well as application of cyclic mechanical loading, affects solute transport within healthy cartilage tissue. Penetration of fluorescently tagged solutes was quantified using confocal microscopy. For all the solutes tested, fluorescence curves were obtained through the articular surface. On average, diffusivities for the solutes of sizes 200 kDa, 150 kDa, 50 kDa, and 25 kDa were 3.3, 3.4, 5.1, and 6.0 µm2/s from 0 to 100 µm from the articular surface. Diffusivities went up to a maximum of 16.5, 18.5, 20.5, and 23.4 µm2/s for the 200 kDa, 150 kDa, 50 kDa, and 25 kDa molecules, respectively, from 225 to 325 µm from the surface. Overall, the effect of loading was very significant, with maximal transport enhancement for each solute ranging from 2.2 to 3.4-fold near 275 µm. Ultimately, solutes of this size do not diffuse uniformly nor are convected uniformly, through the depth of the cartilage tissue. This research potentially holds great clinical significance to discover ways of further optimizing transport into cartilage and leads to effective antibody-based treatments for OA.

Fibronectin mediates enhanced wear protection of lubricin during shear.

Fibronectin (FN) is a glycoprotein found in the superficial zone of cartilage; however, its role in the lubrication and the wear protection of articular joints is unknown. In this work, we have investigated the molecular interactions between FN and various components of the synovial fluid such as lubricin (LUB), hyaluronan (HA), and serum albumin (SA), which are all believed to contribute to joint lubrication. Using a Surface Forces Apparatus, we have measured the normal (adhesion/repulsion) and lateral (friction) forces across layers of individual synovial fluid components physisorbed onto FN-coated mica substrates. Our chief findings are (i) FN strongly tethers LUB and HA to mica, as indicated by high and reversible long-range repulsive normal interactions between surfaces, and (ii) FN and LUB synergistically enhance wear protection of surfaces during shear, as suggested by the structural robustness of FN+LUB layers under pressures up to about 4 MPa. These findings provide new insights into the role of FN in the lubricating properties of synovial fluid components sheared between ideal substrates and represent a significant step forward in our understanding of cartilage damage involved in diseases such as osteoarthritis.

Structure-function relations and rigidity percolation in the shear properties of articular cartilage.

Among mammalian soft tissues, articular cartilage is particularly interesting because it can endure a lifetime of daily mechanical loading despite having minimal regenerative capacity. This remarkable resilience may be due to the depth-dependent mechanical properties, which have been shown to localize strain and energy dissipation. This paradigm proposes that these properties arise from the depth-dependent collagen fiber orientation. Nevertheless, this structure-function relationship has not yet been quantified. Here, we use confocal elastography, quantitative polarized light microscopy, and Fourier-transform infrared imaging to make same-sample measurements of the depth-dependent shear modulus, collagen fiber organization, and extracellular matrix concentration in neonatal bovine articular cartilage. We find weak correlations between the shear modulus |G(∗)| and both the collagen fiber orientation and polarization. We find a much stronger correlation between |G(∗)| and the concentration of collagen fibers. Interestingly, very small changes in collagen volume fraction vc lead to orders-of-magnitude changes in the modulus with |G(∗)| scaling as (vc - v0)(ξ). Such dependencies are observed in the rheology of other biopolymer networks whose structure exhibits rigidity percolation phase transitions. Along these lines, we propose that the collagen network in articular cartilage is near a percolation threshold that gives rise to these large mechanical variations and localization of strain at the tissue's surface.

Tissue-engineered intervertebral discs produce new matrix, maintain disc height, and restore biomechanical function to the rodent spine.

Lower back and neck pain are leading physical conditions for which patients see their doctors in the United States. The organ commonly implicated in this condition is the intervertebral disc (IVD), which frequently herniates, ruptures, or tears, often causing pain and limiting spinal mobility. To date, approaches for replacement of diseased IVD have been confined to purely mechanical devices designed to either eliminate or enable flexibility of the diseased motion segment. Here we present the evaluation of a living, tissue-engineered IVD composed of a gelatinous nucleus pulposus surrounded by an aligned collagenous annulus fibrosus in the caudal spine of athymic rats for up to 6 mo. When implanted into the rat caudal spine, tissue-engineered IVD maintained disc space height, produced de novo extracellular matrix, and integrated into the spine, yielding an intact motion segment with dynamic mechanical properties similar to that of native IVD. These studies demonstrate the feasibility of engineering a functional spinal motion segment and represent a critical step in developing biological therapies for degenerative disc disease.