Elastic and osmotic properties of articular cartilage

Scientific Reports, 2020

Articular cartilage is a load-bearing tissue found in animal and human joints. it is a composite gel-like material in which a fibrous collagen network encapsulates large proteoglycan assemblies that imbibe fluid and "inflate" the network. Here we describe a composite hydrogel consisting of a cross-linked polyvinyl alcohol matrix filled with poly(acrylic acid) microparticles that mimics functional properties and biomechanical behavior of cartilage. the swelling and mechanical behaviors of this biomimetic model system are strikingly similar to that of human cartilage. the development of synthetic composite gel-based articular cartilage analog suggests new avenues to explore material properties, and their change in disease and degeneration, as well as novel strategies for developing composite tissueengineered cartilage constructs for regenerative medicine applications. This manuscript describes a predictive composite hydrogel model of cartilage. A novel aspect of the model is the incorporation of the pre-stress, which has been largely overlooked in previous tissue models. Pre-stress is present even in the absence of external loading, and it defines the load-bearing ability of cartilage. We demonstrate that the swelling and mechanical behavior of our biomimetic model system reproduces remarkably well the behavior of healthy and osteoarthritic human cartilage. From a biological perspective, articular cartilage is a thin connective tissue (2 to 4 mm thick) that caps the ends of bones at joints. Cartilage extracellular matrix (ECM) consists mainly of proteoglycans (PGs), collagens, water, and ions. The most abundant macromolecule in cartilage ECM is Type II collagen, which is organized into fibrils that form a meshwork at larger length scales. The major PG in cartilage is the bottlebrush-shaped aggrecan that "inflates" the collagen matrix . Intracellularly synthesized aggrecan molecules are secreted into the ECM, where they aggregate to form a secondary bottlebrush with hyaluronic acid (HA) stabilized by a link protein . The osmotic swelling pressure of the aggrecan-HA assemblies inflates the surrounding collagen matrix (Fig. ). At swelling equilibrium, the tissue is prestressed, conferring unique mechanical properties on cartilage. At equilibrium, although the total swelling pressure is zero, the collagen matrix is in tension. In diseases, such as osteoarthritis, the lubricating and load-bearing ability is reduced, however, the mechanisms that lead to loss of cartilage's key biomechanical properties are still poorly understood. From a polymer science perspective, cartilage is a composite load-bearing gel, consisting of a relatively stiff, fibrous collagen network that encapsulates large, negatively charged (anionic) proteoglycan (PG) polymer assemblies that exhibit a hierarchical bottlebrush organization. These PG polyelectrolytes draw water in osmotically, causing this phase to swell against the collagen network phase that confines them, increasing the overall stiffness of the cartilage tissue. Under a compressive loading, fluid is expelled from the tissue; after unloading it recovers its original shape and volume. The resistance of cartilage to such deformation and volume changes defines its load-bearing capacity. In degenerative joint diseases (e.g., osteoarthritis) cartilage structure is progressively damaged. Changes in the stiffness of the collagen matrix and/or loss of PGs strongly affect the biomechanical properties (e.g., load-bearing capacity) of the tissue 9,10 . Although a qualitative picture of cartilage behavior being explained by the interplay between the swelling of the PGs and the restraint imposed by the collagen network was first proposed by Ogston 1,2 , a deeper polymer chemistry and physics-based understanding of the determinants of cartilage's key functional properties, such as its load-bearing ability, has eluded adequate description.

Journal of the Mechanical Behavior of Biomedical Materials, 2016

The present study aims to discover the contribution of glycosaminoglycans (GAGs) and collagen fibers to the mechanical properties of the osteoarthritic (OA) cartilage tissue. We used nanoindentation experiments to understand the mechanical behavior of mild and severe osteoarthritic cartilage at micro-and nano-scale at different swelling conditions. Contrast enhanced micro-computed tomography (EPIC-μCT) was used to confirm that mild OA specimens had significantly higher GAGs content compared to severe OA specimens. In micro-scale, the semi-equilibrium modulus of mild OA specimens significantly dropped after immersion in a hypertonic solution and at nano-scale, the histograms of the measured elastic modulus revealed three to four components. Comparing the peaks with those observed for healthy cartilage in a previous study indicated that the first and third peaks represent the mechanical properties of GAGs and the collagen network. The third peak shows considerably stiffer elastic modulus for mild OA samples as compared to the severe OA samples in isotonic conditions. Furthermore, this peak clearly dropped when the tonicity increased, indicating the loss of collagen (pre-) stress in the shrunk specimen. Our observations support the association of the third peak with the collagen network. However, our results did not provide any direct evidence to support the association of the first peak with GAGs. For severe OA specimens, the peak associated with the collagen network did not drop when the tonicity increased, indicating a change in the response of OA cartilage to hypertonicity, likely collagen damage, as the disease progresses to its latest stages.

Annals of the Rheumatic Diseases, 1984

PLoS ONE, 2014

There is a need for materials that are well suited for cartilage tissue engineering. Hydrogels have emerged as promising biomaterials for cartilage repair, since, like cartilage, they have high water content, and they allow cells to be encapsulated within the material in a genuinely three-dimensional microenvironment. In this study, we investigated the mechanical properties of tissue-engineered cartilage constructs using in vitro culture models incorporating human chondrocytes from osteoarthritis patients. We evaluated hydrogels formed from mixtures of photocrosslinkable gelatin-methacrylamide (Gel-MA) and varying concentrations (0-2%) of hyaluronic acid methacrylate (HA-MA). Initially, only small differences in the stiffness of each hydrogel existed. After 4 weeks of culture, and to a greater extent 8 weeks of culture, HA-MA had striking and concentration dependent impact on the changes in mechanical properties. For example, the initial compressive moduli of cell-laden constructs with 0 and 1% HA-MA were 29 and 41 kPa, respectively. After 8 weeks of culture, the moduli of these constructs had increased to 66 and 147 kPa respectively, representing a net improvement of 69 kPa for gels with 1% HA-MA. Similarly the equilibrium modulus, dynamic modulus, failure strength and failure strain were all improved in constructs containing HA-MA. Differences in mechanical properties did not correlate with glycosaminoglycan content, which did not vary greatly between groups, yet there were clear differences in aggrecan intensity and distribution as assessed using immunostaining. Based on the functional development with time in culture using human chondrocytes, mixtures of Gel-MA and HA-MA are promising candidates for cartilage tissueengineering applications.

2016

SUMMARY The finding of other investigators that increased water content is often associated with signs of a torn collagen network in human osteoarthritic (OA) cartilage led to this study. In the Pond-Nuki model of post-traumatic OA experimental but not control femoral condylar cartilage showed evidence of breakdown and stiffening of collagen network as assessed by measurement of swelling properties and indentation behaviour respectively. These changes in the unstable knees occurred despite lack of erosion of that surface cartilage ascertained from carbon black mapping and history. The stiffening rather than softening change was therefore attributed to cartilage oedema of the middle and deep certilagenous zones, wherein breakdown of collagen network has been postulated to occur. Because of insignificant reduction of total hexuronate in these cartilages, a proteoglycan (PG) profile of sedimentation coefficients for aggregate (PGA) and subunit species (PGS) was analysed to see if colla...

Journal of Biomechanics, 2013

The tensile modulus of articular cartilage is much larger than its compressive modulus. This tensioncompression nonlinearity enhances interstitial fluid pressurization and decreases the frictional coefficient. The current set of studies examines the tensile and compressive properties of cylindrical chondrocyte-seeded agarose constructs over different developmental stages through a novel method that combines osmotic loading, video microscopy, and uniaxial unconfined compression testing. This method was previously used to examine tension-compression nonlinearity in native cartilage. Engineered cartilage, cultured under free-swelling (FS) or dynamically loaded (DL) conditions, was tested in unconfined compression in hypertonic and hypotonic salt solutions. The apparent equilibrium modulus decreased with increasing salt concentration, indicating that increasing the bath solution osmolarity shielded the fixed charges within the tissue, shifting the measured moduli along the tensioncompression curve and revealing the intrinsic properties of the tissue. With this method, we were able to measure the tensile (401 783 kPa for FS and 6787 473 kPa for DL) and compressive (161 7 33 kPa for FS and 348 7 203 kPa for DL) moduli of the same engineered cartilage specimens. These moduli are comparable to values obtained from traditional methods, validating this technique for measuring the tensile and compressive properties of hydrogel-based constructs. This study shows that engineered cartilage exhibits tension-compression nonlinearity reminiscent of the native tissue, and that dynamic deformational loading can yield significantly higher tensile properties.

MRS Proceedings, 2014

ABSTRACTCartilage is a complex biological tissue that exhibits gel-like behavior. Its primary biological function is providing compressive resistance to external loading and nearly frictionless lubrication of joints. In this study, we model cartilage extracellular matrix using a biomimetic system. We demonstrate that poly(vinyl) alcohol (PVA) hydrogels are robust biomaterials exhibiting mechanical and swelling properties similar to that of cartilage extracellular matrix. A comparison is made between the macroscopic behavior of PVA gels and literature data reported for cartilage.

Journal of Biomechanics, 2008

used to describe cartilage contact variables. This study provides novel data for the mechanical properties of normal and osteoarthritic human articular cartilage and enhances our ability to model this tissue using simple isotropic hyperelastic materials.

Journal of The Mechanical Behavior of Biomedical Materials

Fibrillar interconnectivity Degeneration Destructuring A B S T R A C T Articular cartilage functions as a load-bearing tissue by virtue of a functional coupling between its hydrated proteoglycan component and its zonally differentiated fibrillar network. How degeneration influences this relationship at the macro-, micro-, and ultrastructural levels is investigated in this study. Healthy bovine patellae (N = 9) and patellae exhibiting varying degrees of degeneration (N = 16) formed the basis of the study. Cartilage-on-bone blocks obtained from each patella were subjected to creep loading under a nominal stress of 4.5 MPa via a rectangular planar indenter which incorporated a narrow channel relief space to create a defined region where the cartilage would not be directly loaded. Following the attainment of creep equilibrium each sample was chemically

Tissue Engineering, 2006

Objective: To generate a cartilage biomaterial using a suspension culture with biophysical properties similar to native articular cartilage. Design: A novel cartilage tissue equivalent (CTE) using a no-scaffold, high-density suspension culture of neonatal porcine chondrocytes was formed on poly 2-hydroxyethyl methacrylate-treated plates for up to 16 weeks. Equilibrium aggregate modulus and hydraulic permeability were measured at 8 and 16 weeks using confined compression stress relaxation experiments. The CTE proteoglycan composition was characterized using sodium and T 1q magnetic resonance imaging methods after 8 weeks. Results: The resultant CTE produces a biomaterial consistent with a hyaline cartilage phenotype in appearance and expression of type II collagen and aggrecan. The equilibrium aggregate modulus and permeability for the 8-week specimens were 41.6 (standard deviation (SD) 4.3) kPa and 2.85 À13 (SD 2.45 À13) m 4 /Ns, respectively, and, for the 16-week specimens, 35.2 (SD 7.6) kPa and 2.67 À13 (SD 1.06 À13) m 4 /Ns, respectively. Average sodium concentration of the 8-week CTE ranged from 260 to 278 mM and average T 1q relaxation times from 105 to 107 ms, indicating proteoglycan content similar to that of native articular cartilage. Conclusion: The high-density culture method produced a CTE with characteristics that approach those of native articular cartilage. The CTE mechanical properties are similar to those of the native cartilage. The CTE developed in this study represents a promising methodological advancement in cartilage tissue engineering and cartilage repair.