Dependence of nanoscale friction and adhesion properties of articular cartilage on contact load

Particulate ECM biomaterial ink is 3D printed and naturally crosslinked to form structurally-layered and lubricated cartilage tissue mimics

Articular cartilage is a layered tissue with a complex, heterogenous structure and lubricated surface which is challenging to reproduce using traditional tissue engineering methods. 3D printing techniques have enabled engineering of complex scaffolds for cartilage regeneration, but constructs fail to replicate the unique zonal layers, and limited cytocompatible crosslinkers exist. To address the need for mechanically robust, layered scaffolds, we developed an extracellular matrix particle-based biomaterial ink (pECM biomaterial ink) which can be extruded, polymerizes via disulfide bonding, and restores surface lubrication. Our cartilage pECM biomaterial ink utilizes functionalized hyaluronan, a naturally occurring glycosaminoglycan, crosslinked directly to decellularized tissue particles (ø 40-100 µm). We experimentally determined that hyaluronan functionalized with thiol groups (t-HA) forms disulfide bonds with the ECM particles to form a 3D network. We show that two inks can be co-printed to create a layered cartilage scaffold with bulk compressive and surface (friction coefficient, adhesion, and roughness) mechanics approaching values measured on native cartilage. We demonstrate that our printing process enables the addition of macropores throughout the construct, increasing the viability of introduced cells by 10%. The delivery of these 3D printed scaffolds to a defect is straightforward, customizable to any shape, and adheres to surrounding tissue.

Effect of Loading on the Adhesion and Frictional Characteristics of Top Layer Articular Cartilage Nanoscale Contact: A Molecular Dynamics Study

Articular cartilage is a water-lubricated naturally occurring biological interface imparting unique mechanical and ultralow frictional properties in bone joints. Although the material of cartilage, synovial fluid composition, and their lubricating modes and properties have been extensively investigated at various scales experimentally, there is still a lack of understanding of load bearing, adhesion, and friction mechanisms of the cartilage-cartilage interface from an atomistic perspective under heavy loads. In this study, the effect of loading on adhesion and frictional behavior in articular cartilage is investigated with a proposed atomistic model for top layer cartilage-cartilage contact in unhydrated conditions using molecular dynamics (MD) simulations. Pull-off tests reveal that cohesive interactions occur at the interface due to formation of heavily interpenetrated atomistic sites leading to stretching and localized pulling of fragments during sliding. Sliding tests show that friction is load- and direction-dependent with the coefficient of friction (COF) obtained in the range of 0.20-0.75 at the interface for sliding in parallel and perpendicular directions to the collagen axis. These values are in good agreement with earlier nanoscale experimental results reported for the top layer cartilage-cartilage interface. The COF reduces with an increase in load and tends to be higher for the parallel sliding case than for the perpendicular case owing to the presence of the constant number of H-bonds. Overall, this work contributes toward understanding sliding in unhydrated biointerfaces, which is the precursor of wear, and provides insights into implant research.

Probing the frictional properties of soft materials at the nanoscale

The understanding of friction in soft materials is of increasing importance due to the demands of industries such as healthcare, biomedical, food and personal care, the incorporation of soft materials into technology, and in the study of interacting biological interfaces. Many of these processes occur at the nanoscale, but even at micrometer length scales there are fundamental aspects of tribology that remain poorly understood. With the advent of Friction Force Microscopy (FFM), there have been many fundamental insights into tribological phenomena at the atomic scale, such as 'stick-slip' and 'super-lubricity'. This review examines the growing field of soft tribology, the experimental aspects of FFM and its underlying theory. Moving to the nanoscale changes the contact mechanics which govern adhesive forces, which in turn play a pivotal role in friction, along with the deformation of the soft interface and dissipative phenomena. We examine recent progress and future prospects in soft nanotribology.

Chondrocyte viability is lost during high-rate impact loading by transfer of amplified strain, but not stress, to pericellular and cellular regions

OBJECTIVE

Deleterious impact loading to cartilage initiates post-traumatic osteoarthritis (OA). While cytokine and enzyme levels regulate disease progression, specific mechanical cues that elucidate cellular OA origins merit further investigation. We defined the dominant pericellular and cellular strain/stress transfer mechanisms following bulk-tissue injury associated with cell death.

METHOD

Using an in vitro model, we investigated rate-dependent loading and spatial localization of cell viability in acute indentation and time-course studies. Atomic force microscopy (AFM) and magnetic resonance imaging (MRI) confirmed depth-wise changes in cartilage micro-/macro-mechanics and structure post-indentation. To understand the transfer of loading to cartilage domains, we computationally modeled full-field strain and stress measures in interstitial matrix, pericellular and cellular regions.

RESULTS

Chondrocyte viability decreased following rapid impact (80%/s) vs slow loading (0.1%/s) or unloaded controls. Viability was lost immediately during impact within regions near the indenter-tissue contact but did not change over 7 days of tissue culture. AFM studies revealed a loss of stiffness following 80%/s loading, and MRI studies confirmed an increased tensile and shear strain, but not relaxometry. Image-based patterns of chondrocyte viability closely matched computational estimates of amplified maximum principal and shear strain in interstitial matrix, pericellular and cellular regions.

CONCLUSION

Rapid indentation worsens chondrocyte death and degrades cartilage matrix stiffness in indentation regions. Cell death at high strain rates may be driven by elevated tensile strains, but not matrix stress. Strain amplification beyond critical thresholds in the pericellular matrix and cells may define a point of origin for early damage in post-traumatic OA.

Rate-dependent adhesion of cartilage and its relation to relaxation mechanisms

Cartilage adhesion has been found to play an important role in friction responses in the boundary lubrication regime, but its underlying mechanisms have only been partially understood. This study investigates the rate dependence of adhesion from pre-to post-relaxation timescales of cartilage and its possible relation to relaxation responses of the tissue. Adhesion tests on cartilage were performed to obtain rate-dependent cartilage adhesion from relaxed to unrelaxed states and corresponding relaxation responses. The rate dependence of cartilage adhesion was analyzed based on experimental relaxation responses. Cartilage adhesion increased about 20 times from relaxed to unrelaxed states. This rate-dependent enhancement correlated well with the load relaxation responses in a characteristic time domain. These experimental results indicated that the degree of recovery (or relaxation) in the vicinity of contact during unloading governed the rate dependence of cartilage adhesion. In addition, the experimentally measured enhancement of adhesion was interpreted with the aid of computationally and analytically predicted adhesion trends in viscoelastic, poroviscoelastic, and cohesive contact models. Agreement between the experimental and predicted trends implied that the enhancement of cartilage adhesion originated from complex combinations of interfacial peeling and negative fluid pressure generated within the contact area during unloading. These findings enhance the current understanding of rate-dependent adhesion mechanisms explored within short time scales and thus could provide new insight into friction responses and stick-induced damage in cartilage.

In Vivo Cellular Infiltration and Remodeling in a Decellularized Ovine Osteochondral Allograft

Interest in decellularized tissues has steadily gained as potential solutions for degenerative diseases and traumatic events, replacing sites of missing tissue, and providing the relevant biochemistry and microstructure for tissue ingrowth and regeneration. Osteoarthritis, a progressive and debilitating disease, is often initiated with the formation of a focal defect in the otherwise smooth surface of articular cartilage. Decellularized cartilage tissue, which maintains the structural complexity of the native extracellular matrix, has the potential to provide a clinically relevant solution to focal defects or large tissue damage, possibly even circumventing or complementing current techniques such as microfracture and mosaicplasty. However, it is currently unclear whether implantation of decellularized cartilage in vivo may provide a mechanically and biochemically relevant platform to promote cell remodeling and repair. We examined whole decellularized osteochondral allografts implanted in the ovine trochlear groove to investigate cellular remodeling and repair tissue quality compared to empty defects and contralateral controls (healthy cartilage). At 3 months postsurgery, cells were observed in both the decellularized tissue and empty defects, although both at significantly lower levels than healthy cartilage. Qualitative and quantitative histological analysis demonstrated maintenance of cartilage features of the decellularized implant similar to healthy cartilage groups. Noninvasive analysis by quantitative magnetic resonance imaging showed no difference in T_1ρ and T₂* between all groups. Investigation of the mechanical properties of repair tissue showed significantly lower elasticity in decellularized implants and empty defects compared to healthy cartilage, but similar tribological quantities. Overall, this study suggests that decellularized cartilage implants are subject to cellular remodeling in an in vivo environment and may provide a potential tissue engineering solution to cartilage defect interventions.

Mapping the Nonreciprocal Micromechanics of Individual Cells and the Surrounding Matrix Within Living Tissues

The biomechanical properties of the extracellular matrix (ECM) play an important role in cell migration, gene expression, and differentiation. Biomechanics measurements of ECM are usually performed on cryotomed tissue sections. However, studies on cell/matrix interplay are impossible to perform due to disruptions in cell viability and tissue architecture from freeze-thaw cycling. We developed a technique to map the stiffness of living cells and surrounding matrix by atomic force microscopy and use fluorescence microscopy to relate those properties to changes in matrix and cell structure in embryonic and adult tissues in situ. Stiffness mapping revealed significant differences between vibratomed (living) and cryotomed tissues. Isolated cells are softer than those in native matrix, suggesting that cell mechanics are profoundly influenced by their three-dimensional environment and processing state. Viable tissues treated by hyaluronidase and cytochalasin D displayed targeted disruption of matrix and cytoskeletal networks, respectively. While matrix stiffness affected cellular stiffness, changes in cell mechanics did not reciprocally influence matrix stiffness.

Surface zone articular chondrocytes modulate the bulk and surface mechanical properties of the tissue-engineered cartilage

The central hypothesis of functional tissue engineering is that an engineered construct can serve as a viable replacement tissue in vivo by replicating the structure and function of native tissue. In the case of articular cartilage, this requires the reproduction of the bulk mechanical and surface lubrication properties of native hyaline cartilage. Cartilage tissue engineering has primarily focused on achieving the bulk mechanical properties of native cartilage such as the compressive aggregate modulus and tensile strength. A scaffold-free self-assembling process has been developed that produces engineered cartilage with compressive properties approaching native tissue levels. Thus, the next step in this process is to begin addressing the friction coefficient and wear properties of these engineered constructs. The superficial zone protein (SZP), also known as lubricin or PRG4, is a boundary mode lubricant that is synthesized by surface zone (SZ) articular chondrocytes. Under conditions of high loading and low sliding speeds, SZP reduces friction and wear at the articular surface. The objective of this investigation was to determine whether increasing the proportion of SZ chondrocytes in cartilage constructs, in the absence of external stimuli such as growth factors and mechanical loading, would enhance the secretion of SZP and improve their frictional properties. In this study, cartilage constructs were engineered through a self-assembling process with varying ratios of SZ and middle zone (MZ) chondrocytes (SZ:MZ): 0:100, 25:75, 50:50, 75:25, and 100:0. Constructs containing different ratios of SZ and MZ chondrocytes did not significantly differ in the glycosaminoglycan composition or compressive aggregate modulus. In contrast, tensile properties and collagen content were enhanced in nearly all constructs containing greater amounts of SZ chondrocytes. Increasing the proportion of SZ chondrocytes had the hypothesized effect of improving the synthesis and secretion of SZP. However, increasing the SZ chondrocyte fraction did not significantly reduce the friction coefficient. These results demonstrate that additional factors, such as SZP-binding macromolecules, surface roughness, and adhesion, need to be examined to modulate the lubrication properties of engineered cartilage.

Synthesis and characterization of a lubricin mimic (mLub) to reduce friction and adhesion on the articular cartilage surface

The lubricating proteoglycan, lubricin, facilitates the remarkable low friction and wear properties of articular cartilage in the synovial joints of the body. Lubricin lines the joint surfaces and plays a protective role as a boundary lubricant in sliding contact; decreased expression of lubricin is associated with cartilage degradation and the pathogenesis of osteoarthritis. An unmet need for early osteoarthritis treatment is the development of therapeutic molecules that mimic lubricin function and yet are also resistant to enzymatic degradation common in the damaged joint. Here, we engineered a lubricin mimic (mLub) that is less susceptible to enzymatic degradation and binds to the articular surface to reduce friction. mLub was synthesized using a chondroitin sulfate backbone with type II collagen and hyaluronic acid (HA) binding peptides to promote interaction with the articular surface and synovial fluid constituents. In vitro and in vivo characterization confirmed the binding ability of mLub to isolated type II collagen and HA, and to the cartilage surface. Following trypsin treatment to the cartilage surface, application of mLub, in combination with purified or commercially available hyaluronan, reduced the coefficient of friction, and adhesion, to control levels as assessed over macro-to micro-scales by rheometry and atomic force microscopy. In vivo studies demonstrate an mLub residency time of less than 1 week. Enhanced lubrication by mLub reduces surface friction and adhesion, which may suppress the progression of degradation and cartilage loss in the joint. mLub therefore shows potential for treatment in early osteoarthritis following injury.

Effects of enzymatic degradation after loading in temporomandibular joint

Synovial fluid of the joint decreases friction between the cartilage surfaces and reduces cartilage wear during articulation. Characteristic changes of synovial fluid have been shown in patients with osteoarthritis (OA) in the temporomandibular joint (TMJ). OA is generally considered to be induced by excessive mechanical stress. However, whether the changes in synovial fluid precede the mechanical overloading or vice versa remains unclear. In the present study, our purpose was to examine if the breakdown of joint lubrication affects the frictional properties of mandibular condylar cartilage and leads to subsequent degenerative changes in TMJ. We measured the frictional coefficient in porcine TMJ by a pendulum device after digestion with hyaluronidase (HAase) or trypsin. Gene expressions of interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2), matrix metalloproteinases (MMPs), type II collagen, and histology were examined after prolonged cyclic loading by an active pendulum system. The results showed that the frictional coefficient increased significantly after HAase (35%) or trypsin (74%) treatment. Gene expression of IL-1β, COX-2, and MMPs-1, -3, and -9 increased significantly in enzyme-treated TMJs after cyclic loading. The increase in the trypsin-treated group was greater than that in the HAase-treated group. Type II collagen expression was reduced in both enzyme-treated groups. Histology revealed surface fibrillation and increased MMP-1 in the trypsin-treated group, as well as increased IL-1β in both enzyme-treated groups after cyclic loading. The findings demonstrated that the compromised lubrication in TMJ is associated with altered frictional properties and surface wear of condylar cartilage, accompanied by release of pro-inflammatory and matrix degradation mediators under mechanical loading.

Role of interstitial fluid pressurization in TMJ lubrication

In temporomandibular joints (TMJs), the disc and condylar cartilage function as load-bearing, shock-absorbing, and friction-reducing materials. The ultrastructure of the TMJ disc and cartilage is different from that of hyaline cartilage in other diarthrodial joints, and little is known about their lubrication mechanisms. In this study, we performed micro-tribometry testing on the TMJ disc and condylar cartilage to obtain their region- and direction-dependent friction properties. Frictional tests with a migrating contact area were performed on 8 adult porcine TMJs at 5 different regions (anterior, posterior, central, medial, and lateral) in 2 orthogonal directions (anterior-posterior and medial-lateral). Some significant regional differences were detected, and the lateral-medial direction showed higher friction than the anterior-posterior direction on both tissues. The mean friction coefficient of condylar cartilage against steel was 0.027, but the disc, at 0.074, displayed a significantly higher friction coefficient. The 2 tissues also exhibited different frictional dependencies on sliding speed and normal loading force. Whereas the friction of condylar cartilage decreased with increased sliding speed and was independent of the magnitude of normal force, friction of the disc showed no dependence on sliding speed but decreased as normal force increased. Further analysis of the Péclet number and frictional coefficients suggested that condylar cartilage relies on interstitial fluid pressurization to a greater extent than the corresponding contact area of the TMJ disc.

Interaction of lubricin with type II collagen surfaces: adsorption, friction, and normal forces

One of the major constituents of the synovial fluid that is thought to be responsible for chondroprotection and boundary lubrication is the glycoprotein lubricin (PRG4); however, the molecular mechanisms by which lubricin carries out its critical functions still remain largely unknown. We hypothesized that the interaction of lubricin with type II collagen, the main component of the cartilage extracellular matrix, results in enhanced tribological and wear properties. In this study, we examined: (i) the molecular details by which lubricin interacts with type II collagen and how binding is related to boundary lubrication and adhesive interactions; and (ii) whether collagen structure can affect lubricin adsorption and its chondroprotective properties. We found that lubricin adsorbs strongly onto denatured, amorphous, and fibrillar collagen surfaces. Furthermore, we found large repulsive interactions between the collagen surfaces in presence of lubricin, which increased with increasing lubricin concentration. Lubricin attenuated the large friction and also the long-range adhesion between fibrillar collagen surfaces. Interestingly, lubricin adsorbed onto and mediated the frictional response between the denatured and native amorphous collagen surfaces equally and showed no preference on the supramolecular architecture of collagen. However, the coefficient of friction was lowest on fibrillar collagen in the presence of lubricin. We speculate that an important role of lubricin in mediating interactions at the cartilage surface is to attach to the cartilage surface and provide a protective coating that maintains the contacting surfaces in a sterically repulsive state.

Single cell spectroscopy: noninvasive measures of small-scale structure and function

The advancement of spectroscopy methods attained through increases in sensitivity, and often with the coupling of complementary techniques, has enabled real-time structure and function measurements of single cells. The purpose of this review is to illustrate, in light of advances, the strengths and the weaknesses of these methods. Included also is an assessment of the impact of the experimental setup and conditions of each method on cellular function and integrity. A particular emphasis is placed on noninvasive and nondestructive techniques for achieving single cell detection, including nuclear magnetic resonance, in addition to physical, optical, and vibrational methods.

Frictional response of normal and osteoarthritic articular cartilage in human femoral head

We evaluated the microscale frictional response of human articular cartilage in different osteoarthritis stages using an atomic force microscope. Four human femoral heads (60-80 years old) with different osteoarthritis stages were explanted, and two cylindrical cartilage samples were sectioned from each femoral head. The microscale frictional coefficient mu of human cartilage in phosphate-buffered saline increased with increasing osteoarthritis stages, resulting in mu = 0.119 +/- 0.036 for stage 0 (normal cartilage), 0.151 +/- 0.039 for stage I, 0.158 +/- 0.041 for stage 2, and 0.409 +/- 0.119 for stage 3. Statistically significant differences of mu values for different osteoarthritis stages were detected only between stage 3 and other stages (p < 0.0001). The average surface roughness Rq significantly increased with increasing osteoarthritis stages, ranging from 137 +/- 25 nm for stage 0 to 533 +/- 196 nm for stage 3. A significant correlation between mu and Rq for different osteoarthritis stages was observed (R2 = 0.981). These results demonstrate a positive correlation between the osteoarthritis stages and cartilage surface roughness, and the dependence of the human cartilage frictional response, on osteoarthritis progression. The results could be due to a decrease in the superficial zone protein concentration during the natural progression of osteoarthritis.

Effect of the charge and roughness of surfaces on normal and friction forces measured in aqueous solutions

We used the atomic force microscope (AFM) to determine how the roughness and charge on a surface affect the adhesion and friction when measured against a smooth surface (colloid probe) in an aqueous solution. The effect of roughness was investigated by coating TiO2 crystal substrates with TiO2 nano- or micro-sized particles, where an increase in the particle size increased the RMS roughness of the substrate. The charge of the substrate was varied by changing the pH of the aqueous solution. Force-separation curves and friction-load data were measured for the smooth colloid probe-rough substrate systems. The adhesion and friction between two surfaces in solution were seen to depend on the surface charge and roughness. A noncharged surface gave the greatest adhesion, while a charged surface gave weaker adhesions. Increasing the roughness of the surface resulted in a stronger adhesion. The magnitude and range of the adhesions were not affected by the measuring velocity in the case of a noncharged substrate but decreased with an increasing velocity for charged surfaces. The friction was seen not to depend on roughness in the case of a noncharged surface. However, in the case of a charged surface, the friction decreased with an increased roughness for low loads and then showed no dependence on the surface roughness for high loads. The results of this experiment show that the adhesion and friction of a system can be decreased via the roughness and charge of the substrate and the ion types in the solution.

Microscale surface friction of articular cartilage in early osteoarthritis

Articular cartilage forms the articulating surface of long bones and facilitates energy dissipation upon loading as well as joint lubrication and wear resistance. In normal cartilage, boundary lubrication between thin films at the cartilage surface reduces friction in the absence of interstitial fluid pressurization and fluid film lubrication by synovial fluid. Inadequate boundary lubrication is associated with degenerative joint conditions such as osteoarthritis (OA), but relations between OA and surface friction, lubrication and wear in boundary lubrication are not well defined. The purpose of the present study was to measure microscale boundary mode friction of the articular cartilage surface in an in vivo experimental model to better understand changes in cartilage surface friction in early OA. Cartilage friction was measured on the articular surface by atomic force microscopy (AFM) under applied loads ranging from 0.5 to 5 μN. Microscale AFM friction analyses revealed depth dependent changes within the top-most few microns of the cartilage surface in this model of early OA. A significant increase of nearly 50% was observed in the mean engineering friction coefficient for OA cartilage at the 0.5 μN load level; no significant differences in friction coefficients were found under higher applied loads. Changes in cartilage surface morphology observed by scanning electron microscopy included cracking and roughening of the surface indicative of disruption and wear accompanied by an apparent disintegration of the thin surface lamina from the underlying matrix. Immunohistochemical staining of lubricin - an important cartilage surface boundary lubricant - did not reveal differences in spatial distribution near the cartilage surface in OA compared to controls. The increase in friction at the 0.5 μN force level is interpreted to reflect changes in the interfacial mechanics of the thin surface lamina of articular cartilage: increased friction implies reduced lubrication efficiency and a higher potential for cartilage surface wear in OA. The effects of mechanical or biochemical changes or loss of the thin surface lamina on the remaining tissue with respect to OA progression is unknown and requires further study, but preservation of the surface lamina seems an important early target for the maintenance of cartilage health and prevention of OA.

Role of lubricin and boundary lubrication in the prevention of chondrocyte apoptosis

Osteoarthritis is a complex disease involving the mechanical breakdown of articular cartilage in the presence of altered joint mechanics and chondrocyte death, but the connection between these factors is not well established. Lubricin, a mucinous glycoprotein encoded by the PRG4 gene, provides boundary lubrication in articular joints. Joint friction is elevated and accompanied by accelerated cartilage damage in humans and mice that have genetic deficiency of lubricin. Here, we investigated the relationship between coefficient of friction and chondrocyte death using ex vivo and in vitro measurements of friction and apoptosis. We observed increases in whole-joint friction and cellular apoptosis in lubricin knockout mice compared with wild-type mice. When we used an in vitro bovine explant cartilage-on-cartilage bearing system, we observed a direct correlation between coefficient of friction and chondrocyte apoptosis in the superficial layers of cartilage. In the bovine explant system, the addition of lubricin as a test lubricant significantly lowered the static coefficient of friction and number of apoptotic chondrocytes. These results demonstrate a direct connection between lubricin, boundary lubrication, and cell survival and suggest that supplementation of synovial fluid with lubricin may be an effective treatment to prevent cartilage deterioration in patients with genetic or acquired deficiency of lubricin.

The fractal structure of equine articular cartilage

The naturally occurring structure of articular cartilage has proven to be an effective means for the facilitation of motion and load support in equine and other animal joints. Cartilage has been found to be a complex and dynamic medium, which has led to an incomplete understanding of the nature and operating mechanisms acting within a joint. Although cartilage has biphasic and triphasic properties, it is believed that the performance of equine articular joints is influenced by the surface roughness of the joint cartilage (Ateshian et al., '98; Chan et al., 2011; Yao and Unsworth, '93). Various joint types with different motions and regimes of lubrication have altered demands on the articular surface that may affect cartilage surface properties. In research performed on freshly harvested samples, equine articular cartilage has been shown to possess a multiscale structure and a fractal dimension. It is thought that by determining the fractal dimension (D) of articular cartilage, a better understanding of the friction, wear, and lubrication mechanisms for biomechanic surfaces can eventually be reached. This study looks at the fractal dimensions of three different articular cartilage surfaces in the equine carpus: the radiocarpal, midcarpal, and carpometacarpal surfaces. The three surfaces provide an ideal comparison of fractal dimensions for a different range of motion, geometry, and loading. In each sample, identical treatment was performed during measurement by a stylus profilometer.

Engineering lubrication in articular cartilage

Despite continuous progress toward tissue engineering of functional articular cartilage, significant challenges still remain. Advances in morphogens, stem cells, and scaffolds have resulted in enhancement of the bulk mechanical properties of engineered constructs, but little attention has been paid to the surface mechanical properties. In the near future, engineered tissues will be able to withstand and support the physiological compressive and tensile forces in weight-bearing synovial joints such as the knee. However, there is an increasing realization that these tissue-engineered cartilage constructs will fail without the optimal frictional and wear properties present in native articular cartilage. These characteristics are critical to smooth, pain-free joint articulation and a long-lasting, durable cartilage surface. To achieve optimal tribological properties, engineered cartilage therapies will need to incorporate approaches and methods for functional lubrication. Steady progress in cartilage lubrication in native tissues has pushed the pendulum and warranted a shift in the articular cartilage tissue-engineering paradigm. Engineered tissues should be designed and developed to possess both tribological and mechanical properties mirroring natural cartilage. In this article, an overview of the biology and engineering of articular cartilage structure and cartilage lubrication will be presented. Salient progress in lubrication treatments such as tribosupplementation, pharmacological, and cell-based therapies will be covered. Finally, frictional assays such as the pin-on-disk tribometer will be addressed. Knowledge related to the elements of cartilage lubrication has progressed and, thus, an opportune moment is provided to leverage these advances at a critical step in the development of mechanically and tribologically robust, biomimetic tissue-engineered cartilage. This article is intended to serve as the first stepping stone toward future studies in functional tissue engineering of articular cartilage that begins to explore and incorporate methods of lubrication.