N. Chowdhury etal.

Tribological properties, encompassing the science of friction, wear, and lubrication, play a pivotal role in the efcient functioning of various materials and systems. Nature’s ingenious designs have provided remarkable examples of precise tribo­logical properties, as evidenced by biological tissues like articular cartilage, which exhibit minimal friction and wear due to the phenomenon of hydration lubrication. However, the innate limitations of these natural tissues, such as the challenge of repair after injury or damage, have driven the exploration of innovative solutions in the realm of bio-tribology [1]. Recent advancements in bio-tribology have spurred the development of potential replacement materials, particularly hydrogels, by leveraging in-depth surface property analysis. Hydrogels, characterized by their biphasic nature and high water content akin to biological tissues, have emerged as promising candidates for mimicking natural tissue behavior. Their unique proper­ties, including low friction characteristics and potential for creating a favorable extracellular matrix (ECM) microenvironment, have sparked interest in their appli­cation for biological tissue regeneration [2].

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Articular cartilage, located in synovial joints, is a durable tissue that serves as a protective layer. The thickness of articular cartilage in humans typically ranges from 1 to 5mm. It covers the surface where bones articulate, providing resilience and smooth movement during joint motion. The extracellular matrix (ECM) of articular cartilage contains dispersed chondrocytes, forming a connective tissue that offers various functions like structural support, resilience, and lubrication. ECM mainly consists of collagen (15–22%), water (60–85%), and proteoglycans (protein chains glycosaminoglycan (GAG)) (4–7%). Figure2.1 shows the layers and cross­sectional view of various zones in cartilage. The articular cartilage has four clearly dened zones: the deep zone, calcied zone, middle zone, and supercial zone [1].

The organization of bers and chondrocytes is the determination of the mechani­cal and structural properties of the tissue. However, the load-carrying ability and frictional characteristics of articular cartilage depend on its surface structure and supercial layer properties. An acellular non-brous, highly viscous synovial uid,

N. Chowdhury etal.

acceptable granular electron-dense material zone exists atop the transitional zone [11, 12]. The surface layer’s regenerative mechanism is dened by the mechanical characteristics and movement of liquid-like synovial uid, GAGS, and lipid throughout the cartilage surface and solid matrix structure when deformed [13]. Articular cartilage does not have progenitor cells, adequate nutrients, or perichon­drium, as proteoglycan is the water-attracting component in articular cartilage. It limits the healing capability of this cartilage and is known as cartilage damage or osteoarthritis. Two primary kinds of imperfections may occur osteochondral defects (C), which extend into the subchondral bone, and chondral defects (B), affecting only the cartilage. Figure2.2a,b shows the cartilage defect and osteochondral defect caused by age, bone defect, and accident [2, 14, 15].

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Research ndings have provided insights into how lubrication occurs in the articular cartilage. In the early understanding of diarthrodial joints, it was hypothesized that articular cartilage relied on the lubrication mechanism of a uid lm, aided by the viscous synovial uid. This uid full-lm lubrication prevented direct contact between the cartilage surfaces. However, subsequent studies revealed that synovial uid is rapidly depleted within a short time as it lters through the porous layers of cartilage [17].