*Review* **Articular and Artificial Cartilage, Characteristics, Properties and Testing Approaches—A Review**

**Mohammad Mostakhdemin 1,\*, Ashveen Nand 2,3 and Maziar Ramezani 1,\***

	- anand2@unitec.ac.nz

**Abstract:** The design and manufacture of artificial tissue for knee joints have been highlighted recently among researchers which necessitates an apt approach for its assessment. Even though most re-searches have focused on specific mechanical or tribological tests, other aspects have remained underexplored. In this review, elemental keys for design and testing artificial cartilage are dis-cussed and advanced methods addressed. Articular cartilage structure, its compositions in load-bearing and tribological properties of hydrogels, mechanical properties, test approaches and wear mechanisms are discussed. Bilayer hydrogels as a niche in tissue artificialization are presented, and recent gaps are assessed.

**Keywords:** articular cartilage; hydrogels; mechanical properties; tribological properties

**Citation:** Mostakhdemin, M.; Nand, A.; Ramezani, M. Articular and Artificial Cartilage, Characteristics, Properties and Testing Approaches —A Review. *Polymers* **2021**, *13*, 2000. https://doi.org/10.3390/ polym13122000

Academic Editor: Bramasta Nugraha

Received: 19 May 2021 Accepted: 14 June 2021 Published: 18 June 2021

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#### **1. Introduction**

The complex structure of healthy articular cartilage facilitates the joint withstanding the imposed pressures and retaining interstitial fluid to lessen stresses on its soft tissue while easing the locomotion and minimizing friction between cartilage mates. Avascular nature of this tissue results in unrecoverable damaged lesions and severe pain over time. Polymeric hydrogels are promising candidate materials for the replacement of the damaged cartilage. Moreover, polymeric scaffolds have been applied in interface tissue engineering and their uses have extended to bone to tendon and muscle to tendon interface reconstruction [1]. Recently, bilayer hydrogels have been developed with distinct techniques as promising artificial cartilage due to their resemblance to the native cartilage structure. Bilayer hydrogels contain bulk and lubricious layers that enhance water retention in their lubricious layer and advance tribological properties such as wear-resistance and coefficient of friction (CoF). The absence of optimum mechanical and tribological properties has been highlighted as a research gap in recent years because promoting mechanical properties results in a reduction in the tribological properties or vice versa.

In this study, summaries of recent research are covered, also essential elements in designing of artificial cartilage, common materials, required tests according to standard regulations and strengthening method are discussed broadly. Recent research has been highlighted and gaps addressed adequately. This review further discusses the fundamental resources that are considered in design of wide ranges of hydrogels specially bilayer hydrogels which have gained researchers' attention due to their promising mechanical and tribological properties.
