## 1 Introduction  
Articular cartilage (AC) may be damaged due to aging, disease, or trauma. AC has a limited regenerative capacity that has been attributed to the lack of innervation and the avascular nature of the tissue [1]. AC defects may progress to the subchondral bone to create an osteochondral defect. If left untreated, such defects can potentially lead to the development of osteoarthritis [2, 3], a cartilage-degenerating disease affecting over 300 million worldwide and representing a burden on the healthcare systems globally [4, 5]. The treatment of AC defects often results in a fibrocartilage tissue with poor long-term outcomes [6], thus presenting a significant clinical challenge. This is partially because the current regenerative and tissue engineering strategies fail to recapitulate the native organization of the AC tissue. AC is a graded tissue that has three main zones: superficial, middle, and deep. These zones are associated with differences in matrix composition, matrix structural organization, and cell number, which result in gradients with complex physical, mechanical, and biological properties [7]. In adult healthy human articular cartilage, the superficial zone is characterized by thickly packed collagen type II fibers, a high tensile strength, and a high cell density, while the deep zone is characterized by a high sulfated glycosaminoglycan (sGAG) content, high compressive strength, and a low cell density as compared to the other layers [8].  
Different biofabrication strategies, such as bioprinting, have been exploited to recapitulate few of the gradients connecting the three zones [9]. Graded scaffolds aim to mimic the highly organized native tissue. Several studies have explored the fabrication of multilayered constructs exhibiting differences in biological [10] or chemical composition [11; 12; 13; 14] and physical or mechanical properties [15; 16; 17]. Different gradient strategies have also been combined to tailor physical (stiffness) and biochemical (growth factor) gradients that better control stem-cell behavior [18].  
To the best of our knowledge, there has been only one study introducing a cell density gradient in a bioprinted construct [19], where swine articular cartilage-derived constructs with different density gradients of rabbit chondrocytes were compared to constructs with equivalent homogeneous cell distribution. In the study, gene expression and sGAG deposition were quantified for the overall constructs. However, limited information was presented regarding the extracellular matrix (ECM) that was deposited by the cells at different densities in each zone. Therefore, further research is warranted to characterize the cell-derived tissue deposition in each of the zones within a graded construct, especially using human material.  
The aims of this paper were (i) to design and fabricate scaffolds with three-zone cell density using bioprinting with an alginate-based bioink containing human articular chondrocytes and a poly-\(\epsilon\)-caprolactone (PCL) support structure, (ii) to characterize the cell viability and cell gradient stability overtime, and (iii) to evaluate the time-dependent deposition of ECM by the cells embedded in the scaffolds. To achieve the cell density gradient, the design of the scaffolds included three zones with three different cell densities. The different zones aimed to mimic both the dimensions and cell density of the superficial, middle, and deep zones as seen in the native structure of articular cartilage.