## 1 Introduction
This drawback can be overcome by the addition of other biomaterials as a reinforcing agent and the introduction of a secondary crosslinking mechanism via chemical crosslinking which will give rise to a more stable hydrogel [13]. The mechanical properties of alginate hydrogel can be improved by the addition of cellulose nanocrystals (CNCs) as a reinforcement material [14]. In general, CNCs have many unique properties that are highly desirable in biomedical applications. Some of which include its high aspect ratio, availability, abundance in nature, and sustainability [15]. In 3D bioprinting applications, CNCs are often used as rheological modifiers and reinforcing agents for hydrogels due to their high crystallinity [14]. Several studies report that the addition of nanocellulose greatly improved the printability of alginate-based hydrogels [16; 17; 18]. During the extrusion process, it was also reported that the printed structures not only retained their shape with minimal spreading but also had improved geometries [19; 20]. While the addition of CNCs as a reinforcing agent improves the strength and printability of hydrogels, a stronger support system is required to produce a tough and flexible hydrogel that can mimic the native articular cartilage [21]. The introduction of a second network can help to achieve this requirement. This technique is also known as the double network (DN) system [21; 22]. A double network hydrogel improves the mechanical properties due to the presence of interpenetrating networks, which is a desirable property for cartilage bioprinting applications [13]. Double network hydrogels are currently studied with the intention to overcome the limitation of weak and brittle hydrogels. A study done by [PERSON] et al. confirmed this hypothesis by successfully forming highly stretchable and tough double network hydrogels for 3D bioprinting [21]. Synthetic polymers like polyethylene glycol diacrylate (PEGDA) can be combined with natural polymers to form a double network hydrogel through photopolymerization. Several literatures have reported good cell viability while using low concentrations (ranging from 0.01% to 0.1%) of Irgacure 2959 (I-2959) and short UV exposure times (ranging from 1 to 10 min) in their study [3; 23; 24; 25]. [PERSON] et al. also reported the promising cell viability of human articular chondrocytes in their photopolymerised bioink with a cell viability of 63.2 \(\pm\) 9.0% in the post printing polymerisation which increased to 89.2 \(\pm\) 3.6% when simultaneous polymerisation was applied [3]. Irgacure 2959 has been the photoinitiator of choice as it is the least toxic among other photoinitiators (Irgacure 184 and Irgacure 651) and causes minimal cell death when tested over a broad range of mammalian cell types [24]. PEGDA is widely recognised as a successful scaffolding material in tissue engineering due to its biocompatibility and low immunogenicity [26]. PEGDA-based hydrogels are also resistant to protein adhesion, making them ideal scaffolds that can promote cell growth and differentiation towards the formation of tissues [27]. Hence, this study attempted to produce bioink using different concentration of CNC, Alg, and PEGDA and to utilise the double network crosslinking technique that may satisfy the requirement for cartilage bioprinting.