## 4 Conclusions  
In this preliminary study, we successfully produced different formulations using CNC, Alg, and PEGDA for bioprinting applications. CNCs extracted from oil palm trunk exhibited a typical rod-like structure with an average diameter and length of around 7 \(\pm\) 2.4 and 113 \(\pm\) 20.7 nm, respectively. Almost all the pre-gel formulations of CNC/Alg/PEGDA displayed shear-thinning behaviour, most prominently in 1% [\(w/v\)] Alg formulations with power-law index \(\eta<1\). For post extrusion gelled formulations, CNC/Alg/PEGDA with  
Figure 10: Scanning electron micrographs of freeze-dried F8 [4% CNC: 4% Alg: 40% PEGDA] construct at magnification of (**A**) 100\(\times\) and (**B**) 801\(\times\). CNC fibres indicated with arrows.  
2.5% and 4% [\(w/v\)] Alg showed storage modulus dominance over loss modulus (G' > G'') suggesting a good shape fidelity. After the constructs were subjected to UV treatment at 365 nm, only the F8 construct [4% CNC: 4% Alg: 40% PEGDA] demonstrated the tough and flexible characteristics that are favourable for cartilage bioprinting. It can possibly mimic the properties of the native articular cartilage due to a similar water content percentage (79.5%) and the small swelling ratio (4.877) that might contribute to a minimal change of the 3D construct geometry. From the SEM analysis, the UV crosslinked constructs revealed a rough and wavy surface, and the pore size ranged from 3 to 20 \(\upmu\)m. Overall, the presence of CNCs in the double network hydrogel is seemingly important and shows positive effects towards the fabrication of a potential bioink. However, further analysis on the CNC/Alg/PEGDA formulation, such as biocompatibility and cytotoxicity, printability using a 3D bioprinter, and the degradation properties, should be evaluated to make a conclusive stand on its suitability as a bioink for cartilage bioprinting applications.  
The following are available online at [[https://www.mdpi.com/article/10.3390/app1200771/s1](https://www.mdpi.com/article/10.3390/app1200771/s1)]([https://www.mdpi.com/article/10.3390/app1200771/s1](https://www.mdpi.com/article/10.3390/app1200771/s1)), Figure S1: Oscillation amplitude sweep (0.1-100%) of F6: [4% CNC: 4% Alg: 10% PEGDA] hydrogel with a fixed frequency of 1 Hz. Figure S2: Oscillation amplitude sweep (0.1-100%) of F7: [4% Alg: 40% PEGDA] hydrogel with a fixed frequency of 1 Hz. Figure S3: Oscillation amplitude sweep (0.1-100%) of F8: [4% CNC: 4% Alg: 40% PEGDA] hydrogel with a fixed frequency of 1 Hz. Figure S4: Oscillation amplitude sweep (0.1-100%) of F9: [2% CNC: 2.5% Alg: 25% PEGDA] hydrogel with a fixed frequency of 1 Hz.  
Conceptualization, Y.B.; formal analysis, A.W.A.; funding acquisition, R.H., K.M.K.I., Z.A.A.H. and Y.B.; investigation, A.W.A.; methodology, A.W.A. and N.J.; project administration, N.J.; resources, R.H. and K.M.K.I.; supervision, Y.B.; validation, Z.A.A.H.; writing--original draft, A.W.A.; writing--review & editing, Y.B. All authors have read and agreed to the published version of the manuscript.  
This work is supported financially by the Ministry of Higher Education Malaysia with Fundamental Research Grant Scheme with Project Code: FRGS/1/2018/STG05/USM/02/6.  
Not applicable.  
All relevant data are within the paper.  
The authors would like to thank the School of Industrial Technology, USM for providing the oil palm trunk fibres and the use of the lab for the isolation and extraction of the cellulose nanocrystals (CNCs) used in this study.  
The authors declare no conflict of interest.