## 3 Results and Discussion
### Optimization of PCL and PCL/CNC Filaments Produced by Wet-Spinning
This surface roughness is attributed to the partial exposure of CNC on the filament surface, which can potentially enhance cell attachment in biological applications. The solution parameters, wet-spinning conditions, and the calculated filament diameters are detailed in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Polymer Percentage (\%)**} & \multicolumn{2}{c}{**Wet-Spinning Conditions**} & \multirow{2}{*}{**Mean Diameter (\(\upmu\)m)**} \\ \cline{3-3} \cline{5-5}  & & **Needle** & & **Flow Rate** & **(\(\upmu\)m)** \\ \hline F1 & 25\% PCL/1\% CNC & & & & 50.53 \(\pm\) 1.04 \\ \hline F2 & 25\% PCL/2\% CNC & & & & 52.67 \(\pm\) 1.09 \\ \hline F3 & 25\% PCL/9\% CNC & & & & 55.00 \(\pm\) 1.32 \\ \hline F4 & 30\% PCL/1\% CNC & & & & 49.88 \(\pm\) 1.09 \\ \hline F5 & 30\% PCL/2\% CNC & 0.61 & 6 & & 50.26 \(\pm\) 1.35 \\ \hline F6 & 30\% PCL/3\% CNC & & & & 53.57 \(\pm\) 1.64 \\ \hline \end{tabular}
\end{table} 
Table 1: Solution parameters (PCL and CNC percentage), wet-spinning conditions (needle diameter and flow rate), and filament average diameters and standard deviations of the filaments obtained. A mechanical analysis for the six filaments was conducted by performing stress-strain curves. For each filament, 10 replicates were tested. All the stress-strain curves showed a configuration similar to the curve representing a normal tendon when subjected to a mechanical tensile test. The values of maximum tension, maximum stress deformation, breaking point, strain, and Young's modulus were assessed and are detailed in Table 2.  
After a comprehensive analysis of the results obtained, the maximum stress deformation values stand out as they proved to be remarkably high, highlighting one of the most distinctive characteristics of the PCL polymer, i.e., its elongation capacity. However, it was observed that the incorporation of CNC in the formulations led to a reduction in deformation as a result of the greater rigidity provided to the filaments by the presence of CNC. Even so, the filaments maintained substantial deformation values. According to the literature, healthy tendons and ligaments operate under failure loads between 7% and 40% [32]. It was found that all the optimized formulations in this study significantly exceeded these values. In general, it can be inferred that the addition of CNC to PCL solutions results in an increase in tensile strength and breaking point, leading to the exclusion of formulations with lower percentages of CNC in all optimized solutions. In addition, the values obtained indicate that increasing the percentage of PCL translates into an increase in the tensile strength and breaking point, pointing to a closer approximation to the main objective when combined with a higher percentage of CNC. The results obtained corroborate the idea that CNC plays a significant role in the mechanical reinforcement of filaments. [PERSON]'s modulus values were similar for all the optimized filaments (0.18-0.27 GPa). The values for Young's modulus found in the literature for the anterior cruciate ligament (ACL), the patellar ligament (PL), and the tendon of the triple semitenthous muscle (STT) were found to be 0.195, 0.426, and 0.163 GPa, respectively [44]. This corroborates the potential of the produced filaments to replace ACL, with similar values to [PERSON]'s modulus. Particularly, the filament produced with 30% (\(w/v\)) PCL and 3% (\(w/v\)) CNC shows a Young's modulus of 0.27 GPa, which represents a 38% increase when compared to the ACL value. Overall, the optimized conditions for producing uniform and mechanically robust PCL/CNC filaments involved a 30% (\(w/v\)) PCL concentration with 3% (\(w/v\)) CNC, a flow rate of 6 mL/h, and a 0.61 mm needle diameter. These conditions provided a balance between filament uniformity and the desired enhancement in mechanical properties for ACL tissue engineering applications.