## 4 Discussion
### Three-Dimensional-Printing Methods and Materials
Finer spatial resolutions may explain why studies utilised these methods predominantly for printing small nodules [36; 45; 46] and underlay the challenges [PERSON] et al. [33] faced in replicating accurate geometries of smaller lung nodules when employing FDM printing methods. Nonetheless, studies that utilised material jetting and SLS suffered from longer printing times, expensive resources, and laborious modelling steps due to requiring supporting materials with subsequent removal [35; 36; 37]. The limited selection of photopolymers available additionally constrains the range of radiodensities achievable with these methods [2]. The advantages and disadvantages of a selection of materials investigated in this review are presented in Table 5. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Material (Printing Method)** & **Advantages** & **Disadvantages** \\ \hline PLA (FDM) & Low melting point [59] & Brittle [59] \\  & Simple print process [32] & Rough surface finish [59] \\  & Non-toxic and biocompatible [59] & Surface texture is unnatural due to \\  & Rigid and strong [60] & laminae or stair-step appearance [43] \\  & Wide variety of colours [59] & Low heat resistance—can warp and \\  & Inexpensive and highly available [59] & melt under sun exposure [61] \\  & Suitable for soft tissue and muscle & Prone to oozing effect [61] \\  & replication as exhibits radiodensities & FDM requires removal of supporting \\  & between 32 and 185 HU at 100\% & material for overhanging parts [62] \\  & infill [40,42,45] & \\ \hline ABS (FDM) & Relatively low attenuations, making it & Prone to shrinkage and warping \\  & suitable as a surrogate for adipose & during cooling after the print [40] \\  & tissue [40] & Requires removal of supporting \\  & Tough, and impact resistant, makes & material for overhanging parts [62] \\  & for robust moulds to encase filler & Toxic [28] \\  & materials [39] & Affected by humidity [28] \\ \hline TPU (FDM and SLS) & Flexible polymer [63] & TPU used with FDM printers is not \\  & Low radiodensities of around & functionally strong as compared to \\  & \(-200\) HU, suitable for representing & SLS [63] \\  & subsolid, minimally attenuating & \\  & lesions [38] & \\  & Higher resolution enabled with SLS & \\  & as compared to FDM printing [63] & \\ \hline Nylon/Polyamide 12 (SLS) & High-detail resolution and strength. & Free unsintered powder may remain \\  & Suitable for small structures requiring & trapped in parts of the model [36] \\  & low radiodensities (\(-700\) to & High-cost printers [36] \\  & \(-130\) HU) [36] & Prone to thermal distortion [36] \\  & Does not require supporting material & Rough and grainy surface finish [36] \\  & due to free powder acting as the & \\  & supporting material [36] & \\ \hline PETG (FDM) & Suitable for cartilage tissue, & Easily scratched and absorbs \\  & exhibiting \(\sim\)170 \(\pm\) 20 HU [40] & moisture [59] \\  & Simple to print, flexible and & Can produce thin hairs on the surface \\  & strong [60] & due to stringing (oozing material) [60] \\  & Glossy and smooth surface finish [60] & \\  & Negligible warping [60] & \\  & Water resistant [60] & \\ \hline Vero PureWhite (Polylet) & Rigid radiopaque photopolymer [64] & Brittle [64] \\  & Fine resolution and accuracy [64] & \\  & Durable [64] & \\  & Suitable for moulds encapsulating & \\  & materials [35] & \\ \hline Filaments doped with high-density & Higher atomic numbers and densities & Long-term damage of the extrusion \\  & enabled to better replicate & nozzle due to abrasion from \\  & granite-PLA, ABS with added Bismuth (FDM), & radiologists of cortical bone, which \\  & Bone meal powder amalgamate (casting) & \\  & materials [10]. & \\  & StoneFil PLA—density of & \\  & 1.54 g/cm\({}^{3}\)[10] & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of 3D-printing materials presented in this review.