## 4 Materials for SLA
### Graphene Composites
#### 4.2.2 Three-Dimensional SLA Printing of Graphene and GO
The 3D-printed specimens exhibited a drop in tensile strength with the addition of graphene (1 and 2 wt%) as compared with pristine resin. The authors explained that the penetration of light is blocked by graphene, preventing complete crosslinking of the photopolymer resin [114,115]. The sample consisting of 2 wt% graphene had a rougher surface, with an appearance of bonded sand particles. Therefore, a low graphene content, less than 5 wt% according to some authors [116], is recommended to maintain the appearance and mechanical strength of the printed object. In the report of [PERSON] et al. [116], a very small amount of graphene powder (0.1-2 wt%) was doped into a 3D photocurable clear resin that could act as an inherent laser absorber and ionization promoter, which resulted in the direct analysis of samples without adding matrix for functional spectrometry devices. An extra solvent can be added to disperse graphene, but in this case, the photocuring behaviour will be changed. In the work of [PERSON] et al. [117], a solvent-free method was applied to fabricate scaffolds for bone tissue engineering. The UV-crosslinkable resin consisting of polylactic acid-polyurethane oligomer, trimethylolpropane trimethacrylate as a reactive diluent, Irgacure 819 and graphene was 3D printed, resulting in nanocomposites with better mechanical properties than commercial resins. Additionally, combining graphene with nanocellulose and incorporating it into UV-curable polyurethane (PU) has proven to impart new hybrid nanomaterials suitable for 3D printing [118]. The versatility of using graphene-PU resin in stereolithography has been explored by [PERSON] et al. [119]. The authors report that mixing graphene nanoplates with lignin results in strong interfacial interaction between functional groups of the lignin graphene and PU, and thus an enhanced filler-matrix compatibility. The tensile strength and hardness of the photocured PU composite increased significantly with a loading of 0.6 wt% lignin graphene. [PERSON] et al. [120] used GO reinforced commercial resin and fabricated 3D-printed structures via SLA. The samples were pyrolyzed under a nitrogen atmosphere to reduce GO to graphene and to decompose the polymer, resulting in 3D structures consisting entirely of graphene. Unfortunately, the graphene-based 3D structures showed high shrinking and brittleness, making them useless for practical applications. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Filler** & **Polymer Matrix** & **Resin Preparation** & \multicolumn{2}{c}{**3D Printing**} & \multicolumn{2}{c}{**Post-Processing**} & \multicolumn{1}{c}{**Applications**} & \multicolumn{1}{c}{**Ref. **} \\ \hline \multirow{4}{*}{ \begin{tabular}{c} Single-layer GO \\ (SLGO) \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} Commercial polymer \\ resin (Pic 100 from \\ EnvisionTEC Inc., \\ Dearthorn, MI, USA) \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} SLGO was dispersed in \\ acetone and ultrasonicated \\ for 2 h addition of \\ resin (Pic 100 from \\ EnvisionTEC Inc., \\ Dearthorn, MI, USA) \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} 3D printing was \\ polymer and \\ ultrasonication for another \\ 2 h active evaporation system \\ heating at 70 \({}^{\circ}\)C for 12 h; \\ 0.2 and 0.5 wt% SLGO \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} 3D printing was \\ performed using a \\ bottom-up mask \\ projection-based \\ SLA system \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} 60 \({}^{\circ}\)C for 6 h and \\ - * \\ 6 h \\ \end{tabular}  } & \multirow{4}{*}{\ \begin{tabular}{c} -* \\ 6 h \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} [110] \\ 6 h \\ \end{tabular}  } \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & &  
Graphene has the potential to become a key candidate as a filler reinforcer in polymer resins for 4D printing to boost the development of smart materials.