## 4 Materials for SLA
### Graphene Composites
#### 4.2.2 Three-Dimensional SLA Printing of Graphene and GO  
At the academic level, graphene has been used as reinforcement in several photopolymer resins for a variety of applications, spanning from the development of semiconductorsto medical devices (Table 2). The properties of 3D printing are largely dependent on the dispersion of nanofillers. Generally, there are two main approaches to disperse graphene or GO in photocurable resins: directly in the liquid monomer, followed by the addition of initiators, or using a three-step method: (1) ultrasonication of a graphene/GO suspension in different solvents (acetone, chloroform, water, dimethyl formamide), (2) addition of polymer, and (3) removal of the solvent. In tuning the properties of 3D-printed nanocomposite, the design of the resin should consider the aspect ratio and surface chemistry of the nanofiller, percolation threshold, and interfacial property of nanofiller and matrix. Often, in carbon-reinforced polymer composites, the addition of the nanofiller results in stronger but more brittle materials. In these cases, the reinforcement effect of increased tensile strength is overshadowed by a decrease in ductility, which leads to concerns about the toughness of the composite. Scientists followed two approaches to attain a more balanced strength/ductility relation, and these are the post-curing of 3D-printed parts and functionalizing GO. Some examples are the works of [PERSON] et al. [110] and [PERSON] et al. [111], who incorporated GO into SLA resin intended for 3D printing. Specimens with only 0.2 wt% GO showed a 62.2% increase in tensile strength without compromising ductility [110]. Furthermore, annealing the composite at 60 \({}^{\circ}\)C for 6 h and at 110 \({}^{\circ}\)C for another 6 h led to an increase of 12.8% in elongation compared with the uncured polymer matrix. The authors stated that this increase in ductility is due to the increasing crystallinity of GOs reinforced polymers. [PERSON] et al. observed an increase in ductility by about 15% at 0.5 wt% GO, but tensile strength decreased by 52% with further addition of GO [111]. Thus, 3D-printed specimens were annealed at 100 \({}^{\circ}\)C, and a drastic increase in strength by 673.6% for the 1 wt% GO nanocomposite was noticed (Figure 4a-c). However, the material became more brittle, showing a decrease in ductility by 82%. It is noteworthy that thermal post-curing facilitated the chemical crosslinking via esterification and decreasing in pore size of the resin, which explains the enhancement in mechanical strength. All these studies pointed out two important research directions in developing successful GO/photopolymers, namely the uniform GO dispersion in SLA resins and annealing treatment. In a recent example, [PERSON] et al. used mild annealing conditions on the SLA printed fibre-graphene oxide polymer composite resin designed for the development of new mechanical metamaterials [112]. After annealing, an enhancement in compression strength of almost 10 times, as compared with pure polymer, was obtained at a reinforcement content of 0.8 wt%. In 2019, [PERSON] et al. [113] explored for the first time the SLA technique as a likely way for the development of the GO/elastomer nanocomposites. The 3D-printed samples showed a decrease in mechanical and thermal properties with the addition of GO in the resin as a result of the filler agglomeration. Overall, the literature on GO-reinforced polymers prepared via SLA is scarce, but graphene-based photopolymer resins are even less studied. The reason for this is the poor dispersion of graphene in the polymer matrix due to the lack of functional groups on  
Figure 4: (**A**) Tensile strength comparison of casted and 3D-printed part. SLA-printed complex-shaped GO nanocomposites: (**B**) nested dodecahedron and (**C**) diagrid ring. Reprinted with permission from [111]. its surface. In the work of [PERSON] et al., Tween20 was added to improve the dispersion of graphene into a graphene/photopolymer resin.