## 4 Materials for SLA
### Resins  
The process of photopolymerization requires the presence of three major components: the resin, the photoinitiator and the light source. The most important ingredient used in SLA printing is resin. The photocurable resins are multifunctional monomers and oligomers, typically from the class of (meth)acrylates, epoxides and thiol-enes [65]. The acrylic resins undergo photopolymerization through a radical mechanism. When exposed to UV light, the photoinitiators produce free radicals that attack the double bond of the (meth)acrylic monomers, which subsequently transfer the radical to another monomer, and the polymer chain grows. In contrast to acrylates and methacrylate, epoxy monomers are polymerized through an ionic mechanism. In this case, the cationic photoinitiators produce acids that react with a bond in epoxy monomers and induce polymerization. Some commercial epoxy monomers used in SLA are bisphenol A diglycidyl ether (DGEBA) or 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ECC) [66]. Those liquid monomers/oligomers are converted into a solid polymer within seconds or a few minutes. The polymerization is initiated by active species (e.g., free radicals or cations), formed when a photoinitiator decompose in the presence of UV (200-400 nm), visible (400-700 nm) and, more rarely, IR (700-1000 nm) light [65]. Other components can be added in low concentrations to the photocurable formulation to improve parameters for printability or the properties of the resin. For instance, diluents (e.g., trimethylolpropane triacrylate, dipropylene glycol diacrylate, monofunctional urethane acrylate) are used to reduce the viscosity of the printing resin, photo-absorbers are useful to control the penetration depth of the incident light, inert dyes can provide specific colours, and inhibitors help to prevent the premature polymerization of the acrylate-based resins [60]. Moreover, the addition of dispersion agents improves the tinctorial strength, reduces the resin viscosity and surface tension and prevent sedimentation of particles, while stabilizers enhance shelf-life stability during resin storage and maintain a stable viscosity. Finally, the incorporation of fillers confers improved the mechanical properties of the printed object. Most often, the exact composition of a commercial resin is not publicly disclosed. Synthetic polymers are by far the most used resin for SLA, as they are affordable, highly versatile and possess a very good light sensitiveness. Materials traders such as 3D Systems, FormLabs, etc., provide a wide range of acrylic, epoxy, vinyl ether photopolymers, polycarbonate-like, acrylonitrile-butadiene-styrene (ABS)-like and polypropylene-like materials. Notably, commercialized photopolymer resins are predominantly fossil-based and relatively expensive. Resins for SLA are continuously developed, making them useful for more-demanding biodegradable, human safety, and environmentally friendly products. Most of the available stereolithography resins form highly crosslinked networks, and the resulting materials are glassy or rigid. It should be noted that the focus is not on obtaininga crosslinking density as higher as possible, which may lead to a brittle structure. For medical purposes, it is often necessary to develop objects with elastomer properties [67]. Biodegradable oligomers appropriate for SLA contain hydrolysate ester- or carbonate linkages in the main chain and contain molecules of D,L-lactide (DLLA), poly(propylenefumarate) (PPF), trimethylene carbonate (TMC) and \(\epsilon\)-caprolactone (CL). The viscosity and glass transition of the macromers are key elements for prototyping and dictates the formulation of resin. Macromers of high molecular weight such as DLLA or PPF is diluted with non-reactive diluents such as N-methylpyrrolidone or water. Importantly, while curing, the material suffers an isotropic shrinkage, which should be taken into consideration when designing the prototype.