## 4 Materials for SLA
### Resins
Despite this prevalence of petroleum-based resins for SLA printing, a progressive increase in the use of natural polymers from renewable resources can be noticed. Naturally derived oligomers bearing hydroxyl groups can be converted into photocatalytic resins through a simple chemical functionalization with methacrylic acid, glycidyl methacrylate and methacrylic anhydride. Most promising is the functionalization of natural polymers such as gelatin, alginate, oligopeptides that enable photocuring. Typically, this reaction occurs in phosphate-buffered saline solution by lightly heating (usually at 50 \({}^{\circ}\)C) a mixture of polymer and functionalization agent (4-5%). Specifically, gelatin, which is a major component of extracellular matrix (ECM) derived from the hydrolysis of collagen, is functionalized with methacrylate groups (GelMA) to form a matrix by photocrosslinking (Table 1). However, GelMA has relatively poor printability. Synthetic materials, most frequently belonging to the class of PEG derivatives, are frequently added to bio-resins based on natural polymers in order to modulate their mechanical properties, crosslinking or printability. For example, poly(ethylene glycol) diacrylate (PEGDA) was shown to enhance the printability and feature resolution of GelMA resin [68]. In another study, poly(\(\epsilon\)-caprolactone) methacrylate was introduced into GelMA resin to improve the 3D printing fidelity and provided the hybrid materials with enhanced swelling and proliferation of seeded cells [69]. The utilization of plant oil polymers (polyesters, polyolefins, polyurethane) in photoactive resin formulations for SLA is rarely reported so far. Vegetable oils contain fatty acids bearing large amounts of unsaturated bonds, which can be chemically modified with photocurable groups with epoxides, acrylates and methacrylates. [PERSON] and co-workers [70] fabricated for the first time 4D scaffolds using epoxidized soybean oil acrylate (ESOA). The printed scaffolds could produce a shape recovery transition, determined by the oscillation of the pendant alkane groups present in the stearic, oleic and linoleic acids, the major fatty acids residues in the soybean oil. Epoxidized soybean oil acrylate has excellent biocompatibility and superior processing compared to PLA- and/or PCL-based liquid resin for stereolithography. Importantly, the printing speed and laser frequency have a significant influence on the thickness and width of the printed scaffold (Figure 2). In another study, renewable ESOA-containing resin was developed to create biodegradable structures with similar mechanical properties to non-degradable commercial resins [71]. The mixture was composed of diluent (isobornyl methacrylate), oligomer (trifunctional epoxidized soybean oil acrylate, ESOA\({}_{3}\), difunctional/trifunctional epoxidized soybean oil methacrylate, ESOMA\({}_{2}\)/ESOMA\({}_{3}\)) and photoinitiator (phenylbis-2,4,6-trimethylbenzoyl phosphine oxide). Trifunctional epoxidized soybean oil methacrylate (Figure 3a) was synthesized via a \"green\" solvent-free synthesis route and demonstrated good layer fusion upon 3D printing (Figure 3b). The resulting polymers showed mechanical performance competitive to commercial counterparts. Other notable work on ESOA-based formulations for SLA 3D printing has been made [72; 73], and the key findings from those studies are reported in Table 1.  
Other vegetable oils, such as linseed oil, castor oil and cardanol oil, possesses inherent biodegradability, negligible toxicity and modifiable functional groups and are potential starting materials for the preparation of resins for 3D printing [74; 75; 76]. The use of multiple resins for a printed object is possible by SLA, but it requires supplementary steps such as removal of unpolymerized resin, cleaning of the platform and partially fabricated object and resin change [61]. If these steps are performed manually, the cellular and biochemical purity may be compromised, but the automatic process has been recently explored [77].