## 4 Discussion  
The aloe vera coating method proposed introduces several changes in the physicochemical properties of the PLA surface and influences its biological activity. The increase of the hydrophilicity of plasma-treated PLA scaffolds enhance the affinity of the surface to support the aloe vera coating in an effective way. The functional groups created during plasma treatment of PLA are mainly hydroxyl and carbonyl ones [13, 41]. These groups are not reactive with the functional groups in the structure of the polysaccharides in aloe vera extracts (mainly hydroxyl and acetyl) [42]. Nevertheless, they provide an increase in the polarity of the surface which is more favorable for attaching the extract by intermolecular forces. The similarity of the WCA results between coated and non-coated groups indicates that the hydrophilicity increases similarly (compared to untreated PLA samples) whether the surface modification consisted only in the application of the plasma treatment or if it is followed by the aloe vera coating. Therefore, any change in the biological properties of the scaffolds needs to be explained by the change in the chemical composition of the surfaces, not by their surface energy. Regarding the surface chemistry, despite being a non-significant difference (p > 0.05), the modification in the O/C ratio of the PLA surface before and after the plasma treatment suggests the prevalence of the oxidation mechanism over the destructive one [32]. It is important to take into account that both mechanisms may take place simultaneously and the predominance of one of them depends on the treatment time, as demonstrated by [PERSON] et al. [43] The prevalence of the oxidation route during the plasma treatment in the present study is also supported by the results from the mechanical characterization, as a non-significant difference (p > 0.05) was detected when the plasma treated samples and the non-treated ones were compared (Figure 2). These results also evidenced that the aloe vera coating does not play a role on the mechanical properties of the final structure. The low thickness of the coating and the absence of etching compounds in the aloe vera extract justify this lack of modification on the mechanical properties of the scaffolds after the coating. With respect to the coated samples, it is worth mentioning that there is a significant decrease of the O/C ratio (p < 0.05) for the samples coated at pH 3 and pH 4, but not for those coated at pH 5. Figure 5: Metabolic activity of human fetal osteoblastic cell line (hFOB) cells on the plasma-treated and coated scaffolds determined by the CCK-8 assay (* p < 0.05, ** p < 0.01 and *** p < 0.001). This trend could be related to the amount and type of bioactive compounds that are able to be fixed on the PLA surface. The O/C ratio expected from the coating depends on the mannose:glucose:galactose ratio of the polysaccharides in the extracts [25]. The composition of aloe vera extracts is complex and it depends on different parameters, such as the geographical location of the plants or the season when the leaves are harvested, as the carbohydrates have a storage biological function in the living plants [25], or the extraction and processing procedures used; so it is difficult to predict a clear expected trend of this parameter. Regarding the enzymatic degradation of the scaffolds, the results confirm an influence of the coating on the degradation rate of the material. As the adsorption of the enzymes to the surface is the first step of any enzymatic degradation process, the modifications introduced on the surface chemistry can potentially modify the enzymatic degradation mechanism of PLA. In fact, the adsorption process has been demonstrated to be a key element to understand the enzymatic degradation of PLA and, in general, of any poly(hydroxyalkanoates), according to [PERSON] et al. [44]. These authors stated that the adsorption of _proteinase_\(K\) on PLA surfaces is an irreversible process that could be related to a hydrophobic interaction between the PLA and the enzymes.