## 1 Introduction  
Polylactic acid (PLA)-based scaffolds have been extensively used for bone regeneration [1; 2; 3; 4; 5; 6], as they have a suitable degradation rate and mechanical properties to replace the tissue in non-load-bearing applications. However, PLA is a hydrophobic material and, as a consequence, its interaction with the extracellular matrix (ECM) is low [7]. As the adsorption of adhesion-signaling proteins from the ECM is a key step to promote a good osseointegration of the scaffolds [8], the limited affinity of this biomaterial to these substances in the ECM is one of the main drawbacks of PLA-based structures.  
Different strategies have been proposed to overcome this limitation and increase the number of applications of PLA-based structures in the field of tissue engineering. An important approach comprises the incorporation of additives, including biocercamics [9], graphene oxide [10] and natural-derived polymers [11]. Another strategy is the surface modification of the scaffold [12] by changing the roughness, wettability and/or surface chemistry of the structures [8]. The processes to change the properties of the surface can be classified into two groups: the functionalization of PLA or the coating of the structure with a bioactive compound able to interact effectively with the proteins in the ECM. In the first group of techniques, it is possible to mention the plasma [13; 14] or alkaline treatment [15] of the parts. These surface treatments can be used to provide anchoring points to link bioactive compounds to the surface of PLA-based scaffolds. Different substances have been proposed with this aim, including chitosan [16], alginate [17], collagen [7; 12; 18] or calcium phosphates [19].  
In this work, the authors have tested the effect of applying an innovative coating based on extracts of aloe vera to 3D-printed PLA-based scaffolds. Aloe vera is a plant from the Lilliaceal family with demonstrated healing properties [20]. In the tissue engineering field, it has been tested mainly for skin regeneration [21; 22]. However, the use of this natural substance in bone tissue engineering remains limited [23] to the best of our knowledge and no references have been found regarding its use to improve the affinity of PLA and promote the attachment and proliferation of osteoblastic cells.  
According to the state of the art on the chemical characterization of aloe vera extracts [24; 25], acetaminophen is the main bioactive compound in the gel of the leaves. It is a mannose-containing polysaccharide that has demonstrated the promotion of collagen synthesis during wound healing [21; 26]. This characteristic could have positive implications for enhancing the osseointegration process in aloe vera coated scaffolds. Besides, there is currently a high availability of this product at affordable prices, which makes it a viable strategy from the economic point of view compared with other options, such as the use of growth factors [27]. For these reasons, this strategy has been proposed in this work as a promising approach to improve the performance of 3D-printed PLA scaffolds in vitro. Plasma treatment was used as a first step to functionalize the surface of the materials and promote aloe vera adhesion.