The in vitro biostability and biocompatibility of ethyl vinyl acetate (EVA) nanocomposites incorporating organically modified montmorillonite (organo-MMT) were investigated as new candidate material for biomedical applications. The in vitro treatment of neat EVA and EVA nanocomposites was performed by immersing the materials in oxidizing and hydrolytic agents, at a temperature of 37 °C, for 4 weeks. The in vitro mechanical properties of the materials under these environmentally challenging conditions were assessed. Based on morphology studies, the degree of MMT dispersion and exfoliation decreased as the nanofiller loading increased. The EVA containing 1 wt% organo-MMT exhibited the best nanofiller dispersion and exfoliation characteristics. The surface degradation features of this nanocomposite were seen to be smoother than those of neat EVA and other EVA nanocomposites. Furthermore, the EVA nanocomposites have improved mechanical properties in comparison with the neat EVA, and these properties were less affected by the in vitro conditions. The best in vitro mechanical properties were achieved when 1 wt% of organo-MMT was added into the EVA. It was postulated that the presence of a better dispersed and exfoliated organo-MMT layered structure introduced a more tortuous path for the diffusion of oxidants and water molecules, thereby decreasing their permeation towards the EVA molecular chains. Therefore, the degradation kinetics within the EVA molecular chains were at a lower rate, which resulted in enhanced biostability. Furthermore, the toughness of the hydrated EVA (exposed to PBS at 37 °C) was greatly enhanced with the addition of the 1 wt% organo-MMT. The biocompatibility assessment suggests that the EVA nanocomposites are not cytotoxic, and thus have fulfilled the prerequisite to be further developed as a biomedical material.

Poly(ethylene-vinyl acetate) (PEVA) nanocomposite incorporating dual clay nanofiller (DCN) of surface modified montmorillonite (S-MMT) and bentonite (Bent) was studied for biomedical applications. In order to overcome agglomeration of the DCN, the S-MMT and Bent were subjected to a physical treatment prior to being mixed with the copolymer to form nanocomposite material. The S-MMT and Bent were physically treated to become S-MMT(P) and Bent(pH-s), respectively, that could be more readily dispersed in the copolymer matrix due to increments in their basal spacing and loosening of their tactoid structure. The biocompatibility of both nanofillers was assessed through a fibroblast cell cytotoxicity assay. The mechanical properties of the neat PEVA, PEVA nanocomposites, and PEVA-DCN nanocomposites were evaluated using a tensile test for determining the best S-MMT(P):Bent(pH-s) ratio. The results were supported by morphological studies by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Biostability evaluation of the samples was conducted by comparing the ambient tensile test data with the in vitro tensile test data (after being immersed in simulated body fluid at 37 °C for 3 months). The results were supported by surface degradation analysis. Our results indicate that the cytotoxicity level of both nanofillers reduced upon the physical treatment process, making them safe to be used in low concentration as dual nanofillers in the PEVA-DCN nanocomposite. The results of tensile testing, SEM, and TEM proved that the ratio of 4:1 (S-MMT(P):Bent(pH-s)) provides a greater enhancement in the mechanical properties of the PEVA matrix. The biostability assessment indicated that the PEVA-DCN nanocomposite can achieve much better retention in tensile strength after being subjected to the simulated physiological fluid for 3 months with less surface degradation effect. These findings signify the potential of the S-MMT(P)/Bent(pH-s) as a reinforcing DCN, with simultaneous function as biostabilizing agent to the PEVA copolymer for implant application.