Surface modification has been crucial for biomedical applications, namely in developing substrates that can respond to stimuli, have tunable cell adhesion and permeability control for bioactive agents. The layer-by-layer method (LbL) is an easy and straightforward approach to construct nanostructured ultrathin coatings by exploiting complementary interactions between building blocks onto solid supports. However, LbL often resorts to electrostatic interactions, which has limited the integration of new bioactive elements onto such class of coatings.
Herein, cashew gum (CG) derivatives have been used alongside chitosan (CHT) to produce LbL films via either electrostatic interactions or Schiff base interactions. CG is a natural product derived from the Anacardium occidentale L. tree, abundant in northeastern Brazil. Its main composition consists of D-galactose (72%), D-glucose (14%), arabinose (4.6%), rhamnose (3.2%), glucuronic acid (4.7%), and around 1% of protein content. It is a non-toxic anionic compound, inexpensive, and biodegradable. CHT is a polycationic polysaccharide obtained by the alkaline deacetylation of chitin, composed of glucosamine and randomly distributed N-acetyl-glucosamine. Its unique properties, such as biocompatibility, biodegradability, antibacterial activity, wound healing properties, makes CHT one of the most used natural polymers in tissue engineering and other biomedical applications.
The used CG derivatives include one carboxymethylated (CGCm) and three oxidized (CGOx, where x represents 20%, 50% or 80%) variations. FTIR and zeta-potential analyses confirmed the success of carboxymethylation and oxidation reactions as well as the positive and negative charge of CHT and CGCm, respectively. QCM-D results demonstrated that the CG derivatives allowed building up coatings with increased thickness compared to unmodified CG. The thickest film was achieved for CGO20 (about 100 nm), suggesting that a higher quantity of CGO20 was required to overcompensate the previous CHT layer. Roughness measurements by AFM showed that the roughest surface is obtained using CGO80 (Rq=25.1 nm). On the one hand, this behaviour may be attributed to the stronger type of interactions (Schiff bases) that are not present in the unmodified CG and CGCm formulations. On the other hand, CGO80 presented islet-like structures at the surface, despite being the formulation that leads to the lowest thickness of all CGOx derivatives. This work shows the possibility to control the thickness and topography of CG-containing multilayered films by increasing the strength of its derivatives, thus widening the potential of CG as bioactive elements in coatings for biomedical devices.