This study presents the impregnation in supercritical carbon dioxide (scCO2) of nanocellulose-based structures with thymol as a natural antimicrobial molecule to prepare bioactive, biosourced materials. First, cellulose nanofibrils (CNFs) were used to produce four types of materials (nanopapers, cryogels from water or tert-butyl alcohol suspensions, and aerogels) of increasing specific surface area up to 160 m2·g-1, thanks to the use of different processes, namely, vacuum filtration, freeze-drying, and supercritical drying. Second, these CNF-based structures were impregnated with thymol in the scCO2 medium using a relatively low temperature and pressure of 40 °C and 100 bar during 1 h. The amount of impregnated thymol in the different CNF materials was investigated by fluorescence spectroscopy, 13C NMR analysis, and gas chromatography. All three methods consistently showed that the amount of impregnated thymol increases with the specific surface area of the material. The antimicrobial activity of the impregnated CNF-based materials was then measured against three reference strains of microorganisms: the Gram-negative Escherichia coli and Gram-positive Staphylococcus epidermidis bacteria, and the yeast Candida albicans using the disk diffusion test method. The latter revealed the leaching of thymol in sufficient amounts to generate antimicrobial activity against the three strains in the case of the cryogel derived from a tert-butyl alcohol suspension and the aerogel, which are the two materials exhibiting the highest specific surface areas. The proposed strategy, therefore, enabled us to precisely steer the amount of active molecule loading and the related antimicrobial activity by adjusting the specific surface area of the biosourced material impregnated in green supercritical conditions. These results are very promising and confirm that supercritical impregnation of active molecules onto nanocellulose three-dimensional (3D) structures can be an interesting solution for the design of active medical devices such as wound dressings.
Keywords: aerogel; antimicrobial; cryogel; impregnation; nanocellulose; supercritical CO2; thymol.