The past two decades have been packed with the development of a new bio-nano-material called microfibrillated cellulose (MFC). Simply obtained by a strong fibrillation of the cellulosic fibers, MFC consists of aggregates of cellulose microfibrils, and has diameters ranging from 20–60 nm and length ≥ 1 μm [1]. These dimensions give it a high surface energy, and with the high amount of hydroxyl groups at its surface, MFC has the ability to form a tight nanoporous network at dried state. The applications of MFC have attracted much interest, and three years ago, it has been applied for the first time as drug delivery system. MFC has been used as spray-dried powder, aerogels, or films [2]. In parallel, our group proposed a new concept on the use of the nanoporous MFC network formed after coating onto paper substrate as sustained release system of active molecules. Promising results were achieved using caffeine as a model molecule [3].The present study aims to get deeper insight into the diffusion of caffeine through MFC-coated papers. The tested samples were prepared using three different strategies as shown Fig. 1a. The MFC network was observed using an Atomic Force Microscope (AFM). Release experiments were conducted in an aqueous medium, and analyzed using a UV spectrophotometer (Fig. 1b). Based on a solution of Fick's second law, a mathematical model was suggested to fit the experimental data. Further experiments were then performed under different conditions to confirm assumptions and validate predictions. The influence of MFC on the identified diffusivity values was then discussed. The relevance of the mathematical model used was finally questioned.