Induction and quantification of fiber alignment in cellulose nanofibril films

Abstract Cellulose nanofibrils (CNF) have been explored as an emerging naturally sourced material for use in the preparation of new biomaterials. CNF fibrils have a high aspect ratio with fibril lengths of ~ 1 µm and diameters of 20–40 nm. The assembly of CNF impacts both bulk mechanical properties as well as localized cellular interaction. The ability to reproducibly tune CNF fiber alignment is an active area of CNF-based biomaterial research. Here, we present a simple CNF fibril alignment strategy based on application of constant unilateral force on thin CNF films drying on a flexible substrate. CNF fibril alignment/orientation was characterized using both Polarized Light Microscopy (PLM) and conventional Scanning Electron Microscopy (SEM) approaches. CNF is optically birefringent; therefore, calculation of the birefringence orientation index (BOI) can infer the extent of CNF fibril alignment with a non-destructive, cost-effective technique. CNF fibril alignment is markedly increased with application of 10.2 N force as assessed by both SEM and PLM analysis. SEM imaging resolved individual CNF and the alignment was analyzed using OrientationJ, an ImageJ plugin, to extract fibril angle whereas PLM microscopy provided a BOI value. Both the fibril alignment and BOI score were in agreement; therefore, it is acceptable to infer fibril organization with PLM techniques. Furthermore, the addition of nanoparticle hydroxyapatite did not diminish the CNF fibril alignment as assessed by both PLM and SEM highlighting the utility of the CNF film fabrication technique. In summary, the application of unilateral force on thin CNF films adhered to latex, is an elegant, scalable, and cost-effective technique for generating CNF films with reproducible fibril alignment.