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.
Cellulose nanofibrils (CNFs) have been explored as an emerging naturally sourced material for the development of new biomaterials. The ability to reproducibly tune CNF fiber alignment is an active area of CNF-based biomaterial research. Typically, CNF-based biomaterials are composites, requiring the ability to control the organization of both CNF and additional additives to achieve desired mechanical and cellular interactions. Here, we present a simple CNF fibril alignment strategy based on the application of constant unilateral force on thin CNF films drying on a flexible substrate to reproducibly modulate CNF fibril alignment while integrating nanoparticle hydroxyapatite, a candidate mineral oxide. CNF fibril alignment/orientation is characterized using Polarized Light Microscopy (PLM), Scanning Electron Microscopy (SEM) and mechanical tensile testing. Collectively, all these characterization tools demonstrate that the application of a 10.2 N unilateral force on a CNF film drying on a flexible substrate results in increased CNF fibril alignment. Furthermore, the addition of nanoparticle hydroxyapatite did not diminish the CNF fibril alignment as assessed by both PLM, SEM, and modulus of elasticity 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 composite films with reproducible fibril alignment.
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