Tendon-inspired hydrogel with segmental change of stiffness modulated by carboxymethylated cellulose nanofibers

A tendon has a unique structure of spatially gradient stiffness. In mimicry of the structure, the great connection between each segment is critical for the spatial structuring of composites because mechanical failure mainly occurs at the binding site. It is challenging to mimic the tendon-inspired hydrogel structure with sequential stiffness by 3D printing a composite ink based on cellulose nanofibers (CNFs) and photoreactive acrylamide (AAm). Due to the difficulty of binding polymeric segments, the monomeric ink and matrix mixed with CNFs are adopted for matrix-assisted printing. Since the carboxymethylation of CNF enhances the nanofibrillation of cellulose pulp, the tensile strength of the carboxymethylated CNF (CM-CNF)/PAAm composite hydrogel can be modulated by reaction time and CM-CNF content. The uniaxially aligned lines of tough CM-CNF/AAm are printed in a soft CM-CNF/AAm matrix, which is produced at different reaction times. Changing composite volume fractions of CM-CNF/AAm (0%, 20%, 40%, 60%, and 100%) during printing enables a sequential change in stiffness (47.5 ± 3, 49.2 ± 2.5, 56.8 ± 7.5, 62.0 ± 5.8, and 81.3 ± 5.3 kPa) displaying a gradient in tensile strength without failure between segments as a tendon presents. Tightly bonded junctions between heterogeneous regions, and the controlled MAP of uniaxially aligned lines to form the tendon-like structure provide a novel approach to the fabrication of functional, tough hydrogels for use in biomedical engineering and soft robotics. Graphical abstract