Three-Dimensional Printing of a Tough Polymer Composite Gel Ink Reinforced with Chemically Modified Cellulose Nanofibrils Showing Self-Healing Behavior

Three-dimensional (3D) gel printing has significant potential for producing structurally accurate biomaterials based on complex biological tissues for use in regenerative biotechnology. The efficiency depends on the chosen gel printing device and the parameters used but also primarily on the selection of the gel ink. In the present study, the dimerizing functions in ureidopyrimidinone (UPy) were exploited to develop a gel ink. A composite gel ink was developed made out of a polymer consisting of UPy and di(ethylene glycol) ethyl ether (DEGEEA) functions and modified cellulose nanofibril-containing UPy groups (CNF- g -UPy) as print performance-enhancing additives. This resulted in a hyper-cross-linked network with superior mechanical strength compared to the matrix itself, with an increase of 3.7× in storage modulus and 2.8× in loss modulus. The CNF- g -UPy/UPy-DEGEEA gel ink exhibited shear-thinning properties and could therefore be extruded to form uniform strands with no merging and retain a relatively stable structure. The self-healing ability was demonstrated by the adhesion of two printed patterns after 2 min at 80 °C. The multiple cross-linkages and self-healing ability of the filaments, therefore, allow for stable printed structures. All of these findings point out the promising potential of such gel ink for many biotechnological applications.