Bioprinting has advanced drastically in the last decade, leading to many new biomedical applications for tissue engineering and regenerative medicine. However, there are still a myriad of challenges to overcome, with vast amounts of research going into bioprinter technology, biomaterials, cell sources, vascularization, innervation, maturation, and complex 4D functionalization. Currently, stereolithographic bioprinting is the primary technique for polymer resin bioinks. However, it lacks the ability to print multiple cell types and multiple materials, control directionality of materials, and place fillers, cells, and other biological components in specific locations among the scaffolds. This study sought to create bioinks from a typical polymer resin, poly(ethylene glycol) diacrylate (PEGDA), for use in extrusion bioprinting to fabricate gradient scaffolds for complex tissue engineering applications. Bioinks were created by adding cellulose nanocrystals (CNCs) into the PEGDA resin at ratios from 95/5 to 60/40 w/w PEGDA/CNCs, in order to reach the viscosities needed for extrusion printing. The bioinks were cast, as well as printed into single-material and multiple-material (gradient) scaffolds using a CELLINK BIOX printer, and crosslinked using lithium phenyl-2,4,6-trimethylbenzoylphosphinate as the photoinitiator. Thermal and mechanical characterizations were performed on the bioinks and scaffolds using thermogravimetric analysis, rheology, and dynamic mechanical analysis. The 95/5 w/w composition lacked the required viscosity to print, while the 60/40 w/w composition displayed extreme brittleness after crosslinking, making both CNC compositions non-ideal. Therefore, only the bioink compositions of 90/10, 80/20, and 70/30 w/w were used to produce gradient scaffolds. The gradient scaffolds were printed successfully and embodied unique mechanical properties, utilizing the benefits of each composition to increase mechanical properties of the scaffold as a whole. The bioinks and gradient scaffolds successfully demonstrated tunability of their mechanical properties by varying CNC content within the bioink composition and the compositions used in the gradient scaffolds. Although stereolithographic bioprinting currently dominates the printing of PEGDA resins, extrusion bioprinting will allow for controlled directionality, cell placement, and increased complexity of materials and cell types, improving the reliability and functionality of the scaffolds for tissue engineering applications.

Poly-[(R)-3-hydroxyalkanoate] biopolymers, or PHAs, are biocompatible and biodegradable polyesters that can be produced by diverse microbial strains. PHA polymers have found widespread uses in applications ranging from sustainable replacements of nonbiodegradable bulk-commodity plastics to biomaterials. However, further expansion into other markets and industries has generally been limited by the inability to chemically modify these polymers. Recently, our lab engineered E. coli LSBJ, a microbial strain able to produce PHA copolymers with controlled unit compositions from simple and accessible fatty acid feedstocks. We envisioned meaningfully broadening the application spectrum of these materials via production of chemically tractable PHA biopolymers containing “click”-ready chemical functionalities. With a myriad of applications in mind, in this study we demonstrate the synthesis and biopolymerization of a panel of ω-azido fatty acids and take the first exploratory steps toward demonstrating their conjugation via a strain-promoted azide–alkyne cycloaddition (SPAAC) reaction. The convenience of accessing these materials will open the door to new applications for functionalized PHA polymers.

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Cellulose nanomaterials provide a promising avenue for combining composite materials with natural polymers or bio-based plastics, however, current studies on rheological characterization at high shear rates is limited. In this study, a capillary rheometer was employed to investigate the rheological characteristics of aqueous nanofibrillated cellulose suspensions (NFCs) at room temperature and shear rates above 1000 s -1 . The shear rate-dependent viscosity of NFCs was characterized and monitored for viscosity-related indicators of fibril decomposition or entanglement due to the stresses induced by the high shear-rate flow. Traditional capillary rheology corrections for entrance pressure loss and irregular flow profiles were attempted to compare apparent and true rheological behaviors. Large entanglements of fibrils made the characterization of the suspensions difficult due to their ability to clog the capillary. However, appropriate preprocessing steps enabled better measurement resolution and enhanced dispersion of cellulose nanofibers, as verified by atomic force microscopy. The suspensions display shear thinning behavior and demonstrate higher entrance pressures for lower shear rates. Similar preprocessing techniques could be implemented on a large scale to enhance fibril distribution in NFC suspensions.

Cellulose nanomaterials provide a promising avenue for combining composite materials with natural polymers or bio-based plastics, however, current studies on rheological characterization at high shear rates is limited. In this study, a capillary rheometer was employed to investigate the rheological characteristics of aqueous nanofibrillated cellulose suspensions (NFCs) at room temperature and shear rates above 1000 s-1. The shear rate-dependent viscosity of NFCs was characterized and monitored for viscosity-related indicators of fibril decomposition or entanglement due to the stresses induced by the high shear-rate flow. Traditional capillary rheology corrections for entrance pressure loss and irregular flow profiles were attempted to compare apparent and true rheological behaviors. Large entanglements of fibrils made the characterization of the suspensions difficult due to their ability to clog the capillary. However, appropriate preprocessing steps enabled better measurement resolution and enhanced dispersion of cellulose nanofibers, as verified by atomic force microscopy. The suspensions display shear thinning behavior and demonstrate higher entrance pressures for lower shear rates. Similar preprocessing techniques could be implemented on a large scale to enhance fibril distribution in NFC suspensions.