Granular, microgel-based materials have garnered interest as promising tissue engineering scaffolds due to their inherent porosity, which can promote cell infiltration. Adapting these materials for 3D bioprinting, while maintaining sufficient void space to enable cell migration, can be challenging, since the rheological properties that determine printability are strongly influenced by microgel packing and void fraction. In this work, a strategy is proposed to decouple printability and void fraction by blending UV-crosslinkable gelatin methacryloyl (GelMA) microgels with sacrificial gelatin microgels to form composite inks. It is observed that inks with an apparent viscosity greater than ≈100 Pa s (corresponding to microgel concentrations ≥5 wt%) have rheological properties that enable extrusion-based printing of multilayered structures in air. By altering the ratio of GelMA to sacrificial gelatin microgels, while holding total concentration constant at 6 wt%, a family of GelMA:gelatin microgel inks is created that allows for tuning of void fraction from 0.20 to 0.57. Furthermore, human umbilical vein endothelial cells (HUVEC) seeded onto printed constructs are observed to migrate into granular inks in a void fraction-dependent manner. Thus, the family of microgel inks holds promise for use in 3D printing and tissue engineering applications that rely upon cell infiltration.

Inadequate rheological properties of gelatin methacrylamide (GelMA) were successfully improved by incorporating cellulose nanofibers (CNFs), such that the printed scaffolds could maintain their structural fidelity during the three-dimensional (3D) bio-printing process. The CNFs provided an outstanding shear thinning property, and the GelMA/CNF inks exhibited high zero shear viscosity and structural fidelity under a low dispensing pressure. After evaluating the printability, composite inks containing 2% w/v CNF were observed to have an optimal concentration of CNF to prepare 3D print stable constructs. Therefore, these inks were used to manufacture human nose and ear structures, producing highly porous structures in the printed composite hydrogels. Furthermore, the mechanical stability of the GelMA/CNF composite hydrogel was increased when CNFs were incorporated, which indicated that CNFs played an important role in enhancing the structural properties of the composite hydrogels. Additionally, the biocompatibility of CNF-reinforced hydrogels was evaluated using a fibroblast cell line.

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A cellulose nanofiber (CNF), one of the most attractive green bioresources, was adopted for construction of microfluidic devices using matrix-assisted three-dimensional (3D) printing. CNF hydrogels can support structures printed using CAD design in a 3D hydrogel environment with the appropriate combination of rheological properties between the CNF hydrogel and ink materials. Amazingly, the structure printed freely in the bulky CNF hydrogels was able to retain its highly resolved 3D features in an ultrathin two-dimensional (2D) paper using a simple drying process. The dimensional change in the CNF hydrogels from 3D to 2D resulted from simple dehydration of the CNFs and provided transparent, stackable paper-based 3D channel devices. As a proof of principle, the rheological properties of the CNF hydrogels, the 3D structure of the ink, the formation of channels by evacuation of the ink, and the highly localized selectivity of the devices are described.

It is not easy to design structures with transparent solutions, especially in light projection three-dimensional (3D) printing, since the penetration of light in solution is limitless. Here, silk fibroin incorporated with melanin nanoparticles (SFM) is used as a transparency modifier of poly(ethylene glycol)-tetraacrylate (PEG4A) solution. The incorporation of melanin into the SF hydrogel is performed in the range of 0.05-0.2% (w/v), and the SFM was added to the PEG4A precursor solution at 0.25-1.0% (w/v). The printing accuracy was examined by comparing the printed and designed feature sizes. The addition of 1.0% (w/v) SFM to a 4% (w/v) PEG4A (PEG4A/SFM) precursor solution effectively reduces the transparency of the solution and improves the printing resolution by confining the light beam to a designed region. This enables the fabrication of hard-to-express features such as hollow blood vessels or vacant tubes. Furthermore, the elastic modulus of the printed PEG4A/SFM composite hydrogel increases 2.5-fold higher than the PEG4A hydrogel without SFM. For the bio-ink, PEG4A/SFM-containing cells show non-cytotoxicity and improve the proliferation rate of embedded cells, confirming the high biocompatibility of PEG4A/SFM hydrogels.