Inkjet micropatterning through horseradish peroxidase-mediated hydrogelation for controlled cell immobilization and microtissue fabrication

Cell Cycle Modulation through Physical Confinement in Micrometer-Thick Hydrogel Sheaths

Recently, surface engineering of the cell membrane with biomaterials has attracted great attention for various biomedical applications. In this study, we investigated the possibility of modulating cell cycle progression using alginate and gelatin-based hydrogel sheaths with a thickness of ∼1 μm. The hydrogel sheath was formed on cell surfaces through cross-linking catalyzed by horseradish peroxidase immobilized on the cell surface. The hydrogel sheath did not decrease the viability (>95% during 2 days of culture) of the human cervical carcinoma cell line (HeLa) expressing the fluorescent ubiquitination-based cell cycle indicator 2 (HeLa/Fucci2). Coating the HeLa/Fucci2 cells with the hydrogel sheath resulted in a cell cycle arrest in the G2/M phase, which can be caused by the reduced F-actin formation. As a result of this cell cycle arrest, an inhibition of cell growth was observed in the HeLa/Fucci2 cells. Taken together, our results demonstrate that the hydrogel sheath coating on the cell surface is a feasible approach to modulating cell cycle progression.

Visible light photocrosslinking of sugar beet pectin for 3D bioprinting applications

Herein, we report the hydrogelation of sugar beet pectin (SBP) via visible light-mediated photocrosslinking and its applications in extrusion-based 3D bioprinting. Rapid hydrogelation (<15 s) was achieved by applying 405 nm visible light to an SBP solution in the presence of tris(bipyridine)ruthenium(II) chloride hexahydrate ([Ru(bpy)₃]²⁺) and sodium persulfate (SPS). The mechanical properties of the hydrogel could be tuned by controlling the visible light irradiation time and concentrations of SBP, [Ru(bpy)₃]²⁺, and SPS. High-fidelity 3D hydrogel constructs were fabricated by extruding inks containing 3.0 wt% SBP, 1.0 mM [Ru(bpy)₃]²⁺, and 1.0 mM SPS. Human hepatoblastoma (HepG2) cells encapsulated in SBP hydrogels remained viable and metabolically active after 14 d of culture. Overall, this study demonstrates the feasibility of applying SBP and a visible light-mediated photocrosslinking system to the 3D bioprinting of cell-laden constructs for tissue engineering applications.

Biomechanical microenvironment in peripheral nerve regeneration: from pathophysiological understanding to tissue engineering development

Peripheral nerve injury (PNI) caused by trauma, chronic disease and other factors may lead to partial or complete loss of sensory, motor and autonomic functions, as well as neuropathic pain. Biological activities are always accompanied by mechanical stimulation, and biomechanical microenvironmental homeostasis plays a complicated role in tissue repair and regeneration. Recent studies have focused on the effects of biomechanical microenvironment on peripheral nervous system development and function maintenance, as well as neural regrowth following PNI. For example, biomechanical factors-induced cluster gene expression changes contribute to formation of peripheral nerve structure and maintenance of physiological function. In addition, extracellular matrix and cell responses to biomechanical microenvironment alterations after PNI directly trigger a series of cascades for the well-organized peripheral nerve regeneration (PNR) process, where cell adhesion molecules, cytoskeletons and mechanically gated ion channels serve as mechanosensitive units, mechanical effector including focal adhesion kinase (FAK) and yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) as mechanotransduction elements. With the rapid development of tissue engineering techniques, a substantial number of PNR strategies such as aligned nerve guidance conduits, three-dimensional topological designs and piezoelectric scaffolds emerge expected to improve the neural biomechanical microenvironment in case of PNI. These tissue engineering nerve grafts display optimized mechanical properties and outstanding mechanomodulatory effects, but a few bottlenecks restrict their application scenes. In this review, the current understanding in biomechanical microenvironment homeostasis associated with peripheral nerve function and PNR is integrated, where we proposed the importance of balances of mechanosensitive elements, cytoskeletal structures, mechanotransduction cascades, and extracellular matrix components; a wide variety of promising tissue engineering strategies based on biomechanical modulation are introduced with some suggestions and prospects for future directions.

Modulation of Cell-Cycle Progression by Hydrogen Peroxide-Mediated Cross-Linking and Degradation of Cell-Adhesive Hydrogels

The cell cycle is known to be regulated by features such as the mechanical properties of the surrounding environment and interaction of cells with the adhering substrates. Here, we investigated the possibility of regulating cell-cycle progression of the cells on gelatin/hyaluronic acid composite hydrogels obtained through hydrogen peroxide (H₂O₂)-mediated cross-linking and degradation of the polymers by varying the exposure time to H₂O₂ contained in the air. The stiffness of the hydrogel varied with the exposure time. Human cervical cancer cells (HeLa) and mouse mammary gland epithelial cells (NMuMG) expressing cell-cycle reporter Fucci2 showed the exposure-time-dependent different cell-cycle progressions on the hydrogels. Although HeLa/Fucci2 cells cultured on the soft hydrogel (Young’s modulus: 0.20 and 0.40 kPa) obtained through 15 min and 120 min of the H₂O₂ exposure showed a G2/M-phase arrest, NMuMG cells showed a G1-phase arrest. Additionally, the cell-cycle progression of NMuMG cells was not only governed by the hydrogel stiffness, but also by the low-molecular-weight HA resulting from H₂O₂-mediated degradation. These results indicate that H₂O₂-mediated cross-linking and degradation of gelatin/hyaluronic acid composite hydrogel could be used to control the cell adhesion and cell-cycle progression.

Development of a Hybrid Nanoink for 3D Bioprinting of Heterogeneous Tumor Models

Despite the rapid progress in applying three-dimensional (3D) printing in the field of tissue engineering, fabrication of heterogeneous and complex 3D tumor models remains a challenge. In this study, we report a hybrid nanoink (AGC) composed of alginate, gelatin methacryloyl (GelMA), and cellulose nanocrystal (CNC), designed for multinozzle microextrusion 3D printing of tumor models. Our results show that the ink consisting of 2 wt % alginate, 4 wt % GelMA, and 6 wt % cellulose nanocrystals (AGC246) possesses a superior shear-thinning property and little hysteresis in viscosity recovery. The fabrication of a colorectal cancer (CRC) model is demonstrated by printing a 3D topological substrate with AGC246 and then seeding/printing endothelial (EA-hy 926) and colorectal carcinoma (HCT 116) cells on top. Direct seeding of cells by dropping a cell suspension onto the 3D substrate with distinctive topological features (villi and trenches) deemed inadequate in either creating a monolayer of endothelial cells or precise positioning of cancer cell clusters, even with surface treatment to promote cell adhesion. In contrast, 3D biopinting of a CRC model using cell-laden AGC153, coupled with dual ultraviolet (UV) and ionic cross-linking, is shown to be successful. Hence, this study brings advancements in 3D bioprinting technology through innovative material and methodology designs, which could enable the fabrication of complex in vitro models for both fundamental studies of disease processes and applications in drug screening.

Influence of Hydrogen Peroxide-Mediated Cross-Linking and Degradation on Cell-Adhesive Gelatin Hydrogels

Hydrogen peroxide (H₂O₂) is widely used for the gelation of aqueous solutions of gelatin derivatives with phenolic hydroxyl groups (Gelatin-Ph) catalyzed by horseradish peroxidase (HRP). Apart from this, H₂O₂ is known to cause degradation/depolymerization of various polymers. Here, we prepared Gelatin-Ph hydrogels from solutions containing Gelatin-Ph and HRP by continuously supplying H₂O₂ from the gas phase and investigated the mechanical properties of resultant hydrogels and the behaviors of rat fibroblast and human adipose-derived stem cells on them. Young's modulus of the hydrogel obtained from 5 w/v % Gelatin-Ph and 1 and 5 U/mL HRP increased when the exposure time to air containing H₂O₂ (16 ppm) was extended from 15 to 30 min. However, further prolonging the exposure time to 60 min reduced Young's modulus to the same magnitude as for the hydrogels exposed to air containing H₂O₂ for 15 min. Interestingly, the cell length and aspect ratio of the cells continued to increase, as the exposure time was extended, without reflecting the decrease in Young's modulus. These results indicate that when preparing Gelatin-Ph hydrogels through HRP/H₂O₂-mediated gelation, it is necessary to consider the effect of the degradation of Gelatin-Ph caused by H₂O₂ on the mechanical properties of the resultant hydrogels and the behaviors of cells on them.

Silk fibroin nanofibers: a promising ink additive for extrusion three-dimensional bioprinting

Here, we investigated the usefulness of silk fibroin nanofibers obtained via mechanical grinding of degummed silkworm silk fibers as an additive in bioinks for extrusion three-dimensional (3D) bioprinting of cell-laden constructs. The nanofibers could be sterilized by autoclaving, and addition of the nanofibers improved the shear thinning of polymeric aqueous solutions, independent of electric charge and the content of cross-linkable moieties in the polymers. The addition of nanofibers to bioinks resulted in the fabrication of hydrogel constructs with higher fidelity to blueprints. Mammalian cells in the constructs showed >85% viability independent of the presence of nanofibers. The nanofibers did not affect the morphologies of enclosed cells. These results demonstrate the great potential of silk fibroin nanofibers obtained via mechanical grinding of degummed silkworm silk fibers as an additive in bioinks for extrusion 3D bioprinting.

Void-free 3D Bioprinting for In-situ Endothelialization and Microfluidic Perfusion

Two major challenges of 3D bioprinting are the retention of structural fidelity and efficient endothelialization for tissue vascularization. We address both of these issues by introducing a versatile 3D bioprinting strategy, in which a templating bioink is deposited layer-by-layer alongside a matrix bioink to establish void-free multimaterial structures. After crosslinking the matrix phase, the templating phase is sacrificed to create a well-defined 3D network of interconnected tubular channels. This void-free 3D printing (VF-3DP) approach circumvents the traditional concerns of structural collapse, deformation and oxygen inhibition, moreover, it can be readily used to print materials that are widely considered "unprintable". By pre-loading endothelial cells into the templating bioink, the inner surface of the channels can be efficiently cellularized with a confluent endothelial layer. This in-situ endothelialization method can be used to produce endothelium with a far greater uniformity than can be achieved using the conventional post-seeding approach. This VF-3DP approach can also be extended beyond tissue fabrication and towards customized hydrogel-based microfluidics and self-supported perfusable hydrogel constructs.

Extrusion-Based Bioprinting through Glucose-Mediated Enzymatic Hydrogelation

We report an extrusion-based bioprinting approach, in which stabilization of extruded bioink is achieved through horseradish peroxidase (HRP)-catalyzed cross-linking consuming hydrogen peroxide (H2O2) supplied from HRP and glucose. The bioinks containing living cells, HRP, glucose, alginate possessing phenolic hydroxyl (Ph) groups, and cellulose nanofiber were extruded to fabricate 3D hydrogel constructs. Lattice- and human nose-shaped 3D constructs were successfully printed and showed good stability in cell culture medium for over a week. Mouse 10T1/2 fibroblasts enclosed in the printed constructs remained viable after 7 days of culture. It was also able to switch a non-cell-adhesive surface of the printed construct to cell-adhesive surface for culturing cells on it through a subsequent cross-linking of gelatin possessing Ph moieties. These results demonstrate the possibility of utilizing the presented cross-linking method for 3D bioprinting.