Low-Concentration Gelatin Methacryloyl Hydrogel with Tunable 3D Extrusion Printability and Cytocompatibility: Exploring Quantitative Process Science and Biophysical Properties

Three-dimensional (3D) bioprinting of hydrogels with a wide spectrum of compositions has been widely investigated. Despite such efforts, a comprehensive understanding of the correlation among the process science, buildability, and biophysical properties of the hydrogels for a targeted clinical application has not been developed in the scientific community. In particular, the quantitative analysis across the entire developmental path for 3D extrusion bioprinting of such scaffolds is not widely reported. In the present work, we addressed this gap by using widely investigated biomaterials, such as gelatin methacryloyl (GelMA), as a model system. Using extensive experiments and quantitative analysis, we analyzed how the individual components of methacrylated carboxymethyl cellulose (mCMC), needle-shaped nanohydroxyapatite (nHAp), and poly(ethylene glycol)diacrylate (PEGDA) with GelMA as baseline matrix of the multifunctional bioink can influence the biophysical properties, printability, and cellular functionality. The complex interplay among the biomaterial ink formulations, viscoelastic properties, and printability toward the large structure buildability (structurally stable cube scaffolds with 15 mm edge) has been explored. Intriguingly, the incorporation of PEGDA into the GelMA/mCMC matrix offered improved compressive modulus (∼40-fold), reduced swelling ratio (∼2-fold), and degradation rates (∼30-fold) compared to pristine GelMA. The correlation among microstructural pore architecture, biophysical properties, and cytocompatibility is also established for the biomaterial inks. These photopolymerizable bio(material)inks served as the platform for the growth and development of bone and cartilage matrix when human mesenchymal stem cells (hMSCs) are either seeded on two-dimensional (2D) substrates or encapsulated on 3D scaffolds. Taken together, this present study unequivocally establishes a significant step forward in the development of a broad spectrum of shape-fidelity compliant bioink for the 3D bioprinting of multifunctional scaffolds and emphasizes the need for invoking more quantitative analysis in establishing process-microstructure-property correlation.

A Printable Magnetic-Responsive Iron Oxide Nanoparticle (ION)-Gelatin Methacryloyl (GelMA) Ink for Soft Bioactuator/Robot Applications

The features or actuation behaviors of nature's creatures provide concepts for the development of biomimetic soft bioactuators/robots with stimuli-responsive capabilities, design convenience, and environmental adaptivity in various fields. Mimosa pudica is a mechanically responsive plant that can convert pressure to the motion of leaves. When the leaves receive pressure, the occurrence of asymmetric turgor in the extensor and flexor sides of the pulvinus from redistributing the water in the pulvinus causes the bending of the pulvinus. Inspired by the actuation of Mimosa pudica, designing soft bioactuators can convert external stimulations to driving forces for the actuation of constructs which has been receiving increased attention and has potential applications in many fields. 4D printing technology has emerged as a new strategy for creating versatile soft bioactuators/robots by integrating printing technologies with stimuli-responsive materials. In this study, we developed a hybrid ink by combining gelatin methacryloyl (GelMA) polymers with iron oxide nanoparticles (IONs). This hybrid ION-GelMA ink exhibits tunable rheology, controllable mechanical properties, magnetic-responsive behaviors, and printability by integrating the internal metal ion-polymeric chain interactions and photo-crosslinking chemistries. This design offers the inks a dual crosslink mechanism combining the advantages of photocrosslinking and ionic crosslinking to rapidly form the construct within 60 s of UV exposure time. In addition, the magnetic-responsive actuation of ION-GelMA constructs can be regulated by different ION concentrations (0-10%). Furthermore, we used the ION-GelMA inks to fabricate a Mimosa pudica-like soft bioactuator through a mold casting method and a direct-ink-writing (DIW) printing technology. Obviously, the pinnule leaf structure of printed constructs presents a continuous reversible shape transformation in an air phase without any liquid as a medium, which can mimic the motion characteristics of natural creatures. At the same time, compared to the model casting process, the DIW printed bioactuators show a more refined and biomimetic transformation shape that closely resembles the movement of the pinnule leaf of Mimosa pudica in response to stimulation. Overall, this study indicates the proof of concept and the potential prospect of magnetic-responsive ION-GelMA inks for the rapid prototyping of biomimetic soft bioactuators/robots with untethered non-contact magneto-actuations.

Enhanced Maturation of 3D Bioprinted Skeletal Muscle Tissue Constructs Encapsulating Soluble Factor-Releasing Microparticles

Several microfabrication technologies have been used to engineer native-like skeletal muscle tissues. However, the successful development of muscle remains a significant challenge in the tissue engineering field. Muscle tissue engineering aims to combine muscle precursor cells aligned within a highly organized 3D structure and biological factors crucial to support cell differentiation and maturation into functional myotubes and myofibers. In this study, the use of 3D bioprinting is proposed for the fabrication of muscle tissues using gelatin methacryloyl (GelMA) incorporating sustained insulin-like growth factor-1 (IGF-1)-releasing microparticles and myoblast cells. This study hypothesizes that functional and mature myotubes will be obtained more efficiently using a bioink that can release IGF-1 sustainably for in vitro muscle engineering. Synthesized microfluidic-assisted polymeric microparticles demonstrate successful adsorption of IGF-1 and sustained release of IGF-1 at physiological pH for at least 21 days. Incorporating the IGF-1-releasing microparticles in the GelMA bioink assisted in promoting the alignment of myoblasts and differentiation into myotubes. Furthermore, the myotubes show spontaneous contraction in the muscle constructs bioprinted with IGF-1-releasing bioink. The proposed bioprinting strategy aims to improve the development of new therapies applied to the regeneration and maturation of muscle tissues.

Antibacterial Activity and Biocompatibility with the Concentration of Ginger Fraction in Biodegradable Gelatin Methacryloyl (GelMA) Hydrogel Coating for Medical Implants

The gingerols and shogaols derived from ginger have excellent antibacterial properties against oral bacteria. However, some researchers have noted their dose-dependent potential toxicity. The aim of this study was to enhance the biofunctionality and biocompatibility of the application of ginger to dental titanium screws. To increase the amount of coating of the n-hexane-fractionated ginger on the titanium surface and to control its release, ginger was loaded in different concentrations in a photo-crosslinkable GelMA hydrogel. To improve coating stability of the ginger hydrogel (GH), the wettability of the surface was modified by pre-calcification (TNC), then GH was applied on the surface. As a result, the ginger fraction, with a high content of phenolic compounds, was effective in the inhibition of the growth of S. mutans and P. gingivalis. The GH slowly released the main compounds of ginger and showed excellent antibacterial effects with the concentration. Although bone regeneration was slightly reduced with the ginger-loading concentration due to the increased contents of polyphenolic compounds, it was strongly supplemented through the promotion of osteosis formation by the hydrogel and TNC coating. Finally, we proved the biosafety and superior biofunctionalities the GH-TNC coating on a Ti implant. However, it is recommended to use an appropriate concentration, because an excessive concentration of ginger may affect the improved biocompatibility in clinical applications.

3D Bioprinting of Smart Oxygen-Releasing Cartilage Scaffolds

Three-dimensional bioprinting is a powerful technique for manufacturing improved engineered tissues. Three-dimensional bioprinted hydrogels have significantly advanced the medical field to repair cartilage tissue, allowing for such constructs to be loaded with different components, such as cells, nanoparticles, and/or drugs. Cartilage, as an avascular tissue, presents extreme difficulty in self-repair when it has been damaged. In this way, hydrogels with optimal chemical and physical properties have been researched to respond to external stimuli and release various bioactive agents to further promote a desired tissue response. For instance, methacryloyl gelatin (GelMA) is a type of modified hydrogel that allows for the encapsulation of cells, as well as oxygen-releasing nanoparticles that, in the presence of an aqueous medium and through controlled porosity and swelling, allow for internal and external environmental exchanges. This review explores the 3D bioprinting of hydrogels, with a particular focus on GelMA hydrogels, to repair cartilage tissue. Recent advances and future perspectives are described.

Tunable metacrylated hyaluronic acid-based hybrid bioinks for stereolithography 3D bioprinting

Hyaluronic acid is a native extra-cellular matrix derivative that promises unique properties, such as anti-inflammatory response and cell-signaling with tissue-specific applications under its bioactive properties. Here, we investigate the importance of the duration of synthesis to obtain photocrosslinkable methacrylated hyaluronic acid (MeHA) with high degree of substitution. MeHA with high degree of substitution can result in rapid photocrosslinking and can be used as a bioink for stereolithographic (SLA) three dimensional 3D bioprinting. Increased degree of substitution results Our findings show that a ten-day synthesis results in an 88% degree of methacrylation (DM), whereas three-day and five-day syntheses result in 32% and 42% DM, respectively. The rheological characterization revealed an increased rate of photopolymerization with increasing DM. Further, we developed a hybrid bioink to overcome the non-cell-adhesive nature of MeHA by combining it with gelatin methacryloyl (GelMA) to fabricate 3D cell-laden hydrogel scaffolds. The hybrid bioink exhibited a 55% enhancement in stiffness compared to MeHA only and enabled cell-adhesion while maintaining high cell viability. Investigations also revealed that the hybrid bioink was a more suitable candidate for stereolithography (SLA) 3D bioprinting than MeHA because of its mechanical strength, printability, and cell-adhesive nature. This research lays out a firm foundation for the development of a stable hybrid bioink with MeHA and GelMA for first-ever use with SLA 3D bioprinting.

Automated 3D Microphysiometry Facilitates High-Content and Highly Reproducible Oxygen Measurements within 3D Cell Culture Models

Microphysiometry is a powerful technique to study metabolic parameters and detect changes to external stimuli. However, applying this technique for automated label-free and real-time measurements within cell-laden three-dimensional (3D) cell culture constructs remains a challenge. Herein, we present an entirely automated microphysiometry setup that combines needle-type microsensors with motorized sample and sensor positioning systems inside a standard tissue-culture incubator. The setup records dissolved oxygen as a metabolic parameter along the z-direction within cell-laden 3D constructs in a minimally invasive manner. The microphysiometry setup was applied to characterize the spatial oxygen distribution within thick cell-laden 3D constructs, study the time-dependent changes on the oxygen tension within 3D breast cancer models following a chemotherapeutic treatment, and identify kinetics and recovery effects after drug exposure over 5 weeks. Our data suggest that the microphysiometry setup enables highly reproducible measurements without human intervention, due to the high degree of automation and positional accuracy. The results demonstrate the applicability of the setup to provide valuable long-term insights into oxygenation within 3D models using minimally invasive, label-free, and entirely automated analysis methods.

Fabrication and Characterization of Biodegradable Gelatin Methacrylate/Biphasic Calcium Phosphate Composite Hydrogel for Bone Tissue Engineering

In the field of bone tissue, maintaining adequate mechanical strength and tissue volume is an important part. Recently, biphasic calcium phosphate (BCP) was fabricated to solve the shortcomings of hydroxyapatite (HA) and beta-tricalcium phosphate (β-TCP), and it is widely studied in the field of bone-tissue engineering. In this study, a composite hydrogel was fabricated by applying BCP to gelatin methacrylate (GelMA). It was tested by using a mechanical tester, to characterize the mechanical properties of the prepared composite hydrogel. The fabricated BCP was analyzed through FTIR and XRD. As a result, a different characteristic pattern from hydroxyapatite (HA) and beta-tricalcium phosphate (β-TCP) was observed, and it was confirmed that it was successfully bound to the hydrogel. Then, the proliferation and differentiation of preosteoblasts were checked to evaluate cell viability. The analysis results showed high cell viability and relatively high bone differentiation ability in the composite hydrogel to which BCP was applied. These features have been shown to be beneficial for bone regeneration by maintaining the volume and shape of the hydrogel. In addition, hydrogels can be advantageous for clinical use, as they can shape the structure of the material for custom applications.

Comparative Analysis of Mesenchymal Stem Cell Cultivation in Fetal Calf Serum, Human Serum, and Platelet Lysate in 2D and 3D Systems

In vitro two-dimensional (2D) and three-dimensional (3D) cultivation of mammalian cells requires supplementation with serum. Mesenchymal stem cells (MSCs) are widely used in clinical trials for bioregenerative medicine and in most cases, in vitro expansion and differentiation of these cells are required before application. Optimized expansion and differentiation protocols play a key role in the treatment outcome. 3D cell cultivation systems are more comparable to in vivo conditions and can provide both, more physiological MSC expansion and a better understanding of intercellular and cell-matrix interactions. Xeno-free cultivation conditions minimize risks of immune response after implantation. Human platelet lysate (hPL) appears to be a valuable alternative to widely used fetal calf serum (FCS) since no ethical issues are associated with its harvest, it contains a high concentration of growth factors and cytokines and it can be produced from expired platelet concentrate. In this study, we analyzed and compared proliferation, as well as osteogenic and chondrogenic differentiation of human adipose tissue-derived MSCs (hAD-MSC) using three different supplements: FCS, human serum (HS), and hPL in 2D. Furthermore, online monitoring of osteogenic differentiation under the influence of different supplements was performed in 2D. hPL-cultivated MSCs exhibited a higher proliferation and differentiation rate compared to HS- or FCS-cultivated cells. We demonstrated a fast and successful chondrogenic differentiation in the 2D system with the addition of hPL. Additionally, FCS, HS, and hPL were used to formulate Gelatin-methacryloyl (GelMA) hydrogels in order to evaluate the influence of the different supplements on the cell spreading and proliferation of cells growing in 3D culture. In addition, the hydrogel constructs were cultivated in media supplemented with three different supplements. In comparison to FCS and HS, the addition of hPL to GelMA hydrogels during the encapsulation of hAD-MSCs resulted in enhanced cell spreading and proliferation. This effect was promoted even further by cultivating the hydrogel constructs in hPL-supplemented media.

Designing Gelatin Methacryloyl (GelMA)-Based Bioinks for Visible Light Stereolithographic 3D Biofabrication

Bioinks play a key role in determining the capability of the biofabricatoin processes and the resolution of the printed constructs. Excellent biocompatibility, tunable physical properties, and ease of chemical or biological modifications of gelatin methacryloyl (GelMA) have made it an attractive choice as bioinks for biomanufacturing of various tissues or organs. However, the current preparation methods for GelMA-based bioinks lack the ability to tailor their physical properties for desired bioprinting methods. Inherently, GelMA prepolymer solution exhibits a fast sol-gel transition at room temperature, which is a hurdle for its use in stereolithography (SLA) bioprinting. Here, synthesis parameters are optimized such as solvents, pH, and reaction time to develop GelMA bioinks which have a slow sol-gel transition at room temperature and visible light crosslinkable functions. A total of eight GelMA combinations are identified as suitable for digital light processing (DLP)-based SLA (DLP-SLA) bioprinting through systematic characterizations of their physical and rheological properties. Out of various types of GelMA, those synthesized in reverse osmosis (RO) purified water (referred to as RO-GelMA) are regarded as most suitable to achieve high DLP-SLA printing resolution. RO-GelMA-based bioinks are also found to be biocompatible showing high survival rates of encapsulated cells in the photocrosslinked gels. Additionally, the astrocytes and fibroblasts are observed to grow and integrate well within the bioprinted constructs. The bioink's superior physical and photocrosslinking properties offer pathways of tuning the scaffold microenvironment and highlight the applicability of developed GelMA bioinks in various tissue engineering and regenerative medicine applications.

Synchronous 3D Bioprinting of Large-Scale Cell-Laden Constructs with Nutrient Networks

Maintaining an adequate supply of nutrients/oxygen is a major challenge in the biofabrication of large tissue constructs. However, building preformed nutrient networks may be an effective strategy for engineering thick tissues. Here, a novel way for bioprinting large-scale tissue constructs with intentional nutrient networks is presented. A special nozzle is developed which can print bioink and sacrificial ink half and half synchronously in a single filament. Nutrient networks of these bioprinted constructs are formed by subsequently dissolving away gelatin, which allows for effective oxygen, nutrient, and waste diffusion, facilitating the cell activity and the generation of functional tissues. Due to the cell-laden bioink and sacrificial ink working together and promoting each other's printability to support themselves, complex soft cell-laden constructs with nutrient networks can easily be printed. Furthermore, two different cell types (osteoblast, human umbilical vein endothelial cells) encapsulated in the bioprinted large-scale constructs (≥1 cm) with nutrient networks show enhanced cell viability and spreading within a period of culture. It is envisioned that the advanced bioprinting technology may have significant potentials in facilitating the engineering of complex structures for tissue-specific needs, and bioprinting large-scale tissue constructs with nutrient networks toward applications in organ transplantation and repair.

Biofabrication of endothelial cell, dermal fibroblast, and multilayered keratinocyte layers for skin tissue engineering

The skin serves a substantial number of physiological purposes and is exposed to numerous biological and chemical agents owing to its large surface area and accessibility. Yet, current skin models are limited in emulating the multifaceted functions of skin tissues due to a lack of effort on the optimization of biomaterials and techniques at different skin layers for building skin frameworks. Here, we use biomaterial-based approaches and bioengineered techniques to develop a 3D skin model with layers of endothelial cell networks, dermal fibroblasts, and multilayered keratinocytes. Analysis of mechanical properties of gelatin methacryloyl (GelMA)-based bioinks mixed with different portions of alginate revealed bioprinted endothelium could be better modeled to optimize endothelial cell viability with a mixture of 7.5% GelMA and 2% alginate. Matrix stiffness plays a crucial role in modulating produced levels of Pro-Collagen I alpha-1 and matrix metalloproteinase-1 in human dermal fibroblasts and affecting their viability, proliferation, and spreading. Moreover, seeding human keratinocytes with gelatin-coating multiple times proves helpful in reducing culture time to create multilayered keratinocytes while maintaining their viability. The ability to fabricate selected biomaterials for each layer of skin tissues has implications in the biofabrication of skin systems for regenerative medicine and disease modeling.

Electro-Assisted Bioprinting of Low-Concentration GelMA Microdroplets

Low-concentration gelatin methacryloyl (GelMA) has excellent biocompatibility to cell-laden structures. However, it is still a big challenge to stably fabricate organoids (even microdroplets) using this material due to its extremely low viscosity. Here, a promising electro-assisted bioprinting method is developed, which can print low-concentration pure GelMA microdroplets with low cost, low cell damage, and high efficiency. With the help of electrostatic attraction, uniform GelMA microdroplets measuring about 100 μm are rapidly printed. Due to the application of lower external forces to separate the droplets, cell damage during printing is negligible, which often happens in piezoelectric or thermal inkjet bioprinting. Different printing states and effects of printing parameters (voltages, gas pressure, nozzle size, etc.) on microdroplet diameter are also investigated. The fundamental properties of low-concentration GelMA microspheres are subsequently studied. The results show that the printed microspheres with 5% w/v GelMA can provide a suitable microenvironment for laden bone marrow stem cells. Finally, it is demonstrated that the printed microdroplets can be used in building microspheroidal organoids, in drug controlled release, and in 3D bioprinting as biobricks. This method shows great potential use in cell therapy, drug delivery, and organoid building.

Aqueous Two-Phase Emulsion Bioink-Enabled 3D Bioprinting of Porous Hydrogels

3D bioprinting technology provides programmable and customizable platforms to engineer cell-laden constructs mimicking human tissues for a wide range of biomedical applications. However, the encapsulated cells are often restricted in spreading and proliferation by dense biomaterial networks from gelation of bioinks. Herein, a cell-benign approach is reported to directly bioprint porous-structured hydrogel constructs by using an aqueous two-phase emulsion bioink. The bioink, which contains two immiscible aqueous phases of cell/gelatin methacryloyl (GelMA) mixture and poly(ethylene oxide) (PEO), is photocrosslinked to fabricate predesigned cell-laden hydrogel constructs by extrusion bioprinting or digital micromirror device-based stereolithographic bioprinting. The porous structure of the 3D-bioprinted hydrogel construct is formed by subsequently removing the PEO phase from the photocrosslinked GelMA hydrogel. Three different cell types (human hepatocellular carcinoma cells, human umbilical vein endothelial cells, and NIH/3T3 mouse embryonic fibroblasts) within the 3D-bioprinted porous hydrogel patterns show enhanced cell viability, spreading, and proliferation compared to the standard (i.e., nonporous) hydrogel constructs. The 3D bioprinting strategy is believed to provide a robust and versatile platform to engineer porous-structured tissue constructs and their models for a variety of applications in tissue engineering, regenerative medicine, drug development, and personalized therapeutics.