Plant seed-inspired cell protection, dormancy, and growth for large-scale biofabrication

4D printed shape-shifting biomaterials for tissue engineering and regenerative medicine applications

The existing 3D printing methods exhibit certain fabrication-dependent limitations for printing curved constructs that are relevant for many tissues. Four-dimensional (4D) printing is an emerging technology that is expected to revolutionize the field of tissue engineering and regenerative medicine (TERM). 4D printing is based on 3D printing, featuring the introduction of time as the fourth dimension, in which there is a transition from a 3D printed scaffold to a new, distinct, and stable state, upon the application of one or more stimuli. Here, we present an overview of the current developments of the 4D printing technology for TERM, with a focus on approaches to achieve temporal changes of the shape of the printed constructs that would enable biofabrication of highly complex structures. To this aim, the printing methods, types of stimuli, shape-shifting mechanisms, and cell-incorporation strategies are critically reviewed. Furthermore, the challenges of this very recent biofabrication technology as well as the future research directions are discussed. Our findings show that the most common printing methods so far are stereolithography (SLA) and extrusion bioprinting, followed by fused deposition modelling, while the shape-shifting mechanisms used for TERM applications are shape-memory and differential swelling for 4D printing and 4D bioprinting, respectively. For shape-memory mechanism, there is a high prevalence of synthetic materials, such as polylactic acid (PLA), poly(glycerol dodecanoate) acrylate (PGDA), or polyurethanes. On the other hand, different acrylate combinations of alginate, hyaluronan, or gelatin have been used for differential swelling-based 4D transformations. TERM applications include bone, vascular, and cardiac tissues as the main target of the 4D (bio)printing technology. The field has great potential for further development by considering the combination of multiple stimuli, the use of a wider range of 4D techniques, and the implementation of computational-assisted strategies.

The Effect of the Mechanical Properties of the 3D Printed Gelatin/Hyaluronic Acid Scaffolds on hMSCs Differentiation Towards Chondrogenesis

BACKGROUND

Tissue engineering, including 3D bioprinting, holds great promise as a therapeutic tool for repairing cartilage defects. Mesenchymal stem cells have the potential to treat various fields due to their ability to differentiate into different cell types. The biomimetic substrate, such as scaffolds and hydrogels, is a crucial factor that affects cell behavior, and the mechanical properties of the substrate have been shown to impact differentiation during incubation. In this study, we examine the effect of the mechanical properties of the 3D printed scaffolds, made using different concentrations of cross-linker, on hMSCs differentiation towards chondrogenesis.

METHODS

The 3D scaffold was fabricated using 3D bioprinting technology with gelatin/hyaluronic acid (HyA) biomaterial ink. Crosslinking was achieved by using different concentrations of 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methlymorpholinium chloride n-hydrate (DMTMM), allowing for control of the scaffold's mechanical properties. The printability and stability were also evaluated based on the concentration of DMTMM used. The effects of the gelatin/HyA scaffold on chondrogenic differentiation was analyzed by utilizing various concentrations of DMTMM.

RESULTS

The addition of HyA was found to improve the printability and stability of 3D printed gelatin/HyA scaffolds. The mechanical properties of the 3D gelatin/HyA scaffold could be regulated through the use of different concentrations of DMTMM cross-linker. In particular, the use of 0.25 mM DMTMM for crosslinking the 3D gelatin/HyA scaffold resulted in enhanced chondrocyte differentiation.

CONCLUSION

The mechanical properties of 3D printed gelatin/HyA scaffolds cross-linked using various concentrations of DMTMM can influence the differentiation of hMSCs into chondrocytes.

3D-Printed Functional Hydrogel by DNA-Induced Biomineralization for Accelerated Diabetic Wound Healing

Chronic wounds in diabetic patients are challenging because their prolonged inflammation makes healing difficult, thus burdening patients, society, and health care systems. Customized dressing materials are needed to effectively treat such wounds that vary in shape and depth. The continuous development of 3D-printing technology along with artificial intelligence has increased the precision, versatility, and compatibility of various materials, thus providing the considerable potential to meet the abovementioned needs. Herein, functional 3D-printing inks comprising DNA from salmon sperm and DNA-induced biosilica inspired by marine sponges, are developed for the machine learning-based 3D-printing of wound dressings. The DNA and biomineralized silica are incorporated into hydrogel inks in a fast, facile manner. The 3D-printed wound dressing thus generates provided appropriate porosity, characterized by effective exudate and blood absorption at wound sites, and mechanical tunability indicated by good shape fidelity and printability during optimized 3D printing. Moreover, the DNA and biomineralized silica act as nanotherapeutics, enhancing the biological activity of the dressings in terms of reactive oxygen species scavenging, angiogenesis, and anti-inflammation activity, thereby accelerating acute and diabetic wound healing. These bioinspired 3D-printed hydrogels produce using a DNA-induced biomineralization strategy are an excellent functional platform for clinical applications in acute and chronic wound repair.

In-situ ionic crosslinking of 3D bioprinted cell-hydrogel constructs for mechanical reinforcement and improved cell growth

Hydrogels are commonly used in 3D bioprinting technology owing to their ability to encapsulate living cells. However, their inherent delicate properties limit their applicability in the fabrication of mechanically reliable tissue engineering constructs. Herein, we propose a novel reinvented layering integration method for the functional enhancement of 3D cell-hydrogel bioprinting. This was implemented by inserting electrospun microfiber sheets with a crosslinker between the 3D bioprinted layers. When surface-modified microfiber sheets were combined with Ca²⁺ ionic crosslinkers, the as-printed cell-hydrogel strand was immediately crosslinked when it contacted the sheet surface. The in-situ crosslinking in the bioprinting process not only improved the overall structural stability, but also reinforced the compressive strength and elastic modulus. The enhanced structural stability guaranteed the shape fidelity of the 3D architecture, which included the internal channel network, resulting in improved perfusion conditions for cell growth. The growth of NIH3T3 fibroblasts in 3D bioconstructs with in-situ crosslinking increased by up to five times compared to that of normally bioprinted constructs. The strengthened structural integrity was distinctly sustainable during the cell culture period owing to the sustained release of Ca²⁺ ions from the embedded microfiber sheets. The synergistic effect of the reinforced mechanical properties with enhanced cell growth is expected to extend the applicability of the proposed hydrogel-based bioprinting technique for soft tissue engineering.

Low-Swelling Adhesive Hydrogel with Rapid Hemostasis and Potent Anti-Inflammatory Capability for Full-Thickness Oral Mucosal Defect Repair

Full-thickness oral mucosal defects are accompanied by significant blood loss and frequent infections. Instead of conventional therapies that separate hemostasis and anti-inflammation in steps, emerging hydrogels can integrate multiple functions for the successive process after defect including hemostasis/inflammatory phase, proliferative phase, and remodeling phase. However, these functions can be easily compromised by rapid swelling and degradation of hydrogels in wet oral environment. Herein, a low-swelling adhesive hydrogel with rapid hemostasis and potent anti-inflammatory capability was developed using a dual cross-linking strategy as well as a safe and facile fabrication method. It was double cross-linked hydrogel consisting of gelatin methacrylate (GelMA), nanoclay, and tannic acid (TA) (referred to as GNT). GNT hydrogel exhibited low-swelling (one-eighth of that of GelMA), excellent stretchability (211.86%), and good adhesive properties (5 times the adhesive strength of GelMA). Physicochemical characterization illuminated the close interactions among the three components. A systematic investigation of the therapeutic effects of GNT hydrogels was performed. In vitro and in vivo experimental results demonstrated the potent hemostatic property and excellent antibacterial and anti-inflammatory effects of GNT hydrogels. The RNA sequencing analysis results for rat full-thickness oral mucosal samples showed that GNT reduced inflammation levels by down-regulating the expression of multiple inflammation-related pathways, including TNF and IL-17 pathways. It also enhanced the expression levels of tissue regeneration-related genes and thus accelerated defective mucosal repair. More importantly, the therapeutic effects of GNT were superior to those of a commercial oral tissue repair membrane when applied for full-thickness oral mucosal defect repair in rabbits. In summary, the prepared low-swelling adhesive GNT hydrogel with rapid hemostasis and potent anti-inflammatory is a promising therapy for full-thickness mucosal defect in the moist and dynamic oral environment.

Three-Dimensional Bioprinting for Cartilage Tissue Engineering: Insights into Naturally-Derived Bioinks from Land and Marine Sources

In regenerative medicine and tissue engineering, the possibility to: (I) customize the shape and size of scaffolds, (II) develop highly mimicked tissues with a precise digital control, (III) manufacture complex structures and (IV) reduce the wastes related to the production process, are the main advantages of additive manufacturing technologies such as three-dimensional (3D) bioprinting. Specifically, this technique, which uses suitable hydrogel-based bioinks, enriched with cells and/or growth factors, has received significant consideration, especially in cartilage tissue engineering (CTE). In this field of interest, it may allow mimicking the complex native zonal hyaline cartilage organization by further enhancing its biological cues. However, there are still some limitations that need to be overcome before 3D bioprinting may be globally used for scaffolds’ development and their clinical translation. One of them is represented by the poor availability of appropriate, biocompatible and eco-friendly biomaterials, which should present a series of specific requirements to be used and transformed into a proper bioink for CTE. In this scenario, considering that, nowadays, the environmental decline is of the highest concerns worldwide, exploring naturally-derived hydrogels has attracted outstanding attention throughout the scientific community. For this reason, a comprehensive review of the naturally-derived hydrogels, commonly employed as bioinks in CTE, was carried out. In particular, the current state of art regarding eco-friendly and natural bioinks’ development for CTE was explored. Overall, this paper gives an overview of 3D bioprinting for CTE to guide future research towards the development of more reliable, customized, eco-friendly and innovative strategies for CTE.

Tethered TGF-β1 in a Hyaluronic Acid-Based Bioink for Bioprinting Cartilaginous Tissues

In 3D bioprinting for cartilage regeneration, bioinks that support chondrogenic development are of key importance. Growth factors covalently bound in non-printable hydrogels have been shown to effectively promote chondrogenesis. However, studies that investigate the functionality of tethered growth factors within 3D printable bioinks are still lacking. Therefore, in this study, we established a dual-stage crosslinked hyaluronic acid-based bioink that enabled covalent tethering of transforming growth factor-beta 1 (TGF-β1). Bone marrow-derived mesenchymal stromal cells (MSCs) were cultured over three weeks in vitro, and chondrogenic differentiation of MSCs within bioink constructs with tethered TGF-β1 was markedly enhanced, as compared to constructs with non-covalently incorporated TGF-β1. This was substantiated with regard to early TGF-β1 signaling, chondrogenic gene expression, qualitative and quantitative ECM deposition and distribution, and resulting construct stiffness. Furthermore, it was successfully demonstrated, in a comparative analysis of cast and printed bioinks, that covalently tethered TGF-β1 maintained its functionality after 3D printing. Taken together, the presented ink composition enabled the generation of high-quality cartilaginous tissues without the need for continuous exogenous growth factor supply and, thus, bears great potential for future investigation towards cartilage regeneration. Furthermore, growth factor tethering within bioinks, potentially leading to superior tissue development, may also be explored for other biofabrication applications.

4D bioprintable self-healing hydrogel with shape memory and cryopreserving properties

Four-dimensional (4D) bioprinting is an emerging biofabrication technology that integrates time as a fourth dimension with three-dimensional (3D) bioprinting for fabricating customizable tissue-engineered implants. 4D bioprinted implants are expected to possess self-healing and shape memory properties for new application opportunities, for instance, fabrication of devices with good shape integrity for minimally invasive surgery. Herein, we developed a self-healing hydrogel composed of biodegradable polyurethane (PU) nanoparticles and photo-/thermo-responsive gelatin-based biomaterials. The self-healing property of hydrogel may be associated with the formation of reversible ionomeric interaction between the COO^-group of PU nanoparticles and NH₃⁺group on the gelatin chains. The self-healing hydrogel demonstrated excellent 3D printability and filament resolution. The UV-crosslinked printed hydrogel showed good stackability (>80 layers), structural stability, elasticity, and tunable modulus (1-60 kPa). The shape-memorizable 4D printed constructs revealed good shape fixity (∼95%) and shape recovery (∼98%) through the elasticity as well as forming and collapsing of water lattice in the hydrogel. The hydrogel and the printing process supported the continuous proliferation of neural stem cells (NSCs) (∼3.7-fold after 14 days). Moreover, the individually bioprinted NSCs and mesenchymal stem cells in the adjacent, self-healed filaments showed mutual migration and such interaction promoted the cell differentiation behavior. The cryopreserved (-20 °C or -80 °C) 4D bioprinted hydrogel after awakening and shape recovery at 37 °C demonstrated cell proliferation similar to that of the non-cryopreserved control. This 4D bioprintable, self-healable hydrogel with shape memory and cryopreserving properties may be employed for customized biofabrication.

A Critical Review on Polymeric Biomaterials for Biomedical Applications

Natural and synthetic polymers have been explored for many years in the field of tissue engineering and regeneration. Researchers have developed many new strategies to design successful advanced polymeric biomaterials. In this review, we summarized the recent notable advancements in the preparation of smart polymeric biomaterials with self-healing and shape memory properties. We also discussed novel approaches used to develop different forms of polymeric biomaterials such as films, hydrogels and 3D printable biomaterials. In each part, the applications of the biomaterials in soft and hard tissue engineering with their in vitro and in vivo effects are underlined. The future direction of the polymeric biomaterials that could pave a path towards successful clinical implications is also underlined in this review.

Improving Cell Viability and Velocity in μ-Extrusion Bioprinting with a Novel Pre-Incubator Bioprinter and a Standard FDM 3D Printing Nozzle

Bioprinting is a promising emerging technology. It has been widely studied by the scientific community for the possibility to create transplantable artificial tissues, with minimal risk to the patient. Although the biomaterials and cells to be used are being carefully studied, there is still a long way to go before a bioprinter can easily and quickly produce printings without harmful effects on the cells. In this sense, we have developed a new μ-extrusion bioprinter formed by an Atom Proton 3D printer, an atmospheric enclosure and a new extrusion-head capable to increment usual printing velocity. Hence, this work has two main objectives. First, to experimentally study the accuracy and precision. Secondly, to study the influence of flow rates on cellular viability using this novel μ-extrusion bioprinter in combination with a standard FDM 3D printing nozzle. Our results show an X, Y and Z axis movement accuracy under 17 μm with a precision around 12 μm while the extruder values are under 5 and 7 μm, respectively. Additionally, the cell viability obtained from different volumetric flow tests varies from 70 to 90%. So, the proposed bioprinter and nozzle can control the atmospheric conditions and increase the volumetric flow speeding up the bioprinting process without compromising the cell viability.

Encapsulation of bacterial cells in cytoprotective ZIF-90 crystals as living composites

Exploiting metal-organic frameworks (MOFs) as selectively permeable shelters for encapsulating engineered cells to form hybrid living materials has attracted increasing attention in recent years. Optimizing the synthesis process to improve encapsulation efficiency (EE) is critical for further technological development and applications. Here, using ZIF-90 and genetically engineered Escherichia coli (E. coli) as a demo, we fabricated E. coli@ZIF-90 living composites in which E. coli cells were encapsulated in ZIF-90 crystals. We illustrated that ZIF-90 could serve as a protective porous cage for cells to shield against toxic bactericides including benzaldehyde, cinnamaldehyde, and kanamycin. Notably, the E. coli cells remained alive and could self-reproduce after removing the ZIF-90 crystal cages in ethylenediaminetetraacetic acid, suggesting a feasible route for protecting and prolonging the lifespan of bacterial cells. Moreover, an aqueous multiple-step deposition approach was developed to improve EE of the E. coli@ZIF-90 composites: the EE increased to 61.9 ± 5.2%, in contrast with the efficiency of the traditional method (21.3 ± 4.4%) prepared with PBS buffer. In short, we develop a simple yet viable strategy to manufacture MOF-based living hybrid materials that promise new applications across diverse fields.

Development of pH-Responsive Polymer Coating as an Alternative to Enzyme-Based Stem Cell Dissociation for Cell Therapy

Cell therapy usually accompanies cell detachment as an essential process in cell culture and cell collection for transplantation. However, conventional methods based on enzymatic cell detachment can cause cellular damage including cell death and senescence during the routine cell detaching step due to an inappropriate handing. The aim of the current study is to apply the pH-responsive degradation property of poly (amino ester) to the surface of a cell culture dish to provide a simple and easy alternative method for cell detachment that can substitute the conventional enzyme treatment. In this study, poly (amino ester) was modified (cell detachable polymer, CDP) to show appropriate pH-responsive degradation under mild acidic conditions (0.05% (w/v) CDP, pH 6.0) to detach stem cells (human adipose tissue-derived stem cells (hADSCs)) perfectly within a short period (less than 10 min). Compared to conventional enzymatic cell detachment, hADSCs cultured on and detached from a CDP-coated cell culture dish showed similar cellular properties. We further performed in vivo experiments on a mouse hindlimb ischemia model (1.0 × 106 cells per limb). The in vivo results indicated that hADSCs retrieved from normal cell culture dishes and CDP-coated cell culture dishes showed analogous therapeutic angiogenesis. In conclusion, CDP could be applied to a pH-responsive cell detachment system as a simple and easy nonenzymatic method for stem cell culture and various cell therapies.

Enhancement of 3D-Printable Materials by Dual-Curing Procedures

Dual-curing thermosetting systems are recently being developed as an alternative to conventional curing systems due to their processing flexibility and the possibility of enhancing the properties of cured parts in single- or multi-stage processing scenarios. Most dual-curing systems currently employed in three-dimensional (3D) printing technologies are aimed at improving the quality and properties of the printed parts. However, further benefit can be obtained from control in the curing sequence, making it possible to obtain partially reacted 3D-printed parts with tailored structure and properties, and to complete the reaction by activation of a second polymerization reaction in a subsequent processing stage. This paves the way for a range of novel applications based on the controlled reactivity and functionality of this intermediate material and the final consolidation of the 3D-printed part after this second processing stage. In this review, different strategies and the latest developments based on the concept of dual-curing are analyzed, with a focus on the enhanced functionality and emerging applications of the processed materials.

Bioinspired cell-in-shell systems in biomedical engineering and beyond: Comparative overview and prospects

With the development in tissue engineering, cell transplantation, and genetic technologies, living cells have become an important therapeutic tool in clinical medical care. For various cell-based technologies including cell therapy and cell-based sensors in addition to fundamental studies on single-cell biology, the cytoprotection of individual living cells is a prerequisite to extend cell storage life or deliver cells from one place to another, resisting various external stresses. Nature has evolved a biological defense mechanism to preserve their species under unfavorable conditions by forming a hard and protective armor. Particularly, plant seeds covered with seed coat turn into a dormant state against stressful environments, due to mechanical and water/gas constraints imposed by hard seed coat. However, when the environmental conditions become hospitable to seeds, seed coat is ruptured, initiating seed germination. This seed dormancy and germination mechanism has inspired various approaches that artificially induce cell sporulation via chemically encapsulating individual living cells within a thin but tough shell forming a 3D "cell-in-shell" structure. Herein, the recent advance of cell encapsulation strategies along with the potential advantages of the 3D "cell-in-shell" system is reviewed. Diverse coating materials including polymeric shells and hybrid shells on different types of cells ranging from microbes to mammalian cells will be discussed in terms of enhanced cytoprotective ability, control of division, chemical functionalization, and on-demand shell degradation. Finally, current and potential applications of "cell-in-shell" systems for cell-based technologies with remaining challenges will be explored.

Improved cell viability for large-scale biofabrication with photo-crosslinkable hydrogel systems through a dual-photoinitiator approach

Biofabrication with various hydrogel systems allows the production of tissue or organ constructs in vitro to address various challenges in healthcare and medicine. In particular, photocrosslinkable hydrogels have great advantages such as excellent spatial and temporal selectivity and low processing cost and energy requirements. However, inefficient polymerization kinetics of commercialized photoinitiators upon exposure to UV-A radiation or visible light increase processing time, often compromising cell viability. In this study, we developed a hydrogel crosslinking system which exhibited efficient crosslinking properties and desired mechanical properties with high cell viability, through a dual-photoinitiator approach. Through the co-existence of Irgacure 2959 and VA-086, the overall crosslinking process was completed with a minimal UV dosage during a significantly reduced crosslinking time, producing mechanically robust hydrogel constructs, while most encapsulated cells within the hydrogel constructs remained viable. Moreover, we fabricated a large PEGDA hydrogel construct with a single microchannel as a proof of concept for hydrogels with vasculature to demonstrate the versatility of the system. Our dual-photoinitiator approach allowed the production of this photocrosslinkable hydrogel system with microchannels, significantly improving cell viability and processing efficiency, yet maintaining good mechanical stability. Taken together, we envision the concurrent use of photoinitiators, Irgacure 2959 and VA-086, opening potential avenues for the utilization of various photocrosslinkable hydrogel systems in perfusable large artificial tissue for in vivo and ex vivo applications with improved processing efficiency and cell viability.

3D Freeform Printing of Nanocomposite Hydrogels through in situ Precipitation in Reactive Viscous Fluid

Composite hydrogels have gained great attention as three-dimensional (3D) printing biomaterials because of their enhanced intrinsic mechanical strength and bioactivity compared to pure hydrogels. In most conventional printing methods for composite hydrogels, particles are preloaded in ink before printing, which often reduces the printability of composite ink with little mechanical improvement due to poor particle-hydrogel interaction of physical mixing. In contrast, the in situ incorporation of nanoparticles into a hydrogel during 3D printing achieves uniform distribution of particles with remarkable mechanical reinforcement, while precursors dissolved in inks do not influence the printing process. Herein, we introduced a "printing in liquid" technique coupled with a hybridization process, which allows 3D freeform printing of nanoparticle-reinforced composite hydrogels. A viscoplastic matrix for this printing system provides not only support for printed hydrogel filaments but also chemical reactants to induce various reactions in printed objects for in situ modification. Nanocomposite hydrogel scaffolds were successfully fabricated through this 3D freeform printing of hyaluronic acid (HAc)-alginate (Alg) hydrogel inks through a two-step crosslinking strategy. The first ionic crosslinking of Alg provided structural stability during printing, while the secondary crosslinking of photo-curable HAc improved the mechanical and physiological stability of the nanocomposite hydrogels. For in situ precipitation during 3D printing, phosphate ions were dissolved in the hydrogel ink and calcium ions were added to the viscoplastic matrix. The composite hydrogels demonstrated a significant improvement in mechanical strength, biostability, as well as biological performance compared to pure HAc. Moreover, the multi-material printing of composites with different calcium phosphate contents was achieved by adjusting the ionic concentration of inks. Our method greatly accelerates the 3D printing of various functional or hybridized materials with complex geometries through the design and modification of printing materials coupled with in situ post-printing functionalization and hybridization in reactive viscoplastic matrices.

3D Freeform Printing of Nanocomposite Hydrogels through in situ Precipitation in Reactive Viscous Fluid

Composite hydrogels have gained great attention as three-dimensional (3D) printing biomaterials because of their enhanced intrinsic mechanical strength and bioactivity compared to pure hydrogels. In most conventional printing methods for composite hydrogels, particles are preloaded in ink before printing, which often reduces the printability of composite ink with little mechanical improvement due to poor particle-hydrogel interaction of physical mixing. In contrast, the in situ incorporation of nanoparticles into a hydrogel during 3D printing achieves uniform distribution of particles with remarkable mechanical reinforcement, while precursors dissolved in inks do not influence the printing process. Herein, we introduced a "printing in liquid" technique coupled with a hybridization process, which allows 3D freeform printing of nanoparticle-reinforced composite hydrogels. A viscoplastic matrix for this printing system provides not only support for printed hydrogel filaments but also chemical reactants to induce various reactions in printed objects for in situ modification. Nanocomposite hydrogel scaffolds were successfully fabricated through this 3D freeform printing of hyaluronic acid (HAc)-alginate (Alg) hydrogel inks through a two-step crosslinking strategy. The first ionic crosslinking of Alg provided structural stability during printing, while the secondary crosslinking of photo-curable HAc improved the mechanical and physiological stability of the nanocomposite hydrogels. For in situ precipitation during 3D printing, phosphate ions were dissolved in the hydrogel ink and calcium ions were added to the viscoplastic matrix. The composite hydrogels demonstrated a significant improvement in mechanical strength, biostability, as well as biological performance compared to pure HAc. Moreover, the multi-material printing of composites with different calcium phosphate contents was achieved by adjusting the ionic concentration of inks. Our method greatly accelerates the 3D printing of various functional or hybridized materials with complex geometries through the design and modification of printing materials coupled with in situ post-printing functionalization and hybridization in reactive viscoplastic matrices.