Nanoengineered Colloidal Inks for 3D Bioprinting

Injectable Hydrogels: A Paradigm Tailored with Design, Characterization, and Multifaceted Approaches

Biomaterials denoting self-healing and versatile structural integrity are highly curious in the biomedicine segment. The injectable and/or printable 3D printing technology is explored in a few decades back, which can alter their dimensions temporarily under shear stress, showing potential healing/recovery tendency with patient-specific intervention toward the development of personalized medicine. Thus, self-healing injectable hydrogels (IHs) are stunning toward developing a paradigm for tissue regeneration. This review comprises the designing of IHs, rheological characterization and stability, several benchmark consequences for self-healing IHs, their translation into tissue regeneration of specific types, applications of IHs in biomedical such as anticancer and immunomodulation, wound healing and tissue/bone regeneration, antimicrobial potentials, drugs, gene and vaccine delivery, ocular delivery, 3D printing, cosmeceuticals, and photothermal therapy as well as in other allied avenues like agriculture, aerospace, electronic/electrical industries, coating approaches, patents associated with therapeutic/nontherapeutic avenues, and numerous futuristic challenges and solutions.

Anisotropic hydrogel scaffold by flow-induced stereolithography 3D printing technique

Essential organs, such as the heart and liver, contain a unique porous network that allows oxygen and nutrients to be exchanged, with distinct random to ordered regions displaying varying degrees of strength. A novel technique, referred to here as flow-induced lithography, was developed. This technique generates tunable anisotropic three-dimensional (3D) structures. The ink for this bioprinting technique was made of titanium dioxide nanorods (Ti) and kaolinite nanoclay (KLT) dispersed in a GelMA/PEGDA polymeric suspension. By controlling the flow rate, aligned particle microstructures were achieved in the suspensions. The application of UV light to trigger the polymerization of the photoactive prepolymer freezes the oriented particles in the polymer network. Because the viability test was successful in shearing suspensions containing cells, the flow-induced lithography technique can be used with both acellular scaffolds and cell-laden structures. Fabricated hydrogels show outstanding mechanical properties resembling human tissues, as well as significant cell viability (> 95 %) over one week. As a result of this technique and the introduction of bio-ink, a novel approach has been pioneered for developing anisotropic tissue implants utilizing low-viscosity biomaterials.

Granular Biphasic Colloidal Hydrogels for 3D Bioprinting

Granular hydrogels composed of hydrogel microparticles are promising candidates for 3D bioprinting due to their ability to protect encapsulated cells. However, to achieve high print fidelity, hydrogel microparticles need to jam to exhibit shear-thinning characteristics, which is crucial for 3D printing. Unfortunately, this overpacking can significantly impact cell viability, thereby negating the primary advantage of using hydrogel microparticles to shield cells from shear forces. To overcome this challenge, a novel solution: a biphasic, granular colloidal bioink designed to optimize cell viability and printing fidelity is introduced. The biphasic ink consists of cell-laden polyethylene glycol (PEG) hydrogel microparticles embedded in a continuous gelatin methacryloyl (GelMA)-nanosilicate colloidal network. Here, it is demonstrated that this biphasic bioink offers outstanding rheological properties, print fidelity, and structural stability. Furthermore, its utility for engineering complex tissues with multiple cell types and heterogeneous microenvironments is demonstrated, by incorporating β-islet cells into the PEG microparticles and endothelial cells in the GelMA-nanosilicate colloidal network. Using this approach, it is possible to induce cell patterning, enhance vascularization, and direct cellular function. The proposed biphasic bioink holds significant potential for numerous emerging biomedical applications, including tissue engineering and disease modeling.

Pimobendan controlled release guar gum printlets: Tailoring drug doses for personalised veterinary medicines

Treating chronic heart diseases in dogs is challenging due to variations in mass within and between species. Pimobendan (PBD), a veterinary drug only, is prescribed in specific cases of chronic heart disease in dogs and is available on the market in only a few different doses. Furthermore, the therapy itself is challenging due to the large size of the chewable tablets and the requirement for twice-daily administration. The development of customised and on-demand PBD medicines by three-dimensional (3D) printing has been proposed to circumvent these disadvantages. In this study, we designed controlled-release flavoured printlets containing PBD. We evaluated the use of two natural polymers, guar or xanthan gums, as the main component of the printing inks. Guar gum showed the better rheological behavior and printability by semisolid extrusion. The printlets were produced in three different shapes and sizes to allow dose customisation. Guar gum printlets showed a PBD controlled release profile, regardless of their shape or size. Therefore, we have demonstrated a novel approach for controlling PBD drug release and tailoring the dose by employing a natural polymer to produce 3D-printed tablets. This study represents a significant step towards the development of 3D-printed guar gum controlled-release formulations for veterinary applications.

A sustainable and self-healable silk fibroin nanocomposite with antibacterial and drug eluting properties for 3D printed wound dressings

The development of self-healable and 3D printable hydrogels with decent biocompatibility, mechanical durability, adhesiveness to tissues, and antibacterial activity is of great importance for wound healing applications. In this study, we present a sustainable and environmentally friendly composite hydrogel consisting of silk fibroin (SF), oxidized salep (OS), and kappa carrageenan nanoparticles (NPs) for efficient wound care. The injectable nanocomposite hydrogel is highly stretchable and exhibits strong tissue adhesiveness and self-healing response through Schiff-base cross-linking between OS and SF. The tunable shear-thinning viscoelastic properties of the hydrogel facilitate 3D bioprinting with excellent shape adaptability (97.7 ± 1.1% recovery), enabling the fabrication of complex-shaped constructs. In vitro release kinetics of tetracycline (TC) encapsulated in kappa carrageenan NPs indicate a distinctive Korsmeyer-Peppas profile, including an initial burst release followed by a triphasic pattern controlled by the embedded NPs within the hydrogel matrix. The composite hydrogel shows a remarkable broad-spectrum antibacterial activity with substantial zones of inhibition against S. aureus (34.00 ± 1.00 mm) and E. coli (27.60 ± 2.08 mm) after 24 h of incubation at 37 °C. The addition of TC further enhances the zones of inhibition by approximately 45% for S. aureus and 27% for E. coli. The control group without kappa NP incorporation shows no zone of inhibition, underscoring the critical role of the nanoparticles in imparting antibacterial activity to the hydrogel. Cytocompatibility assays show the high viability of fibroblast (L929) cells (>90%) in vitro. In vivo biocompatibility studies through subcutaneous implantation also do not show malignancy, infection, abscess, necrosis, epidermal or dermal modifications, or inflammation of the wounds after 14 days post-injection. H&E staining shows that the biodegradation of the developed hydrogel facilitates the growth of non-inflammatory cells, leading to the substitution of the injected hydrogel with autologous tissue. The detailed analyses affirm that the multifunctional injectable hydrogel with self-healing and antibacterial properties has high potential for wound healing and skin tissue engineering.

Incorporation of Cell-Adhesive Proteins in 3D-Printed Lipoic Acid-Maleic Acid-Poly(Propylene Glycol)-Based Tough Gel Ink for Cell-Supportive Microenvironment

In extrusion-based 3D printing, the use of synthetic polymeric hydrogels can facilitate fabrication of cellularized and implanted scaffolds with sufficient mechanical properties to maintain the structural integrity and physical stress within the in vivo conditions. However, synthetic hydrogels face challenges due to their poor properties of cellular adhesion, bioactivity, and biofunctionality. New compositions of hydrogel inks have been designed to address this limitation. A viscous poly(maleate-propylene oxide)-lipoate-poly(ethylene oxide) (MPLE) hydrogel is recently developed that shows high-resolution printability, drug-controlled release, excellent mechanical properties with adhesiveness, and biocompatibility. In this study, the authors demonstrate that the incorporation of cell-adhesive proteins like gelatin and albumin within the MPLE gel allows printing of biologically functional 3D scaffolds with rapid cell spreading (within 7 days) and high cell proliferation (twofold increase) as compared with MPLE gel only. Addition of proteins (10% w/v) supports the formation of interconnected cell clusters (≈1.6-fold increase in cell areas after 7-day) and spreading of cells in the printed scaffolds without additional growth factors. In in vivo studies, the protein-loaded scaffolds showed excellent biocompatibility and increased angiogenesis without inflammatory response after 4-week implantation in mice, thus demonstrating the promise to contribute to the printable tough hydrogel inks for tissue engineering.

Toughening colloidal gels using rough building blocks

Colloidal gels, commonly used as mesoporous intermediates or functional materials, suffer from brittleness, often showing small yield strains on the order of 1% or less for gelled colloidal suspensions. The short-range adhesive forces in most such gels are central forces-combined with the smooth morphology of particles, the resistance to yielding and shear-induced restructuring is limited. In this study, we propose an innovative approach to improve colloidal gels by introducing surface roughness to the particles to change the yield strain, giving rise to non-central interactions. To elucidate the effects of particle roughness on gel properties, we prepared thermoreversible gels made from rough or smooth silica particles using a reliable click-like-chemistry-based surface grafting technique. Rheological and optical characterization revealed that rough particle gels exhibit enhanced toughness and self-healing properties. These remarkable properties can be utilized in various applications, such as xerogel fabrication and high-fidelity extrusion 3D-printing, as we demonstrate in this study.

Gelatin methacryloyl and Laponite bioink for 3D bioprinted organotypic tumor modeling

Three-dimensional (3D)in vitrotumor models that can capture the pathophysiology of human tumors are essential for cancer biology and drug development. However, simulating the tumor microenvironment is still challenging because it consists of a heterogeneous mixture of various cellular components and biological factors. In this regard, current extracellular matrix (ECM)-mimicking hydrogels used in tumor tissue engineering lack physical interactions that can keep biological factors released by encapsulated cells within the hydrogel and improve paracrine interactions. Here, we developed a nanoengineered ion-covalent cross-linkable bioink to construct 3D bioprinted organotypic tumor models. The bioink was designed to implement the tumor ECM by creating an interpenetrating network composed of gelatin methacryloyl (GelMA), a light cross-linkable polymer, and synthetic nanosilicate (Laponite) that exhibits a unique ionic charge to improve retention of biological factors released by the encapsulated cells and assist in paracrine signals. The physical properties related to printability were evaluated to analyze the effect of Laponite hydrogel on bioink. Low GelMA (5%) with high Laponite (2.5%-3.5%) composite hydrogels and high GelMA (10%) with low Laponite (1.0%-2.0%) composite hydrogels showed acceptable mechanical properties for 3D printing. However, a low GelMA composite hydrogel with a high Laponite content could not provide acceptable cell viability. Fluorescent cell labeling studies showed that as the proportion of Laponite increased, the cells became more aggregated to form larger 3D tumor structures. Reverse transcription-polymerase chain reaction (RT-qPCR) and western blot experiments showed that an increase in the Laponite ratio induces upregulation of growth factor and tissue remodeling-related genes and proteins in tumor cells. In contrast, cell cycle and proliferation-related genes were downregulated. On the other hand, concerning fibroblasts, the increase in the Laponite ratio indicated an overall upregulation of the mesenchymal phenotype-related genes and proteins. Our study may provide a rationale for using Laponite-based hydrogels in 3D cancer modeling.

Rheology as a Tool for Fine-Tuning the Properties of Printable Bioinspired Gels

Over the last decade, efforts have been oriented toward the development of suitable gels for 3D printing, with controlled morphology and shear-thinning behavior in well-defined conditions. As a multidisciplinary approach to the fabrication of complex biomaterials, 3D bioprinting combines cells and biocompatible materials, which are subsequently printed in specific shapes to generate 3D structures for regenerative medicine or tissue engineering. A major interest is devoted to the printing of biomimetic materials with structural fidelity after their fabrication. Among some requirements imposed for bioinks, such as biocompatibility, nontoxicity, and the possibility to be sterilized, the nondamaging processability represents a critical issue for the stability and functioning of the 3D constructs. The major challenges in the field of printable gels are to mimic at different length scales the structures existing in nature and to reproduce the functions of the biological systems. Thus, a careful investigation of the rheological characteristics allows a fine-tuning of the material properties that are manufactured for targeted applications. The fluid-like or solid-like behavior of materials in conditions similar to those encountered in additive manufacturing can be monitored through the viscoelastic parameters determined in different shear conditions. The network strength, shear-thinning, yield point, and thixotropy govern bioprintability. An assessment of these rheological features provides significant insights for the design and characterization of printable gels. This review focuses on the rheological properties of printable bioinspired gels as a survey of cutting-edge research toward developing printed materials for additive manufacturing.

3D Printability of Silk/Hydroxyapatite Composites for Microprosthetic Applications

Micro-prosthetics requires the fabrication of mechanically robust and personalized components with sub-millimetric feature accuracy. Three-dimensional (3D) printing technologies have had a major impact on manufacturing such miniaturized devices for biomedical applications; however, biocompatibility requirements greatly constrain the choice of usable materials. Hydroxyapatite (HA) and its composites have been widely employed to fabricate bone-like structures, especially at the macroscale. In this work, we investigate the rheology, printability, and prosthetic mechanical properties of HA and HA-silk protein composites, focusing on the roles of composition and water content. We correlate key linear and nonlinear shear rheological parameters to geometric outcomes of printing and explain how silk compensates for the inherent brittleness of printed HA components. By increasing ink ductility, the inclusion of silk improves the quality of printed items through two mechanisms: (1) reducing underextrusion by lowering the required elastic modulus and, (2) reducing slumping by increasing the ink yield stress proportional to the modulus. We demonstrate that the elastic modulus and compressive strength of parts fabricated from silk-HA inks are higher than those for rheologically comparable pure-HA inks. We construct a printing map to guide the manufacturing of HA-based inks with excellent final properties, especially for use in biomedical applications for which sub-millimetric features are required.

3D Printing of Hybrid-Hydrogel Materials for Tissue Engineering: a Critical Review

Purpose

Key natural polymers, known as hydrogels, are an important group of materials in design of tissue-engineered constructs that can provide suitable habitat for cell attachment and proliferation. However, in comparison to tissues within the body, these hydrogels display poor mechanical properties. Such properties cause challenges in 3D printing of hydrogel scaffolds as well as their surgical handling after fabrication. For this reason, the purpose of this study is to critically review the 3D printing processes of hydrogels and their characteristics for tissue engineering application.

Methods

A search of Google Scholar and PubMed has been performed from 2003 to February 2022 using a combination of keywords. A review of the types of 3D printing is presented. Additionally, different types of hydrogels and nano-biocomposite materials for 3D printing application are critically reviewed. The rheological properties and crosslinking mechanisms for the hydrogels are assessed.

Results

Extrusion-based 3D printing is the most common practice for constructing hydrogel-based scaffolds, and it allows for the use of varying types of polymers to enhance the properties and printability of the hydrogel-based scaffolds. Rheology has been found to be exceedingly important in the 3D printing process; however, shear-thinning and thixotropic characteristics should also be present in the hydrogel. Despite these features of extrusion-based 3D printing, there are limitations to its printing resolution and scale.

Conclusion

Combining natural and synthetic polymers and a variety of nanomaterials, such as metal, metal oxide, non-metal, and polymeric, can enhance the properties of hydrogel and provide additional functionality to their 3D-printed constructs.

Electrically stimulated 3D bioprinting of gelatin-polypyrrole hydrogel with dynamic semi-IPN network induces osteogenesis via collective signaling and immunopolarization

In recent years, three-dimensional (3D) bioprinting of conductive hydrogels has made significant progress in the fabrication of high-resolution biomimetic structures with gradual complexity. However, the lack of an effective cross-linking strategy, ideal shear-thinning, appropriate yield strength, and higher print fidelity with excellent biofunctionality remains a challenge for developing cell-laden constructs, hindering the progress of extrusion-based 3D printing of conductive polymers. In this study, a highly stable and conductive bioink was developed based on polypyrrole-grafted gelatin methacryloyl (GelMA-PPy) with a triple cross-linking (thermo-photo-ionically) strategy for direct ink writing-based 3D printing applications. The triple-cross-linked hydrogel with dynamic semi-inner penetrating polymer network (semi-IPN) displayed excellent shear-thinning properties, with improved shape fidelity and structural stability during 3D printing. The as-fabricated hydrogel ink also exhibited "plug-like non-Newtonian" flow behavior with minimal disturbance. The bioprinted GelMA-PPy-Fe hydrogel showed higher cytocompatibility (93%) of human bone mesenchymal stem cells (hBMSCs) under microcurrent stimulation (250 mV/20 min/day). Moreover, the self-supporting and tunable mechanical properties of the GelMA-PPy bioink allowed 3D printing of high-resolution biological architectures. As a proof of concept, we printed a full-thickness rat bone model to demonstrate the structural stability. Transcriptomic analysis revealed that the 3D bioprinted hBMSCs highly expressed gene hallmarks for NOTCH/mitogen-activated protein kinase (MAPK)/SMAD signaling while down-regulating the Wnt/β-Catenin and epigenetic signaling pathways during osteogenic differentiation for up to 7 days. These results suggest that the developed GelMA-PPy bioink is highly stable and non-toxic to hBMSCs and can serve as a promising platform for bone tissue engineering applications.

Convergence of 3D Bioprinting and Nanotechnology in Tissue Engineering Scaffolds

Three-dimensional (3D) bioprinting has emerged as a promising scaffold fabrication strategy for tissue engineering with excellent control over scaffold geometry and microstructure. Nanobiomaterials as bioinks play a key role in manipulating the cellular microenvironment to alter its growth and development. This review first introduces the commonly used nanomaterials in tissue engineering scaffolds, including natural polymers, synthetic polymers, and polymer derivatives, and reveals the improvement of nanomaterials on scaffold performance. Second, the 3D bioprinting technologies of inkjet-based bioprinting, extrusion-based bioprinting, laser-assisted bioprinting, and stereolithography bioprinting are comprehensively itemized, and the advantages and underlying mechanisms are revealed. Then the convergence of 3D bioprinting and nanotechnology applications in tissue engineering scaffolds, such as bone, nerve, blood vessel, tendon, and internal organs, are discussed. Finally, the challenges and perspectives of convergence of 3D bioprinting and nanotechnology are proposed. This review will provide scientific guidance to develop 3D bioprinting tissue engineering scaffolds by nanotechnology.

3D bioprinting and its innovative approach for biomedical applications

3D bioprinting or additive manufacturing is an emerging innovative technology revolutionizing the field of biomedical applications by combining engineering, manufacturing, art, education, and medicine. This process involved incorporating the cells with biocompatible materials to design the required tissue or organ model in situ for various in vivo applications. Conventional 3D printing is involved in constructing the model without incorporating any living components, thereby limiting its use in several recent biological applications. However, this uses additional biological complexities, including material choice, cell types, and their growth and differentiation factors. This state-of-the-art technology consciously summarizes different methods used in bioprinting and their importance and setbacks. It also elaborates on the concept of bioinks and their utility. Biomedical applications such as cancer therapy, tissue engineering, bone regeneration, and wound healing involving 3D printing have gained much attention in recent years. This article aims to provide a comprehensive review of all the aspects associated with 3D bioprinting, from material selection, technology, and fabrication to applications in the biomedical fields. Attempts have been made to highlight each element in detail, along with the associated available reports from recent literature. This review focuses on providing a single platform for cancer and tissue engineering applications associated with 3D bioprinting in the biomedical field.

A facile 3D bio-fabrication of customized tubular scaffolds using solvent-based extrusion printing for tissue-engineered tracheal grafts

Tracheal implantation remains a major therapeutic challenge due to the unavailability of donors and the lack of biomimetic tubular grafts. Fabrication of biomimetic tracheal scaffolds of suitable materials with matched rigidity, enhanced flexibility and biocompatibility has been a major challenge in the field of tracheal reconstruction. In this study, customized tubular grafts made up of FDA-approved polycaprolactone ( PCL ) and polyurethane ( PU ) were fabricated using a novel solvent-based extrusion 3D printing. The printed scaffolds were investigated by various physical, thermal, and mechanical characterizations such as contact angle measurement, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), radial compression, longitudinal compression, and cyclic radial compression. In this study, the native goat trachea was used as a reference for the fabrication of different types of scaffolds (cylindrical, bellow-shaped, and spiral-shaped). The mechanical properties of the goat trachea were also compared to find suitable formulations of PCL / PU . Spiral-shaped scaffolds were found to be an ideal shape based on longitudinal compression and torsion load maintaining clear patency. To check the long-term implantation, in vitro degradation test was performed for all the 3D printed scaffolds and it was found that blending of PU with PCL reduced the degradation behavior. The printed scaffolds were further evaluated for biocompatibility assay, live/dead assay, and cell adhesion assay using bone marrow-derived human mesenchymal stem cells (hMSCs). From biomechanical and biological assessments, PCL 70 / PU 30 of spiral-shaped scaffolds could be a suitable candidate for the development of tracheal regenerative applications.

Stereolithography apparatus and digital light processing-based 3D bioprinting for tissue fabrication

Three-dimensional (3D) bioprinting has emerged as a class of promising techniques in biomedical research for a wide range of related applications. Specifically, stereolithography apparatus (SLA) and digital light processing (DLP)-based vat-polymerization techniques are highly effective methods of bioprinting, which can be used to produce high-resolution and architecturally sophisticated structures. Our review aims to provide an overview of SLA- and DLP-based 3D bioprinting strategies, starting from factors that affect these bioprinting processes. In addition, we summarize the advances in bioinks used in SLA and DLP, including naturally derived and synthetic bioinks. Finally, the biomedical applications of both SLA- and DLP-based bioprinting are discussed, primarily centered on regenerative medicine and tissue modeling engineering.

Synthesis of Hydrogels and Their Progress in Environmental Remediation and Antimicrobial Application

As a kind of efficient adsorptive material, hydrogel has a wide application prospect within different fields, owing to its unique 3D network structures composed of polymers. In this paper, different synthetic strategies, crosslinking methods and their corresponding limitations and outstanding contributions of applications in the fields of removing environmental pollutants are reviewed to further provide a prospective view of their applications in water resources sustainability. Furthermore, the applications within the biomedical field, especially in wound dressing, are also reviewed in this paper, mainly due to their unique water retention ability, antibacterial ability, and good biocompatibility. Finally, the development direction of hydrogels in the fields of environmental remediation and biomedicine were summarized and prospected.

Systematic evaluation of agarose- and agar-based bioinks for extrusion-based bioprinting of enzymatically active hydrogels

Extrusion-based 3D bioprinting enables the production of customized hydrogel structures that can be employed in flow reactors when printing with enzyme-containing inks. The present study compares inks based on either low-melt agarose or agar at different concentrations (3-6%) and loaded with the thermostable enzyme esterase 2 from the thermophilic organism Alicyclobacillus acidocaldarius (AaEst2) with regard to their suitability for the fabrication of such enzymatically active hydrogels. A customized printer setup including a heatable nozzle and a cooled substrate was established to allow for clean and reproducible prints. The inks and printed hydrogel samples were characterized using rheological measurements and compression tests. All inks were found to be sufficiently printable to create lattices without overhangs, but printing quality was strongly enhanced at 4.5% polymer or more. The produced hydrogels were characterized regarding mechanical strength and diffusibility. For both properties, a strong correlation with polymer concentration was observed with highly concentrated hydrogels being more stable and less diffusible. Agar hydrogels were found to be more stable and show higher diffusion rates than comparable agarose hydrogels. Enzyme leaching was identified as a major drawback of agar hydrogels, while hardly any leaching from agarose hydrogels was detected. The poor ability of agar hydrogels to permanently immobilize enzymes indicates their limited suitability for their employment in perfused biocatalytic reactors. Batch-based activity assays showed that the enzymatic activity of agar hydrogels was roughly twice as high as the activity of agarose hydrogels which was mostly attributed to the increased amount of enzyme leaching. Agarose bioinks with at least 4.5% polymer were identified as the most suitable of the investigated inks for the printing of biocatalytic reactors with AaEst2. Drawbacks of these inks are limited mechanical and thermal stability, not allowing the operation of a reactor at the optimum temperature of AaEst2 which is above the melting point of the employed low-melt agarose.

Nanocomposite bioinks for 3D bioprinting

Three-dimensional (3D) bioprinting is an advanced technology to fabricate artificial 3D tissue constructs containing cells and hydrogels for tissue engineering and regenerative medicine. Nanocomposite reinforcement endows hydrogels with superior properties and tailored functionalities. A broad range of nanomaterials, including silicon-based, ceramic-based, cellulose-based, metal-based, and carbon-based nanomaterials, have been incorporated into hydrogel networks with encapsulated cells for improved performances. This review emphasizes the recent developments of cell-laden nanocomposite bioinks for 3D bioprinting, focusing on their reinforcement effects and mechanisms, including viscosity, shear-thinning property, printability, mechanical properties, structural integrity, and biocompatibility. The cell-material interactions are discussed to elaborate on the underlying mechanisms between the cells and the nanomaterials. The biomedical applications of cell-laden nanocomposite bioinks are summarized with a focus on bone and cartilage tissue engineering. Finally, the limitations and challenges of current cell-laden nanocomposite bioinks are identified. The prospects are concluded in designing multi-component bioinks with multi-functionality for various biomedical applications. STATEMENT OF SIGNIFICANCE: 3D bioprinting, an emerging technology of additive manufacturing, has been one of the most innovative tools for tissue engineering and regenerative medicine. Recent developments of cell-laden nanocomposite bioinks for 3D bioprinting, and cell-materials interactions are the subject of this review paper. The reinforcement effects and mechanisms of nanocomposites on viscosity, printability and biocompatibility of bioinks and 3D printed scaffolds are addressed mainly for bone and cartilage tissue engineering. It provides detailed information for further designing and optimizing multi-component bioinks with multi-functionality for specialized biomedical applications.

Self-Healing Injectable Hydrogels for Tissue Regeneration

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.

Nanoengineered Ink for Designing 3D Printable Flexible Bioelectronics

Flexible electronics require elastomeric and conductive biointerfaces with native tissue-like mechanical properties. The conventional approaches to engineer such a biointerface often utilize conductive nanomaterials in combination with polymeric hydrogels that are cross-linked using toxic photoinitiators. Moreover, these systems frequently demonstrate poor biocompatibility and face trade-offs between conductivity and mechanical stiffness under physiological conditions. To address these challenges, we developed a class of shear-thinning hydrogels as biomaterial inks for 3D printing flexible bioelectronics. These hydrogels are engineered through a facile vacancy-driven gelation of MoS₂ nanoassemblies with naturally derived polymer-thiolated gelatin. Due to shear-thinning properties, these nanoengineered hydrogels can be printed into complex shapes that can respond to mechanical deformation. The chemically cross-linked nanoengineered hydrogels demonstrate a 20-fold rise in compressive moduli and can withstand up to 80% strain without permanent deformation, meeting human anatomical flexibility. The nanoengineered network exhibits high conductivity, compressive modulus, pseudocapacitance, and biocompatibility. The 3D-printed cross-linked structure demonstrates excellent strain sensitivity and can be used as wearable electronics to detect various motion dynamics. Overall, the results suggest that these nanoengineered hydrogels offer improved mechanical, electronic, and biological characteristics for various emerging biomedical applications including 3D-printed flexible biosensors, actuators, optoelectronics, and therapeutic delivery devices.

A Bioink Derived From Human Placenta Supporting Angiogenesis

Bioprinting is an emerging approach for constructing sophisticated tissue analogues with detailed architectures such as vascular networks, which requires bioink fulfill the highly printable property and provide a cell-friendly microenvironment mimicking native extracellular matrix (ECM). Here, we developed a human placental ECM-derived bioink (hp-bioink) meeting the requirements of 3D printing for printability and bioactivity. We first decellularized the human placenta, followed by enzymatic digestion, dialysis, lyophilization, and re-solubilization to convert the extracts into hp-bioink. Then, we demonstrated that 3%-5% of hp-bioink can be printed with self-standing and 1%-2% of hp-bioink can be embedded with suspended hydrogels. Moreover, hp-bioink supports HUVEC assembly in vitro and angiogenesis in mice in vivo. Our research enriched the bank of human-derived bioink, and provided a new opportunity to further accelerate bioprinting research and application.

Non-destructive mechanical assessment for optimization of 3D bioprinted soft tissue scaffolds

Characterizing the mechanical properties of engineered tissue constructs provides powerful insight into the function of engineered tissues for their desired application. Current methods of mechanical characterization of soft hydrogels used in tissue engineering are often destructive and ignore the effect of 3D bioprinting on the overall mechanical properties of a whole tissue construct. This work reports on using a non-destructive method of viscoelastic analysis to demonstrate the influence of bioprinting strategy on mechanical properties of hydrogel tissue scaffolds. Structure-function relationships are developed for common 3D bioprinting parameters such as printed fiber size, printed scaffold pattern, and bioink formulation. Further studies include mechanical properties analysis during degradation, real-time monitoring of crosslinking, mechanical characterization of multi-material scaffolds, and monitoring the effect of encapsulated cell growth on the mechanical strength of 3D bioprinted scaffolds. We envision this method of characterization opening a new wave of understanding and strategy in tissue engineering.

Two-dimensional nanomaterials-added dynamism in 3D printing and bioprinting of biomedical platforms: Unique opportunities and challenges

The nanomaterials research spectrum has seen the continuous emergence of two-dimensional (2D) materials over the years. These highly anisotropic and ultrathin materials have found special attention in developing biomedical platforms for therapeutic applications, biosensing, drug delivery, and regenerative medicine. Three-dimensional (3D) printing and bioprinting technologies have emerged as promising tools in medical applications. The convergence of 2D nanomaterials with 3D printing has extended the application dynamics of available biomaterials to 3D printable inks and bioinks. Furthermore, the unique properties of 2D nanomaterials have imparted multifunctionalities to 3D printed constructs applicable to several biomedical applications. 2D nanomaterials such as graphene and its derivatives have long been the interest of researchers working in this area. Beyond graphene, a range of emerging 2D nanomaterials, such as layered silicates, black phosphorus, transition metal dichalcogenides, transition metal oxides, hexagonal boron nitride, and MXenes, are being explored for the multitude of biomedical applications. Better understandings on both the local and systemic toxicity of these materials have also emerged over the years. This review focuses on state-of-art 3D fabrication and biofabrication of biomedical platforms facilitated by 2D nanomaterials, with the comprehensive summary of studies focusing on the toxicity of these materials. We highlight the dynamism added by 2D nanomaterials in the printing process and the functionality of printed constructs.

Applying extrusion-based 3D printing technique accelerates fabricating complex biphasic calcium phosphate-based scaffolds for bone tissue regeneration

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Biphasic calcium phosphates offer a chemically similar biomaterial to the natural bone, which can significantly accelerate bone formation and reconstruction.

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Robocasting is a suitable technique to produce porous scaffolds supporting cell viability, proliferation, and differentiation.

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This review discusses materials and methods utilized for BCP robocasting, considering recent advancements and existing challenges in using additives for bioink preparation.

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Commercialization and marketing approach, in-vitro and in-vivo evaluations, biologic responses, and post-processing steps are also investigated.

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Possible strategies and opportunities for the use of BCP toward injured bone regeneration along with clinical applications are discussed.

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The study proposes that BCP possesses an acceptable level of bone substituting, considering its challenges and struggles.

Background

Aim of review

Key scientific concepts of review

Printable Alginate Hydrogels with Embedded Network of Halloysite Nanotubes: Effect of Polymer Cross-Linking on Rheological Properties and Microstructure

Rapidly growing 3D printing of hydrogels requires network materials which combine enhanced mechanical properties and printability. One of the most promising approaches to strengthen the hydrogels consists of the incorporation of inorganic fillers. In this paper, the rheological properties important for 3D printability were studied for nanocomposite hydrogels based on a rigid network of percolating halloysite nanotubes embedded in a soft alginate network cross-linked by calcium ions. Particular attention was paid to the effect of polymer cross-linking on these properties. It was revealed that the system possessed a pronounced shear-thinning behavior accompanied by a viscosity drop of 4-5 orders of magnitude. The polymer cross-links enhanced the shear-thinning properties and accelerated the viscosity recovery at rest so that the system could regain 96% of viscosity in only 18 s. Increasing the cross-linking of the soft network also enhanced the storage modulus of the nanocomposite system by up to 2 kPa. Through SAXS data, it was shown that at cross-linking, the junction zones consisting of fragments of two laterally aligned polymer chains were formed, which should have provided additional strength to the hydrogel. At the same time, the cross-linking of the soft network only slightly affected the yield stress, which seemed to be mainly determined by the rigid percolation network of nanotubes and reached 327 Pa. These properties make the alginate/halloysite hydrogels very promising for 3D printing, in particular, for biomedical purposes taking into account the natural origin, low toxicity, and good biocompatibility of both components.

3D Bioprinted Multicellular Vascular Models

3D bioprinting is an emerging additive manufacturing technique to fabricate constructs for human disease modeling. However, current cell-laden bioinks lack sufficient biocompatibility, printability, and structural stability needed to translate this technology to preclinical and clinical trials. Here, a new class of nanoengineered hydrogel-based cell-laden bioinks is introduced, that can be printed into 3D, anatomically accurate, multicellular blood vessels to recapitulate both the physical and chemical microenvironments of native human vasculature. A remarkably unique characteristic of this bioink is that regardless of cell density, it demonstrates a high printability and ability to protect encapsulated cells against high shear forces in the bioprinting process. 3D bioprinted cells maintain a healthy phenotype and remain viable for nearly one-month post-fabrication. Leveraging these properties, the nanoengineered bioink is printed into 3D cylindrical blood vessels, consisting of living co-culture of endothelial cells and vascular smooth muscle cells, providing the opportunity to model vascular function and pathophysiology. Upon cytokine stimulation and blood perfusion, this 3D bioprinted vessel is able to recapitulate thromboinflammatory responses observed only in advanced in vitro preclinical models or in vivo. Therefore, this 3D bioprinted vessel provides a potential tool to understand vascular disease pathophysiology and assess therapeutics, toxins, or other chemicals.

Converging 2D Nanomaterials and 3D Bioprinting Technology: State-of-the-Art, Challenges, and Potential Outlook in Biomedical Applications

The development of next-generation of bioinks aims to fabricate anatomical size 3D scaffold with high printability and biocompatibility. Along with the progress in 3D bioprinting, 2D nanomaterials (2D NMs) prove to be emerging frontiers in the development of advanced materials owing to their extraordinary properties. Harnessing the properties of 2D NMs in 3D bioprinting technologies can revolutionize the development of bioinks by endowing new functionalities to the current bioinks. First the main contributions of 2D NMS in 3D bioprinting technologies are categorized here into six main classes: 1) reinforcement effect, 2) delivery of bioactive molecules, 3) improved electrical conductivity, 4) enhanced tissue formation, 5) photothermal effect, 6) and stronger antibacterial properties. Next, the recent advances in the use of each certain 2D NMs (1) graphene, 2) nanosilicate, 3) black phosphorus, 4) MXene, 5) transition metal dichalcogenides, 6) hexagonal boron nitride, and 7) metal-organic frameworks) in 3D bioprinting technology are critically summarized and evaluated thoroughly. Third, the role of physicochemical properties of 2D NMSs on their cytotoxicity is uncovered, with several representative examples of each studied 2D NMs. Finally, current challenges, opportunities, and outlook for the development of nanocomposite bioinks are discussed thoroughly.

Clay Minerals as Bioink Ingredients for 3D Printing and 3D Bioprinting: Application in Tissue Engineering and Regenerative Medicine

The adaptation and progress of 3D printing technology toward 3D bioprinting (specifically adapted to biomedical purposes) has opened the door to a world of new opportunities and possibilities in tissue engineering and regenerative medicine. In this regard, 3D bioprinting allows for the production of tailor-made constructs and organs as well as the production of custom implants and medical devices. As it is a growing field of study, currently, the attention is heeded on the optimization and improvement of the mechanical and biological properties of the so-called bioinks/biomaterial inks. One of the strategies proposed is the use of inorganic ingredients (clays, hydroxyapatite, graphene, carbon nanotubes and other silicate nanoparticles). Clays have proven to be useful as rheological and mechanical reinforcement in a wide range of fields, from the building industry to pharmacy. Moreover, they are naturally occurring materials with recognized biocompatibility and bioactivity, revealing them as optimal candidates for this cutting-edge technology. This review deals with the use of clays (both natural and synthetic) for tissue engineering and regenerative medicine through 3D printing and bioprinting. Despite the limited number of studies, it is possible to conclude that clays play a fundamental role in the formulation and optimization of bioinks and biomaterial inks since they are able to improve their rheology and mechanical properties, thus improving printability and construct resistance. Additionally, they have also proven to be exceptionally functional ingredients (enhancing cellular proliferation, adhesion, differentiation and alignment), controlling biodegradation and carrying/releasing actives with tissue regeneration therapeutic activities.

An injectable bioink with rapid prototyping in the air andin-situmild polymerization for 3D bioprinting

Bioprinting is an attractive technology for building tissues from scratch to explore entire new cell configurations, which brings numerous opportunities for biochemical research such as engineering tissues for therapeutic tissue repair or drug screening. However, bioprinting is faced with the limited number of suitable bioinks that enable bioprinting with excellent printability, high structural fidelity, physiological stability, and good biocompatibility, particularly in the case of extrusion-based bioprinting. Herein, we demonstrate a composite bioink based on gelatin, bacterial cellulose (BC), and microbial transglutaminase (mTG enzyme) with outstanding printing controllability and durable architectural integrity. BC, as a rheology modifier and mechanical enhancer component, endows the bioink with shear-thinning behavior. Moreover, the printed structure becomes robust under physiological conditions owing to thein situchemical crosslinking catalyzed by mTG enzyme. Lattice, bowl, meniscus, and ear structures are printed to demonstrate the printing feasibility of such a composite bioink. Furthermore, the 3D-printed cell-laden constructs are proved to be a conducive biochemical environment that supports growth and proliferation of the encapsulated cellsin vitro. In addition, thein vivostudies convince that the composite bioink possesses excellent biocompatibility and biodegradation. It is believed that the innovation of this new composite bioink will push forward the bioprinting technology onto a new stage.

A Dual-sensitive Hydrogel Based on Poly(Lactide-co-Glycolide)-Polyethylene Glycol-Poly(Lactide-co-Glycolide) Block Copolymers for 3D Printing

The thermo-sensitive hydrogel formed by triblock copolymers of polyethylene glycols and aliphatic polyesters serves as a promising candidate for bioink due to its excellent biodegradability and biocompatibility. However, the thermo-crosslinking alone cannot achieve a robust hydrogel to support the 3D printed constructs without collapse. Herein, a photo-crosslinkable group was introduced into the triblock copolymers to achieve a dual-sensitive hydrogel. A triblock copolymer poly(lactide-co-glycolide)-polyethylene glycol-poly(lactide-co-glycolide) decorated with acrylate group in the chain end was prepared. The obtained aqueous solutions of the copolymers could transform into hydrogels with excellent shear thinning properties and rapid elastic recovery properties spontaneously on the increase of temperature. The resulted thermogels also allowed for photo-crosslinking by exposure to ultraviolet radiation, with storage modulus dramatically increased to stable the printed constructs. Through a two-step crosslinking strategy, complicated tissue-like constructs with high shape fidelity can be printed using the dual-sensitive inks. Moreover, the mechanical strength, swelling ratio, and printability of the hydrogels can be tuned by varying the substitution rate of the acrylate group without compromising the inks' extrudability. We expect that the dual-sensitive hydrogels may be used as bioinks to print large constructs for applications in tissue engineering.

Development of Nanosilicate-Hydrogel Composites for Sustained Delivery of Charged Biopharmaceutics

Nanocomposite hydrogels containing two-dimensional nanosilicates (NS) have emerged as a new technology for the prolonged delivery of biopharmaceuticals. However, little is known about the physical-chemical properties governing the interaction between NS and proteins and the release profiles of NS-protein complexes in comparison to traditional poly(ethylene glycol) (PEG) hydrogel technologies. To fill this gap in knowledge, we fabricated a nanocomposite hydrogel composed of PEG and laponite and identified simple but effective experimental conditions to obtain sustained protein release, up to 23 times slower as compared to traditional PEG hydrogels, as determined by bulk release experiments and fluorescence correlation spectroscopy. Slowed protein release was attributed to the formation of NS-protein complexes, as NS-protein complex size was inversely correlated with protein diffusivity and release rates. While protein electrostatics, protein concentration, and incubation time were important variables to control protein-NS complex formation, we found that one of the most significant and less appreciated variable to obtain a sustained release of bioactive proteins was the buffer chosen for preparing the initial suspension of NS particles. The buffer was found to control the size of nanoparticles, the absorption potential, morphology, and stiffness of hydrogels. From these studies, we conclude that the PEG-laponite composite fabricated is a promising new platform for sustained delivery of positively charged protein therapeutics.

Design of a Waterborne Polyurethane-Urea Ink for Direct Ink Writing 3D Printing

In this work, polycaprolactone-polyethylene glycol (PCL-PEG) based waterborne polyurethane-urea (WBPUU) inks have been developed for an extrusion-based 3D printing technology. The WBPUU, synthesized from an optimized ratio of hydrophobic polycaprolactone diol and hydrophilic polyethylene glycol (0.2:0.8) in the soft segment, is able to form a physical gel at low solid contents. WBPUU inks with different solid contents have been synthesized. The rheology of the prepared systems was studied and the WBPUUs were subsequently used in the printing of different pieces to demonstrate the relationship between their rheological properties and their printing viability, establishing an optimal window of compositions for the developed WBPUU based inks. The results showed that the increase in solid content results in more structured inks, presenting a higher storage modulus as well as lower tan δ values, allowing for the improvement of the ink's shape fidelity. However, an increase in solid content also leads to an increase in the yield point and viscosity, leading to printability limitations. From among all printable systems, the WBPUU with a solid content of 32 wt% is proposed to be the more suitable ink for a successful printing performance, presenting both adequate printability and good shape fidelity, which leads to the realization of a recognizable and accurate 3D construct and an understanding of its relationship with rheological parameters.

Physico-Chemical Challenges in 3D Printing of Polymeric Nanocomposites and Hydrogels for Biomedical Applications

Additive manufacturing techniques (i.e., 3D printing) are rapidly becoming one of the most popular methods for the preparation of materials to be employed in many different fields, including biomedical applications. The main reason is the unique flexibility resulting from both the method itself and the variety of starting materials, requiring the combination of multidisciplinary competencies for the optimization of the process. In particular, this is the case of additive manufacturing processes based on the extrusion or jetting of nanocomposite materials, where the unique properties of nanomaterials are combined with those of a flowing matrix. This contribution focuses on the physico-chemical challenges typically faced in the 3D printing of polymeric nanocomposites and polymeric hydrogels intended for biomedical applications. The strategies to overcome those challenges are outlined, together with the characterization approaches that could help the advance of the field.

Bioinks for 3D Bioprinting: A Scientometric Analysis of Two Decades of Progress

This scientometric analysis of 393 original papers published from January 2000 to June 2019 describes the development and use of bioinks for 3D bioprinting. The main trends for bioink applications and the primary considerations guiding the selection and design of current bioink components (i.e., cell types, hydrogels, and additives) were reviewed. The cost, availability, practicality, and basic biological considerations (e.g., cytocompatibility and cell attachment) are the most popular parameters guiding bioink use and development. Today, extrusion bioprinting is the most widely used bioprinting technique. The most reported use of bioinks is the generic characterization of bioink formulations or bioprinting technologies (32%), followed by cartilage bioprinting applications (16%). Similarly, the cell-type choice is mostly generic, as cells are typically used as models to assess bioink formulations or new bioprinting methodologies rather than to fabricate specific tissues. The cell-binding motif arginine-glycine-aspartate is the most common bioink additive. Many articles reported the development of advanced functional bioinks for specific biomedical applications; however, most bioinks remain the basic compositions that meet the simple criteria: Manufacturability and essential biological performance. Alginate and gelatin methacryloyl are the most popular hydrogels that meet these criteria. Our analysis suggests that present-day bioinks still represent a stage of emergence of bioprinting technology.

Medical application of biomimetic 4D printing

Additive manufacturing has attracted a lot of attention in fabrication of bio medical devices and structures in recent years. 4D printing, a new class of 3D printing where time is considered as a 4th dimension, allows us to build biological structures such as scaffolds, implants, and stents with dynamic performance mimicking the body's natural tissues. In order to properly exploit the capabilities of this fabrication method, understanding and exploiting the shape memory materials is critical. These 'smart' materials are responsive to the external stimuli which eliminates the need for utilizing the sensors, and batteries. These stimuli-triggered 'smart' materials possess a dynamic behavior unlike the static scaffolds based on conventional manufacturing techniques. In this review, recent advances on application of 4D printing for manufacturing of this type of materials and other high-performance biomaterials for medical applications have been discussed.

Chitin nanocrystals assisted 3D printing of polycitrate thermoset bioelastomers

Citrate-based thermoset bioelastomer has numerous tissue engineering applications. However, its insoluble and unmeltable features restricted processing techniques for fabricating complex scaffolds. Herein, direct ink writing (DIW) was explored for 3D printing of poly(1, 8-octanediol-co-Pluronic F127 citrate) (POFC) bioelastomer scaffolds considering that POFC prepolymer (pre-POFC) was waterborne and could form a stable emulsion. The pre-POFC emulsion couldn't be printed, however, chitin nanocrystal (ChiNC) could be as a rheological modifier to tune the flow behavior of pre-POFC emulsion, and thus DIW printing of POFC scaffolds was successfully realized; moreover, ChiNC was also as a supporting agent to prevent collapse of filaments during thermocuring, and simultaneously as a biobased nanofiller to reinforce scaffolds. The rheological analyses showed the pre-POFC/ChiNC inks fulfilled the requirements for DIW printing. The printed scaffolds exhibited low swelling, and good performances in strength and resilence. Furthermore, the entire process was easily performed and eco-friendly.

Bioinks-materials used in printing cells in designed 3D forms

Use of materials to activate non-functional or damaged organs and tissues goes back to early ages. The first materials used for this purpose were metals, and in time, novel materials such as ceramics, polymers and composites were introduced to the field to serve in medical applications. In the last decade, the advances in material sciences, cell biology, technology and engineering made 3D printing of living tissues or organ models in the designed structure and geometry possible by using cells alone or together with hydrogels through additive manufacturing. This review aims to give a brief information about the chemical structures and properties of bioink materials and their applications in the production of 3D tissue constructs.

3D bioprinting of mechanically tuned bioinks derived from cardiac decellularized extracellular matrix

3D bioprinting is a powerful technique for engineering tissues used to study cell behavior and tissue properties in vitro. With the right formulation and printing parameters, bioinks can provide native biological and mechanical cues while allowing for versatile 3D structures that recapitulate tissue-level organization. Bio-based materials that support cellular adhesion, differentiation, and proliferation - including gelatin, collagen, hyaluronic acid, and alginate - have been successfully used as bioinks. In particular, decellularized extracellular matrix (dECM) has become a promising material with the unique ability to maintain both biochemical and topographical micro-environments of native tissues. However, dECM has shown technical limitations for 3D printing (3DP) applications posed by its intrinsically low mechanical stability. Herein, we report hydrogel bioinks composed of partially digested, porcine cardiac decellularized extracellular matrix (cdECM), Laponite-XLG nanoclay, and poly(ethylene glycol)-diacrylate (PEG-DA). The Laponite facilitated extrusion-based 3DP, while PEG-DA enabled photo-polymerization after printing. Improving upon previously reported bioinks derived from dECM, our bioinks combine extrudability, shape fidelity, rapid cross-linking, and cytocompatibility in a single formulation (> 97% viability of encapsulated human cardiac fibroblasts and > 94% viability of human induced pluripotent stem cell derived cardiomyocytes after 7 days). The compressive modulus of the cured hydrogel bioinks was tunable from 13.4-89 kPa by changing the concentration of PEG-DA in the bioink formulation. Importantly, this span of mechanical stiffness encompasses ranges of tissue stiffness from healthy (compressive modulus ~5-15 kPa) to fibrotic (compressive modulus ~30-100 kPa) cardiac tissue states. The printed constructs demonstrated shape fidelity, adaptability to different printing conditions, and high cell viability following extrusion and photo-polymerization, highlighting the potential for applications in modeling both healthy and fibrotic cardiac tissue.

Nanoengineered shear-thinning and bioprintable hydrogel as a versatile platform for biomedical applications

The development of bioinks based on shear-thinning and self-healing hydrogels has recently attracted significant attention for constructing complex three-dimensional physiological microenvironments. For extrusion-based bioprinting, it is challenging to provide high structural reliability and resolution of printed structures while protecting cells from shear forces during printing. Herein, we present shear-thinning and printable hydrogels based on silicate nanomaterials, laponite (LA), and glycosaminoglycan nanoparticles (GAGNPs) for bioprinting applications. Nanocomposite hydrogels (GLgels) were rapidly formed within seconds due to the interactions between the negatively charged groups of GAGNPs and the edges of LA. The shear-thinning behavior of the hydrogel protected encapsulated cells from aggressive shear stresses during bioprinting. The bioinks could be printed straightforwardly into shape-persistent and free-standing structures with high aspect ratios. Rheological studies demonstrated fast recovery of GLgels over multiple strain cycles. In vitro studies confirmed the ability of GLgels to support cell growth, proliferation, and spreading. In vitro osteogenic differentiation of pre-osteoblasts murine bone marrow stromal cells encapsulated inside the GLgels was also demonstrated through evaluation of ALP activity and calcium deposition. The subcutaneous implantation of the GLgel in rats confirmed its in vivo biocompatibility and biodegradability. The engineered shear-thinning hydrogel with osteoinductive characteristics can be used as a new bioink for 3D printing of constructs for bone tissue engineering applications.

Advancements in Therapeutics via 3D Printed Multifunctional Architectures from Dispersed 2D Nanomaterial Inks

2D nanomaterials (2DNMs) possess fascinating properties and are found in multifarious devices and applications including energy storage devices, new generation of battery technologies, sensor devices, and more recently in biomedical applications. Their use in biomedical applications such as tissue engineering, photothermal therapy, neural regeneration, and drug delivery has opened new horizons in treatment of age-old ailments. It is also a rapidly developing area of advanced research. A new approach of integrating 3D printing (3DP), a layer-by-layer deposition technique for building structures, along with 2DNM multifunctional inks, has gained considerable attention in recent times, especially in biomedical applications. With the ever-growing demand in healthcare industry for novel, efficient, and rapid technologies for therapeutic treatment methods, 3DP structures of 2DNMs provide vast scope for evolution of a new generation of biomedical devices. Recent advances in 3DP structures of dispersed 2DNM inks with established high-performance biomedical properties are focused on. The advantages of their 3D structures, the sustainable formulation methods of such inks, and their feasible printing methods are also covered. Subsequently, it deals with the therapeutic applications of some already researched 3DP structures of 2DNMs and concludes with highlighting the challenges as well as the future directions of research in this area.

3D bioprinting for orthopaedic applications: Current advances, challenges and regulatory considerations

In the era of personalised medicine, novel therapeutic approaches raise increasing hopes to address currently unmet medical needs by developing patient-customised treatments. Three-dimensional (3D) bioprinting is rapidly evolving and has the potential to obtain personalised tissue constructs and overcome some limitations of standard tissue engineering approaches. Bioprinting could support a wide range of biomedical applications, such as drug testing, tissue repair or organ transplantation. There is a growing interest for 3D bioprinting in the orthopaedic field, with remarkable scientific and technical advances. However, the full exploitation of 3D bioprinting in medical applications still requires efforts to anticipate the upcoming challenges in translating bioprinted products from bench to bedside. In this review we summarised current trends, advances and challenges in the application of 3D bioprinting for bone and cartilage tissue engineering. Moreover, we provided a detailed analysis of the applicable regulations through the 3D bioprinting process and an overview of available standards covering bioprinting and additive manufacturing.

Nano-Silicate-Reinforced and SDF-1α-Loaded Gelatin-Methacryloyl Hydrogel for Bone Tissue Engineering

Purpose

Autologous bone grafts are the gold standard for treating bone defects. However, limited bone supply and morbidity at the donor site restrict its extensive use. Therefore, developing bone graft materials as an alternative to autologous grafts has gained considerable attention. Injectable hydrogels endowed with osteogenic potential have the ability to fill irregular bone defects using minimally invasive procedures and have thus been attracting researchers’ attention. However, from a clinical perspective, most fabrication methods employed for the current injectable osteogenic hydrogels are difficult and inconvenient. In the current study, we fabricated an injectable osteogenic hydrogel using a simple and convenient strategy.

Materials and Methods

Gelatin-methacryloyl (GelMA) pre-polymer was synthetized. Nano silicate (SN) and stromal cell-derived factor-1 alpha (SDF-1α) were introduced into the pre-polymer to achieve injectability, controlled release property, excellent osteogenic ability, and efficient stem cell homing.

Results

The GelMA-SN-SDF-1α demonstrated excellent injectability via a 17-G needle at room temperature. The loaded SDF-1α exhibited a long-term controlled release pattern and efficiently stimulated MSC migration and homing. The GelMA-SN-SDF-1α hydrogel amplified cell spreading, migration, osteogenic-related biomarker expression, and matrix mineralization. The GelMA-SN-SDF-1α hydrogel filled critical-sized calvaria defects in rats and demonstrated excellent bone regeneration ability, as assessed using micro-CT scanning and histomorphometric staining.

Conclusion

The GelMA-SN-SDF-1α hydrogel provides a simple and convenient strategy for the fabrication of injectable osteogenic graft materials.

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