A comparative study on collagen type I and hyaluronic acid dependent cell behavior for osteochondral tissue bioprinting

Double crosslinked hyaluronic acid and collagen as a potential bioink for cartilage tissue engineering

The avascular nature of hyaline cartilage results in limited spontaneous self-repair and regenerative capabilities when damaged. Recent advances in three-dimensional bioprinting have enabled the precise dispensing of cell-laden biomaterials, commonly referred to as 'bioinks', which are emerging as promising solutions for tissue regeneration. An effective bioink for cartilage tissue engineering needs to create a micro-environment that promotes cell differentiation and supports neocartilage tissue formation. In this study, we introduced an innovative bioink composed of photocurable acrylated type I collagen (COLMA), thiol-modified hyaluronic acid (THA), and poly(ethylene glycol) diacrylate (PEGDA) for 3D bioprinting cartilage grafts using human nasal chondrocytes. Both collagen and hyaluronic acid, being key components of the extracellular matrix (ECM) in the human body, provide essential biological cues for tissue regeneration. We evaluated three formulations - COLMA, COLMA+THA, and COLMA+THA+PEGDA - for their printability, cell viability, structural integrity, and capabilities in forming cartilage-like ECM. The addition of THA and PEGDA significantly enhanced these properties, showcasing the potential of this bioink in advancing applications in cartilage repair and reconstructive surgery.

In Situ Bioprinting: Process, Bioinks, and Applications

Traditional tissue engineering methods face challenges, such as fabrication, implantation of irregularly shaped scaffolds, and limited accessibility for immediate healthcare providers. In situ bioprinting, an alternate strategy, involves direct deposition of biomaterials, cells, and bioactive factors at the site, facilitating on-site fabrication of intricate tissue, which can offer a patient-specific personalized approach and align with the principles of precision medicine. It can be applied using a handled device and robotic arms to various tissues, including skin, bone, cartilage, muscle, and composite tissues. Bioinks, the critical components of bioprinting that support cell viability and tissue development, play a crucial role in the success of in situ bioprinting. This review discusses in situ bioprinting techniques, the materials used for bioinks, and their critical properties for successful applications. Finally, we discuss the challenges and future trends in accelerating in situ printing to translate this technology in a clinical settings for personalized regenerative medicine.

3D bioprinting of microorganisms: principles and applications

In recent years, the ability to create intricate, live tissues and organs has been made possible thanks to three-dimensional (3D) bioprinting. Although tissue engineering has received a lot of attention, there is growing interest in the use of 3D bioprinting for microorganisms. Microorganisms like bacteria, fungi, and algae, are essential to many industrial bioprocesses, such as bioremediation as well as the manufacture of chemicals, biomaterials, and pharmaceuticals. This review covers current developments in 3D bioprinting methods for microorganisms. We go over the bioink compositions designed to promote microbial viability and growth, taking into account factors like nutrient delivery, oxygen supply, and waste elimination. Additionally, we investigate the most important bioprinting techniques, including extrusion-based, inkjet, and laser-assisted approaches, as well as their suitability with various kinds of microorganisms. We also investigate the possible applications of 3D bioprinted microbes. These range from constructing synthetic microbial consortia for improved metabolic pathway combinations to designing spatially patterned microbial communities for enhanced bioremediation and bioprocessing. We also look at the potential for 3D bioprinting to advance microbial research, including the creation of defined microenvironments to observe microbial behavior. In conclusion, the 3D bioprinting of microorganisms marks a paradigm leap in microbial bioprocess engineering and has the potential to transform many application areas. The ability to design the spatial arrangement of various microorganisms in functional structures offers unprecedented possibilities and ultimately will drive innovation.

Nicotinamide mononucleotide based hyaluronic acid methacryloyl hybrid hydrogel regulating stem cells fate for bone regeneration via SIRT1/RUNX2 signaling

Efficient bone reconstruction, especially of the critical size after bone damage, remains a challenge in the clinic. Bone marrow mesenchymal stem cell (BMSC) osteogenic differentiation is considered as a promising strategy for bone repair. Nicotinamide adenine dinucleotide (NAD+) regulating BMSC fate and cellular function enhance osteogenesis, but is hardly delivered and lack of targeting. Herein, a novel and biocompatible scaffold was fabricated to locally deliver a precursor of NAD+, nicotinamide mononucleotide (NMN) to the bone defect site, and its bone repair capability and healing mechanism were clarified. NMN-based hyaluronic acid methacryloyl hybrid hydrogel scaffold (denoted as NMN/HAMA) was prepared via photopolymerization. In vitro RT-qPCR analysis, western blotting, Elisa and alizarin red S staining assays demonstrated that the NMN/HAMA hybrid hydrogel regulated BMSCs cellular function in favour of osteogenic differentiation and mineralization by upregulating the mRNA and proteins expression of the osteogenic genes type I pro-collagen (Col-1), bone morphogenic protein 4 (BMP4), and runt-related transcription factor 2 (RUNX2) via the SIRT1 pathway. Implantation of such hybrid hydrogels significantly enhanced bone regeneration in rodent critical calvarial defect models. Furthermore, restoration of the bone defect with NMN administration was inhibited in Prx1 Cre+; SIRT1flox/flox mice, confirming that the NMN/HAMA hybrid hydrogel scaffold promoted bone regeneration via the SIRT1-RUNX2 pathway. These results imply that NMN-based scaffold may be a promising and economic strategy for the treatment of bone defects.

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.

Embedded 3D bioprinting - An emerging strategy to fabricate biomimetic & large vascularized tissue constructs

Three-dimensional bioprinting is an advanced tissue fabrication technique that allows printing complex structures with precise positioning of multiple cell types layer-by-layer. Compared to other bioprinting methods, extrusion bioprinting has several advantages to print large-sized tissue constructs and complex organ models due to large build volume. Extrusion bioprinting using sacrificial, support and embedded strategies have been successfully employed to facilitate printing of complex and hollow structures. Embedded bioprinting is a gel-in-gel approach developed to overcome the gravitational and overhanging limits of bioprinting to print large-sized constructs with a micron-scale resolution. In embedded bioprinting, deposition of bioinks into the microgel or granular support bath will be facilitated by the sol-gel transition of the support bath through needle movement inside the granular medium. This review outlines various embedded bioprinting strategies and the polymers used in the embedded systems with advantages, limitations, and efficacy in the fabrication of complex vascularized tissues or organ models with micron-scale resolution. Further, the essential requirements of support bath systems like viscoelasticity, stability, transparency and easy extraction to print human scale organs are discussed. Additionally, the organs or complex geometries like vascular constructs, heart, bone, octopus and jellyfish models printed using support bath assisted printing methods with their anatomical features are elaborated. Finally, the challenges in clinical translation and the future scope of these embedded bioprinting models to replace the native organs are envisaged.

3D-Printed Collagen-Nanocellulose Hybrid Bioscaffolds with Tailored Properties for Tissue Engineering Applications

Hybrid collagen (Coll) bioscaffolds have emerged as a promising solution for tissue engineering (TE) and regenerative medicine. These innovative bioscaffolds combine the beneficial properties of Coll, an important structural protein of the extracellular matrix, with various other biomaterials to create platforms for long-term cell growth and tissue formation. The integration or cross-linking of Coll with other biomaterials increases mechanical strength and stability and introduces tailored biochemical and physical factors that mimic the natural tissue microenvironment. This work reports on the fabrication of chemically cross-linked hybrid bioscaffolds with enhanced properties from the combination of Coll, nanofibrillated cellulose (NFC), carboxymethylcellulose (CMC), and citric acid (CA). The bioscaffolds were prepared by 3D printing ink containing Coll-NFC-CMC-CA followed by freeze-drying, dehydrothermal treatment, and neutralization. Cross-linking through the formation of ester bonds between the polymers and CA in the bioscaffolds was achieved by exposing the bioscaffolds to elevated temperatures in the dry state. The morphology, pores/porosity, chemical composition, structure, thermal behavior, swelling, degradation, and mechanical properties of the bioscaffolds in the dry and wet states were investigated as a function of Coll concentration. The bioscaffolds showed no cytotoxicity to MG-63 human bone osteosarcoma cells as tested by different assays measuring different end points. Overall, the presented hybrid Coll bioscaffolds offer a unique combination of biocompatibility, stability, and structural support, making them valuable tools for TE.

Polysaccharide-based biomaterials in a journey from 3D to 4D printing

3D printing is a state-of-the-art technology for the fabrication of biomaterials with myriad applications in translational medicine. After stimuli-responsive properties were introduced to 3D printing (known as 4D printing), intelligent biomaterials with shape configuration time-dependent character have been developed. Polysaccharides are biodegradable polymers sensitive to several physical, chemical, and biological stimuli, suited for 3D and 4D printing. On the other hand, engineering of mechanical strength and printability of polysaccharide-based scaffolds along with their aneural, avascular, and poor metabolic characteristics need to be optimized varying printing parameters. Multiple disciplines such as biomedicine, chemistry, materials, and computer sciences should be integrated to achieve multipurpose printable biomaterials. In this work, 3D and 4D printing technologies are briefly compared, summarizing the literature on biomaterials engineering though printing techniques, and highlighting different challenges associated with 3D/4D printing, as well as the role of polysaccharides in the technological shift from 3D to 4D printing for translational medicine.

Bioprinting: a focus on improving bioink printability and cell performance based on different process parameters

Three dimensional (3D) bioprinting is an emerging biofabrication technique that shows great potential in the field of tissue engineering, regenerative medicine and advanced drug delivery. Despite the current advancement of bioprinting technology, it faces several obstacles such as the challenge of optimizing the printing resolution of 3D constructs while retaining cell viability before, during, and after bioprinting. Therefore, it is of great significance to fully understand factors that influence the shape fidelity of printed structures and the performance of cells encapsulated in bioinks. This review presents a comprehensive analysis of bioprinting process parameters that influence bioink printability and cell performance, including bioink properties (composition, concentration, and component ratio), printing speed and pressure, nozzle charateristics (size, length, and geometry), and crosslinking parameters (crosslinker types, concentration, and crosslinking time). Key examples are provided to analyze how these parameters could be tailored to achieve the optimal printing resolution as well as cell performance. Finally, future prospects of bioprinting technology, including correlating process parameters to particular cell types with predefined applications, applying statistical analysis and artificial intelligence (AI)/machine learning (ML) technique in parameter screening, and optimizing 4D bioprinting process parameters, are highlighted.

Copper-containing chitosan-based hydrogels enabled 3D-printed scaffolds to accelerate bone repair and eliminate MRSA-related infection

Bone defect combined with drug-resistant bacteria-related infection is a thorny challenge in clinic. Herein, 3D-printed polyhydroxyalkanoates/β-tricalcium phosphate (PHA/β-TCP, PT) scaffolds were prepared by fused deposition modeling. Then copper-containing carboxymethyl chitosan/alginate (CA/Cu) hydrogels were integrated with the scaffolds via a facile and low-cost chemical crosslinking method. The resultant PT/CA/Cu scaffolds could not only promote proliferation but also osteogenic differentiation of preosteoblasts in vitro. Moreover, PT/CA/Cu scaffolds exhibited a strong antibacterial activity towards a broad-spectrum of bacteria including methicillin-resistant Staphylococcus aureus (MRSA) through inducing the intercellular generation of reactive oxygen species. In vivo experiments further demonstrated that PT/CA/Cu scaffolds significantly accelerated bone repair of cranial defects and efficiently eliminated MRSA-related infection, showing potential for application in infected bone defect therapy.

Classification, processing, and applications of bioink and 3D bioprinting: A detailed review

With the advancement in 3D bioprinting technology, cell culture methods can design 3D environments which are both, complex and physiologically relevant. The main component in 3D bioprinting, bioink, can be split into various categories depending on the criterion of categorization. Although the choice of bioink and bioprinting process will vary greatly depending on the application, general features such as material properties, biological interaction, gelation, and viscosity are always important to consider. The foundation of 3D bioprinting is the exact layer-by-layer implantation of biological elements, biochemicals, and living cells with the spatial control of the implantation of functional elements onto the biofabricated 3D structure. Three basic strategies underlie the 3D bioprinting process: autonomous self-assembly, micro tissue building blocks, and biomimicry or biomimetics. Tissue engineering can benefit from 3D bioprinting in many ways, but there are still numerous obstacles to overcome before functional tissues can be produced and used in clinical settings. A better comprehension of the physiological characteristics of bioink materials and a higher level of ability to reproduce the intricate biologically mimicked and physiologically relevant 3D structures would be a significant improvement for 3D bioprinting to overcome the limitations.

Cell Encapsulation and 3D Bioprinting for Therapeutic Cell Transplantation

The promise of cell therapy has been augmented by introducing biomaterials, where intricate scaffold shapes are fabricated to accommodate the cells within. In this review, we first discuss cell encapsulation and the promising potential of biomaterials to overcome challenges associated with cell therapy, particularly cellular function and longevity. More specifically, cell therapies in the context of autoimmune disorders, neurodegenerative diseases, and cancer are reviewed from the perspectives of preclinical findings as well as available clinical data. Next, techniques to fabricate cell-biomaterials constructs, focusing on emerging 3D bioprinting technologies, will be reviewed. 3D bioprinting is an advancing field that enables fabricating complex, interconnected, and consistent cell-based constructs capable of scaling up highly reproducible cell-biomaterials platforms with high precision. It is expected that 3D bioprinting devices will expand and become more precise, scalable, and appropriate for clinical manufacturing. Rather than one printer fits all, seeing more application-specific printer types, such as a bioprinter for bone tissue fabrication, which would be different from a bioprinter for skin tissue fabrication, is anticipated in the future.

Design and bioprinting for tissue interfaces

Tissue interfaces include complex gradient structures formed by transitioning of biochemical and mechanical properties in micro-scale. This characteristic allows the communication and synchronistic functioning of two adjacent but distinct tissues. It is particularly challenging to restore the function of these complex structures by transplantation of scaffolds exclusively produced by conventional tissue engineering methods. Three-dimensional (3D) bioprinting technology has opened an unprecedented approach for precise and graded patterning of chemical, biological and mechanical cues in a single construct mimicking natural tissue interfaces. This paper reviews and highlights biochemical and biomechanical design for 3D bioprinting of various tissue interfaces, including cartilage-bone, muscle-tendon, tendon/ligament-bone, skin, and neuro-vascular/muscular interfaces. Future directions and translational challenges are also provided at the end of the paper.

Recent advances of three-dimensional bioprinting technology in hepato-pancreato-biliary cancer models

Hepato-pancreato-biliary (HPB) cancer is a serious category of cancer including tumors originating in the liver, pancreas, gallbladder and biliary ducts. It is limited by two-dimensional (2D) cell culture models for studying its complicated tumor microenvironment including diverse contents and dynamic nature. Recently developed three-dimensional (3D) bioprinting is a state-of-the-art technology for fabrication of biological constructs through layer-by-layer deposition of bioinks in a spatially defined manner, which is computer-aided and designed to generate viable 3D constructs. 3D bioprinting has the potential to more closely recapitulate the tumor microenvironment, dynamic and complex cell-cell and cell-matrix interactions compared to the current methods, which benefits from its precise definition of positioning of various cell types and perfusing network in a high-throughput manner. In this review, we introduce and compare multiple types of 3D bioprinting methodologies for HPB cancer and other digestive tumors. We discuss the progress and application of 3D bioprinting in HPB and gastrointestinal cancers, focusing on tumor model manufacturing. We also highlight the current challenges regarding clinical translation of 3D bioprinting and bioinks in the field of digestive tumor research. Finally, we suggest valuable perspectives for this advanced technology, including combination of 3D bioprinting with microfluidics and application of 3D bioprinting in the field of tumor immunology.

Advances in Stimuli-responsive Hydrogels for Tissue Engineering and Regenerative Medicine Applications: A Review Towards Improving Structural Design for 3D Printing

The physicochemical properties of polymeric hydrogels render them attractive for the development of 3D printed prototypes for tissue engineering in regenerative medicine. Significant effort has been made to design hydrogels with desirable attributes that facilitate 3D printability. In addition, there is significant interest in exploring stimuli-responsive hydrogels to support automated 3D printing into more structurally organised prototypes such as customizable bio-scaffolds for regenerative medicine applications. Synthesizing stimuli-responsive hydrogels is dependent on the type of design and modulation of various polymeric materials to open novel opportunities for applications in biomedicine and bio-engineering. In this review, the salient advances made in the design of stimuli-responsive polymeric hydrogels for 3D printing in tissue engineering are discussed with a specific focus on the different methods of manipulation to develop 3D printed stimuli-responsive polymeric hydrogels. Polymeric functionalisation, nano-enabling and crosslinking are amongst the most common manipulative attributes that affect the assembly and structure of 3D printed bio-scaffolds and their stimuli- responsiveness. The review also provides a concise incursion into the various applications of stimuli to enhance the automated production of structurally organized 3D printed medical prototypes.

Hydrogel: A Potential Material for Bone Tissue Engineering Repairing the Segmental Mandibular Defect

Free flap surgery is currently the only successful method used by surgeons to reconstruct critical-sized defects of the jaw, and is commonly used in patients who have had bony lesions excised due to oral cancer, trauma, infection or necrosis. However, donor site morbidity remains a significant flaw of this strategy. Various biomaterials have been under investigation in search of a suitable alternative for segmental mandibular defect reconstruction. Hydrogels are group of biomaterials that have shown their potential in various tissue engineering applications, including bone regeneration, both through in vitro and in vivo pre-clinical animal trials. This review discusses different types of hydrogels, their fabrication techniques, 3D printing, their potential for bone regeneration, outcomes, and the limitations of various hydrogels in preclinical models for bone tissue engineering. This review also proposes a modified technique utilizing the potential of hydrogels combined with scaffolds and cells for efficient reconstruction of mandibular segmental defects.

Tissues and organ printing: An evolution of technology and materials

Since its beginnings, three-dimensional printing (3DP) technology has been successful because of ongoing advances in operating principles, the range of materials and cost-saving measures. However, the 3DP technological progressions in the biomedical sector have majorly taken place in the last decade after the evolution of novel 3DP systems, generally categorised as bioprinters and biomaterials to provide a replacement, transplantation or regeneration of the damaged organs and tissue constructs of the human body. There is now substantial scientific literature accessible to support the benefits of digital healthcare procedures with the help of bioprinters. It is of the highest significance to know the fundamental principles of the available printers and the compatibility of biomaterials as their feedstock, notwithstanding the huge potential of bioprinting systems to manufacture organs and other human body components. This paper provides a precise and helpful reading of the different categories of bioprinters, suitable biomaterials, numerical simulations and modelling and examples of much acknowledged clinical practices. The paper will also cite the prominent issues that still have not received desired solutions. Overall, the article will be of great use for all the professionals, scholars and engineers concerned with the 3DP, bioprinting and biomaterials.

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.

Pluripotent stem cells for skeletal tissue engineering

Here, we review the use of human pluripotent stem cells for skeletal tissue engineering. A number of approaches have been used for generating cartilage and bone from both human embryonic stem cells and induced pluripotent stem cells. These range from protocols relying on intrinsic cell interactions and signals from co-cultured cells to those attempting to recapitulate the series of steps occurring during mammalian skeletal development. The importance of generating authentic tissues rather than just differentiated cells is emphasized and enabling technologies for doing this are reported. We also review the different methods for characterization of skeletal cells and constructs at the tissue and single-cell level, and indicate newer resources not yet fully utilized in this field. There have been many challenges in this research area but the technologies to overcome these are beginning to appear, often adopted from related fields. This makes it more likely that cost-effective and efficacious human pluripotent stem cell-engineered constructs may become available for skeletal repair in the near future.

Three-Dimensional In Vitro Cell Culture Models for Efficient Drug Discovery: Progress So Far and Future Prospects

Despite tremendous advancements in technologies and resources, drug discovery still remains a tedious and expensive process. Though most cells are cultured using 2D monolayer cultures, due to lack of specificity, biochemical incompatibility, and cell-to-cell/matrix communications, they often lag behind in the race of modern drug discovery. There exists compelling evidence that 3D cell culture models are quite promising and advantageous in mimicking in vivo conditions. It is anticipated that these 3D cell culture methods will bridge the translation of data from 2D cell culture to animal models. Although 3D technologies have been adopted widely these days, they still have certain challenges associated with them, such as the maintenance of a micro-tissue environment similar to in vivo models and a lack of reproducibility. However, newer 3D cell culture models are able to bypass these issues to a maximum extent. This review summarizes the basic principles of 3D cell culture approaches and emphasizes different 3D techniques such as hydrogels, spheroids, microfluidic devices, organoids, and 3D bioprinting methods. Besides the progress made so far in 3D cell culture systems, the article emphasizes the various challenges associated with these models and their potential role in drug repositioning, including perspectives from the COVID-19 pandemic.

Development of agarose-gelatin bioinks for extrusion-based bioprinting and cell encapsulation

Three-dimensional bioprinting continues to advance as an attractive biofabrication technique to employ cell-laden hydrogel scaffolds in the creation of precise, user-defined constructs that can recapitulate the native tissue environment. Development and characterisation of new bioinks to expand the existing library helps to open avenues that can support a diversity of tissue engineering purposes and fulfil requirements in terms of both printability and supporting cell attachment. In this paper, we report the development and characterisation of agarose-gelatin hydrogel blends as a bioink for extrusion-based bioprinting. Four different agarose-gelatin hydrogel blend formulations with varying gelatin concentration were systematically characterised to evaluate suitability as a potential bioink for extrusion-based bioprinting. Additionally, autoclave and filter sterilisation methods were compared to evaluate their effect on bioink properties. Finally, the ability of the agarose-gelatin bioink to support cell viability and culture after printing was evaluated using SH-SY5Y cells encapsulated in bioprinted droplets of the agarose-gelatin. All bioink formulations demonstrate rheological, mechanical and swelling properties suitable for bioprinting and cell encapsulation. Autoclave sterilisation significantly affected the rheological properties of the agarose-gelatin bioinks compared to filter sterilisation. SH-SY5Y cells printed and differentiated into neuronal-like cells using the developed agarose-gelatin bioinks demonstrated high viability (>90%) after 23 days in culture. This study demonstrates the properties of agarose-gelatin as a printable and biocompatible material applicable for use as a bioink.

An Overview of Extracellular Matrix-Based Bioinks for 3D Bioprinting

As a microenvironment where cells reside, the extracellular matrix (ECM) has a complex network structure and appropriate mechanical properties to provide structural and biochemical support for the surrounding cells. In tissue engineering, the ECM and its derivatives can mitigate foreign body responses by presenting ECM molecules at the interface between materials and tissues. With the widespread application of three-dimensional (3D) bioprinting, the use of the ECM and its derivative bioinks for 3D bioprinting to replicate biomimetic and complex tissue structures has become an innovative and successful strategy in medical fields. In this review, we summarize the significance and recent progress of ECM-based biomaterials in 3D bioprinting. Then, we discuss the most relevant applications of ECM-based biomaterials in 3D bioprinting, such as tissue regeneration and cancer research. Furthermore, we present the status of ECM-based biomaterials in current research and discuss future development prospects.

3D Bioprinted Scaffolds for Bone Tissue Engineering: State-Of-The-Art and Emerging Technologies

Treating large bone defects, known as critical-sized defects (CSDs), is challenging because they are not spontaneously healed by the patient’s body. Due to the limitations associated with conventional bone grafts, bone tissue engineering (BTE), based on three-dimensional (3D) bioprinted scaffolds, has emerged as a promising approach for bone reconstitution and treatment. Bioprinting technology allows for incorporation of living cells and/or growth factors into scaffolds aiming to mimic the structure and properties of the native bone. To date, a wide range of biomaterials (either natural or synthetic polymers), as well as various cells and growth factors, have been explored for use in scaffold bioprinting. However, a key challenge that remains is the fabrication of scaffolds that meet structure, mechanical, and osteoconductive requirements of native bone and support vascularization. In this review, we briefly present the latest developments and discoveries of CSD treatment by means of bioprinted scaffolds, with a focus on the biomaterials, cells, and growth factors for formulating bioinks and their bioprinting techniques. Promising state-of-the-art pathways or strategies recently developed for bioprinting bone scaffolds are highlighted, including the incorporation of bioactive ceramics to create composite scaffolds, the use of advanced bioprinting technologies (e.g., core/shell bioprinting) to form hybrid scaffolds or systems, as well as the rigorous design of scaffolds by taking into account of the influence of such parameters as scaffold pore geometry and porosity. We also review in-vitro assays and in-vivo models to track bone regeneration, followed by a discussion of current limitations associated with 3D bioprinting technologies for BTE. We conclude this review with emerging approaches in this field, including the development of gradient scaffolds, four-dimensional (4D) printing technology via smart materials, organoids, and cell aggregates/spheroids along with future avenues for related BTE.

Biofabrication of the osteochondral unit and its applications: Current and future directions for 3D bioprinting

Multiple prevalent diseases, such as osteoarthritis (OA), for which there is no cure or full understanding, affect the osteochondral unit; a complex interface tissue whose architecture, mechanical nature and physiological characteristics are still yet to be successfully reproduced in vitro. Although there have been multiple tissue engineering-based approaches to recapitulate the three dimensional (3D) structural complexity of the osteochondral unit, there are various aspects that still need to be improved. This review presents the different pre-requisites necessary to develop a human osteochondral unit construct and focuses on 3D bioprinting as a promising manufacturing technique. Examples of 3D bioprinted osteochondral tissues are reviewed, focusing on the most used bioinks, chosen cell types and growth factors. Further information regarding the applications of these 3D bioprinted tissues in the fields of disease modelling, drug testing and implantation is presented. Finally, special attention is given to the limitations that currently hold back these 3D bioprinted tissues from being used as models to investigate diseases such as OA. Information regarding improvements needed in bioink development, bioreactor use, vascularisation and inclusion of additional tissues to further complete an OA disease model, are presented. Overall, this review gives an overview of the evolution in 3D bioprinting of the osteochondral unit and its applications, as well as further illustrating limitations and improvements that could be performed explicitly for disease modelling.

Bacterial cellulose-based composites for biomedical and cosmetic applications: Research progress and existing products

Bacterial cellulose (BC) is a promising unique material for various biomedical and cosmetic applications due to its morphology, mechanical strength, high purity, high water uptake, non-toxicity, chemical controllability, and biocompatibility. Today, extensive investigation is into the manufacturing of BC-based composites with other components such as nanoparticles, synthetic polymers, natural polymers, carbon materials, and biomolecules, which will allow the development of a wide range of biomedical and cosmetic products. Moreover, the addition of different reinforcement substances into BC and the organized arrangement of BC nano-fibers have proven a promising improvement in their properties for biomedical applications. This review paper highlights the progress in synthesizing BC-based composites and their applications in biomedical fields, such as wound healing, drug delivery, tissue engineering, and cancer treatment. It emphasizes high-performance BC-based materials and cosmetic applications. Furthermore, it presents challenges yet to be defeated and future possibilities for BC-based composites for biomedical and cosmetic applications.

3D Printed Multiphasic Scaffolds for Osteochondral Repair: Challenges and Opportunities

Osteochondral (OC) defects are debilitating joint injuries characterized by the loss of full thickness articular cartilage along with the underlying calcified cartilage through to the subchondral bone. While current surgical treatments can provide some relief from pain, none can fully repair all the components of the OC unit and restore its native function. Engineering OC tissue is challenging due to the presence of the three distinct tissue regions. Recent advances in additive manufacturing provide unprecedented control over the internal microstructure of bioscaffolds, the patterning of growth factors and the encapsulation of potentially regenerative cells. These developments are ushering in a new paradigm of 'multiphasic' scaffold designs in which the optimal micro-environment for each tissue region is individually crafted. Although the adoption of these techniques provides new opportunities in OC research, it also introduces challenges, such as creating tissue interfaces, integrating multiple fabrication techniques and co-culturing different cells within the same construct. This review captures the considerations and capabilities in developing 3D printed OC scaffolds, including materials, fabrication techniques, mechanical function, biological components and design.

Bioprinting of Collagen Type I and II via Aerosol Jet Printing for the Replication of Dense Collagenous Tissues

Collagen has grown increasingly present in bioprinting, however collagen bioprinting has mostly been limited to the extrusion printing of collagen type I to form weak collagen hydrogels. While these weak collagen hydrogels have their applications, synthetic polymers are often required to reinforce gel-laden constructs that aim to replicate dense collagenous tissues found in vivo. In this study, aerosol jet printing (AJP) was used to print and process collagen type I and II into dense constructs with a greater capacity to replicate the dense collagenous ECM found in connective tissues. Collagen type I and II was isolated from animal tissues to form solutions for printing. Collagen type I and II constructs were printed with 576 layers and measured to have average effective elastic moduli of 241.3 ± 94.3 and 196.6 ± 86.0 kPa (±SD), respectively, without any chemical modification. Collagen type II solutions were measured to be less viscous than type I and both collagen type I and II exhibited a drop in viscosity due to AJP. Circular dichroism and SDS-PAGE showed collagen type I to be more vulnerable to structural changes due to the stresses of the aerosol formation step of aerosol jet printing while the collagen type II triple helix was largely unaffected. SEM illustrated that distinct layers remained in the aerosol jet print constructs. The results show that aerosol jet printing should be considered an effective way to process collagen type I and II into stiff dense constructs with suitable mechanical properties for the replication of dense collagenous connective tissues.

3D Bioprinted Implants for Cartilage Repair in Intervertebral Discs and Knee Menisci

Cartilage defects pose a significant clinical challenge as they can lead to joint pain, swelling and stiffness, which reduces mobility and function thereby significantly affecting the quality of life of patients. More than 250,000 cartilage repair surgeries are performed in the United States every year. The current gold standard is the treatment of focal cartilage defects and bone damage with nonflexible metal or plastic prosthetics. However, these prosthetics are often made from hard and stiff materials that limits mobility and flexibility, and results in leaching of metal particles into the body, degeneration of adjacent soft bone tissues and possible failure of the implant with time. As a result, the patients may require revision surgeries to replace the worn implants or adjacent vertebrae. More recently, autograft - and allograft-based repair strategies have been studied, however these too are limited by donor site morbidity and the limited availability of tissues for surgery. There has been increasing interest in the past two decades in the area of cartilage tissue engineering where methods like 3D bioprinting may be implemented to generate functional constructs using a combination of cells, growth factors (GF) and biocompatible materials. 3D bioprinting allows for the modulation of mechanical properties of the developed constructs to maintain the required flexibility following implantation while also providing the stiffness needed to support body weight. In this review, we will provide a comprehensive overview of current advances in 3D bioprinting for cartilage tissue engineering for knee menisci and intervertebral disc repair. We will also discuss promising medical-grade materials and techniques that can be used for printing, and the future outlook of this emerging field.

Resolution of 3D bioprinting inside bulk gel and granular gel baths

Three-dimensional (3D) bioprinting has rapidly developed in the last decade, playing an increasingly important role in applications including pharmacokinetics research, tissue engineering, and organ regeneration. As a cutting-edge technology in 3D printing, gel bath-supported 3D bioprinting enables the freeform construction of complex structures with soft and water-containing materials, facilitating the in vitro fabrication of live tissue or organ models. To realize in vivo-like organs or tissues in terms of biological functions and complex structures by 3D printing, high resolution and fidelity are prerequisites. Although a wide range of gel matrices have recently been developed as supporting materials, the effect of bath properties and printing parameters on the print resolution is still not clearly understood. This review systematically introduces the decisive factors for resolution in both bulk gel bath systems and granular microgel bath systems, providing guidelines for high-resolution 3D bioprinting based on bath properties and printing parameters.

Biomaterials in bone and mineralized tissue engineering using 3D printing and bioprinting technologies

This review focuses on recently developed printable biomaterials for bone and mineralized tissue engineering. 3D printing or bioprinting is an advanced technology to design and fabricate complex functional 3D scaffolds, mimicking native tissue forin vivoapplications. We categorized the biomaterials into two main classes: 3D printing and bioprinting. Various biomaterials, including natural, synthetic biopolymers and their composites, have been studied. Biomaterial inks or bioinks used for bone and mineralized tissue regeneration include hydrogels loaded with minerals or bioceramics, cells, and growth factors. In 3D printing, the scaffold is created by acellular biomaterials (biomaterial inks), while in 3D bioprinting, cell-laden hydrogels (bioinks) are used. Two main classes of bioceramics, including bioactive and bioinert ceramics, are reviewed. Bioceramics incorporation provides osteoconductive properties and induces bone formation. Each biopolymer and mineral have its advantages and limitations. Each component of these composite biomaterials provides specific properties, and their combination can ameliorate the mechanical properties, bioactivity, or biological integration of the 3D printed scaffold. Present challenges and future approaches to address them are also discussed.

Biomimetic and mechanically supportive 3D printed scaffolds for cartilage and osteochondral tissue engineering using photopolymers and digital light processing

Successful 3D scaffold designs for musculoskeletal tissue engineering necessitate full consideration of the form and function of the tissues of interest. When designing structures for engineering cartilage and osteochondral tissues, one must reconcile the need to develop a mechanically robust system that maintains the health of cells embedded in the scaffold. In this work, we present an approach that decouples the mechanical and biochemical needs and allows for the independent development of the structural and cellular niches in a scaffold. Using the highly tuned capabilities of digital light processing-based stereolithography, structures with complex architectures are achieved over a range of effective porosities and moduli. The 3D printed structure is infilled with mesenchymal stem cells and soft biomimetic hydrogels, which are specifically formulated with extracellular matrix analogs and tethered growth factors to provide selected biochemical cues for the guided differentiation towards chondrogenesis and osteogenesis. We demonstrate the ability to utilize these structures to (a) infill a focal chondral defect and mitigate macroscopic and cellular level changes in the cartilage surrounding the defect, and (b) support the development of a stratified multi-tissue scaffold for osteochondral tissue engineering.

3D Bioprinting of Hydrogels for Cartilage Tissue Engineering

Three-dimensional (3D) bioprinting is an emerging technology based on 3D digital imaging technology and multi-level continuous printing. The precise positioning of biological materials, seed cells, and biological factors, known as "additive biomanufacturing", can provide personalized therapy strategies in regenerative medicine. Over the last two decades, 3D bioprinting hydrogels have significantly advanced the field of cartilage and bone tissue engineering. This article reviews the development of 3D bioprinting and its application in cartilage tissue engineering, followed by a discussion of the current challenges and prospects for 3D bioprinting. This review presents foundational information on the future optimization of the design and manufacturing process of 3D additive biomanufacturing.

Collagen Bioinks for Bioprinting: A Systematic Review of Hydrogel Properties, Bioprinting Parameters, Protocols, and Bioprinted Structure Characteristics

Bioprinting is a modern tool suitable for creating cell scaffolds and tissue or organ carriers from polymers that mimic tissue properties and create a natural environment for cell development. A wide range of polymers, both natural and synthetic, are used, including extracellular matrix and collagen-based polymers. Bioprinting technologies, based on syringe deposition or laser technologies, are optimal tools for creating precise constructs precisely from the combination of collagen hydrogel and cells. This review describes the different stages of bioprinting, from the extraction of collagen hydrogels and bioink preparation, over the parameters of the printing itself, to the final testing of the constructs. This study mainly focuses on the use of physically crosslinked high-concentrated collagen hydrogels, which represents the optimal way to create a biocompatible 3D construct with sufficient stiffness. The cell viability in these gels is mainly influenced by the composition of the bioink and the parameters of the bioprinting process itself (temperature, pressure, cell density, etc.). In addition, a detailed table is included that lists the bioprinting parameters and composition of custom bioinks from current studies focusing on printing collagen gels without the addition of other polymers. Last but not least, our work also tries to refute the often-mentioned fact that highly concentrated collagen hydrogel is not suitable for 3D bioprinting and cell growth and development.

High-Resolution Bioprinting of Recombinant Human Collagen Type III

The development of commercial collagen inks for extrusion-based bioprinting has increased the amount of research on pure collagen bioprinting, i.e., collagen inks not mixed with gelatin, alginate, or other more common biomaterial inks. New printing techniques have also improved the resolution achievable with pure collagen bioprinting. However, the resultant collagen constructs still appear too weak to replicate dense collagenous tissues, such as the cornea. This work aims to demonstrate the first reported case of bioprinted recombinant collagen films with suitable optical and mechanical properties for corneal tissue engineering. The printing technology used, aerosol jet® printing (AJP), is a high-resolution printing method normally used to deposit conductive inks for electronic printing. In this work, AJP was employed to deposit recombinant human collagen type III (RHCIII) in overlapping continuous lines of 60 µm to form thin layers. Layers were repeated up to 764 times to result in a construct that was considered a few hundred microns thick when swollen. Samples were subsequently neutralised and crosslinked using EDC:NHS crosslinking. Nanoindentation and absorbance measurements were conducted, and the results show that the AJP-deposited RHCIII samples possess suitable mechanical and optical properties for corneal tissue engineering: an average effective elastic modulus of 506 ± 173 kPa and transparency ≥87% at all visible wavelengths. Circular dichroism showed that there was some loss of helicity of the collagen due to aerosolisation. SDS-PAGE and pepsin digestion were used to show that while some collagen is degraded due to aerosolisation, it remains an inaccessible substrate for pepsin cleavage.

Bioprinting of human nasoseptal chondrocytes-laden collagen hydrogel for cartilage tissue engineering

Skin cancer patients often have tumorigenic lesions on their noses. Surgical resection of the lesions often results in nasal cartilage removal. Cartilage grafts taken from other anatomical sites are used for the surgical reconstruction of the nasal cartilage, but donor-site morbidity is a common problem. Autologous tissue-engineered nasal cartilage grafts can mitigate the problem, but commercially available scaffolds define the shape and sizes of the engineered grafts during tissue fabrication. Moreover, the engineered grafts suffer from the inhomogeneous distribution of the functional matrix of cartilage. Advances in 3D bioprinting technology offer the opportunity to engineer cartilages with customizable dimensions and anatomically shaped configurations without the inhomogeneous distribution of cartilage matrix. Here, we report the fidelity of Freeform Reversible Embedding of Suspended Hydrogel (FRESH) bioprinting as a strategy to generate customizable and homogenously distributed functional cartilage matrix engineered nasal cartilage. Using FRESH and in vitro chondrogenesis, we have fabricated tissue-engineered nasal cartilage from combining bovine type I collagen hydrogel and human nasoseptal chondrocytes. The engineered nasal cartilage constructs displayed molecular, biochemical and histological characteristics akin to native human nasal cartilage.

3D printed hybrid bone constructs of PCL and dental pulp stem cells loaded GelMA

Fabrication of scaffolds using polymers and then cell seeding is a routine protocol of tissue engineering applications. Synthetic polymers have adequate mechanical properties to substitute for some bone tissue, but they are generally hydrophobic and have no specific cell recognition sites, which leads to poor cell affinity and adhesion. Some natural polymers, have high cell affinity but are mechanically weak and do not have the strength required as a bone supporting material. In the present study, 3D printed hybrid scaffolds were fabricated using PCL and GelMA carrying dental pulp stem cells (DPSCs), which is printed in the gaps between the PCL struts. This cell loaded GelMA was shown to support osteoinductivity, while the PCL provided mechanical strength needed to mimic the bone tissue. 3D printed PCL/GelMA and GelMA scaffolds were highly stable during 21 days of incubation in PBS. The compressive moduli of the hybrid scaffolds were in the range of the compressive moduli of trabecular bone. DPSCs were homogeneously distributed throughout the entire hydrogel component and exhibited high cell viability in both scaffolds during 21 days of incubation. Upon osteogenic differentiation DPSCs expressed two key matrix proteins, osteopontin and osteocalcin. Alizarin red staining showed mineralized nodules, which demonstrates osteogenic differentiation of DPSCs within GelMA. This construct yielded a very high cell viability, osteogenic differentiation and mineralization comparable to cell culture without compromising mechanical strength suitable for bone tissue engineering applications. Thus, 3D printed, cell loaded PCL/GelMA hybrid scaffolds have a great potential for use in bone tissue engineering applications.

3D Bioprinting of Human Tissues: Biofabrication, Bioinks, and Bioreactors

The field of tissue engineering has progressed tremendously over the past few decades in its ability to fabricate functional tissue substitutes for regenerative medicine and pharmaceutical research. Conventional scaffold-based approaches are limited in their capacity to produce constructs with the functionality and complexity of native tissue. Three-dimensional (3D) bioprinting offers exciting prospects for scaffolds fabrication, as it allows precise placement of cells, biochemical factors, and biomaterials in a layer-by-layer process. Compared with traditional scaffold fabrication approaches, 3D bioprinting is better to mimic the complex microstructures of biological tissues and accurately control the distribution of cells. Here, we describe recent technological advances in bio-fabrication focusing on 3D bioprinting processes for tissue engineering from data processing to bioprinting, mainly inkjet, laser, and extrusion-based technique. We then review the associated bioink formulation for 3D bioprinting of human tissues, including biomaterials, cells, and growth factors selection. The key bioink properties for successful bioprinting of human tissue were summarized. After bioprinting, the cells are generally devoid of any exposure to fluid mechanical cues, such as fluid shear stress, tension, and compression, which are crucial for tissue development and function in health and disease. The bioreactor can serve as a simulator to aid in the development of engineering human tissues from in vitro maturation of 3D cell-laden scaffolds. We then describe some of the most common bioreactors found in the engineering of several functional tissues, such as bone, cartilage, and cardiovascular applications. In the end, we conclude with a brief insight into present limitations and future developments on the application of 3D bioprinting and bioreactor systems for engineering human tissue.

3D bioprinting of tissue-specific osteoblasts and endothelial cells to model the human jawbone

Jawbone differs from other bones in many aspects, including its developmental origin and the occurrence of jawbone-specific diseases like MRONJ (medication-related osteonecrosis of the jaw). Although there is a strong need, adequate in vitro models of this unique environment are sparse to date. While previous approaches are reliant e.g. on scaffolds or spheroid culture, 3D bioprinting enables free-form fabrication of complex living tissue structures. In the present work, production of human jawbone models was realised via projection-based stereolithography. Constructs were bioprinted containing primary jawbone-derived osteoblasts and vasculature-like channel structures optionally harbouring primary endothelial cells. After 28 days of cultivation in growth medium or osteogenic medium, expression of cell type-specific markers was confirmed on both the RNA and protein level, while prints maintained their overall structure. Survival of endothelial cells in the printed channels, co-cultured with osteoblasts in medium without supplementation of endothelial growth factors, was demonstrated. Constructs showed not only mineralisation, being one of the characteristics of osteoblasts, but also hinted at differentiation to an osteocyte phenotype. These results indicate the successful biofabrication of an in vitro model of the human jawbone, which presents key features of this special bone entity and hence appears promising for application in jawbone-specific research.

Bioresorbable Polymers: Advanced Materials and 4D Printing for Tissue Engineering

Three-dimensional (3D) printing is a valuable tool in the production of complexes structures with specific shapes for tissue engineering. Differently from native tissues, the printed structures are static and do not transform their shape in response to different environment changes. Stimuli-responsive biocompatible materials have emerged in the biomedical field due to the ability of responding to other stimuli (physical, chemical, and/or biological), resulting in microstructures modifications. Four-dimensional (4D) printing arises as a new technology that implements dynamic improvements in printed structures using smart materials (stimuli-responsive materials) and/or cells. These dynamic scaffolds enable engineered tissues to undergo morphological changes in a pre-planned way. Stimuli-responsive polymeric hydrogels are the most promising material for 4D bio-fabrication because they produce a biocompatible and bioresorbable 3D shape environment similar to the extracellular matrix and allow deposition of cells on the scaffold surface as well as in the inside. Subsequently, this review presents different bioresorbable advanced polymers and discusses its use in 4D printing for tissue engineering applications.

Modeling the Mechanobiology of Cancer Cell Migration Using 3D Biomimetic Hydrogels

Understanding how cancer cells migrate, and how this migration is affected by the mechanical and chemical composition of the extracellular matrix (ECM) is critical to investigate and possibly interfere with the metastatic process, which is responsible for most cancer-related deaths. In this article we review the state of the art about the use of hydrogel-based three-dimensional (3D) scaffolds as artificial platforms to model the mechanobiology of cancer cell migration. We start by briefly reviewing the concept and composition of the extracellular matrix (ECM) and the materials commonly used to recreate the cancerous ECM. Then we summarize the most relevant knowledge about the mechanobiology of cancer cell migration that has been obtained using 3D hydrogel scaffolds, and relate those discoveries to what has been observed in the clinical management of solid tumors. Finally, we review some recent methodological developments, specifically the use of novel bioprinting techniques and microfluidics to create realistic hydrogel-based models of the cancer ECM, and some of their applications in the context of the study of cancer cell migration.

Advances in biofabrication techniques for collagen-based 3D in vitro culture models for breast cancer research

Collagen is the most abundant component of the extracellular matrix (ECM), therefore it represents an ideal biomaterial for the culture of a variety of cell types. Recently, collagen-based scaffolds have shown promise as 3D culture platforms for breast cancer-based research. Two-dimensional (2D) in vitro culture models, while useful for gaining preliminary insights, are ultimately flawed as they do not adequately replicate the tumour microenvironment. As a result, they do not facilitate proper 3D cell-cell/cell-matrix interactions and often an exaggerated response to therapeutic agents occurs. The ECM plays a crucial role in the development and spread of cancer. Alterations within the ECM have a significant impact on the pathogenesis of cancer, the initiation of metastasis and ultimate progression of the disease. 3D in vitro culture models that aim to replicate the tumour microenvironment have the potential to offer a new frontier for cancer research with cell growth, morphology and genetic properties that more closely match in vivo cancers. While initial 3D in vitro culture models used in breast cancer research consisted of simple hydrogel platforms, recent advances in biofabrication techniques, including freeze-drying, electrospinning and 3D bioprinting, have enabled the fabrication of biomimetic collagen-based platforms that more closely replicate the breast cancer ECM. This review highlights the current application of collagen-based scaffolds as 3D in vitro culture models for breast cancer research, specifically for adherence-based scaffolds (i.e. matrix-assisted). Finally, the future perspectives of 3D in vitro breast cancer models and their potential to lead to an improved understanding of breast cancer diagnosis and treatment are discussed.

Bioink Formulations for Bone Tissue Regeneration

Unlike the conventional techniques used to construct a tissue scaffolding, three-dimensional (3D) bioprinting technology enables fabrication of a porous structure with complex and diverse geometries, which facilitate evenly distributed cells and orderly release of signal factors. To date, a range of cell-laden materials, such as natural or synthetic polymers, have been deployed by the 3D bioprinting technique to construct the scaffolding systems and regenerate substitutes for the natural extracellular matrix (ECM). Four-dimensional (4D) bioprinting technology has attracted much attention lately because it aims to accommodate the dynamic structural and functional transformations of scaffolds. However, there remain challenges to meet the technical requirements in terms of suitable processability of the bioink formulations, desired mechanical properties of the hydrogel implants, and cell-guided functionality of the biomaterials. Recent bioprinting techniques are reviewed in this article, discussing strategies for hydrogel-based bioinks to mimic native bone tissue-like extracellular matrix environment, including properties of bioink formulations required for bioprinting, structure requirements, and preparation of tough hydrogel scaffolds. Stimulus mechanisms that are commonly used to trigger the dynamic structural and functional transformations of the scaffold are analyzed. At the end, we highlighted the current challenges and possible future avenues of smart hydrogel-based bioink/scaffolds for bone tissue regeneration.

3D Bioprinting of In Vitro Models Using Hydrogel-Based Bioinks

Coronavirus disease 2019 (COVID-19), which has recently emerged as a global pandemic, has caused a serious economic crisis due to the social disconnection and physical distancing in human society. To rapidly respond to the emergence of new diseases, a reliable in vitro model needs to be established expeditiously for the identification of appropriate therapeutic agents. Such models can be of great help in validating the pathological behavior of pathogens and therapeutic agents. Recently, in vitro models representing human organs and tissues and biological functions have been developed based on high-precision 3D bioprinting. In this paper, we delineate an in-depth assessment of the recently developed 3D bioprinting technology and bioinks. In particular, we discuss the latest achievements and future aspects of the use of 3D bioprinting for in vitro modeling.

Comparison of the Translational Potential of Human Mesenchymal Progenitor Cells from Different Bone Entities for Autologous 3D Bioprinted Bone Grafts

Reconstruction of segmental bone defects by autologous bone grafting is still the standard of care but presents challenges including anatomical availability and potential donor site morbidity. The process of 3D bioprinting, the application of 3D printing for direct fabrication of living tissue, opens new possibilities for highly personalized tissue implants, making it an appealing alternative to autologous bone grafts. One of the most crucial hurdles for the clinical application of 3D bioprinting is the choice of a suitable cell source, which should be minimally invasive, with high osteogenic potential, with fast, easy expansion. In this study, mesenchymal progenitor cells were isolated from clinically relevant human bone biopsy sites (explant cultures from alveolar bone, iliac crest and fibula; bone marrow aspirates; and periosteal bone shaving from the mastoid) and 3D bioprinted using projection-based stereolithography. Printed constructs were cultivated for 28 days and analyzed regarding their osteogenic potential by assessing viability, mineralization, and gene expression. While viability levels of all cell sources were comparable over the course of the cultivation, cells obtained by periosteal bone shaving showed higher mineralization of the print matrix, with gene expression data suggesting advanced osteogenic differentiation. These results indicate that periosteum-derived cells represent a highly promising cell source for translational bioprinting of bone tissue given their superior osteogenic potential as well as their minimally invasive obtainability.

Current Advances in 3D Bioprinting Technology and Its Applications for Tissue Engineering

Three-dimensional (3D) bioprinting technology has emerged as a powerful biofabrication platform for tissue engineering because of its ability to engineer living cells and biomaterial-based 3D objects. Over the last few decades, droplet-based, extrusion-based, and laser-assisted bioprinters have been developed to fulfill certain requirements in terms of resolution, cell viability, cell density, etc. Simultaneously, various bio-inks based on natural-synthetic biomaterials have been developed and applied for successful tissue regeneration. To engineer more realistic artificial tissues/organs, mixtures of bio-inks with various recipes have also been developed. Taken together, this review describes the fundamental characteristics of the existing bioprinters and bio-inks that have been currently developed, followed by their advantages and disadvantages. Finally, various tissue engineering applications using 3D bioprinting are briefly introduced.

3D Cell Printing of Tissue/Organ-Mimicking Constructs for Therapeutic and Drug Testing Applications

The development of artificial tissue/organs with the functional maturity of their native equivalents is one of the long-awaited panaceas for the medical and pharmaceutical industries. Advanced 3D cell-printing technology and various functional bioinks are promising technologies in the field of tissue engineering that have enabled the fabrication of complex 3D living tissue/organs. Various requirements for these tissues, including a complex and large-volume structure, tissue-specific microenvironments, and functional vasculatures, have been addressed to develop engineered tissue/organs with native relevance. Functional tissue/organ constructs have been developed that satisfy such criteria and may facilitate both in vivo replenishment of damaged tissue and the development of reliable in vitro testing platforms for drug development. This review describes key developments in technologies and materials for engineering 3D cell-printed constructs for therapeutic and drug testing applications.

Bioprinting of Collagen: Considerations, Potentials, and Applications

Collagen is the most abundant extracellular matrix protein that is widely used in tissue engineering (TE). There is little research done on printing pure collagen. To understand the bottlenecks in printing pure collagen, it is imperative to understand collagen from a bottom-up approach. Here it is aimed to provide a comprehensive overview of collagen printing, where collagen assembly in vivo and the various sources of collagen available for TE application are first understood. Next, the current printing technologies and strategy for printing collagen-based materials are highlighted. Considerations and key challenges faced in collagen printing are identified. Finally, the key research areas that would enhance the functionality of printed collagen are presented.

Emulating Human Tissues and Organs: A Bioprinting Perspective Toward Personalized Medicine

The lack of in vitro tissue and organ models capable of mimicking human physiology severely hinders the development and clinical translation of therapies and drugs with higher in vivo efficacy. Bioprinting allow us to fill this gap and generate 3D tissue analogues with complex functional and structural organization through the precise spatial positioning of multiple materials and cells. In this review, we report the latest developments in terms of bioprinting technologies for the manufacturing of cellular constructs with particular emphasis on material extrusion, jetting, and vat photopolymerization. We then describe the different base polymers employed in the formulation of bioinks for bioprinting and examine the strategies used to tailor their properties according to both processability and tissue maturation requirements. By relating function to organization in human development, we examine the potential of pluripotent stem cells in the context of bioprinting toward a new generation of tissue models for personalized medicine. We also highlight the most relevant attempts to engineer artificial models for the study of human organogenesis, disease, and drug screening. Finally, we discuss the most pressing challenges, opportunities, and future prospects in the field of bioprinting for tissue engineering (TE) and regenerative medicine (RM).

Effects of Processing Parameters of 3D Bioprinting on the Cellular Activity of Bioinks

In this review, few established cell printing techniques along with their parameters that affect the cell viability during bioprinting are considered. 3D bioprinting is developed on the principle of additive manufacturing using biomaterial inks and bioinks. Different bioprinting methods impose few challenges on cell printing such as shear stress, mechanical impact, heat, laser radiation, etc., which eventually lead to cell death. These factors also cause alteration of cells phenotype, recoverable or irrecoverable damages to the cells. Such challenges are not addressed in detail in the literature and scientific reports. Hence, this review presents a detailed discussion of several cellular bioprinting methods and their process-related impacts on cell viability, followed by probable mitigation techniques. Most of the printable bioinks encompass cells within hydrogel as scaffold material to avoid the direct exposure of the harsh printing environment on cells. However, the advantages of printing with scaffold-free cellular aggregates over cell-laden hydrogels have emerged very recently. Henceforth, optimal and favorable crosslinking mechanisms providing structural rigidity to the cell-laden printed constructs with ideal cell differentiation and proliferation, are discussed for improved understanding of cell printing methods for the future of organ printing and transplantation.

Fabrication of a Polycaprolactone/Alginate Bipartite Hybrid Scaffold for Osteochondral Tissue Using a Three-Dimensional Bioprinting System

Osteochondral defects, including damage to both the articular cartilage and the subchondral bone, are challenging to repair. Although many technological advancements have been made in recent years, there are technical difficulties in the engineering of cartilage and bone layers, simultaneously. Moreover, there is a great need for a valuable in vitro platform enabling the assessment of osteochondral tissues to reduce pre-operative risk. Three-dimensional (3D) bioprinting systems may be a promising approach for fabricating human tissues and organs. Here, we aimed to develop a polycaprolactone (PCL)/alginate bipartite hybrid scaffold using a multihead 3D bioprinting system. The hybrid scaffold was composed of PCL, which could improve the mechanical properties of the construct, and alginate, encapsulating progenitor cells that could differentiate into cartilage and bone. To differentiate the bipartite hybrid scaffold into osteochondral tissue, a polydimethylsiloxane coculture system for osteochondral tissue (PCSOT) was designed and developed. Based on evaluation of the biological performance of the novel hybrid scaffold, the PCL/alginate bipartite scaffold was successfully fabricated; importantly, our findings suggest that this PCSOT system may be applicable as an in vitro platform for osteochondral tissue engineering.