A novel bioprinting method and system for forming hybrid tissue engineering constructs

Preclinical models for studying corticosteroid-induced osteonecrosis of the femoral head

Nontraumatic osteonecrosis of the femoral head (ONFH) is a refractory condition that commonly results in femoral head collapse and degenerative arthritis of the hip. In the early stages, surgical procedures for hip preservation, including core decompression (CD), have been developed to prevent progressive collapse of the femoral head. Optimization of bone regeneration and biological augmentation may further enhance the therapeutic efficacy of CD for ONFH. Thus, combining CD with cell-based therapy has recently been proposed. In fact, patients treated with cell-based therapy using autologous bone marrow concentrate demonstrate improved survivorship of the femoral head, compared with conventional CD alone. Preclinical research studies to investigate adjunctive therapies for CD often utilize the rabbit model of corticosteroid-induced ONFH. Mesenchymal stem cells (MSCs) are known to promote osteogenesis and angiogenesis, and decrease inflammation in bone. Local drug delivery systems have the potential to achieve targeted therapeutic effects by precisely controlling the drug release rate. Scaffolds can provide an osteoconductive structural framework to facilitate the repair of osteonecrotic bone tissue. We focused on the combination of both cell-based and scaffold-based therapies for bone tissue regeneration in ONFH. We hypothesized that combining CD and osteoconductive scaffolds would provide mechanical strength and structural cell guidance; and that combining CD and genetically modified (GM) MSCs to express relevant cytokines, chemokines, and growth factors would promote bone tissue repair. We developed GM MSCs that overexpress the anti-inflammatory, pro-reconstructive cytokines platelet-derived growth factor-BB to provide MSCs with additional benefits and investigated the efficacy of combinations of these GM MSCs and scaffolds for treatment of ONFH in skeletally mature male New Zealand white rabbits. In the future, the long-term safety, efficacy, durability, and cost-effectiveness of these and other biological and mechanical treatments must be demonstrated for the patients affected by ONFH.

Development of Biocompatible 3D-Printed Artificial Blood Vessels through Multidimensional Approaches

Within the human body, the intricate network of blood vessels plays a pivotal role in transporting nutrients and oxygen and maintaining homeostasis. Bioprinting is an innovative technology with the potential to revolutionize this field by constructing complex multicellular structures. This technique offers the advantage of depositing individual cells, growth factors, and biochemical signals, thereby facilitating the growth of functional blood vessels. Despite the challenges in fabricating vascularized constructs, bioprinting has emerged as an advance in organ engineering. The continuous evolution of bioprinting technology and biomaterial knowledge provides an avenue to overcome the hurdles associated with vascularized tissue fabrication. This article provides an overview of the biofabrication process used to create vascular and vascularized constructs. It delves into the various techniques used in vascular engineering, including extrusion-, droplet-, and laser-based bioprinting methods. Integrating these techniques offers the prospect of crafting artificial blood vessels with remarkable precision and functionality. Therefore, the potential impact of bioprinting in vascular engineering is significant. With technological advances, it holds promise in revolutionizing organ transplantation, tissue engineering, and regenerative medicine. By mimicking the natural complexity of blood vessels, bioprinting brings us one step closer to engineering organs with functional vasculature, ushering in a new era of medical advancement.

From materials to clinical use: advances in 3D-printed scaffolds for cartilage tissue engineering

Osteoarthritis caused by articular cartilage defects is a particularly common orthopedic disease that can involve the entire joint, causing great pain to its sufferers. A global patient population of approximately 250 million people has an increasing demand for new therapies with excellent results, and tissue engineering scaffolds have been proposed as a potential strategy for the repair and reconstruction of cartilage defects. The precise control and high flexibility of 3D printing provide a platform for subversive innovation. In this perspective, cartilage tissue engineering (CTE) scaffolds manufactured using different biomaterials are summarized from the perspective of 3D printing strategies, the bionic structure strategies and special functional designs are classified and discussed, and the advantages and limitations of these CTE scaffold preparation strategies are analyzed in detail. Finally, the application prospect and challenges of 3D printed CTE scaffolds are discussed, providing enlightening insights for their current research.

Is it possible to 3D bioprint load-bearing bone implants? A critical review

Rehabilitative capabilities of any tissue engineered scaffold rely primarily on the triad of (i) biomechanical properties such as mechanical properties and architecture, (ii) chemical behavior such as regulation of cytokine expression, and (iii) cellular response modulation (including their recruitment and differentiation). The closer the implant can mimic the native tissue, the better it can rehabilitate the damage therein. Among the available fabrication techniques, only 3D bioprinting (3DBP) can satisfactorily replicate the inherent heterogeneity of the host tissue. However, 3DBP scaffolds typically suffer from poor mechanical properties, thereby, driving the increased research interest in development of load-bearing 3DBP orthopedic scaffolds in recent years. Typically, these scaffolds involve multi-material 3D printing, comprising of at-least one bioink and a load-bearing ink; such that mechanical and biological requirements of the biomaterials are decoupled. Ensuring high cellular survivability and good mechanical properties are of key concerns in all these studies. 3DBP of such scaffolds is in early developmental stages, and research data from only a handful of preliminary animal studies are available, owing to limitations in print-capabilities and restrictive materials library. This article presents a topically focused review of the state-of-the-art, while highlighting aspects like available 3DBP techniques; biomaterials' printability; mechanical and degradation behavior; and their overall bone-tissue rehabilitative efficacy. This collection amalgamates and critically analyses the research aimed at 3DBP of load-bearing scaffolds for fulfilling demands of personalized-medicine. We highlight the recent-advances in 3DBP techniques employing thermoplastics and phosphate-cements for load-bearing applications. Finally, we provide an outlook for possible future perspectives of 3DBP for load-bearing orthopedic applications. Overall, the article creates ample foundation for future research, as it gathers the latest and ongoing research that scientists could utilize.

Trends in Photopolymerizable Bioinks for 3D Bioprinting of Tumor Models

Three-dimensional (3D) bioprinting technologies involving photopolymerizable bioinks (PBs) have attracted enormous attention in recent times owing to their ability to recreate complex structures with high resolution, mechanical stability, and favorable printing conditions that are suited for encapsulating cells. 3D bioprinted tissue constructs involving PBs can offer better insights into the tumor microenvironment and offer platforms for drug screening to advance cancer research. These bioinks enable the incorporation of physiologically relevant cell densities, tissue-mimetic stiffness, and vascularized channels and biochemical gradients in the 3D tumor models, unlike conventional two-dimensional (2D) cultures or other 3D scaffold fabrication technologies. In this perspective, we present the emerging techniques of 3D bioprinting using PBs in the context of cancer research, with a specific focus on the efforts to recapitulate the complexity of the tumor microenvironment. We describe printing approaches and various PB formulations compatible with these techniques along with recent attempts to bioprint 3D tumor models for studying migration and metastasis, cell-cell interactions, cell-extracellular matrix interactions, and drug screening relevant to cancer. We discuss the limitations and identify unexplored opportunities in this field for clinical and commercial translation of these emerging technologies.

The efficiency of genetically modified mesenchymal stromal cells combined with a functionally graded scaffold for bone regeneration in corticosteroid-induced osteonecrosis of the femoral head in rabbits

Core decompression (CD) with mesenchymal stromal cells (MSCs) is an effective therapy for early-stage osteonecrosis of the femoral head (ONFH). Preconditioning of MSCs, using inflammatory mediators, is widely used in immunology and various cell therapies. We developed a three-dimensional printed functionally graded scaffold (FGS), made of β-TCP and PCL, for cell delivery at a specific location. The present study examined the efficacy of CD treatments with genetically modified (GM) MSCs over-expressing PDGF-BB (PDGF-MSCs) or GM MSCs co-over-expressing IL-4 and PDGF-BB and preconditioned for three days of exposure to lipopolysaccharide and tumor necrosis factor-alpha (IL-4-PDGF-pMSCs) using the FGS for treating steroid-induced ONFH in rabbits. We compared CD without cell-therapy, with IL-4-PDGF-pMSCs alone, and with FGS loaded with PDGF-MSCs or IL-4-PDGF-pMSCs. For the area inside the CD, the bone volume in the CD alone was higher than in both FGS groups. The IL-4-PDGF-pMSCs alone and FGS + PDGF-MSCs reduced the occurrence of empty lacunae and improved osteoclastogenesis. There was no significant difference in angiogenesis among the four groups. The combined effect of GM MSCs or pMSCs and the FGS was not superior to the effect of each alone. To establish an important adjunctive therapy for CD for early ONFH in the future, it is necessary and essential to develop an FGS that delivers biologics appropriately and provides structural and mechanical support.

Additive Manufacturing and Physicomechanical Characteristics of PEGDA Hydrogels: Recent Advances and Perspective for Tissue Engineering

In this brief review, we discuss the recent advancements in using poly(ethylene glycol) diacrylate (PEGDA) hydrogels for tissue engineering applications. PEGDA hydrogels are highly attractive in biomedical and biotechnology fields due to their soft and hydrated properties that can replicate living tissues. These hydrogels can be manipulated using light, heat, and cross-linkers to achieve desirable functionalities. Unlike previous reviews that focused solely on material design and fabrication of bioactive hydrogels and their cell viability and interactions with the extracellular matrix (ECM), we compare the traditional bulk photo-crosslinking method with the latest three-dimensional (3D) printing of PEGDA hydrogels. We present detailed evidence combining the physical, chemical, bulk, and localized mechanical characteristics, including their composition, fabrication methods, experimental conditions, and reported mechanical properties of bulk and 3D printed PEGDA hydrogels. Furthermore, we highlight the current state of biomedical applications of 3D PEGDA hydrogels in tissue engineering and organ-on-chip devices over the last 20 years. Finally, we delve into the current obstacles and future possibilities in the field of engineering 3D layer-by-layer (LbL) PEGDA hydrogels for tissue engineering and organ-on-chip devices.

Biodegradable Poly(ester) Urethane Acrylate Resins for Digital Light Processing: From Polymer Synthesis to 3D Printed Tissue Engineering Constructs

Digital light processing (DLP) is an accurate and fast additive manufacturing technique to produce a variety of products, from patient-customized biomedical implants to consumer goods. However, DLP's use in tissue engineering has been hampered due to a lack of biodegradable resin development. Herein, a library of biodegradable poly(esters) capped with urethane acrylate (with variations in molecular weight) is investigated as the basis for DLP printable resins for tissue engineering. The synthesized oligomers show good printability and are capable of creating complex structures with mechanical moduli close to those of medium-soft tissues (1-3 MPa). While fabricated films from different molecular weight resins show few differences in surface topology, wettability, and protein adsorption, the adhesion and metabolic activity of NCTC clone 929 (L929) cells and human dermal fibroblasts (HDFs) are significantly different. Resins from higher molecular weight oligomers provide greater cell adhesion and metabolic activity. Furthermore, these materials show compatibility in a subcutaneous in vivo pig model. These customizable, biodegradable, and biocompatible resins show the importance of molecular tuning and open up new possibilities for the creation of biocompatible constructs for tissue engineering.

Closed-loop vasculature network design for bioprinting large, solid tissue scaffolds

Vascularization is an indispensable requirement for fabricating large solid tissues and organs. The natural vasculature derived from medical imaging modalities for large tissues and organs are highly complex and convoluted. However, the present bioprinting capabilities limit the fabrication of such complex natural vascular networks. Simplified bioprinted vascular networks, on the other hand, lack the capability to sustain large solid tissues. This work proposes a generalized and adaptable numerical model to design the vasculature by utilizing the tissue/organ anatomy. Starting with processing the patient's medical images, organ structure, tissue-specific cues, and key vasculature tethers are determined. An open-source abdomen magnetic resonance image dataset was used in this work. The extracted properties and cues are then used in a mathematical model for guiding the vascular network formation comprising arterial and venous networks. Next, the generated three-dimensional networks are used to simulate the nutrient transport and consumption within the organ over time and the regions deprived of the nutrients are identified. These regions provide cues to evolve and optimize the vasculature in an iterative manner to ensure the availability of the nutrient transport throughout the bioprinted scaffolds. The mass transport of six components of cell culture media-glucose, glycine, glutamine, riboflavin, human serum albumin, and oxygen was studied within the organ with designed vasculature. As the vascular structure underwent iterations, the organ regions deprived of these key components decreased significantly highlighting the increase in structural complexity and efficacy of the designed vasculature. The numerical method presented in this work offers a valuable tool for designing vascular scaffolds to guide the cell growth and maturation of the bioprinted tissues for faster regeneration post bioprinting.

Properties and Printability of the Synthesized Hydrogel Based on GelMA

Gelatin methacryloyl (GelMA) has recently attracted increasing attention. Unlike other hydrogels, it allows for the adjustment of the mechanical properties using such factors as degree of functionalization, concentration, and photocrosslinking parameters. In this study, GelMA with a high degree of substitution (82.75 ± 7.09%) was synthesized, and its suitability for extrusion printing, cytocompatibility, and biocompatibility was studied. Satisfactory printing quality was demonstrated with the 15% concentration hydrogel. The high degree of functionalization led to a decrease in the ability of human adipose-derived stem cells (ADSCs) to adhere to the GelMA surface. During the first 3 days after sowing, proliferation was observed. Degradation in animals after subcutaneous implantation was slowed down.

Functional engineering strategies of 3D printed implants for hard tissue replacement

Three-dimensional printing technology with the rapid development of printing materials are widely recognized as a promising way to fabricate bioartificial bone tissues. In consideration of the disadvantages of bone substitutes, including poor mechanical properties, lack of vascularization and insufficient osteointegration, functional modification strategies can provide multiple functions and desired characteristics of printing materials, enhance their physicochemical and biological properties in bone tissue engineering. Thus, this review focuses on the advances of functional engineering strategies for 3D printed biomaterials in hard tissue replacement. It is structured as introducing 3D printing technologies, properties of printing materials (metals, ceramics and polymers) and typical functional engineering strategies utilized in the application of bone, cartilage and joint regeneration.

Engineering Spatiotemporal Control in Vascularized Tissues

A major challenge in engineering scalable three-dimensional tissues is the generation of a functional and developed microvascular network for adequate perfusion of oxygen and growth factors. Current biological approaches to creating vascularized tissues include the use of vascular cells, soluble factors, and instructive biomaterials. Angiogenesis and the subsequent generation of a functional vascular bed within engineered tissues has gained attention and is actively being studied through combinations of physical and chemical signals, specifically through the presentation of topographical growth factor signals. The spatiotemporal control of angiogenic signals can generate vascular networks in large and dense engineered tissues. This review highlights the developments and studies in the spatiotemporal control of these biological approaches through the coordinated orchestration of angiogenic factors, differentiation of vascular cells, and microfabrication of complex vascular networks. Fabrication strategies to achieve spatiotemporal control of vascularization involves the incorporation or encapsulation of growth factors, topographical engineering approaches, and 3D bioprinting techniques. In this article, we highlight the vascularization of engineered tissues, with a focus on vascularized cardiac patches that are clinically scalable for myocardial repair. Finally, we discuss the present challenges for successful clinical translation of engineered tissues and biomaterials.

Systematic review on the application of 3D-bioprinting technology in orthoregeneration: current achievements and open challenges

BACKGROUND

Joint degeneration and large or complex bone defects are a significant source of morbidity and diminished quality of life worldwide. There is an unmet need for a functional implant with near-native biomechanical properties. The potential for their generation using 3D bioprinting (3DBP)-based tissue engineering methods was assessed. We systematically reviewed the current state of 3DBP in orthoregeneration.

METHODS

This review was performed using PubMed and Web of Science. Primary research articles reporting 3DBP of cartilage, bone, vasculature, and their osteochondral and vascular bone composites were considered. Full text English articles were analyzed.

RESULTS

Over 1300 studies were retrieved, after removing duplicates, 1046 studies remained. After inclusion and exclusion criteria were applied, 114 articles were analyzed fully. Bioink material types and combinations were tallied. Cell types and testing methods were also analyzed. Nearly all papers determined the effect of 3DBP on cell survival. Bioink material physical characterization using gelation and rheology, and construct biomechanics were performed. In vitro testing methods assessed biochemistry, markers of extracellular matrix production and/or cell differentiation into respective lineages. In vivo proof-of-concept studies included full-thickness bone and joint defects as well as subcutaneous implantation in rodents followed by histological and µCT analyses to demonstrate implant growth and integration into surrounding native tissues.

CONCLUSIONS

Despite its relative infancy, 3DBP is making an impact in joint and bone engineering. Several groups have demonstrated preclinical efficacy of mechanically robust constructs which integrate into articular joint defects in small animals. However, notable obstacles remain. Notably, researchers encountered pitfalls in scaling up constructs and establishing implant function and viability in long term animal models. Further, to translate from the laboratory to the clinic, standardized quality control metrics such as construct stiffness and graft integration metrics should be established with investigator consensus. While there is much work to be done, 3DBP implants have great potential to treat degenerative joint diseases and provide benefit to patients globally.

Advances in three-dimensional bioprinted stem cell-based tissue engineering for cardiovascular regeneration

Three-dimensional (3D) bioprinting of cellular or biological components are an emerging field to develop tissue structures that mimic the spatial, mechanochemical and temporal characteristics of cardiovascular tissues. 3D multi-cellular and multi-domain organotypic biological constructs can better recapitulate in vivo physiology and can be utilized in a variety of applications. Such applications include in vitro cellular studies, high-throughput drug screening, disease modeling, biocompatibility analysis, drug testing and regenerative medicine. A major challenge of 3D bioprinting strategies is the inability of matrix molecules to reconstitute the complexity of the extracellular matrix and the intrinsic cellular morphologies and functions. An important factor is the inclusion of a vascular network to facilitate oxygen and nutrient perfusion in scalable and patterned 3D bioprinted tissues to promote cell viability and functionality. In this review, we summarize the new generation of 3D bioprinting techniques, the kinds of bioinks and printing materials employed for 3D bioprinting, along with the current state-of-the-art in engineered cardiovascular tissue models. We also highlight the translational applications of 3D bioprinting in engineering the myocardium cardiac valves, and vascular grafts. Finally, we discuss current challenges and perspectives of designing effective 3D bioprinted constructs with native vasculature, architecture and functionality for clinical translation and cardiovascular regeneration.

Development and Application of Three-Dimensional Bioprinting Scaffold in the Repair of Spinal Cord Injury

Spinal cord injury (SCI) is a disabling and destructive central nervous system injury that has not yet been successfully treated at this stage. Three-dimensional (3D) bioprinting has become a promising method to produce more biologically complex microstructures, which fabricate living neural constructs with anatomically accurate complex geometries and spatial distributions of neural stem cells, and this is critical in the treatment of SCI. With the development of 3D printing technology and the deepening of research, neural tissue engineering research using different printing methods, bio-inks, and cells to repair SCI has achieved certain results. Although satisfactory results have not yet been achieved, they have provided novel ideas for the clinical treatment of SCI. Considering the potential impact of 3D bioprinting technology on neural studies, this review focuses on 3D bioprinting methods widely used in SCI neural tissue engineering, and the latest technological applications of bioprinting of nerve tissues for the repair of SCI are discussed. In addition to introducing the recent progress, this work also describes the existing limitations and highlights emerging possibilities and future prospects in this field.

Hybprinting for musculoskeletal tissue engineering

This review presents bioprinting methods, biomaterials, and printing strategies that may be used for composite tissue constructs for musculoskeletal applications. The printing methods discussed include those that are suitable for acellular and cellular components, and the biomaterials include soft and rigid components that are suitable for soft and/or hard tissues. We also present strategies that focus on the integration of cell-laden soft and acellular rigid components under a single printing platform. Given the structural and functional complexity of native musculoskeletal tissue, we envision that hybrid bioprinting, referred to as hybprinting, could provide unprecedented potential by combining different materials and bioprinting techniques to engineer and assemble modular tissues.

Next Evolution in Organ-Scale Biofabrication: Bioresin Design for Rapid High-Resolution Vat Polymerization

The field of bioprinting has made significant advancements in recent years and allowed for the precise deposition of biomaterials and cells. However, within this field lies a major challenge, which is developing high resolution constructs, with complex architectures. In an effort to overcome these challenges a biofabrication technique known as vat polymerization is being increasingly investigated due to its high fabrication accuracy and control of resolution (µm scale). Despite the progress made in developing hydrogel precursors for bioprinting techniques, such as extrusion-based bioprinting, there is a major lack in developing hydrogel precursor bioresins for vat polymerization. This is due to the specific unique properties and characteristics required for vat polymerization, from lithography to the latest volumetric printing. This is of major concern as the shortage of bioresins available has a significant impact on progressing this technology and exploring its full potential, including speed, resolution, and scale. Therefore, this review discusses the key requirements that need to be addressed in successfully developing a bioresin. The influence of monomer architecture and bioresin composition on printability is described, along with key fundamental parameters that can be altered to increase printing accuracy. Finally, recent advancements in bioresins are discussed together with future directions.

(Bio)manufactured Solutions for Treatment of Bone Defects with Emphasis on US-FDA Regulatory Science Perspective

Bone defects, with second highest demand for surgeries around the globe, may lead to serious health issues and negatively influence patient lives. The advances in biomedical engineering and sciences have led to the development of several creative solutions for bone defect treatment. This review provides a brief summary of bone graft materials, an organized overview of top-down and bottom-up (bio)manufacturing approaches, plus a critical comparison between advantages and limitations of each method. We specifically discuss additive manufacturing techniques and their operation mechanisms in detail. Next, we review the hybrid methods and promising future directions for bone grafting, while giving a comprehensive US-FDA regulatory science perspective, biocompatibility concepts and assessments, and clinical considerations to translate a technology from a research laboratory to the market. The topics covered in this review could potentially fuel future research efforts in bone tissue engineering, and perhaps could also provide novel insights for other tissue engineering applications.

Emerging Technologies in Multi-Material Bioprinting

Bioprinting, within the emerging field of biofabrication, aims at the fabrication of functional biomimetic constructs. Different 3D bioprinting techniques have been adapted to bioprint cell-laden bioinks. However, single-material bioprinting techniques oftentimes fail to reproduce the complex compositions and diversity of native tissues. Multi-material bioprinting as an emerging approach enables the fabrication of heterogeneous multi-cellular constructs that replicate their host microenvironments better than single-material approaches. Here, bioprinting modalities are reviewed, their being adapted to multi-material bioprinting is discussed, and their advantages and challenges, encompassing both custom-designed and commercially available technologies are analyzed. A perspective of how multi-material bioprinting opens up new opportunities for tissue engineering, tissue model engineering, therapeutics development, and personalized medicine is offered.

Bioprinting Au Natural: The Biologics of Bioinks

The development of appropriate bioinks is a complex task, dependent on the mechanical and biochemical requirements of the final construct and the type of printer used for fabrication. The two most common tissue printers are micro-extrusion and digital light projection printers. Here we briefly discuss the required characteristics of a bioink for each of these printing processes. However, physical printing is only a short window in the lifespan of a printed construct-the system must support and facilitate cellular development after it is printed. To that end, we provide a broad overview of some of the biological molecules currently used as bioinks. Each molecule has advantages for specific tissues/cells, and potential disadvantages are discussed, along with examples of their current use in the field. Notably, it is stressed that active researchers are trending towards the use of composite bioinks. Utilizing the strengths from multiple materials is highlighted as a key component of bioink development.

Recent Advancements in 3D Printing and Bioprinting Methods for Cardiovascular Tissue Engineering

Recent decades have seen a plethora of regenerating new tissues in order to treat a multitude of cardiovascular diseases. Autografts, xenografts and bioengineered extracellular matrices have been employed in this endeavor. However, current limitations of xenografts and exogenous scaffolds to acquire sustainable cell viability, anti-inflammatory and non-cytotoxic effects with anti-thrombogenic properties underline the requirement for alternative bioengineered scaffolds. Herein, we sought to encompass the methods of biofabricated scaffolds via 3D printing and bioprinting, the biomaterials and bioinks recruited to create biomimicked tissues of cardiac valves and vascular networks. Experimental and computational designing approaches have also been included. Moreover, the in vivo applications of the latest studies on the treatment of cardiovascular diseases have been compiled and rigorously discussed.

Converging functionality: Strategies for 3D hybrid-construct biofabrication and the role of composite biomaterials for skeletal regeneration

The evolution of additive manufacturing (AM) technologies, biomaterial development and our increasing understanding of cell biology has created enormous potential for the development of personalized regenerative therapies. In the context of skeletal tissue engineering, physical and biological demands play key roles towards successful construct implantation and the achievement of bone, cartilage and blood vessel tissue formation. Nevertheless, meeting such physical and biological demands to mimic the complexity of human tissues and their functionality is still a significant ongoing challenge. Recent studies have demonstrated that combination of AM technologies and advanced biomaterials has great potential towards skeletal tissue engineering. This review aims to analyze how the most prominent technologies and discoveries in the field converge towards the development of advanced constructs for skeletal regeneration. Particular attention is placed on hybrid biofabrication strategies, combining bioinks for cell delivery with biomaterial inks providing physical support. Hybrid biofabrication has been the focus of recent emerging strategies, however there has been limited review and analysis of these techniques and the challenges involved. Furthermore, we have identified that there are multiple hybrid fabrication strategies, here we present a category system where each strategy is reviewed highlighting their distinct advantages, challenges and potential applications. In addition, bioinks and biomaterial inks are the main components of the hybrid biofabrication strategies, where it is recognized that such platforms still lack optimal physical and biological functionality. Thus, this review also explores the development of composite materials specifically targeting the enhancement of physical and biological functionality towards improved skeletal tissue engineering. STATEMENT OF SIGNIFICANCE: Biofabrication strategies capable of recreating the complexity of native tissues could open new clinical possibilities towards patient-specific regenerative therapies and disease models. Several reviews target the existing additive manufacturing (AM) technologies that may be utilised for biomedical purposes. However, this work presents a unique perspective, describing how such AM technologies have been recently translated towards hybrid fabrication strategies, targeting the fabrication of constructs with converging physical and biological properties. Furthermore, we address composite bioinks and biomaterial inks that have been engineered to overcome traditional limitations, and might be applied to the hybrid fabrication strategies outlined. This work offers ample perspectives and insights into the current and future challenges for the fabrication of skeletal tissues aiming towards clinical and biomedical applications.

Hierarchical biomaterials via photopatterning-enhanced direct ink writing

Recent advances in additive manufacturing (AM) technologies provide tools to fabricate biological structures with complex three-dimensional (3D) organization. Deposition-based approaches have been exploited to manufacture multimaterial constructs. Stimulus-triggered approaches have been used to fabricate scaffolds with high resolution. Both features are useful to produce biomaterials that mimic the hierarchical organization of human tissues. Recently, multitechnology biofabrication approaches have been introduced that integrate benefits from different AM techniques to enable more complex materials design. However, few methods allow for tunable properties at both micro- and macro-scale in materials that are conducive for cell growth. To improve the organization of biofabricated constructs, we integrated direct ink writing (DIW) with digital light processing (DLP) to form multimaterial constructs with improved spatial control over final scaffold mechanics. Polymer-nanoparticle hydrogels were combined with methacryloyl gelatin (GelMA) to engineer dual inks that were compatible with both DIW and DLP. The shear-thinning and self-healing properties of the dual inks enabled extrusion-based 3D printing. The inclusion of GelMA provided a handle for spatiotemporal control of cross-linking with DLP. Exploiting this technique, complex multimaterial constructs were printed with defined mechanical reinforcement. In addition, the multitechnology approach was used to print live cells for biofabrication applications. Overall, the combination of DIW and DLP is a simple and efficient strategy to fabricate hierarchical biomaterials with user-defined control over material properties at both micro- and macro-scale.

Multimaterial bioprinting and combination of processing techniques towards the fabrication of biomimetic tissues and organs

Tissue reconstruction requires the utilization of multiple biomaterials and cell types to replicate the delicate and complex structure of native tissues. Various three-dimensional (3D) bioprinting techniques have been developed to fabricate customized tissue structures; however, there are still significant challenges, such as vascularization, mechanical stability of printed constructs, and fabrication of gradient structures to be addressed for the creation of biomimetic and complex tissue constructs. One approach to address these challenges is to develop multimaterial 3D bioprinting techniques that can integrate various types of biomaterials and bioprinting capabilities towards the fabrication of more complex structures. Notable examples include multi-nozzle, coaxial, and microfluidics-assisted multimaterial 3D bioprinting techniques. More advanced multimaterial 3D printing techniques are emerging, and new areas in this niche technology are rapidly evolving. In this review, we briefly introduce the basics of individual 3D bioprinting techniques and then discuss the multimaterial 3D printing techniques that can be developed based on combination of these techniques for the engineering of complex and biomimetic tissue constructs. We also discuss the perspectives and future directions to develop state-of-the-art multimaterial 3D bioprinting techniques for engineering tissues and organs.

Bio-assembling and Bioprinting for Engineering Microvessels from the Bottom Up

Blood vessels are essential in transporting nutrients, oxygen, metabolic wastes, and maintaining the homeostasis of the whole human body. Mass of engineered microvessels is required to deliver nutrients to the cells included in the constructed large three-dimensional (3D) functional tissues by diffusion. It is a formidable challenge to regenerate microvessels and build a microvascular network, mimicking the cellular viabilities and activities in the engineered organs with traditional or existing manufacturing techniques. Modular tissue engineering adopting the "bottom-up" approach builds one-dimensional (1D) or two-dimensional (2D) modular tissues in micro scale first and then uses these modules as building blocks to generate large tissues and organs with complex but indispensable microstructural features. Building the microvascular network utilizing this approach could be appropriate and adequate. In this review, we introduced existing methods using the "bottom-up" concept developed to fabricate microvessels including bio-assembling powered by different micromanipulation techniques and bioprinting utilizing varied solidification mechanisms. We compared and discussed the features of the artificial microvessels engineered by these two strategies from multiple aspects. Regarding the future development of engineering the microvessels from the bottom up, potential directions were also concluded.

3D Tissue and Organ Printing-Hope and Reality

Three-dimensional (3D) bioprinting is an emerging, groundbreaking strategy in tissue engineering, allowing the fabrication of living constructs with an unprecedented degree of complexity and accuracy. While this technique greatly facilitates the structuring of native tissue-like architectures, many challenges still remain to be faced. In this review, the fruits of recent research that demonstrate how advanced bioprinting technologies, together with inspiring creativity, can be used to address these challenges are presented and discussed. Next, the future of the field is discussed, in terms of expected developments, as well as possible directions toward the realization of the vision of fully functional, engineered tissues, and organs. Last, a few hypothetical scenarios for the role 3D bioprinting may play in future tissue engineering are depicted, with an emphasis on its impact on tomorrow's regenerative medicine.

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.

Effect of porosity of a functionally-graded scaffold for the treatment of corticosteroid-associated osteonecrosis of the femoral head in rabbits

Background/Objective: Core decompression (CD) with scaffold and cell-based therapies is a promising strategy for providing both mechanical support and regeneration of the osteonecrotic area for early stage osteonecrosis of the femoral head (ONFH). We designed a new 3D printed porous functionally-graded scaffold (FGS) with a central channel to facilitate delivery of transplanted cells in a hydrogel to the osteonecrotic area. However, the optimal porous structural design for the FGS for the engineering of bone in ONFH has not been elucidated. The aim of this study was to fabricate and evaluate two different porous structures (30% or 60% porosity) of the FGSs in corticosteroid-associated ONFH in rabbits.

Methods

Two different FGSs with 30% or 60% porosity containing a 1-mm central channel were 3D printed using polycaprolactone and β-tricalcium phosphate. The FGS was 3-mm diameter and 32-mm length and was composed of three segments: 1-mm in length for the non-porous proximal segment, 22-mm in length for the porous (30% versus 60%) middle segment, and 9-mm in length for the 15% porous distal segment. Eighteen male New Zealand White rabbits were given a single dose of 20 ​mg/kg methylprednisolone acetate intramuscularly. Four weeks later, rabbits were divided into three groups: the CD group, the 30% porosity FGS group, and the 60% porosity FGS group. In the CD group, a 3-mm diameter drill hole was created into the left femoral head. In the FGS groups, a 30% or 60% porosity implant was inserted into the bone tunnel. Eight weeks postoperatively, femurs were harvested and microCT, mechanical, and histological analyses were performed.

Results

The actual porosity and pore size of the middle segments were 26.4% ​± ​2.3% and 699 ​± ​56 ​μm in the 30% porosity FGS, and 56.0% ​± ​4.5% and 999 ​± ​71 ​μm in the 60% porosity FGS, respectively using microCT analysis. Bone ingrowth ratio in the 30% porosity FGS group was 73.9% ​± ​15.8%, which was significantly higher than 39.5% ​± ​13.0% in the CD group on microCT (p ​< ​0.05). Bone ingrowth ratio in the 60% porosity FGS group (61.3% ​± ​30.1%) showed no significant differences compared to the other two groups. The stiffness at the bone tunnel site in the 30% porosity FGS group was 582.4 ​± ​192.3 ​N/mm³, which was significantly higher than 338.7 ​± ​164.6 ​N/mm³ in the 60% porosity FGS group during push-out testing (p ​< ​0.05). Hematoxylin and eosin staining exhibited thick and mature trabecular bone around the porous FGS in the 30% porosity FGS group, whereas thinner, more immature trabecular bone was seen around the porous FGS in the 60% porosity FGS group.

Conclusion

These findings indicate that the 30% porosity FGS may enhance bone regeneration and have superior biomechanical properties in the bone tunnel after CD in ONFH, compared to the 60% porosity FGS.

Translation potential statement

The translational potential of this article: This FGS implant holds promise for improving outcomes of CD for early stage ONFH.

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).

Development of a Dual Hydrogel Model System for Vascularization

Numerous hydrogel-based culture systems are used to create in vitro model for prevascularization. Hydrogels used to induce a microenvironment conducive to microvessel formation are typically soft and fast degradable, but often suffer from maintaining a lasting perfusable channel in vitro. Here, a dual hydrogel system that consists of photo-crosslinkable gelatin methacrylate (GelMA) and polyethylene glycol dimethacrylate (PEGDMA) is reported. GelMA hydrogels present soft and rapidly degradable properties and show microporous structures while PEGDMA is relatively stiff, almost nondegradable in vitro, and less porous. The dual hydrogel system is sequentially photo-crosslinked to construct an endothelial cell (EC)-lined perfusable PEGDMA channel and surrounding GelMA for endothelial vascular networks. Such dual hydrogel system exhibits seamless integration of the stiff PEGDMA channel and the surrounding soft GelMA, and facilitates rapid EC sprouting and extensive microvessel formation from a stable endothelium on the PEGDMA channel into the GelMA. Furthermore, diffusivity of biomolecules in the perfusable dual hydrogel system is affected by both the structural and physicochemical properties of the hydrogel system and the microvascular networks formed in the system. The establishment of the dual hydrogel system for vascularization holds great promise as an in vitro angiogenesis model and prevascularization strategy of large tissue constructs.

Recent Advances in Formulating and Processing Biomaterial Inks for Vat Polymerization-Based 3D Printing

3D printing and bioprinting have become a key component in precision medicine. They have been used toward the fabrication of medical devices with patient-specific shapes, production of engineered tissues for in vivo regeneration, and preparation of in vitro tissue models used for screening therapeutics. In particular, vat polymerization-based 3D (bio)printing as a unique strategy enables more sophisticated architectures to be rapidly built. This progress report aims to emphasize the recent advances made in vat polymerization-based 3D printing and bioprinting, including new biomaterial ink formulations and novel vat polymerization system designs. While some of these approaches have not been utilized toward the combination with biomaterial inks, it is anticipated their rapid translation into biomedical applications.

3D bioprinting of oligo(poly[ethylene glycol] fumarate) for bone and nerve tissue engineering

3D bioprinting is a promising new tissue restoration technique that enables the precise deposition of cells and growth factors in order to more closely mimic the structure and function of native organs. In this study, we report the development of a new bioink using oligo(poly[ethylene glycol] fumarate) (OPF), a photo-crosslinkable, and biodegradable polymer, for 3D bioprinting. In addition to OPF, a small portion of gelatin was also incorporated into the bioink to make it bio-printable. After immersion in the cell medium, gelatin was eluted away to create a bioprinted scaffold of pure OPF. Excellent cell viability, spreading, and long-term proliferation of encapsulated cells was observed using both bone and nerve cells as examples. These results demonstrate that OPF bioink has great potential in future 3D bioprinting applications that aim to replicate complex, layered tissues, and/or organs.

Acoustic Patterning of Growth Factor for Three-Dimensional Tissue Engineering

Temporal and spatial presentations of biological cues are critical for tissue engineering. There is a great need in improving the incorporation of bioagent(s) (specifically growth factor(s) [GF(s)]) onto three-dimensional scaffolds. In this study, we developed a process to combine additive manufacturing (AM) technology with acoustic droplet ejection (ADE) technology to control GF distribution. More specifically, we implemented ADE to control the distribution of recombinant human bone morphogenetic protein-2 (rhBMP-2) onto polycaprolactone (PCL)-based tissue engineering constructs (TECs). Three substrates were used in this study: (1) succinimide-terminated PCL (PCL-N-hydroxysuccinimide [NHS]) as model material, (2) alkali-treated PCL (PCL-NaOH) as first control material, and (3) fibrin-coated PCL (PCL-Fibrin) as second control material. It was shown that our process enables a pattern of BMP-2 spots of ∼250 μm in diameter with ∼700 μm center-to-center spacing. An initial concentration of BMP-2 higher than 300 μg/L was required to retain a detectable amount of GF on the substrate after a wash with phosphate-buffered solution. However, to obtain detectable osteogenic differentiation of C2C12 cells, the initial concentration of BMP-2 higher than 750 μg/L was needed. The cells on PCL-NHS samples showed spatial alkaline phosphatase staining correlating with local patterns of BMP-2, although the intensity was lower than the controls (PCL-NaOH and PCL-Fibrin). Our results have demonstrated that the developed AM-ADE process holds great promise in creating TECs with highly controlled GF patterning. Impact statement The combined process of additive manufacturing with acoustic droplet ejection to control growth factor (GF) distribution across three-dimensional (3D) porous scaffolds that is presented in this study enables creating 3D tissue engineering constructs with highly controlled GF patterning. Such constructs enable temporal and spatial presentations of biological cues for enhancing cell migration and differentiation and eventually the formation of targeted tissues in vitro and in vivo.

3D Bioprinting for Vascularized Tissue-Engineered Bone Fabrication

Vascularization in bone tissues is essential for the distribution of nutrients and oxygen, as well as the removal of waste products. Fabrication of tissue-engineered bone constructs with functional vascular networks has great potential for biomimicking nature bone tissue in vitro and enhancing bone regeneration in vivo. Over the past decades, many approaches have been applied to fabricate biomimetic vascularized tissue-engineered bone constructs. However, traditional tissue-engineered methods based on seeding cells into scaffolds are unable to control the spatial architecture and the encapsulated cell distribution precisely, which posed a significant challenge in constructing complex vascularized bone tissues with precise biomimetic properties. In recent years, as a pioneering technology, three-dimensional (3D) bioprinting technology has been applied to fabricate multiscale, biomimetic, multi-cellular tissues with a highly complex tissue microenvironment through layer-by-layer printing. This review discussed the application of 3D bioprinting technology in the vascularized tissue-engineered bone fabrication, where the current status and unique challenges were critically reviewed. Furthermore, the mechanisms of vascular formation, the process of 3D bioprinting, and the current development of bioink properties were also discussed.

Vat polymerization-based bioprinting-process, materials, applications and regulatory challenges

Over the years, the field of bioprinting has attracted attention for its highly automated fabrication system that enables the precise patterning of living cells and biomaterials at pre-defined positions for enhanced cell-matrix and cell-cell interactions. Notably, vat polymerization (VP)-based bioprinting is an emerging bioprinting technique for various tissue engineering applications due to its high fabrication accuracy. Particularly, different photo-initiators (PIs) are utilized during the bioprinting process to facilitate the crosslinking mechanism for fabrication of high-resolution complex tissue constructs. The advancements in VP-based printing have led to a paradigm shift in fabrication of tissue constructs from cell-seeding of tissue scaffolds (non-biocompatible fabrication process) to direct bioprinting of cell-laden tissue constructs (biocompatible fabrication process). This paper, presenting a first-time comprehensive review of the VP-based bioprinting process, provides an in-depth analysis and comparison of the various biocompatible PIs and highlights the important considerations and bioprinting requirements. This review paper reports a detailed analysis of its printing process and the influence of light-based curing modality and PIs on living cells. Lastly, this review also highlights the significance of VP-based bioprinting, the regulatory challenges and presents future directions to transform the VP-based printing technology into imperative tools in the field of tissue engineering and regenerative medicine. The readers will be informed on the current limitations and achievements of the VP-based bioprinting techniques. Notably, the readers will realize the importance and value of highly-automated platforms for tissue engineering applications and be able to develop objective viewpoints towards this field.

Stereolithography 3D Bioprinting

Stereolithography (SLA) 3D bioprinting has emerged as a prominent bioprinting method addressing the requirements of complex tissue fabrication. This chapter addresses the advancement in SLA 3D bioprinting in concurrent with the development of novel photocrosslinkable biomaterials with enhanced physical and chemical properties. We discuss the cytocompatible photoinitiators operating in the wide spectrum of the ultraviolet (UV) and the visible light and high-resolution dynamic mask projection systems with a suitable illumination source. The potential of SLA 3D bioprinting has been explored in various themes, like bone and neural tissue engineering and in the development of controlled microenvironments to study cell behavior. The flexible design and versatility of SLA bioprinting makes it an attractive bioprinting process with myriad possibilities and clinical applications.

Engineered 3D Polymer and Hydrogel Microenvironments for Cell Culture Applications

The realization of biomimetic microenvironments for cell biology applications such as organ-on-chip, in vitro drug screening, and tissue engineering is one of the most fascinating research areas in the field of bioengineering. The continuous evolution of additive manufacturing techniques provides the tools to engineer these architectures at different scales. Moreover, it is now possible to tailor their biomechanical and topological properties while taking inspiration from the characteristics of the extracellular matrix, the three-dimensional scaffold in which cells proliferate, migrate, and differentiate. In such context, there is therefore a continuous quest for synthetic and nature-derived composite materials that must hold biocompatible, biodegradable, bioactive features and also be compatible with the envisioned fabrication strategy. The structure of the current review is intended to provide to both micro-engineers and cell biologists a comparative overview of the characteristics, advantages, and drawbacks of the major 3D printing techniques, the most promising biomaterials candidates, and the trade-offs that must be considered in order to replicate the properties of natural microenvironments.

An introduction to bone tissue engineering

Bone tissue has the capability to regenerate itself; however, defects of a critical size prevent the bone from regenerating and require additional support. To aid regeneration, bone scaffolds created out of autologous or allograft bone can be used, yet these produce problems such as fast degradation rates, reduced bioactivity, donor site morbidity or the risk of pathogen transmission. The development of bone tissue engineering has been used to create functional alternatives to regenerate bone. This can be achieved by producing bone tissue scaffolds that induce osteoconduction and integration, provide mechanical stability, and either integrate into the bone structure or degrade and are excreted by the body. A range of different biomaterials have been used to this end, each with their own advantages and disadvantages. This review will introduce the requirements of bone tissue engineering, beginning with the regeneration process of bone before exploring the requirements of bone tissue scaffolds. Aspects covered include the manufacturing process as well as the different materials used and the incorporation of bioactive molecules, growth factors and cells.

Review of alginate-based hydrogel bioprinting for application in tissue engineering

The dawn of 3D printing in medicine has given the field the hope of vitality in many patients fighting a multitude of diseases. Also entitled bioprinting, this appertains to its sequential printing of precursor ink, embodying cells and polymer/composite in a predetermined trajectory. The precursor ink, in addition to cells, is predominantly constituted of hydrogels due to its biodegradability and ability to mimic the body's anatomy and mechanical features, e.g. bones, etc. This review paper is devoted to explicating the bioprinting (3D/4D) of alginate hydrogels, which are extracts from brown algae, through extrusion additive manufacturing. Alginates are salt derivatives of alginic acid and constitute long chains of polysaccharides, which provides pliability and gelling adeptness to their structure. Alginate hydrogel (employed for extrusion) can be pristine or composite relying on the requisite properties (target application controlled or in vivo environment), e.g. alginate-natural (gelatin/agarose/collagen/hyaluronic acid/etc) and alginate-synthetic (polyethylene glycol (PEG)/pluronic F-127/etc). Extrusion additive manufacturing of alginate is preponderate among others with its uncomplicated processing, material efficiency (cut down on wastage), and outspread adaptability for viscosities (0.03-6 * 10⁴ Pa.s), but the procedure is limited by resolution (200 μm) in addition to accuracy. However, 3D-fabricated biostructures display rigidness (unvarying with conditions) i.e. lacks a smart response, which is reassured by accounting time feature as a noteworthy accessory to printing, interpreted as 4D bioprinting. This review propounds the specific processing itinerary for alginate (meanwhile traversing across its composites/blends with natural and synthetic consideration) in extrusion along with its pre-/during/post-processing parameters intrinsic to the process. Furthermore, propensity is also presented in its (alginate extrusion processing) application for tissue engineering, i.e. bones, cartilage (joints), brain (neural), ear, heart (cardiac), eyes (corneal), etc, due to a worldwide quandary over accessibility to natural organs for diverse types of diseases. Additionally, the review contemplates recently invented advance printing, i.e. 4D printing for biotic species, with its challenges and future opportunities.