Bioprinted osteon-like scaffolds enhance in vivo neovascularization

Advances in three-dimensional printing of hydrogel formulations for vascularized tissue and organ regeneration

Over the last decades, three-dimensional (3D) printing has emerged as one of the most promising alternative tissue and organ regeneration technologies. Recent advances in 3D printing technology, particularly in hydrogel-derived bioink formulations, offer promising solutions for fabricating intricate, biomimetic scaffolds that promote vascularization. In this review, we presented numerous studies that have been conducted to fabricate 3D-printed hydrogel vascularized constructs with significant advancements in printing integumentary systems, cardiovascular systems, vascularized bone tissues, skeletal muscles, livers, and kidneys. Furthermore, this work also discusses the engineering considerations, current challenges, proposed solutions, and future outlooks of 3D bioprinting.

Engineering Biomimetic Microvascular Capillary Networks in Hydrogel Fibrous Scaffolds via Microfluidics-Assisted Co-Axial Wet-Spinning

The microvascular bed plays a crucial role in establishing nutrient exchange and waste removal, as well as maintaining tissue metabolic activity in the human body. However, achieving microvascularization of engineered 3D tissue constructs is still an unsolved challenge. In this work, we developed biomimetic cell-laden hydrogel microfibers recapitulating oriented microvascular capillary-like networks by using a 3D bioprinting technique combined with microfluidics-assisted coaxial wet-spinning. Highly packed and aligned bundles embedding a coculture of human bone marrow-derived mesenchymal stem cells (MSCs) and human umbilical vein endothelial cells (HUVECs) were produced by simultaneously extruding two different bioinks. To this aim, core-shell fibers were wet-spun in a coagulation bath to collect the scaffolds later on a rotary drum. Initially, the versatility of the proposed system was assessed for the extrusion of multimaterial core-shell hydrogel fibers. Subsequently, the platform was validated for the in vitro biofabrication of samples promoting optimal cell alignment along the fiber axis. After 3 weeks of culture, such fiber configuration resulted in the development of an oriented capillary-like network within the fibrin-based core and in the endothelial-specific CD31 marker expression upon MSC/HUVEC maturation. Synergistically, the vertical arrangement of the coaxial nozzle coupled with the rotation of the fiber collector facilitated the rapid creation of tightly packed bundles characterized by a dense, oriented, and extensively branched capillary network. Notably, such findings suggest that the proposed biofabrication strategy can be used for the microvascularization of tissue-specific 3D constructs.

Leveraging printability and biocompatibility in materials for printing implantable vessel scaffolds

Vessel scaffolds are crucial for treating cardiovascular diseases (CVDs). It is currently feasible to fabricate vessel scaffolds from a variety of materials using traditional fabrication methods, but the risks of thrombus formation, chronic inflammation, and atherosclerosis associated with these scaffolds have led to significant limitations in the clinical usages. Bioprinting, as an emerging technology, has great potential in constructing implantable vessel scaffolds. During the fabrication of the constructs, the biomaterials used for bioprinting have offered significant contributions for the successful fabrications of the vessel scaffolds. Herein, we review recent advances in biomaterials for bioprinting implantable vessel scaffolds. First, we briefly introduce the requirements for implantable vessel scaffolds and its conventional manufacturing methods. Next, a brief overview of the classic methods for bioprinting vessel scaffolds is presented. Subsequently, we provide an in-depth analysis of the properties of the representative natural, synthetic, composite and hybrid biomaterials that can be used for bioprinting implantable vessel scaffolds. Ultimately, we underscore the necessity of leveraging biocompatibility and printability for biomaterials, and explore the unmet needs and potential applications of these biomaterials in the field of bioprinted implantable vessel scaffolds.

3D morphometry of endothelial cells angiogenesis in an extracellular matrix composite hydrogel

Human umbilical vein endothelial cells (HUVECs) play a fundamental role in angiogenesis. Herein, we introduce digital holographic microscopy (DHM) for the 3D quantitative morphological analysis of HUVECs in extracellular matrix (ECM)-based biomaterials as an angiogenesis model. The combination of volumetric information from DHM and the physicochemical and cytobiocompatibility data provided by fluorescence microscopy and cytology offers a comprehensive understanding of the angiogenesis-related parameters of HUVECs within the ECM. DHM enables label-free, non-contact, and non-invasive 3D monitoring of living samples in real time, in a quantitative manner. In this study, the human amniotic membrane (HAM) is decellularized, pulverized, and combined with sodium alginate hydrogel to provide an in vitro substrate for modeling HUVEC angiogenesis. Our results demonstrate that modifying alginate hydrogel with HAM enhances its biofunctionality due to the presence of ECM components. Moreover, the DHM results reveal an increase in its porous properties, which, in turn, aids in interpreting the tubulation results.

A 3-Dimensional Scaffolding System Recapitulates the Hierarchical Osteon Structure

The bone is composed of solid cortical bone and honeycomb-like trabecular bone. Although the cortical bone provides the substantial mechanical strength of the bone, few studies have focused on its regeneration. As the structural and functional units of the cortical bone, osteons play critical roles in bone turnover. Composed of osteocytes, lamellae, lacunocanalicular network, and Haversian canals, osteons exhibit a delicate and hierarchical architecture. Studies have attempted to reconstruct the osteonal structure with artificial approaches; however, hardly the four elements were recapitulated simultaneously. In this work, a series of bioengineering techniques, including electrospinning, micropatterning, and laser-directed microfabrication, were employed to develop a three-dimensional scaffolding system, which successfully recapitulated the osteon structure in vitro. The physiological morphology and bioactivity of osteocytes were emulated, the intercellular communications between osteocytes were identified, and the concentric lamellae and Haversian canals were simulated as well. This work constructed an in vivo-like platform for osteon study, providing convenience for exploring the interaction among the osteonal elements.

3D Stem Cell Spheroids with 2D Hetero-Nanostructures for In Vivo Osteogenic and Immunologic Modulated Bone Repair

3D stem cell spheroids have immense potential for various tissue engineering applications. However, current spheroid fabrication techniques encounter cell viability issues due to limited oxygen access for cells trapped within the core, as well as nonspecific differentiation issues due to the complicated environment following transplantation. In this study, functional 3D spheroids are developed using mesenchymal stem cells with 2D hetero-nanostructures (HNSs) composed of single-stranded DNA (ssDNA) binding carbon nanotubes (sdCNTs) and gelatin-bind black phosphorus nanosheets (gBPNSs). An osteogenic molecule, dexamethasone (DEX), is further loaded to fabricate an sdCNTgBP-DEX HNS. This approach aims to establish a multifunctional cell-inductive 3D spheroid with improved oxygen transportation through hollow nanotubes, stimulated stem cell growth by phosphate ions supplied from BP oxidation, in situ immunoregulation, and osteogenesis induction by DEX molecules after implantation. Initial transplantation of the 3D spheroids in rat calvarial bone defect shows in vivo macrophage shifts to an M2 phenotype, leading to a pro-healing microenvironment for regeneration. Prolonged implantation demonstrates outstanding in vivo neovascularization, osteointegration, and new bone regeneration. Therefore, these engineered 3D spheroids hold great promise for bone repair as they allow for stem cell delivery and provide immunoregulative and osteogenic signals within an all-in-one construct.

Printing of 3D biomimetic structures for the study of bone metastasis: A review

Bone metastasis primarily occurs when breast, prostate, or lung cancers disseminate tumoral cells into bone tissue, leading to a range of complications in skeletal tissues and, in severe cases, paralysis resulting from spinal cord compression. Unfortunately, our understanding of pathophysiological mechanisms is incomplete and the translation of bone metastasis research into the clinic has been slow, mainly due to the lack of credible ex vivo and in vivo models to study the disease progression. Development of reliable and rational models to study how tumor cells become circulating cells and then invade and sequentially colonize the bone are in great need. Advances in tissue engineering technologies offers reliable 3D tissue alternatives which answer relevant research questions towards the understanding of cancer evolution and key functional properties of metastasis progression as well as prognosis of therapeutic approach. Here we performed an overview of cellular mechanisms involved in bone metastasis including a short summary of normal bone physiology and metastasis initiation and progression. Also, we comprehensively summarized current advances and methodologies in fabrication of reliable bone tumor models based on state-of-the-art printing technologies which recapitulate structural and biological features of native tissue. STATEMENT OF SIGNIFICANCE: This review provides a comprehensive summary of the collective findings in relation to various printed bone metastasis models utilized for investigating specific bone metastasis diseases, related characteristic functions and chemotherapeutic drug screening. These tumoral models are comprehensively evaluated and compared, in terms of their ability to recapitulate physiological metastasis microenvironment. Various biomaterials (natural and synthetic polymers and ceramic based substrates) and printing strategies and design architecture of models used for printing of 3D bone metastasis models are discussed here. This review clearly out-lines current challenges and prospects for 3D printing technologies in bone metastasis research by focusing on the required perspective models for clinical application of these technologies in chemotherapeutic drug screening.

Integrating bioprinting, cell therapies and drug delivery towards in vivo regeneration of cartilage, bone and osteochondral tissue

The biological and biomechanical functions of cartilage, bone and osteochondral tissue are naturally orchestrated by a complex crosstalk between zonally dependent cells and extracellular matrix components. In fact, this crosstalk involves biomechanical signals and the release of biochemical cues that direct cell fate and regulate tissue morphogenesis and remodelling in vivo. Three-dimensional bioprinting introduced a paradigm shift in tissue engineering and regenerative medicine, since it allows to mimic native tissue anisotropy introducing compositional and architectural gradients. Moreover, the growing synergy between bioprinting and drug delivery may enable to replicate cell/extracellular matrix reciprocity and dynamics by the careful control of the spatial and temporal patterning of bioactive cues. Although significant advances have been made in this direction, unmet challenges and open research questions persist. These include, among others, the optimization of scaffold zonality and architectural features; the preservation of the bioactivity of loaded active molecules, as well as their spatio-temporal release; the in vitro scaffold maturation prior to implantation; the pros and cons of each animal model and the graft-defect mismatch; and the in vivo non-invasive monitoring of new tissue formation. This work critically reviews these aspects and reveals the state of the art of using three-dimensional bioprinting, and its synergy with drug delivery technologies, to pattern the distribution of cells and/or active molecules in cartilage, bone and osteochondral engineered tissues. Most notably, this work focuses on approaches, technologies and biomaterials that are currently under in vivo investigations, as these give important insights on scaffold performance at the implantation site and its interaction/integration with surrounding tissues.

Polyhydroxybutyrate-based osteoinductive mineralized electrospun structures that mimic components and tissue interfaces of the osteon for bone tissue engineering

Scaffolds for bone tissue engineering should enable regeneration of bone tissues with its native hierarchically organized extracellular matrix (ECM) and multiple tissue interfaces. To achieve this, inspired by the structure and properties of bone osteon, we fabricated polyhydroxybutyrate (PHB)-based mineralized electrospun fibrous scaffolds. After studying multiple PHB-based fibers, we chose 7%PHB/1%Gelatin fibers (PG) to fabricate mineralized fibers that mimic mineralized collagen fibers in bone. The mineralized PG (mPG) surface had a rough, hydrophilic layer of low crystalline calcium phosphate which was biocompatible to bone marrow stromal cells (BMSCs), induced their proliferation and was osteoinductive. Subsequently, by modulating the electrospinning process, we fabricated mPG-based novel higher order fibrous scaffolds that mimic the macroscale geometries of osteons of bone ECM. Inspired by the aligned collagen fibers in bone lamellae, we fabricated mPG scaffolds with aligned fibers that could direct anisotropic elongation of mouse BMSC (mBMSCs). Further, we fabricated electrospun mPG-based osteoinductive tubular constructs which can mimic cylindrical bone components like osteons or lamellae or be used as long bone analogues based on their dimensions. Finally, to regenerate tissue interfaces in bone, we introduced a novel bi-layered scaffold-based approach. An electrospun bi-layered tubular construct that had PG in the outer layer and 7%PHB/0.5%Polypyrrole fibers (PPy) in the inner layer was fabricated. The bi-layered tubular construct underwent preferential surface mineralization only on its outer layer. This outer mineralized layer supported osteogenesis while the inner PPy layer could support neural cell growth. Thus, the bi-layered tubular construct may be used to regenerate haversian canal in the osteons which hosts nerve fibers. Overall, the study introduced novel techniques to fabricate biomimetic structures that can regenerate components of bone osteon and its multiple tissue interfaces. The study lays foundation for the fabrication of a modular scaffold that can regenerate bone with its hierarchical structure and complex tissue interfaces.

Synergy between 3D-extruded electroconductive scaffolds and electrical stimulation to improve bone tissue engineering strategies

In this work, we propose a simple, reliable, and versatile strategy to create 3D electroconductive scaffolds suitable for bone tissue engineering (TE) applications with electrical stimulation (ES). The proposed scaffolds are made of 3D-extruded poly(ε-caprolactone) (PCL), subjected to alkaline treatment, and of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), anchored to PCL with one of two different crosslinkers: (3-glycidyloxypropyl)trimethoxysilane (GOPS) and divinyl sulfone (DVS). Both cross-linkers allowed the formation of a homogenous and continuous coating of PEDOT:PSS to PCL. We show that these PEDOT:PSS coatings are electroconductive (11.3-20.1 S cm-1), stable (up to 21 days in saline solution), and allow the immobilization of gelatin (Gel) to further improve bioactivity. In vitro mineralization of the corresponding 3D conductive scaffolds was greatly enhanced (GOPS(NaOH)-Gel - 3.1 fold, DVS(NaOH)-Gel - 2.0 fold) and cell colonization and proliferation were the highest for the DVS(NaOH)-Gel scaffold. In silico modelling of ES application in DVS(NaOH)-Gel scaffolds indicates that the electrical field distribution is homogeneous, which reduces the probability of formation of faradaic products. Osteogenic differentiation of human bone marrow derived mesenchymal stem/stromal cells (hBM-MSCs) was performed under ES. Importantly, our results clearly demonstrated a synergistic effect of scaffold electroconductivity and ES on the enhancement of MSC osteogenic differentiation, particularly on cell-secreted calcium deposition and the upregulation of osteogenic gene markers such as COL I, OC and CACNA1C. These scaffolds hold promise for future clinical applications, including manufacturing of personalized bone TE grafts for transplantation with enhanced maturation/functionality or bioelectronic devices.

Long-term day-by-day tracking of microvascular networks sprouting in fibrin gels: From detailed morphological analyses to general growth rules

Understanding and controlling of the evolution of sprouting vascular networks remains one of the basic challenges in tissue engineering. Previous studies on the vascularization dynamics have typically focused only on the phase of intense growth and often lacked spatial control over the initial cell arrangement. Here, we perform long-term day-by-day analysis of tens of isolated microvasculatures sprouting from endothelial cell-coated spherical beads embedded in an external fibrin gel. We systematically study the topological evolution of the sprouting networks over their whole lifespan, i.e., for at least 14 days. We develop a custom image analysis toolkit and quantify (i) the overall length and area of the sprouts, (ii) the distributions of segment lengths and branching angles, and (iii) the average number of branch generations-a measure of network complexity. We show that higher concentrations of vascular endothelial growth factor (VEGF) lead to earlier sprouting and more branched networks, yet without significantly affecting the speed of growth of individual sprouts. We find that the mean branching angle is weakly dependent on VEGF and typically in the range of 60°-75°, suggesting that, by comparison with the available diffusion-limited growth models, the bifurcating tips tend to follow local VEGF gradients. At high VEGF concentrations, we observe exponential distributions of segment lengths, which signify purely stochastic branching. Our results-due to their high statistical relevance-may serve as a benchmark for predictive models, while our new image analysis toolkit, offering unique features and high speed of operation, could be exploited in future angiogenic drug tests.

Research progress of vascularization strategies of tissue-engineered bone

The bone defect caused by fracture, bone tumor, infection, and other causes is not only a problematic point in clinical treatment but also one of the hot issues in current research. The development of bone tissue engineering provides a new way to repair bone defects. Many animal experimental and rising clinical application studies have shown their excellent application prospects. The construction of rapid vascularization of tissue-engineered bone is the main bottleneck and critical factor in repairing bone defects. The rapid establishment of vascular networks early after biomaterial implantation can provide sufficient nutrients and transport metabolites. If the slow formation of the local vascular network results in a lack of blood supply, the osteogenesis process will be delayed or even unable to form new bone. The researchers modified the scaffold material by changing the physical and chemical properties of the scaffold material, loading the growth factor sustained release system, and combining it with trace elements so that it can promote early angiogenesis in the process of induced bone regeneration, which is beneficial to the whole process of bone regeneration. This article reviews the local vascular microenvironment in the process of bone defect repair and the current methods of improving scaffold materials and promoting vascularization.

Directly coaxial bioprinting of 3D vascularized tissue using novel bioink based on decellularized human amniotic membrane

Despite several progressions in the biofabrication of large-scale engineered tissues, direct biopri nting of perfusable three-dimensional (3D) vasculature remained unaddressed. Developing a feasible method to generate cell-laden thick tissue with an effective vasculature network to deliver oxygen and nutrient is crucial for preventing the formation of necrotic spots and tissue death. In this study, we developed a novel technique to directly bioprint 3D cell-laden prevascularized construct. We developed a novel bioink by mixing decellularized human amniotic membrane (dHAM) and alginate (Alg) in various ratios. The bioink with encapsulated human vein endothelial cells (HUVECs) and a crosslinker, CaCl2, were extruded via sheath and core nozzle respectively to directly bioprint a perfusable 3D vasculature construct. The various concentration of bioink was assessed from several aspects like biocompatibility, porosity, swelling, degradation, and mechanical characteristics, and accordingly, optimized concentration was selected (Alg 4 %w/v - dHAM 0.6 %w/v). Then, the crosslinked bioink without microchannel and the 3D bioprinted construct with various microchannel distances (0, 1.5 mm, 3 mm) were compared. The 3D bioprinted construct with a 1.5 mm microchannels distance demonstrated superiority owing to its 492 ± 18.8 % cell viability within 14 days, excellent tubulogenesis, remarkable expression of VEGFR-2 which play a crucial role in endothelial cell proliferation, migration, and more importantly angiogenesis, and neovascularization. This perfusable bioprinted construct also possess appropriate mechanical stability (32.35 ± 5 kPa Young's modulus) for soft tissue. Taking these advantages into the account, our new bioprinting method possesses a prominent potential for the fabrication of large-scale prevascularized tissue to serve for regenerative medicine applications like implantation, drug-screening platform, and the study of mutation disease.

Preclinical Testing Techniques: Paving the Way for New Oncology Screening Approaches

Prior to clinical trials, preclinical testing of oncology drug candidates is performed by evaluating drug candidates with in vitro and in vivo platforms. For in vivo testing, animal models are used to evaluate the toxicity and efficacy of drug candidates. However, animal models often display poor translational results as many drugs that pass preclinical testing fail when tested with humans, with oncology drugs exhibiting especially poor acceptance rates. The FDA Modernization Act 2.0 promotes alternative preclinical testing techniques, presenting the opportunity to use higher complexity in vitro models as an alternative to in vivo testing, including three-dimensional (3D) cell culture models. Three-dimensional tissue cultures address many of the shortcomings of 2D cultures by more closely replicating the tumour microenvironment through a combination of physiologically relevant drug diffusion, paracrine signalling, cellular phenotype, and vascularization that can better mimic native human tissue. This review will discuss the common forms of 3D cell culture, including cell spheroids, organoids, organs-on-a-chip, and 3D bioprinted tissues. Their advantages and limitations will be presented, aiming to discuss the use of these 3D models to accurately represent human tissue and as an alternative to animal testing. The use of 3D culture platforms for preclinical drug development is expected to accelerate as these platforms continue to improve in complexity, reliability, and translational predictivity.

An engineered lamellar bone mimicking full-scale hierarchical architecture for bone regeneration

Lamellar bone, compactly and ingeniously organized in the hierarchical pattern with 6 ordered scales, is the structural motif of mature bone. Each hierarchical scale exerts an essential role in determining physiological behavior and osteogenic bioactivity of bone. Engineering lamellar bone with full-scale hierarchy remains a longstanding challenge. Herein, using bioskiving and mineralization, we attempt to engineer compact constructs resembling full-scale hierarchy of lamellar bone. Through systematically investigating the effect of mineralization on physicochemical properties and bioactivities of multi-sheeted collagen matrix fabricated by bioskiving, the hierarchical mimicry and hierarchy-property relationship are elucidated. With prolongation of mineralization, hierarchical mimicry and osteogenic bioactivity of constructs are performed in a bidirectional manner, i.e. first rising and then descending, which is supposed to be related with transformation of mineralization mechanism from nonclassical to classical crystallization. Construct mineralized 9 days can accurately mimic each hierarchical scale and efficiently promote osteogenesis. Bioinformatic analysis further reveals that this construct potently activates integrin α5-PI3K/AKT signaling pathway through mechanical and biophysical cues, and thereby repairing critical-sized bone defect. The present study provides a bioinspired strategy for completely resembling complex hierarchy of compact mineralized tissue, and offers a critical research model for in-depth studying the structure-function relationship of bone.

Bioprinted PDLSCs with high-concentration GelMA hydrogels exhibit enhanced osteogenic differentiation in vitro and promote bone regeneration in vivo

OBJECTIVES

We aimed to explore the osteogenic potential of periodontal ligament stem cells (PDLSCs) in bioprinted methacrylate gelatine (GelMA) hydrogels in vitro and in vivo.

MATERIALS AND METHODS

PDLSCs in GelMA hydrogels at various concentrations (3%, 5%, and 10%) were bioprinted. The mechanical properties (stiffness, nanostructure, swelling, and degradation properties) of bioprinted constructs and the biological properties (cell viability, proliferation, spreading, osteogenic differentiation, and cell survival in vivo) of PDLSCs in bioprinted constructs were evaluated. Then, the effect of bioprinted constructs on bone regeneration was investigated using a mouse cranial defect model.

RESULTS

Ten percent GelMA printed constructs had a higher compression modulus, smaller porosity, lower swelling rate, and lower degradation rate than 3% GelMA. PDLSCs in bioprinted 10% GelMA bioprinted constructs showed lower cell viability, less cell spreading, upregulated osteogenic differentiation in vitro, and lower cell survival in vivo. Moreover, upregulated expression of ephrinB2 and EphB4 protein and their phosphorylated forms were found in PDLSCs in 10% GelMA bioprinted constructs, and inhibition of eprhinB2/EphB4 signalling reversed the enhanced osteogenic differentiation of PDLSCs in 10% GelMA. The in vivo experiment showed that 10% GelMA bioprinted constructs with PDLSCs contributed to more new bone formation than 10% GelMA constructs without PDLSCs and constructs with lower GelMA concentrations.

CONCLUSIONS

Bioprinted PDLSCs with high-concentrated GelMA hydrogels exhibited enhanced osteogenic differentiation partially through upregulated ephrinB2/EphB4 signalling in vitro and promoted bone regeneration in vivo, which might be more appropriate for future bone regeneration applications.

CLINICAL RELEVANCE

Bone defects are a common clinical oral problem. Our results provide a promising strategy for bone regeneration through bioprinting PDLSCs in GelMA hydrogels.

Keeping It Organized: Multicompartment Constructs to Mimic Tissue Heterogeneity

Tissue engineering aims at replicating tissues and organs to develop applications in vivo and in vitro. In vivo, by engineering artificial constructs using functional materials and cells to provide both physiological form and function. In vitro, by engineering three-dimensional (3D) models to support drug discovery and enable understanding of fundamental biology. 3D culture constructs mimic cell-cell and cell-matrix interactions and use biomaterials seeking to increase the resemblance of engineered tissues with its in vivo homologues. Native tissues, however, include complex architectures, with compartmentalized regions of different properties containing different types of cells that can be captured by multicompartment constructs. Recent advances in fabrication technologies, such as micropatterning, microfluidics or 3D bioprinting, have enabled compartmentalized structures with defined compositions and properties that are essential in creating 3D cell-laden multiphasic complex architectures. This review focuses on advances in engineered multicompartment constructs that mimic tissue heterogeneity. It includes multiphasic 3D implantable scaffolds and in vitro models, including systems that incorporate different regions emulating in vivo tissues, highlighting the emergence and relevance of 3D bioprinting in the future of biological research and medicine.

Review on Additives in Hydrogels for 3D Bioprinting of Regenerative Medicine: From Mechanism to Methodology

The regeneration of biological tissues in medicine is challenging, and 3D bioprinting offers an innovative way to create functional multicellular tissues. One common way in bioprinting is bioink, which is one type of the cell-loaded hydrogel. For clinical application, however, the bioprinting still suffers from satisfactory performance, e.g., in vascularization, effective antibacterial, immunomodulation, and regulation of collagen deposition. Many studies incorporated different bioactive materials into the 3D-printed scaffolds to optimize the bioprinting. Here, we reviewed a variety of additives added to the 3D bioprinting hydrogel. The underlying mechanisms and methodology for biological regeneration are important and will provide a useful basis for future research.

New Challenges and Prospective Applications of Three-Dimensional Bioactive Polymeric Hydrogels in Oral and Craniofacial Tissue Engineering: A Narrative Review

Regenerative medicine, and dentistry offers enormous potential for enhancing treatment results and has been fueled by bioengineering breakthroughs over the previous few decades. Bioengineered tissues and constructing functional structures capable of healing, maintaining, and regenerating damaged tissues and organs have had a broad influence on medicine and dentistry. Approaches for combining bioinspired materials, cells, and therapeutic chemicals are critical in stimulating tissue regeneration or as medicinal systems. Because of its capacity to maintain an unique 3D form, offer physical stability for the cells in produced tissues, and replicate the native tissues, hydrogels have been utilized as one of the most frequent tissue engineering scaffolds during the last twenty years. Hydrogels' high water content can provide an excellent conditions for cell viability as well as an architecture that mimics real tissues, bone, and cartilage. Hydrogels have been used to enable cell immobilization and growth factor application. This paper summarizes the features, structure, synthesis and production methods, uses, new challenges, and future prospects of bioactive polymeric hydrogels in dental and osseous tissue engineering of clinical, exploring, systematical and scientific applications.

3D printing of biomaterials for vascularized and innervated tissue regeneration

Neurovascular networks play significant roles in the metabolism and regeneration of many tissues and organs in the human body. Blood vessels can transport sufficient oxygen, nutrients, and biological factors, while nerve fibers transmit excitation signals to targeted cells. However, traditional scaffolds cannot satisfy the requirement of stimulating angiogenesis and innervation in a timely manner due to the complexity of host neurovascular networks. Three-dimensional (3D) printing, as a versatile and favorable technique, provides an effective approach to fabricating biological scaffolds with biomimetic architectures and multimaterial compositions, which are capable of regulating multiple cell behaviors. This review paper presents a summary of the current progress in 3D-printed biomaterials for vascularized and innervated tissue regeneration by presenting skin, bone, and skeletal muscle tissues as an example. In addition, we highlight the crucial roles of blood vessels and nerve fibers in the process of tissue regeneration and discuss the future perspectives for engineering novel biomaterials. It is expected that 3D-printed biomaterials with angiogenesis and innervation properties can not only recapitulate the physiological microenvironment of damaged tissues but also rapidly integrate with host neurovascular networks, resulting in accelerated functional tissue regeneration.

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.

Mussel-Inspired Adhesive Hydrogels Based on Laponite-Confined Dopamine Polymerization as a Transdermal Patch

Transdermal patch for local drug delivery has attained huge attention as an attractive alternative to existing drug delivery techniques as it is painless and user-friendly. However, most adhesive hydrogels either do not have adequate adhesion with the skin or cause discomfort while being removed from the skin surface due to excessive adhesion. To address this challenge, we developed an adhesive hydrogel based on laponite-confined dopamine polymerization as a transdermal patch. Laponite RDS nanoclay was used to control the hydrogel's viscous behavior and dopamine polymerization. The laponite polymerized polydopamine (l-PDA) was incorporated into poly(vinyl alcohol) (PVA) to make the PVA-l-PDA hydrogel. The laponite-confined polymerization improved the hydrogels' water contact angle and adhesion strength. The adhesion strength of the PVA-l-PDA hydrogel was adequate to adhere to the evaluated goat skin, glass, and polypropylene surfaces. Notably, the PVA-l-PDA hydrogel was easy to peel off from the skin. Further, we evaluated the drug release profile in goat skin using lidocaine as a model drug. We observed the controlled release of lidocaine from the PVA-l-PDA hydrogel compared to the PVA-PDA hydrogel. In addition, the nanoclay-confined adhesive hydrogel did not show any cytotoxic effect in fibroblasts. Altogether, PVA-l-PDA hydrogels offer appropriate adhesive strength, toughness, and biocompatibility. Thus, the PVA-l-PDA hydrogel has the potential to be an efficient transdermal patch.

Development of osteon-like scaffold-cell construct by quadruple coaxial extrusion-based 3D bioprinting of nanocomposite hydrogel

Despite advances in bone tissue engineering, fabricating a scaffold which can be used as an implant for large bone defects remains challenge. One of the great importance in fabricating a biomimetic bone implant is considering the possibility of the integration of the structure and function of implants with hierarchical structure of bone. Herein, we propose a method to mimic the structural unit of compact bone, osteon, with spatial pattern of human umbilical vein endothelial cells (HUVECs) and human mesenchymal stem cells (hMSCs) in the adjacent layers that mimic Haversian canal and lamella, respectively. To this end, coaxial extrusion-based bioprinting technique via a customized quadruple-layer core-shell nozzle was employed. 3D implant scaffold-cell construct was fabricated by using polyethylene glycol as a hollowing agent in the first layer, gelatin methacryloyl (GelMA) and alginate blended hydrogel encapsulating HUVEC cells with vascular endothelial growth factor nanoparticles in the second layer (vasculogenic layer) to mimic vascular vessel, and GelMA and alginate blended hydrogel containing hMSCs cells in the outer osteogenic layer to imitate lamella. Two types of bone minerals, whitlockite and hydroxyapatite, were incorporated in osteogenic layer to induce osteoblastic differentiation and enhance mechanical properties (the young's modules of nanocomposite increased from 35 kPa to 80 kPa). In-vitro evaluations demonstrated high cell viability (94 % within 10 days) and proliferation. Furthermore, ALP enzyme activity increased considerably within 2 weeks and mineralized extra cellular matrix considerably produced within 3 weeks. Also, a significant increase in osteogenic markers was observed indicating the presence of differentiated osteoblast cells. Therefore, the work indicates the potential of single step 3D bioprinting process to fabricate biomimetic osteons to use as bone grafts for regeneration.

The effect of culture conditions on the bone regeneration potential of osteoblast-laden 3D bioprinted constructs

Three Dimensional (3D) bioprinting is one of the most recent additive manufacturing technologies and enables the direct incorporation of cells within a highly porous 3D-bioprinted construct. While the field has mainly focused on developing methods for enhancing printing resolution and shape fidelity, little is understood about the biological impact of bioprinting on cells. To address this shortcoming, this study investigated the in vitro and in vivo response of human osteoblasts subsequent to bioprinting using gelatin methacryloyl (GelMA) as the hydrogel precursor. First, bioprinted and two-dimensional (2D) cultured osteoblasts were compared, demonstrating that the 3D microenvironment from bioprinting enhanced bone-related gene expression. Second, differentiation regimens of 2-week osteogenic pre-induction in 2D before bioprinting and/or 3-week post-printing osteogenic differentiation were assessed for their capacity to increase the bioprinted construct's biofunctionality towards bone regeneration. The combination of pre-and post-induction regimens showed superior osteogenic gene expression and mineralisation in vitro. Moreover, a rat calvarial model using microtomography and histology demonstrated bone regeneration potential for the pre-and post-differentiation procedure. This study shows the positive impact of bioprinting on cells for osteogenic differentiation and the increased in vivo osteogenic potential of bioprinted constructs via a pre-induction method. STATEMENT OF SIGNIFICANCE: 3D bioprinting, one of the most recent technologies for tissue engineering has mostly focussed on developing methods for enhancing printing properties, little is understood on the biological impact of bioprinting and /or subsequent in vitro maturation methods on cells. Therefore, we addressed these fundamental questions by investigating osteoblast gene expression in bioprinted construct and assessed the efficacy of several induction regimen towards osteogenic differentiation in vitro and in vivo. Osteogenic induction of cells prior to seeding in scaffolds used in conventional tissue engineering applications has been demonstrated to increase the osteogenic potential of the resulting construct. However, to the best of our knowledge, pre-induction methods have not been investigated in 3D bioprinting.

Building Osteogenic Microenvironments with a Double-Network Composite Hydrogel for Bone Repair

The critical factor determining the in vivo effect of bone repair materials is the microenvironment, which greatly depends on their abilities to promote vascularization and bone formation. However, implant materials are far from ideal candidates for guiding bone regeneration due to their deficient angiogenic and osteogenic microenvironments. Herein, a double-network composite hydrogel combining vascular endothelial growth factor (VEGF)-mimetic peptide with hydroxyapatite (HA) precursor was developed to build an osteogenic microenvironment for bone repair. The hydrogel was prepared by mixing acrylated β-cyclodextrins and octacalcium phosphate (OCP), an HA precursor, with gelatin solution, followed by ultraviolet photo-crosslinking. To improve the angiogenic potential of the hydrogel, QK, a VEGF-mimicking peptide, was loaded in acrylated β-cyclodextrins. The QK-loaded hydrogel promoted tube formation of human umbilical vein endothelial cells and upregulated the expression of angiogenesis-related genes, such as Flt1 , Kdr , and VEGF , in bone marrow mesenchymal stem cells. Moreover, QK could recruit bone marrow mesenchymal stem cells. Furthermore, OCP in the composite hydrogel could be transformed into HA and release calcium ions facilitating bone regeneration. The double-network composite hydrogel integrated QK and OCP showed obvious osteoinductive activity. The results of animal experiments showed that the composite hydrogel enhanced bone regeneration in skull defects of rats, due to perfect synergistic effects of QK and OCP on vascularized bone regeneration. In summary, improving the angiogenic and osteogenic microenvironments by our double-network composite hydrogel shows promising prospects for bone repair.

An open-source bioink database for microextrusion 3D printing

3D printing has rapidly become a critical enabling technology in tissue engineering and regenerative medicine for the fabrication of complex engineered tissues. 3D bioprinting, in particular, has advanced greatly to facilitate the incorporation of a broad spectrum of biomaterials along with cells and biomolecules of interest forin vitrotissue generation. The increasing complexity of novel bioink formulations and application-dependent printing conditions poses a significant challenge for replicating or innovating new bioprinting strategies. As the field continues to grow, it is imperative to establish a cohesive, open-source database that enables users to search through existing 3D printing formulations rapidly and efficiently. Through the efforts of the NIH/NIBIB Center for Engineering Complex Tissues, we have developed, to our knowledge, the first bioink database for extrusion-based 3D printing. The database is publicly available and allows users to search through and easily access information on biomaterials and cells specifically used in 3D printing. In order to enable a community-driven database growth, we have established an open-source portal for researchers to enter their publication information for addition into the database. Although the database has a broad range of capabilities, we demonstrate its utility by performing a comprehensive analysis of the printability domains of two well-established biomaterials in the printing world, namely poly(ϵ-caprolactone) and gelatin methacrylate. The database allowed us to rapidly identify combinations of extrusion pressure, temperature, and speed that have been used to print these biomaterials and more importantly, identify domains within which printing was not possible. The data also enabled correlation analysis between all the printing parameters, including needle size and type, that exhibited compatibility for cell-based 3D printing. Overall, this database is an extremely useful tool for the 3D printing and bioprinting community to advance their research and is an important step towards standardization in the field.

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.

Computer-aided patterning of PCL microspheres to build modular scaffolds featuring improved strength and neovascularized tissue integration

In the past decade, modular scaffolds prepared by assembling biocompatible and biodegradable building blocks (e.g. microspheres) have found promising applications in tissue engineering (TE) towards the repair/regeneration of damaged and impaired tissues. Nevertheless, to date this approach has failed to be transferred to the clinic due to technological limitations regarding microspheres patterning, a crucial issue for the control of scaffold strength, vascularization and integration in vivo. In this work, we propose a robust and reliable approach to address this issue through the fabrication of polycaprolactone (PCL) microsphere-based scaffolds with in-silico designed microarchitectures and high compression moduli. The scaffold fabrication technique consists of four main steps, starting with the manufacture of uniform PCL microspheres by fluidic emulsion technique. In the second step, patterned polydimethylsiloxane (PDMS) moulds were prepared by soft lithography. Then, layers of 500 µm PCL microspheres with geometrically inspired patterns were obtained by casting the microspheres onto PDMS moulds followed by their thermal sintering. Finally, three-dimensional porous scaffolds were built by the alignment, stacking and sintering of multiple (up to six) layers. The so prepared scaffolds showed excellent morphological and microstructural fidelity with respect to the in-silico models, and mechanical compression properties suitable for load bearing TE applications. Designed porosity and pore size features enabled in vitro human endothelial cells adhesion and growth as well as tissue integration and blood vessels invasion in vivo. Our results highlighted the strong impact of spatial patterning of microspheres on modular scaffolds response, and pay the way about the possibility to fabricate in silico-designed structures featuring biomimetic composition and architectures for specific TE purposes.

3D bioprinting of osteon-mimetic scaffolds with hierarchical microchannels for vascularized bone tissue regeneration

The integration of three-dimensional (3D) bioprinted scaffold's structure and function for critical-size bone defect repair is of immense significance. Inspired by the basic component of innate cortical bone tissue--osteons, many studies focus on biomimetic strategy. However, the complexity of hierarchical microchannels in the osteon, the requirement of mechanical strength of bone, and the biological function of angiogenesis and osteogenesis remain challenges in the fabrication of osteon-mimetic scaffolds. Therefore, we successfully built mimetic scaffolds with vertically central medullary canals, peripheral Haversian canals, and transverse Volkmann canals structures simultaneously by 3D bioprinting technology using polycaprolactone and bioink loading with bone marrow mesenchymal stem cells (BMSCs) and bone morphogenetic protein-4 (BMP-4). Subsequently, endothelial progenitor cells (EPCs) were seeded into the canals to enhance angiogenesis. The porosity and compressive properties of bioprinted scaffolds could be well controlled by altering the structure and canal numbers of the scaffolds. The osteon-mimetic scaffolds showed satisfactory biocompatibility and promotion of angiogenesis and osteogenesis in vitro and prompted the new blood vessels and new bone formation in vivo. In summary, this study proposes a biomimetic strategy for fabricating structured and functionalized 3D bioprinted scaffolds for vascularized bone tissue regeneration.

Endothelialized microvessels fabricated by microfluidics facilitate osteogenic differentiation and promote bone repair

In bone tissue engineering, vascularization is one of the critical factors that limit the effect of biomaterials for bone repair. While various approaches have been tried to build vascular networks in bone grafts, lack of endothelialization still constitutes a major technical hurdle. In this study, we have developed a facile technique to fabricate endothelialized biomimetic microvessels (BMVs) from alginate-collagen composite hydrogels within a single step using microfluidic technology. BMVs with different sizes could be readily prepared by adjusting the flow rate of microfluids. All BMVs supported perfusion and outward penetration of substances in the tube. Endothelial cells could adhere and proliferate on the inner wall of tubes. It was also found that the expression of CD31 and secretion of BMP-2 and PDGF-BB were higher in the rat umbilical vein endothelial cells (RUVECs) in BMVs than those cultured on hydrogel. When co-cultured with bone marrow mesenchymal stem cells (BMSCs), endothelialized BMVs promoted the osteogenic differentiation of BMSCs compared to those in acellular BMV group. In vivo, markedly enhanced new bone formation was achieved by endothelialized BMVs in a rat critical-sized calvarial defect model compared to those with non-endothelialized BMVs or without BMVs. Together, findings from both in vitro and in vivo studies have proven that endothelialized BMVs function to facilitate osteogenesis and promote bone regeneration, and therefore might present an effective strategy in bone tissue engineering. STATEMENT OF SIGNIFICANCE: In bone tissue engineering, limited vascularization is one of the critical factors that limit the effect of biomaterials for bone repair. In this study, we developed a facile technique to fabricate endothelialized biomimetic microvessels (BMVs) from alginate-collagen composite hydrogels within a single step using microfluidic technology. Both in vitro and in vivo studies have proven that endothelialized BMVs function to facilitate osteogenesis and promote bone regeneration, and therefore might present an effective strategy in bone tissue engineering.

Fabrication of a bio-instructive scaffold conferred with a favorable microenvironment allowing for superior implant osseointegration and accelerated in situ vascularized bone regeneration via type H vessel formation

The potential translation of bio-inert polymer scaffolds as bone substitutes is limited by the lack of neovascularization upon implantation and subsequently diminished ingrowth of host bone, most likely resulted from the inability to replicate appropriate endogenous crosstalk between cells. Human umbilical vein endothelial cell-derived decellularized extracellular matrix (HdECM), which contains a collection of angiocrine biomolecules, has recently been demonstrated to mediate endothelial cells(ECs) - osteoprogenitors(OPs) crosstalk. We employed the HdECM to create a PCL (polycaprolactone)/fibrin/HdECM (PFE) hybrid scaffold. We hypothesized PFE scaffold could reconstitute a bio-instructive microenvironment that reintroduces the crosstalk, resulting in vascularized bone regeneration. Following implantation in a rat femoral bone defect, the PFE scaffold demonstrated early vascular infiltration and enhanced bone regeneration by microangiography (μ-AG) and micro-computational tomography (μ-CT). Based on the immunofluorescence studies, PFE mediated the endogenous angiogenesis and osteogenesis with a substantial number of type H vessels and osteoprogenitors. In addition, superior osseointegration was observed by a direct host bone-PCL interface, which was likely attributed to the formation of type H vessels. The bio-instructive microenvironment created by our innovative PFE scaffold made possible superior osseointegration and type H vessel-related bone regeneration. It could become an alternative solution of improving the osseointegration of bone substitutes with the help of induced type H vessels, which could compensate for the inherent biological inertness of synthetic polymers.

From 3D printing to 3D bioprinting: the material properties of polymeric material and its derived bioink for achieving tissue specific architectures

The application of 3D printing technologies fields for biological tissues, organs, and cells in the context of medical and biotechnology applications requires a significant amount of innovation in a narrow printability range. 3D bioprinting is one such way of addressing critical design challenges in tissue engineering. In a more general sense, 3D printing has become essential in customized implant designing, faithful reproduction of microenvironmental niches, sustainable development of implants, in the capacity to address issues of effective cellular integration, and long-term stability of the cellular constructs in tissue engineering. This review covers various aspects of 3D bioprinting, describes the current state-of-the-art solutions for all aforementioned critical issues, and includes various illustrative representations of technologies supporting the development of phases of 3D bioprinting. It also demonstrates several bio-inks and their properties crucial for being used for 3D printing applications. The review focus on bringing together different examples and current trends in tissue engineering applications, including bone, cartilage, muscles, neuron, skin, esophagus, trachea, tympanic membrane, cornea, blood vessel, immune system, and tumor models utilizing 3D printing technology and to provide an outlook of the future potentials and barriers.

Use of Electrospun Phenylalanine/Poly-ε-Caprolactone Chiral Hybrid Scaffolds to Promote Endothelial Remodeling

The fabrication of tissue-engineered vascular grafts to replace damaged vessels is a promising therapy for cardiovascular diseases. Endothelial remodeling in the lumen of TEVGs is critical for successful revascularization. However, the construction of well-functioning TEVGs remains a fundamental challenge. Herein, chiral hybrid scaffolds were prepared by electrospinning using D/L-phenylalanine based gelators [D(L)PHEG] and poly-ε-caprolactone (PCL). The chirality of scaffolds significantly affected the endothelial remodeling progress of TEVGs. Compared with L-phenylalanine based gelators/poly-ε-caprolactone (L/PCL) and PCL, D-phenylalanine based gelators/poly-ε-caprolactone (D/PCL) scaffolds enhanced cell adhesion, and proliferation and upregulated the expression of fibronectin-1, and vinculin. These results suggests that chiral hybrid scaffolds can promote endothelial remodeling of TEVGs by upregulating adhesion-associated protein levels. This study offers an innovative strategy for endothelial remodeling of TEVGs by fabricating chiral hybrid scaffolds, and provides new insight for the treatment of cardiovascular diseases.

Bioprinting Scaffolds for Vascular Tissues and Tissue Vascularization

In recent years, tissue engineering has achieved significant advancements towards the repair of damaged tissues. Until this day, the vascularization of engineered tissues remains a challenge to the development of large-scale artificial tissue. Recent breakthroughs in biomaterials and three-dimensional (3D) printing have made it possible to manipulate two or more biomaterials with complementary mechanical and/or biological properties to create hybrid scaffolds that imitate natural tissues. Hydrogels have become essential biomaterials due to their tissue-like physical properties and their ability to include living cells and/or biological molecules. Furthermore, 3D printing, such as dispensing-based bioprinting, has progressed to the point where it can now be utilized to construct hybrid scaffolds with intricate structures. Current bioprinting approaches are still challenged by the need for the necessary biomimetic nano-resolution in combination with bioactive spatiotemporal signals. Moreover, the intricacies of multi-material bioprinting and hydrogel synthesis also pose a challenge to the construction of hybrid scaffolds. This manuscript presents a brief review of scaffold bioprinting to create vascularized tissues, covering the key features of vascular systems, scaffold-based bioprinting methods, and the materials and cell sources used. We will also present examples and discuss current limitations and potential future directions of the technology.

3D printing of self-standing and vascular supportive multimaterial hydrogel structures for organ engineering

Three dimensional printable formulation of self-standing and vascular-supportive structures using multi-materials suitable for organ engineering is of great importance and highly challengeable, but, it could advance the 3D printing scenario from printable shape to functional unit of human body. In this study, the authors report a 3D printable formulation of such self-standing and vascular-supportive structures using an in-house formulated multi-material combination of albumen/alginate/gelatin-based hydrogel. The rheological properties and relaxation behavior of hydrogels were analyzed before the printing process. The suitability of the hydrogel in 3D printing of various customizable and self-standing structures, including a human ear model, was examined by extrusion-based 3D printing. The structural, mechanical, and physicochemical properties of the printed scaffolds were studied systematically. Results supported the 3D printability of the formulated hydrogel with self-standing structures, which are customizable to a specific need. In vitro cell experiment showed that the formulated hydrogel has excellent biocompatibility and vascular supportive behavior with the extent of endothelial sprout formation when tested with human umbilical vein endothelial cells. In conclusion, the present study demonstrated the suitability of the extrusion-based 3D printing technique for manufacturing complex shapes and structures using multi-materials with high fidelity, which have great potential in organ engineering.

Current Trends on Protein Driven Bioinks for 3D Printing

In the last decade, three-dimensional (3D) extrusion bioprinting has been on the top trend for innovative technologies in the field of biomedical engineering. In particular, protein-based bioinks such as collagen, gelatin, silk fibroin, elastic, fibrin and protein complexes based on decellularized extracellular matrix (dECM) are receiving increasing attention. This current interest is the result of protein's tunable properties, biocompatibility, environmentally friendly nature and possibility to provide cells with the adequate cues, mimicking the extracellular matrix's function. In this review we describe the most relevant stages of the development of a protein-driven bioink. The most popular formulations, molecular weights and extraction methods are covered. The different crosslinking methods used in protein bioinks, the formulation with other polymeric systems or molecules of interest as well as the bioprinting settings are herein highlighted. The cell embedding procedures, the in vitro, in vivo, in situ studies and final applications are also discussed. Finally, we approach the development and optimization of bioinks from a sequential perspective, discussing the relevance of each parameter during the pre-processing, processing, and post-processing stages of technological development. Through this approach the present review expects to provide, in a sequential manner, helpful methodological guidelines for the development of novel bioinks.

3D Printing and Bioprinting to Model Bone Cancer: The Role of Materials and Nanoscale Cues in Directing Cell Behavior

Bone cancer, both primary and metastatic, is characterized by a low survival rate. Currently, available models lack in mimicking the complexity of bone, of cancer, and of their microenvironment, leading to poor predictivity. Three-dimensional technologies can help address this need, by developing predictive models that can recapitulate the conditions for cancer development and progression. Among the existing tools to obtain suitable 3D models of bone cancer, 3D printing and bioprinting appear very promising, as they enable combining cells, biomolecules, and biomaterials into organized and complex structures that can reproduce the main characteristic of bone. The challenge is to recapitulate a bone-like microenvironment for analysis of stromal-cancer cell interactions and biological mechanics leading to tumor progression. In this review, existing approaches to obtain in vitro 3D-printed and -bioprinted bone models are discussed, with a focus on the role of biomaterials selection in determining the behavior of the models and its degree of customization. To obtain a reliable 3D bone model, the evaluation of different polymeric matrices and the inclusion of ceramic fillers is of paramount importance, as they help reproduce the behavior of both normal and cancer cells in the bone microenvironment. Open challenges and future perspectives are discussed to solve existing shortcomings and to pave the way for potential development strategies.

Vascularization Strategies in Bone Tissue Engineering

Bone is a highly vascularized tissue, and its development, maturation, remodeling, and regeneration are dependent on a tight regulation of blood vessel supply. This condition also has to be taken into consideration in the context of the development of artificial tissue substitutes. In classic tissue engineering, bone-forming cells such as primary osteoblasts or mesenchymal stem cells are introduced into suitable scaffolds and implanted in order to treat critical-size bone defects. However, such tissue substitutes are initially avascular. Because of the occurrence of hypoxic conditions, especially in larger tissue substitutes, this leads to the death of the implanted cells. Therefore, it is necessary to devise vascularization strategies aiming at fast and efficient vascularization of implanted artificial tissues. In this review article, we present and discuss the current vascularization strategies in bone tissue engineering. These are based on the use of angiogenic growth factors, the co-implantation of blood vessel forming cells, the ex vivo microfabrication of blood vessels by means of bioprinting, and surgical methods for creating surgically transferable composite tissues.

3D Bioprinting of Vascularized Tissues for in vitro and in vivo Applications

With a limited supply of organ donors and available organs for transplantation, the aim of tissue engineering with three-dimensional (3D) bioprinting technology is to construct fully functional and viable tissue and organ replacements for various clinical applications. 3D bioprinting allows for the customization of complex tissue architecture with numerous combinations of materials and printing methods to build different tissue types, and eventually fully functional replacement organs. The main challenge of maintaining 3D printed tissue viability is the inclusion of complex vascular networks for nutrient transport and waste disposal. Rapid development and discoveries in recent years have taken huge strides toward perfecting the incorporation of vascular networks in 3D printed tissue and organs. In this review, we will discuss the latest advancements in fabricating vascularized tissue and organs including novel strategies and materials, and their applications. Our discussion will begin with the exploration of printing vasculature, progress through the current statuses of bioprinting tissue/organoids from bone to muscles to organs, and conclude with relevant applications for in vitro models and drug testing. We will also explore and discuss the current limitations of vascularized tissue engineering and some of the promising future directions this technology may bring.

Three-dimensional bioactive hydrogel-based scaffolds for bone regeneration in implant dentistry

Bone tissue requires a range of complex mechanisms to allow the restoration of its structure and function. Bone healing is a signaling cascade process, involving cells secreting cytokines, growth factors, and pro-inflammatory factors in the defect site that will, subsequently, recruit surrounding stem cells to migrate, proliferate, and differentiate into bone-forming cells. Bioactive functional scaffolds could be applied to improve the bone healing processes where the organism is not able to fully regenerate the lost tissue. However, to be optimal, such scaffolds should act as osteoconductors - supporting bone-forming cells, providing nutrients, and sustaining the arrival of new blood vessels, and act as osteoinducers - slowly releasing signaling molecules that stimulate mesenchymal stem cells to differentiate and deposit mineralized bone matrix. Different compositions and shapes of scaffolds, cutting-edge technologies, application of signaling molecules to promote cell differentiation, and high-quality biomaterials are reaching favorable outcomes towards osteoblastic differentiation of stem cells in in vitro and in vivo researches for bone regeneration. Hydrogel-based biomaterials are being pointed as promising for bone tissue regeneration; however, despite all the research and high-impact scientific publications, there are still several challenges that prevent the use of hydrogel-based scaffolds for bone regeneration being feasible for their clinical application. Hence, the objective of this review is to consolidate and report, based on the current scientific literature, the approaches for bone tissue regeneration using bioactive hydrogel-based scaffolds, cell-based therapies, and three-dimensional bioprinting to define the key challenges preventing their use in clinical applications.

Osteon: Structure, Turnover, and Regeneration

Bone is composed of dense and solid cortical bone and honeycomb-like trabecular bone. Although cortical bone provides the majority of mechanical strength for a bone, there are few studies focusing on cortical bone repair or regeneration. Osteons (the Haversian system) form structural and functional units of cortical bone. In recent years, emerging evidences have shown that the osteon structure (including osteocytes, lamellae, lacunocanalicular network, and Haversian canals) plays critical roles in bone mechanics and turnover. Therefore, reconstruction of the osteon structure is crucial for cortical bone regeneration. This article provides a systematic summary of recent advances in osteons, including the structure, function, turnover, and regenerative strategies. First, the hierarchical structure of osteons is illustrated and the critical functions of osteons in bone dynamics are introduced. Next, the modeling and remodeling processes of osteons at a cellular level and the turnover of osteons in response to mechanical loading and aging are emphasized. Furthermore, several bioengineering approaches that were recently developed to recapitulate the osteon structure are highlighted. Impact statement This review provides a comprehensive summary of recent advances in osteons, especially the roles in bone formation, remodeling, and regeneration. Besides introducing the hierarchical structure and critical functions of osteons, we elucidate the modeling and remodeling of osteons at a cellular level. Specifically, we highlight the bioengineering approaches that were recently developed to mimic the hierarchical structure of osteons. We expect that this review will provide informative insights and attract increasing attentions in orthopedic community, shedding light on cortical bone regeneration in the future.

Fabrication of centimeter-sized 3D constructs with patterned endothelial cells through assembly of cell-laden microbeads as a potential bone graft

Modular tissue engineering is a promising biofabrication strategy to create engineered bone grafts in a bottom-up manner, in which cell-laden micro-modules are prepared as basic building blocks to assemble macroscopic tissues via different integrating mechanisms. In this study, we prepared collagen microbeads loaded with human bone marrow derived mesenchymal stem cells (BMSCs) using a microfluidic approach. The cell-laden microbeads were characterized for size change, cell activity, osteogenesis, as well as their self-assembly properties to generate centimeter-sized constructs. Moreover, using the cell-laden beads as a supporting medium, induced pluripotent stem cell-derived endothelial cells (iPSC-EC) were patterned inside bead aggregates through extrusion-based 3D printing. This fabrication approach that combines modular tissue engineering and supports 3D printing has the potential to create 3D engineered bone grafts with a pre-existing, customized vasculature.

Biomaterials for Bioprinting Microvasculature

Microvasculature functions at the tissue and cell level, regulating local mass exchange of oxygen and nutrient-rich blood. While there has been considerable success in the biofabrication of large- and small-vessel replacements, functional microvasculature has been particularly challenging to engineer due to its size and complexity. Recently, three-dimensional bioprinting has expanded the possibilities of fabricating sophisticated microvascular systems by enabling precise spatiotemporal placement of cells and biomaterials based on computer-aided design. However, there are still significant challenges facing the development of printable biomaterials that promote robust formation and controlled 3D organization of microvascular networks. This review provides a thorough examination and critical evaluation of contemporary biomaterials and their specific roles in bioprinting microvasculature. We first provide an overview of bioprinting methods and techniques that enable the fabrication of microvessels. We then offer an in-depth critical analysis on the use of hydrogel bioinks for printing microvascularized constructs within the framework of current bioprinting modalities. We end with a review of recent applications of bioprinted microvasculature for disease modeling, drug testing, and tissue engineering, and conclude with an outlook on the challenges facing the evolution of biomaterials design for bioprinting microvasculature with physiological complexity.

Utilization of Carbon Nanotubes in Manufacturing of 3D Cartilage and Bone Scaffolds

Cartilage and bone injuries are prevalent ailments, affecting the quality of life of injured patients. Current methods of treatment are often imperfect and pose the risk of complications in the long term. Therefore, tissue engineering is a rapidly developing branch of science, which aims at discovering effective ways of replacing or repairing damaged tissues with the use of scaffolds. However, both cartilage and bone owe their exceptional mechanical properties to their complex ultrastructure, which is very difficult to reproduce artificially. To address this issue, nanotechnology was employed. One of the most promising nanomaterials in this respect is carbon nanotubes, due to their exceptional physico-chemical properties, which are similar to collagens-the main component of the extracellular matrix of these tissues. This review covers the important aspects of 3D scaffold development and sums up the existing research tackling the challenges of scaffold design. Moreover, carbon nanotubes-reinforced bone and cartilage scaffolds manufactured using the 3D bioprinting technique will be discussed as a novel tool that could facilitate the achievement of more biomimetic structures.

Concurrent multi-lineage differentiation of mesenchymal stem cells through spatial presentation of growth factors

Severe tendon and ligament injuries are estimated to affect between 300 000 and 400 000 people annually. Surgical repairs of these injuries often have poor long-term clinical outcomes because of resection of the interfacial tissue-the enthesis-and subsequent stress concentration at the attachment site. A healthy enthesis consists of distinct regions of bone, fibrocartilage, and tendon, each with distinct cell types, extracellular matrix components, and architecture, which are important for tissue function. Tissue engineering, which has been proposed as a potential strategy for replacing this tissue, is currently limited by its inability to differentiate multiple lineages of cells from a single stem cell population within a single engineered construct. In this study, we develop a multi-phasic gelatin methacrylate hydrogel construct system for spatial presentation of proteins, which is then validated for multi-lineage differentiation towards the cell types of the bone-tendon enthesis. This study determines growth factor concentrations for differentiation of mesenchymal stem cells towards osteoblasts, chondrocytes/fibrochondrocytes, and tenocytes, which maintain similar differentiation profiles in 3D hydrogel culture as assessed by qPCR and immunofluorescence staining. Finally, it is shown that this method is able to guide heterogeneous and spatially confined changes in mesenchymal stem cell genes and protein expressions with the tendency to result in osteoblast-, fibrochondrocyte-, and tenocyte-like expression profiles. Overall, we demonstrate the utility of the culture technique for engineering other musculoskeletal tissue interfaces and provide a biochemical approach for recapitulating the bone-tendon enthesis in vitro.

3D Bioprinting: The Emergence of Programmable Biodesign

Until recently, bioprinting was largely limited to highly interdisciplinary research teams, as the process requires significant input from specialists in the fields of materials science, engineering, and cell biology. With the advent of commercially available high-performance bioprinters, the field has become accessible to a wider range of research groups, who can now buy the hardware off the shelf instead of having to build it from scratch. As a result, bioprinting has rapidly expanded to address a wide array of research foci, which include organotypic in vitro models, complex engineered tissues, and even bioprinted microbial systems. Moreover, in the early days, the range of suitable bioinks was limited. Now, there is a plethora of viable options to suit many cell phenotypes. This rapidly evolving dynamic environment creates endless opportunities for scientists to design and construct highly complex biological systems. However, this scientific diversity presents its own set of challenges, such as defining standardized protocols for characterizing bioprinted structures, which is essential for eventual organ replacement. In this progress report, the current state-of-the-art in the field of bioprinting is discussed, with a special emphasis on recent hardware developments, bioprinting for regenerative medicine, and late-breaking nontraditional topics.