The stiffness and structure of three-dimensional printed hydrogels direct the differentiation of mesenchymal stromal cells toward adipogenic and osteogenic lineages

Photo-annealable agarose microgels for jammed microgel printing: Transforming thermogelling hydrogel to a functional bioink

Three-dimensional (3D) printing of hydrogel structures using jammed microgel inks offer distinct advantages of improved printing functionalities, as these inks are strain-yielding and self-recovering types. However, interparticle binding in granular hydrogel inks is a challenge to overcome the limited integrity and reduced macroscale modulus prevalent in the 3D printed microgel scaffolds. In this study, we prepared chemically annealable agarose microgels through a process of xerogel rehydration, applying a low-cost and high throughput method of spray drying. The crosslinked jammed microgel matrix is found to have superior mechanical properties with a Young's modulus of 2.23 MPa and extensibility up to 7.2 %, surpassing those of traditional biopolymer-based and microgel-based inks. Furthermore, this study addresses the complexities encountered in the existing system of printing thermoresponsive agarose bioink using this jammed microgel printing approach. The jammed agarose microgel ink exhibited to be self-recovering, yield stress fluid and validated the temperature-independent printing. Furthermore, the 3D printed jammed microgel scaffold demonstrated good cell responsiveness as evaluated through the viability and morphological study in-vitro with mesenchymal stem cells cultured in it. This unique fabrication approach offers exciting possibilities to expand on microgel printing for varied requirements in tissue engineering.

Design and evaluation of a bilayered dermal/hypodermal 3D model using a biomimetic hydrogel formulation

Due to the limitations of the current skin wound treatments, it is highly valuable to have a wound healing formulation that mimics the extracellular matrix (ECM) and mechanical properties of natural skin tissue. Here, a novel biomimetic hydrogel formulation has been developed based on a mixture of Agarose-Collagen Type I (AC) combined with skin ECM-related components: Dermatan sulfate (DS), Hyaluronic acid (HA), and Elastin (EL) for its application in skin tissue engineering (TE). Different formulations were designed by combining AC hydrogels with DS, HA, and EL. Cell viability, hemocompatibility, physicochemical, mechanical, and wound healing properties were investigated. Finally, a bilayered hydrogel loaded with fibroblasts and mesenchymal stromal cells was developed using the Ag-Col I-DS-HA-EL (ACDHE) formulation. The ACDHE hydrogel displayed the best in vitro results and acceptable physicochemical properties. Also, it behaved mechanically close to human native skin and exhibited good cytocompatibility. Environmental scanning electron microscopy (ESEM) analysis revealed a porous microstructure that allows the maintenance of cell growth and ECM-like structure production. These findings demonstrate the potential of the ACDHE hydrogel formulation for applications such as an injectable hydrogel or a bioink to create cell-laden structures for skin TE.

Advancing bone regeneration: Unveiling the potential of 3D cell models in the evaluation of bone regenerative materials

Bone, a rigid yet regenerative tissue, has garnered extensive attention for its impressive healing abilities. Despite advancements in understanding bone repair and creating treatments for bone injuries, handling nonunions and large defects remains a major challenge in orthopedics. The rise of bone regenerative materials is transforming the approach to bone repair, offering innovative solutions for nonunions and significant defects, and thus reshaping orthopedic care. Evaluating these materials effectively is key to advancing bone tissue regeneration, especially in difficult healing scenarios, making it a critical research area. Traditional evaluation methods, including two-dimensional cell models and animal models, have limitations in predicting accurately. This has led to exploring alternative methods, like 3D cell models, which provide fresh perspectives for assessing bone materials' regenerative potential. This paper discusses various techniques for constructing 3D cell models, their pros and cons, and crucial factors to consider when using these models to evaluate bone regenerative materials. We also highlight the significance of 3D cell models in the in vitro assessments of these materials, discuss their current drawbacks and limitations, and suggest future research directions. STATEMENT OF SIGNIFICANCE: This work addresses the challenge of evaluating bone regenerative materials (BRMs) crucial for bone tissue engineering. It explores the emerging role of 3D cell models as superior alternatives to traditional methods for assessing these materials. By dissecting the construction, key factors of evaluating, advantages, limitations, and practical considerations of 3D cell models, the paper elucidates their significance in overcoming current evaluation method shortcomings. It highlights how these models offer a more physiologically relevant and ethically preferable platform for the precise assessment of BRMs. This contribution is particularly significant for "Acta Biomaterialia" readership, as it not only synthesizes current knowledge but also propels the discourse forward in the search for advanced solutions in bone tissue engineering and regeneration.

A Versatile Approach to Stabilize Liquid-Liquid Interfaces using Surfactant Self-Assembly

Stabilizing liquid-liquid interfaces, whether between miscible or immiscible liquids, is crucial for a wide range of applications, including energy storage, microreactors, and biomimetic structures. In this study, a versatile approach for stabilizing the water-oil interface is presented using the morphological transitions that occur during the self-assembly of anionic, cationic, and nonionic surfactants mixed with fatty acid oils. The morphological transitions underlying this approach are characterized and extensively studied through small-angle X-ray scattering (SAXS), rheometry, and microscopy techniques. Dissipative particle dynamics (DPD) as a simulation tool is adopted to investigate these morphological transitions both in the equilibrium ternary system as well as in the dynamic condition of the water-oil interface. Such a versatile strategy holds promise for enhancing applications such as liquid-in-liquid 3D printing. Moreover, it has the potential to revolutionize a wide range of fields where stabilizing liquid-liquid interfaces not only offers unprecedented opportunities for fine-tuning nanostructural morphologies but also imparts interesting practical features to the resulting liquid shapes. These features include perfusion capabilities, self-healing, and porosity, which could have significant implications for various industries.

A 3D Bioprinted Human Neurovascular Unit Model of Glioblastoma Tumor Growth

A 3D bioprinted neurovascular unit (NVU) model is developed to study glioblastoma (GBM) tumor growth in a brain-like microenvironment. The NVU model includes human primary astrocytes, pericytes and brain microvascular endothelial cells, and patient-derived glioblastoma cells (JHH-520) are used for this study. Fluorescence reporters are used with confocal high content imaging to quantitate real-time microvascular network formation and tumor growth. Extensive validation of the NVU-GBM model includes immunostaining for brain relevant cellular markers and extracellular matrix components; single cell RNA sequencing (scRNAseq) to establish physiologically relevant transcriptomics changes; and secretion of NVU and GBM-relevant cytokines. The scRNAseq reveals changes in gene expression and cytokines secretion associated with wound healing/angiogenesis, including the appearance of an endothelial mesenchymal transition cell population. The NVU-GBM model is used to test 18 chemotherapeutics and anti-cancer drugs to assess the pharmacological relevance of the model and robustness for high throughput screening.

Three-dimensional bioprinting of tissue-engineered skin: Biomaterials, fabrication techniques, challenging difficulties, and future directions: A review

As an emerging new manufacturing technology, Three-dimensional (3D) bioprinting provides the potential for the biomimetic construction of multifaceted and intricate architectures of functional integument, particularly functional biomimetic dermal structures inclusive of cutaneous appendages. Although the tissue-engineered skin with complete biological activity and physiological functions is still cannot be manufactured, it is believed that with the advances in matrix materials, molding process, and biotechnology, a new generation of physiologically active skin will be born in the future. In pursuit of furnishing readers and researchers involved in relevant research to have a systematic and comprehensive understanding of 3D printed tissue-engineered skin, this paper furnishes an exegesis on the prevailing research landscape, formidable obstacles, and forthcoming trajectories within the sphere of tissue-engineered skin, including: (1) the prevalent biomaterials (collagen, chitosan, agarose, alginate, etc.) routinely employed in tissue-engineered skin, and a discerning analysis and comparison of their respective merits, demerits, and inherent characteristics; (2) the underlying principles and distinguishing attributes of various current printing methodologies utilized in tissue-engineered skin fabrication; (3) the present research status and progression in the realm of tissue-engineered biomimetic skin; (4) meticulous scrutiny and summation of the extant research underpinning tissue-engineered skin inform the identification of prevailing challenges and issues.

Nanofibrous polyelectrolyte complex incorporated BSA-alginate composite bioink for 3D bioprinting of bone mimicking constructs

Although several bioinks have been developed for 3D bioprinting applications, the lack of optimal printability, mechanical properties, and adequate cell response has limited their practical applicability. Therefore, this work reports the development of a composite bioink consisting of bovine serum albumin (BSA), alginate, and self-assembled nanofibrous polyelectrolyte complex aggregates of gelatin and chitosan (PEC-GC). The nanofibrous PEC-GC aggregates were prepared and incorporated into the bioink in varying concentrations (0 % to 3 %). The bioink samples were bioprinted and crosslinked post-printing by calcium chloride. The average nanofiber diameter of PEC-GC was 62 ± 15 nm. It was demonstrated that PEC-GC improves the printability and cellular adhesion of the developed bioink and modulates the swelling ratio, degradation rate, and mechanical properties of the fabricated scaffold. The in vitro results revealed that the bioink with 2 % PEC-GC had the best post-printing cell viability of the encapsulated MG63 osteosarcoma cells and well oragnized stress fibers, indicating enhanced cell adhesion. The cell viability was >90 %, as observed from the MTT assay. The composite bioink also showed osteogenic potential, as confirmed by the estimation of alkaline phosphatase activity and collagen synthesis assay. This study successfully fabricated a high-shape fidelity bioink with potential in bone tissue engineering.

In Situ Bioprinting: Process, Bioinks, and Applications

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

Effect of Hydrogel Stiffness on Chemoresistance of Breast Cancer Cells in 3D Culture

Chemotherapy is one of the most common strategies for cancer treatment, whereas drug resistance reduces the efficiency of chemotherapy and leads to treatment failure. The mechanism of emerging chemoresistance is complex and the effect of extracellular matrix (ECM) surrounding cells may contribute to drug resistance. Although it is well known that ECM plays an important role in orchestrating cell functions, it remains exclusive how ECM stiffness affects drug resistance. In this study, we prepared agarose hydrogels of different stiffnesses to investigate the effect of hydrogel stiffness on the chemoresistance of breast cancer cells to doxorubicin (DOX). Agarose hydrogels with a stiffness range of 1.5 kPa to 112.3 kPa were prepared and used to encapsulate breast cancer cells for a three-dimensional culture with different concentrations of DOX. The viability of the cells cultured in the hydrogels was dependent on both DOX concentration and hydrogel stiffness. Cell viability decreased with DOX concentration when the cells were cultured in the same stiffness hydrogels. When DOX concentration was the same, breast cancer cells showed higher viability in high-stiffness hydrogels than they did in low-stiffness hydrogels. Furthermore, the expression of P-glycoprotein mRNA in high-stiffness hydrogels was higher than that in low-stiffness hydrogels. The results suggested that hydrogel stiffness could affect the resistance of breast cancer cells to DOX by regulating the expression of chemoresistance-related genes.

Controllable and biocompatible 3D bioprinting technology for microorganisms: Fundamental, environmental applications and challenges

3D bioprinting is a new 3D manufacturing technology, that can be used to accurately distribute and load microorganisms to form microbial active materials with multiple complex functions. Based on the 3D printing of human cells in tissue engineering, 3D bioprinting technology has been developed. Although 3D bioprinting technology is still immature, it shows great potential in the environmental field. Due to the precise programming control and multi-printing pathway, 3D bioprinting technology provides a high-throughput method based on micron-level patterning for a wide range of environmental microbiological engineering applications, which makes it an on-demand, multi-functional manufacturing technology. To date, 3D bioprinting technology has been employed in microbial fuel cells, biofilm material preparation, microbial catalysts and 4D bioprinting with time dimension functions. Nevertheless, current 3D bioprinting technology faces technical challenges in improving the mechanical properties of materials, developing specific bioinks to adapt to different strains, and exploring 4D bioprinting for intelligent applications. Hence, this review systematically analyzes the basic technical principles of 3D bioprinting, bioinks materials and their applications in the environmental field, and proposes the challenges and future prospects of 3D bioprinting in the environmental field. Combined with the current development of microbial enhancement technology in the environmental field, 3D bioprinting will be developed into an enabling platform for multifunctional microorganisms and facilitate greater control of in situ directional reactions.

Synergistic coupling between 3D bioprinting and vascularization strategies

Three-dimensional (3D) bioprinting offers promising solutions to the complex challenge of vascularization in biofabrication, thereby enhancing the prospects for clinical translation of engineered tissues and organs. While existing reviews have touched upon 3D bioprinting in vascularized tissue contexts, the current review offers a more holistic perspective, encompassing recent technical advancements and spanning the entire multistage bioprinting process, with a particular emphasis on vascularization. The synergy between 3D bioprinting and vascularization strategies is crucial, as 3D bioprinting can enable the creation of personalized, tissue-specific vascular network while the vascularization enhances tissue viability and function. The review starts by providing a comprehensive overview of the entire bioprinting process, spanning from pre-bioprinting stages to post-printing processing, including perfusion and maturation. Next, recent advancements in vascularization strategies that can be seamlessly integrated with bioprinting are discussed. Further, tissue-specific examples illustrating how these vascularization approaches are customized for diverse anatomical tissues towards enhancing clinical relevance are discussed. Finally, the underexplored intraoperative bioprinting (IOB) was highlighted, which enables the direct reconstruction of tissues within defect sites, stressing on the possible synergy shaped by combining IOB with vascularization strategies for improved regeneration.

3D Printable Gelatin Methacryloyl (GelMA)-Dextran Aqueous Two-Phase System with Tunable Pores Structure and Size Enables Physiological Behavior of Embedded Cells In Vitro

The restricted porosity of most hydrogels established for in vitro 3D tissue engineering applications limits embedded cells with regard to their physiological spreading, proliferation, and migration behavior. To overcome these confines, porous hydrogels derived from aqueous two-phase systems (ATPS) are an interesting alternative. However, while developing hydrogels with trapped pores is widespread, the design of bicontinuous hydrogels is still challenging. Herein, an ATPS consisting of photo-crosslinkable gelatin methacryloyl (GelMA) and dextran is presented. The phase behavior, monophasic or biphasic, is tuned via the pH and dextran concentration. This, in turn, allows the formation of hydrogels with three distinct microstructures: homogenous nonporous, regular disconnected-pores, and bicontinuous with interconnected-pores. The pore size of the latter two hydrogels can be tuned from ≈4 to 100 µm. Cytocompatibility of the generated ATPS hydrogels is confirmed by testing the viability of stromal and tumor cells. Their distribution and growth pattern are cell-type specific but are also strongly defined by the microstructure of the hydrogel. Finally, it is demonstrated that the unique porous structure is sustained when processing the bicontinuous system by inkjet and microextrusion techniques. The proposed ATPS hydrogels hold great potential for 3D tissue engineering applications due to their unique tunable interconnected porosity.

Macro, Micro, and Nano-Inspired Bioactive Polymeric Biomaterials in Therapeutic, and Regenerative Orofacial Applications

Introducing dental polymers has accelerated biotechnological research, advancing tissue engineering, biomaterials development, and drug delivery. Polymers have been utilized effectively in dentistry to build dentures and orthodontic equipment and are key components in the composition of numerous restorative materials. Furthermore, dental polymers have the potential to be employed for medication administration and tissue regeneration. To analyze the influence of polymer-based investigations on practical medical trials, it is required to evaluate the research undertaken in this sector. The present review aims to gather evidence on polymer applications in dental, oral, and maxillofacial reconstruction.

How Hydrogel Stiffness Affects Adipogenic Differentiation of Mesenchymal Stem Cells under Controlled Morphology

Matrix stiffness has been disclosed as an essential regulator of cell fate. However, it is barely studied how the matrix stiffness affects stem cell functions when cell morphology changes. Thus, in this study, the effect of hydrogel stiffness on adipogenic differentiation of human bone-marrow-derived mesenchymal stem cells (hMSCs) with controlled morphology was investigated. Micropatterns of different size and elongation were prepared by a photolithographical micropatterning technique. The hMSCs were cultured on the micropatterns and showed a different spreading area and elongation following the geometry of the underlying micropatterns. The cells with controlled morphology were embedded in agarose hydrogels of different stiffnesses. The cells showed a different level of adipogenic differentiation that was dependent on both hydrogel stiffness and cell morphology. Adipogenic differentiation became strong when the cell spreading area decreased and hydrogel stiffness increased. Adipogenic differentiation did not change with cell elongation. Therefore, cell spreading area and hydrogel stiffness could synergistically affect adipogenic differentiation of hMSCs, while cell elongation did not affect adipogenic differentiation. A change of cell morphology and hydrogel stiffness was accompanied by actin filament alignment that was strongly related to adipogenic differentiation. The results indicated that cell morphology could affect cellular sensitivity to hydrogel stiffness. The results will provide useful information for the elucidation of the interaction of stem cells and their microenvironmental biomechanical cues.

3D Cell Models in Radiobiology: Improving the Predictive Value of In Vitro Research

Cancer is intrinsically complex, comprising both heterogeneous cellular composition and extracellular matrix. In vitro cancer research models have been widely used in the past to model and study cancer. Although two-dimensional (2D) cell culture models have traditionally been used for cancer research, they have many limitations, such as the disturbance of interactions between cellular and extracellular environments and changes in cell morphology, polarity, division mechanism, differentiation and cell motion. Moreover, 2D cell models are usually monotypic. This implies that 2D tumor models are ineffective at accurately recapitulating complex aspects of tumor cell growth, as well as their radiation responses. Over the past decade there has been significant uptake of three-dimensional (3D) in vitro models by cancer researchers, highlighting a complementary model for studies of radiation effects on tumors, especially in conjunction with chemotherapy. The introduction of 3D cell culture approaches aims to model in vivo tissue interactions with radiation by positioning itself halfway between 2D cell and animal models, and thus opening up new possibilities in the study of radiation response mechanisms of healthy and tumor tissues.

Current advancements in bio-ink technology for cartilage and bone tissue engineering

In tissue engineering, the fate of a particular organ/tissue regeneration and repair mainly depends on three pillars - 3D architecture, cells used, and stimulus provided. 3D cell supportive structure development is one of the crucial pillars necessary for defining organ/tissue geometry and shape. In recent years, the advancements in 3D bio-printing (additive manufacturing) made it possible to develop very precise 3D architectures with the help of industrial software like Computer-Aided Design (CAD). The main requirement for the 3D printing process is the bio-ink, which can act as a source for cell support, proliferation, drug (growth factors, stimulators) delivery, and organ/tissue shape. The selection of the bio-ink depends upon the type of 3D tissue of interest. Printing tissues like bone and cartilage is always challenging because it is difficult to find printable biomaterial that can act as bio-ink and mimic the strength of the natural bone and cartilage tissues. This review describes different biomaterials used to develop bio-inks with different processing variables and cell-seeding densities for bone and cartilage 3D printing applications. The review also discusses the advantages, limitations, and cell bio-ink compatibility in each biomaterial section. The emphasis is given to bio-inks reported for 3D printing cartilage and bone and their applications in orthopedics and orthodontists. The critical/important performance and the architectural morphology requirements of desired bone and cartilage bio-inks were compiled in summary.

Global hotspots and emerging trends in 3D bioprinting research

Three-dimensional (3D) bioprinting is an advanced tissue engineering technique that has received a lot of interest in the past years. We aimed to highlight the characteristics of articles on 3D bioprinting, especially in terms of research hotspots and focus. Publications related to 3D bioprinting from 2007 to 2022 were acquired from the Web of Science Core Collection database. We have used VOSviewer, CiteSpace, and R-bibliometrix to perform various analyses on 3,327 published articles. The number of annual publications is increasing globally, a trend expected to continue. The United States and China were the most productive countries with the closest cooperation and the most research and development investment funds in this field. Harvard Medical School and Tsinghua University are the top-ranked institutions in the United States and China, respectively. Dr. Anthony Atala and Dr. Ali Khademhosseini, the most productive researchers in 3D bioprinting, may provide cooperation opportunities for interested researchers. Tissue Engineering Part A contributed the largest publication number, while Frontiers in Bioengineering and Biotechnology was the most attractive journal with the most potential. As for the keywords in 3D bioprinting, Bio-ink, Hydrogels (especially GelMA and Gelatin), Scaffold (especially decellularized extracellular matrix), extrusion-based bioprinting, tissue engineering, and in vitro models (organoids particularly) are research hotspots analyzed in the current study. Specifically, the research topics "new bio-ink investigation," "modification of extrusion-based bioprinting for cell viability and vascularization," "application of 3D bioprinting in organoids and in vitro model" and "research in personalized and regenerative medicine" were predicted to be hotspots for future research.

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

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

Characterization of Biocompatibility of Functional Bioinks for 3D Bioprinting

Three-dimensional (3D) bioprinting with suitable bioinks has become a critical tool for fabricating 3D biomimetic complex structures mimicking physiological functions. While enormous efforts have been devoted to developing functional bioinks for 3D bioprinting, widely accepted bioinks have not yet been developed because they have to fulfill stringent requirements such as biocompatibility and printability simultaneously. To further advance our knowledge of the biocompatibility of bioinks, this review presents the evolving concept of the biocompatibility of bioinks and standardization efforts for biocompatibility characterization. This work also briefly reviews recent methodological advances in image analyses to characterize the biocompatibility of bioinks with regard to cell viability and cell-material interactions within 3D constructs. Finally, this review highlights a number of updated contemporary characterization technologies and future perspectives to further advance our understanding of the biocompatibility of functional bioinks for successful 3D bioprinting.

Out of Box Thinking to Tangible Science: A Benchmark History of 3D Bio-Printing in Regenerative Medicine and Tissues Engineering

Advancements and developments in the 3D bioprinting have been promising and have met the needs of organ transplantation. Current improvements in tissue engineering constructs have enhanced their applications in regenerative medicines and other medical fields. The synergistic effects of 3D bioprinting have brought technologies such as tissue engineering, microfluidics, integrated tissue organ printing, in vivo bioprinted tissue implants, artificial intelligence and machine learning approaches together. These have greatly impacted interventions in medical fields, such as medical implants, multi-organ-on-chip models, prosthetics, drug testing tissue constructs and much more. This technological leap has offered promising personalized solutions for patients with chronic diseases, and neurodegenerative disorders, and who have been in severe accidents. This review discussed the various standing printing methods, such as inkjet, extrusion, laser-assisted, digital light processing, and stereolithographic 3D bioprinter models, adopted for tissue constructs. Additionally, the properties of natural, synthetic, cell-laden, dECM-based, short peptides, nanocomposite and bioactive bioinks are briefly discussed. Sequels of several tissue-laden constructs such as skin, bone and cartilage, liver, kidney, smooth muscles, cardiac and neural tissues are briefly analyzed. Challenges, future perspectives and the impact of microfluidics in resolving the limitations in the field, along with 3D bioprinting, are discussed. Certainly, a technology gap still exists in the scaling up, industrialization and commercialization of this technology for the benefit of stakeholders.

3D Printability of Silk/Hydroxyapatite Composites for Microprosthetic Applications

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

Bibliometric and visualization analysis of stem cell therapy for meniscal regeneration from 2012 to 2022

Background: Meniscus injuries, a common joint disease caused by long-term wear, trauma and inflammation, usually cause chronic dysfunction and pain in the joint. Current clinical surgeries mainly aim to remove the diseased tissue to alleviate patient suffering instead of helping with meniscus regeneration. As an emerging treatment, stem cell therapy has been verified to facilitate meniscus regeneration effectively. The purpose of this study is to investigate the publication conditions of stem cell therapy for meniscal regeneration and to visualize the research trends and frontiers. Methods: Relevant publications relevant to stem cells for meniscal regeneration was retrieved SCI-Expanded of the Web of Science database from 2012 to 2022. Research trends in the field were analysed and visualized by CiteSpace and VOSviewer. Results: A total of 354 publications were collected and analysed. The United States contributed the largest number of publications (118, 34.104%). Tokyo Medical Dental University has contributed the largest number of publications (34) among all full-time institutions. Stem cell research therapy has published the largest number of researches on stem cells for meniscal regeneration (17). SEKIYA. I contributed the majority of publications in this field (31), while Horie, M was the most frequently cited authors (166). #1 tissue engineering, #2 articular cartilage, #3 anterior cruciate ligament, #4 regenerative medicine, #5 scaffold are the chief keywords. This indicates that the current research hotspot has been transformed from basic surgical research to tissue engineering. Conclusion: Stem cell therapy is a promising therapeutic method for meniscus regeneration. This is the first visualized and bibliometric study to thoroughly construct the development trends and knowledge structure in the research field of stem cell therapy for meniscal regeneration in the past 10 years. The results thoroughly summarize and visualize the research frontiers, which will shed light on the research direction of stem cell therapy for meniscal regeneration.

3D bioprinting and its innovative approach for biomedical applications

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

Bioprinting: From Technique to Application in Tissue Engineering and Regenerative Medicine

Among the different approaches present in regenerative medicine and tissue engineering, the one that has attracted the most interest in recent years is the possibility of printing functional biological tissues. Bioprinting is a technique that has been applied to create cellularized three-dimensional structures that mimic biological tissues and thus allow their replacement. Hydrogels are interesting materials for this type of technique. Hydrogels based on natural polymers are known due to their biocompatible properties, in addition to being attractive biomaterials for cell encapsulation. They provide a threedimensional aqueous environment with biologically relevant chemical and physical signals, mimicking the natural environment of the extracellular matrix (ECM). Bioinks are ink formulations that allow the printing of living cells. The controlled deposition of biomaterials by bioinks needs to maintain cell viability and offer specific biochemical and physical stimuli capable of guiding cell migration, proliferation, and differentiation. In this work, we analyze the theoretical and practical issues of bioprinting, citing currently used methods, their advantages, and limitations. We present some important molecules that have been used to compose bioinks, as well as the cellular responses that have been observed in different tissues. Finally, we indicate future perspectives of the method.

Alternative Methods as Tools for Obesity Research: In Vitro and In Silico Approaches

The study of adipogenesis is essential for understanding and treating obesity, a multifactorial problem related to body fat accumulation that leads to several life-threatening diseases, becoming one of the most critical public health problems worldwide. In this review, we propose to provide the highlights of the adipogenesis study based on in vitro differentiation of human mesenchymal stem cells (hMSCs). We list in silico methods, such as molecular docking for identification of molecular targets, and in vitro approaches, from 2D, more straightforward and applied for screening large libraries of substances, to more representative physiological models, such as 3D and bioprinting models. We also describe the development of physiological models based on microfluidic systems applied to investigate adipogenesis in vitro. We intend to identify the main alternative models for adipogenesis evaluation, contributing to the direction of preclinical research in obesity. Future directions indicate the association of in silico and in vitro techniques to bring a clear picture of alternative methods based on adipogenesis as a tool for obesity research.

Recent trends in bioartificial muscle engineering and their applications in cultured meat, biorobotic systems and biohybrid implants

Recent advances in tissue engineering and biofabrication technology have yielded a plethora of biological tissues. Among these, engineering of bioartificial muscle stands out for its exceptional versatility and its wide range of applications. From the food industry to the technology sector and medicine, the development of this tissue has the potential to affect many different industries at once. However, to date, the biofabrication of cultured meat, biorobotic systems, and bioartificial muscle implants are still considered in isolation by individual peer groups. To establish common ground and share advances, this review outlines application-specific requirements for muscle tissue generation and provides a comprehensive overview of commonly used biofabrication strategies and current application trends. By solving the individual challenges and merging various expertise, synergetic leaps of innovation that inspire each other can be expected in all three industries in the future.

Remodeling of Architected Mesenchymal Microtissues Generated on Mechanical Metamaterials

Mechanical metamaterials constitute a nascent category of architected structures comprising arranged periodic components with tailored geometrical features. These materials are now being employed as advanced medical implants due to their extraordinary mechanical properties over traditional devices. Nevertheless, to achieve desired tissue integration and regeneration, it is critical to study how the microarchitecture affects interactions between metamaterial scaffolds and living biological tissues. Based on human induced pluripotent stem cell technology and multiphoton lithography, we report the establishment of an in vitro microtissue model to study the integration and remodeling of human mesenchymal tissues on metamaterial scaffolds with different unit geometries. Microtissues showed distinct tissue morphologies and cellular behaviors between architected octet-truss and bowtie structures. Under the active force generated from mesenchymal tissues, the octet-truss and bowtie metamaterial scaffolds demonstrated unique instability phenomena, significantly different from uniform loading using conventional mechanical testing.

3D-bioprinting of aortic valve interstitial cells: impact of hydrogel and printing parameters on cell viability

Calcific aortic valve disease (CAVD) is a frequent cardiac pathology in the aging society. Although valvular interstitial cells (VICs) seem to play a crucial role, mechanisms of CAVD are not fully understood. Development of tissue-engineered cellular models by 3D-bioprinting may help to further investigate underlying mechanisms of CAVD. VIC were isolated from ovine aortic valves and cultured in Dulbecco's modified Eagle's Medium (DMEM). VIC of passages six to ten were dissolved in a hydrogel consisting of 2% alginate and 8% gelatin with a concentration of 2 × 10⁶VIC ml^-1. Cell-free and VIC-laden hydrogels were printed with an extrusion-based 3D-bioprinter (3D-Bioplotter^®Developer Series, EnvisionTec, Gladbeck, Germany), cross-linked and incubated for up to 28 d. Accuracy and durability of scaffolds was examined by microscopy and cell viability was tested by cell counting kit-8 assay and live/dead staining. 3D-bioprinting of scaffolds was most accurate with a printing pressure ofP< 400 hPa, nozzle speed ofv< 20 mm s^-1, hydrogel temperature ofT_H= 37 °C and platform temperature ofT_P= 5 °C in a 90° parallel line as well as in a honeycomb pattern. Dissolving the hydrogel components in DMEM increased VIC viability on day 21 by 2.5-fold compared to regular 0.5% saline-based hydrogels (p< 0.01). Examination at day 7 revealed dividing and proliferating cells. After 21 d the entire printed scaffolds were filled with proliferating cells. Live/dead cell viability/cytotoxicity staining confirmed beneficial effects of DMEM-based cell-laden VIC hydrogel scaffolds even 28 d after printing. By using low pressure printing methods, we were able to successfully culture cell-laden 3D-bioprinted VIC scaffolds for up to 28 d. Using DMEM-based hydrogels can significantly improve the long-term cell viability and overcome printing-related cell damage. Therefore, future applications 3D-bioprinting of VIC might enable the development of novel tissue engineered cellular 3D-models to examine mechanisms involved in initiation and progression of CAVD.

Tissues and organ printing: An evolution of technology and materials

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

Alginate-Based Composites for Corneal Regeneration: The Optimization of a Biomaterial to Overcome Its Limits

For many years, corneal transplantation has been the first-choice treatment for irreversible damage affecting the anterior part of the eye. However, the low number of cornea donors and cases of graft rejection highlighted the need to replace donor corneas with new biomaterials. Tissue engineering plays a fundamental role in achieving this goal through challenging research into a construct that must reflect all the properties of the cornea that are essential to ensure correct vision. In this review, the anatomy and physiology of the cornea are described to point out the main roles of the corneal layers to be compensated and all the requirements expected from the material to be manufactured. Then, a deep investigation of alginate as a suitable alternative to donor tissue was conducted. Thanks to its adaptability, transparency and low immunogenicity, alginate has emerged as a promising candidate for the realization of bioengineered materials for corneal regeneration. Chemical modifications and the blending of alginate with other functional compounds allow the control of its mechanical, degradation and cell-proliferation features, enabling it to go beyond its limits, improving its functionality in the field of corneal tissue engineering and regenerative medicine.

Bioink Formulation and Machine Learning-Empowered Bioprinting Optimization

Bioprinting enables the fabrication of complex, heterogeneous tissues through robotically-controlled placement of cells and biomaterials. It has been rapidly developing into a powerful and versatile tool for tissue engineering. Recent advances in bioprinting modalities and biofabrication strategies as well as new materials and chemistries have led to improved mimicry and development of physiologically relevant tissue architectures constituted with multiple cell types and heterogeneous spatial material properties. Machine learning (ML) has been applied to accelerate these processes. It is a new paradigm for bioprinting. In this review, we explore current trends in bioink formulation and how ML has been used to accelerate optimization and enable real-time error detection as well as to reduce the iterative steps necessary for bioink formulation. We examined how rheometric properties, including shear storage, loss moduli, viscosity, shear-thinning property of biomaterials affect the printability of a bioink. Furthermore, we scrutinized the interplays between yield shear stress and the printability of a bioink. Moreover, we systematically surveyed the application of ML in precision in situ surgical site bioprinting, closed-loop AI printing, and post-printing optimization.

Effects of Y-27632 on the osteogenic and adipogenic potential of human dental pulp stem cells in vitro

BACKGROUND

Human dental pulp stem cells (hDPSCs) possess mesenchymal stem cell properties, originating from migrating neural crest cells. hDPSCs have received extensive attention in the field of tissue engineering and regenerative medicine due to their accessibility and ability to differentiate in several cell phenotypes. In this study, we cultured hDPSCs with Y-27632 to observe their biological behaviors changes.

METHODS

The hDPSCs were separately cultured with Y-27632 (0, 0.156, 0.312, 0.625, 1.25, 2.50, 5, 10, 20, 40 μm) for 24, 48, 72 h to select the suitable concentration and time using CCK-8. Then, the hDPSCs were cultured with 2.50 μm Y-27632 for 48 h to analyzed the biological behaviors changes by 5-Ethynyl-2'-deoxyuridine (EdU), plate cloning, transwell, scratch, and Annexin V FITC/PI assays, separately. Additionally, osteogenic calcium nodules and lipid droplets were analyzed using alizarin red staining and oil red O staining, respectively. qRT-PCR was used to analyze the expression of osteogenesis, adipogenesis, stemness maintenance, and inflammation related genes.

RESULTS

The hDPSCs proliferation was significantly enhanced after cultured with 2.50 μm Y-27632 for 48 h, but there was no significant difference in migration and apoptosis. Observation of alkaline phosphatase (ALP) activity, osteogenic and adipogenic differentiation abilities of hDPSCs, Y-27632 treatment clearly decreased the ALP activity and osteogenic differentiation ability, increased the adipogenic differentiation ability. Furthermore, Y-27632 decreased the CD73, CD90, CD105, CD166, TLR4, and NF-κB p65 genes expression, but increased the IL-8 gene expression.

CONCLUSIONS

The biological behaviors of hDPSCs could be changed when they cultured with Y-27632.

3D Bioprinting of Pectin-Cellulose Nanofibers Multicomponent Bioinks

Pectin has found extensive interest in biomedical applications, including wound dressing, drug delivery, and cancer targeting. However, the low viscosity of pectin solutions hinders their applications in 3D bioprinting. Here, we developed multicomponent bioinks prepared by combining pectin with TEMPO-oxidized cellulose nanofibers (TOCNFs) to optimize the inks' printability while ensuring stability of the printed hydrogels and simultaneously print viable cell-laden inks. First, we screened several combinations of pectin (1%, 1.5%, 2%, and 2.5% w/v) and TOCNFs (0%, 0.5%, 1%, and 1.5% w/v) by testing their rheological properties and printability. Addition of TOCNFs allowed increasing the inks' viscosity while maintaining shear thinning rheological response, and it allowed us to identify the optimal pectin concentration (2.5% w/v). We then selected the optimal TOCNFs concentration (1% w/v) by evaluating the viability of cells embedded in the ink and eventually optimized the writing speed to be used to print accurate 3D grid structures. Bioinks were prepared by embedding L929 fibroblast cells in the ink printed by optimized printing parameters. The printed scaffolds were stable in a physiological-like environment and characterized by an elastic modulus of E = 1.8 ± 0.2 kPa. Cells loaded in the ink and printed were viable (cell viability >80%) and their metabolic activity increased in time during the in vitro culture, showing the potential use of the developed bioinks for biofabrication and tissue engineering applications.

Advances in Filament Structure of 3D Bioprinted Biodegradable Bone Repair Scaffolds

Conventional bone repair scaffolds can no longer meet the high standards and requirements of clinical applications in terms of preparation process and service performance. Studies have shown that the diversity of filament structures of implantable scaffolds is closely related to their overall properties (mechanical properties, degradation properties, and biological properties). To better elucidate the characteristics and advantages of different filament structures, this paper retrieves and summarizes the state of the art in the filament structure of the three-dimensional (3D) bioprinted biodegradable bone repair scaffolds, mainly including single-layer structure, double-layer structure, hollow structure, core-shell structure and bionic structures. The eximious performance of the novel scaffolds was discussed from different aspects (material composition, ink configuration, printing parameters, etc.). Besides, the additional functions of the current bone repair scaffold, such as chondrogenesis, angiogenesis, anti-bacteria, and anti-tumor, were also concluded. Finally, the paper prospects the future material selection, structural design, functional development, and performance optimization of bone repair scaffolds.

Extrusion-based 3D (Bio)Printed Tissue Engineering Scaffolds: Process-Structure-Quality Relationships

Biological additive manufacturing (Bio-AM) has emerged as a promising approach for the fabrication of biological scaffolds with nano- to microscale resolutions and biomimetic architectures beneficial to tissue engineering applications. However, Bio-AM processes tend to introduce flaws in the construct during fabrication. These flaws can be traced to material nonhomogeneity, suboptimal processing parameters, changes in the (bio)printing environment (such as nozzle clogs), and poor construct design, all with significant contributions to the alteration of a scaffold's mechanical properties. In addition, the biological response of endogenous and exogenous cells interacting with the defective scaffolds could become unpredictable. In this review, we first described extrusion-based Bio-AM. We highlighted the salient architectural and mechanotransduction parameters affecting the response of cells interfaced with the scaffolds. The process phenomena leading to defect formation and some of the tools for defect detection are reviewed. The limitations of the existing developments and the directions that the field should grow in order to overcome said limitations are discussed.

Fabricating Robust Constructs with Internal Phase Nanostructures via Liquid-in-Liquid 3D Printing

The ability to print soft materials into predefined architectures with programmable nanostructures and mechanical properties is a necessary requirement for creating synthetic biomaterials that mimic living tissues. However, the low viscosity of common materials and lack of required mechanical properties in the final product present an obstacle to the use of traditional additive manufacturing approaches. Here, a new liquid-in-liquid 3D printing approach is used to successfully fabricate constructs with internal nanostructures using in situ self-assembly during the extrusion of an aqueous solution containing surfactant and photocurable polymer into a stabilizing polar oil bath. Subsequent photopolymerization preserves the nanostructures created due to surfactant self-assembly at the immiscible liquid-liquid interface, which is confirmed by small-angle X-ray scattering. Mechanical properties of the photopolymerized prints are shown to be tunable based on constituent components of the aqueous solution. The reported 3D printing approach expands the range of low-viscosity materials that can be used in 3D printing, and enables robust constructs production with internal nanostructures and spatially defined features. The reported approach has broad applications in regenerative medicine by providing a platform to print self-assembling biomaterials into complex tissue mimics where internal supramolecular structures and their functionality control biological processes, similar to natural extracellular matrices.

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

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

Cell encapsulated and microenvironment modulating microbeads containing alginate hydrogel system for bone tissue engineering

Functional tissue regeneration using synthetic biomaterials requires proliferation and heterotypic differentiation of stem/progenitor cells within a specialized heterogeneous (biophysical-biochemical) microenvironment. The current techniques have limitations to develop synthetic hydrogels, mimicking native extracellular matrix porosity along with heterogeneous microenvironmental cues of matrix mechanics, degradability, microstructure and cell-cell interactions. Here, we have developed a microenvironment modulating system to fabricate in situ porous hydrogel matrix with two or more distinct tailored microenvironmental niches within microbeads and the hydrogel matrix for multicellular tissue regeneration. Electrosprayed pectin-gelatin blended microbeads and crosslinked alginate hydrogel system help to tailor microenvironmental niches of encapsulated cells where two different cells are surrounded by a specific microenvironment. The effect of different microenvironmental parameters associated with the microbead/hydrogel matrix was evaluated using human umbilical-cord mesenchymal stem cells (hUCMSCs). The osteogenic differentiation of hUCMSCs in the hydrogel matrix was evaluated for bone tissue regeneration. This will be the first report on microenvironment modulating microbead-hydrogel system to encapsulate two/more types of cells in a hydrogel, where each cell is surrounded with distinct niches for heterogeneous tissue regeneration.

Development of a chitosan and hyaluronic acid hydrogel with potential for bioprinting utilization: A preliminary study

Bioprinting is a technique that has been applied in the areas of tissue engineering and regenerative medicine (TERM). Natural polymer-based hydrogels are known for their favorable biocompatible properties, as well as attractive biomaterials for cell encapsulation. These hydrogels provide an aqueous three-dimensional environment with biologically relevant chemical and physical signals, mimicking the natural environment of the extracellular matrix (ECM). Chitosan (CHI) and hyaluronic acid (HA) have been widely researched for biomedical applications. Bioinks are "ink" formulations, usually hydrogels, that allow the printing of living cells. This work proposes the development of a low cost and simple chitosan CHI-AH hydrogel with potential to become a bioink. At physiological temperature, the biomaterials form a hydrogel. The material developed was characterized by the analysis of morphology, cytotoxicity, and cell viability. FTIR showed the characteristic vibrational bands of chitosan and HA. No difference in swelling was observed between the different formulations studied, although SEM showed architectural differences between the hydrogels obtained. Extract cytotoxicity testing showed that the hydrogel is not cytotoxic. The direct toxicity test also revealed the absence of toxicity, but the cells had difficulty migrating into the gel, probably because of its density. These data were confirmed by SEM. Further testing are ongoing to better understand the gel's characteristics to improve the limitations found so far.

Review of Bioprinting in Regenerative Medicine: Naturally Derived Bioinks and Stem Cells

Regenerative medicine offers the potential to repair or substitute defective tissues by constructing active tissues to address the scarcity and demands for transplantation. The method of forming 3D constructs made up of biomaterials, cells, and biomolecules is called bioprinting. Bioprinting of stem cells provides the ability to reliably recreate tissues, organs, and microenvironments to be used in regenerative medicine. 3D bioprinting is a technique that uses several biomaterials and cells to tailor a structure with clinically relevant geometries and sizes. This technique's promise is demonstrated by 3D bioprinted tissues, including skin, bone, cartilage, and cardiovascular, corneal, hepatic, and adipose tissues. Several bioprinting methods have been combined with stem cells to effectively produce tissue models, including adult stem cells, embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and differentiation techniques. In this review, technological challenges of printed stem cells using prevalent naturally derived bioinks (e.g., carbohydrate polymers and protein-based polymers, peptides, and decellularized extracellular matrix), recent advancements, leading companies, and clinical trials in the field of 3D bioprinting are delineated.

Recent trends in natural polysaccharide based bioinks for multiscale 3D printing in tissue regeneration: A review

Biofabrication by three-dimensional (3D) printing has been an attractive technology in harnessing the possibility to print anatomical shaped native tissues with controlled architecture and resolution. 3D printing offers the possibility to reproduce complex microarchitecture of native tissues by printing live cells in a layer by layer deposition to provide a biomimetic structural environment for tissue formation and host tissue integration. Plant based biomaterials derived from green and sustainable sources have represented to emulate native physicochemical and biological cues in order to direct specific cellular response and formation of new tissues through biomolecular recognition patterns. This comprehensive review aims to analyze and identify the most commonly used plant based bioinks for 3D printing applications. An overview on the role of different plant based biomaterial of terrestrial origin (Starch, Nanocellulose and Pectin) and marine origin (Ulvan, Alginate, Fucoidan, Agarose and Carrageenan) used for 3D printing applications are discussed elaborately. Furthermore, this review will also emphasis in the functional aspects of different 3D printers, appropriate printing material, merits and demerits of numerous plant based bioinks in developing 3D printed tissue-like constructs. Additionally, the underlying potential benefits, limitations and future perspectives of plant based bioinks for tissue engineering (TE) applications are also discussed.

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

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

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

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

3D-Printed Objects for Multipurpose Applications

3D printing is a popular nonconventional manufacturing technique used to print 3D objects by using conventional and nonconventional materials. The application and uses of 3D printing are rapidly increasing in each dimension of the engineering and medical sectors. This article overviews the multipurpose applications of 3D printing based on current research. In the beginning, various popular methods including fused deposition method, stereolithography 3D printing method, powder bed fusion method, digital light processing method, and metal transfer dynamic method used in 3D printing are discussed. Popular materials utilized randomly in printing techniques such as hydrogel, ABS, steel, silver, and epoxy are overviewed. Engineering applications under the current development of the printing technique which include electrode, 4D printing technique, twisting object, photosensitive polymer, and engines are focused. Printing of medical equipment including artificial tissues, scaffolds, bioprinted model, prostheses, surgical instruments, COVID-19, skull, and heart is of major focus. Characterization techniques of the printed 3D products are mentioned. In addition, potential challenges and future prospects are evaluated based on the current scenario. This review article will work as a masterpiece for the researchers interested to work in this field.

Biophysical and Biochemical Cues of Biomaterials Guide Mesenchymal Stem Cell Behaviors

Mesenchymal stem cells (MSCs) have been widely used in the fields of tissue engineering and regenerative medicine due to their self-renewal capabilities and multipotential differentiation assurance. However, capitalizing on specific factors to precisely guide MSC behaviors is the cornerstone of biomedical applications. Fortunately, several key biophysical and biochemical cues of biomaterials that can synergistically regulate cell behavior have paved the way for the development of cell-instructive biomaterials that serve as delivery vehicles for promoting MSC application prospects. Therefore, the identification of these cues in guiding MSC behavior, including cell migration, proliferation, and differentiation, may be of particular importance for better clinical performance. This review focuses on providing a comprehensive and systematic understanding of biophysical and biochemical cues, as well as the strategic engineering of these signals in current scaffold designs, and we believe that integrating biophysical and biochemical cues in next-generation biomaterials would potentially help functionally regulate MSCs for diverse applications in regenerative medicine and cell therapy in the future.

Collagen-Based Thiol-Norbornene Photoclick Bio-Ink with Excellent Bioactivity and Printability

As the essential foundation of bioprinting technology, cell-laden bio-ink is confronted with the inevitable contradiction between printability and bioactivity. For example, type I collagen has been widely applied for its excellent biocompatibility; however, its relatively low self-assembly speed restricts the performance in high-precision bioprinting of cell-laden structures. In this study, we synthesize norbornene-functionalized neutral soluble collagen (NorCol) by the reaction of acid-soluble collagen (Col) and carbic anhydride in the aqueous phase. NorCol retains collagen triple-helical conformation and can be quickly orthogonally cross-linked to build a cell-laden hydrogel via a cell-friendly thiol-ene photoclick reaction. Moreover, the additional carboxyl groups produced in the reaction of carbic anhydride and collagen obviously improve the solubility of NorCol in neutral buffer and miscibility of NorCol with other polymers such as alginate and gelatin. It enables hybrid bio-ink to respond to multiple stimuli, resulting in continuous cross-linked NorCol networks in hybrid hydrogels. For the first time, the collagen with a triple helix structure and gelatin can be mixed and printed, keeping the integrity of the printed construct after gelatin's dissolution. The molecular interaction among giant collagen molecules allows NorCol hydrogel formation at a low concentration, which leads to excellent cell spreading, migration, and proliferation. These properties give NorCol flexible formability and excellent biocompatibility in temperature-, ion-, and photo-based bioprinting. We speculate that NorCol is a promising bio-ink for emerging demands in tissue engineering, regenerative medicine, and personalized therapeutics.