A tailored three-dimensionally printable agarose-collagen blend allows encapsulation, spreading, and attachment of human umbilical artery smooth muscle cells

Advances in agar-based composites: A comprehensive review

This review article explores the developments and applications in agar-based composites (ABCs), emphasizing various constituents such as metals, clay/ceramic, graphene, and polymers across diversified fields like wastewater treatment, drug delivery, food packaging, the energy sector, biomedical engineering, bioplastics, agriculture, and cosmetics. The focus is on agar as a sustainable and versatile biodegradable polysaccharide, highlighting research that has advanced the technology of ABCs. A bibliometric analysis is conducted using the Web of Science database, covering publications from January 2020 to March 2024, processed through VOSviewer Software Version 1.6.2. This analysis assesses evolving trends and scopes in the literature, visualizing co-words and themes that underscore the growing importance and potential of ABCs in various applications. This review paper contributes by showcasing the existing state-of-the-art knowledge and motivating further development in this promising field.

3D-Printed Polymeric Biomaterials for Health Applications

3D printing, also known as additive manufacturing, holds immense potential for rapid prototyping and customized production of functional health-related devices. With advancements in polymer chemistry and biomedical engineering, polymeric biomaterials have become integral to 3D-printed biomedical applications. However, there still exists a bottleneck in the compatibility of polymeric biomaterials with different 3D printing methods, as well as intrinsic challenges such as limited printing resolution and rates. Therefore, this review aims to introduce the current state-of-the-art in 3D-printed functional polymeric health-related devices. It begins with an overview of the landscape of 3D printing techniques, followed by an examination of commonly used polymeric biomaterials. Subsequently, examples of 3D-printed biomedical devices are provided and classified into categories such as biosensors, bioactuators, soft robotics, energy storage systems, self-powered devices, and data science in bioplotting. The emphasis is on exploring the current capabilities of 3D printing in manufacturing polymeric biomaterials into desired geometries that facilitate device functionality and studying the reasons for material choice. Finally, an outlook with challenges and possible improvements in the near future is presented, projecting the contribution of general 3D printing and polymeric biomaterials in the field of healthcare.

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.

Biomaterials for extrusion-based bioprinting and biomedical applications

Amongst the range of bioprinting technologies currently available, bioprinting by material extrusion is gaining increasing popularity due to accessibility, low cost, and the absence of energy sources, such as lasers, which may significantly damage the cells. New applications of extrusion-based bioprinting are systematically emerging in the biomedical field in relation to tissue and organ fabrication. Extrusion-based bioprinting presents a series of specific challenges in relation to achievable resolutions, accuracy and speed. Resolution and accuracy in particular are of paramount importance for the realization of microstructures (for example, vascularization) within tissues and organs. Another major theme of research is cell survival and functional preservation, as extruded bioinks have cells subjected to considerable shear stresses as they travel through the extrusion apparatus. Here, an overview of the main available extrusion-based printing technologies and related families of bioprinting materials (bioinks) is provided. The main challenges related to achieving resolution and accuracy whilst assuring cell viability and function are discussed in relation to specific application contexts in the field of tissue and organ fabrication.

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

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

Ionic Strength Impacts the Physical Properties of Agarose Hydrogels

Agarose is a natural polysaccharide known for its ability to form thermoreversible hydrogels. While the effects of curing temperature and polysaccharide concentration on mechanical properties have been discussed in the literature, the role of ionic strength has been less studied. In the present manuscript, we investigate the effects of supporting salt concentration and the role of cation (i.e. Na+ or Li+, neighbors in the Hofmeister series), on the setting and performance of agarose hydrogels. Compressive and rheological measurements show that the supporting salts reduce the immediate elastic response of agarose hydrogels, with Li+ showing a stronger effect than Na+ at high ionic strength, while they significantly increase the extent of linear stress-strain response (i.e., linear elasticity). The presence of increasing amounts of added supporting salt also leads to a reduction in hysteresis during mechanical deformation due to loading and unloading cycles, which is more pronounced with Li+ than with Na+. The combination of rheological measurements and NMR relaxometry shows a mesh size in agarose hydrogels in the order of 6-17 nm, with a thickness of the water layer bound to the biopolymer of about 3 nm. Of note, the different structuring of the water within the hydrogel network due to the different alkali seems to play a role for the final performance of the hydrogels.

Controlled Quenching of Agarose Defines Hydrogels with Tunable Structural, Bulk Mechanical, Surface Nanomechanical, and Cell Response in 2D Cultures

The scaffolding of agarose hydrogel networks depends critically on the rate of cooling (quenching) after heating. Efforts are made to understand the kinetics and evolution of biopolymer self-assembly upon cooling, but information is lacking on whether quenching might affect the final hydrogel structure and performance. Here, a material strategy for the fine modulation of quenching that involves temperature-curing steps of agarose is reported. Combining microscopy techniques, standard and advanced macro/nanomechanical tools, it is revealed that agarose accumulates on the surface when the curing temperature is set at 121 °C. The inhomogeneity can be mostly recovered when it is reduced to 42 °C. This has a drastic effect on the stiffness of the surface, but not on the viscoelasticity, roughness, and wettability. When hydrogels are strained at small/large deformations, the curing temperature has no effect on the viscoelastic response of the hydrogel bulk but does play a role in the onset of the non-linear region. Cells cultured on these hydrogels exhibit surface stiffness-sensing that affects cell adhesion, spreading, F-actin fiber tension, and assembly of vinculin-rich focal adhesions. Collectively, the results indicate that the temperature curing of agarose is an efficient strategy to produce networks with tunable mechanics and is suitable for mechanobiology studies.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

Hydrogels with tailor-made swelling-shrinkable properties have aroused considerable interest in numerous biomedical domains. For example, as swelling is a key issue for blood and wound extrudates absorption, the transference of nutrients and metabolites, as well as drug diffusion and release, hydrogels with high swelling capacity have been widely applicated in full-thickness skin wound healing and tissue regeneration, and drug delivery. Nevertheless, in the fields of tissue adhesives and internal soft-tissue wound healing, and bioelectronics, non-swelling hydrogels play very important functions owing to their stable macroscopic dimension and physical performance in physiological environment. Moreover, the negative swelling behavior (i.e., shrinkage) of hydrogels can be exploited to drive noninvasive wound closure, and achieve resolution enhancement of hydrogel scaffolds. In addition, it can help push out the entrapped drugs, thus promote drug release. However, there still has not been a general review of the constructions and biomedical applications of hydrogels from the viewpoint of swelling-shrinkable properties. Therefore, this review summarizes the tactics employed so far in tailoring the swelling-shrinkable properties of hydrogels and their biomedical applications. And a relatively comprehensive understanding of the current progress and future challenge of the hydrogels with different swelling-shrinkable features is provided for potential clinical translations.

Vertebrate and Invertebrate Animal and New In Vitro Models for Studying Neisseria Biology

The history of Neisseria research has involved the use of a wide variety of vertebrate and invertebrate animal models, from insects to humans. In this review, we itemise these models and describe how they have made significant contributions to understanding the pathophysiology of Neisseria infections and to the development and testing of vaccines and antimicrobials. We also look ahead, briefly, to their potential replacement by complex in vitro cellular models.

Enhanced intradermal delivery of Dragon's blood in biocompatible nanosuspensions hydrogel patch for skin photoprotective effect

Dragon's Blood is a member of the Chinese medicinal herb, having anti-oxygen and anti-inflammatory activity for the photoprotective effect. However, the poor water solubility of raw Dragon's Blood powder has limited its intradermal delivery process. In this study, we evaluated nanosuspensions to enhance intradermal delivery of Dragon's Blood exerting a photoprotective effect. The prepared nanosuspension was added to a composite hydrogel patch matrix for better skin application. In the present research, we used biocompatible materials hyaluronic acid and amino acid surfactants as nanosuspension stabilizers and agar/gelatin/sodium polyacrylate as hydrogel patch matrix. The prepared Dragon's Blood nanosuspension had a particle size of 447.0 ± 48.6 nm. The micro-structures morphology and viscoelasticity characteristics by SEM and rheological testing confirmed a sufficient crosslinked hydrogel network. The skin retention amount of Dragon's Blood nanosuspension was 1.48 times of raw Dragon's Blood powder water suspension, and the skin penetration amount of Dragon's Blood nanosuspension was only about 1/3 of Dragon's Blood DMSO solution. In the UVB-irradiated HaCaT cell phototoxicity model, Dragon's Blood nanosuspension also significantly increased cell viability by about 1 time of the model group and decreased the production of reactive oxygen species about 1/2 times of model group. In vivo safety and efficiency evaluation experiment illustrated that DB-NS hydrogel patch processes have favorable safety and photoprotective effect with no skin irritancy and phototoxicity. Furthermore, DB-NS and DB-NS hydrogel patches could protect skin from UVA and UVB irritating skin reactions. Overall, our study of the combined use of biocompatible and biodegradable materials as excipients of nanosuspension and hydrogel patch could be used as an effective additive of Intradermal delivery and skin photoprotection.

Triphasic 3D In Vitro Model of Bone-Tendon-Muscle Interfaces to Study Their Regeneration

The transition areas between different tissues, known as tissue interfaces, have limited ability to regenerate after damage, which can lead to incomplete healing. Previous studies focussed on single interfaces, most commonly bone-tendon and bone-cartilage interfaces. Herein, we develop a 3D in vitro model to study the regeneration of the bone-tendon-muscle interface. The 3D model was prepared from collagen and agarose, with different concentrations of hydroxyapatite to graduate the tissues from bones to muscles, resulting in a stiffness gradient. This graduated structure was fabricated using indirect 3D printing to provide biologically relevant surface topographies. MG-63, human dermal fibroblasts, and Sket.4U cells were found suitable cell models for bones, tendons, and muscles, respectively. The biphasic and triphasic hydrogels composing the 3D model were shown to be suitable for cell growth. Cells were co-cultured on the 3D model for over 21 days before assessing cell proliferation, metabolic activity, viability, cytotoxicity, tissue-specific markers, and matrix deposition to determine interface formations. The studies were conducted in a newly developed growth chamber that allowed cell communication while the cell culture media was compartmentalised. The 3D model promoted cell viability, tissue-specific marker expression, and new matrix deposition over 21 days, thereby showing promise for the development of new interfaces.

Influence of the physico-chemical bioink composition on the printability and cell biological properties in 3D-bioprinting of a liver tumor cell line

The selection of a suitable matrix material is crucial for the development of functional, biomimetic tissue and organ models. When these tissue models are fabricated with 3D-bioprinting technology, the requirements do not only include the biological functionality and physico-chemical properties, but also the printability. In our work, we therefore present a detailed study of seven different bioinks with the focus on a functional liver carcinoma model. Agarose, gelatin, collagen and their blends were selected as materials based on their benefits for 3D cell culture and Drop-on-Demand (DoD) bioprinting. The formulations were characterized for their mechanical (G' of 10-350 Pa) and rheological (viscosity 2-200 Pa*s) properties as well as albumin diffusivity (8-50 μm²/s). The cellular behavior was exemplarily shown for HepG2 cells by monitoring viability, proliferation and morphology over 14 days, while the printability on a microvalve DoD printer was evaluated by drop volume monitoring in flight (100-250 nl), camera imaging of the wetting behavior and microscopy of the effective drop diameter (700 µm and more). We did not observe negative effects on cell viability or proliferation, which is due to the very low shear stresses inside the nozzle (200-500 Pa). With our method, we could identify the strengths and weaknesses of each material, resulting in a material portfolio. By specifically selecting certain materials or blends, cell migration and possible interaction with other cells can be directed as indicated by the results of our cellular experiments.

Addition of collagen type I in agarose created a dose-dependent effect on matrix production in engineered cartilage

Extracellular-matrix composition impacts mechanical performance in native and engineered tissues. Previous studies showed collagen type I-agarose blends increased cell-matrix interactions and extra-cellular matrix production. However, long-term impacts on protein production and mechanical properties of engineered cartilage are unknown. Our objective was to characterize the effect of collagen type I on matrix production of chondrocytes embedded in agarose hydrogels. We hypothesized that the addition of collagen would improve long-term mechanical properties and matrix production (e.g., collagen and glycosaminoglycans) through increased bioactivity. Agarose hydrogels (2% w/v) were mixed with varying concentrations of collagen type I (0, 2, 5 mg/mL). Juvenile bovine chondrocytes were added to the hydrogels to assess matrix production over 4 weeks through biochemical assays, and mechanical properties were assessed through unconfined compression. We observed a dose-dependent effect on cell bioactivity, where 2 mg/mL of collagen improved bioactivity, but 5 mg/mL had a negative impact on bioactivity. This resulted in higher modulus for scaffolds supplemented with lower collagen concentration as compared to the higher collagen concentration, but not when compared to the control. In conclusion, the addition of collagen to agarose constructs provided a dose-dependent impact on improving glycosaminoglycan production but did not improve collagen production or compressive mechanics.

Stem Cell-Laden Hydrogel-Based 3D Bioprinting for Bone and Cartilage Tissue Engineering

Tremendous advances in tissue engineering and regenerative medicine have revealed the potential of fabricating biomaterials to solve the dilemma of bone and articular defects by promoting osteochondral and cartilage regeneration. Three-dimensional (3D) bioprinting is an innovative fabrication technology to precisely distribute the cell-laden bioink for the construction of artificial tissues, demonstrating great prospect in bone and joint construction areas. With well controllable printability, biocompatibility, biodegradability, and mechanical properties, hydrogels have been emerging as an attractive 3D bioprinting material, which provides a favorable biomimetic microenvironment for cell adhesion, orientation, migration, proliferation, and differentiation. Stem cell-based therapy has been known as a promising approach in regenerative medicine; however, limitations arise from the uncontrollable proliferation, migration, and differentiation of the stem cells and fortunately could be improved after stem cells were encapsulated in the hydrogel. In this review, our focus was centered on the characterization and application of stem cell-laden hydrogel-based 3D bioprinting for bone and cartilage tissue engineering. We not only highlighted the effect of various kinds of hydrogels, stem cells, inorganic particles, and growth factors on chondrogenesis and osteogenesis but also outlined the relationship between biophysical properties like biocompatibility, biodegradability, osteoinductivity, and the regeneration of bone and cartilage. This study was invented to discuss the challenge we have been encountering, the recent progress we have achieved, and the future perspective we have proposed for in this field.

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

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

Effects of N-Acetyl-L-Cystein Antioxidant on Ex Vivo Culture of Vitrified Premature Mouse Ovarian Tissue

Optimization of practical ways to obtain mature follicles from cryopreserved ovarian tissues, especially in patients suffering from ovarian dysfunction, is very important. In vitro ovarian tissue culture allows faster screening of follicle development and reduces follicle isolation damage. During ovarian tissue culture, controlling oxidative stress is critical to support better follicular development and less damage. Immature Naval Medical Research Institute (NMRI) mouse ovaries (8-days-old) were randomly distributed into four cultured groups; non-vitrified, vitrified, non-vitrified N-acetyl-L-cysteine (NAC)⁺, and vitrified NAC⁺. Ovaries of vitrified groups along with non-vitrified ovaries were cultured on agar gel in the presence or absence of NAC for 5 days. Afterward, morphological evaluations, mRNA expressions of Gdf9, Bmp6, Lif, Amh, Bax, and Bcl2 genes, malondialdehyde, and total antioxidant capacities were compared between four groups at the first and last day of culture. Good preservation of tissue integrity and an increase of follicular development were observed in all groups. In addition, the expression of Gdf9, Lif, Bax, and Bcl2 genes were increased and Amh was decreased in groups cultured in the presence of NAC compared to groups cultured without NAC. Although total antioxidant capacity was not significantly different between the experimental groups, the lipid peroxidation and apoptotic index were significantly reduced in the presence of NAC. Thus, it appears that NAC antioxidant acts as a contributory factor for the ex vivo culture of ovarian tissue and reduces oxidative stress, apoptotic index, and improves follicular development, especially in non-vitrified groups.

3D or not 3D: a guide to assess cell viability in 3D cell systems

Cell viability is the primary integrative parameter used for various purposes, particularly when fabricating tissue equivalents (e.g., using bioprinting or scaffolding techniques), optimizing conditions to cultivate cells, testing chemicals, drugs, and biomaterials, etc. Most of the conventional methods were originally designed for a monolayer (2D) culture; however, 2D approaches fail to adequately assess a tissue-engineered construct's viability and drug effects and recapitulate the host-pathogen interactions and infectivity. This study aims at revealing the influence of particular 3D cell systems' parameters such as the components' concentration, gel thickness, cell density, etc. on the cell viability and applicability of standard assays. Here, we present an approach to achieving adequate and reproducible results on the cell viability in 3D collagen- and fibrin-based systems using the Live/Dead, AlamarBlue, and PicoGreen assays. Our results have demonstrated that a routine precise analysis of 3D systems should be performed using a combination of at least three methods based on different cell properties, e.g. the metabolic activity, proliferative capacity, morphology, etc.

An Overview on Recent Progress of the Hydrogels: From Material Resources, Properties to Functional Applications

Hydrogels, as the most typical elastomer materials with three-dimensional network structures, have attracted wide attention owing to their outstanding features in fields of sensitive stimulus response, low surface friction coefficient, good flexibility and bio-compatibility. Because of numerous fresh polymer materials (or polymerization monomers), hydrogels with various structure diversities and excellent properties are emerging, and the development of hydrogels is very vigorous over the past decade. This review focuses on state-of-the-art advances, systematically reviews the recent progress on construction of novel hydrogels utilized several kinds of typical polymerization monomers, and explores the main chemical and physical cross-linking methods to develop the diversity of hydrogels. Following the aspects mentioned above, the classification and emerging applications of hydrogels, such as pH response, ionic response, electrical response, thermal response, biomolecular response, and gas response, are extensively summarized. Finally, we have done this review with the promises and challenges for the future evolution of hydrogels and their biological applications. cross-linking methods; functional applications; hydrogels; material resources This article is protected by copyright. All rights reserved.

Biomaterial-based strategies for in vitro neural models

In vitro models have been used as a complementary tool to animal studies in understanding the nervous system's physiological mechanisms and pathological disorders, while also serving as platforms to evaluate the safety and efficiency of therapeutic candidates. Following recent advances in materials science, micro- and nanofabrication techniques and cell culture systems, in vitro technologies have been rapidly gaining the potential to bridge the gap between animal and clinical studies by providing more sophisticated models that recapitulate key aspects of the structure, biochemistry, biomechanics, and functions of human tissues. This was made possible, in large part, by the development of biomaterials that provide cells with physicochemical features that closely mimic the cellular microenvironment of native tissues. Due to the well-known material-driven cellular response and the importance of mimicking the environment of the target tissue, the selection of optimal biomaterials represents an important early step in the design of biomimetic systems to investigate brain structures and functions. This review provides a comprehensive compendium of commonly used biomaterials as well as the different fabrication techniques employed for the design of neural tissue models. Furthermore, the authors discuss the main parameters that need to be considered to develop functional platforms not only for the study of brain physiological functions and pathological processes but also for drug discovery/development and the optimization of biomaterials for neural tissue engineering.

Biomaterials from the sea: Future building blocks for biomedical applications

Marine resources have tremendous potential for developing high-value biomaterials. The last decade has seen an increasing number of biomaterials that originate from marine organisms. This field is rapidly evolving. Marine biomaterials experience several periods of discovery and development ranging from coralline bone graft to polysaccharide-based biomaterials. The latter are represented by chitin and chitosan, marine-derived collagen, and composites of different organisms of marine origin. The diversity of marine natural products, their properties and applications are discussed thoroughly in the present review. These materials are easily available and possess excellent biocompatibility, biodegradability and potent bioactive characteristics. Important applications of marine biomaterials include medical applications, antimicrobial agents, drug delivery agents, anticoagulants, rehabilitation of diseases such as cardiovascular diseases, bone diseases and diabetes, as well as comestible, cosmetic and industrial applications.

Epithelial-stromal cell interactions and extracellular matrix mechanics drive the formation of airway-mimetic tubular morphology in lung organoids

Complex human airway cellular organization where extracellular matrix (ECM) and epithelial and stromal lineages interact present challenges for organ study in vitro. Current in vitro lung models that focus on the lung epithelium do not represent complex airway morphology and cell-ECM interactions seen in vivo. Models including stromal populations often separate them via a semipermeable barrier precluding cell–cell interaction or the effect of ECM mechanics. We investigated the effect of stromal cells on basal epithelial cell-derived bronchosphere structure and function through a triple culture of human bronchial epithelial, lung fibroblast, and airway smooth muscle cells. Epithelial–stromal cross-talk resulted in epithelial cell-driven branching tubules with stromal cells surrounding epithelial cells termed bronchotubules. Agarose– Matrigel scaffold (Agrigel) formed a mechanically tuneable ECM, with adjustable viscoelasticity and stiffness enabling long-term tubule survival. Bronchotubule models may enable research into how epithelial–stromal cell and cell–ECM communication drive tissue patterning, repair, and development of disease.

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Healthy lung epithelial and fibroblast cell coculture in Matrigel forms tubules

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Tubules collapse in 4 days

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Addition of healthy airway smooth muscle cells allows for a contractile phenotype

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Triple culture in stiffer matrix maintains tubular organoid structure for 20 days

A biofabrication method to align cells within bioprinted photocrosslinkable and cell-degradable hydrogel constructs via embedded fibers

The extracellular matrix (ECM) is composed of biochemical and biophysical cues that control cell behaviors and bulk mechanical properties. For example, anisotropy of the ECM and cell alignment are essential in the directional properties of tissues such as myocardium, tendon, and the knee meniscus. Technologies are needed to introduce anisotropic behavior into biomaterial constructs that can be used for the engineering of tissues as models and towards translational therapies. To address this, we developed an approach to align hydrogel fibers within cell-degradable bioink filaments with extrusion printing, where shear stresses during printing align fibers and photocrosslinking stabilizes the fiber orientation. Suspensions of hydrogel fibers were produced through the mechanical fragmentation of electrospun scaffolds of norbornene-modified hyaluronic acid, which were then encapsulated with meniscal fibrochondrocytes, mesenchymal stromal cells, or cardiac fibroblasts within gelatin-methacrylamide bioinks during extrusion printing into agarose suspension baths. Bioprinting parameters such as the needle diameter and the bioink flow rate influenced shear profiles, whereas the suspension bath properties and needle translation speed influenced filament diameters and uniformity. When optimized, filaments were formed with high levels of fiber alignment, which resulted in directional cell spreading during culture over one week. Controls that included bioprinted filaments without fibers or non-printed hydrogels of the same compositions either with or without fibers resulted in random cell spreading during culture. Further, constructs were printed with variable fiber and resulting cell alignment by varying print direction or using multi-material printing with and without fibers. This biofabrication technology advances our ability to fabricate constructs containing aligned cells towards tissue repair and the development of physiological tissue models.

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.

FRESH bioprinting technology for tissue engineering - the influence of printing process and bioink composition on cell behavior and vascularization

The rapid and tailored biofabrication of natural materials is of high interest for the field of tissue engineering and regenerative medicine. Scaffolds require both high biocompatibility and tissue-dependent mechanical strength to function as basis for tissue-engineered implants. Thus, natural hydrogels such as fibrin are promising but their rapid biofabrication remains challenging. Printing of low viscosity and slow polymerizing solutions with good spatial resolution can be achieved by freeform reversible embedding of suspended hydrogels (FRESH) bioprinting of cell-laden natural hydrogels. In this study, fibrin and hyaluronic acid were used as single components as well as blended ink mixtures for the FRESH bioprinting. Rheometry revealed that single materials were less viscous than the blended bioink showing higher values for viscosity over a shear rate of 10-1000 s^-1. While fibrin showed viscosities between 0.1624 and 0.0017 Pa·s, the blended ink containing fibrin and hyaluronic acid were found to be in a range of 0.1-1 Pa·s. In 3D vascularization assays, formation of vascular structures within the printed constructs was investigated indicating that the printing process did not harm cells and allowed formation of vasculature comparable to moulded control samples. Best values for vascularization were achieved in bioinks consisting of 1.0% fibrin-0.5% hyaluronic acid. The vascular structure area and length were three times higher compared to other tested bioinks, and structure volume as well as number of branches revealed almost four times higher values. In this study, we combined the benefits of the FRESH printing technique with in vitro vascularization, showing that it is possible to achieve a mechanically stable small-scale hydrogel construct incorporating vascular network formation.

A facile, versatile hydrogel bioink for 3D bioprinting benefits long-term subaqueous fidelity, cell viability and proliferation

Both of the long-term fidelity and cell viability of three-dimensional (3D)-bioprinted constructs are essential to precise soft tissue repair. However, the shrinking/swelling behavior of hydrogels brings about inadequate long-term fidelity of constructs, and bioinks containing excessive polymer are detrimental to cell viability. Here, we obtained a facile hydrogel by introducing 1% aldehyde hyaluronic acid (AHA) and 0.375% N-carboxymethyl chitosan (CMC), two polysaccharides with strong water absorption and water retention capacity, into classic gelatin (GEL, 5%)-alginate (ALG, 1%) ink. This GEL-ALG/CMC/AHA bioink possesses weak temperature dependence due to the Schiff base linkage of CMC/AHA and electrostatic interaction of CMC/ALG. We fabricated integrated constructs through traditional printing at room temperature and in vivo simulation printing at 37°C. The printed cell-laden constructs can maintain subaqueous fidelity for 30 days after being reinforced by 3% calcium chloride for only 20 s. Flow cytometry results showed that the cell viability was 91.38 ± 1.55% on day 29, and the cells in the proliferation plateau at this time still maintained their dynamic renewal with a DNA replication rate of 6.06 ± 1.24%. This work provides a convenient and practical bioink option for 3D bioprinting in precise soft tissue repair.

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.

3-Dimensional Bioprinting of Cardiovascular Tissues: Emerging Technology

Three-dimensional (3D) bioprinting may overcome challenges in tissue engineering. Unlike conventional tissue engineering approaches, 3D bioprinting has a proven ability to support vascularization of larger scale constructs and has been used for several cardiovascular applications. An overview of 3D bioprinting techniques, in vivo translation, and challenges are described.

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 tumor model – a platform for anticancer drug development

While still attractive, the currently available 2D cell culture models present several limitations and if possible should be supplemented with their 3D counterparts, that is with spheroids/organoids or bio-printed structures. Those alternatives can sometimes show widely different results compared to the simpler 2D cell culture, especially during cytotoxicity testing that is often used during cancer drug development and in the rising field of personalized medicine. Although some of the methods like spheroid formation and basic alginate based bio-prints were already available for years, they still require huge amounts of optimization and troubleshooting to be used effectively. Proficient use of dedicated tools and software can help to overcome some of the difficulties associated with those seemingly well described models. In this article we compare the most popular and currently available methods of acquiring 3D bio-models while describing their limitations and shortcomings as well as technical hurdles that one has to overcome to succeed in the use of this complex model.

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

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

Current Trends in In Vitro Modeling to Mimic Cellular Crosstalk in Periodontal Tissue

Clinical evidence indicates that in physiological and therapeutic conditions a continuous remodeling of the tooth root cementum and the periodontal apparatus is required to maintain tissue strength, to prevent damage, and to secure teeth anchorage. Within the tooth's surrounding tissues, tooth root cementum and the periodontal ligament are the key regulators of a functional tissue homeostasis. While the root cementum anchors the periodontal fibers to the tooth root, the periodontal ligament itself is the key regulator of tissue resorption, the remodeling process, and mechanical signal transduction. Thus, a balanced crosstalk of both tissues is mandatory for maintaining the homeostasis of this complex system. However, the mechanobiological mechanisms that shape the remodeling process and the interaction between the tissues are largely unknown. In recent years, numerous 2D and 3D in vitro models have sought to mimic the physiological and pathophysiological conditions of periodontal tissue. They have been proposed to unravel the underlying nature of the cell-cell and the cell-extracellular matrix interactions. The present review provides an overview of recent in vitro models and relevant biomaterials used to enhance the understanding of periodontal crosstalk and aims to provide a scientific basis for advanced regenerative strategies.

3D bioprinting for skin tissue engineering: Current status and perspectives

Skin and skin appendages are vulnerable to injury, requiring rapidly reliable regeneration methods. In recent years, 3D bioprinting has shown potential for wound repair and regeneration. 3D bioprinting can be customized for skin shape with cells and other materials distributed precisely, achieving rapid and reliable production of bionic skin substitutes, therefore, meeting clinical and industrial requirements. Additionally, it has excellent performance with high resolution, flexibility, reproducibility, and high throughput, showing great potential for the fabrication of tissue-engineered skin. This review introduces the common techniques of 3D bioprinting and their application in skin tissue engineering, focusing on the latest research progress in skin appendages (hair follicles and sweat glands) and vascularization, and summarizes current challenges and future development of 3D skin printing.

Biosynthetic, biomimetic, and self-assembled vascularized Organ-on-a-Chip systems

Organ-on-a-Chip (OOC) devices have seen major advances in the last years with respect to biological complexity, physiological composition and biomedical relevance. In this context, integration of vasculature has proven to be a crucial element for long-term culture of thick tissue samples as well as for realistic pharmacokinetic, toxicity and metabolic modelling. With the emergence of digital production technologies and the reinvention of existing tools, a multitude of design approaches for guided angio- and vasculogenesis is available today. The underlying production methods can be categorized into biosynthetic, biomimetic and self-assembled vasculature formation. The diversity and importance of production approaches, vascularization strategies as well as biomaterials and cell sourcing are illustrated in this work. A comprehensive technological review with a strong focus on the challenge of producing physiologically relevant vascular structures is given. Finally, the remaining obstacles and opportunities in the development of vascularized Organ-on-a-Chip platforms for advancing drug development and predictive disease modelling are noted.

Bioprinting: From Tissue and Organ Development to in Vitro Models

Bioprinting techniques have been flourishing in the field of biofabrication with pronounced and exponential developments in the past years. Novel biomaterial inks used for the formation of bioinks have been developed, allowing the manufacturing of in vitro models and implants tested preclinically with a certain degree of success. Furthermore, incredible advances in cell biology, namely, in pluripotent stem cells, have also contributed to the latest milestones where more relevant tissues or organ-like constructs with a certain degree of functionality can already be obtained. These incredible strides have been possible with a multitude of multidisciplinary teams around the world, working to make bioprinted tissues and organs more relevant and functional. Yet, there is still a long way to go until these biofabricated constructs will be able to reach the clinics. In this review, we summarize the main bioprinting activities linking them to tissue and organ development and physiology. Most bioprinting approaches focus on mimicking fully matured tissues. Future bioprinting strategies might pursue earlier developmental stages of tissues and organs. The continuous convergence of the experts in the fields of material sciences, cell biology, engineering, and many other disciplines will gradually allow us to overcome the barriers identified on the demanding path toward manufacturing and adoption of tissue and organ replacements.

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.

Physical and Chemical Factors Influencing the Printability of Hydrogel-based Extrusion Bioinks

Bioprinting researchers agree that "printability" is a key characteristic for bioink development, but neither the meaning of the term nor the best way to experimentally measure it has been established. Furthermore, little is known with respect to the underlying mechanisms which determine a bioink's printability. A thorough understanding of these mechanisms is key to the intentional design of new bioinks. For the purposes of this review, the domain of printability is defined as the bioink requirements which are unique to bioprinting and occur during the printing process. Within this domain, the different aspects of printability and the factors which influence them are reviewed. The extrudability, filament classification, shape fidelity, and printing accuracy of bioinks are examined in detail with respect to their rheological properties, chemical structure, and printing parameters. These relationships are discussed and areas where further research is needed, are identified. This review serves to aid the bioink development process, which will continue to play a major role in the successes and failures of bioprinting, tissue engineering, and regenerative medicine going forward.

Warp-Knitted Spacer Fabrics: A Versatile Platform to Generate Fiber-Reinforced Hydrogels for 3D Tissue Engineering

Mesenchymal stem cells (MSCs) possess huge potential for regenerative medicine. For tissue engineering approaches, scaffolds and hydrogels are routinely used as extracellular matrix (ECM) carriers. The present study investigated the feasibility of using textile-reinforced hydrogels with adjustable porosity and elasticity as a versatile platform for soft tissue engineering. A warp-knitted poly (ethylene terephthalate) (PET) scaffold was developed and characterized with respect to morphology, porosity, and mechanics. The textile carrier was infiltrated with hydrogels and cells resulting in a fiber-reinforced matrix with adjustable biological as well as mechanical cues. Finally, the potential of this platform technology for regenerative medicine was tested on the example of fat tissue engineering. MSCs were seeded on the construct and exposed to adipogenic differentiation medium. Cell invasion was detected by two-photon microscopy, proliferation was measured by the PrestoBlue assay. Successful adipogenesis was demonstrated using Oil Red O staining as well as measurement of secreted adipokines. In conclusion, the given microenvironment featured optimal mechanical as well as biological properties for proliferation and differentiation of MSCs. Besides fat tissue, the textile-reinforced hydrogel system with adjustable mechanics could be a promising platform for future fabrication of versatile soft tissues, such as cartilage, tendon, or muscle.

Three-dimensional printing of extracellular matrix (ECM)-mimicking scaffolds: A critical review of the current ECM materials

The loss of tissues and organs through injury and disease has stimulated the development of therapeutics that have the potential to regenerate and replace the affected tissue. Such therapeutics have the benefit of reducing the reliance and demand for life-saving organ transplants. Of the several regenerative strategies, 3D printing has emerged as the forerunner in regenerative attempts due to the fact that biologically and anatomically correct 3D structures can be fabricated according to the specified need. Despite the progress in this field, improvement is still limited by the difficulty in fabricating scaffolds that adequately mimic the native cellular microenvironment. In response, despite the complexities of the native extracellular matrix (ECM), the inclusion of ECM components into bioinks has emerged as a cutting-edge research area in terms of providing possible ECM-mimicking abilities of the 3D printed constructs. Furthermore, the development of ECM-mimicking scaffolds can potentially assist in improving personalized patient treatments. This review provides a critical analysis of selected naturally occurring ECM components as well as synthetic self-assembling peptides in their ability to provide the required ECM microenvironment for tissue regeneration. The success and possible short comings of each material, as well as the specific characteristics of each bioink, are evaluated.

Hydrogels for Bioprinting: A Systematic Review of Hydrogels Synthesis, Bioprinting Parameters, and Bioprinted Structures Behavior

Nowadays, bioprinting is rapidly evolving and hydrogels are a key component for its success. In this sense, synthesis of hydrogels, as well as bioprinting process, and cross-linking of bioinks represent different challenges for the scientific community. A set of unified criteria and a common framework are missing, so multidisciplinary research teams might not efficiently share the advances and limitations of bioprinting. Although multiple combinations of materials and proportions have been used for several applications, it is still unclear the relationship between good printability of hydrogels and better medical/clinical behavior of bioprinted structures. For this reason, a PRISMA methodology was conducted in this review. Thus, 1,774 papers were retrieved from PUBMED, WOS, and SCOPUS databases. After selection, 118 papers were analyzed to extract information about materials, hydrogel synthesis, bioprinting process, and tests performed on bioprinted structures. The aim of this systematic review is to analyze materials used and their influence on the bioprinting parameters that ultimately generate tridimensional structures. Furthermore, a comparison of mechanical and cellular behavior of those bioprinted structures is presented. Finally, some conclusions and recommendations are exposed to improve reproducibility and facilitate a fair comparison of results.

Advances in Extrusion 3D Bioprinting: A Focus on Multicomponent Hydrogel-Based Bioinks

3D bioprinting involves the combination of 3D printing technologies with cells, growth factors and biomaterials, and has been considered as one of the most advanced tools for tissue engineering and regenerative medicine (TERM). However, despite multiple breakthroughs, it is evident that numerous challenges need to be overcome before 3D bioprinting will eventually become a clinical solution for a variety of TERM applications. To produce a 3D structure that is biologically functional, cell-laden bioinks must be optimized to meet certain key characteristics including rheological properties, physico-mechanical properties, and biofunctionality; a difficult task for a single component bioink especially for extrusion based bioprinting. As such, more recent research has been centred on multicomponent bioinks consisting of a combination of two or more biomaterials to improve printability, shape fidelity and biofunctionality. In this article, multicomponent hydrogel-based bioink systems are systemically reviewed based on the inherent nature of the bioink (natural or synthetic hydrogels), including the most current examples demonstrating properties and advances in application of multicomponent bioinks, specifically for extrusion based 3D bioprinting. This review article will assist researchers in the field in identifying the most suitable bioink based on their requirements, as well as pinpointing current unmet challenges in the field.

Tissue mimetic hyaluronan bioink containing collagen fibers with controlled orientation modulating cell migration and alignment

Biofabrication is providing scientists and clinicians the ability to produce engineered tissues with desired shapes and gradients of composition and biological cues. Typical resolutions achieved with extrusion-based bioprinting are at the macroscopic level. However, for capturing the fibrillar nature of the extracellular matrix (ECM), it is necessary to arrange ECM components at smaller scales, down to the micron and the molecular level. Herein, we introduce a bioink containing the tyramine derivative of hyaluronan (HA; henceforth known as THA) and collagen (Col) type 1. In this bioink, similar to connective tissues, Col is present in the fibrillar form, and HA functions as a viscoelastic space filler. THA was enzymatically cross-linked under mild conditions allowing simultaneous Col fibrillogenesis, thus achieving a homogeneous distribution of Col fibrils within the viscoelastic HA-based matrix. The THA-Col composite displayed synergistic properties in terms of storage modulus and shear thinning, translating into good printability. Shear-induced alignment of the Col fibrils along the printing direction was achieved and quantified via immunofluorescence and second-harmonic generation. Cell-free and cell-laden constructs were printed and characterized, analyzing the influence of the controlled microscopic anisotropy on human bone marrow-derived mesenchymal stromal cell (hMSC) migration. Anisotropic HA-Col showed cell-instructive properties modulating hMSC adhesion, morphology, and migration from micropellets stimulated by the presence and the orientation of Col fibers. Actin filament staining showed that hMSCs embedded in aligned constructs displayed increased cytoskeleton alignment along the fibril direction. Based on gene expression of cartilage/bone markers and ECM production, hMSCs embedded in the isotropic bioink displayed chondrogenic differentiation comparable with standard pellet culture by means of proteoglycan production (safranin O staining and proteoglycan quantification). The possibility of printing matrix components with control over microscopic alignment brings biofabrication one step closer to capturing the complexity of native tissues.

Hand-held bioprinting for de novo vascular formation applicable to dental pulp regeneration

Aim of the study: Deep carious lesions may cause irreversible pulpitis and the current endodontic treatment typically removes the whole dental pulp tissue, which finally reduces lifespan of the teeth. Nowadays, the most frequent treatment is based on removing the infected tissue and filling the root canal with inert synthetic materials. Tissue engineering approaches are important alternatives to the current treatment, because they can potentially maintain the biological function of the tooth instead of sacrificing it.Materials and Methods: In this study, we propose a tissue engineering approach based on a hand-held in situ bioprinting strategy. Our approach enabled bioprinting of cell-loaded collagen-based bioinks with suitable rheological, structural and biological properties, which allowed for vasculogenesis in the root canal.Results: The rheological properties of the bioprintable bioink were measured by oscillatory amplitude sweep testing and were corroborated by macroscopic evaluation after in vitro culture, in which printed bioinks maintained their original form without contraction. Moreover, we showed evidence for successful vasculogenesis in bioprintable bioinks with comparable quality and quantity to control fibrin and collagen non-bioprintable hydrogels.Conclusions: We conclude that hand-held bioprinting holds potential for in situ treatment of dental diseases with successful evidence for vascular tube formation, as an asset for maintenance of the biological function of the tooth.

Assessment methodologies for extrusion-based bioink printability

Extrusion-based bioprinting is one of the leading manufacturing techniques for tissue engineering and regenerative medicine. Its primary limitation is the lack of materials, known as bioinks, which are suitable for the bioprinting process. The degree to which a bioink is suitable for bioprinting has been described as its 'printability.' However, a lack of clarity surrounding the methodologies used to evaluate a bioink's printability, as well as the usage of the term itself, have hindered the field. This article presents a review of measures used to assess the printability of extrusion-based bioinks in an attempt to assist researchers during the bioink development process. Many different aspects of printability exist and many different measurements have been proposed as a consequence. Researchers often do not evaluate a new bioink's printability at all, while others simply do so qualitatively. Several quantitative measures have been presented for the extrudability, shape fidelity, and printing accuracy of bioinks. Different measures have been developed even within these aspects, each testing the bioink in a slightly different way. Additionally, other relevant measures which had little or no examples of quantifiable methods are also to be considered. Looking forward, further work is needed to improve upon current assessment methodologies, to move towards a more comprehensive view of printability, and to standardize these printability measurements between researchers. Better assessment techniques will naturally lead to a better understanding of the underlying mechanisms which affect printability and better comparisons between bioinks. This in turn will help improve upon the bioink development process and the bioinks available for use in bioprinting.