Characterization of cell viability during bioprinting processes

3D neural tissue models: From spheroids to bioprinting

Three-dimensional (3D) in vitro neural tissue models provide a better recapitulation of in vivo cell-cell and cell-extracellular matrix interactions than conventional two-dimensional (2D) cultures. Therefore, the former is believed to have great potential for both mechanistic and translational studies. In this paper, we review the recent developments in 3D in vitro neural tissue models, with a particular focus on the emerging bioprinted tissue structures. We draw on specific examples to describe the merits and limitations of each model, in terms of different applications. Bioprinting offers a revolutionary approach for constructing repeatable and controllable 3D in vitro neural tissues with diverse cell types, complex microscale features and tissue level responses. Further advances in bioprinting research would likely consolidate existing models and generate complex neural tissue structures bearing higher fidelity, which is ultimately useful for probing disease-specific mechanisms, facilitating development of novel therapeutics and promoting neural regeneration.

3D Bioprinting for Cartilage and Osteochondral Tissue Engineering

Significant progress has been made in the field of cartilage and bone tissue engineering over the last two decades. As a result, there is real promise that strategies to regenerate rather than replace damaged or diseased bones and joints will one day reach the clinic however, a number of major challenges must still be addressed before this becomes a reality. These include vascularization in the context of large bone defect repair, engineering complex gradients for bone-soft tissue interface regeneration and recapitulating the stratified zonal architecture present in many adult tissues such as articular cartilage. Tissue engineered constructs typically lack such spatial complexity in cell types and tissue organization, which may explain their relatively limited success to date. This has led to increased interest in bioprinting technologies in the field of musculoskeletal tissue engineering. The additive, layer by layer nature of such biofabrication strategies makes it possible to generate zonal distributions of cells, matrix and bioactive cues in 3D. The adoption of biofabrication technology in musculoskeletal tissue engineering may therefore make it possible to produce the next generation of biological implants capable of treating a range of conditions. Here, advances in bioprinting for cartilage and osteochondral tissue engineering are reviewed.

Bioinks for bioprinting functional meniscus and articular cartilage

3D bioprinting can potentially enable the engineering of biological constructs mimicking the complex geometry, composition, architecture and mechanical properties of different tissues and organs. Integral to the successful bioprinting of functional articular cartilage and meniscus is the identification of suitable bioinks and cell sources to support chondrogenesis or fibrochondrogenesis, respectively. Such bioinks must also possess the appropriate rheological properties to be printable and support the generation of complex geometries. This review will outline the parameters required to develop bioinks for such applications and the current recent advances in 3D bioprinting of functional meniscus and articular cartilage. The paper will conclude by discussing key scientific and technical hurdles in this field and by defining future research directions for cartilage and meniscus bioprinting.

Additive Manufacturing of Vascular Grafts and Vascularized Tissue Constructs

There is a great need for engineered vascular grafts among patients with cardiovascular diseases who are in need of bypass therapy and lack autologous healthy blood vessels. In addition, because of the severe worldwide shortage of organ donors, there is an increasing need for engineered vascularized tissue constructs as an alternative to organ transplants. Additive manufacturing (AM) offers great advantages and flexibility of fabrication of cell-laden, multimaterial, and anatomically shaped vascular grafts and vascularized tissue constructs. Various inkjet-, extrusion-, and photocrosslinking-based AM techniques have been applied to the fabrication of both self-standing vascular grafts and porous, vascularized tissue constructs. This review discusses the state-of-the-art research on the use of AM for vascular applications and the key criteria for biomaterials in the AM of both acellular and cellular constructs. We envision that new smart printing materials that can adapt to their environment and encourage rapid endothelialization and remodeling will be the key factor in the future for the successful AM of personalized and dynamic vascular tissue applications.

Mechanically Tunable Bioink for 3D Bioprinting of Human Cells

This study introduces a thermogelling bioink based on carboxylated agarose (CA) for bioprinting of mechanically defined microenvironments mimicking natural tissues. In CA system, by adjusting the degree of carboxylation, the elastic modulus of printed gels can be tuned over several orders of magnitudes (5-230 Pa) while ensuring almost no change to the shear viscosity (10-17 mPa) of the bioink solution; thus enabling the fabrication of 3D structures made of different mechanical domains under identical printing parameters and low nozzle shear stress. Human mesenchymal stem cells printed using CA as a bioink show significantly higher survival (95%) in comparison to when printed using native agarose (62%), a commonly used thermogelling hydrogel for 3D-bioprinting applications. This work paves the way toward the printing of complex tissue-like structures composed of a range of mechanically discrete microdomains that could potentially reproduce natural mechanical aspects of functional tissues.

Design and Fabrication of a Low-Cost Three-Dimensional Bioprinter

Three-dimensional (3D) bioprinting offers innovative research vectors for tissue engineering. However, commercially available bioprinting platforms can be cost prohibitive to small research facilities, especially in an academic setting. The goal is to design and fabricate a low-cost printing platform able to deliver cell-laden fluids with spatial accuracy along the X, Y, and Z axes of 0.1 mm. The bioprinter consists of three subassemblies: a base unit, a gantry, and a shuttle component. The platform utilizes four stepper motors to position along three axes and a fifth stepper motor actuating a pump. The shuttle and gantry are each driven along their respective horizontal axes via separate single stepper motor, while two coupled stepper motors are used to control location along the vertical axis. The current shuttle configuration allows for a 5 mL syringe to be extruded within a work envelope of 180 mm × 160 mm × 120 mm (X, Y, Z). The shuttle can easily be reconfigured to accommodate larger volume syringes. An attachment for a laser pen is located such that printing material may be light-activated pre-extrusion. Positional fidelity was established with calipers possessing a resolution to the nearest hundredth millimeter. The motors associated with the X and Y axes were calibrated to approximately 0.02 mm per motor impulse. The Z axis has a theoretical step distance of ∼51 nm, generating 0.04% error over a 10 mm travel distance. The A axis, or pump motor, has an impulse distance of 0.001 mm. The volume extruded by a single impulse is dictated by the diameter of the syringe used. With a 5 mL syringe possessing an inner diameter of 12.35 mm, the pump pushes as little as 0.119 μL. While the Z axis is tuned to the highest resolution settings for the motor driver, the X, Y, and A axes can obtain higher or lower resolution via physical switches on the motor drivers.

'Printability' of Candidate Biomaterials for Extrusion Based 3D Printing: State-of-the-Art

Regenerative medicine has been highlighted as one of the UK's 8 'Great Technologies' with the potential to revolutionize patient care in the 21st Century. Over the last decade, the concept of '3D bioprinting' has emerged, which allows the precise deposition of cell laden bioinks with the aim of engineering complex, functional tissues. For 3D printing to be used clinically, there is the need to produce advanced functional biomaterials, a new generation of bioinks with suitable cell culture and high shape/print fidelity, to match or exceed the physical, chemical and biological properties of human tissue. With the rapid increase in knowledge associated with biomaterials, cell-scaffold interactions and the ability to biofunctionalize/decorate bioinks with cell recognition sequences, it is important to keep in mind the 'printability' of these novel materials. In this illustrated review, we define and refine the concept of 'printability' and review seminal and contemporary studies to highlight the current 'state of play' in the field with a focus on bioink composition and concentration, manipulation of nozzle parameters and rheological properties.

Directing the Self-assembly of Tumour Spheroids by Bioprinting Cellular Heterogeneous Models within Alginate/Gelatin Hydrogels

Human tumour progression is a dynamic process involving diverse biological and biochemical events such as genetic mutation and selection in addition to physical, chemical, and mechanical events occurring between cells and the tumour microenvironment. Using 3D bioprinting we have developed a method to embed MDA-MB-231 triple negative breast cancer cells, and IMR-90 fibroblast cells, within a cross-linked alginate/gelatin matrix at specific initial locations relative to each other. After 7 days of co-culture the MDA-MB-231 cells begin to form multicellular tumour spheroids (MCTS) that increase in size and frequency over time. After ~15 days the IMR-90 stromal fibroblast cells migrate through a non-cellularized region of the hydrogel matrix and infiltrate the MDA-MB-231 spheroids creating mixed MDA-MB-231/IMR-90 MCTS. This study provides a proof-of-concept that biomimetic in vitro tissue co-culture models bioprinted with both breast cancer cells and fibroblasts will result in MCTS that can be maintained for durations of several weeks.

A brief review of extrusion-based tissue scaffold bio-printing

Extrusion-based bio-printing has great potential as a technique for manipulating biomaterials and living cells to create three-dimensional (3D) scaffolds for damaged tissue repair and function restoration. Over the last two decades, advances in both engineering techniques and life sciences have evolved extrusion-based bio-printing from a simple technique to one able to create diverse tissue scaffolds from a wide range of biomaterials and cell types. However, the complexities associated with synthesis of materials for bio-printing and manipulation of multiple materials and cells in bio-printing pose many challenges for scaffold fabrication. This paper presents an overview of extrusion-based bio-printing for scaffold fabrication, focusing on the prior-printing considerations (such as scaffold design and materials/cell synthesis), working principles, comparison to other techniques, and to-date achievements. This paper also briefly reviews the recent development of strategies with regard to hydrogel synthesis, multi-materials/cells manipulation, and process-induced cell damage in extrusion-based bio-printing. The key issue and challenges for extrusion-based bio-printing are also identified and discussed along with recommendations for future, aimed at developing novel biomaterials and bio-printing systems, creating patterned vascular networks within scaffolds, and preserving the cell viability and functions in scaffold bio-printing. The address of these challenges will significantly enhance the capability of extrusion-based bio-printing.

Enhanced bone tissue regeneration using a 3D printed microstructure incorporated with a hybrid nano hydrogel

Three-dimensional (3D) functional constructs with biomimetic mechanical and chemical properties are ideal for various regenerative medicine applications. These properties of 3D fabricated constructs mainly depend on the intrinsic characteristics of the materials and fabrication method. In this respect, the current use of hydrogels for musculoskeletal tissue repair is not ideal due to the lack of suitable mechanical properties, as well as the high biomimetic requirement for success. To overcome this limitation, we developed a novel functionalized hydrogel with bioactive gold nanoparticles (GNPs), reinforcing a 3D printed microstructure via fused deposition modeling (FDM) for bone tissue regeneration. We used biodegradable thermoplastic polylactic acid (PLA) as the 3D printed microstructure in combination with photo-curable gelatin hydrogels as the encapsulation matrix for the incorporation of cyclic RGD conjugated GNPs (RGNP), and investigated their mechanical properties. In addition, human adipose-derived stem cells (ADSCs) were encapsulated within the gelatin hydrogel and examined for viability, morphology, and osteogenic differentiation in vitro. The results showed that the stiffness of the composite hydrogel on reinforcing a 3D printed microstructure can be readily modulated to simulate the stiffness of the human mandibular condyle. ADSCs encapsulated in the composite structures remained viable within the hydrogel and showed excellent spreading on the 3D printed PLA microstructure. More importantly, osteogenic differentiation with incorporated RGNPs promoted significantly higher gene expression of osteogenic specific factors. Therefore, reinforced composite hydrogels are suitable for stem cell differentiation control and bone tissue regeneration.

A cell-printing approach for obtaining hASC-laden scaffolds by using a collagen/polyphenol bioink

In the cell-printing process, bioink has been considered as an extremely important component for successful fabrication of macroscale cell-laden structures. Bioink should be non-toxic, biocompatible, and printable. To date, alginate has been widely used as a whole or partial component of bioink because it is non-toxic to embedded cells and even it can provide good printability with rapid gelation under calcium ions. However, alginate bioinks do not possess cell-activating ability. To overcome the shortcomings of alginate-based bioinks, a new collagen bioink, which was mixed with human adipose stem cells (hASCs) and crosslinked with a polyphenol (tannic acid), was proposed. The feasibility of the bioink was demonstrated using several in vitro assessments for comparison of the macroscale porous cell-laden collagen/polyphenol structure containing the hASCs with the conventional alginate-based cell-laden structure. The levels of the metabolic activity, including the cell viability and cell proliferation, of the cell-laden collagen structure were significantly higher than those of the control (alginate-based cell-laden structure). The results show that the newly designed bioink and cell-laden structure are potentially new outstanding components for regeneration of various tissues.

Biofabrication and Bone Tissue Regeneration: Cell Source, Approaches, and Challenges

The growing occurrence of bone disorders and the increase in aging population have resulted in the need for more effective therapies to meet this request. Bone tissue engineering strategies, by combining biomaterials, cells, and signaling factors, are seen as alternatives to conventional bone grafts for repairing or rebuilding bone defects. Indeed, skeletal tissue engineering has not yet achieved full translation into clinical practice because of several challenges. Bone biofabrication by additive manufacturing techniques may represent a possible solution, with its intrinsic capability for accuracy, reproducibility, and customization of scaffolds as well as cell and signaling molecule delivery. This review examines the existing research in bone biofabrication and the appropriate cells and factors selection for successful bone regeneration as well as limitations affecting these approaches. Challenges that need to be tackled with the highest priority are the obtainment of appropriate vascularized scaffolds with an accurate spatiotemporal biochemical and mechanical stimuli release, in order to improve osseointegration as well as osteogenesis.

Polyvinylpyrrolidone-Based Bio-Ink Improves Cell Viability and Homogeneity during Drop-On-Demand Printing

Drop-on-demand (DOD) bioprinting has attracted huge attention for numerous biological applications due to its precise control over material volume and deposition pattern in a contactless printing approach. 3D bioprinting is still an emerging field and more work is required to improve the viability and homogeneity of printed cells during the printing process. Here, a general purpose bio-ink was developed using polyvinylpyrrolidone (PVP) macromolecules. Different PVP-based bio-inks (0%-3% w/v) were prepared and evaluated for their printability; the short-term and long-term viability of the printed cells were first investigated. The Z value of a bio-ink determines its printability; it is the inverse of the Ohnesorge number (Oh), which is the ratio between the Reynolds number and a square root of the Weber number, and is independent of the bio-ink velocity. The viability of printed cells is dependent on the Z values of the bio-inks; the results indicated that the cells can be printed without any significant impairment using a bio-ink with a threshold Z value of ≤9.30 (2% and 2.5% w/v). Next, the cell output was evaluated over a period of 30 min. The results indicated that PVP molecules mitigate the cell adhesion and sedimentation during the printing process; the 2.5% w/v PVP bio-ink demonstrated the most consistent cell output over a period of 30 min. Hence, PVP macromolecules can play a critical role in improving the cell viability and homogeneity during the bioprinting process.

Recent cell printing systems for tissue engineering

Three-dimensional (3D) printing in tissue engineering has been studied for the bio mimicry of the structures of human tissues and organs. Now, it is being applied to 3D cell printing, which can position cells and biomaterials, such as growth factors, at desired positions in the 3D space. However, there are some challenges of 3D cell printing, such as cell damage during the printing process and the inability to produce a porous 3D shape owing to the embedding of cells in the hydrogel-based printing ink, which should be biocompatible, biodegradable, and non-toxic, etc. Therefore, researchers have been studying ways to balance or enhance the post-print cell viability and the print-ability of 3D cell printing technologies by accommodating several mechanical, electrical, and chemical based systems. In this mini-review, several common 3D cell printing methods and their modified applications are introduced for overcoming deficiencies of the cell printing process.

Principles of the Kenzan Method for Robotic Cell Spheroid-Based Three-Dimensional Bioprinting<sup/>

Bioprinting is a technology with the prospect to change the way many diseases are treated, by replacing the damaged tissues with live de novo created biosimilar constructs. However, after more than a decade of incubation and many proofs of concept, the field is still in its infancy. The current stagnation is the consequence of its early success: the first bioprinters, and most of those that followed, were modified versions of the three-dimensional printers used in additive manufacturing, redesigned for layer-by-layer dispersion of biomaterials. In all variants (inkjet, microextrusion, or laser assisted), this approach is material ("scaffold") dependent and energy intensive, making it hardly compatible with some of the intended biological applications. Instead, the future of bioprinting may benefit from the use of gentler scaffold-free bioassembling methods. A substantial body of evidence has accumulated, indicating this is possible by use of preformed cell spheroids, which have been assembled in cartilage, bone, and cardiac muscle-like constructs. However, a commercial instrument capable to directly and precisely "print" spheroids has not been available until the invention of the microneedles-based ("Kenzan") spheroid assembling and the launching in Japan of a bioprinter based on this method. This robotic platform laces spheroids into predesigned contiguous structures with micron-level precision, using stainless steel microneedles ("kenzans") as temporary support. These constructs are further cultivated until the spheroids fuse into cellular aggregates and synthesize their own extracellular matrix, thus attaining the needed structural organization and robustness. This novel technology opens wide opportunities for bioengineering of tissues and organs.

Strategy to Achieve Highly Porous/Biocompatible Macroscale Cell Blocks, Using a Collagen/Genipin-bioink and an Optimal 3D Printing Process

Recently, a three-dimensional (3D) bioprinting process for obtaining a cell-laden structure has been widely applied because of its ability to fabricate biomimetic complex structures embedded with and without cells. To successfully obtain a cell-laden porous block, the cell-delivering vehicle, bioink, is one of the significant factors. Until now, various biocompatible hydrogels (synthetic and natural biopolymers) have been utilized in the cell-printing process, but a bioink satisfying both biocompatibility and print-ability requirements to achieve a porous structure with reasonable mechanical strength has not been issued. Here, we propose a printing strategy with optimal conditions including a safe cross-linking procedure for obtaining a 3D porous cell block composed of a biocompatible collagen-bioink and genipin, a cross-linking agent. To obtain the optimal processing conditions, we modified the 3D printing machine and selected an optimal cross-linking condition (∼1 mM and 1 h) of genipin solution. To show the feasibility of the process, 3D pore-interconnected cell-laden constructs were manufactured using osteoblast-like cells (MG63) and human adipose stem cells (hASCs). Under these processing conditions, a macroscale 3D collagen-based cell block of 21 × 21 × 12 mm³ and over 95% cell viability was obtained. In vitro biological testing of the cell-laden 3D porous structure showed that the embedded cells were sufficiently viable, and their proliferation was significantly higher; the cells also exhibited increased osteogenic activities compared to the conventional alginate-based bioink (control). The results indicated the fabrication process using the collagen-bioink would be an innovative platform to design highly biocompatible and mechanically stable cell blocks.

Three-dimensional direct cell bioprinting for tissue engineering

Bioprinting is a relatively new technology where living cells with or without biomaterials are printed layer-by-layer in order to create three-dimensional (3D) living structures. In this article, novel bioprinting methodologies are developed to fabricate 3D biological structures directly from computer models using live multicellular aggregates. Multicellular aggregates made out of at least two cell types from fibroblast, endothelial and smooth muscle cells are prepared and optimized. A novel bioprinting approach is proposed in order to continuously extrude cylindrical multicellular aggregates through the bioprinter's glass microcapillaries. The multicellular aggregates are first aspirated into a capillary and then compressed to form a continuous cylindrical multicellular bioink. To overcome surface tension-driven droplet formation, the required compression ratio is calculated. Based on the developed bioprinting strategies, multicellular aggregates and their support structures are bioprinted to form 3D tissue constructs with predefined shapes. The effect of the bioprinting process was examined for fusion, cell viability at different compression ratios, and f-actin cytoskeletal organization. The results show that the bioprinted 3D constructs fuse rapidly and have high cell viability after printing. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2530-2544, 2017.

Bioink properties before, during and after 3D bioprinting

Bioprinting is a process based on additive manufacturing from materials containing living cells. These materials, often referred to as bioink, are based on cytocompatible hydrogel precursor formulations, which gel in a manner compatible with different bioprinting approaches. The bioink properties before, during and after gelation are essential for its printability, comprising such features as achievable structural resolution, shape fidelity and cell survival. However, it is the final properties of the matured bioprinted tissue construct that are crucial for the end application. During tissue formation these properties are influenced by the amount of cells present in the construct, their proliferation, migration and interaction with the material. A calibrated computational framework is able to predict the tissue development and maturation and to optimize the bioprinting input parameters such as the starting material, the initial cell loading and the construct geometry. In this contribution relevant bioink properties are reviewed and discussed on the example of most popular bioprinting approaches. The effect of cells on hydrogel processing and vice versa is highlighted. Furthermore, numerical approaches were reviewed and implemented for depicting the cellular mechanics within the hydrogel as well as for prediction of mechanical properties to achieve the desired hydrogel construct considering cell density, distribution and material-cell interaction.

Effect of bioink properties on printability and cell viability for 3D bioplotting of embryonic stem cells

3D cell printing is an emerging technology for fabricating complex cell-laden constructs with precise and pre-designed geometry, structure and composition to overcome the limitations of 2D cell culture and conventional tissue engineering scaffold technology. This technology enables spatial manipulation of cells and biomaterials, also referred to as 'bioink', and thus allows study of cellular interactions in a 3D microenvironment and/or in the formation of functional tissues and organs. Recently, many efforts have been made to develop new bioinks and to apply more cell sources for better biocompatibility and biofunctionality. However, the influences of printing parameters on the shape fidelity of 3D constructs as well as on cell viability after the cell printing process have been poorly characterized. Furthermore, parameter optimization based on a specific cell type might not be suitable for other types of cells, especially cells with high sensibility. In this study, we systematically studied the influence of bioink properties and printing parameters on bioink printability and embryonic stem cell (ESC) viability in the process of extrusion-based cell printing, also known as bioplotting. A novel method was established to determine suitable conditions for bioplotting ESCs to achieve both good printability and high cell viability. The rheological properties of gelatin/alginate bioinks were evaluated to determine the gelation properties under different bioink compositions, printing temperatures and holding times. The bioink printability was characterized by a newly developed semi-quantitative method. The results demonstrated that bioinks with longer gelation times would result in poorer printability. The live/dead assay showed that ESC viability increased with higher printing temperatures and lower gelatin concentrations. Furthermore, an exponential relationship was obtained between ESC viability and induced shear stress. By defining the proper printability and acceptable viability ranges, a combined parameters region was obtained. This study provides guidance for parameter optimization and the fine-tuning of 3D cell printing processes regarding both bioink printability and cell viability after bioplotting, especially for easily damaged cells, like ESCs.

Bioprinting and Differentiation of Stem Cells

The 3D bioprinting of stem cells directly into scaffolds offers great potential for the development of regenerative therapies; in particular for the fabrication of organ and tissue substitutes. For this to be achieved; the lineage fate of bioprinted stem cell must be controllable. Bioprinting can be neutral; allowing culture conditions to trigger differentiation or alternatively; the technique can be designed to be stimulatory. Such factors as the particular bioprinting technique; bioink polymers; polymer cross-linking mechanism; bioink additives; and mechanical properties are considered. In addition; it is discussed that the stimulation of stem cell differentiation by bioprinting may lead to the remodeling and modification of the scaffold over time matching the concept of 4D bioprinting. The ability to tune bioprinting properties as an approach to fabricate stem cell bearing scaffolds and to also harness the benefits of the cells multipotency is of considerable relevance to the field of biomaterials and bioengineering.

Engineering zonal cartilage through bioprinting collagen type II hydrogel constructs with biomimetic chondrocyte density gradient

Background

Cartilage tissue engineering is a promising approach for repairing and regenerating cartilage tissue. To date, attempts have been made to construct zonal cartilage that mimics the cartilaginous matrix in different zones. However, little attention has been paid to the chondrocyte density gradient within the articular cartilage. We hypothesized that the chondrocyte density gradient plays an important role in forming the zonal distribution of extracellular matrix (ECM).

Methods

In this study, collagen type II hydrogel/chondrocyte constructs were fabricated using a bioprinter. Three groups were created according to the total cell seeding density in collagen type II pre-gel: Group A, 2 × 10⁷ cells/mL; Group B, 1 × 10⁷ cells/mL; and Group C, 0.5 × 10⁷ cells/mL. Each group included two types of construct: one with a biomimetic chondrocyte density gradient and the other with a single cell density. The constructs were cultured in vitro and harvested at 0, 1, 2, and 3 weeks for cell viability testing, reverse-transcription quantitative PCR (RT-qPCR), biochemical assays, and histological analysis.

Results

We found that total ECM production was positively correlated with the total cell density in the early culture stage, that the cell density gradient distribution resulted in a gradient distribution of ECM, and that the chondrocytes’ biosynthetic ability was affected by both the total cell density and the cell distribution pattern.

Conclusions

Our results suggested that zonal engineered cartilage could be fabricated by bioprinting collagen type II hydrogel constructs with a biomimetic cell density gradient. Both the total cell density and the cell distribution pattern should be optimized to achieve synergistic biological effects.

Influence of Flow Behavior of Alginate-Cell Suspensions on Cell Viability and Proliferation

Tissue scaffolds with living cells fabricated by three-dimensional bioprinting/plotting techniques are becoming more prevalent in tissue repair and regeneration. In the bioprinting process, cells are subject to process-induced forces (such as shear force) that can result in cell damage and loss of cell function. The flow behavior of the biomaterial solutions that encapsulate living cells in this process plays an important role. This study used a rheometer to examine the flow behavior of alginate solution and alginate-Schwann cell (RSC96), alginate-fibroblast cell (NIH-3T3), and alginate-skeletal muscle cell (L8) suspensions during shearing with respect to effects on cell viability and proliferation. The flow behavior of all the alginate-cell suspensions varied with alginate concentration and cell density and had a significant influence on the viability and proliferation of the cells once sheared as well as on the recovery of the sheared cells. These findings provide a mean to preserve cell viability and/or retain cell proliferation function in the bioprinting process by regulating the flow behavior of cell-biomaterial suspensions and process parameters.

Current Status of Bioinks for Micro-Extrusion-Based 3D Bioprinting

Recent developments in 3D printing technologies and design have been nothing short of spectacular. Parallel to this, development of bioinks has also emerged as an active research area with almost unlimited possibilities. Many bioinks have been developed for various cells types, but bioinks currently used for 3D printing still have challenges and limitations. Bioink development is significant due to two major objectives. The first objective is to provide growth- and function-supportive bioinks to the cells for their proper organization and eventual function and the second objective is to minimize the effect of printing on cell viability, without compromising the resolution shape and stability of the construct. Here, we will address the current status and challenges of bioinks for 3D printing of tissue constructs for in vitro and in vivo applications.

New strategy for enhancing in situ cell viability of cell-printing process via piezoelectric transducer-assisted three-dimensional printing

Tissue engineering has become one of the great applications of three-dimensional cell printing because of the possibility of fabricating complex cell-laden scaffolds. Three typical methods (inkjet, micro-extrusion, and laser-assisted bio-printing) have been used to fabricate structures. Of these, micro-extrusion is a comparatively easy method, but has some drawbacks such as low in situ cell viability after fabricating cell-laden structures because of the high wall shear stress in micro-sized nozzles. To overcome this shortcoming, we suggest an innovative cell printing method, which is assisted by a piezoelectric transducer (PZT). The PZT assistance in the dispensing process enhances the printing efficiency and cell viability by decreasing the wall shear stress within a nozzle because the PZT effect can lower the shear viscosity of the bioink via micro-scale vibration. In this study, 5 wt% cell-laden alginate was used as a bioink, and various PZT conditions (frequencies up to ∼400 Hz and amplitudes up to ∼40.5 μm) were simultaneously applied to the cell-printing process to examine the effectiveness of the PZT. The PZT-assisted cell-printing method was found to be highly effective in direct cell printing and could achieve cell-laden structures with high in situ cell viability.

Printing of Three-Dimensional Tissue Analogs for Regenerative Medicine

Three-dimensional (3-D) cell printing, which can accurately deposit cells, biomaterial scaffolds and growth factors in precisely defined spatial patterns to form biomimetic tissue structures, has emerged as a powerful enabling technology to create live tissue and organ structures for drug discovery and tissue engineering applications. Unlike traditional 3-D printing that uses metals, plastics and polymers as the printing materials, cell printing has to be compatible with living cells and biological matrix. It is also required that the printing process preserves the biological functions of the cells and extracellular matrix, and to mimic the cell-matrix architectures and mechanical properties of the native tissues. Therefore, there are significant challenges in order to translate the technologies of traditional 3-D printing to cell printing, and ultimately achieve functional outcomes in the printed tissues. So it is essential to develop new technologies specially designed for cell printing and in-depth basic research in the bioprinted tissues, such as developing novel biomaterials specifically for cell printing applications, understanding the complex cell-matrix remodeling for the desired mechanical properties and functional outcomes, establishing proper vascular perfusion in bioprinted tissues, etc. In recent years, many exciting research progresses have been made in the 3-D cell printing technology and its application in engineering live tissue constructs. This review paper summarized the current development in 3-D cell printing technologies; focus on the outcomes of the live printed tissues and their potential applications in drug discovery and regenerative medicine. Current challenges and limitations are highlighted, and future directions of 3-D cell printing technology are also discussed.

Hydrogel microfabrication technology toward three dimensional tissue engineering

The development of biologically relevant three-dimensional (3D) tissue constructs is essential for the alternative methods of organ transplantation in regenerative medicine, as well as the development of improved drug discovery assays. Recent technological advances in hydrogel microfabrication, such as micromolding, 3D bioprinting, photolithography, and stereolithography, have led to the production of 3D tissue constructs that exhibit biological functions with precise 3D microstructures. Furthermore, microfluidics technology has enabled the development of the perfusion culture of 3D tissue constructs with vascular networks. In this review, we present these hydrogel microfabrication technologies for the in vitro reconstruction and cultivation of 3D tissues. Additionally, we discuss current challenges and future perspectives of 3D tissue engineering.

Controlling Shear Stress in 3D Bioprinting is a Key Factor to Balance Printing Resolution and Stem Cell Integrity

A microvalve-based bioprinting system for the manufacturing of high-resolution, multimaterial 3D-structures is reported. Applying a straightforward fluid-dynamics model, the shear stress at the nozzle site can precisely be controlled. Using this system, a broad study on how cell viability and proliferation potential are affected by different levels of shear stress is conducted. Complex, multimaterial 3D structures are printed with high resolution. This work pioneers the investigation of shear stress-induced cell damage in 3D bioprinting and might help to comprehend and improve the outcome of cell-printing studies in the future.

3D Printing of Scaffold for Cells Delivery: Advances in Skin Tissue Engineering

Injury or damage to tissue and organs is a major health problem, resulting in about half of the world’s annual healthcare expenditure every year. Advances in the fields of stem cells (SCs) and biomaterials processing have provided a tremendous leap for researchers to manipulate the dynamics between these two, and obtain a skin substitute that can completely heal the wounded areas. Although wound healing needs a coordinated interplay between cells, extracellular proteins and growth factors, the most important players in this process are the endogenous SCs, which activate the repair cascade by recruiting cells from different sites. Extra cellular matrix (ECM) proteins are activated by these SCs, which in turn aid in cellular migrations and finally secretion of growth factors that can seal and heal the wounds. The interaction between ECM proteins and SCs helps the skin to sustain the rigors of everyday activity, and in an attempt to attain this level of functionality in artificial three-dimensional (3D) constructs, tissue engineered biomaterials are fabricated using more advanced techniques such as bioprinting and laser assisted printing of the organs. This review provides a concise summary of the most recent advances that have been made in the area of polymer bio-fabrication using 3D bio printing used for encapsulating stem cells for skin regeneration. The focus of this review is to describe, in detail, the role of 3D architecture and arrangement of cells within this system that can heal wounds and aid in skin regeneration.

Biofabrication of bone tissue: approaches, challenges and translation for bone regeneration

The rising incidence of bone disorders has resulted in the need for more effective therapies to meet this demand, exacerbated by an increasing ageing population. Bone tissue engineering is seen as a means of developing alternatives to conventional bone grafts for repairing or reconstructing bone defects by combining biomaterials, cells and signalling factors. However, skeletal tissue engineering has not yet achieved full translation into clinical practice as a consequence of several challenges. The use of additive manufacturing techniques for bone biofabrication is seen as a potential solution, with its inherent capability for reproducibility, accuracy and customisation of scaffolds as well as cell and signalling factor delivery. This review highlights the current research in bone biofabrication, the necessary factors for successful bone biofabrication, in addition to the current limitations affecting biofabrication, some of which are a consequence of the limitations of the additive manufacturing technology itself.

MODELING MECHANICAL CELL DAMAGE IN THE BIOPRINTING PROCESS EMPLOYING A CONICAL NEEDLE

Biofabrication technologies involve the incorporation of living cells into various bioproducts by employing different cell manipulation techniques. Among them, bioprinting, delivering cell suspension through a fine needle under pressurized air, has been widely used because of its capability of precise process control. In the cell-printing process of bioprinting, cells are exposed to fluid stresses due to the velocity gradient in the fine needle. If the stresses exceed a certain level, the cell membrane may be overstretched, leading to membrane failure and thus causing mechanical cell damage. Modeling the mechanical cell damage in the bioprinting process is a challenging task due to the complex fluid flow and cell deformation involved. This paper introduces a novel method based on computational fluid dynamics (CFD) to represent the mechanical cell damage in the bioprinting process using a conical needle. Specifically, the cell deformation and movement during the cell-fluid interaction processes were represented by the immersed boundary method (IBM). A strain energy density (SED)-based cell damage criterion was developed and used to determine cell damage. Experiments were performed by using 3T3 fibroblasts and the results agree well with the proposed model.

NUMERICAL SIMULATION OF PRESSURE-INDUCED CELL PRINTING

Nowadays, because of great biomedical applications of state-of-the art prototyping (bio-printing), many studies have been conducted in this field with focus on three-dimensional prototyping. There are several mechanisms for bio-printing of live cells such as piezoelectric and thermal and pneumatic inkjeting systems. Cell viability should be preserved during the bio-printing process. Lots of researches have been carried out to investigate and compare cell viability through different prototyping mechanisms. In order to quantify percentage of the cells that are killed during the proto-typing process, applied stresses on the cell and consequently its deformation should be calculated. A maximum strain energy density that the cell can tolerate is reported in the range of 25 Kj ⋅ m-3 to 100 Kj ⋅ m-3. This can be considered as a criteria to find the percentage of the damaged cells during the bio-printing processes. In this study, the bio-printing of the cell has been modeled and the cell viability have been investigated. Firstly, it is shown that in modeling of the bio-printing process, the effects of dynamic flow on calculating the applied stress on the cell is not negligible and must be considered. In the next step, the percentage of damaged endothelial cell aggregate under 80 kPa applied pressure (64 MPa/m) and 200 micron nozzle diameter is reported. Based on findings of this study, the percentage of endothelial cells viability under mentioned condition is reported 76%. The proposed method of this study can be utilized to examine the cell viability and performance of each prototyping systems.

Bioprinting is changing regenerative medicine forever

3D printing, or solid freeform fabrication, applied to regenerative medicine brings technologies from several industries together to help solve unique challenges in both basic science and tissue engineering. By more finely organizing cells and supporting structures precisely in 3D space, we will gain critical knowledge of cell-cell communications and cell-environment interactions. As we increase the scale, we will move toward complex tissue and organ structures where several cell phenotypes will functionally and structurally interact, thus recapitulating the form and function of native tissues and organs.

3D bioprinting of tissues and organs

Additive manufacturing, otherwise known as three-dimensional (3D) printing, is driving major innovations in many areas, such as engineering, manufacturing, art, education and medicine. Recent advances have enabled 3D printing of biocompatible materials, cells and supporting components into complex 3D functional living tissues. 3D bioprinting is being applied to regenerative medicine to address the need for tissues and organs suitable for transplantation. Compared with non-biological printing, 3D bioprinting involves additional complexities, such as the choice of materials, cell types, growth and differentiation factors, and technical challenges related to the sensitivities of living cells and the construction of tissues. Addressing these complexities requires the integration of technologies from the fields of engineering, biomaterials science, cell biology, physics and medicine. 3D bioprinting has already been used for the generation and transplantation of several tissues, including multilayered skin, bone, vascular grafts, tracheal splints, heart tissue and cartilaginous structures. Other applications include developing high-throughput 3D-bioprinted tissue models for research, drug discovery and toxicology.

3D printing of HEK 293FT cell-laden hydrogel into macroporous constructs with high cell viability and normal biological functions

3D printing has evolved into a versatile technology for fabricating tissue-engineered constructs with spatially controlled cells and biomaterial distribution to allow biomimicking of in vivo tissues. In this paper, we reported a novel study of 3D printing of cell lines derived from human embryonic kidney tissue into a macroporous tissue-like construct. Nozzle temperature, chamber temperature and the composition of the matrix material were studied to achieve high cell viability (>90%) after 3D printing and construct formation. Long-term construct stability with a clear grid structure up to 30 days was observed. Cells continued to grow as cellular spheroids with strong cell-cell interactions. Two transfected cell lines of HEK 293FT were also 3D printed and showed normal biological functions, i.e. protein synthesis and gene activation in responding to small molecule stimulus. With further refinement, this 3D cell printing technology may lead to a practical fabrication of functional embryonic tissues in vitro.

Arthroscopic airbrush assisted cell implantation for cartilage repair in the knee: a controlled laboratory and human cadaveric study

OBJECTIVE

The objective of this study was to investigate the feasibility of arthroscopic airbrush assisted cartilage repair.

METHODS

An airbrush device (Baxter) was used to spray both human expanded osteoarthritic chondrocytes and choncrocytes with their pericellular matrix (chondrons) at 1 × 10(6) cells/ml fibrin glue (Tissucol, Baxter) in vitro. Depth-dependent cell viability was assessed for both methods with confocal microscopy. Constructs were cultured for 21 days to assess matrix production. A controlled human cadaveric study (n = 8) was performed to test the feasibility of the procedure in which defects were filled with either arthroscopic airbrushing or needle extrusion. All knees were subjected to 60 min of continuous passive motion and scored on outline attachment and defect filling.

RESULTS

Spraying both chondrocytes and chondrons in fibrin glue resulted in a homogenous cell distribution throughout the scaffold. No difference in viability or matrix production between application methods was found nor between chondrons and chondrocytes. The cadaveric study revealed that airbrushing was highly feasible, and that defect filling through needle extrusion was more difficult to perform based on fibrin glue adhesion and gravity-induced seepage. Defect outline and coverage scores were consistently higher for extrusion, albeit not statistically significant.

CONCLUSION

Both chondrons and chondrocytes can be evenly distributed in a sprayed fibrin glue scaffold without affecting viability while supporting matrix production. The airbrush technology is feasible, easier to perform than needle extrusion and allows for reproducible arthroscopic filling of cartilage defects.

Biomaterials for integration with 3-D bioprinting

Bioprinting has emerged in recent years as an attractive method for creating 3-D tissues and organs in the laboratory, and therefore is a promising technology in a number of regenerative medicine applications. It has the potential to (i) create fully functional replacements for damaged tissues in patients, and (ii) rapidly fabricate small-sized human-based tissue models, or organoids, for diagnostics, pathology modeling, and drug development. A number of bioprinting modalities have been explored, including cellular inkjet printing, extrusion-based technologies, soft lithography, and laser-induced forward transfer. Despite the innovation of each of these technologies, successful implementation of bioprinting relies heavily on integration with compatible biomaterials that are responsible for supporting the cellular components during and after biofabrication, and that are compatible with the bioprinting device requirements. In this review, we will evaluate a variety of biomaterials, such as curable synthetic polymers, synthetic gels, and naturally derived hydrogels. Specifically we will describe how they are integrated with the bioprinting technologies above to generate bioprinted constructs with practical application in medicine.