Electrohydrodynamic jetting of mouse neuronal cells

The 3D Bioprinted Scaffolds for Wound Healing

Skin tissue engineering and regeneration aim at repairing defective skin injuries and progress in wound healing. Until now, even though several developments are made in this field, it is still challenging to face the complexity of the tissue with current methods of fabrication. In this review, short, state-of-the-art on developments made in skin tissue engineering using 3D bioprinting as a new tool are described. The current bioprinting methods and a summary of bioink formulations, parameters, and properties are discussed. Finally, a representative number of examples and advances made in the field together with limitations and future needs are provided.

Inkjet Bioprinting of Biomaterials

The inkjet technique has the capability of generating droplets in the picoliter volume range, firing thousands of times in a few seconds and printing in the noncontact manner. Since its emergence, inkjet technology has been widely utilized in the publishing industry for printing of text and pictures. As the technology developed, its applications have been expanded from two-dimensional (2D) to three-dimensional (3D) and even used to fabricate components of electronic devices. At the end of the twentieth century, researchers were aware of the potential value of this technology in life sciences and tissue engineering because its picoliter-level printing unit is suitable for depositing biological components. Currently inkjet technology has been becoming a practical tool in modern medicine serving for drug development, scaffold building, and cell depositing. In this article, we first review the history, principles and different methods of developing this technology. Next, we focus on the recent achievements of inkjet printing in the biological field. Inkjet bioprinting of generic biomaterials, biomacromolecules, DNAs, and cells and their major applications are introduced in order of increasing complexity. The current limitations/challenges and corresponding solutions of this technology are also discussed. A new concept, biopixels, is put forward with a combination of the key characteristics of inkjet printing and basic biological units to bring a comprehensive view on inkjet-based bioprinting. Finally, a roadmap of the entire 3D bioprinting is depicted at the end of this review article, clearly demonstrating the past, present, and future of 3D bioprinting and our current progress in this field.

Development of 3D bioprinting: From printing methods to biomedical applications

Biomanufacturing of tissues/organs in vitro is our big dream, driven by two needs: organ transplantation and accurate tissue models. Over the last decades, 3D bioprinting has been widely applied in the construction of many tissues/organs such as skins, vessels, hearts, etc., which can not only lay a foundation for the grand goal of organ replacement, but also be served as in vitro models committed to pharmacokinetics, drug screening and so on. As organs are so complicated, many bioprinting methods are exploited to figure out the challenges of different applications. So the question is how to choose the suitable bioprinting method? Herein, we systematically review the evolution, process and classification of 3D bioprinting with an emphasis on the fundamental printing principles and commercialized bioprinters. We summarize and classify extrusion-based, droplet-based, and photocuring-based bioprinting methods and give some advices for applications. Among them, coaxial and multi-material bioprinting are highlighted and basic principles of designing bioinks are also discussed.

Bio-electrosprayed human neural stem cells are viable and maintain their differentiation potential

Background: Bio-electrospray (BES) is a jet-based delivery system driven by an electric field that has the ability to form micro to nano-sized droplets. It holds great potential as a tissue engineering tool as it can be used to place cells into specific patterns. As the human central nervous system (CNS) cannot be studied in vivo at the cellular and molecular level, in vitro CNS models are needed. Human neural stem cells (hNSCs) are the CNS building block as they can generate both neurones and glial cells. Methods: Here we assessed for the first time how hNSCs respond to BES. To this purpose, different hNSC lines were sprayed at 10 kV and their ability to survive, grow and differentiate was assessed at different time points. Results: BES induced only a small and transient decrease in hNSC metabolic activity, from which the cells recovered by day 6, and no significant increase in cell death was observed, as assessed by flow cytometry. Furthermore, bio-electrosprayed hNSCs differentiated as efficiently as controls into neurones, astrocytes and oligodendrocytes, as shown by morphological, protein and gene expression analysis. Conclusions: This study highlights the robustness of hNSCs and identifies BES as a suitable technology that could be developed for the direct deposition of these cells in specific locations and configurations.

Bio-electrosprayed human neural stem cells are viable and maintain their differentiation potential

Background: Bio-electrospray (BES) is a jet-based delivery system driven by an electric field that has the ability to form micro to nano-sized droplets. It holds great potential as a tissue engineering tool as it can be used to place cells into specific patterns. As the human central nervous system (CNS) cannot be studied in vivo at the cellular and molecular level, in vitro CNS models are needed. Human neural stem cells (hNSCs) are the CNS building block as they can generate both neurones and glial cells. Methods: Here we assessed for the first time how hNSCs respond to BES. To this purpose, different hNSC lines were sprayed at 10 kV and their ability to survive, grow and differentiate was assessed at different time points. Results: BES induced only a small and transient decrease in hNSC metabolic activity, from which cells recovered by day 6, and no significant increase in cell death was observed, as assessed by flow cytometry. Furthermore, bio-electrosprayed hNSCs differentiated as efficiently as controls into neurones, astrocytes and oligodendrocytes as shown by morphological, protein and gene expression analysis. Conclusions: This study highlights the robustness of hNSCs and identifies BES as a suitable technology that could be developed for the direct deposition of these cells in specific locations and configurations.

3D bioprinting processes: A perspective on classification and terminology

This article aims to provide further classification of cell-compatible bioprinting processes and examine the concept of 3D bioprinting within the general technology field of 3D printing. These technologies are categorized into four distinct process categories, namely material jetting, vat photopolymerization, material extrusion and free-form spatial printing. Discussion will be presented on the definition of classification with example of techniques grouped under the same category. The objective of this article is to establish a basic framework for standardization of process terminology in order to accelerate the implementation of bioprinting technologies in research and commercial landscape.

3D bioprinting of tissues and organs for regenerative medicine

3D bioprinting is a pioneering technology that enables fabrication of biomimetic, multiscale, multi-cellular tissues with highly complex tissue microenvironment, intricate cytoarchitecture, structure-function hierarchy, and tissue-specific compositional and mechanical heterogeneity. Given the huge demand for organ transplantation, coupled with limited organ donors, bioprinting is a potential technology that could solve this crisis of organ shortage by fabrication of fully-functional whole organs. Though organ bioprinting is a far-fetched goal, there has been a considerable and commendable progress in the field of bioprinting that could be used as transplantable tissues in regenerative medicine. This paper presents a first-time review of 3D bioprinting in regenerative medicine, where the current status and contemporary issues of 3D bioprinting pertaining to the eleven organ systems of the human body including skeletal, muscular, nervous, lymphatic, endocrine, reproductive, integumentary, respiratory, digestive, urinary, and circulatory systems were critically reviewed. The implications of 3D bioprinting in drug discovery, development, and delivery systems are also briefly discussed, in terms of in vitro drug testing models, and personalized medicine. While there is a substantial progress in the field of bioprinting in the recent past, there is still a long way to go to fully realize the translational potential of this technology. Computational studies for study of tissue growth or tissue fusion post-printing, improving the scalability of this technology to fabricate human-scale tissues, development of hybrid systems with integration of different bioprinting modalities, formulation of new bioinks with tuneable mechanical and rheological properties, mechanobiological studies on cell-bioink interaction, 4D bioprinting with smart (stimuli-responsive) hydrogels, and addressing the ethical, social, and regulatory issues concerning bioprinting are potential futuristic focus areas that would aid in successful clinical translation of this technology.

Essential steps in bioprinting: From pre- to post-bioprinting

An increasing demand for directed assembly of biomaterials has inspired the development of bioprinting, which facilitates the assembling of both cellular and acellular inks into well-arranged three-dimensional (3D) structures for tissue fabrication. Although great advances have been achieved in the recent decade, there still exist issues to be addressed. Herein, a review has been systematically performed to discuss the considerations in the entire procedure of bioprinting. Though bioprinting is advancing at a rapid pace, it is seen that the whole process of obtaining tissue constructs from this technique involves multiple-stages, cutting across various technology domains. These stages can be divided into three broad categories: pre-bioprinting, bioprinting and post-bioprinting. Each stage can influence others and has a bearing on the performance of fabricated constructs. For example, in pre-bioprinting, tissue biopsy and cell expansion techniques are essential to ensure a large number of cells are available for mass organ production. Similarly, medical imaging is needed to provide high resolution designs, which can be faithfully bioprinted. In the bioprinting stage, compatibility of biomaterials is needed to be matched with solidification kinetics to ensure constructs with high cell viability and fidelity are obtained. On the other hand, there is a need to develop bioprinters, which have high degrees of freedom of movement, perform without failure concerns for several hours and are compact, and affordable. Finally, maturation of bioprinted cells are governed by conditions provided during the post-bioprinting process. This review, for the first time, puts all the bioprinting stages in perspective of the whole process of bioprinting, and analyzes their current state-of-the art. It is concluded that bioprinting community will recognize the relative importance and optimize the parameter of each stage to obtain the desired outcomes.

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 Organ Regeneration

Regenerative medicine holds the promise of engineering functional tissues or organs to heal or replace abnormal and necrotic tissues/organs, offering hope for filling the gap between organ shortage and transplantation needs. Three-dimensional (3D) bioprinting is evolving into an unparalleled biomanufacturing technology due to its high-integration potential for patient-specific designs, precise and rapid manufacturing capabilities with high resolution, and unprecedented versatility. It enables precise control over multiple compositions, spatial distributions, and architectural accuracy/complexity, therefore achieving effective recapitulation of microstructure, architecture, mechanical properties, and biological functions of target tissues and organs. Here we provide an overview of recent advances in 3D bioprinting technology, as well as design concepts of bioinks suitable for the bioprinting process. We focus on the applications of this technology for engineering living organs, focusing more specifically on vasculature, neural networks, the heart and liver. We conclude with current challenges and the technical perspective for further development of 3D organ bioprinting.

Design and Printing Strategies in 3D Bioprinting of Cell-Hydrogels: A Review

Bioprinting is an emerging technology that allows the assembling of both living and non-living biological materials into an ideal complex layout for further tissue maturation. Bioprinting aims to produce engineered tissue or organ in a mechanized, organized, and optimized manner. Various biomaterials and techniques have been utilized to bioprint biological constructs in different shapes, sizes and resolutions. There is a need to systematically discuss and analyze the reported strategies employed to fabricate these constructs. We identified and discussed important design factors in bioprinting, namely shape and resolution, material heterogeneity, and cellular-material remodeling dynamism. Each design factors are represented by the corresponding process capabilities and printing parameters. The process-design map will inspire future biomaterials research in these aspects. Design considerations such as data processing, bio-ink formulation and process selection are discussed. Various printing and crosslinking strategies, with relevant applications, are also systematically reviewed. We categorized them into 5 general bioprinting strategies, including direct bioprinting, in-process crosslinking, post-process crosslinking, indirect bioprinting and hybrid bioprinting. The opportunities and outlook in 3D bioprinting are highlighted. This review article will serve as a framework to advance computer-aided design in bioprinting technologies.

Fabrication of hASCs-laden structures using extrusion-based cell printing supplemented with an electric field

In this study, we proposed a hybrid cell-printing technique that combines a conventional extrusion-based cell-printing process with an electrohydrodynamic jet. The electric field stabilized the extruded struts of cell-embedding-hydrogel and reduced the damage to dispensed cells caused by the high wall shear stress in the dispensing nozzle. The new cell-printing process was optimized in terms of various processing parameters, applied electric field strength, nozzle movement speed, and distance between the nozzle tip and working stage. Using the optimal cell-embedding hydrogel composition (1×10(6)cellsmL(-1) in 4wt% alginate) and cell-printing process parameters (applied voltage, 1kV; nozzle movement speed, 12mms(-1); distance, 0.7mm; current, 10.67±1.1nA), we achieved rapid and stable fabrication of a cell-laden structure without loss of cell viability or proliferation, the values of which were similar to those of the process without an electric field. Furthermore, by applying the same pneumatic pressure to fabricate cell-laden structures, considerably higher volume flow rate and cell viability at the same volume flow rate were achieved by the modified process compared with conventional extrusion-based cell-printing processes. To assess the feasibility of the method, the hydrogel containing human adipose stem cells (hASCs) and alginate (4wt%) was fabricated into a cell-laden porous structure in a layer-by-layer manner. The cell-laden structure exhibited reasonable initial hASC viability (87%), which was similar to that prior to processing of the cell-embedding-hydrogel.

STATEMENT OF SIGNIFICANCE

The extrusion-based cell-printing process has shortcomings, such as unstable flow and potential loss of cell viability. The unsteady flow can occur due to the high cell concentration, viscosity, and surface tension of bioinks. Also, cell viability post extrusion can be significantly reduced by damage of the cells due to the high wall shear stress in the extrusion nozzle. To overcome these limitations, we suggested an innovative cell-printing process that combines a conventional extrusion-based cellprinting process with an electric field. The electric field in the cell-printing process stabilized the extruded struts of bioink and dramatically reduced the damage to dispensed cells caused by the high wall shear stress in the dispensing nozzle.

A comprehensive review on droplet-based bioprinting: Past, present and future

Droplet-based bioprinting (DBB) offers greater advantages due to its simplicity and agility with precise control on deposition of biologics including cells, growth factors, genes, drugs and biomaterials, and has been a prominent technology in the bioprinting community. Due to its immense versatility, DBB technology has been adopted by various application areas, including but not limited to, tissue engineering and regenerative medicine, transplantation and clinics, pharmaceutics and high-throughput screening, and cancer research. Despite the great benefits, the technology currently faces several challenges such as a narrow range of available bioink materials, bioprinting-induced cell damage at substantial levels, limited mechanical and structural integrity of bioprinted constructs, and restrictions on the size of constructs due to lack of vascularization and porosity. This paper presents a first-time review of DBB and comprehensively covers the existing DBB modalities including inkjet, electrohydrodynamic, acoustic, and micro-valve bioprinting. The recent notable studies are highlighted, the relevant bioink biomaterials and bioprinters are expounded, the application areas are presented, and the future prospects are provided to the reader.

Controlled single-cell deposition and patterning by highly flexible hollow cantilevers

Single-cell patterning represents a key approach to decouple and better understand the role and mechanisms of individual cells of a given population. In particular, the bottom-up approach of engineering neuronal circuits with a controlled topology holds immense promises to perceive the relationships between connectivity and function. In order to accommodate these efforts, highly flexible SU-8 cantilevers with integrated microchannels have been fabricated for both additive and subtractive patterning. By directly squeezing out single cells onto adhesive surfaces, controlled deposition with a spatial accuracy of 5 μm could be achieved, while subtractive patterning has been realized by selective removal of targeted single cells. Complex cell patterns were created on substrates pre-patterned with cell-adhesive and repulsive areas, preserving the original pattern geometry for long-term studies. For example, a circular loop with a diameter of 530 μm has been realized using primary hippocampal neurons, which were fully connected to their respective neighbors along the loop. Using the same cantilevers, the versatility of the technique has also been demonstrated via in situ modification of already mature neuronal cultures by both detaching individual cells of the population and adding fresh ones, incorporating them into the culture.

Current advances and future perspectives in extrusion-based bioprinting

Extrusion-based bioprinting (EBB) is a rapidly growing technology that has made substantial progress during the last decade. It has great versatility in printing various biologics, including cells, tissues, tissue constructs, organ modules and microfluidic devices, in applications from basic research and pharmaceutics to clinics. Despite the great benefits and flexibility in printing a wide range of bioinks, including tissue spheroids, tissue strands, cell pellets, decellularized matrix components, micro-carriers and cell-laden hydrogels, the technology currently faces several limitations and challenges. These include impediments to organ fabrication, the limited resolution of printed features, the need for advanced bioprinting solutions to transition the technology bench to bedside, the necessity of new bioink development for rapid, safe and sustainable delivery of cells in a biomimetically organized microenvironment, and regulatory concerns to transform the technology into a product. This paper, presenting a first-time comprehensive review of EBB, discusses the current advancements in EBB technology and highlights future directions to transform the technology to generate viable end products for tissue engineering and regenerative medicine.

Mechanisms, Capabilities, and Applications of High-Resolution Electrohydrodynamic Jet Printing

This review gives an overview of techniques used for high-resolution jet printing that rely on electrohydrodynamically induced flows. Such methods enable the direct, additive patterning of materials with a resolution that can extend below 100 nm to provide unique opportunities not only in scientific studies but also in a range of applications that includes printed electronics, tissue engineering, and photonic and plasmonic devices. Following a brief historical perspective, this review presents descriptions of the underlying processes involved in the formation of liquid cones and jets to establish critical factors in the printing process. Different printing systems that share similar principles are then described, along with key advances that have been made in the last decade. Capabilities in terms of printable materials and levels of resolution are reviewed, with a strong emphasis on areas of potential application.

Application and Analysis of Biological Electrospray in Tissue Engineering

Nan-fiber scaffolds are suitable tools for tissue engineering. Electro spinning materials together with cells presents not adequate to obtain a high cellular zing tissue constructs as the shear force, tensile force, and other physical effects excited in the electro spinning process, which are harmful to cellular differentiation, development and function. However, this limitation has been overcome by a micro integration system of simultaneously bio-electro spraying human adipose stem cells (ASCs) and electro spinning Polyvinyl alcohol (PVA). Then it was compared to the single electro spinning nan-fiber scaffolds in relation to cell viability, which showed that the scaffolds by micro integration approach has a larger number of surviving cells and more suitable for cell growth and proliferation. In addition, the relationship between different parameters of biological electrospray (voltage, flow rate and distance of the needle from the collecting board) and droplet size of cell suspension was elucidated and the droplets with a near-mono distribution (<50um) could be generated to deposit a single living cell within a droplet. The association of bio-electro spraying with electro spinning (a scaffold preparation technique) has been demonstrated to be a promising and suitable tissue engineering approach in producing nan-fiber based three-dimensional (3-D) cell seeded scaffolds.

Bio-electrospraying of human mesenchymal stem cells: An alternative for tissue engineering

Bio-electrospraying (BES) is a technique used for the processing of cells and can be applied to tissue engineering. The association of BES with scaffold production techniques has been shown to be an interesting strategy for the production of biomaterials with cells homogeneously distributed in the entire structure. Various studies have evaluated the effects of BES on different cell types. However, until the present moment, no studies have evaluated the impact of BES time on mesenchymal stem cells (MSC). Therefore, the aim of this work was to standardise the different parameters of BES (voltage, flow rate, and distance of the needle from the collecting plate) in relation to cell viability and then to evaluate the impact of BES time in relation to viability, proliferation, DNA damage, maintenance of plasticity and the immunophenotypic profile of MSC. Using 15 kV voltage, 0.46 ml/h flow rate and 4 cm distance, it was possible to form a stable and continuous jet of BES without causing a significant reduction in cell viability. Time periods between 15 and 60 min of BES did not cause alterations of viability, proliferation, plasticity, and immunophenotypic profile of the MSC. Time periods above 30 min of BES resulted in DNA damage; however, the DNA was able to repair itself within five hours. These results indicate that bio-electrospraying is an adequate technique for processing MSC which can be safely applied to tissue engineering and regenerative medicine.

Electrosprayed nanoparticles for drug delivery and pharmaceutical applications

Nanotechnology based Pharma has emerged significantly and has influenced the Pharma industry up to a considerable extent. Nanoparticles technology holds a good share of the nanotech Pharma and is significant in comparison with the other domains. Electrospraying technology answers the potential needs of nanoparticle production such as scalability, reproducibility, effective encapsulation etc. Many drugs have been electrosprayed with and without polymer carriers. Drug release characteristics are improved with the incorporation of biodegradable polymer carriers which sustain the release of encapsulated drug. Electrospraying is acknowledged as an important technique for the preparation of nanoparticles with respect to pharmaceutical applications. Herein we attempted to consolidate the reports pertaining to electrospraying and their corresponding therapeutic application area.

Recent advancements in tissue engineering for stem cell-based cardiac therapies

Advances in cardiac tissue engineering have recently focused on utilizing stem cells to regenerate infarcted and scarred myocardium. Due to their proliferative nature and tremendous potential for differentiation, stem cells are presently being investigated for clinical applications. Unfortunately, limiting factors such as massive cell death and poor retention have hampered clinical outcomes. Consequently, the development of an efficient delivery system for stem cells to the target site is essential. The use of innovative tissue engineering techniques has opened up new horizons within the field of cellular cardiomyoplasty. This paper will present a comprehensive overview of the recent advancements in stem cell technology destined for myocardial tissue repair. In addition, the multidisciplinary approach to tissue engineering presented here will provide the reader with insight into the clinical realization of cellular cardiomyoplasty.

Bio-electrosprayed living composite matrix implanted into mouse models

We show that composite de novo structures can be generated using bio-electrosprays. Mouse lung fibroblasts are bio-electrosprayed directly with a biopolymer to form cell-bearing matrices, which are viable even when implanted subcutaneously into murine hosts. Generated cell-bearing matrices are assessed in-vitro and found to undergo all expected cellular behaviour. Subsequent in-vivo studies demonstrate the implanted living matrices integrating as expected with the surrounding microenvironment. The in-vitro and in-vivo studies elucidate and validate the ability for either bio-electrosprays or cell electrospinning to form a desired living architecture for undergoing investigation for repairing, replacing and rejuvenating damaged and/or ageing tissues.

Bio-electrospraying primary cardiac cells: in vitro tissue creation and functional study

Manifestations of myocardial infarctions have been recognized as one of the major killers in the Western world. Therefore, advancing and developing novel cardiac tissue repair and replacement therapeutics have great implications to our health sciences and well-being. There are several approaches for forming cardiac tissues, non-jet-based and jet-based methodologies. A unique advantage of jet-based approaches is the possibility to handle living cells with a matrix for cell distribution and deposition in suspension, either as single or heterogeneous cell populations. Our previous studies on bio-electrospraying of cardiac cells have shown great promise. Here, we show for the first time the ability to bio-electrospray the three major cell types of the myocardium, both independently and simultaneously, for forming a fully functional cardiac tissue. Several samples are characterized in vitro and found to be indistinguishable in comparison to controls. Thus, we are describing a swiftly emerging novel biotechnique for direct cardiac tissue generation. Moreover, the present investigations pave the way for the development and optimization of a bio-patterning approach for the fabrication of biologically viable cardiac tissue grafts for the potential treatment of severe heart failure after myocardial infarction.

Electric field assisted manipulation of microdroplets on a superhydrophobic surface

The efficient manipulation of low-volume droplets offers many potential applications in relation to chemical and biomedical tests and protocols. A novel approach to the manipulation of a microdroplet on a superhydrophobic surface is introduced in the present communication. The microdroplet was first picked up onto a hydrophilic needle, transported from one location to another, and finally released under the action of an electric field force. Three key parameters in this process, the radius of the droplet, the distance between the two electrodes, and the required voltage, were investigated. This study should be helpful for the design of microfluidic devices.

Bio-electrospraying: A potentially safe technique for delivering progenitor cells

Bio-electrospraying is fast becoming an attractive tool for in situ cell delivery into scaffolds for tissue engineering applications, with several cell types been successfully electrosprayed. Bone marrow derived mesenchymal progenitor/stem cells (BMSC), which are an important cell source for tissue engineering, have not been explored in detail and the effect of electrospraying on their "stemness" is not known. This study therefore investigates the effects of electrospraying on BMSC viability, proliferation, and multilineage differentiation potential. Electrospraying a BMSC suspension at flow rate of 6 mL/h and voltages of 7.5-15 kV could successfully generate a continuous, stable and linearly directed electrospray of cells. Morphological observation, trypan blue tests and alamar blue based metabolic assays revealed about 88% of these electrosprayed cells were viable, and proliferated at rates similar to native BMSCs. However, at higher voltages, electrospraying became unstable and reduced cell viability, possibly due to electrical or thermal damage to the cells. BMSCs electrosprayed at 7.5 kV also retained their multipotency and could be successfully differentiated into adipogenic, chondrogenic, and osteogenic lineages, demonstrating similar morphology and gene expression levels as induced native BMSCs. These results indicate that bio-electrospraying could be safely used as a progenitor/stem cell delivery technique for tissue engineering and regenerative medicine applications.

Integrating novel technologies to fabricate smart scaffolds

Tissue engineering aims at restoring or regenerating a damaged tissue by combining cells, derived from a patient biopsy, with a 3D porous matrix functioning as a scaffold. After isolation and eventual in vitro expansion, cells are seeded on the 3D scaffolds and implanted directly or at a later stage in the patient's body. 3D scaffolds need to satisfy a number of requirements: (i) biocompatibility, (ii) biodegradability and/or bioresorbability, (iii) suitable mechanical properties, (iv) adequate physicochemical properties to direct cell-material interactions matching the tissue to be replaced and (v) ease in regaining the original shape of the damaged tissue and the integration with the surrounding environment. Still, it appears to be a challenge to satisfy all the aforementioned requisites with the biomaterials and the scaffold fabrication technologies nowadays available. 3D scaffolds can be fabricated with various techniques, among which rapid prototyping and electrospinning seem to be the most promising. Rapid prototyping technologies allow manufacturing scaffolds with a controlled, completely accessible pore network--determinant for nutrient supply and diffusion--in a CAD/CAM fashion. Electrospinning (ESP) allows mimicking the extracellular matrix (ECM) environment of the cells and can provide fibrous scaffolds with instructive surface properties to direct cell faith into the proper lineage. Yet, these fabrication methods have some disadvantages if considered alone. This review aims at summarizing conventional and novel scaffold fabrication techniques and the biomaterials used for tissue engineering and drug-delivery applications. A new trend seems to emerge in the field of scaffold design where different scaffolds fabrication technologies and different biomaterials are combined to provide cells with mechanical, physicochemical and biological cues at the macro-, micro- and nano-scale. If merged together, these integrated technologies may lead to the generation of a new set of 3D scaffolds that satisfies all of the scaffolds' requirements for tissue-engineering applications and may contribute to their success in a long-term scenario.

Jet-based methods to print living cells

Cell printing has been popularized over the past few years as a revolutionary advance in tissue engineering has potentially enabled heterogeneous 3-D scaffolds to be built cell-by-cell. This review article summarizes the state-of-the-art cell printing techniques that utilize fluid jetting phenomena to deposit 2- and 3-D patterns of living eukaryotic cells. There are four distinct categories of jetbased approaches to printing cells. Laser guidance direct write (LG DW) was the first reported technique to print viable cells by forming patterns of embryonic-chick spinal-cord cells on a glass slide (1999). Shortly after this, modified laser-induced forward transfer techniques (LIFT) and modified ink jet printers were also used to print viable cells, followed by the most recent demonstration using an electrohydrodynamic jetting (EHDJ) method. The low cost of some of these printing technologies has spurred debate as to whether they could be used on a large scale to manufacture tissue and possibly even whole organs. This review summarizes the published results of these cell printers (cell viability, retained genotype and phenotype), and also includes a physical description of the various jetting processes with a discussion of the stresses and forces that may be encountered by cells during printing. We conclude the review by comparing and contrasting the different jet-based techniques, while providing a map for future experiments that could lead to significant advances in the field of tissue engineering.

Stable electric-field driven cone-jetting of concentrated biosuspensions

Electrospraying, or electrohydrodynamic jetting, is one of several jet-based technologies being explored to process living biological organisms. One of the key advantages of electrospraying is its ability to deposit advanced materials with high resolution that cannot be obtained with other competing technologies, such as ink-jet printing. However, to generate a controlled droplet size distribution in the micrometre range necessary for precision drop and placement of materials requires jetting in stable cone-jet mode. In this paper, we describe the experimental set-up and conditions by which electrospray jetting in stable cone-jet is achieved and use this methodology to process a highly concentrated biological suspension having 10(7) cells ml(-1), the highest cellular loading processed to this day by a jetting approach in this jet based category. The areas of study to which this technology may be applied span the physical and the life sciences.

New ways to print living cells promise breakthroughs for engineering complex tissues in vitro

The ability to control the placement of cells and the assembly of networks in vitro has tremendous potential for understanding the regulation of development as well as for generating artificial tissues. To date, most engineering tools that can place materials with precision are not compatible with the requirements of living cells, and so approaches to tissue engineering have focused on patterning substrates as a way of controlling cell growth rather than patterning cells directly. In this issue of Biochemical Journal, however, Eagles et al. adapt electrohydrodynamic printing technology to 'print' living cells from a neuronal cell line on to a substrate. The importance of this approach is that it has the potential for unprecedented control over the position of cells in culture by directly placing them, thus allowing for the systematic assembly of cell networks.