Tissue engineering with electric fields: investigation of the shape of mammalian cell aggregates formed at interdigitated oppositely castellated electrodes

Engineered 3D liver-tissue model with minispheroids formed by a bioprinting process supported with in situ electrical stimulation

Three-dimensional (3D) bioprinting, an effective technique for building cell-laden structures providing native extracellular matrix environments, presents challenges, including inadequate cellular interactions. To address these issues, cell spheroids offer a promising solution for improving their biological functions. Particularly, minispheroids with 50-100 μm diameters exhibit enhanced cellular maturation. We propose a one-step minispheroid-forming bioprinting process incorporating electrical stimulation (E-MS-printing). By stimulating the cells, minispheroids with controlled diameters were generated by manipulating the bioink viscosity and stimulation intensity. To validate its feasibility, E-MS-printing process was applied to fabricate an engineered liver model designed to mimic the hepatic lobule unit. E-MS-printing was employed to print the hepatocyte region, followed by bioprinting the central vein using a core-shell nozzle. The resulting constructs displayed native liver-mimetic structures containing minispheroids, which facilitated improved hepatic cell maturation, functional attributes, and vessel formation. Our results demonstrate a new potential 3D liver model that can replicate native liver tissues.

Rapid Formation of Aggregates with Uniform Numbers of Cells Based on Three-dimensional Dielectrophoresis

We applied a fabrication method for the formation of island organization of cells based on a three-dimensional (3D) device for negative dielectrophoresis (n-DEP) to produce cell aggregates with uniform numbers of cells rapidly and simply. The intersections formed by rotating the interdigitated array (IDA) with two combs of band electrodes on the upper substrate by 90° relative to the IDA with two combs on the lower substrate were prepared in the device. The AC voltage was applied to a comb on the upper substrate and a comb on the lower substrate, while AC voltage with opposite phase was applied to another comb on the upper substrate and another comb on the lower substrate. Cells dispersed randomly were directed toward the intersections with relatively lower electric fields due to n-DEP, which formed by AC voltage applied bands with the identical phase, resulting in the formation of island patterns of cells. The cells accumulated at intersections were promoted to form the cell aggregates due to the close contact together. The production of cell aggregations adhered together was easily found by the dispersion behavior after switching the applied frequency to convert the cellular pattern. When cells were accumulated at the intersections by n-DEP for 45 min, almost accumulations of cells were adhered together, and hence a formations of cell aggregations. By using the present method, we can rapidly and simply fabricate cell aggregations with a uniform number of cells.

Electrical Differentiation of Mesenchymal Stem Cells into Schwann-Cell-Like Phenotypes Using Inkjet-Printed Graphene Circuits

Graphene-based materials (GBMs) have displayed tremendous promise for use as neurointerfacial substrates as they enable favorable adhesion, growth, proliferation, spreading, and migration of immobilized cells. This study reports the first case of the differentiation of mesenchymal stem cells (MSCs) into Schwann cell (SC)-like phenotypes through the application of electrical stimuli from a graphene-based electrode. Electrical differentiation of MSCs into SC-like phenotypes is carried out on a flexible, inkjet-printed graphene interdigitated electrode (IDE) circuit that is made highly conductive (sheet resistance < 1 kΩ/sq) via a postprint pulse-laser annealing process. MSCs immobilized on the graphene printed IDEs and electrically stimulated/treated (etMSCs) display significant enhanced cellular differentiation and paracrine activity above conventional chemical treatment strategies [≈85% of the etMSCs differentiated into SC-like phenotypes with ≈80 ng mL^-1 of nerve growth factor (NGF) secretion vs. 75% and ≈55 ng mL^-1 for chemically treated MSCs (ctMSCs)]. These results help pave the way for in vivo peripheral nerve regeneration where the flexible graphene electrodes could conform to the injury site and provide intimate electrical simulation for nerve cell regrowth.

Magnetically shaped cell aggregates: from granular to contractile materials

In recent decades, significant advances have been made in the description and modelling of tissue morphogenesis. By contrast, the initial steps leading to the formation of a tissue structure, through cell-cell adhesion, have so far been described only for small numbers of interacting cells. Here, through the use of remote magnetic forces, we succeeded at creating cell aggregates of half million cells, instantaneously and for several cell types, not only those known to form spheroids. This magnetic compaction gives access to the cell elasticity, found in the range of 800 Pa. The magnetic force can be removed at any time, allowing the cell mass to evolve spontaneously thereafter. The dynamics of contraction of these cell aggregates just after their formation (or, in contrast, their spreading for non-interacting monocyte cells) provides direct information on cell-cell interactions and allows retrieving the adhesion energy, in between 0.05 and 2 mJ m(-2), depending on the cell type tested, and in the case of cohesive aggregates. Thus, we show, by probing a large number of cell types, that cell aggregates behave like complex materials, undergoing a transition from a wet granular to contractile network, and that this transition is controlled by cell-cell interactions.

Dielectrophoresis: an assessment of its potential to aid the research and practice of drug discovery and delivery

Dielectrophoresis (DEP) is an electrokinetic technique with proven ability to discriminate and selectively manipulate cells based on their phenotype and physiological state, without the need for biological tags and markers. The DEP response of a cell is predominantly determined by the physico-chemical properties of the plasma membrane, subtle changes of which can be detected from two so-called 'cross-over' frequencies, f(xo1) and f(xo2). Membrane capacitance and structural changes can be monitored by measurement of f(xo1) at sub-megahertz frequencies, and current indications suggest that f(xo2), located above 100 MHz, is sensitive to changes of trans-membrane ion fluxes. DEP lends itself to integration in microfluidic devices and can also operate at the nanoscale to manipulate nanoparticles. Apart from measurements of f(xo1) and f(xo2), other examples where DEP could contribute to drug discovery and delivery include its ability to: enrich stem cells according to their differentiation potential, and to engineer artificial cell structures and nano-structures.

Three-dimensional cell culture based on microfluidic techniques to mimic living tissues

This mini-review consists of microfluidic fabrication methods of cellular spheroids and cell-laden hydrogels, and their applications for tissue engineering. Using microfluidic devices, cellular spheroids and cell-laden hydrogels with controllable design are formed reproducibly. Owing to their size uniformity, they are used as building blocks for bottom-up tissue engineering to construct uniform and arbitrarily shaped tissues. Thus, cellular spheroids and cell-laden hydrogels based on microfluidic techniques are powerful tools to create tissues for human implantation and the treatment of diseases.

Formation of embryoid bodies using dielectrophoresis

Embryoid body (EB) formation forms an important step in embryonic stem cell differentiation invivo. In murine embryonic stem cell (mESC) cultures EB formation is inhibited by the inclusion of leukaemic inhibitory factor (LIF) in the medium. Assembly of mESCs into aggregates by positive dielectrophoresis (DEP) in high field regions between interdigitated oppositely castellated electrodes was found to initiate EB formation. Embryoid body formation in aggregates formed with DEP occurred at a more rapid rate-in fact faster compared to conventional methods-in medium without LIF. However, EB formation also occurred in medium in which LIF was present when the cells were aggregated with DEP. The optimum characteristic size for the electrodes for EB formation with DEP was found to be 75-100 microns; aggregates smaller than this tended to merge, whilst aggregates larger than this tended to split to form multiple EBs. Experiments with ESCs in which green fluorescent protein (GFP) production was targeted to the mesodermal gene brachyury indicated that differentiation within embryoid bodies of this size may preferentially occur along the mesoderm lineage. As hematopoietic lineages during normal development derive from mesoderm, the finding points to a possible application of DEP formed EBs in the production of blood-based products from ESCs.

A review of the responses of two- and three-dimensional engineered tissues to electric fields

The application of external biophysical signals is one approach to tissue engineering that is explored less often than more traditional additions of exogenous biochemical and chemical factors to direct cell and tissue outcomes. The study of bioelectromagnetism and the field of electrotherapeutics have evolved over the years, and we review biocompatible electric stimulation devices and their successful application to tissue growth. Specifically, information on capacitively coupled alternating current, inductively coupled alternating current, and direct current devices is described. Cell and tissue responses from the application of these devices, including two- and three-dimensional in vitro studies and in vivo studies, are reviewed with regard to cell proliferation, adhesion, differentiation, morphology, and migration and tissue function. The current understanding of cellular mechanisms related to electric stimulation is detailed. The advantages of electric stimulation are compared with those pf other techniques, and areas in which electric fields are used as an adjuvant therapy for healing and regeneration are discussed.

Construction by dielectrophoresis of microbial aggregates for the study of bacterial cell dormancy

A study of the effect of aggregate size on the resuscitation of dormant M. smegmatis was conducted by constructing cell aggregates with defined sizes and shapes using dielectrophoresis and monitoring the resuscitation process under controlled laboratorial conditions in a long-term cell feeding system. Differently sized cell aggregates were created on the surface of indium tin oxide coated microelectrodes, their heights and shapes controlled by the strength of the induced electric field and the shape of the microelectrodes. Both two-dimensional (ring-patterned) and three-dimensional cell aggregates were produced. The cell aggregates were maintained under sterile conditions at 37 degrees C for up to 14 days by continuously flushing Sauton's medium through the chamber. Resuscitation of dormant M. smegmatis was evaluated by the production of the fluorescent dye 5-cyano-2,3-ditolyltetrazolium chloride. The results confirm that the resuscitation of dormant M. smegmatis is triggered by the accumulation of a resuscitation promoting factor inside the aggregates by diffusion limitation.

Review article-dielectrophoresis: status of the theory, technology, and applications

A review is presented of the present status of the theory, the developed technology and the current applications of dielectrophoresis (DEP). Over the past 10 years around 2000 publications have addressed these three aspects, and current trends suggest that the theory and technology have matured sufficiently for most effort to now be directed towards applying DEP to unmet needs in such areas as biosensors, cell therapeutics, drug discovery, medical diagnostics, microfluidics, nanoassembly, and particle filtration. The dipole approximation to describe the DEP force acting on a particle subjected to a nonuniform electric field has evolved to include multipole contributions, the perturbing effects arising from interactions with other cells and boundary surfaces, and the influence of electrical double-layer polarizations that must be considered for nanoparticles. Theoretical modelling of the electric field gradients generated by different electrode designs has also reached an advanced state. Advances in the technology include the development of sophisticated electrode designs, along with the introduction of new materials (e.g., silicone polymers, dry film resist) and methods for fabricating the electrodes and microfluidics of DEP devices (photo and electron beam lithography, laser ablation, thin film techniques, CMOS technology). Around three-quarters of the 300 or so scientific publications now being published each year on DEP are directed towards practical applications, and this is matched with an increasing number of patent applications. A summary of the US patents granted since January 2005 is given, along with an outline of the small number of perceived industrial applications (e.g., mineral separation, micropolishing, manipulation and dispensing of fluid droplets, manipulation and assembly of micro components). The technology has also advanced sufficiently for DEP to be used as a tool to manipulate nanoparticles (e.g., carbon nanotubes, nano wires, gold and metal oxide nanoparticles) for the fabrication of devices and sensors. Most efforts are now being directed towards biomedical applications, such as the spatial manipulation and selective separationenrichment of target cells or bacteria, high-throughput molecular screening, biosensors, immunoassays, and the artificial engineering of three-dimensional cell constructs. DEP is able to manipulate and sort cells without the need for biochemical labels or other bioengineered tags, and without contact to any surfaces. This opens up potentially important applications of DEP as a tool to address an unmet need in stem cell research and therapy.

Application of low-frequency alternating current electric fields via interdigitated electrodes: effects on cellular viability, cytoplasmic calcium, and osteogenic differentiation of human adipose-derived stem cells

Electric stimulation is known to initiate signaling pathways and provides a technique to enhance osteogenic differentiation of stem and/or progenitor cells. There are a variety of in vitro stimulation devices to apply electric fields to such cells. Herein, we describe and highlight the use of interdigitated electrodes to characterize signaling pathways and the effect of electric fields on the proliferation and osteogenic differentiation of human adipose-derived stem cells (hASCs). The advantage of the interdigitated electrode configuration is that cells can be easily imaged during short-term (acute) stimulation, and this identical configuration can be utilized for long-term (chronic) studies. Acute exposure of hASCs to alternating current (AC) sinusoidal electric fields of 1 Hz induced a dose-dependent increase in cytoplasmic calcium in response to electric field magnitude, as observed by fluorescence microscopy. hASCs that were chronically exposed to AC electric field treatment of 1 V/cm (4 h/day for 14 days, cultured in the osteogenic differentiation medium containing dexamethasone, ascorbic acid, and β-glycerol phosphate) displayed a significant increase in mineral deposition relative to unstimulated controls. This is the first study to evaluate the effects of sinusoidal AC electric fields on hASCs and to demonstrate that acute and chronic electric field exposure can significantly increase intracellular calcium signaling and the deposition of accreted calcium under osteogenic stimulation, respectively.

Creation of arrays of cell aggregates in defined patterns for developmental biology studies using dielectrophoresis

It is shown that dielectrophoresis--the movement of particles in non-uniform electric fields--can be used to create engineered skin with artificial placodes of different sizes and shapes, in different spatial patterns. Modeling of the electric field distribution and image analysis of the cell aggregates produced showed that the aggregation is highly predictable. The cells in the aggregates remain viable, and reorganization and compaction of the cells in the aggregates occurs when the artificial skin is subsequently cultured. The system developed could be of considerable use for the in vitro study of developmental processes where local variations in cell density and direct cell-cell contacts are important.

The use of electric fields in tissue engineering: A review

The use of electric fields for measuring cell and tissue properties has a long history. However, the exploration of the use of electric fields in tissue engineering is only very recent. A review is given of the various methods by which electric fields may be used in tissue engineering, concentrating on the assembly of artificial tissues from its component cells using electrokinetics. A comparison is made of electrokinetic techniques with other physical cell manipulation techniques which can be used in the construction of artificial tissues.