nDEP-driven cell patterning and bottom-up construction of cell aggregates using a new bioelectronic chip

An opto-thermal approach for assembling yeast cells by laser heating of a trapped light absorbing particle

Cell assembly has important applications in biomedical research, which can be achieved with laser-heating induced thermal convective flow. In this paper, an opto-thermal approach is developed to assemble the yeast cells dispersed in solution. At first, polystyrene (PS) microbeads are used instead of cells to explore the method of microparticle assembly. The PS microbeads and light absorbing particles (APs) are dispersed in solution and form a binary mixture system. Optical tweezers are used to trap an AP at the substrate glass of the sample cell. Due to the optothermal effect, the trapped AP is heated and a thermal gradient is generated, which induces a thermal convective flow. The convective flow drives the microbeads moving toward and assembling around the trapped AP. Then, the method is used to assemble the yeast cells. The results show that the initial concentration ratio of yeast cells to APs affects the eventual assembly pattern. The binary microparticles with different initial concentration ratios assemble into aggregates with different area ratios. The experiment and simulation results show that the dominant factor in the area ratio of yeast cells in the binary aggregate is the velocity ratio of the yeast cells to the APs. Our work provides an approach to assemble the cells, which has a potential application in the analysis of microbes.

Dielectrophoretic Traps for Efficient Bead and Cell Trapping and Formation of Aggregates of Controlled Size and Composition

We present a microfluidic dielectrophoretic-actuated system designed to trap chosen single-cell and form controlled cell aggregates. A novel method is proposed to characterize the efficiency of the dielectrophoretic trapping, considering the flow speed but also the heat generated by the traps as limiting criteria in cell-safe manipulation. Two original designs with different manufacturing processes are experimentally compared. The most efficient design is selected and the cell membrane integrity is monitored by fluorescence imaging to guarantee a safe-cell trapping. Design rules are suggested to adapt the traps to multiple-cells trapping and are experimentally validated as we formed aggregates of controlled size and composition with two different types of cells. We provide hereby a simple manufactured tool allowing the controlled manipulation of particles for the composition of multicellular assemblies.

Efficient Cell Impedance Measurement by Dielectrophoretic Cell Accumulation and Evaluation of Chondrogenic Phenotypes

The quantitative and functional analyses of cells are important for cell-based therapies. In this study, to establish the quantitative cell analysis method, we propose an impedance measurement method supported by dielectrophoretic cell accumulation. An impedance measurement and dielectrophoresis device was constructed using opposing comb-shaped electrodes. Using dielectrophoresis, cells were accumulated to form chain-like aggregates on the electrodes to improve the measurement sensitivity of the electrical impedance device. To validate the proposed method, the electrical impedance and capacitance of primary and de-differentiated chondrocytes were measured. As a result, the impedance of the chondrocytes decreased with an increase in the passage number, whereas the capacitance increased. Therefore, the impedance measurement method proposed in this study has the potential to identify chondrocyte phenotypes.

Engineering Biological Tissues from the Bottom-Up: Recent Advances and Future Prospects

Tissue engineering provides a powerful solution for current organ shortages, and researchers have cultured blood vessels, heart tissues, and bone tissues in vitro. However, traditional top-down tissue engineering has suffered two challenges: vascularization and reconfigurability of functional units. With the continuous development of micro-nano technology and biomaterial technology, bottom-up tissue engineering as a promising approach for organ and tissue modular reconstruction has gradually developed. In this article, relevant advances in living blocks fabrication and assembly techniques for creation of higher-order bioarchitectures are described. After a critical overview of this technology, a discussion of practical challenges is provided, and future development prospects are proposed.

Reactive Sputtered Silicon Nitride as an Alternative Passivation Layer for Microelectrode Arrays in Sensitive Bioimpedimetric Cell Monitoring

Microelectrode arrays (MEAs) are widely used to study the behavior of cells noninvasively and in real time. While the design of MEAs focuses mainly on the electrode material or its application-dependent modification, the passivation layer, which is crucial to define the electrode area and to insulate the conducting paths, remains largely unnoticed. Because often most cells are in direct contact with the passivation layer rather than the electrode material, biocompatible photoresists such as SU-8 are almost exclusively used. However, SU-8 is not without limitations in terms of optical transmission, optimal cell support, or compatibility within polymer-based microfluidic lab on chip systems. Here, we established a silicon nitride (SiN) passivation by physical vapor deposition (PVD), which was optimized and evaluated for impedance spectroscopy-based monitoring of cells. Surface characteristics, biocompatibility, and electrical insulation capability were investigated and compared to SU8 in detail. To investigate the influence of the SiN passivation on the impedimetric analysis of cells, HEK-293 A and MCF-7 were chosen as adherent cell models and measured on microelectrodes of 50-200 μm in diameter. The results clearly revealed an overall suitability of SiN as alternative passivation. While for the smallest electrode size a cell line dependent comparable or slightly decreased cell signal could be observed in comparison with SU-8, a significant higher cell signal was observed for microelectrodes larger than 50 μm in diameter. Furthermore, a high suitability for the bonding of PEGDA and PDMS microfluidic structures on the SiN passivation layer without any leakage could be demonstrated.

Dielectrophoretic Micro-Organization of Chondrocytes to Regenerate Mechanically Anisotropic Cartilaginous Tissue

Recently, many studies have focused on the repair and regeneration of damaged articular cartilage using tissue engineering. In tissue engineering therapy, cells are cultured in vitro to create a three-dimensional (3-D) tissue designed to replace the damaged cartilage. Although tissue engineering is a useful approach to regenerating cartilage, mechanical anisotropy has not been reconstructed from a cellular organization level. This study aims to create mechanically anisotropic cartilaginous tissue using dielectrophoretic cell patterning and gel-sheet lamination. Bovine chondrocytes were patterned in a hydrogel to form line-array cell clusters via negative dielectrophoresis (DEP). The results indicate that the embedded chondrocytes remained viable and reconstructed cartilaginous tissue along the patterned cell array. Moreover, the agarose gel, in which chondrocytes were patterned, demonstrated mechanical anisotropy. In summary, our DEP cell patterning and gel-sheet lamination techniques would be useful for reconstructing mechanically anisotropic cartilage tissues.

Defining Cell Cluster Size by Dielectrophoretic Capture at an Array of Wireless Electrodes of Several Distinct Lengths

Clusters of biological cells play an important role in normal and disease states, such as in the release of insulin from pancreatic islets and in the enhanced spread of cancer by clusters of circulating tumor cells. We report a method to pattern cells into clusters having sizes correlated to the dimensions of each electrode in an array of wireless bipolar electrodes (BPEs). The cells are captured by dielectrophoresis (DEP), which confers selectivity, and patterns cells without the need for physical barriers or adhesive interactions that can alter cell function. Our findings demonstrate that this approach readily achieves fine control of cell cluster size over a broader range set by other experimental parameters. These parameters include the magnitude of the voltage applied externally to drive capture at the BPE array, the rate of fluid flow, and the time allowed for DEP-based cell capture. Therefore, the reported method is anticipated to allow the influence of cluster size on cell function to be more fully investigated.

Dielectrophoresis-assisted creation of cell aggregates under flow conditions using planar electrodes

We present a microfluidic platform allowing dielectrophoresis-assisted formation of cell aggregates of controlled size and composition under flow conditions. When specific experimental conditions are met, negative dielectrophoresis allows efficient concentration of cells towards electric field minima and subsequent aggregation. This bottom-up assembly strategy offers several advantages with respect to the targeted application: first, dielectrophoresis offers precise control of spatial cell organization, which can be adjusted by optimizing electrode design. Then, it could contribute to accelerate the establishment of cell-cell interactions by favoring close contact between neighboring cells. The trapping geometry of our chip is composed of eight electrodes arranged in a circle. Several parameters have been tested in simulations to find the best configurations for trapping in flow. Those configurations have been tested experimentally with both polystyrene beads and human embryonic kidney cells. The final design and experimental setup have been optimized to trap cells and release the created aggregates on demand.

Modification of porous PLGA microspheres by poly-l-lysine for use as tissue engineering scaffolds

Due to their good biocompatibility, biodegradability and special shapes, porous poly(lactic-co-glycolic acid) (PLGA) microspheres show a wide application in the field of tissue engineering. Herein we demonstrate a simple and low-cost method for modifying porous PLGA microspheres with poly-l-lysine (PLL) to promote cell growth on the microspheres. Porous PLGA microspheres were first treated by sodium hydroxide (NaOH) solution to introduce carboxyl groups on their surface. Then, the hydrolyzed microspheres (PLGA-H) were immerged in PLL solution to yield PLL-impregnated microspheres (PLGA-PLL). Cell experiments showed that although the cytotoxicity of microspheres was slightly increased after PLL modification, their cell viability was still higher than 85%. Compared with PLGA and PLGA-H microspheres, PLGA-PLL microspheres were more favorable for MG63 cell to attach and proliferate due to their increased initial cell attachment numbers and enhanced cell-matrix interactions. This new modification method of porous PLGA microspheres proposes a route toward efficient repair of tissue defects at reduced risk and cost level.

Study of a Microfluidic Chip Integrating Single Cell Trap and 3D Stable Rotation Manipulation

Single cell manipulation technology has been widely applied in biological fields, such as cell injection/enucleation, cell physiological measurement, and cell imaging. Recently, a biochip platform with a novel configuration of electrodes for cell 3D rotation has been successfully developed by generating rotating electric fields. However, the rotation platform still has two major shortcomings that need to be improved. The primary problem is that there is no on-chip module to facilitate the placement of a single cell into the rotation chamber, which causes very low efficiency in experiment to manually pipette single 10-micron-scale cells into rotation position. Secondly, the cell in the chamber may suffer from unstable rotation, which includes gravity-induced sinking down to the chamber bottom or electric-force-induced on-plane movement. To solve the two problems, in this paper we propose a new microfluidic chip with manipulation capabilities of single cell trap and single cell 3D stable rotation, both on one chip. The new microfluidic chip consists of two parts. The top capture part is based on the least flow resistance principle and is used to capture a single cell and to transport it to the rotation chamber. The bottom rotation part is based on dielectrophoresis (DEP) and is used to 3D rotate the single cell in the rotation chamber with enhanced stability. The two parts are aligned and bonded together to form closed channels for microfluidic handling. Using COMSOL simulation and preliminary experiments, we have verified, in principle, the concept of on-chip single cell traps and 3D stable rotation, and identified key parameters for chip structures, microfluidic handling, and electrode configurations. The work has laid a solid foundation for on-going chip fabrication and experiment validation.

Digital microfluidic platform for dielectrophoretic patterning of cells encapsulated in hydrogel droplets

In this article, we present a method for cell patterning and culture within a hydrogel droplet on a digital microfluidic (DMF) platform.