This article reviews recent developments in microfluidic impedance flow cytometry for high-throughput electrical property characterization of single cells. Four major perspectives of microfluidic impedance flow cytometry for single-cell characterization are included in this review: (1) early developments of microfluidic impedance flow cytometry for single-cell electrical property characterization; (2) microfluidic impedance flow cytometry with enhanced sensitivity; (3) microfluidic impedance and optical flow cytometry for single-cell analysis and (4) integrated point of care system based on microfluidic impedance flow cytometry. We examine the advantages and limitations of each technique and discuss future research opportunities from the perspectives of both technical innovation and clinical applications.

Circulating tumour cells (CTCs) in a blood circulation system are associated with cancer metastasis. The analysis of the drug-resistance gene expression of cancer patients’ CTCs holds promise for selecting a more effective therapeutic regimen for an individual patient. However, the current CTC isolation schemes might not be able to harvest CTCs with sufficiently high purity for such applications. To address this issue, this study proposed to integrate the techniques of optically induced dielectrophoretic (ODEP) force-based cell manipulation and fluorescent microscopic imaging in a microfluidic system to further purify CTCs after the conventional CTC isolation methods. In this study, the microfluidic system was developed, and its optimal operating conditions and performance for CTC isolation were evaluated. The results revealed that the presented system was able to isolate CTCs with cell purity as high as 100%, beyond what is possible using the previously existing techniques. In the analysis of CTC gene expression, therefore, this method could exclude the interference of leukocytes in a cell sample and accordingly contribute to higher analytical sensitivity, as demonstrated in this study. Overall, this study has presented an ODEP-based microfluidic system capable of simply and effectively isolating a specific cell species from a cell mixture.

Transdermal drug delivery is a convenient route to transport pharmaceuticals. However, its application is limited to a few compounds. Microneedle (MN) patches have demonstrated a high efficiency for delivering poorly permeable drugs and gained significant attention due to the advantages of being painless and convenient for patients as well as providing efficient delivery. This study presents dissolvable polyvinylpyrrolidone (PVP)-based MNs that do not require needle removal while providing the rapid release of encapsulated insulin. PVP with two molecular weights, PVP10 and PVP360, were used to fabricate the needle portion of the MNs, and PVP360/sodium carboxymethyl cellulose (CMC) was used for preparation of the backing layer. The detailed characteristics and in vitro skin penetration capability of a series of PVP360/PVP10 MNs were analyzed. The penetration depths of PVP MNs were evaluated using optical coherence tomography. The PVP360/PVP10 = 1 : 3 MN patch was chosen for ex vivo and in vivo studies. Ex vivo drug release profiles show that the permeation efficiency of Lissamine green B (LGB)- and fluorescein isothiocyanate-labeled BSA (FITC-BSA)-loaded PVP MN patches was much higher than the groups of patches without MNs. Furthermore, to estimate the feasibility of PVP MNs for diabetes treatment, insulin-loaded PVP MN patches were administered to diabetic mice to evaluate glycemic control. The relative pharmacologic availability revealed that the MN patch has an immediate and effective effect on hypoglycemic administration. This study demonstrates that dissolvable PVP MNs may serve as a promising device to deliver macromolecules or protein drugs for efficient transdermal drug delivery.

Although microfluidic cell culture systems are versatile tools for cellular assays, their use has yet to set in motion an evolutionary shift away from conventional cell culture methods. This situation is mainly due to technical hurdles: the operational barriers to the end-users, the lack of compatible detection schemes capable of reading out the results of a microfluidic-based cellular assay, and the lack of fundamental data to bridge the gap between microfluidic and conventional cell culture models. To address these issues, we propose a high-throughput, perfusion, three-dimensional (3-D) microfluidic cell culture system encompassing 30 microbioreactors. This integrated system not only aims to provide a user-friendly cell culture tool for biologists to perform assays but also to enable them to obtain precise data. Its technical features include (i) integration of a heater chip based on transparent indium tin oxide glass, providing stable thermal conditions for cell culturing; (ii) a microscale 3-D culture sample loading scheme that is both efficient and precise; (iii) a non-mechanical pneumatically driven multiplex medium perfusion mechanism; and (iv) a microplate reader–compatible waste medium collector array for the subsequent high throughput bioassays. In this study, we found that the 3-D culture sample loading method provided uniform sample loading [coefficient of variation (CV): 3.2%]. In addition, the multiplex medium perfusion mechanism led to reasonably uniform (CV: 3.6–6.9%) medium pumping rates in the 30 microchannels. Moreover, we used the proposed system to perform a successful cell culture-based chemosensitivity assay. To determine the effects of cell culture models on the cellular proliferation, and the results of chemosensitivity assays, we compared our data with that obtained using three conventional cell culture models. We found that the nature of the cell culture format could lead to different evaluation outcomes. Consequently, when establishing a cell culture model for in vitro cell-based assays, it might be necessary to investigate the fundamental physiological variations of the cultured cells in different culture systems to avoid any misinterpretation of data. As a whole, we have developed an integrated microfluidic cell culture system that overcomes several technical hurdles commonly encountered in the practical application of microfluidic cell culture systems, and we have obtained fundamental information to reconcile differences found with data acquired using conventional methods.

This study reports the utilisation of an optically switched dielectrophoretic (ODEP) force for the manipulation and assembly of cell-encapsulating alginate microbeads in a microfluidic perfusion cell culture system for bottom-up tissue engineering. One of the key features of this system is the ODEP force-based mechanism, which allows a commercial projector to be coupled with a computer to manipulate and assemble cell-encapsulating microbeads in an efficient, manageable, and user-friendly manner. Another distinctive feature is the design of the microfluidic cell culture chip, which allows the patterned cell-encapsulating microbeads to be cultivated on site under culture medium perfusion conditions. For demonstrating its application in bottom-up cartilage tissue engineering, chondrocyte-encapsulating alginate microbeads varying in encapsulated cell densities were generated. The manipulation forces associated with operating the alginate microbeads were experimentally evaluated. The results revealed that the measured manipulation forces increased with increases in both the applied electric voltage and the number of cells in the alginate microbeads. Nevertheless, the observed manipulation force was found to be independent of the size of the cell-free alginate microbeads. It can be speculated that the friction force may influence the estimation of the ODEP force within the experimental conditions investigated. In this study, chondrocyte-encapsulating alginate microbeads with three different cell densities were manipulated and assembled in the proposed microfluidic system to form a compact sheet-like cell culture construct that imitates the cell distribution in the cross-section of native articular cartilage. Moreover, the demonstration case also showed that the cell viability of the cultured cells in the microfluidic system remained as high as 96 ± 2%. In this study, four sheet-like cell culture constructs were stacked to create a larger assembled cell culture construct. The cell distribution inside the cell culture construct was further confirmed by a confocal microscopy observation, which showed that the distribution was similar to that in native articular cartilage. As a whole, the proposed system holds great promise as a platform for engineering tissue constructs with easily tunable inner cell distributions.