Lab-on-a-chip technology is an emerging field evolving from the recent advances of micro- and nanotechnologies. The technology allows the integration of various components into a single microdevice. Microfluidics, the science and engineering of fluid flow in microscale, is the enabling underlying concept for lab-on-a-chip technology. The present paper reviews the design, fabrication and characterization of drug delivery systems based on this amazing technology. The systems are categorized and discussed according to the scales at which the drug is administered. Starting with the fundamentals on scaling laws of mass transfer and basic fabrication techniques, the paper reviews and discusses drug delivery devices for cellular, tissue and organism levels. At the cellular level, a concentration gradient generator integrated with a cell culture platform is the main drug delivery scheme of interest. At the tissue level, the synthesis of smart particles as drug carriers using lab-on-a-chip technology is the main focus of recent developments. At the organism level, microneedles and implantable devices with fluid-handling components are the main drug delivery systems. For drug delivery to a small organism that can fit into a microchip, devices similar to those of cellular level can be used.

We report a low-cost approach for sample concentration in a microfluidic paperbased analytical device using ion concentration polarization. This device platform can attain liquid sample filling by capillary suction through the microporous paper and ion selective transport through a nanoporous Nafion membrane under a DC electric field. The device is demonstrated for a fast depletion of fluorescent dye samples and an effective microfluidic concentrator for fluorescent dye with about 40-fold concentration enhancement achieved in 200 seconds. The device fabrication is simply based on paper cutting and lamination without need of lithography and printing of hydrophobic material such as wax. The lamination approach presented in this paper has the potential to be transferred to a large-scale production of paper-based analytical devices.

Individualized measurement of sweat loss under heat stress is important in assessing physical performance and preventing heat-related illness for athletes or individuals working in extreme environments. The objective of this work was to develop a low-cost and easy-to-fabricate wearable sensor that enables accurate real-time measurement of sweat rate. A capacitive-type sensor was fabricated from two conducting parallel plates, plastic insulating layers, and a central microfluidic channel formed by laser cutting a plastic film. The device has no microfabricated electrodes and is assembled using adhesive tape. Sensor accuracy was validated at different flow rates and confirmed using an equivalent circuit model of the device. On-body measurements demonstrate the feasibility of real-time measurements and show good agreement with values determined from a conventional sweat collection device.

Analysis of microRNA (miRNA), a pivotal primary regulator of fundamental cellular processes, at the single-cell level is essential to elucidate regulated gene expression precisely. Most single-cell gene sequencing methods use the polymerase chain reaction (PCR) to increase the concentration of the target gene for detection, thus requiring a barcoding process for cell identification and creating a challenge for real-time, large-scale screening of sequences in cells to rapidly profile physiological samples. In this study, a rapid, PCR-free, single-cell miRNA assay is developed from a continuous-flow microfluidic process employing a DNA hybridization chain reaction to amplify the target miRNA signal. Individual cells are encapsulated with DNA amplifiers in water-in-oil droplets and then lysed. The released target miRNA interacts with the DNA amplifiers to trigger hybridization reactions, producing fluorescence signals. Afterward, the target sequences are recycled to trigger a cyclic cascade reaction and significantly amplify the fluorescence signals without using PCR thermal cycling. Multiple DNA amplifiers with distinct fluorescence signals can be encapsulated simultaneously in a droplet to measure multiple miRNAs from a single cell simultaneously. Moreover, this process converts the lab bench PCR assay to a real-time droplet assay with the post-reaction fluorescence signal as a readout to allow flow cytometry-like continuous-flow measurement of sequences in a single cell with an ultrahigh throughput (300-500 cells per minute) for rapid biomedical identification.

Secreted proteins determine a range of cellular functionalities correlated with human health and disease progression. Because of cell heterogeneity, it is essential to measure low abundant protein secretions from individual cells to determine single‐cell activities. In this study, an integrated platform consisting of smart hydrogel immunosensors for the sensitive detection of single‐cell secretions is developed. A single cell and smart hydrogel microparticles are encapsulated within a droplet. After incubation, target secreted proteins from the cell are captured in the smart hydrogel particle for immunoassay. The temperature‐induced volume phase transition of the hydrogel biosensor allows the concentration of analytes within the gel matrix to increase, enabling high‐sensitivity measurements. Distinct heterogeneity for live cell secretions is determined from 6000 cells within 1 h. This method is tested for low abundant essential secretions, such as interleukin‐6, interleukin‐8, and monocyte chemoattractant protein‐1 secretions of both suspended cells (HL60) and adherent cells (MCF7 and MDA‐MB‐231). This platform is highly flexible and can be used to simultaneously measure a wide range of clinically relevant cellular secretions; it thus represents a novel tool for precise biological assays.