Multi-resistive pulse sensor microfluidic device

Resistive pulse sensors have been used to characterise everything from whole cells to small molecules. Their integration into microfluidic devices has simplified sample handling whilst increasing throughput. Typically, these devices measure a limited size range, making them prone to blockages in complex sample matrixes. To prolong their life and facilitate their use, samples are often filtered or prepared to match the sample with the sensor diameter. Here, we advance our tuneable flow resistive pulse sensor which utilises additively manufactured parts. The sensor allows parts to be easily changed, washed and cleaned, its simplicity and versatility allow components from existing nanopore fabrication techniques such as glass pipettes to be integrated into a single device. This creates a multi-nanopore sensor that can simultaneously measure particles from 0.1 to 30 μm in diameter. The orientation and controlled fluid flow in the device allow the sensors to be placed in series, whereby smaller particles can be measured in the presence of larger ones without the risk of being blocked. We illustrate the concept of a multi-pore flow resistive pulse sensor, by combining an additively manufactured tuneable sensor, termed sensor 1, with a fixed nanopore sensor, termed sensor 2. Sensor 1 measures particles as small as 10 μm in diameter, whilst sensor 2 can be used to characterise particles as small as 100 nm, depending upon its dimensions. We illustrate the dual pore sensor by measuring 1 and 10 μm particles simultaneously.

A Tunable Three-Dimensional Printed Microfluidic Resistive Pulse Sensor for the Characterization of Algae and Microplastics

Technologies that can detect and characterize particulates in liquids have applications in health, food, and environmental monitoring. Simply counting the numbers of cells or particles is not sufficient for most applications; other physical properties must also be measured. Typically, it is necessary to compromise between the speed of a sensor and its chemical and biological specificity. Here, we present a low-cost and high-throughput multiuse counter that classifies a particle's size, concentration, and shape. We also report how the porosity/conductivity or the particle can influence the signal. Using an additive manufacturing process, we have assembled a reusable flow resistive pulse sensor capable of being tuned in real time to measure particles from 2 to 30 μm across a range of salt concentrations, i.e., 2.5 × 10^-4 to 0.1 M. The device remains stable for several days with repeat measurements. We demonstrate its use for characterizing algae with spherical and rod structures as well as microplastics shed from tea bags. We present a methodology that results in a specific signal for microplastics, namely, a conductive pulse, in contrast to particles with smooth surfaces such as calibration particles or algae, allowing the presence of microplastics to be easily confirmed and quantified. In addition, the shapes of the signal and of the particle are correlated, giving an extra physical property to characterize suspended particulates. The technology can rapidly screen volumes of liquid, 1 mL/min, for the presence of microplastics and algae.

Optical guiding-based cell focusing for Raman flow cell cytometer

We report the use of an optical guiding arrangement generated in a microfluidic channel to produce a stream of single cells in a line for single-cell Raman spectroscopic analysis. The optical guiding arrangement consisted of dual-line optical tweezers, generated using a 1064 nm laser, aligned in the shape of a '' symbol. By controlling the laser power in the tweezers and the flow rate in the microfluidic channel, a single line flow of cells could be produced in the tail of the guiding arrangement, where the 514.5 nm Raman excitation beam was also located. Furthermore, by resonantly exciting the Raman spectrum, a good-quality Raman spectrum could be recorded from the flowing single cells as they passed through the Raman excitation focal spot without the need to trap the cells. As a proof of concept, it was shown that red blood cells (RBCs) could be guided to the tail of the optical guide and the Raman spectra of the resonantly excited cells could be recorded in a continuous manner without trapping the cells at a cell flow rate of ∼500 cells per h. From the recorded spectra, we were able to distinguish between RBCs containing hemoglobin in the normal form (normal-RBCs) and the met form (met-RBCs) from a mixture of RBCs comprising met-RBCs and normal-RBCs in a ratio of 1 : 9.

Conductivity-based detection techniques in nanofluidic devices

This review covers conductivity detection in fabricated nanochannels and nanopores. Improvements in nanoscale sensing are a direct result of advances in fabrication techniques, which produce devices with channels and pores with reproducible dimensions and in a variety of materials. Analytes of interest are detected by measuring changes in conductance as the analyte accumulates in the channel or passes transiently through the pore. These detection methods take advantage of phenomena enhanced at the nanoscale, such as ion current rectification, surface conductance, and dimensions comparable to the analytes of interest. The end result is the development of sensing technologies for a broad range of analytes, e.g., ions, small molecules, proteins, nucleic acids, and particles.

Automated long-term monitoring of parallel microfluidic operations applying a machine vision-assisted positioning method

As microfluidics has been applied extensively in many cell and biochemical applications, monitoring the related processes is an important requirement. In this work, we design and fabricate a high-throughput microfluidic device which contains 32 microchambers to perform automated parallel microfluidic operations and monitoring on an automated stage of a microscope. Images are captured at multiple spots on the device during the operations for monitoring samples in microchambers in parallel; yet the device positions may vary at different time points throughout operations as the device moves back and forth on a motorized microscopic stage. Here, we report an image-based positioning strategy to realign the chamber position before every recording of microscopic image. We fabricate alignment marks at defined locations next to the chambers in the microfluidic device as reference positions. We also develop image processing algorithms to recognize the chamber positions in real-time, followed by realigning the chambers to their preset positions in the captured images. We perform experiments to validate and characterize the device functionality and the automated realignment operation. Together, this microfluidic realignment strategy can be a platform technology to achieve precise positioning of multiple chambers for general microfluidic applications requiring long-term parallel monitoring of cell and biochemical activities.

A microfluidic origami chip for synthesis of functionalized polymeric nanoparticles

This report demonstrates a microfluidic origami chip to synthesize monodisperse, doxorubicin-loaded poly(lactic-co-glycolic acid) nanoparticles with diameters of ~100 nm, a size optimized for cellular uptake and anticancer efficacy, but difficult to achieve with existing approaches. This three-dimensional design in a microchannel may allow for the fabrication of polymeric nanoparticles in this size regime with ease.