Materials and Fabrication Techniques for Nano- and Microfluidic Devices

The objectives of this research has been the development of various microfluidic devices for different applications, these applications include cell capture and integrating with sensing platforms. Novel microfluidic devices have been developed, these have been functionalised to contain antibodies that are able to capture cells that are pumped through the device. The device has been demonstrated as working successfully by microscopic analysis of the captured cancer cells in the device. This work demonstrates the successful proof of concept that these novel devices are functional and are ready to be taken forward for further development. A microfluidic adapter containing activated charcoal filter paper has been created that integrates with a DMSO-melanin pH sensor in order to overcome stability problems that occur when the sensor is used with blood samples. It is theorised that anions in the blood interfere with the silver reference electrode on the sensor. Anions bind to activated charcoal and as such can be removed by this filter paper. An increase in signal stability was observed however further investigation is required to see whether this impacts on the pH sensitivity of the sensor. Photonic measurements on novel waveguide chips have been demonstrated an initial proof of concept for the creation of a sensing platform that is based on this technology. The waveguide chips have been shown to be sensitive to changes in the refractive index measured when the device was tested with different samples including water, air and gold nanoparticles. The waveguide chips are also sensitive to changes in temperature. Future work involving functionalising these chips in order to make them respond to specific changes and analytes shows promise for the development of a novel sensor platform that has potentially useful environmental and clinical applications. Research into using square wave voltammetry and screen printed electrodes to investigate bacterial characteristics has been expanded and the suitability of using this technique for identifying bacteria has been investigated by testing multiple different bacterial species with these electrodes. This work has also introduced screen printed electrodes with gold working electrodes as well as the carbon working electrodes that have been used in previous work carried out in this research group. The voltammograms produced using the gold working electrode were better than those that were produced using the carbon working electrode as the shape of the voltammograms did show some correlation with the bacterial species that they were tested with. This could be investigated further by expanding on the number of different species tested and increasing the number of replicates.

Femtosecond-laser micromachining is an enabling technology that allows maskless realization of fully integrated photonic, microfluidic and micromechanical devices. Indeed, direct femtosecond laser writing can be used in transparent materials both for the fabrication of high-quality waveguides, and, possibly combined with chemical etching, for the realization of microfluidic channels. Moreover it can be used for surface structuring and functionalization, for the realization of high aspect-ratio microholes and microchannels by direct surface ablation, and for the fabrication of 3D micro/nanostructures by means of two-photon-polymerization. In this presentation, the basics of femtosecond laser writing will be summarized, and the main aspects influencing induced material modifications will be discussed. The main focus will be on femtosecond laser micromachining in transparent materials (typically glasses and crystals) where nonlinear absorption of light results in strongly localized permanent modifications of the material itself. At lower laser fluence "gentle" modifications can be induced, allowing direct writing of high quality optical waveguides. Waveguides characteristics can be optimized to make them suitable for the realization of integrated quantum circuits. At higher laser fluence nanogratings can be formed inside the material that allow selective chemical etching of microchannels. By combining waveguides and microchannels fully-integrated micro-optofluidic devices for sensing can be realized. Some typical applications will be presented, with particular focus on quantum technologies, to better illustrate the versatility and peculiarities of the technique. The results obtained show the strong potential of this microfabrication technology, paving the way to increasingly compact and multifunctional integrated quantum photonic and microfluidic devices.

Wearable microfluidic detection devices for noninvasive sweat analysis are highly desirable for human health monitoring. Herein, we present a bioenzyme-free microfluidic colorimetric detection device for simultaneous detection of creatinine (Cre), urea, and Mg2+ in sweat, enabling comprehensive kidney function and mental stress assessment. The device leverages a ternary catalytic system composed of H2MoO5 nanomaterial, Cu2+, and Cre, which exhibits significantly enhanced oxidase-like activity. This system directly catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to generate a blue product (ox-TMB), providing a robust platform for Cre detection without the need for bioenzymes. For urea and Mg2+ detection, 4-dimethylaminobenzaldehyde and chromium black T were employed as effective chromogenic reagents, respectively. A flexible multichannel microfluidic chip was fabricated, incorporating specific colorimetric reagent-modified paper chips into the corresponding reaction reservoirs. This design allows for the simultaneous detection of Cre, urea, and Mg2+ in sweat with linear ranges of 10-100 μM, 10-70 mM, and 0.5-8 mM, respectively. The device's performance was validated using actual sweat samples, with detection results analyzed via a smartphone application, demonstrating high accuracy and reliability. This work introduces a simple, low-cost, and stable bioenzyme-free microfluidic colorimetric detection device, offering significant potential for in situ and intelligent sweat analysis. The integration of smartphone technology further enhances its practicality, making it a promising tool for real-time health monitoring and point-of-care testing.

Integration of isothermal amplification methods in microfluidic devices: Recent advances

This paper discusses a low-cost technology for the fabrication of microfluidic devices on glass substrate using metal embossing technique. The fabrication technique demonstrated is a much simpler approach of embossing on glass using thermo-compression process with a patterned metal layer to define device structure. Well established printed circuit board fabrication photo-process is used to realize the desired planar geometry on metal layer deposited over a glass substrate. The depth of the channel is defined by the thickness of the metal deposited by electro-plating. The embossing technology offers a relatively safer approach conducive to batch processing to enable repeatable, high-yield, low-cost devices fabricated using low-cost equipment. Major challenges of achieving adhesion of the deposited thick nickel layer without peel-off and control of the thermo-compression process to achieve reliable and repeatable embossing without structural distortions were addressed. To prove its suitability for manufacturing, experiments were carried out with full wafer of 6″ × 6″ square glass wafer and optimal process steps for low-cost microfluidic device manufacturing have been well established.

Automation Highlights from the Literature

Life Sciences Discovery and Technology Highlights

In this thesis, nanometer scale semiconductor lasers and micrometer scale polymer based dye lasers are our focus in bringing the miniaturized lasers to applications in data transmission; ultra-small chemical / biological sensors; and ultra-compact spectroscopic sources. Combining the advantage of electrically driven semiconductor lasers and the advantage of a broad emission spectrum of dye molecules would utilize the highly dense multi-functional lab-on-a-chip by integrating microfluidic PCR, microfluidic fluorescent detection system, and compact visible and NIR detectors which are commercially available. On the other hand, in the meantime of pushing the size limit of the laser cavities, new phenomena with the nanoscale lasers enable further exploration and understanding in fundamental physics. In the first part of this thesis, two sub-micron scale semiconductor lasers are presented. The smallest lasers utilizing the disk structures—with diameters of approximately 600 nm—were realized in the InGaP/InGaAlP quantum well material system at room temperature, featuring ultra-small mode volumes of approximately 0.03 cubic?microns, and exhibiting single-mode operation at low threshold powers. And the first visible photonic crystal ultra-small mode volume lasers, with cavity volumes of approximately 0.01 cubic?microns, are realized in the same material system. They are ideally suited for use as spectroscopic sources and both of them can be lithographically tuned from 650 – 690 nm. In the second part of this thesis, two sub-millimeter-scale polymer-based dye lasers—a poly(dimethylsiloxane) (PDMS)-based mechanically tunable DFB dye laser and a poly(methylmethacrylate) (PMMA)-based second-order circular grating distributed feedback dye laser—are presented. Both of them are compatible with microfluidic technology, which gives freedom in integrating the lasers with the microfluidic chips. Compared to the soft lithography used in the PDMS-based dye laser, the nanoimprint lithography used in the PMMA-based dye laser would be more useful for fabricating ultra-small dye lasers and enabling mass production in the near future. At the end of the thesis, a nano-linewidth metal grating mask pattern transferred transient grating (MPT-TG) technique is described as a potential technique using the ultra-small lasers for molecular-dynamics study in solutions.

Three-color photographic process: R. Berthon

A prototype of a low-cost and easy-to-use thin layer coulometric microfluidic device based on an ion-selective membrane for calcium detection is described. The microfluidic device was fabricated and consequently assembled with inexpensive materials without using sophisticated and centralized fabrication laboratory facilities. The linear range of the device is found to be 10-100 μM for a 60 s current integration time. Preliminary validations showed that the microfluidic device is suitable for the quantification of calcium in mineral water.

There is a great interest in low-cost, versatile microfluidic platforms of which the fabrication processes are rapid, straightforward, and translatable to industrial mass productions. In addition, it is beneficial for microfluidic devices to be reconfigurable in the field, so that multiple functions can be realized by a minimum number of devices. Here, we present a versatile acrylic-tape platform which allows highly accessible rapid prototyping of microfluidic devices, as well as device reconfiguration to realize different functions. The clean-room-free fabrication and sealing process only requires a laser cutter, acrylic, and tapes and can be done by an untrained person in the field. We experimentally characterized the relationship between the capillary flow speed and the channel height, the latter of which can be well controlled by the fabrication process. Reconfiguration of microfluidic functions was demonstrated on a single acrylic-tape device, thanks to the reversible sealing enabled by functional tapes. Different pumping mechanisms, including on-chip pumps for better portability and syringe pumps for precise fluid control, have been employed for the demonstration of two-phase flow and droplet generation, respectively. The low-cost and versatile acrylic-tape microfluidic devices are promising tools for applications in a wide range of fields, especially for point-of-care biomedical and clinical applications.

Adhesive tape-assisted etching of silk fibroin film with LiBr aqueous solution for microfluidic devices

In this work, an Android application for measurement of nitrite concentration and pH determination in combination with a low-cost paper-based microfluidic device is presented. The application uses seven sensing areas, containing the corresponding immobilized reagents, to produce selective color changes when a sample solution is placed in the sampling area. Under controlled conditions of light, using the flash of the smartphone as a light source, the image captured with the built-in camera is processed using a customized algorithm for multidetection of the colored sensing areas. The developed image-processing allows reducing the influence of the light source and the positioning of the microfluidic device in the picture. Then, the H (hue) and S (saturation) coordinates of the HSV color space are extracted and related to pH and nitrite concentration, respectively. A complete characterization of the sensing elements has been carried out as well as a full description of the image analysis for detection. The results show good use of a mobile phone as an analytical instrument. For the pH, the resolution obtained is 0.04 units of pH, 0.09 of accuracy, and a mean squared error of 0.167. With regard to nitrite, 0.51% at 4.0 mg L(-1) of resolution and 0.52 mg L(-1) as the limit of detection was achieved.

Microfluidic devices offer precise control over solution mixing and gradient generation, essential for cell-based assays in cryobiology and biomedical research. However, traditional fabrication methods are time-consuming, costly, and require specialized expertise, which limits accessibility. To address these challenges, we developed a cost-effective, reliable, and fully 3D-printed microfluidic device workflow to facilitate rapid and inexpensive prototyping using a consumer-grade printer and biocompatible plastic resins. Here, we demonstrate this workflow with a fluidic mixing device capable of generating programmable concentration gradients and solution combinations. Commercial mixing devices cost more than $300 each and cannot be customized. By utilizing affordable resin materials and an innovative open-channel design sealed with transparent adhesive tape, we overcame common fabrication issues such as channel clogging, enabling rapid and reproducible fabrication of complex microfluidic architectures, all at a materials cost of less than $5. Here we demonstrate this workflow, integrating dual-syringe pumps to create linear osmotic gradients, ranging from iso-osmotic (~300 mOsm/kg) to hyperosmotic (~9,000 mOsm/kg) conditions, followed by a return to isotonicity over defined intervals. To ensure automation and reproducibility, we developed an open-source Python-based software tool that precisely regulates syringe pump activation, flow rates, and gradient timing. The device's performance was validated through continuous osmometric measurements, which confirmed both the linearity and accuracy of gradient generation, and colorimetric measurements to confirm mixing efficacy. This accessible and cost-effective microfluidic platform significantly improves the reproducibility of osmotic exposure studies and shows potential for various biomedical applications, including drug screening and precise chemical modulation.

Recently, micro-electro-mechanical-systems (MEMS) technology and micromachining techniques have enabled miniaturization of biomedical devices and systems. Not only do these techniques facilitate the development of miniaturized instrumentation for biomedical analysis, but they also open a new era for integration of microdevices for performing accurate and sensitive diagnostic assays. A so-called “micro-total-analysis-system”, which integrates sample pretreatment, transport, reaction, and detection on a small chip in an automatic format, can be realized by combining functional microfluidic components manufactured by specific MEMS technologies. Among the promising applications using microfluidic technologies, nucleic acid-based detection has shown considerable potential recently. For instance, micro-polymerase chain reaction chips for rapid DNA amplification have attracted considerable interest. In addition, microfluidic devices for rapid sample pretreatment prior to nucleic acid-based detection have also achieved significant progress in the recent years. In this review paper, microfluidic systems for sample preparation, nucleic acid amplification and detection for fast diagnosis will be reviewed. These microfluidic devices and systems have several advantages over their large-scale counterparts, including lower sample/reagent consumption, lower power consumption, compact size, faster analysis, and lower per unit cost. The development of these microfluidic devices and systems may provide a revolutionary platform technology for fast sample pretreatment and accurate, sensitive diagnosis.