Recent research for microfluidic devices that are fabricated on paper and textile substrates have largely been prompted by the desire to develop point-of-care (POC) diagnostic kits. Using textiles as a platform for microfluidics takes advantage of wicking either by drawing fluids through the space between threads or thread internal cavity structure. Designing smart threads layout or combining fibrous with hydrophilic and hydrophobic features can be used to realize flow control. Paper-based microfluidic devices without any imposed channels (printed etc.) will allow for wicking isotropically. However, the individual fiber/yarns of the fabric allow for definition of channels on their own. Besides, fabric microfluidic devices have some other advantages, e.g., flexibility, performing wetting/non-wetting contrast without additional barrier, 3D structure. Fig. 1 shows one example of fabric-based microfluidics: the cotton yarns on hydrophobic fabric transfer dyes in different zones.
Fabrication of Fabric Microfluidics
Fabrication methods of paper microfluidics can also be applied to fabric platforms. Since the porous structure of a piece of fabric provides an interconnected network of microchannels, by changing the layout or the combination of yarns, more versatile and precise fluid flow control inside the fabric microfluidic devices is enabled.
Cellulose materials like cotton cloths have gained extensive interest due to their cheap, easy-to-get properties. Instead of introducing other threads into the fabric, partial surface treatments are simple ways to modify the flow pattern in microfluidic networks. A cloth-based device with confined detection zones and restricted flow regions was fabricated by depositing hydrophobic materials such as wax. Similar to photolithography, complicated patterns can also be achieved by multiple surface treatments. As illustrated in Fig. 2, after spraying hydrophobic materials over the platform, surfaces with clear boundaries were created by laser scanning.
Chemical modification was also studied to enable fabrics with specific functionality, such as colorimetry and conductivity. Since capillary forces control the fluid flow towards detection zones, where dye or conductive materials were usually immobilized via printing or in situ polymerization, fabric-based microfluidic devices have the potential to be adapted to portable, miniaturized and robust analytical devices.
Unlike the photolithography method which is widely used to make flow patterns, a fabric-based microchannel network with heterogeneous surface properties can be easily achieved by weaving hydrophobic and hydrophilic yarns together. Moreover, standard textile production provides a large design space especially with the intrinsic yarn properties (i.e. natural and synthetic fibers, surface chemistry). For example, simultaneous coflow of immiscible fluids was demonstrated in an amphiphilic microchannels by weaving polypropylene fibers and poly(ethylene terephthalate) together, as shown in Fig. 3.
Applications and Challenges
Fabric-based microfluidic devices have demonstrated great potential in wearable devices for health care detecting body signals. However, fabrics usually have no significant chemical, optical or electrical properties that allow for their direct use as sensors. Therefore, most applications require a transducer material which may be colorimetric or electrical capable of converting an on-fabric chemical reaction into measurable signals.
Typical colorimetric fabric-based sensors that detect biomolecules are related to chemical color-change reactions is shown in Fig. 4. In this paper, a microfluidic cloth-based analytical devices (μCADs) for glucose sensing was fabricated. Functional materials, such as glucose oxidase enzyme, horseradish peroxidase enzyme and potassium iodide, were sequentially loaded on a pre-treated cloth. The photograph showed multiple sampling zones at each glucose concentration. When glucose is oxidized and produces peroxide which then reacts with the color transducer, potassium iodide, a color change is triggered. Biomolecules such as lactate, bovine serum albumin (BSA), nitrite ion, IgG antibody and human chorionic gonadotropin have already been successfully demonstrated based on similar mechanism. Despite the portability, cost-effectiveness, and ease of operation of these devices, background fabric color (fabric impurities) and color distribution often sacrifice the quantitative readout. Chemiluminescence is one way to alleviate background noise, but it also requires more precise reaction control.
The transduction of electrochemical signals can be performed in versatile modes including amperometric, potentiometric and impedimetric based on the sensing principles. Generally speaking, amperometric measures currents generated by redox reactions, usually enzyme based sensing, at fixed potentials. Potentiometric measures the difference potential between reference and working electrodes and is widely used for ion sensing. While impedimetric measures capacitance or resistance changes created by some binding events without electron transfer process, like the binding between antigen and antibody.
These approaches require the incorporation of electrodes onto the fabric (Fig. 5). These can be made either through printing or in situ polymerization of conductive materials. Electrical measurements often provide faster and more sensitive detection compared to colorimetric devices. However, the accumulation of non-target analytes may potentially serve as physical barrier which impedes electron transfer.
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