Go with the capillary flow. Simple thread-based microfluidics

Studies on microfluidic thread-based analytical devices (μTADs) have been developed for a decade. Threads, made of natural cellulose or synthesized polymer fibres, are manufactured in the form which possesses a large number of capillary gaps for liquid transport, and show differences in structural morphologies and surface chemistry. Threads have been used to design simple thread-based microfluidics with different analytical functions. Due to their low-cost, portability, biocompatibility and ease of integration with other components, threads show a great application basis to fabricate point-of-care μTADs. To date, there are a limited number of published studies on μTADs in contrast to paper-based analytical devices. To facilitate further advances of μTADs, this review summarizes the development of these devices from the aspects of engineering control of capillary flow on threads and biological applications. Particularly, as liquid flow on threads affects the transport and detection efficiencies, strategies of controlling liquid flow on threads are firstly reviewed. This includes thread surface modification, proper thread type selection, and microfluidic channel design. Then various detection methods including colorimetry, fluorescence, electrochemistry and surface-enhanced Raman spectroscopy applied on μTADs are reviewed. The biological applications of μTADs in blood separation and analysis, immunoassay, wearable sensing, as well as compatibility with smartphones for biosensing are also systematically summarized. Finally, the future perspectives of μTADs regarding external collaboration to satisfy the “REASSURED” criteria are proposed. This review is expected to expand understandings of μTAD designs and applications, and to clarify the challenges and future development directions of μTADs.