The goal of this project was to develop valves for microfluidic devices and to move towards the development of microfluidic nanomachinery. Valves for paper strips are in development that will enable new and better disease diagnostics. For microfluidic nanomachines, we have successfully developed colour-switching microdroplet chromatophores. We are currently testing approaches to scale these to make flexible artificial chameleon skin.
Paper-based immunoassays are often used to detect biomarkers of disease or physiological state. One example is the home pregnancy test, which was used by 6% of US women in the last year alone. The emergence of low-cost paper microfluidics, where channels are defined in paper using wax printing,1 promises increased analytical performance through the better control of fluid flows. For example, adding signal enhancement steps can decrease the limits of detection by factors of 5 or more.
In the ANTNAM ERC proof-of-concept at Cambridge, we are developing actuating nano-transducers (ANTs) to valve fluid flow in such paper devices. ANTs are metal nanoparticles coated with a phase-changing polymer gel (pNIPAM). When heated above 32°C the gel collapses to a few-nm-thick compact layer, but when cooled back below 32°C the gel explosively refills with water.2 By loading these into paper, the hydro-phobicity/philicity can be switched to open or close a paper channel to fluid flow. When hydrophilic, fluid wicks into the paper channel as normal. When hydrophobic, the fluid is repelled and the channel is closed. The heating can be either direct from heating pads or indirect from illumination with light (even sunlight can be sufficient).
Robust valving has the potential to make new paper immunoassays commercially practical. In particular, this technology will enable the automation of assays that require multiple timed reagent addition/washing steps. The detection of malaria protein is one example, and we are currently targeting this for the proof of principle.
Initial results are promising, with flow rate valving by a factor of 50. We are working to improve the switching further, but already this should be sufficient for many applications. We also believe that it will be possible to throttle flow, which should enable quantitative sensing rather than the standard qualitative yes/no assays. We are working closely with Cambridge Enterprise on the intellectual property [Baumberg, et al, A reversible cycle phase change fluid. PC927751GB], and have obtained additional funding to continue this development for a further year. Our goal is to have a demonstrator device by the end of 2019. At that point we will more widely engage with companies, as our early engagement work showed that a demonstrator is vital for this stage.
