Our paper "A suspending-droplet mode paper-based microfluidic platform for low-cost, rapid, and
Paper-based microfluidic devices (μPADs) have attracted increasing attention as a new platform for point-of-care (POC) testing and other rapid tests, because of the convenience and low cost in use. However, there is still a major limitation in their functionality, mainly on the capability in handling multi-step reactions. Current μPADs commonly use water-soaking paper to fabricate the channels, so that pump-free liquid driving could be realized utilizing the capillary effect; but such design leaves no room for storage or on-demand manipulation of different liquids on the chip. On the other hand, in many real assays, different reagents need to be held in discrete reservoirs and mixed at specific time points. This major limitation has restricted the applications of μPADs in POC assays.
In this paper we present a new type of μPADs that can address the aforementioned limitation. These devices drive the liquids using surface tension and gravity rather than the capillary force used in conventional μPADs, and can support multiple steps of manipulations on demand. Different from the conventional μPADs that are made in water-soaking paper substrates, the paper substrate we used was art paper, which could prevent the absorption of liquid for a period of time. In such a design, liquid drops are suspended on the surface of the device in designed reservoirs; when the chip is tilted, the liquid drops will move to other reservoirs according to the guidance of channels defined on the surface. To differentiate it from reported μPAD devices that are fabricated with water-permeable paper, we name this new type of paper-based devices suspending-droplet mode paper-based microfluidic devices (SD-μPAD). To fabricate the superhydrophobic barrier pattern on the SD-μPADs, we developed a new method using Teflon micro-templates to produce inexpensive and precisely patterned superhydrophobic coating on paper.
As a demonstration of this new design, we developed a method using the reaction characteristics of iridium(III) complex for rapid, onsite detection of lead(II) ions in liquid samples. Because of the large Stokes shift of the iridium(III) complex probe, inexpensive optical filter systems can be employed, and we were able to make an inexpensive, battery-powered compact device for routine portable detection using a smartphone as a detector, allowing the rapid analysis and interpretation of results on site as well as the automatic dissemination of data to professional institutes, even in poor rural areas of developing countries.
