Low-cost Microfluidic Paper based Analytical Devices (μPADs): Fabrication Methods, Flow Control, and Applications

The term microfluidic paper-based analytical devices (μPADs) was coined by Prof. Whitesides’ research group at Harvard University1,2 to define a new type of analytical device made from paper that meets the ASSURED3,4 criteria established by the World Health Organization (WHO) for an ideal diagnostic device in the developing world. The ASSURED criteria defined in 2004 include the following components: affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free, and deliverable to endusers.

μPADs meet the ASSURED requirements by being affordable with low fabrication costs. Typically, μPADs cost less than $1,5 with some as inexpensive as $0.002.6 The porous nature of the paper provides capillary action for fluid handling, reducing the need for external pump equipment. The paper is available in flexible sheets of various thicknesses that are compatible with existing printing technologies. It can be chemically modified and/or functionalized with a range of biomolecules, including DNA, proteins, and other small molecules, enabling a wide variety of tests either novel2 or adapted from previously developed paper-based diagnostics such as dip sticks and lateral flow assays.7 With its white base colour, the paper is an ideal substrate for colourimetric assays, allowing for simple visual interpretation of test results without requiring extensive user training. Additionally, the paper is thin and relatively light, making it easy to stack, store, and transport to the end-users. 

Nowadays, the ASSURED criteria have been revised to the REASSURED criteria, which further incorporate requirements for real-time connectivity, ease of sample collection, and environmental friendliness.8 Considering their advantages, μPADs can still satisfy the REASSURED criteria, particularly with recent advances in electronics and their incorporation into μPADs.9–11