Microfluidic cloth-based analytical devices: Emerging technologies and applications

Highlights

• Different fabrication methods for μCADs are introduced and compared.

• Cloth-based microfluidic functional components are discussed.

• The use of electrodes in electroanalytical μCADs are described.

• The detection methods and corresponding applications associated with μCADs are compared and categorized.

• The current development of wearable μCADs is demonstrated.

Abstract

Cloth (or fabric) is an omnipresent material that has various applications in everyday life, and has become one of the things people are most familiar with. It has some attractive properties such as low cost, ability to transport fluid by capillary force, high tensile strength and durability, good wet strength, and great biocompatibility and biodegradability. Hence, cloth is an ideal material for the development of economical and user-friendly diagnostic devices for many applications including food detection, environmental monitoring, disease diagnosis and public health. Microfluidic cloth-based analytical devices (μCADs) (or microfluidic fabric-based analytical devices (μFADs)) first emerged in 2011 as a low-cost alternative to conventional laboratory testing, with the goal of improving point of care testing and disease screening in the developing world. In this review, we examine the advances in the development of μCADs from 2011 to 2020, especially highlighting emerging technologies and applications related to the μCADs. First, different fabrication methods for μCADs are introduced and compared. Second, a series of cloth-based microfluidic functional components are discussed, including microvalves, fluid velocity control elements, micromixers, and microfilters. Then, electroanalytical μCADs are described, especially focusing on the use of cloth-based electrodes. Next, various detection methods for μCADs, together with their corresponding applications, are compared and categorized. In addition, the current development of wearable μCADs is also demonstrated. Finally, the future outlook and trends in this field are discussed.

Introduction

Since the first microelectromechanical system (MEMS) (Manz et al., 1990) was described over twenty-five years ago, microfluidics has become an empowering technology with wide potential applications in many fields such as chemistry, biology, medicine, physics and so on (Yager et al., 2006; Haeberle and Zengerle, 2007; Yeo et al., 2011; Sackmann et al., 2014; Yamada et al., 2015; Shang et al., 2017). In general, microfluidic systems can be defined as having one dimension of the channel less than 1 mm down to tens of micrometres. Most of microfluidic devices are regularly and successfully constructed from silicon, glass, and polymers (like polycarbonate, polymethylmethacrylate, and SU-8) by using lithographic techniques. However, in these systems, tedious, complex and relatively expensive microfabrication technologies have to be involved. Moreover, these small microfluidic devices usually need to be linked with a separate, cumbersome pumping system to drive the flow. All these factors remain a large obstacle to the development of more economical and scalable approaches for the fabrication of microfluidic devices.

As we know, polydimethylsiloxane (PDMS) is a relatively low-cost substrate for polymer-based microfluidics, and soft lithography also becomes a predominant technology for the fabrication of PDMS-based microfluidic devices (Whitesides, 2006; Berthier et al., 2012). However, these microfluidic devices have not yet solved problems that are otherwise insoluble. The disconnect stems from the fact that end-users of microfluidic devices want technology to be simple, cheap and invisible, and are only interested in solving their own problems whereas developers of technology are often fundamentally not interested in solving real problems (Whitesides, 2013). However, this is changing, and for this, capillary-based microfluidics may be the answer (Yetisen et al., 2013). Currently, tremendous efforts have focused on replacement of PDMS with alternative fiber materials. Cheap, porous materials can effectively wick liquids by capillary action, which eliminates the need for external power sources to generate flow.

In 2007, Whitesides and coworkers introduced the concept of using patterned paper as a microfluidic platform for diagnostic applications (Martinez et al., 2007). Since then, microfluidic paper-based analytical devices (μPADs) have been drawing increasing interests from researchers in different fields (Martinez et al., 2010; Yetisen et al., 2013; Gong and Sinton, 2017). μPADs combine some of capabilities of conventional microfluidic devices with the simplicity of paper strip tests. Especially, fully-developed μPADs have the potential for more rapid, less expensive and more highly multiplexed analyses (Martinez et al., 2010). However, μPADs have some important limitations as follows (Nilghaz et al., 2015; Liu et al., 2016a, 2017): (1) the choice of paper doesn't seem very wide; (2) the paper substrate is still relatively expensive; (3) paper is mechanically weaker, nondurable, low-flexibility, and especially loses its strength when it is too wet; and (4) the interstitial spaces in paper are usually relatively less.

As part of the continuous efforts in exploring new substrate materials for the fabrication of simple, inexpensive analytical devices, cloth (or fabric) has been used as a superior alternative to paper for the fabrication of more robust microfluidic devices (i.e. microfluidic cloth-based analytical devices (μCADs) or microfluidic fabric-based analytical devices (μFADs)). In 2011, Dendukuri and coworkers first introduced the concept of μCADs woven using silk yarns (Bhandari et al., 2011). Ever since, μCADs have been demonstrated in recent literature to have a significant potential for the development of simple, low-cost diagnostic devices. Cloth-based microfluidic technologies are affordable, user-friendly, robust, rapid, and scalable for fabricating, thus holding great potential to deliver point-of-care diagnostics to resource-limited settings or developing countries. In this review, we introduce the advances in the cloth-based microfluidics since 2011. At present, the development of μCADs is still in its infancy, and this review is important for researchers to have a comprehensive and objective understanding of the current situation and future trends of cloth-based microfluidic technologies. In this paper, the advantages of μCADs are emphasized, which may cause more researchers to pay close attention to this subfield of microfluidics. Meanwhile, this review also reveals the limitations of the existing technologies and provides some suggestions for the future technological development. Furthermore, the application of μCADs in combination with other detection technologies is discussed, which may provide a good reference value for the diversified development and application of future μCADs. We hope that this review will be helpful to researchers working in this area who wish to learn about the advances achieved to date and as well to readers who are interested in initiating work in this field.