Desktop Fabrication of Lab-On-Chip Devices on Flexible Substrates: A Brief Review

Abstract

Flexible microfluidic devices are currently in demand because they can be mass-produced in resource-limited settings using simple and inexpensive fabrication tools. Finding new ways to fabricate microfluidic platforms on flexible substrates has been a hot area. Integration of customized detection tools for different lab-on-chip applications has made this area challenging. Significant advancements have occurred in the area over the last decade; therefore, there is a need to review such interesting fabrication tools employed on flexible substrates, such as paper and plastics. In this short review, we review individual fabrication tools and their combinations that have been used to develop such platforms in the past five years. These tools are not only simple and low-cost but also require minimal skills for their operation. Moreover, key examples of plastic-based flexible substrates are also presented, because a diverse range of plastic materials have prevailed recently for a variety of lab-on-chip applications. This review should attract audience of various levels, i.e., from hobbyists to scientists, and from high school students to postdoctoral researchers, to produce their own flexible devices in their own settings.

Keywords: biosensors; desktop fabrication; flexible devices; lab-on-chip; microfluidics.

Figure 1

Schematics showing different types of microfluidic platforms: (a) Polydimethylsiloxane (PDMS) based microchannels showing a continuous flow channel microfluidic platform. (b) Droplet microfluidic system to create immiscible liquid droplets working as micro-reactors. (c) Digital microfluidic system where liquid droplets move under the influence of electrical potential. (d) Paper-based microfluidic system which works based on capillary action that moves liquid through hydrophilic channels (Adapted with permission from IOP Science) [1].

Figure 2

(a) Wax patterning on a paper substrate, followed by heating step, to create hydrophobic borders across the substrate. (b) A typical example of a µPAD device for the minimum hydrophobic barrier test. Black boundaries show printed wax which prevents liquid to flow out of channel. (c) A paper-based device with multichannel system of various widths (adapted with the permission from Creative Commons Attribution-Non Commercial-No Derivatives License) [17].

Figure 3

An example of wearable sensor demonstrated on a balloon which shows adaptability to curvilinear surfaces. The multiple layers consist of an Ecoflex layer (inner), polyurethane (intermediate), a printable sensor (Ag/AgCl with Ecoflex composite) and a flexible insulator layer (upper) (Adapted with the permission from Wiley Materials) [18].

Figure 4

Types of fabrication methods to construct lab-on-chip devices (Adapted with the permission from Elsevier) [28].

Figure 5

Wax dipping method and detection of analytes: (a) Procedure for patterning paper by wax dipping (left). (b) Shape and size of an iron mask. (c) Detection of protein and glucose on a paper microfluidic device by the wax dipping method (Adapted with the permission from Elsevier) [29].

Figure 6

Wax on plastic platforms: (a) Wax printed microwells and stem cells applications. (b) Flow on fluorescently labeled DNA (green) and mixing of food colors (red and blue) in a wax-on-plastic microfluidic system. (c) Fabrication of a multilayer electrochemical sensor using a combination of wax and inkjet printing. (d) Device showing the gravitation flow of liquid through a wax printed channel on a transparent film for detection of Fe²⁺. (Adapted with permission from RSC and Springer Nature) [13,34,35,36,37].

Figure 7

(a) Quenching of the fluorescence band in the presence of different concentrations of Ag⁺ in mg/L under UV light at 365 nm. (b) Corresponding calibration curve obtained in the presence of various concentrations of Ag⁺. (Adapted with permission from American Chemical Society) [45].

Figure 8

Fabrication of 2D and 3D µPADs through vinyl cutter (Adapted with the permission from Elsevier) [57].

Figure 9

(a) Illustration of the continuous ink technical pen. (b) Technical pens fed continuously by ink. (c) The effects of different plotting speeds and numbers of passes on the water resistance of patterns in a continuous (top) and noncontinuous (bottom) plotting system. (d) Deflection of the plotted patterns in both the x and y directions. (e) comparison of the dimensions of the printed spots by the pen-plotter. (f) A representative image shows the performance of the continuous plotting system in high-throughput fabrication of paper-based microfluidics. (Adapted with the permission of American Chemical Society) [63].

Figure 10

3D electrochemical origami paper-based analytical device (omPAD) integrated with a custom CMOS potentiostat (Adapted with the permission from IEEE) [67].

Figure 11

Schematic of the microchannel fabrication and microchannel enclosing using wax printing and laser cutter where (a) printing wax pattern on a fiter paper, (b) heating step to penetrate wax through paper, (c) laser fabrication for adhesive seals, (d) inkjet printing of QR code and scale (e) enclosing microchannels by combining device layers, and (f) final device construct. (Adapted with the permission from Springer Nature) [68].

Figure 12

(a) Image of paper-based µPAD, which is fabricated by screen-printing of crayon eye liner on filter paper. (b) Schematic shows the process of sticking the µPAD to the paper-based heater using a piece of double-sided adhesive tape (Adapted with the permission of Springer Nature) [69].

Figure 13

Schematic representation of the fabrication steps of the micro-fluidic chip on PMMA where (a) device layers, assembly and application on human sweat monitoring and (b) sensor structure and surface (Adapted with the permission of Elsevier) [71].