**4. Microfluidic devices as diagnostic platforms for SARS-CoV-2/ COVID-19**

Microfluidic devices utilize fluid systems on the micro-scale that behave differently than that of fluids at volumes seen in day-to-day life. The main difference between micro and macro-scale fluid dynamics is seen in the readiness of micro-scale fluids to achieve a state of flow known as laminar flow [2]. Laminar flow describes a state of fluid flow where the fluid moves in continuous parallel layers, without any disruption between the layers of fluid [20]. In practice, this means that even at lower velocities, the fluid is not subjected to unwanted lateral mixing and the particles within the fluid itself are moving in straight, parallel, lines with one another. This directly contrasts with the type of flow ordinarily seen in macro-scale volumes, turbulent flow, which would instead experience rapid and chaotic variations in pressure and flow velocity across any given period. In turbulent fluids, particles are unevenly distributed and are subjected to random changes, which are undesirable traits when attempting to measure or work with systems that require a great deal of accuracy [20]. In the context of microfluidics, by taking advantage of the readily achievable laminar flow states, researchers and scientists can perform extremely useful and precise experiments that would normally be impossible with larger volumes [20, 21]. Interestingly, microfluidic technology was originally invented in the 1950s by Siemens-Elema, not as a diagnostic technique but as a subset of printing technology used to efficiently transport ink [21].

It was not until the turn of the millennium that this microfluidic technology really began seeing more practical medical applications [21]. As seen in **Figure 4**, the *Perspective Chapter: Microfluidic Technologies for On-Site Detection and Quantification… DOI: http://dx.doi.org/10.5772/intechopen.105950*

**Figure 4.** *A timeline of the historical milestones achieved with microfluidics [21].*

number of microfluidic applications has seen a steady increase since the early 2000s, largely in part to the extremely fast development of computing power and computational hardware, which allowed for many combinations of microfluidic technology, integrated with digital processing [19–21]. Therefore, the number of potential future microfluidic applications appears limitless.

## **4.1 Microfluidic classification and scientific relevance**

Over the years, microfluidics has grown in a way that several classifications of microfluidic technology exist to aid in organizing the myriad of its applications. The biggest classification of microfluidics distinguishes microfluidic systems between continuous-flow and droplet-based systems, sometimes called segmented flow.

As seen in **Figure 5**, Continuous-flow microfluidic systems utilize the characteristics of laminar flow to facilitate experimental processes that often require controlled mixing of micro-scale reagent and sample volumes [20]. Moreover, these continuousflow systems can serve as ideal platforms for other slow-processing experiments such as microfluidic-based PCR quantification and even real-time sample separation techniques. Conversely, droplet microfluidics allows for more instances of chaotic mixing to occur in a highly controlled environment, essentially facilitating numerous independent mixing phenomena to occur [20].

The differences in how these techniques can be achieved or improved upon with microfluidics depend on the micro-channel design of the system. In continuous-flow microfluidics, the four main categories of channel design include serpentine, spiral, oscillating-flow, and straight microchannels [22]. Each design channel design serves its own unique purpose when being used in microfluidics. For instance, serpentine

#### **Figure 5.**

*A visualization of (a) droplet, and (b) continuous-flow microfluidics fluid dynamic mixing [20].*

microchannels can be used to extend the time in which a fluid is exposed to external factors, such as UV light for crosslinking or heat to promote a particular reaction, across a more unified and even distribution of the fluid [18, 22]. Oscillatory continuous-flow microfluidics has been used to increase the effectiveness of liquid-liquid separations as well as aid in sample concentration techniques to improve detection outcomes in downstream processes [22]. These continuous-flow adaptations allow microfluidic systems to intrinsically offer more freedom for the experimental design than conventional techniques and can use a smaller working volume to make better use of smaller and costlier reagents. In general, continuous-flow microfluidic systems have seen an increase in popularity over the years due to their modularity, but do not compare in popularity to droplet microfluidic systems.

Droplet microfluidic systems forego the continuous laminar flow physics that is innate in continuous-flow systems, but instead make use of the micro-scale laminar flow physics to facilitate the production of individual droplet microenvironments [23]. These droplet microenvironments, shown in **Figure 6**, are discrete microlitre volumes that are formed through the controlled flow of a sheath (transporter) fluid across one or more sample fluids at a common junction [23]. Thus, repeatedly partitioning the sample fluids in a controlled manner and encapsulating the mixture of sample fluids into droplets. What makes droplet microfluidics so important and unique, even compared to continuous-flow microfluidics, is that it can allow for both high throughput experimentation and an increase in resolving power [2, 24]. Droplet microfluidic systems have been demonstrated to effectively process, in high throughput, experimental analysis in resolutions as high as single-cell resolutions; something many other platforms cannot achieve on their own [23, 24]. The various forms of droplet microfluidic systems can be classified into three main categories: high and ultrahigh throughput droplet microfluidics (htDM/uhtDM), digital droplet microfluidics (dDM), and controlled droplet microfluidics (cDM) [23]. In htDM and uhtDM, the main purpose of the system is to generate as many stable and uniform droplet microenvironments as possible to facilitate increased resolution and turnaround time. The production of thousands of comparable droplet microenvironments in a controlled manner promotes increased reaction efficiency as well as both qualitative and quantitative analysis through integration with digital technology. This integration between htDM and uhtDM with digital technology does not necessarily overlap with the dDM classification but does experience some similarities in that they can both utilize automation to enhance their analyses [24]. dDM systems aim to achieve complete automation through their integration with droplet microfluidics. However, it does not often utilize high throughput processing [23, 24]. Instead, purely dDM systems often generate multiple types of droplets in one system and have these different droplets interact with one another, in a highly controlled manner [23]. By utilizing components of both htDM/uhtDM systems and dDM systems,

*Perspective Chapter: Microfluidic Technologies for On-Site Detection and Quantification… DOI: http://dx.doi.org/10.5772/intechopen.105950*

#### **Figure 6.**

*A graphic highlighting the various droplet microfluidic classifications [23].*

researchers can achieve systems that are classified as controlled droplet microfluidic systems. These controlled droplet microfluidic systems gain some of the advantages of the other two systems and offer even greater modularity than either of those systems independently. As a result, the use of digital integration along with controlled droplet microfluidic systems microfluidics permits microfluidic platforms to be one of the most promising cost-reducing diagnostic platforms in both current and future diagnostic research.

#### **4.2 Advantages of microfluidic integration in diagnostic tools**

The increased efficiency of sample-reagent interactions is a particularly attractive characteristic of microfluidic devices which drives research towards improved diagnostic applications of microfluidic technology [25]. Normally, diagnostic technologies utilizing sample-reagent interactions between molecules of relatively weak binding affinities are regarded as non-optimal, in macro-scale volumes, due to the inefficiencies and potential overconsumption of resources [25]. Resources such as reagents, samples, and equipment can often be extremely resource-intensive and expensive, which can result in fewer tests per unit volume of sample and may discourage researchers from moving forward with a particular combination of cost-effective and practical diagnostic material. Ideal detection molecules for diagnostic purposes should offer an acceptable balance between cost, binding efficiency, and selectivity to the target molecule of interest. Finding this balance of characteristics in a detection molecule often requires lengthy amounts of research and testing, which would further incur operational costs. However, these limitations associated with non-microfluidic technologies can be improved upon with the use of microfluidic technology. Therefore, microfluidic integration can overcome many of the limitations associated with non-microfluidic systems and gain some advantages unique to microfluidics.

#### *4.2.1 Cost-reduction*

Conventional diagnostic techniques often require costly materials as well as logistical and operational costs, all of which create barriers limiting the access of these techniques to resource-limited settings [26]. Furthermore, due to the COVID-19 pandemic, unmet demand for conventional diagnostic tests had significantly increased the prices of individual tests, leaving several countries and low-income individuals with fewer options for obtaining these potentially lifesaving items [19, 26]. In 2021, it was found that a single COVID-19 rapid antigen test could have costed consumers upwards of \$20 USD, per test [2, 19, 25]. These essential diagnostic tools are often subject to the whim of the companies that produce and distribute the tests, which can only further limit access to lower-income individuals [16, 17]. By integrating microfluidic technology with previously existing platforms, microfluidics has the potential to reduce the overall costs of mass-producing rapid and accurate diagnostic tests for the detection and quantification of SARS-CoV-2.

#### *4.2.1.1 Fabrication*

To better optimize the costs for mass-producing diagnostic tests, microfluidic integration may help to reduce costs by permitting more variety in the materials used to fabricate tests. As seen in **Table 1**, commonly used materials in producing several microfluidic integrated systems including silicon, glass, polymers, and paper; each material, offers its own respective advantages and disadvantages [2, 26].

Silicon and glass were two of the most used materials in the fabrication of microfluidic devices due to their abundance [2]. These two materials offered great modularity in the type of potential applications the microfluidic systems would supplement. However, both suffered from high material costs [2, 26]. As technology advanced and other synthesized materials became more affordable, these two abundant microfluidic materials became less and less viable for the mass-production of diagnostic tests [16, 26]. As a result, materials such as polymers and paper became more attractive microfluidic substrates. Coupled with their simplified fabrication and processing methods, both paper-based and polymer microfluidic devices share stronger potential as on-site SARS-CoV-2 diagnostic tests, as compared to glass or silicon-based systems [16, 18, 19].

In addition to the reduction of microfluidic substrate costs, microfluidics reduces the inherent costs associated with reagent use since only microlitre volumes are used in the system. Each individual microfluidic test would, ideally, only require microlitre volumes of any reagents and samples to be effectively analyzed. Lower fluid volumes required for the successful operation of each test reduce the cost of fabrication per test and might allow for a better redistribution of financial resources.

#### *4.2.1.2 Operations*

A microfluid integrated diagnostic device has the potential to greatly reduce operational costs associated with processing samples. By reducing the number of experienced operators required to pilot the device or removing the need for expensive equipment to process samples, a large portion of the cost (per test) incurred by the consumer can be greatly reduced. This operational cost reduction can be achieved by developing a system with self-contained microfluidic tests that are qualitatively and quantitatively analyzed by a small portable test analyzer, with a single operator.

By developing a portable microfluidic analyzer that can independently process multiple diagnostic tests, in sequence or in parallel, only one unit operator would be required to perform on-site diagnoses. Depending on the design and software of the analyzer, the operator might not always require a great deal of experience to collect, process, and record samples. In the case where diagnostic sample processing

