Microfluidic functions

CHAPTER 1. Introduction

1.3 Lab-on-a-disc

1.3.2 Microfluidic functions

In order to achieve bio- or chemical- assays composed of complex multiple liquid handling, various unit operations are equipped on lab-disc system. Herein, several microfluidic functions are presented with relevant examples.

Centrifugal pumping

By spinning a disc, a centrifugal force is generated to induce that the radial transport of fluids outwards from the center towards the outer edge of the disc. The flow of fluids in a lab-disc system depends on the rotational speed, radial location and dimensions of the fluidic channels and reservoir, as well as fluidic properties.^{8, 24} The average flow velocity (U) in a micro-channel by centrifugal pumping can be derived as:

𝑈 =𝐷_ℎ²𝜌𝜔²𝑟̅∆𝑟

32𝜇𝐿 (8)

where 𝐷_ℎ is the hydraulic diameter of the channel (defined as 4A/P, A is the cross-sectional area and P is the wetted perimeter of the channel), 𝜌 is the density of the liquid, 𝜔 is the angular velocity of the disc, 𝑟̅ is the average distance of the liquid in the channels from the center of the disc, ∆𝑟 is the radial extent of the fluid, 𝜇 is the viscosity of the fluid, and L is the length of the liquid in the microchannel (Figure 1.3).²⁴ From this derivation, the volumetric flow rate, Q, can be given as 𝑈 ∙ 𝐴.

Duffy et al. demonstrated that the theoretical flow rates calculated by equation (8) are well- matched with the experimental flow rates in a wide range of flow rates (5 nL/s to 0.1 mL/s) taken from

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the corresponding rotational speeds of 400 to 1600 rpm and with the following channel dimensions:

widths from 20 to 500 μm; depths from 16 to 340 μm; lengths from 12.5 to 182 mm.⁹⁴ This study reports that the centrifugal pumping is insensitive to the many physicochemical properties of the fluid, such as ionic strength and pH, meaning it can be used to pump various biological fluids including blood and urine.

Valving

The valving technique is crucial unit operation in lab-disc systems, actually even in any microfluidic system, for the integration of assays incorporating multiple procedures (Figure 1.4).

Basically, valving techniques can be categorized into two type of valves, passive valves and active valves, by the presence or absence of external actuations such as heating, and pressing.

There are several passive valves including capillary valves, hydrophobic valves, and siphon valves. Capillary valves are actuated by the balance between the capillary pressure of micro-channel and centrifugally-induced pressure. If the centrifugally-induced pressure is larger than the capillary pressure, the valve can be opened, otherwise the valve will remain closed.⁸ The capillary force is proportional to the dimensions of the micro-channel, and the centrifugal-induced pressure can be tuned depending on the rotational speed. Hydrophobic valves rely on the phenomena where a hydrophobic region disturbs the transfer of aqueous fluids. A hydrophobic region can be made by a sudden narrowing in a hydrophobic channel, or surface functionalization that increases the hydrophobicity.⁸ The valve will also open when the centrifugally-induced pressure is larger than the critical value of the capillary pressure at the hydrophobic barriers. A siphon valve relies on a disc-based siphon structure which is actuated by spinning the disc, that can modulate the state of valves, and can transfer the liquids.⁸ In order to open a siphon valve, a siphon channel must be hydrophilic to be primed with liquids. A siphon valve can be useful to particular steps that requires high spin speeds what cannot use the other passive valves.

Although the passive valves can be easily controlled without additional actuations by changing the rotational speed of the disc, it might restrict the dimensions and location of fluidic channels, and the rotating speeds, so it cannot be applied to integrate sequential operations of complex bio- or chemical assays. In addition, the passive valves are not vapor tight. In other words, they cannot prevent the evaporation of stored liquid that can change the composition of analytes of assays. Therefore, various active valves have been developed that make it possible to integrate more complex assays on the disc.

For example, Park et al. reported the laser irradiated ferrowax microvalves (LIFMs) that are controlled by laser irradiations.²⁰ LIFMs are composed of paraffin wax mixed with 10 nm-sized iron oxide nanoparticles so that the normally closed/opened LIFMs change their state to the opposite state (closed

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to open, open to closed) when the paraffin wax is melted by the heat released from iron oxide nanoparticles under laser irradiation. Recently, Kim et al. developed the reversible and thermally stable diaphragm valves (ID valves) that are controlled by a simple push and twist motion.^21-22 This valve provides highly repeatable and reversible actuations, so it is advantageous in the integration of highly complex assays on a disc, such as the N-fold serial dilution of analytes.²² Overall, passive valves are often utilized in relatively simple and compact applications, whereas active valves are suitable for more complex assays.

Metering

Metering is necessary to appropriately carry out most assays as well as microfluidic applications. Certainly, the lab-disc system provides robust automatic volumetric metering. Once the chamber has primarily filled with fluids by spinning the disc, the overflow of the residual fluids is channeled out to a waste chamber, as shown in Figure 1.5(a). Although the volume of the metering chamber must be pre-determined, precise metering can still be achieved. A previous study reported the metering of 300 nL with a 5% coefficient of variability.³³

Mixing

Mixing is an important unit-operation especially for biomedical applications. For example, the sample of bacterial cells should be mixed well with lysis buffer to lyse the bacteria, then to release the bacterial DNA and RNA in order to carry out a downstream analysis. Though, also in the conventional microfluidic platform, lots of studies have attempted to achieve efficient mixing in micro-channels,⁹⁵ it still be limited to diffusive mixing due to low Reynolds numbers with laminar flow so that the mixing is very slow. On the other hand, a lab-disc can simply implement the convective mixing with rapid oscillations of the disc body even in low Reynolds number regimes (Figure 1.5(b)).³⁷ Surely in larger- scale fluidic chambers, the fluids can be actively mixed by the flow inertia, counteracting viscous damping. In addition, mixing on a disc is enhanced by introducing supporting materials such as micro- particles, paramagnetic particles with magnetic actuations³², and artificial air-bubbles.⁹⁶

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Figure 1.4. Various valving techniques equipped on a lab-on-a-disc system. (a-c) three kinds of passive valves;⁸ (a) Capillary valve, (b) Hydrophobic valve, (c) Siphon valve, (d-e) active valves;

(d) Laser irradiated ferrowax microvalves (LIFMs),²⁰ (e) reversible and thermally stable diaphragm valves (ID valves).^21-22

Figure 1.5. Efficient metering and mixing on a lab-on-a-disc system. (a) Schematic diagram for the explanation of metering operation on a disc,³³ (b) Rapid oscillations of disc by changing the spinning direction enhance the mixing.³⁷

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