Volume Control in A Microfluidic Device
In order to effectively carry out microfluidic experiments, it is necessary to use the most suitable method to control the different volume control techniques of the microfluidic flow. This page introduces the existing microfluidic technology, mainly including syringe pump, peristaltic pump, and other techniques.
Microfluidic Volume Definition
Chips, pipes, and accessories in microfluidics will account for the total volume of the system, called the "internal volume". The internal volume is the sum of two different volumes-the swept volume and the dead volume.
Microfluidic "Sweep" Volume
The swept volume is the portion of the internal volume that is directly in the flow path-when fluid flows through the device, the fluid must flow through this volume. It is usually best to keep this volume as low as possible.
Microfluidic Dead Volume
The dead volume is the part of the internal volume outside the flow path. This means that the liquid entering the volume will not be recovered or immediately recovered. It is a "buffer tank". All microfluidic component manufacturers try to minimize the dead volume in their products.
Microfluidic Internal Volume
The internal volume is the sum of the clearance volume and the dead volume. If the connection to the pipe is not optimized, an additional internal volume can be created. For the liquid to flow directly into the chip completely from the tube, all tubes need to be fully seated and tightened at all times.
Syringe pumps are widely used in standard laboratories. The flow rate is equal to the cross-section of the syringe multiplied by the linear velocity imposed by the mechanical system. A wide range of flow rates can be achieved by changing the volume of the syringe or the linear velocity of the piston.
However, the flow pulsation generated by mechanical actuation can affect the experiment, especially in the case of low flow rates of rigid pipes or viscous liquids. The response and set-up time may be very long, and it usually takes more than 15 minutes to reach a stable flow.
In addition, the actual traffic in the system will not be monitored, and users can only access the traffic sequence. This may lead to deviations in the results.
Its pressure is not controlled. If the microsystem is blocked, the pressure will rise. Exceeding a certain limit may damage the syringe or the microsystem. In this case, the experiment needs to be constantly monitored, so it becomes difficult to automate the experiment.
The liquid is contained in the hose of the peristaltic pump, and the alternating compression and relaxation will suck the liquid and move it forward. The flow arterial movement is very high, resulting from the instability of about 20% CV. Compared with the syringe pump, the flow arterial movement is more obvious. To obtain different flow rates, the inner diameter of the hose must be changed.
The principle is to generate electroosmotic flow in a porous medium made of glass. The main advantage is that there are no moving parts and can be directly controlled by electrical signals. The main disadvantage is that it can only be used with ionic water or alcohol.
The integrated micropump is mainly based on the principle of peristalsis and a flexible membrane made of PDMS. The flow rate range is usually low. The main advantage is the opportunity to control the fluid within the pL range. It is particularly useful for applications where very small amounts of fluid are to be tested.
Figure 1. (a) Schematic illustration of a droplet-based microfluidic system containing integrated pneumatic micropumps. (b) Cross-sectional view of A-A′ in part (a). The microfluidic device comprises three layers: a flexible polydimethylsiloxane (PDMS) membrane, a planar glass substrate and a micro-patterned PDMS substrate. (Choi J. W, et al. 2014)
- Choi J. W, et al. (2014). "Integrated Pneumatic Micro-pumps for High-throughput Droplet-based Microfluidics." RSC Adv. 4: 20341-20345.
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