Microfluidics is mainly about handling and manipulating very small amounts of liquids in micron-scale channels and chambers. The small size of the microchannels requires precise and reliable fluid delivery systems. In the past few decades, a wide range of methods for delivering fluids to microchannels has been developed. Therefore, researchers, scientists, and engineers have a broad range of options to choose from depending on the requirements. For example, in a microfluidic-based point of care diagnostic device, the fluid delivery rate might not be of high importance as much as the portability and affordability whereas in a research setting for single-cell analysis, the fluid delivery mechanism should be highly accurate with minimum fluctuations demanding a different fluid delivery method.
Syringe pumps are designed to deliver very precise amounts of fluids. As the name implies, a syringe pump delivers the fluid by pushing a syringe. As opposed to pressure pumps, syringe pumps adjust the flow by changing the flow-rate rather than the pressure. Therefore, syringe pumps do not need a flow-meter to control the flow rate. In most of the syringe pumps, there is a built-in library of syringe types and volumes where you can pick the syringe that you are using. Then, the pump automatically calculates the surface area of the syringe and the speed at which the syringe should be pushed to deliver the desired flow rate. However, in some older models, the syringe volume and the syringe surface area should be entered manually. In this case, you can find the surface area of common syringe types here.
Syringe pumps are easy to use and are the most common type of fluid delivery systems in microfluidics. They support from a single syringe to multi-channel syringes allowing for multiple reagents to be delivered at once. More advanced models can both infuse and withdraw the fluids or be programmed through Labview or Matlab softwares to create complex flow profiles to deliver the fluid with a particular pattern. For example, the blood flow is pulsatile in the arteries and a syringe pump could be programmed to perfuse a heart-on-a-chip in a pulsatile manner rather than a constant flow.
Although syringe pumps are an ideal option for research labs and R&D settings, they are not suitable for point of care applications and portable devices.
In pressure pumps, the fluid delivery is controlled by adjusting the pressure. Often the target fluid is in a sealed reservoir where it gets pressurized by the air above it and gets delivered to the microchannels. The reservoir can be used to agitate the sample before injecting them to the microfluidic chips and prevent sedimentation. The reservoir also allows for easy control of the temperature. Some pressure pumps need a pressure or vacuum line to pressurize the reservoir but more advanced do not require any pressure source. The major drawback of pressure pumps is that they need a flow control unit which in turn increases the cost of the system. Often, an in-line flow meter should connect to the system to read the flow rate. The operator can then play around with the pressure until the desired flow rate is reached.
In peristaltic pumps, a piece of tubing is coiled around the rotor. The tubing is flexible and by continuous and periodic compressing of the tube, the pump moves the fluid. These pumps are relatively inexpensive and can be used for recirculation of the samples. However, peristaltic pumps are usually associated with pulsation. The amount of pulsation in these pumps is often higher than syringe pumps making them unsuitable for applications in which precision is of importance. Additionally, they are not programmable and cannot create complex flow profiles.
The abovementioned methods are all active techniques meaning that they require an external power source to operate. Passive methods, however, do not require any power source to deliver the sample to the microfluidic chips. This is beneficial for point of care and diagnostics applications that need to be affordable and portable. Although not using external power sources makes passive methods ideal for point of care devices, it compromises the precision. Passive methods are not as precise and accurate as active methods. Also, active techniques can deliver the samples even in high-pressure drop chips but passive approaches are limited in their maximum capacity for fluid delivery.
Passive techniques could use a wide variety of natural forces such as inertia or capillary forces for fluid delivery. Some of the approaches include:
Capillary microfluidics is one of the most widely used methods in passively-driven microfluidic chips. Capillary action refers to the movement of a fluid confined by solid walls as a result of intermolecular forces between the two. The capillary force could be strong enough to overcome friction and gravity. The transport of fluids in microchannels using capillary forces could be similar to the transport of water and nutrients in plants.
A column of a liquid creates hydrostatic pressure. This hydrostatic pressure could be used to drive the samples through the channels or pressurize them. Often a tube -which is long enough to create the hydrostatic pressure sufficient to overcome the pressure drop in the microchannel- gets connected to the ports and drives the sample through the microchannel as a result of the pressure of the fluid column.
Osmosis is the process by which the molecules transport through a permeable membrane from a region with high concentration to another one of low concentration to equalize the concentration. This can be used in microfluidic chips to generate osmosis-based pressure. This technique often requires a more complex setup compared to other passive methods.
Here, the sample is pushed through the microchannels by manually being pressurized. This can happen by either pressing the reservoir that is storing the sample with a finger or by introducing the sample via a pipette. Although this approach is the easiest passive fluid delivery technique, it cannot be used when the pressure drop is too high.
This method takes advantage of creating a negative pressure to suck the liquid through the microchannels.