What is the responsiveness in the case of a microfluidic system and how to improve it?
According to the Computer Science and Communication Dictionary, responsiveness refers to the specific ability of a system of functional units to complete assigned tasks within a given time. In the case of microfluidics, we define responsiveness as the time it takes for the pressure or the flow-rate in the fluidic reservoir to reach a given set point. Three different times can be defined to describe the responsiveness of a microfluidic system:
- the response time of a microfluidic system
- the rising time of a microfluidic system
- the settling time of a microfluidic system
The global responsiveness is the sum of these three times. Microfluidic Flow Controllers display major benefits in terms of responsiveness compared to other types of fluid handling solutions such as syringe or peristaltic pumps.
The response time of a microfluidic system
The response time is the elapsed time between the sending of a command from the computer system and the beginning of a response. In microfluidics, response time is the duration between a command and the the start of flow (the first reaction of the microfluidic pump). When using a pressure controller, flow will start once a sufficient pressure rise occurs in the fluidic reservoir. The electrical and mechanical response times of the hardware components such as valves need to be as low as possible in order to decrease the response time.
The rising time of a microfluidic system
The rising time is the time a system takes to move from a starting value to a set value. It gives an idea of how quickly the system is able to change its value. However, it does not give any information about the fact that the setpoint is reached. Some microfluidic devices may have an excellent response time but also provide long transient states until flow is stable. This is typical of syringe pumps with particularly highly resistant systems as well as other pressure pumps with poorly developed algorithms.
In microfluidics, the rising time is the time elapsed to go from 10% to 90% (or sometimes 95%) of the requested pressure or flow-rate.
1. The settling time of a microfluidic system
The settling time is the elapsed time between the command and the moment the pressure or flow-rate gets and stays within a given range around the target value. It gives information about how fast the system reaches the set-point in a stable way. The settling time includes the response time, plus the rising time and finally, the time needed to be within the specified error margin.
In microfluidics, the settling time is generally defined as the time needed for the response of flow to reach and remain within an error band of ±5% of the final value of the flow. This is a function of the delivery system as well as the fluidic system resistance.
2. Advantages of pressure based fluid delivery solutions
When choosing a microfluidic instrument, it is important to not only look at the response time but also at the settling time to conduct the experiment at steady state as fast as desired. Pressure based flow controllers present several benefits compared to other technologies. The valves used in our instruments usually have response times below 10 ms, which is lower than most motors used in syringe pumps. Very reactive systems such as the Flow-EZTM and its FASTAB technology have very low rising and settling times. The graph below shows the response times of a MFCS flow controller with one of the best syringe pumps on the market.
The impact of the response time in microfluidic systems
One limitation to all systems comes from the fluidic system itself. The microfluidic resistance and hydraulic capacitance (coming from the elasticity that is present in the channel walls, especially when using PDMS) induce a delay in the response in terms of flow-rate. One can think of it this way: at the beginning, before reaching steady state, one has to pump a sufficient amount of energy into the compliant parts of the system so that all the energy coming from the source can be dedicated to moving the fluid. If the resistance of the system is very high, it can take some time to do that first step. It is possible to define a typical response time like its electrical analogue:
Typical response times for microfluidic systems range from less than a micro second to several seconds. In order to get very fast response times, the best strategy is to use very short and rigid connecting tubes.