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Microfluidic Instruments Stability

Definition of stability in microfluidic instrumentation

In microfluidic instrumentation, stability refers to the ability of an instrument to maintain a certain physical property at a constant value, while rejecting any perturbations in the environment. It is a very important parameter, as even small variations in physical quantities during microfluidic experiments can dramatically change the results. A stable instrument ensures that results are repeatable and reproducible.

How to measure a microfluidic instrument's stability?

The stability of a microfluidic instrument can be quantified by setting the microfluidic instrument to a constant value, measuring the physical quantity across a period of time and using statistical measures of dispersion on the samples acquired. We generally use the three metrics below.

RangeStandard deviationCoefficient of variation
formula formula formula
LaTeX: \Delta = x_{max} - x_{min}LaTeX: s = \sqrt{\frac{1}{N-1}\sum_{i=1}^N (x_i - \bar{x})^2}LaTeX: CV = \frac{s}{\bar{x}}
Xmax: Highest measured valueN: Number of sampless: Standard deviation
X: mean of samples
Unit: same as the sampleUnit: same as the sampleUnit: dimensionless

1. The use of the range of metric for measure a microfluidic instrument's stability

This metric is the difference between the highest and the lowest value measured during the characterization. It is sensitive to outliers and not very representative of the microfluidic instrument’s stability, but it can be important if the experiment has narrow specifications and can be disturbed or damaged by deviations, even for a short amount of time.

2. The use of the standard deviation metric for measure a microfluidic instrument's stability

Standard deviation metric is an estimate of the typical distance between each sample and the mean. It is more representative of the microfluidic instrument’s stability than the range, as it gauges the performance across all data points.

3. The use of the coefficient of variation for measure a microfluidic instrument stability

The coefficient of variation metric is the ratio between the standard deviation and the mean of the samples. It is the most useful value to characterize a microfluidic instrument’s stability, as it is independent of scale and unit.

higher stability graph

What is resolution and what is its effect on the microfluidic instrument's stability?

The stability of a microfluidic instrument depends on the resolution and response time of its sensors and actuators as well as the algorithms for communication and feedback. The resolution is the smallest difference in the physical quantity that the sensor is able to detect, or the smallest variation that can be produced on the actuator’s output. The better the resolution, the more closely the microfluidic instrument can follow its inputs and control its outputs to regulate the quantity of interest. The response time refers to the time it takes for the sensor to produce a new reading, or the time it takes for the actuator’s output to reach the requested value. The shorter the response time, the faster the instrument can run its feedback loop to compensate any variations in the quantity of interest.

How to regulate the flow of a microfluidic intrument?

The main quantity that must be regulated in microfluidics is the flow rate. Our microfluidics instruments control the flow rate using pneumatic pressure. Microfluidics instruments are able to compensate for variations in the input pressure and the ambient pressure and temperature, in order to generate a very precise and stable pressure inside a sealed reservoir, causing the fluid to flow out of this reservoir into the microfluidic chip.

Pressure regulators improve microfluidic instrument's stability

The stability provided by pressure regulators for microfluidics is generally better than the resolution of the internal pressure sensor, thanks to the technique of oversampling. By reading the sensor at a very fast rate (from 1000 to 2500 Hz), the microfluidic instrument obtains multiple samples corresponding to the same real pressure value, since the pressure changes at a slower pace. These samples contain electronic noise inherent to the sensor, but that noise is zero-centered, which means we can greatly attenuate it by averaging the samples, thus achieving a more precise measurement of the pressure. The regulation algorithm uses an integrator, so it naturally averages the sensor measurements and is able to maintain the pressure with a dispersion much smaller than the sensor’s resolution.

Stable pressure is often not enough to maintain a stable flow rate. For microfluidics chips with a low fluidic resistance fluidic resistance, the pressure required to maintain the flow is small, so any changes in the height of liquid inside the reservoir (either because it is emptied over time or because it was moved during the experiment) will change the flow rate due to the hydrostatic pressure. This pressure is in the order of 10 mbar (0,15 PSI) for most experiments.

On the other hand, if the channels in the microfluidic chip are very narrow, they can become clogged with particles, thus increasing the fluidic resistance and decreasing the flow rate if the pressure remains constant. These particles can come and go at random, leading to an unpredictable flow rate.

That is why Fluigent also offers microfluidic flow rate sensors and regulation algorithms . When used with the pressure regulators, microfluidic flow rate sensors are able to maintain the desired flow rate, rejecting every pneumatic or fluidic perturbation. Microfluidic flow rate sensors achieve the same functionality as a syringe pump, but with a much more stable and reliable output.

To obtain the best stability for your experiments, make sure to select instruments with the appropriate range. It is possible to add fluidic resistance to your microfluidic setup in order to increase the amount of pressure necessary to produce the desired flow rate and more closely match the range of the experimental setup to that of the instrument.

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