CASE
STUDY

Measuring cell density
in a microfluidic
system utilizing opticaldensity

September 2022
Author:Florian Siemenroth

Case study: Measuring cell density in a microfluidic system
September 2022

Parallelization& scalabibilty

Increased efficiency(time & reagents)

Inline monitoring

Microfluidic systems become more and more important in biotechnological applications in thepharma, cosmetics and food & beverage markets due to its small footprint. Therefore, these systems
are an easy option for parallelization, modularity and scalability as well as low power and reagentconsumption.
Culturing cells is a highly sensitive procedure. Some important parameters need to be monitored
closely and accurately, i.e. temperature, dissolved oxygen, CO2, pH, concentration of feed, VOC andcell density.
The cell density is often measured by an optical measuring principle taking the optical density (OD)
value generated by turbidity in the cell suspension and its potential for transmitting respectively
absorbing light.
This case study shall share the idea of a cost-efficient, microfluidic setup for microbioreactors incell culturing processes taking the OD and monitoring the cell growth.

What is microfluidics?
Microfluidics is the fine art of creation and manipulation of small portions of fluids, often realized by flow
within small, sub-millimetre-scale channels. These small dimensions allow the fluid flow to be controlledwith exquisite precision (Seifert, Thiele; 2020).

About the mp6 micropump
The available, industrialized and commercialized example is the mp6 micropump by Bartels MikrotechnikGmbH. This micro pump is a positive displacement membrane pump utilizing piezo buzzers. The alternating
displacement of the piezo acutators lead to the following typical fluidic values of the pump:▪
▪

Liquids (eta = 1 mPas): q = 5 - 8000 µl/min in free flow and p > 600 mbar
Gas: q > 25 ml/min in free flow and p > 150 mbar

All values are approximate and no guarantee of specific technical properties.
Changes in the course of technical progress are possible without notice.

Case study: Measuring cell density in a microfluidic system
September 2022

Introduction
Especially the upstream process in batch, feed batch or even perfusion processes in biotechnologicalapplications, e.g. pharmaceuticals research, development and production or food and beverage, is a very
important part. Within this process, cells are grown at a very high level and affect later processes.
Therefore, the upstream process needs to be monitored and adjusted at all times offering the best
circumstances for growing cells being as efficient as possible. Reagents are often expensive and time isshort, so a wasting behaviour should be avoided as much as possible. Monitoring the temperature, dissolved
oxygen, CO2, pH, concentration of feed, VOC and cell density is essential.

I. Cell Density
The cell density is the number of cells in a defined volume. An increasing cell density, i.e. a higher numberof cells in the same volume, gives a good input on the behaviour of cells is especially on their growth. In
most cases the goal is that the cell growth is defined by a low time and feed or reagent consumption. Thatleads to an adjusted feeding process depending on the cell growth. The higher the cell density, the higher
the turbidity and therefore the absorbance of light.
If one measures the cell suspension optically, the gradient of absorbance can be higher or lower, which
describes a higher or lower cell growth and the operator or even the bioreactor system itself can react tothe cell culture circumstances. This measuring principle is based on the optical density (OD).

II. Optical Density (OD)
The optical density in cell culturing applications is defined as:
𝐼0
𝐸𝜆 = log10( )𝐼At a wavelength of λ = 260 nm. Here [Eλ]is the absorbance and [I] is the intensity. In this case an opticalsensor incl. light source is used working at a wavelength of λ = 350 nm.

III. Experiment
The experiment set-up is shown in Fig. 1. A recirculation setup including the mp6 micropump driven by
the mp-Multiboard2 and a mp-Highdriver4 running the cell suspension from a microbioreactor in a loop.
The optical sensor, Optek OPB350, is monitoring the absorbance respectively intensity inline but without
being in contact with the liquid.

Figure 1 Sketch of experimental setup
All values are approximate and no guarantee of specific technical properties.
Changes in the course of technical progress are possible without notice.

Case study: Measuring cell density in a microfluidic system
September 2022
In this experiment yeast is used as cell material. It has a number of cells related to its weight of 1010
cells/mg. For the suspensions 150 mg is dispersed in 50 ml, which leads to 1.5*1010 cells in total and3*107 cells/ml. Fig. 2 shows three differently dispersed suspensions.

yeast suspension
1400
1200

Intensity

1000
800
600
400
200
00204060

80

100

120

140160120140160120

140160Time [s]yeast suspension 50 %
1400
1200

Intensity

1000
8006004002000020406080100

Time [s]yeast suspension 25 %
1400

1200

Intensity

1000
8006004002000020406080100

Time [s]Figure 2 Absorbance of differently dispersed yeast suspensions always compared to pure water
All values are approximate and no guarantee of specific technical properties.
Changes in the course of technical progress are possible without notice.

Case study: Measuring cell density in a microfluidic system
September 2022

One can see the increasing intensity for less dense suspension. Also, the OD for the densest suspension
0.38, the OD for the second suspension is 0.17 and the OD for the suspension with the smallest density is0.08. So, the transparency increases which fits to the fact of more and more less dense suspension. Inaddition, the comparison between the cell suspension and pure water shows a decreasing gap from densest
to lowest density and shows the plausibility of the principle at the same time as the intensity value forpure water is constant at 1200.
In addition to this case study, we have created a video that further shows what a microfluidic systemmeasuring cell density can look like. You can find it on the Bartels Mikrotechnik YouTube channel:
https://www.youtube.com/watch?v=T0hh2LR3gNQ

Components and systems used:••••

mp6 micropump by Bartels Mikrotechnik
mp-Multiboard2 incl. mp-Highdriver4 by Bartels Mikrotechnik
Microbioreactor
Optek OPB350

All values are approximate and no guarantee of specific technical properties.
Changes in the course of technical progress are possible without notice.

Case study: Measuring cell density in a microfluidic system
September 2022

Bartels Mikrotechnik is a globally active manufacturer and development service provider inthe field of microfluidics. In the microEngineering division, the company supports industrial
customers in the modification, adaptation and new development of high-performance andmarket-oriented product solutions through the innovative means of microsystems technology.
The second division, microComponents, produces and distributes microfluidic products andsystems, especially for miniaturized and portable applications. Our key products aremicropumps that convey smallest quantities of gases or liquids and are used in a variety ofways in biotechnology, pharmaceuticals, medical technology and numerous other applications.
Bartels Mikrotechnik with passion for microfluidics!

Follow Bartels:

Contact us:
Bartels Mikrotechnik GmbH
Konrad-Adenauer-Allee 1144263 Dortmund Germany

www.bartels-mikrotechnik.de
info@bartels-mikrotechnik.de
Tel: +49-231-47730-500

All values are approximate and no guarantee of specific technical properties.
Changes in the course of technical progress are possible without notice.