Miniaturized Flow Sensors for Medical Microfluidic Devices

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In medical diagnostics, the development towards near-patient lab diagnostics (point-of-care testing (POCT)) continues rapidly. The notion is that testing should happen as punctually as possible and preferably directly at the patient’s bedside. For more multifaceted examinations, precise microfluidic systems are necessary.

One well-established example of a POCT is a urine test using a paper strip. In these tests, the various components of urine – such as white and red blood cells, glucose and the pH value – cause color variations on the reactive surfaces of the test strip. By comparing the color patterns to a reference scale, the nurse can determine qualitative information regarding the concentration of the many substances in the sampled urine.

In an even simpler exemplary process, the oxygen saturation of blood can be established non-invasively via the skin by clipping an optical sensor to the fingertip of the patient. Earlier, before the arrival of these pulse oximeters, a blood sample had to be taken and transported to the central lab for testing.

More intricate tests – for instance, to detect particular viruses or bacteria – still rely on the specialist personnel and elaborate infrastructure of a central lab. Such tests frequently require extra steps with regards to sample preparation or pre-treatment, special temperature conditions, or complex devices for analysis. In order to perform these tests directly at the patients’ bedside in the hospital, the test processes must be shortened and interaction with the user reduced. In ideal cases, the whole test is done self-reliantly on a single microfluidic system; i.e. the entire ‘lab’ is incorporated into a chip and thus becomes a ‘lab on a chip’.

Microfluidic POCT

Over the last few decades, a lot of research has been performed in the field of microfluidics, predominantly in the biosciences, with the aim to miniaturize and automate diagnostic tests, lab experiments, and (bio)chemical processes. Rather than pipetting discrete quantities of liquids from one container to another, currently the liquid in a microfluidic system flows via the miniature channels of a manifold.

The liquid in these channels is usually moved by external pumps or pressure sources to which the fluidic chip is linked. To regulate the applied pressures or stabilize the flow produced by the pump, the flow rate is measured using flow sensors. Thanks to their exceptional sensitivity even at very minimal flow rates, Sensirion’s microthermal mass flow meters for liquids set the industry standard in the precise monitoring of liquid flows in microfluidic systems.

The dimensions of the channels in the fluidic system have been considerably decreased down to the micrometer range, with the liquid volumes in the microliter or even nanoliter range. This enables the microfluidics to greatly decrease the sample and reagent quantities, enable faster reactions and thereby boost throughput. Plus, the smaller size of the fluidic system also means smaller devices and lower costs.

Both are essential conditions in order to conduct tests in a decentralized manner directly at the place of care; i.e. at the bedside, in a doctor’s office, or on the same hospital floor. This, in turn, streamlines the logistics and delivers faster results for improved and more targeted treatment of patients.

Possible Applications in Medical Technology and Other Fields

Commercially available microfluidic POCT devices can be employed, for instance, to measure proteins used as biomarkers in the diagnosis of heart attacks. Other devices examine blood samples to establish the composition of white and red blood cells, and their subtypes. Another application in medical technology is cytometry to establish the concentration of T helper cells for checking the immune systems of HIV/AIDS patients.

To conclude, efforts are also happening to transfer molecular-biological approaches for the detection of, for example, antibiotic resistant bacteria via their DNA to point-of-care systems. Even genetic fingerprints for forensic purposes can be developed using POCT devices.

However, microfluidics has a progressively important role to play in areas other than the clinical field, such as industrial process monitoring and research and development. In particular, microbiology is unquestionably important today in the food and drinks production sector; it is used to track the quality of yeast cells that ferment malt to beer, and the bacterial populations in milk, which must not surpass specific threshold values.

Until now, discrete samples have been diverted from the production process and transported to laboratories for testing; currently new types of miniaturized flow cytometers enable direct verification of separate production lots or even uninterrupted monitoring on the production line.

Improved Performance Through Precise Flow Monitoring

Due to its origins, the field of microfluidics is closely connected with the biosciences and related technologies, such as cell manipulation, cell sorting, and DNA analysis. Microfluidics is also applicable in other miniaturized systems that use liquids; for instance, microscopic chemical reactors and micro fuel cells for portable energy generation.

Precise Flow Monitoring

Precise Flow Monitoring

In all these developing application areas, the precise monitoring and control of liquid flows is vital to the dependable operation of the respective device. With a minute form factor of just 10 x 10 mm2, Sensirion’s LPG10 flow sensor delivers excellent accuracy and speed in the measurement of very low flow rates.

The sensor has a planar microfluidic glass substrate and allows compact integration in any fluidic system. The established microthermal measurement technique in an advanced design enables flow measurements of just a few milliliters down to single microliters per minute, or even lower. Glass as the only wetted material ensures ideal compatibility with pharmaceutical and biological processes.

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The sensor provides a direct and extremely precise measurement of the flow rate at every pertinent point in the fluidic system. The consistent detection of common errors, such as air bubbles, clogging, or leaks, is evidently integrated.

Sensirion Inc.

This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.

For more information on this source, please visit Sensirion Inc.

Last updated: Apr 3, 2019 at 10:24 AM

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