The lack of standard metrology tools impedes the pace of microfluidics development

HENNE VAN HEEREN, MICROFLUIDICS ASSOCIATION

Microfluidics — the manipulation of fluid (often biofluid) to reveal certain inherent traits — is the foundational technology for myriad miniaturized devices, ranging from microreactors for the chemical industry to medical diagnostic devices. The COVID-19 pandemic has shown the importance of fast, reliable, and specific testing capability. A missing link in the widespread acceptance of this technology has been the lack of reliable standards for the manufacturing of microfluidic devices, and the optics and photonics industry has a significant role to play in building these standards.

The microfluidics industry is working hard to develop standards that will help to decrease the transition time from prototype to commercial availability. To accomplish this objective, a group of organizations on the microfluidics supply chain are promoting microfluidics standards and guidelines. The group, which includes photonics-focused companies, formed the foundation of the Microfluidics Association (www.microfluidics-association.org).

The association interviewed a large number of microfluidics experts about their perception of weaknesses in the supply chain. One thing was clear: The lack of standard metrology tools and protocols is impeding fast industrial development. A detailed analysis, including a breakdown of the expertise that could reliably produce such tools, showed that the answer may come from the photonics industry. To begin, critical parts of microfluidic devices are inaccessible for mechanical testing, too sensitive to be touched by measurement tools, or too small for mechanical measurement. Optical sensing is seen as fast, reliable, and easy to automate. And practically all microfluidic products are made from transparent materials that light can penetrate, such as glass, cyclo olefin polymer, and polymethyl methacrylate.

Generally, a microfluidic device consists of a chip in which channels and other 3D structures have been inscribed, along with a cover layer with access ports through which fluids can be introduced. The most-used microfluidic fabrication processes are lithography-based processing, injection molding, embossing, and stamping. The first process uses glass as a base material, while the other three use polymers.

Certain metrology challenges, however, are faced by all suppliers of microfluidic products:

The flow resistivity and pressure decay inside the device. Deviation from the intended cross-sectional shapes within the device — between design and manufacture — leads to an increase or decrease of the flow resistivity and thereby influences the performance of the device. These deviations can be caused by variations in etching, in the case of glass products, or by deformation during the bonding of polymer plates. Surface properties of the inside of the channels, such as roughness or wettability, will also influence flow resistivity and pressure decay. The lack of contrast under inspection might be a problem, and photonic components might capture this detail.

For all of these tests that would streamline the design and construction of microfluidic devices, there are no industry-wide accepted protocols and dedicated tools.

The microfluidics industry therefore needs tools and protocols for measuring channel dimensions (in 3D). The ideal moment during manufacture for the most accurate and precise measurement is after etching, in the case of glass, and after bonding, in the case of polymers. The ability to measure the entire area of the wafer, or even a sample area, in a short amount of time is desirable, and it would provide an indication of how successfully homogeneity and stability have been achieved over the wafer. It would also help predict internal flow resistivity and pressure decay.

The optical transmission of the materials used. Most microfluidic-based diagnostic devices use optical detection of biochemical reactions inside the microfluidic channels. Measurement of the optical transmission of the material used is therefore essential. Optical transmission methods depend on the specificity of an optical path, the light wavelength, and the detector. The measurement result depends very much on the material properties and the fabrication process.

For all of these tests that would streamline the design and construction of microfluidic devices, there are no industry-wide dedicated tools and accepted protocols. Protocols derived from the efficient production of such systems — which the optics industry could help facilitate through testing and analysis — would not only be important for the user, but would also map out ways to overcome instabilities in the process of manufacturing. Such standardization is essential for further quality improvement and increased availability of microfluidic technology.

Meet the author

Henne van Heeren is owner of enablingMNT and secretary of the Microfluidics Association. He worked as a production manager and business development manager at micro- and nanotechnology companies for 17 years. Over the last 19 years, he has worked as a business consultant. Van Heeren was one of the founding members of the Microfluidics Association; email: henne@enablingmnt.com.

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