Cover image illustrating a flow of biomolecules on a surface.
The patterning of molecules is an important and growing field (>3000 scientific articles and >8000 patents published every year), critical in applications as varied as DNA/protein microarrays, tissue engineering, biosensors, biomaterials or fundamental cell studies.
In collaboration with the multi-scale robotics lab at ETH Zurich, my team at IBM Research – Zurich has published an article reviewing the role of mass transport in the precise patterning of biological molecules on technological substrates at the microscale. This work was featured on the cover of the March 5 issue of Chemical Society Reviews, a highly-cited peer-reviewed scientific journal published by the Royal Society of Chemistry.
Coffee ring stains
All patterning processes require a liquid medium that contains the molecule of interest and that is in contact with the substrate to be patterned. This process takes place in two steps: (i) the molecule is transported to the surface interface, and, (ii) a chemical reaction binds the molecule to the surface.
While the reaction step has been thoroughly explored, the transport step has been largely neglected. This is striking, as it is often the most important factor defining good quality patterns.
Stain left by drying of a coffee droplet.
Depositing droplets on surfaces, such as in inkjet printing, is the most common method for surface patterning. The liquid of the droplet evaporates faster near the borders than in the center. As a consequence, internal flows are established inside the droplet that transport the content of the liquid toward the edge. The result after total evaporation is a stain with a prominent surrounding line and little material deposited in the central area. This is typically seen in dried stains of coffee or other beverages, hence the name “coffee ring effect.”
Such scenarios of uncontrolled or unwanted transport are unfortunately still common in methods used for biopatterning both in industry and research. The resulting spots often cannot be used for accurate quantification as required, for example, in diagnostics, only allowing a qualitative or semi-quantitative assay reading.
In our article, we articulate the fundamentals of mass transport in a biopatterning context. One of the key messages is that by accurately controlling flows on surfaces, very significant gains in terms of patterning time and uniformity can be obtained. This is something that is becoming increasingly explored with microfluidics, which can establish extremely accurate laminar flows on surfaces. Additionally, our article outlines the advances of more established methods that are starting to thoroughly explore the deposition of biomolecules (e.g. electrochemical deposition).
Toward quantitative medicine and biology
We believe that with an increase in throughput needs in biology, screening and diagnostics applications, there will be increased innovation in biopatterning substrates, assay implementations and manufacturability, all of which will need to overcome some of the underlying transport-phenomena-related limitations and help champion the idea of precise biology.
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