Microfluidics with a splash of color

Lithography techniques enable the manufacturing of microfluidic chips with detailed, high-resolution patterns. But they don’t produce finished channels. Rather, they carve paths into substrates that need to be sealed—for example, with a lid, as shown on the left in the figure below—to enclose fluid flows. Researchers choose from various sealing techniques that depend on the materials and geometries involved. Any missteps in the delicate process can cause clogs, leaks, or other problems.

But what if the sealing step could be avoided altogether? Using a recently developed lithographic technique, Detao Qin and coworkers in Easan Sivaniah’s group at the Institute for Integrated Cell-Material Sciences at Kyoto University in Japan directly printed microfluidic channels inside thin polymer films. The approximately 850 nm films were mounted on different substrates to control their flexibility and transparency. And because the channels have porous layered structures that produce Bragg reflections, illustrated on the right in the figure below, they display different colors depending on the device preparation and the fluid properties.

The lithographic technique underlying the microfluidic devices, organized microfibrillation (OM), was developed in 2019 by Masateru Ito, Andrew Gibbons, and coworkers, including Qin and other authors on the new paper. The researchers illuminated a photosensitive polymer on a reflective surface so the light formed a standing wave and created alternating layers of cross-linked and uncross-linked polymer. The solvent they used to wash away the uncross-linked polymer applied a stress because it has different solubility in the two materials, and the stress caused supportive microfibrils to form between the cross-linked layers. The wavelength of the illuminating light set the spacing between the layers, and the amount of cross-linking determined the fibril density.

With their OM technique in hand, the researchers turned to another use for the layered structures: directing fluid flows. The porous structure of each channel sets the flow velocity—it’s independent of the channel’s overall size and shape, which determine the flow profile and velocity in typical microfluidic devices. The microstructure also affects the device’s light-scattering properties, so for a given liquid, areas with different porosities are visually distinguishable, as shown in the opening image. The colors are also a proxy for flow speed; redder colors indicate larger channels through which fluid moves more easily.

The refractive index of the fluid flowing through a channel also affects how the device scatters light, so liquids flowing through identical channels can be differentiated by their appearance. The devices are even useful for biomolecular fluids with indistinguishable optical properties: Multiple channels with different porosities can be used to separate molecules based on their sizes. (D. Qin et al., Nat. Commun. 13, 2281, 2022 .)