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Introduction to Microfluidic Chips Fabrication

In recent years, microfluidic chips have been regarded as an effective tool for achieving fast, highly sensitive, and high throughput analysis in many therapeutic fields. Moreover, microfluidic chips can be controlled by the fluid or particle micro-environment to carry out the site-specific biochemical analysis. Meanwhile, a wide variety of microfluidic chip materials, including but not limited, silicon, glass, ceramics, as well as polymers, have been introduced for fabricating microfluidic chips. Among them, glass, silicon, and a polymer (PDMS) have become the most popular materials and are broadly used for low-cost, portable analyses. Also, recent reports have revealed that liquid metal is a perfect material and can be injected into the microchannel for producing an electrode in microfluidic chips.

Fig.1 Scheme of vacuum filling of liquid metal into a microfluidic channel. (Zhu, 2020)

Liquid Metal Layers

Currently, a battery of liquid metals has been treated as suitable materials for the development of novel microfluidic chips. For instance, gallium-based liquid metals have high surface tension, high thermal, as well as high electrical conductivity. In the meantime, as a liquid, these metals are safer, less toxic, and have negligible vapor pressure compared to other metals, like mercury. Moreover, a thin layer of gallium oxide quickly forms on the metal surface when exposed to oxygen. This layer can change many physical and chemical properties of gallium, including its rheological properties, and thus plays an important role in regulating the function of microfluidics. In addition, researchers have indicated that liquid metals injection structures can be used for producing tiny microfluidic components, such as heaters, pumps, or electrodes.

Metal Layer Coated Microfluidic Chips

Up to now, a large number of liquid metal layer coated microfluidic chips have been generated and the features of strong reconfiguration, low consumption, high scalability, and high integration make these new chips have broad application prospects. In general, liquid metal-based microfluidic chips are designed by using the dispersed phase and continuous phase to produce liquid metal droplets on a three-channel generator. Furthermore, this chip can use microchannels filled with liquid metals as a non-contact electrode to induce an electric field through the droplet channel, thereby realizing the separation and sorting in the microfluidic system.

Besides, liquid metals serve as the sacrificial ink for fabricating 3D microfluidic chips. In terms of detailed procedures, it can be divided into three steps. The first step is to use a 3D printer to combine the liquid metal with the substrate. In this condition, the liquid metal oxide layer can further stabilize the microchannel shape. The second step is to encapsulate and print by casting and curing polymers such as PDMS. The third step is to empty the microchannel for sample analysis by flushing the acidic solution into the channel.

Reference

1. Zhu, L.; et al. Liquid metal-based soft microfluidics. Small. 2020, 16(9): 1903841.

For Research Use Only. Not For Clinical Use.