Spontaneous droplet generation via surface wetting

Integrated Droplet-Based Digital Loop-Mediated Isothermal Amplification Microfluidic Chip with Droplet Generation, Incubation, and Continuous Fluorescence Detection

This study integrated sample partition, incubation, and continuous fluorescence detection on a single microfluidic chip for droplet-based digital Loop-Mediated Isothermal Amplification (LAMP) of nucleic acids. This integration eliminated the need to transfer reactions between different platforms, avoiding sample contamination and loss. Prior to the reaction, filling the channels with an oil phase and adding a glass cover slip on top of the chip overcame the problem of bubble generation in the channels during the LAMP reaction due to heating. Additionally, using two fluorescence intensity thresholds enabled simultaneous detection and counting of positive and negative droplets within a single fluorescence detection channel. The chip can partition approximately 6000 droplets from a 5 µL sample within 10 min, with a droplet diameter of around 110 µm and a coefficient of variation (CV) value of 0.82%. Staphylococcus aureus was quantified via the proposed platform. The results demonstrated a highly accurate correlation coefficient (R = 0.9998), and the detection limit reached a concentration of 1.7 × 102 copies/µL. The entire process of the droplet digital LAMP reaction, from droplet generation to incubation to quantitative results, took a maximum of 70 min.

On-Chip Nucleic Acid Purification Followed by ddPCR for SARS-CoV-2 Detection

We developed a microfluidic chip integrated with nucleic acid purification and droplet-based digital polymerase chain reaction (ddPCR) modules to realize a 'sample-in, result-out' infectious virus diagnosis. The whole process involved pulling magnetic beads through drops in an oil-enclosed environment. The purified nucleic acids were dispensed into microdroplets by a concentric-ring, oil-water-mixing, flow-focusing droplets generator driven under negative pressure conditions. Microdroplets were generated with good uniformity (CV = 5.8%), adjustable diameters (50-200 μm), and controllable flow rates (0-0.3 μL/s). Further verification was provided by quantitative detection of plasmids. We observed a linear correlation of R² = 0.9998 in the concentration range from 10 to 10⁵ copies/μL. Finally, this chip was applied to quantify the nucleic acid concentrations of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The measured nucleic acid recovery rate of 75 ± 8.8% and detection limit of 10 copies/μL proved its on-chip purification and accurate detection abilities. This chip can potentially be a valuable tool in point-of-care testing.

Auto Flow-Focusing Droplet Reinjection Chip-Based Integrated Portable Droplet System (iPODs)

Droplet microfluidics provides powerful tools for biochemical applications. However, precise fluid control is usually required in the process of droplet generation and detection, which hinders droplet-based applications in point-of-care testing (POCT). Here, we present a droplet reinjection method capable of droplet distribution without precise fluid control and external pumps by which the droplets can be passively aligned and detected one by one at intervals. By further integrating the surface-wetting-based droplet generation chip, an integrated POrtable Droplet system (iPODs) is developed. The iPODs integrates multiple functions such as droplet generation, online reaction, and serial reading. Using the iPODs, monodisperse droplets can be generated at a flow rate of 800 Hz with a narrow size distribution (CV <2.2%). Droplets are kept stable, and the fluorescence signal can be significantly identified after the reaction. The spaced droplet efficiency in the reinjection chip is nearly 100%. In addition, we validate digital loop-mediated isothermal amplification (dLAMP) within 80 min with a simple operation workflow. The results show that iPODs has good linearity (R² = 0.999) at concentrations ranging from 10¹ to 10⁴ copies/μL. Thus, the developed iPODs highlights its potential to be a portable, low-cost, and easy-to-deploy toolbox for droplet-based applications.

Permanent Hydrophobic Surface Treatment Combined with Solvent Vapor-Assisted Thermal Bonding for Mass Production of Cyclic Olefin Copolymer Microfluidic Chips

A hydrophobic surface modification followed by solvent vapor-assisted thermal bonding was developed for the fabrication of cyclic olefin copolymer (COC) microfluidic chips. The modifier species 1H,1H,2H,2H-perfluorooctyl trichlorosilane (FOTS) was used to achieve the entrapment functionalization on the COC surface, and a hydrophobic surface was developed through the formation of a Si-O-Si crosslink network. The COC surface coated with 40 vol % cyclohexane, 59 vol % acetone, and 1 vol % FOTS by ultrasonic spray 10 and 20 times maintained its hydrophobicity with the water contact angle increasing from ∼86 to ∼115° after storage for 3 weeks. The solvent vapor-assisted thermal bonding was optimized to achieve high bond strength and good channel integrity. The results revealed that the COC chips exposed to 60 vol % cyclohexane and 40 vol % acetone for 120 s have the highest bond strength, with a burst pressure of ∼17 bar, which is sufficient for microfluidics applications such as droplet generation. After bonding, the channel maintained its integrity without any channel collapse. The hydrophobicity was also maintained, proved by the water contact angle of ∼115° on the bonded film, as well as the curved shape of water flow in the chip channel by capillary test. The combined hydrophobic treatment and solvent bonding process show significant benefits for scale-up production compared to conventional hydrophilic treatment for bonding and hydrophobic treatment using surface grafting or chemical vapor deposition since it does not require nasty chemistry, long-term treatment, vacuum chamber, and can be integrated into production line easily. Such a process can also be extended to permanent hydrophilic treatment combined with the bonding process and will lay a foundation for low-cost mass production of plastic microfluidic cartridges.