A fast and plug-and-play mechanical connector for microfluidic chips

This article introduces a fast and plug-and-play mechanical connector for microfluidic chips. With this connection device, fast and pluggable connection between microfluidic chips and liquid storage tanks can be achieved, allowing microfluidic flow between the storage tank and the chip and between chips while ensuring airtightness and avoiding liquid leakage. The reason for liquid leakage and the maximum sealing pressure are theoretically analyzed from the perspective of surface tension. The relationship between different liquids (surface tension coefficient), air gap distance, and leakage pressure is explored. Furthermore, the effect of fast connection method and capillary connection method on liquid flow inside the chip is compared. The experiment shows that the flow rate using the fast connection method can reach at least 70% of that using the capillary connection method, and can gradually increase to 96% with increasing pressure and flow rate.

for different types and sizes of cells.Even because of the special properties of some cells, the overall structure of the chip needs to be adjusted [6].In addition, the pipeline connection of the chip places high demands on operators and experimental environments: on the one hand, most chips now use capillary and hose connections to connect to the chip [7].The pipeline adapted to the chip is easy to block the line of sight under the microscope and affect observation [8]; on the other hand, it is inconvenient for mutual connection between capillaries, chip ports and hoses with different diameters requiring different sizes of conical joints and capillaries.This greatly increases the complexity of accessories.Moreover, in order to ensure the airtightness of the system, the connection speed is relatively slow.
Regarding the problem of mutual connection between chips, Zhao proposed a connection method based on preparing a superhydrophobic layer.They divided the chip into a motherboard and a module and spin-coated a double-sized layer of silica particles and silica-based oligomer adhesive on the surface of the motherboard and module to prepare a superhydrophobic coating.At the same time, the motherboard and module are connected by means of pins or magnets, effectively removing most of the pipelines and capillaries.From the experimental results, the huge difference in wettability between the inside and surface of the channel allows the liquid to flow smoothly to the next chip and effectively prevents the introduction of bubbles.Sander used a circular ring for sealing and simplified the assembly of chips and brackets by creating a semi-doughnut-shaped groove in a 3D-printed bracket.The design of these grooves allows 80% of the height of the O-ring to be located inside the bracket.The width of the groove is 110% of the diameter of the O-ring, allowing rubber material to be squeezed by the chip to form an air-tight and liquid-tight connection between the chip and bracket.However, these methods have not completely solved the problem of time cost for pipeline connection.In some experiments that require rapid changes in environment, chip connection is still a major reason limiting its use.Therefore, fast plug-and-play use of microfluidic chips remains a major challenge.
In this work, a fast plug-and-play microfluidic connection device was designed to allow microfluidic flow between the storage tank and between chips while avoiding liquid leakage.The microfluidic connection device is aligned with the chip through an alignment mechanism while ensuring that multiple pairs of microfluidic ports are vertically aligned, allowing fluid to pass quickly through the connection device.The fixation between the microfluidic connection device and the chip is assisted by a fixture.An air window is opened in the microfluidic connection device for observation under a microscope.Furthermore, the maximum pressure that allows fluid to pass normally in the chip under different pressure conditions was simulated.

Materials
A micro operating system consists of the following parts: a multi-channel solenoid valve controller and two solenoid valves, a dual-channel pressure controller based on pneumatic principle, a flow sensor, a self-designed and processed microfluidic connection device, an auxiliary fixed fixture, and a host for pressure data acquisition, image processing and motion control.

Fabrication of microfluidic chips
Due to its good biocompatibility, hydrophobicity and air permeability, polydimethylsiloxane (PDMS, Sylgard184, Dow Corning) was chosen to manufacture microfluidic chips.Microfluidic chips are made by soft lithography.First, a pre-designed chip mold is made using a 3D printer (Formlabs， Form3+).Then, the prepolymer and curing agent are mixed separately in a weight ratio of 9:1 to prepare the PDMS mixture.The PDMS mixture is poured onto the 3D printed mold and degassed and bubble-eliminated in a vacuum drying oven.It is then cured for 1.5 hours in an oven set at 60℃.The PDMS replica is peeled off and placed in an oven set at 80℃ for 2 hours of hard film curing.A drill is used to drill inlet and outlet pipeline connection holes on the flat PDMS chip.Then, oxygen plasma treatment is used to bond the PDMS chip with glass, and the resulting PDMS chip is used as a platform for concept testing.

Design and fabrication of microfluidic connection devices
According to the size of the microfluidic chip, the entire microfluidic connection device is determined to be a 40*30*5mm rectangular parallelepiped, which can just cover the entire PDMS chip.Four 1mm diameter circular holes are designed at the position corresponding to the PDMS chip port on the connection device, and a 1/4-28 threaded hole is opened on the other side of the circular hole.At the same time, in order to facilitate alignment of the connection device with the PDMS chip, three baffles for blocking and positioning are added on the side with circular holes.When moving and determining the position of the connection device, just press it against the PDMS chip and slide it from the empty side until resistance is felt to stop, ensuring that multiple pairs of microfluidic ports are vertically aligned.In short, three baffles are used to constrain relative motion between the connection device and PDMS chip.In order to ensure tight connection between the connection device and PDMS chip, a fixture is used for auxiliary fixation to achieve passive assembly between them.According to Table 1, iron was chosen based on comparison of different material characteristics (transparency, biocompatibility, price, processing difficulty, etc.), because it is opaque and requires observation under a microscope during experimental process.An optical window was additionally opened with a designed size of 4*2*5mm.[7] Vanden D S, Lucklum F, Bunge F, et al. 2018 [J].Micromachines.3D printing solutions for microfluidic chip-to-world connections.9(2):p71.

Figure 1 .
Figure 1.Experimental environment.(a)pressure controller (b) solenoid valve controller (c) host (d) liquid storage tank (e) microfluidic chips and clamp (f) flow sensor (g) Schematic diagram illustrating system pipeline connections (h) Combination of microfluidic chips and connecting devices.