Team:TU Eindhoven/Microfluidics/Methods

Methods

The fabrication of a microfluidic device is a two-step process where two types of lithography are involved. First the photolithography process is described and then soft lithography. Check our Protocol Page for more information about the precise processes.

Photolithography

The goal of photolithography is to create small features on a silicon wafer using a photoresist. The photoresist is spun on the silicon wafer with a spin coater. Different spin programs are used to get control over the height of the channels. To create the master mold for the droplet devices first a spin speed of 500 rpm is applied for 10 seconds after which the spin speed is ramped up with 330 rpm/s to a final speed of 1450 rpm and held for 30 seconds. This results in channels of 16 µm in height.

Figure 1. An overview of photolithography.

SU-8 2010 negative photoresist is used to create the master. After UV light exposure the SU-8 becomes insoluble due to crosslinking between the Bisphenol A epoxy oligomers. When developed the unexposed parts are dissolved leaving a pattern as is depicted in Figure 1. The used photomasks which have been used can be found in the Droplet Device or Cell Encapsulation Device Pages. The designes were all made in AutoCAD v2014.

Soft Litography

Once the production of the mastermold has been finished PDMS can be applied to the master. PDMS is the basis of the final devices. It consists of a mixture of siloxane oligomers (base agent) and cross-linkers (curing agent) (R is usually CH₃ or H) at a specific ratio. The most commonly used weight ratio is 10:1 and is also used in these devices. After applying this mixture to the master mold it is baked. Baking allows the solution to form a highly cross-linked network. The pattern of the master mold becomes embedded into the PDMS.

Figure 1. TU Eindhoven iGEM 2014 in a microfluidic device.

Once the PDMS has been baked it can be peeled of the master. The devices are cut out of the PDMS slab. The inlets and outlets are punched with the desirable diameter the inlets and outlets should be. Finally the PDMS devices are treated with oxygen plasma (Figure 3). This method introduces silanol groups and removes the methyl groups therefore changing the surface from hydrophobic to hydrophilic. The silanol groups interact with the silica (SiO₂) groups at the surface of the microscope glasses. Thus treatment with oxygen plasma allows for a better binding to silicate glass surfaces forming an irreversibly seal to create leak-tight channels.

Once the microfluidics devices are bound properly to the microscope glasses they are treated with aquapel. Aquapel is a water-repellant and makes the channels hydrophobic again so the fluids used in the devices will not be interacting with the sides of the channel and can maintain a stationary flow.

Bibliography

Song H, Chen DL, Ismagilov RF. Reactions in droplets in microfluidic channels. Angew Chem Int Ed Engl. 2006;45:7336–7356

Mazutis, L., Gilbert, J., Ung. W.L., Weitz, D.A., Griffiths, A.D. & Heyman J.A. (2013). Single-cell analysis and sorting using droplet-based microfluidics. Nature, 8(5), pp. 870-91.

Song H, Chen DL, Ismagilov RF. Reactions in droplets in microfluidic channels. Angew Chem Int Ed Engl. 2006;45:7336–7356