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 using a spin coater. Different spin programs are used to gain 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.

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 (Figure 2). 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 designs 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 crosslinkers (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 our devices. After applying this mixture to the master mold it is baked. Baking allows the solution to form a highly cross-linked network (Figure 2). The pattern of the master mold becomes embedded into the PDMS.

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 a desirable diameter. Finally, the PDMS devices are treated with oxygen plasma (Figure 3). This method introduces silanol groups and removes the methyl groups, resulting in a hydrophilic surface instead of a hydrophobic. The silanol groups interact with the silica (SiO₂) groups at the surface of the microscope slides (Figure 3). 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 that the fluids used in the devices will not be interacting with the sides of the channel and can maintain a stationary flow.

Flow

The flow highly depends on the Reynolds number. This is a dimensionless quantity that describes the behavior of the fluid flow. The Reynolds number is defined by Re = ρvd/µ where ρ is the density of the fluid (g/cm³), v is the velocity of the fluid (µm/s), d is the diameter of the channel (µm) and µ is the viscosity of the fluid (g/cm s^-1). It is also known as the ratio between viscous and stationary forces. Because the channels are in the order of 10^-6 meters, the Reynolds number is Re << 1. In the small channels of the device viscous forces dominate. This results in a laminar flow with a poiseuille profile [3].

Bibliography

[1] H. Huang and D. Densmore, (2014), “Lab Chip.” DOI:10.1039/C4LC00509K

[2] D. J. Campbell, K. J. Beckman, C. E. Calderon, P. W. Doolan, R. H. Moore, A. B. Elis, G. C. Lisensky, (1999), "Replication and Compression of Bulk Surface Structures with Polydimethylsiloxane Elastomer." J. Chem. Educ. Vol. 76 , 537.

[3] Mcdonald, J. Cooper, David C. Duffy, Janelle R. Anderson, Daniel T. Chiu, Hongkai Wu, Olivier J. A. Schueller, and George M. Whitesides, (2000), "Fabrication of microfluidic systems in poly(dimethylsiloxane)." Electrophoresis 21.1: 27-40.