B. Murienne - Master Project Thesis - Infoscience - EPFL

Figure 4. Photolithography process [51].
The substrate, for example a silicon wafer, is coated with a photoresist and exposed to a light
source through a mask. The mask defines the micropatterns by only allowing only some parts of
the photoresist to be exposed to the light source. The photoresist is a light-sensitive material
which can be positive or negative and changes its properties depending on its exposure to light.
During the development, either the exposed or unexposed parts of the resist are removed. For a
positive resist, the developer removes the parts of the resist exposed to light, whereas for a
negative resist, only the exposed parts stay. Other processes such as etching and lift-off can also
be used after photolithography to create different structures. Etching is the process of transferring
a pattern from the photoresist to the layer below it, whereas lift-off transfers the photoresist
pattern to the layer above it [51]. Using different masks and techniques, several patterns can be
combined on a single object to create a more complex structure.
2.3 Stretch devices
Stretch devices are designed to stretch cells that have been previously cultured on elastic
membranes as monolayers. Each device consists of three concentric cylinders: an indenter ring, a
membrane holder with an O-ring and a screw-top as shown on Figure 5 (a,c). There exists two
different types of stretch devices, circular stretchers which induce isotropic stretch and elliptical
stretchers which induce anisotropic stretch as represented on Figure 5 (b,d). The elastic
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