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Soft lithography

Almost all animal cells recognize topography and stiffness of their microenvironment. While cell reactions to these micromechanical signals are well documented, the underlying sensing and signal processing processes are still poorly understood. To study these processes we prepare microstructured substrates for cell cultivation. Exploiting the possibilities of nanofabrication in the Helmholtz Nanoelectronic Facility and the moldability of silicone rubber artificial environments for cells are prepared with top down defined geometry and mechanical stiffness. This process is called “Soft Lithography” or “’Soft Replica Molding”. Its principles are as follows:

1) A microstructure is designed and a chromium mask carrying the microstructure is produced by electron beam lithography.

2) Using photolithography molds for replication are produced. At present we are fabricating silicon/silicondioxide molds exhibiting a regular grid of microdots (50-400 nm height, few micrometer diameter) for creating almost flat and very soft substrates for cells. Alternatively we are producing microholes in thick film photoresist that are used to mold micopillars of up to 45 μm height and few micrometer diameter.

3) In the last step the actual molding is done. Here silicone rubber formulations comprising two components, base and cross-linker, are used. These components are fluid. After mixing they cross-link only upon heating to form a rubber whose stiffness is set by the ratio of base to cross-linker. Even extremely soft rubbers resembling gland tissue (few hundred pascals of stiffness) can be produced in this way. In micromolding the still fluid silicone mixture is placed onto the micromold and the whole sandwich is heated. After heat-induced cross-linking the now “solid” silicone rubber is carefully peeled off the mold and is now ready for use in life science research.

Heart muscle cell Heart muscle cell stretched between elastic microcolumns, scanning electron microscopy image.

Heart muscle cell 2DHeart muscle cell on a micro-structured surface. Symmetric grid underneath the cell was generated by imprinting micro-structures of 100 nm depth. The cell deforms the regular grid at its adhesion points; cell forces become visible. The distance between two points amounts to 3.5 ?m.

Wafer preparationWafer preparation under clean room conditions for the duplication of micro-structured elastic surfaces.