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Microstructures fabricated from SU-8 photoresist have excellent mechanical properties, thermal stability, etch resistance and are chemically stable. SU-8 is also highly transparent under near UV and visible light. These properties make it ideal for microfluidic and bioMEMS applications. Yet native SU-8 is also highly hydrophobic and has a low surface energy. Since water use is also required in these devices, it would imply that microfluidic systems fabricated from native SU-8 require active pumping. Surface modification eliminates that need.
The polymer surface has to be rendered hydrophilic to enhance capillary flow. To this end, wet chemical or plasma-based processes may be used. Oxygen plasma activation of polymer surfaces is widely used to achieve a highly hydrophilic SU-8 surface as a result of CO and COO groups formed on the surface. This yields completely wettable surfaces with water contact angles of less than five degrees. A plasma treatment increases surface energy and often increases the surface roughness.
Methods for SU-8 surface functionalization, especially for bioanalytical applications, are described in the recent literature. For example, functional films such as enzyme-immobilized layers or biocompatible layers have been attached to SU-8. Biomolecules on SU-8 are of interest because of their application in bioMEMS, biomolecule immobilization related assays, biosensors, membrane bioreactors, clinical diagnostics, molecular biology, agriculture, environmental science and the chemical / biochemical industry.
Diagram: R.A.Singh ME Dept., Nat. U of Singapore
In electrowetting, liquid surface tensions are modified by applying a field potential. With no voltage applied, colored oil lies flat between water and the electrode within a defined well. When a voltage is applied between the electrode and water, the tension changes and the water then forces the oil aside.
In electrowetting displays (EWD), the use of an active matrix TFT back-plane can control each pixel allowing for high speed and video content. Because electrowetting is low power, this is an attractive technology for many applications. Electrowetting displays can also be used in both reflective and transmissive modes and offer higher brightness compared with other reflective technologies.
Electrowetting is also used for a wide range of microfluidic lab-on-chip applications.
Courtesy of University of Cincinnati, Plastics Electronics 2010
