Poly(dimethylsiloxane) (PDMS) is broadly utilized for the development of disposable lab-on-a-chip systems. For many microfluidic applications hydrophilic surface characteristics are indispensable. In this work, an easy, reliable and stable surface hydrophilization procedure based on poly(ethylene glycol) (PEG)-silanization is presented that overcomes the hydrophobic nature of PDMS. Furthermore, the long-term stability of the grafting as well as the effect of different parameters (e.g., applied solvent) on the hydrophilicity according to the measured contact angles (CAs) are analyzed within different media. Finally, the successful hydrophilization of different PDMS microfluidic devices and the effect of its performance are demonstrated (e.g., in a micro-emulsification component).Effect of the surface properties of PDMS-structured devices on the performance of drop emulsification with regard to the CA: (a) bare, hydrophobic and (b) PEG-grafted, hydrophilic PDMS.
Experimental investigations of the effect of microchannel geometry on high-pressure dispersion and emulsification were carried out. Customized microchannels of varying geometric principles were fabricated in silicon and steel. In order to characterize the process efficiency of microchannel geometries, the effects of the process parameters (mean velocity, Reynolds number, and local pressure drop) were examined and correlated to the dispersion and emulsification results. It is demonstrated that high pressure losses focused at a small channel length and high velocity gradients lead to high stress intensities and, in consequence, to low particle or droplet sizes. Thus, 2D orifices were successfully further improved regarding their process efficiency by adding a third-dimension constriction.
Three innovative micro actuator concepts on the basis of the differential SMA principle are presented in this paper: a high adaptive multi-actuator system, which is driven by numerous identical single actuators connected in parallel and in series, a micro gripper for handling and assembling of complex hybrid micro systems and a micro actuator system in medical tools for the percutaneous resection of aortic valves. The SMA material is used in the form of 50 lm thin NiTi foils because of their well-defined properties and high strength. In order to integrate them into micro systems, different manufacturing methods have been applied and improved at the Institute for Microtechnology. Laser cutting and wet chemical etching are used for example to microstructure the actuator elements. Different methods for electrical and mechanical connections of the actuators are employed like soldering by the use of an additional gold layer. A batch fabrication process of SMA actuators is realized by embedding NiTi foil elements into SU-8 structures. To optimize the design of SMA actuator elements according to its application different simulation procedures are used.
The mergence of partial aspects and functional components of micro actuators and micro fluidic technology allows the development of complex micro systems, which are more and more interesting for MEMS application, especially for BioMEMS. This enormous potential is shown in this article showing the realization of an electro magnetic micro pump. The basic build-up consists of a polymer magnet integrated into a pump chamber of a fluidic PDMS device, which is located above a double layer micro coil. By applying a current, the polymer magnet performs a bidirectional movement, which results in a pumping effect by the two arranged passive check valves being perpendicularly arranged to the flow channels. The valve membrane is flexible and opens the channel towards the flow direction. The advantage of this configuration is that leakage can be avoided by the special geometrical configuration of the fluid chamber and the valves. The fabrication process includes UV depth lithography using AZ9260, electroforming of copper for the double layer spiral coil and Epon SU-8 for insulation, embedding and manufacturing of the valve seat. Furthermore, the fluidic devices are realized by replica molding of PDMS using a multilayer SU-8 master. Furthermore, a new technology for realizing micro polymer magnets was optimized and deployed. Using these fabrication processes, a magnetic micro actuator has already been developed based on the movable plunger principle, which forms the basic set-up of the micro pump. This actuator is monolithically fabricated and successfully tested. In addition, the fluidic system of the micro pump was successfully fabricated and tested. In order to connect the valve seats based on SU-8 to the PDMS fluidic chamber and the valve lips, a special bonding process was developed. The combination of the fluidic system with the electromagnetic part is currently under investigation. The dimension of the micro pump is about 10 x 6 x 3 mm.
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