Microactuators are a key component of microsystems. Common actuation technologies include piezoelectric, electrostatic and thermal, but pressurised gases or liquids are rarely used.
Pressurised gas or liquid
Actuators driven by pressurised gases or liquids (pneumatic- and hydraulic-based) have high-force-density potentials that can be very useful in microsystem applications; however they are rarely used in microsystems.
Some attempts to fabricate such actuators have been reported with the majority utilising inflatable pneumatic balloon architecture. Balloon-based actuators have short actuation strokes, nonlinear actuation behaviour, and are not desirable for large-scale applications. Piston-cylinder actuators are more favoured, as they can achieve relatively large strokes (their actuation force is not a function of the stroke of the actuator) and their actuation force is a linear function of the applied pressure.
Microcylinder fabrication challenges
Very little research has been conducted in the area of piston-cylinder actuators. This is mainly because of the lack of adequate microseal technologies and the difficulty of fabricating pneumatic or hydraulic microactuators using standard micromachining processes.
Overcoming such challenges, researchers at the Katholieke Universiteit Leuven in Belgium have developed a fabrication process for piston-cylinder pneumatic and hydraulic actuators based on etching techniques, UV-definable polymers, and low-temperature bonding. The production process was validated by the successful fabrication of piston-cylinder microactuators, which were then evaluated and their performance tested.
Silicon cylinders
The fabrication process will be fully described in an upcoming issue of the Journal of Microelectromechanical Systems. Briefly, a substrate of silicon dioxide (SiO2) is potassium hydroxide (KOH) etched to form trenches. Next, a molybdenum (Mo) layer of about 3 micro-metres thickness is sputtered on the wafer. The wafer is then covered by a thick layer of Ormocomp, a hybrid inorganic-organic epoxy polymer, with excess polymer removed by a planarisation.
In the next step, a plasma etch is used to reduce the height of the pistons so that they will not make contact with the cover glass plate that is bonded to the wafer in the subsequent step. Finally, the wafer is diced, and the pistons are released by etching the sacrificial Mo layer under the pistons. At this stage, the pistons are able to move within the Si cylinder. A significant advantage of this process is that the devices are fabricated in a batch process rather than using piecewise processes such as micromilling, which makes it suitable for large scale production.
Prototype
The actuator prototype that the researchers fabricated had a piston with a cross section of 15 mm² and a stroke of 750 micrometres. Using both pressurised air and water as driving fluid, it was determined that the actuation force was 0,1 N at a supply pressure of 1,6 MPa. According to the researchers, such actuation force is a significant improvement over alternative actuation mechanisms, including electrostatic and thermal actuators, with similar dimensions.
Forecast
Since the materials used can theoretically accommodate higher pressures, future work will focus on increasing the actuation force. At this stage, Frost & Sullivan believes that the actuation force demonstrated opens the prospect of using this technology in a number of applications, such as tools for minimally invasive surgery, tools for microassembly, tactile displays, and other microrobotic applications.
For more information on Frost & Sullivan’s technical and market analysis contact Patrick Cairns, Frost & Sullivan, +27 (0)18 464 2402, [email protected], www.frost.com
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