In the microfluidic field of micro-electro-mechanical systems (MEMS), two types of conventional actuators are known. An actuator of the first type uses electro-chemicals or induced electric fields to drive or separate liquid and the feature is immovability of elements thereof, such as fixed electrodes, which operates by applying electrical potential to induce an electrical field for realizing driving or separation of liquid without employment of movable parts. Examples include electrophoretic actuation unit and dielectrophoretic actuation unit. An actuator of the second type is operated by using electro-mechanical moving parts to drive liquid, such as a piezoelectric device that makes use of mechanical elements thereof to drive liquid, the feature of which resides on movability of elements thereof. Integrated design and manufacturing of the above MEMS actuation units are of vital importance for protein chips, micro-fluidic systems or lab-on-a-chips of the biomedical field.
By the first driving way of electro-chemicals or induced electric field, the electrophoretic actuation or dielectrophoretic actuation is operated with alternating current power and requires electrical voltage as high as several hundreds or even over one thousand volts. These make them not suitable for applications of biomedical systems that are implanted in human body or are arranged very close to human body. On the other hand, the second driving way using, e.g., the piezoelectric materials, allows manufacturing by bonding blocks of piezoelectric material and other parts together. However, the piezoelectric device has a bulky size, which cannot be easily reduced. The piezoelectric device can also be manufactured by thin film growth method, which, however, suffers process incompatibility and as a consequence, the piezoelectric driving and manufacturing process thereof cannot be easily integrated with the newly-developed biomedical systems that are arranged close to human body. In other words, (electric) field-based or piezoelectrics-based driving mechanisms are subject to severe limitation in the applications of biomedical micro-fluidic systems, and new electro-thermal actuation principles as well as their applicable devices are required accordingly.
As to electro-thermal driving, it originates from the idea of thermo-buckled actuation. With proper layout designs of heating resistors, electrical power accompanying application of electrical voltage or current can be consumed at portions that have great electrical resistances, and the portions are heated up. When the heating causes the structures adjacent to the portions with a large buckling deformation, realistic actuation can be affected by this deformation consequently. A micro actuation unit making use of such a phenomenon is referred to a thermo-buckled micro actuation unit.
The earliest thermo-buckled micro actuation unit made of metal was made by LIGA technology. Silicon-based material is later employed to eliminate the limitation of rare and expensive synchrotron X-ray sources. Special configuration of the heated surfaces is thus realized so that the silicon-based thermo-buckled micro actuation unit proved to have up-and-down movement in an uni-directional way. The conventional thermo-buckled devices just as mentioned above, made of metal or polysilicon, have a very high operation temperature of at least 400° C. Thus, the thermal driving device is often used in optical MEMS applications, for the high temperature induced during the operation of the thermo-buckled device does not seriously affect the normal operation of the optical devices. However, these conventional thermal driving devices are not suitable for biomedical applications due to the high operation temperature thereof.
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