This work describes the development of low-power bubble-actuated planar micro gate valves. The work includes the design of the gate valve, the development of electrolysis bubbles as a low power actuator, and a novel fabrication procedure that allows these valves to be integrated with other planar fluidic devices and controlling circuitry. The operation of the valves is demonstrated and valve performance is measured in terms of leakage, power consumption, and cycle time.
The low-power, bubble-actuated, planar micro gate valves consist of a silicon gate that is thrust across a channel by the expansion of bubbles generated locally on the microscale. Unlike most previous microvalves, these bubble actuated microvalves are entirely planar. Therefore, all fluid and mechanical movement occurs in the plane of the wafer. Planarity minimizes fabrication complexity and simplifies integration with other planar microfluidic devices.
Electrolysis was chosen as a micro-bubble generation method to conserve energy. As the size of thermally generated bubbles shrink, the surface to volume ratio, and thus the relative importance of heat transfer increases. As a result, thermally generated bubbles become much less efficient on the micro-scale. In contrast, electrolysis bubbles are formed in an isothermal process, losing no energy to heat transfer. Electrolysis bubbles are shown to actuate microvalves with as little as116µJ. In addition, since surface tension decreases with increasing temperature, the iso-thermal electrolysis bubbles are capable of applying more force than comparably sized vapor bubbles.
While the electrolysis bubbles require very little power, they are slow to disappear greatly limiting valving frequency. In addition, electrolysis bubbles require that the working fluid be water or an aqueous solution. However, most biological fluids are water based. The low-power bubble actuated micro gate valves have been designed to take advantage of surface tension to remove actuation bubbles from actuation chambers and thereby increase actuation speed and frequency. Actuation frequencies as high as 3Hz have been achieved. In addition, to obviate the need for to maintain a bubble to keep the valve in place, a bi-stable valve that requires energy only when switching, suspended from elastic buckling beams, is demonstrated. The valves and their actuators have all been fabricated in a novel silicon on epoxy (SOE) process. This process allows integration of multiple microfluidic elements with each other and with surface micromachined features such as bubble generators or control-lingcircuitry. The process also allows the moving pieces such as floating gates and buckling beams to be fabricated in-situ without requiring post assembly.
The valves designs here are not completely optimal due to fabrication limitations encountered during this research. Fluid leakage past the valve is great enough that closed to open flow resistance ratio is limited to 5:1. Methods of reducing leakage, increasing bi-stability and closed loop control are identified and discussed.