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A Thermopneumatic Microfluidic System
Charles Grosjean, Xing Yang, and Yu-Chong Taii

Abstract. A self-contained planar microfluidic system using thermopneumatic actuation has been demonstrated. Using a novel suspended silicon island heater fabricated by DRIE, and a precision machined acrylic fluidic substrate with a matching silicone rubber membrane, a system of channels, valves, and a pump has been demonstrated with self-contained actuation using air as a working fluid.

Previously, we reported a thermopneumatic peristaltic pump [1]. That design is planar and integration with channels is a natural consequence of the fabrication process. Thus, a planar system of pumps, valves, reservoirs, and channels can be made without assembling the components individually. However, for a system with multiple reagents, the pumps can take up a large area. Conceptually, the peristaltic design can be changed into a diaphragm pump with active valves by changing the operational sequence and resizing the chambers. With active inlet/outlet valves and a central pumping chamber, a generic pumping structure can be made with a plurality of generic input/output ports. Fluids can be routed from any combination of ports with the proper electronic control. A system of many pumps and valves can be reduced to a single pumping core and a number of valves greatly reducing the complexity of a microfluidic system.

Due to the potentially large number of valves in such a system, an optimized heater and working fluid are required. Both low power consumption and reasonably fast actuation are required. Air [2] offers good transient performance, but is less efficient than other liquid working fluids. A tradeoff must be made between thermal isolation to reduce heat lost to the substrate and efficient thermal transfer to the working fluid. To achieve this, a silicon island or heat spreader was suspended on a silicon nitride membrane. The process is shown in Figure 1. The use of DRIE allows great flexibility in the dimensions of the membrane boundary and the island itself, and taking advantage of DRIE "lag", multilevel structures can be formed without additional masking steps. The silicon island is nominally 100 µm thick with a 0.7 µm LPCVD silicon nitride membrane. Cr/Au is used for the resistive heater. To characterize the thermal performance of the heater, an Infrascopeª I was used to measure the steady state temperature distribution. Figure 2 demonstrates that the structure has very good thermal isolation due to the silicon nitride membrane. The small 1.4 mm diameter heater reaches a temperature of 110ûC with only 75 mW of power. The substrate stays at room temperature. As pumping rate is dependent on the speed of the actuator, transient response was characterized electrically by measuring the time constant of the heater resistance in response to a step input and using a laser interferometer to measure the deflection of a thermopneumatic actuator. For the smaller valve actuator, Figure 3 shows that the slowest time constant (cooling) is under 0.25 s. This is several times faster than our previous work [2] using gold on silicon nitride membrane heaters. The measured electrical time constant for heating is 98 ms which correlates well with the 120 ms measured time constant using the interferometer. The electrical measurement only characterizes the heater temperature and the not the working fluid itself which is why itÕs faster.

A pump was tested by combining a heater, a silicone rubber membrane die, a backplate to form a sealed thermopneumatic chamber, and an acrylic pump/valve/channel substrate. For testing, a simple self-contained system with two valves and a single pumping core was used. Bi-directional pumping of liquids was demonstrated with flow rates between 2 and 3 µl/min. The components for the system are shown in Figure 4.

[1] C. Grosjean and Y.C. Tai, "A Thermopneumatic Peristaltic Micropump," 1999 International Conference on Solid-State Sensors and Actuators (Transducers Ô99), Sendai, Japan, June 1999.
[2] C. Grosjean, X. Yang, and Y.C. Tai, "A Practical Thermopneumatic Valve," Twelfth IEEE International Conference on Micro Electro Mechanical Systems (MEMS Ô99), Florida, USA, pp. 147-152, Jan. 17-21, 1999.

Submitting Author: Charles Grosjean, California Institute of Technology, Department of Electrical Engineering, 136-93, Pasadena, CA 91125 USA. Tel: 626/395-2227, Fax: 626/584-9104, Email: charlesg@mems.caltech.edu

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Figure 1. Process flow for the suspended silicon island heater. Each die (shown in Figure 2) contains One pumping heater (3.8 mm) and six valve heaters (1.4 mm).

Figure 2. Heater die (right) and thermal image of a valve heater with 75 mW applied power. Note the excellent thermal isolation (high temperature for small power input) and localized temperature.

Figure 3. Transient response of thermal actuator (heater and membrane die bonded together) in response to a 2V step input. The rise (heating) time is 0.12 s and the fall (cooling) time is 0.24 s.

Figure 4. An assembled thermopneumatic actuator with clear silicone membranes (left) and the acrylic fluidic substrate with a pumping chamber in the center, two channels, and two valve cavities. The array of holes on top and bottom are for electrical connections to the thermopneumatic actuator.