Polymer MEMS for Micro Fluid Delivery Systems
Ellis Meng and Yu-Chong Tai
Department of Electrical Engineering
California Institute of Technology
M/C 136-93, Pasadena, CA 91126, USA
Introduction
Polymer materials for microelectromechanical systems (MEMS)
technology have been gaining in popularity to meet biocompatibility
requirements of biological and chemical applications. Silicone rubbers havebeen explored extensively by many in a wide range of applications from cellsorting [1] to capillary electrophoresis [2]. However, the major drawback ofusing this material is the incompatibility with other microfabrication
techniques and processes. Parylenes have long been used in integrated
circuits and as a printed circuit board coating for its excellent electrical andmechanical properties. Much attention has now been shifted to the fabricationof biocompatible devices using parylene as a structural material.
Neuroscientists have taken advantage of the biocompatibility of parylene in
the form of coatings on implantable microelectrodes [3]. In addition, parylene
is compatible with microfabrication techniques and forms a conformal,
pinhole-free coating at room temperature.
A miniature prototype fluid delivery system incorporating polymers as
structural materials is presented here using a micropump as the fluid actuator,a thermal flow sensor as a fluidic control device, and micromachined couplers
as fluidic interconnects. This type of system is important in the developmentof micro-dispensers for lab-on-a-chip.
Experimental
Parylene Check-Valved Diaphragm Pump.
Various micro
mechanical diaphragm pumps have been designed to achieve the maximum
flow rate possible using microfabricated parts. Typically, microfabricatedpumps operate with maximum flow rates in the nL/min-µL/min range [4].
Flow rectification in these pumps was accomplished by using high-flow
parylene check valves. These one-way valves consist of thin film parylene
valve caps that are tethered over an orifice to a silicon substrate. Flow is only
permitted when the valve cap experiences forward flow across the valve capand seat. Operation of such a valve is shown in Figure 1. It has been shownthat such a valve is capable of providing a four-fold improvement in flow
handling capability when compared to similar microfabricated valves [5].
Bossed silicone membranes and gaskets were also used as pump components.
Device fabrication details are given in [6] and additional information can befound in [7]. Using this pump, it is possible to achieve flow rates of 13
mL/min. Figure 2 shows an assembled pump and 3D cross sectional view ofa pump assembly.

Parylene Thermal Flow Sensing Array. Flow sensors based on thermal
operating principles have long been popular for their ease of use andfabrication. Sensors for use with biological applications, however, require
low operating temperatures and biocompatibility. A thermal flow sensing
array constructed of parylene and platinum has been demonstrated [8] andsatisfies both of these requirements. Various views of the sensor are seen in
Figure 3 and Figure 4. This device operates based on the flow ratedependent convective heat transfer from a heated sensing element to passing
fluid. In addition, the sensor is capable of multiple modes of operation andcan sense flows as low as 0.5 µL/min. In one operating mode, called time-offlight, flow rate is measured by tracking a heat pulse applied to a heater at asensor downstream. The detected signal measured in terms of resistancechange over time is shown for various flow rates in Figure 5. The
characteristic response shows that as the flow rate increases, the time at which
the peak signal is detected, or “top time,” decreases. Flow rates are set andsupplied by a precision syringe pump (KD Scientific Inc. Model KDS 100)
fitted with precision syringes (Hamilton Company 1700 Series GASTIGHT
syringes).

(a)(b)
Figure 3. (a) Top view of packaged flow sensing array and (b) SEM ofplatinum flow sensing elements suspended on a parylene membrane.

(a)(b)
Figure 4. (a) Backside of packaged flow sensing array showing microfluidic
couplers and (b) exploded 3D drawing of flow sensor components andassembly.

Figure 1. Diagrams and photos showing parylene check valve operations inboth closed and opened modes.

(a)(b)
Figure 2. (a) Top view of an assembled micro check-valved diaphragm pumpand (b) 3D schematic showing pump components.

Figure 5. Sensor response at various flow rates measured in terms ofresistance change over time for time-of-flight operation.

Microfluidic Couplers. Microfluidic devices require both fluidic andelectrical interfaces with the outside world. While electrical interconnects arewell developed, the problem of making fluidic connections to microfluidic
devices with µm- or mm-scale orifices is still not completely solved. The
fluidic coupling approach used here is to connect fluidic devices to theexternal environment using micromachined silicon and polymer couplers [9].
Several types of micromachined fluidic couplers are shown in Figure 6.
These couplers are designed to work with conventional PEEK and fused silica
capillary tubing and are constructed of silicon or polyolefin. These couplers
are extremely robust and support pressures up to 104 kPa (1500 psi).

(a)
(b)
(c)
(d)
Figure 6. Three types of couplers: (a) silicon bulk, (b) polymer molded, (c)and silicon post couplers; and (d) a silicon post coupler with fused silica
tubing attached to a microfluidic chip.
System Assembly. The discrete microfluidic devices previously
presented were joined using micromachined couplers to create a micro fluiddelivery system. Here, the check-valved diaphragm pump serves as a fluid
actuator and is connected to a downstream flow sensing array using siliconpost couplers and PEEK tubing. The flow sensing elements are connected to adata acquisition unit (Hewlett Packard HP34970). Filtered deionized water
was pumped through the system and out through a calibrated pipette where theflow rate is measured by a stop watch. The flow sensor was operated in timeof flight mode using a 3 volt pulse with 1 second duration. While closed loop
control is possible, it is not implemented for simplicity. A schematic diagram
detailing the system components and data acquisition setup is shown in
Figure 7.

Figure 7.
Schematic diagram of flow delivery system layout andexperimental testing setup.
Results and Discussion
A working microfluidic system composed of mostly polymer
components has been successfully assembled and tested. Performance os thesystem can be seen in Figure 8 which shows the sensor output in time-offlight mode for flow driven by the micro diaphragm pump. The 0 Hz tracecorresponds to 0 µL/min flow and the 10 Hz trace corresponds to a flow rateof 46 µL/min. The frequency refers to the actuation frequency of the micropump actuator. Values for top times and peak response are indicated.
Compared to typical time-of-flight sensor response curves, there is noticeableroughness in the 10 Hz signal. This is attributed to the non-continuous flow
produced by the reciprocating actuator in the diaphragm pump. In a typical
pumping cycle, only the pump mode contributes to the overall flow while thesupply mode replenishes fluid in the pump in preparation for the next
pumping cycle. At lower flow rates, and thus lower actuation frequencies,
this pulsed flow effect on the sensor output is much more pronounced [7].
This system demonstrates the feasibility and usefulness of an integrated
dosing system. A further refinement to this system would be to scale theentire system down to chip level. This is ideal in applications where small
flow rates (µL/min to nL/min) are desired. Parylene-based MEMS technology

can be used to make reservoirs, valves, pumps, and other components allwithin a surface micromachined channel for performing various fluidic
functions. This multi-layer parylene technique has been demonstrated in [10]in which a continuous flow micro fluid delivery system was fabricated.

Figure 8.
operation.

Flow sensor response to pumped fluid flow in time-of-flight

Conclusions
Polymer materials have been readily incorporated into MEMS devices.
Various polymer components such as parylene check valves, silicone
membranes, and parylene supported sensors have been fabricated using
MEMS technology. In turn, these discrete devices have been successfully
integrated into a functional microfluidic system capable of delivery fluids inthe mL/min to µL/min range. Integrated approaches to making completely
functional surface-micromachined parylene based fluidic systems have been
proven feasible and are being investigated.
Acknowledgement. This work was funded in part by the Engineering
Research Centers Program of the National Science Foundation under Award
Number EEC-9402726. We would like to thank Mr. Trevor Roper forassistance in fabrication and Mr. Tuan Hoang for help with experiments andproofreading.
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