A CHECK-VALVED SILICONE DIAPHRAGM PUMP
Ellis Meng, Xuan-Qi Wang, Howen Mak, and Yu-Chong Tai
Caltech Micromachining Lab, 136-93, Caltech, Pasadena, CA 91126, USA
ABSTRACT
Two generations of check-valved silicone rubber
diaphragm pumps are presented.
Significant
improvements have been made from pump to pumpincluding the design and fabrication of a double-sided
check valve, a bossed silicone membrane, and silicone
gaskets. Water flow rates of up to 13 ml/min and amaximum back pressure of 5.9 kPa were achieved
through pneumatic operation with an external
compressed air source. Using a custom designed
solenoid actuator, flow rates of up to 4.5 ml/min and amaximum back pressure of 2.1 kPa have been
demonstrated.

and was implemented as a membrane material in
microvalves [13] and micropumps [14] due to its highelongation (100-1000%). Thus, large deflections andefficient use of pump chamber volume are realized.
Silicone rubber (Sylgard 184; Dow Corning) was also
used in this pump to create molded gaskets.
The testing apparatus involved the use of Plexiglas
pump seats in addition to a combination of gluing andclamping.
Actuation was supplied by a custom
designed solenoid. For the second generation pump,
pneumatic operation with a pressurized air source
controlled by a three-way solenoid valve was also
examined.

INTRODUCTION

DESIGN AND FABRICATION

A reciprocating diaphragm with two passive checkvalves is a common micropump scheme [1-3]. Otherschemes include diffuser [4, 5], electrohydrodynamic[6, 7], rotary [8], and gear pumps [9]. In diaphragmpumps, fluidic transport is achieved by alternating theproduction of under- and over-pressures in the pumpchamber, resulting in a supply mode and a pump mode,respectively. During these modes, pumping occurs only
when enough pressure difference is produced toovercome the cracking pressure of the rectifying valves.
Since microactuators are limited by a small stroke
capability, low cracking pressure, reverse leakage, andflow resistance are highly desirable characteristics incheck valves.

The first generation pump was assembled by gluing
chips together (Fig. 1). As this made it difficult toassemble and repair, the second generation (Fig. 2) wasPreviously, Shoji et al [10] and Smith and Hök [11]demonstrated valves designed for use with micropumps,
however, both exhibited some degree of reverse leakage
and had a limited flow rate. Parylene check valvesmodified from [12] satisfied the above requirementswith the additional advantage of negligible stiction andsurface tension effects. The geometric configuration of
the moving membrane element of the check valve, inaddition to the low Young’s modulus of Parylene (~2.8GPa), resulted in high deflections and, thus, negligible
flow resistance.
In fact, the flow resistancecharacteristics of the check valve were nearly identicalto those of a plain orifice. Pumps designed with thesevalves were capable of reverse blocking even in theneutral state unlike valveless pumps [4-7].
It is also necessary to minimize the pressure required todeflect the pumping membrane. Since only weak forces
are available, a membrane material with low Young’s
modulus is ideal. Silicone rubber (MRTV1; American
Safety Technologies, Inc.) has a low Young’s modulus

Check Valve

Epoxy

PlexiglasSpacersSiliconeMembraneFigure 1: 2D Cross Section of 1st GenerationPump
Silicone Gasket

Silicone
Membrane Chip(a)Si/SiliconeSpacer

Check Valve

Plexiglas Spacer

Silicone Gasket(b)Figure 2: (a) & (b) 3D Cross Sections of 2ndGeneration Pump

designed to eliminate the need for glue by taking
advantage of the sealing property of silicone rubbers.
Alternating layers of silicone and silicon allowed thepump to be watertight when clamped.
Parylene Check ValveIn the first pump, check valves were fabricated on thesame side of the wafer and thus only able to rectify flowin one direction. Pump assembly was complicated byhaving to dice individual valves and glue them, facingin opposite directions, to a Plexiglas structural support.
As a result, the second generation pump check valves
were made with a double-sided process shown in Fig. 3.

to sharp corners in the Parylene, the photoresist washard baked to round off the convex corners. A second
layer of Parylene (3 µm) was then deposited andmasked with 1000 Å of aluminum during patterning inoxygen plasma. This Al masking layer was replacedwith thick photoresist (10 µm of AZ 4620) in the
double-sided process to avoid overheating thephotoresist and Parylene layers during thermal
evaporation. To release the check valve, the 20 µm
silicon layer obstructing the orifice was etched away inBrF3. After dicing, the Al masking layer and sacrificial
photoresist layer were removed to complete thefabrication process. Top views of the check valves used(a)(b)

Figure 4: Check Valves (a) at Rest & (b) Deflectedby Applied Pressure
are shown in Figure 4.
Silicone Membrane
To efficiently use pump chamber volume, it wasnecessary to match the chamber geometry with that ofthe membrane. A large orifice etched into silicon
formed a spacer chip that defined the pump chamberwhen situated between the check valves and pumping
membrane. First generation KOH etched silicone
membranes (7 ´ 7 mm2) and spacers were square
making high compression ratios difficult to achieve. By
using DRIE, a cylindrical pump chamber and matchingcircular diaphragm (8 mm in diameter) could berealized. In addition, it was easier to define cylindricalboss structures without the need for cornercompensation.

Figure 3: Double-Sided Check Valve Process Flow
The fabrication process started with a thermally
oxidized (1.5 µm thick) silicon wafer. Cavities thatwould eventually become orifices were defined using aKOH etch. 20 µm of silicon was left to serve as astructural support. A circular region of silicon wasexposed and roughened using BrF3 gas phase etching.
This reduced stiction of the Parylene tethers to thesubstrate and enhanced adhesion of the anchors. A-174
adhesion promoter was applied followed by thedeposition of a 2 µm layer of Parylene-C. The Parylene
was patterned in an oxygen plasma and then asacrificial photoresist layer (5 µm of AZ 4400) wasspun and patterned. To prevent check valve failure due

20 µm silicon membranes are formed via KOH or DRIEetching. Silicone rubber is spin coated to a thickness of80 µm (first generation) and 140 µm (secondgeneration). An SF6 plasma is used to release theFigure 6: Bossed Silicone Membrane Process Flow

(a)
(b)
Figure 7: KOH Etched Silicone Membrane(a)(b)
Figure 8: DRIE Etched Silicone Membrane
membranes. The fabrication process and various
membrane designs are shown in Figures 6-8. The firstgeneration spacer is simply an 8´8 mm2 orifice etched
into silicon via KOH. Second generation spacers areformed by removing the membrane portion of themembrane chips. The silicone on the surrounding areas
is left as a means to seal the structure during assembly.

Figure 11: Silicon Gasket Process Flowof photoresist (AZ 4620) was used as a mask for bothetching steps. The first etch established the thickness ofthe gasket (~100 µm), and the second produced pits toform alignment pegs (400 µm). These pegs mated withpits in a Plexiglas pump seat. A layer of Parylene orplasma deposited Teflon (CHF3) can be used as moldrelease layers. The remaining gaskets were moldedfrom precision milled Delrin masters.
PackagingSilicone Gasket
Alignment Posts(a)(b)
Figure 9: (a) Gasket on Check Valve Chip & (b)3D View of Gasket
Molded silicone gaskets were introduced in the second
pump to seal the check valves to the pump seat (Fig. 9).
To prevent silicone membranes from adhering to theParylene check valves during pump operation, it wasnecessary to insert a machined Plexiglas spacer.
However, this created the need for another silicone
gasket to seal the spacer to the check valve chip (Fig.
10).

(a)
(b)
Figure 10: (a) Gasket & (b) 3D View of Gasket
Molding silicone into a DRIE etched silicon master(Fig.11) formed the gasket shown in Figure 9. 10 µm

The first generation pump was assembled through a
combination of clamping and gluing. The membranechip was glued to a Plexiglas plate and clamped to therest of the pump which consisted of chips stacked andglued together. Tygon tubing was connected to theinput and output to facilitate testing. A solenoidactuator was then properly positioned beneath themembrane.

(a)(b)
Figure 13: Top View of (a) Single Inlet/Outlet & (b)Check Valve Array Pump
The second generation pump (Fig. 13) was entirely
clamped and only required adhesive to connect thetubing and actuator to the packaging. This modular
design allowed easy access to and replacement ofindividual components. Two different pump seats wereprecision milled from Plexiglas for the two different
check valve configurations. Plexiglas backing plates
allowed the pump to be clamped together and provided
a means for attaching the actuator. A different backing
plate was used during pneumatic testing.

TESTING AND RESULTS
Check Valve
Performance of the new check valves was characterized
for both air and water (Fig. 14). Flow rate versus
applied pressure plots with the check valve in place andremoved (orifice only) are displayed together. As
shown in [12], the twist-up tether configuration of thecheck valves allowed for enough deflection to achievenearly atmospheric pressure at the exit. Check valve
cracking pressures were below sensor resolution.

Flow Rate (SLPM)

5.0
4.5
4.0
3.5
3.0

3 Check Valves

2.52.03 Orifices
2 Check Valves

1.5
1.02 Orifices
1 Check Valve

0.50.01 Orifice010203040

Applied Pressure (kPa)(a)
120100Flow Rate (ccm)

5.0
4.5
Flow Rate (SLPM)

4.0
3.5
3.0
2.560

3 Check Valves
3 Orifices
2 Check Valves
2 Orifices
1 Check Valve
1 Orifice

4020

2.0
1.50

Check Valve

1.00Orifice

246406080(b)Figure 15: Flow Rate vs. Pressure in Check Valve
Array for (a) Air & (b) Water

0.0020Applied Pressure (kPa)

0.5
810

Applied Pressure (kPa)(a)
45Pump40
Flow Rate (ccm)8035

1st Generation
The solenoid actuator produced a reciprocating motion
where an integrated plunger was retracted and extended

3025
2015

Check Valve

10Orifice

5005

1015

2025

Applied Pressure (kPa)(b)
Figure 14: Flow Rate vs. Applied Pressure Plotsfor (a) Air & (b) Water
A novel check valve array, consisting of and inlet andoutlet each comprised of three individual check valves,
was also characterized. Figure 15 shows the flow rateincrease as the number of check valves was increased.
Similar data for check valves used in the firstgeneration pump, which have orifice dimensions 780µm ´ 780 µm, was presented in [12]. A slightadvantage in performance for water flow was observedin the array case. The orifice dimensions for thesolitary input/outlet check valves were 770 µm ´ 770µm and for the check valves in the array, 370 µm ´ 370µm. Effective orifice areas were 5.93 ´ 10-3 mm2 and4.11 ´ 10-3 mm2, respectively. Thus, a check valve
array was preferred over a single check valve design tomaximize flow rates.

Figure 16: Solenoid Actuation Scheme
as current was turned on and off, respectively (Fig. 16).
By using the plunger to deflect the silicone membrane
repeatedly, water pumping was achieved. Flow rateversus frequency and back pressure plots are displayedin Figure 17. Careful priming was necessary to achieve
optimal performance. As expected, flow rate was
highly dependent on actuation frequency and wasapproximately linear for low frequencies. However,
above 10 Hz, this linear relationship was lost as themembrane was unable to match actuator movement.
The maximum achievable flow rate was 3.4 µl/min at
10 Hz and maximum sustainable back pressure was 2.1
kPa which occurs for both 10 and 20 Hz.

1400012000

3500
Flow Rate ( l/min)

Flow Rate (µ l/min)

4000

3000
2500
2000
1500
1000

10000
8000
6000
4000
2000

500
0000

20406080

Frequency (Hz)

51015

100

2530(a)

(a)
12000

2000
1800

10000

1600

1 Hz

1400

Flow Rate ( µ l/min)

Flow Rate (µl/min)

20

Frequency (Hz)10 Hz

120020 Hz

1000
800
600
400
200

1 Hz5 Hz8000

10 Hz

6000
4000
20000
0.0

0.5

1.0

1.5

2.00

2.50

Back Pressure (kPa)

22nd Generation: Pneumatic Pumping
Pneumatic operation was performed to verify the designof the second generation pump. Both check valve
schemes were tested using an inflation pressure of 6.9
kPa and the results are shown in Figures 18 and 19. Up
to 4.4 µl/min at 13 Hz and a back pressure of 3.6 kPa at
5 Hz were possible with the single inlet and outlet
design. A nearly threefold increase in flow rate of 13µl/min at 11 Hz and maximum back pressure of 5.9 kPaat 5 Hz was achieved in the array pump. Thus, thecheck valve array pump outperformed the single checkvalve pump in both categories. The same nonlinear
behavior in the flow rate dependency on frequency wasalso observed, occurring for the same reasons as before.

456(b)
Figure 19: 2nd Generation Pump with Check
Valve Array (a) Flow Rate vs. Frequency & (b)Back Pressure Plots for Pneumatic Actuation
2nd Generation: Solenoid Actuated Pumping
To eliminate the fall off in flow rate at high frequencies,
it was necessary to physically connect the actuator tothe pumping membrane. Thus, a new membrane with acircular silicon boss structure was implemented. This
allowed the plunger of the actuator to be attached to thepump chamber via a silicon boss on the silicone
membrane. The improvement in flow rate versus
frequency performance for the single inlet/outlet pumpis shown in Fig. 20. A maximum flow rate of 4.5µl/min at 14 Hz was possible. However, the maximum
back pressure decreased slightly to 1.32 kPa at 1 and 5Hz.
5000

4500

4500

4000

4000
Flow Rate (µl/min)

5000

3500
3000
2500
200015003500
3000
2500
2000
1500
1000

1000

500

500

00051015202530354045050

510Frequency (Hz)

152025Frequency (Hz)

(a)

(a)

8000

6000

7000

1 Hz

1 Hz

5000

5 Hz

6000

Flow Rate (µl/min)

Flow Rate ( µ l/min)3

Back Pressure (kPa)(b)
Figure 17: 1st Generation Pump (a) Flow Rate vs.
Frequency & (b) Back Pressure Plots

Flow Rate ( l/min)

110 Hz

5000
4000
3000
2000

5 Hz

4000

10 Hz
15 Hz

3000

20 Hz

2000
1000

1000

0001234

Back Pressure (kPa)(b)
Figure 18: 2nd Generation Pump with Single
Check Valve (a) Flow Rate vs. Frequency & (b)Back Pressure Plots for Pneumatic Actuation

0.00.51.01.5Back Pressure (kPa)(b)
Figure 20: Solenoid Actuated Pump (a) Flow
Rate vs. Frequency & (b) Back Pressure PlotsDISCUSSIONACKNOWLEDGMENTS
In an ideal situation, the volume of liquid pumped per
actuation cycle is a constant regardless of the actuation
frequency. However, as can be seen in Figures 21a-d,
900

500

Volume Pumped Per Cycle (Volume Pumped Per Cycle (l)l)

600400300200100800700600
500
4003002001000002040608001001020304050(a)

(b)
1800

500400300200100l)
600

1600

Volume Pumped Per Cycle (Volume Pumped Per Cycle (l)

700

1400
1200
1000
800
600
4002000051015REFERENCES

Frequency (Hz)Frequency (Hz)0This work is supported by the DARPAMICROFLUMES program under Naval Ocean Systems
Center Contract N66001-96-C-83632. The authors
would like to thank Mr. Charles Grosjean and Dr. Qiao
Lin for helpful discussions; Mr. Tuan Hoang for help
with testing and proofreading; and Dr. Thomas Tsao,
Mr. Trevor Roper, and Mr. Hung Bui for assistancewith fabrication.

20250Frequency (Hz)102030Frequency (Hz)

(c)

(d)

Figure 21: Volume Pumped Per Cycle vs. Frequencyfor (a) 1st Generation Pump, 2nd Generation Pump
Actuated (b) Pneumatically and (c) with a Solenoid,and (d) 2nd Generation Pump with Check Valve Array
Actuated Pneumaticallythe volume decreases with increasing frequency. In thepneumatically actuated pump, since flow rate is equal tothe pressure drop over the fluidic resistance (Q = P/R),
the volume pumped per cycle is inversely proportional
to the frequency. Solenoid actuation, in which themembrane and plunger are physically connected,
produces a more complicated behavior that cannot becompletely described by this simple analysis. The datasuggests that the membrane deflection, which occurs ina finite amount of time, might be a contributing factor
as it also decreases with increasing frequency.
Discrepancies between the maximum flow rateachieved for pneumatic versus solenoid actuation were
due to the inability of the solenoid actuator to produce
deflections equal to the height of the pump chamber.
For the pumps presented, a solenoid actuator wassufficient to demonstrate the device however, integrated
thermopneumatic or piezoelectric actuators are also
being investigated.
CONCLUSION
Two silicone diaphragm pumps are presented. Multiple
check valve configurations were explored and a checkvalve array is optimal for pumping higher volumes. A
greater reduction in dead volume and better actuator arenecessary to achieve air pumping.

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