PAPER

www.rsc.org/loc | Lab on a Chip

A low power, on demand electrothermal valve for wireless drug delivery
applications
Po-Ying Li,a Tina K. Givrad,b Roya Sheybani,b Daniel P. Holschneider,bc Jean-Michel I. Maarekb
and Ellis Meng*ab
Received 27th May 2009, Accepted 2nd October 2009First published as an Advance Article on the web 19th October 2009
DOI: 10.1039/b910248e
We present a low power, on demand Parylene MEMS electrothermal valve. A novel U-shaped thermal
resistive element requires low power (mW) and enables rapid valve opening (ms). Using both
finite element analysis and valve opening experiments, a robust resistive element design for improved
valve opening performance in water was obtained. In addition, a thermistor, as an inrush current
limiter, was added into the valve circuit to provide variable current ramping. Wireless activation of thevalve using RF inductive power transfer was demonstrated.

Introduction
One-shot, disposable microvalves have been widely investigatedfor many microfluidic applications such as space exploration,1polymerase chain reaction (PCR),2 lab-on-a-chip,3–5 and drugdelivery6,7 (Table 1). These valves utilize many activation
methods to achieve valve opening and, among them, electrothermal activation is the most common strategy.2,5–7 Electrothermal valves usually consist of a temporary membrane(or plug) that blocks the flow path and a heater that melts themembrane to open the valve. Depending on the membranematerials used, power consumption can be as high as 2250 mW
for a Pt/Ti membrane6 to 67 mW for a PMMA membrane.7
Other valve operation principles include smart alloys,1microspheres,4 and hydrogels.3
Generally, one-shot microvalves should have low power
consumption, a short time constant, and reliability.8 In
implantable drug delivery systems, low power consumption is
critical; these systems rely on batteries or radio frequency (RF)coils for power. Activation of the valve occurs over a period
referred to as the time constant9 and is highly dependent on thedesign. Devices with widely varying time constants, from ms tomin, may be selected to suit a particular application. Reliable
valve operation and performance are also important design
requirements, especially in the case of implantable devices. Our
application involves the use of a valve for gating radiotracer
delivery in neuroimaging of small animals and requires a singleuse, low power, on demand valve.
Previously, an implantable infusion pump system was investigated that was based on an alternative neuroimaging strategy toachieve real-time neuroimaging of rats.10,11 By triggering an offthe-shelf solenoid valve, pressurized radioactive tracer followed
by euthanasia were rapidly infused into the systemic circulation.
aMing Hsieh Department of Electrical Engineering, University of Southern
California, Los Angeles, CA, 90089, USAb
Department of Biomedical Engineering, University of Southern California,
Los Angeles, CA, 90089, USA. E-mail: ellis.meng@usc.edu; Fax: +1-213821-3897; Tel: +1-213-740-6952
cDepartment of Psychiatry, Neurology, Cell and Neurobiology, Universityof Southern California, Los Angeles, CA, 90033, USA

This journal is ª The Royal Society of Chemistry 2010

Areas of elevated cerebral blood flow were immediately labeled
and brains were collected, fixed, and imaged using autoradiography. Labeled regions correspond to high activity areas linked
to the particular behavior being studied. Thus, animal behavior
can be assessed using this combination strategy involving drugdelivery followed by imaging.
Many constraints are placed on the valve to meet the specifications for this neuroimaging paradigm. First, our target animal
is the transgenic mouse in which many disease models areavailable. Due to the lack of small form factor valves, the priorsystem was demonstrated in rats which are much larger thanmice (350 g vs. 40 g). The rat pump weighed 32.5 g (10% of the
rat’s weight) which was largely due to the solenoid valve andbattery required to operate it. By using a microfabricated electrothermal valve, a pump weighing only 2.5 g was possible andsuitable for use in mice. This weight requirement also impacts theoverall power budget. The target system weight does not allow
for a battery, thus wireless approaches were necessary.
Furthermore, the low power coupling in air of the wirelesssystem12 necessitates low power consumption (mW). To minimize diffusion and nonspecific labeling, the valve must open
immediately (ms) and contribute minimal fluidic resistance toallow rapid radiotracer release. Finally, after the valve is opened,no valve debris should enter the blood stream or obstruct flow.
Therefore, in this study, a microelectromechanical systems(MEMS)-based valve was designed to specifically target theserequirements.

DesignThe electrothermal valve consists of either two-layer (metal/
Parylene) or sandwiched (Parylene/metal/Parylene) thin film
composite membranes (Fig. 1(a)–(c)). The metal layer contains
a resistive element and was deposited on flexible Parylene
membranes. In some versions, an additional overlying Parylene
layer was added. The thin film Parylene resists loading by
the pressure source (drug reservoir) and supports the thin film
metal structures while the resistive metal element, once energized,
melts the Parylene membrane and opens the valve. Parylene wasselected as the membrane material for its mechanical flexibility
Lab Chip, 2010, 10, 101–110 | 101

Diffusion
N/A
490
67
2.8
N/A
15000
1005
No
LiquidPressure Driven
N/A
N/A
N/A
N/A
N/A
8000
N/A
No
LiquidPressure Driven
100.0 (14.5)
70
3000
N/A
N/A
180000540000No
Liquid102 | Lab Chip, 2010, 10, 101–110

PET is the abbreviation of polyethylene terephthalate. b PMMA is the abbreviation of polymethylmethacrylate.
aPressure Driven
275.8 (40)
70
200
3
N/A
20000
4000
Yes
LiquidMass Transport Mechanism
Max. Pressure (kPa (psi))
Melting Temperature ( C)
Opening Power (mW)
Applied Voltage (V)Applied Current (mA)
Valving Time (ms)Energy (mJ)
Biocompatible Construction
Delivered Material (Solid/Liquid/
Gas)

Pressure Driven
10000.0 (1450)
137
13000
29.2
439
3000
47400
No
Gas

Pressure Driven
N/A
N/A
N/A
N/A
500
N/A
N/A
No
LiquidDiffusion
N/A
1770
2250
N/A
1000
0.01
0.025
Yes
Solid/Liquid

Pt/Ti/Pt
10/100/10
N/A
N/A
Transistor
N/A
Phenolic Resistor
N/A
Thermal Element Materials
Thermal Element Thickness (Å)

Cu/Ti
200000/2000

Auø25000Pt/Ti/Pt
50  50  0.01
Hydrogel300  700  50
Paraffin1000  500  N/A
Membrane or Plug Materials
Valve Dimension L  W  T (mm)Solder Paste
420  420  N/Aa

PET
N/A  N/A  100

Microsphere200  50  N/A

Au
10000

Our Device

Paryleneø500  (2, 10, or 20)(thick)Pt or Ti
2000, 5000, 7500,
or 10000
Pressure Driven
689.4 (100)
290
30
6
4
66
1.98
Yes
LiquidPMMA
500  500  100b

Luo et al.7
Maloney et al.6
Beebe et al.3
Griss et al.4
Aracil et al.5
Liu et al.2
Bejhed et al.1

Table 1 Comparison of the Parylene electrothermal valve with one-shot valves reported in research literature(Young’s modulus  4 GPa), biocompatibility (USP class VI
material) and compatibility with microfabrication.13–15
The five different resistive elements share a common U-shaped
geometry but have different element line widths (Table 2). In
these designs, a tongue-like flap was included in the metal layer;
the purpose of this structure is to strengthen the fold in the
Parylene film formed when the freed membrane flap is bent backby the escape of the pressurized contents of the gated fluid
reservoir. Fig. 1(d) and (e) demonstrate the valve operation
process. Power is transmitted from an activated external primarycoil to a secondary coil connected to the valve. Then, current
passes through the resistive element and generates Joule heating
to melt the adjacent Parylene surrounding the element. The
element is designed such that Parylene is melted in a narrow
region through part of the circumference of the cannula lumen.
Thus, the remainder of the Parylene film is pushed away by thefluid, folding back against the cannula. This piece is supported bya metal-strengthened flap which prevents the Parylene film fromwashing away or obstructing the fluidic path. Compared toprevious electrothermal valves,16,17 this improved design requires
melting of only the limited Parylene area surrounding the resistive element (instead of melting of the whole valve area) which
contributes to a decrease in the power consumption and anincrease in the valve opening speed.

Theory and modeling
The valve opening process is achieved through heat transfer fromthe resistive heating element to the valve membrane material.
The mechanical strength of the resistive element plays animportant role in valve operation and it must withstand thethermally-induced stress from its supporting membrane during
the opening process. Previously, we modeled the mechanicalperformance of a similar Parylene valve with a different resistive
element pattern and demonstrated the mechanical robustness of
this Pt resistive element under 1 atm of applied pressure.16 In thisstudy, two finite element models (FEM) were developed toevaluate heat transfer and thermal stress in our electrothermal
valves. First, two transient FEM thermal simulations each withdifferent resistive element geometries were studied for the
purpose of improving valve opening by optimizing the heattransfer path to the membrane material. The mechanicalperformance of the improved valve geometry was further investigated by varying dimensions in six FEM thermal stresssimulations.

Thermal modeling
Four major heat transfer routes exist from the resistive element:(1) to the electrical leads, (2) to the Parylene membrane,(3) through the Parylene membrane into fluid (air or water), and(4) through the Parylene membrane into the cannula (Fig. 2(a)and (b)). To simplify the discussion, route 1 and 4 are omitted
due to the separation distance and thermal conductivity of these
thermal sinks. Route 2 is defined as the heat propagation within
the valve membrane and is the critical heat transfer route forvalve opening. However, this horizontal heat transfer path(x-direction) within the membrane is much longer than route 3 inthe vertical propagation direction (z-direction, radius ofThis journal is ª The Royal Society of Chemistry 2010

Fig. 1 Schematics and photographs of the Parylene electrothermal valve showing (a) the electrothermal valve layout, (b) a single fabricated valve,(c) close-up of a valve element, (d) cross-sectional view of the valve assembly, and (e) illustration of valve operation.

membrane compared to thickness of membrane). If a highthermal conductivity fluid (relative to the thermal conductivity of
Parylene, e.g. water) is used, most of the heat will flow into thisfluid. Under these conditions, opening the valve requires a muchhigher input power. However, higher applied power may induce
larger membrane deformation and increase the risk of element
fracture before valve opening.
Melting of the entire valve area has been a common valve
opening strategy in previous electrothermal valve designs.6,7,16,17
However, higher heat and power are required and the thin film
resistive element may fracture prematurely. Modifying theresistive element design such that it is constrained to the perimeter of the valve area, minimizes the area over which heat transfer
is necessary to open the valve and lowers the overall power
consumption.
These factors are evident in transient thermal FEM simulations(COSMOSWorks 2007, SolidWorks Co., Concord, MA) thatwere performed. Two electrothermal valve designs with the same
Parylene membrane and different resistive element geometries(A: a serpentine element trace patterned over the entire valve area,
and B: an U-shaped trace limited to the perimeter of the valve)were investigated (Fig. 2(c) and (d)). The simulation parameters
are listed in Table 3. The same power was applied to bothelements and air was used as the contact medium. Valve opening
occurs when the Parylene membrane temperature is greater thanthe melting point. Valve A opened after 400 ms and valve
B melted in less than 13.3 ms. By 133 ms, valve B reached a stabletemperature distribution while the temperature of valve A wasstill increasing at 400 ms. In addition, a narrow temperature
distribution pattern was observed for valve B. As described in thedesign section, this U-shaped element melts only a small quantityof Parylene instead of melting of the entire valve area. Therefore,
based on these simulations, the later design strategy was selected
for reduced power consumption and faster valve opening.

Thermal stress modeling
Thermal stress modeling was performed on six valve designs
having U-shaped elements (COSMOSWorks 2007, SolidWorks
This journal is ª The Royal Society of Chemistry 2010

Co., Concord, MA). Valves differed in wire style (straight orserpentine), element material (Pt or Ti), wire width (5 mm or
20 mm), and wire thickness (1000 Å or 3000 Å) (Table 4). In thesesimulations, electrical power was applied to the resistive traces(40.5 mW) and the corresponding thermal-induced stress distributions over the entire valve area were obtained (Fig. 3). The
maximum thermal stresses in the traces of each valve design aresummarized in Table 4. Valve 4 (straight, Ti, 20 mm wide, andthick 3000 Å wire) possessed the lowest thermal stress(276.2 MPa). In general, lower thermal stresses were present indesigns with straight traces. The results suggest that mechanically robust traces are dependent on mechanical properties ofthe metal and the layout geometry (straight, wider, and thicker
traces are preferred). Although the FEM software cannot
simulate dynamic stresses that occur during the valve melting
and opening, these simpler thermal stress simulations can suggest
improvements to the overall design.
A compromise exists between applied power and valve
opening performance. While a faster opening may be desired,high applied power risks premature element fracture before
complete valve opening. Air was used as the contact medium inthe simulations. However, if water or other liquids are appliedinstead, a higher applied power will be required to open thevalve. In both cases, higher power is associated with an increasein temperature. The elevated temperature induces large deformation of the Parylene membrane and high stress on the thinmetal film that may result in premature fracture.

Fabrication
Valve membrane fabrication
Two electrothermal valves consisting of Parylene/metal or Parylene/metal/Parylene layers were constructed using a surface
microfabrication process similar to that reported by Li et al.16,17
First, the bottom Parylene C layer was deposited on a 3 inch
silicon wafer with native oxide; the oxide layer facilitates Parylene release at the end of the process. Following lithography,
metal (Pt or Ti) was e-beam evaporated. Then the resistive
Lab Chip, 2010, 10, 101–110 | 103

E

450
40
1100
CircularRounded

D

450
5
1178
Circular
Squareelement and contact pad were revealed by liftoff. In the case of
the Parylene/metal/Parylene construction, another layer of Parylene was deposited and the contact pads were opened using
reactive ion etching (O2 plasma). Valves were manually dicedusing a fine blade and peeled off the silicon substrate.
Valve packaging
Briefly, a valve was aligned and attached to two short glass tubes(length of 5 mm and inner diameter of 1.0 mm) using lasermachined double-sided adhesive rings (Fig. 4(a)). Electrical wires
were joined to the contact pads using conductive epoxy(Fig. 4(b)), and a bead of epoxy was applied at the joint tosupport the assembly (Fig. 4(c)). Silicone rubber segments(Silastic Laboratory Tubing, Dow Corning, Midland, MI) wereconnected to the ends of the glass tubes (Fig. 4(d)). Finally, theentire joint was encapsulated with silicone prepolymer (Sylgard
184, Dow Corning, Midland, MI) and cured at 65  C for one
hour (Fig. 4(e)). The assembly process was performed undera stereo microscope.

C

450
5
1878
Serpentine
Square

B

450
20
1152
Circular
SquareExperimental methods
Valve opening performance under various experimental conditions was investigated: material and fabrication (element ID andvalve structure), contact medium, mechanical conditions(applied pressure), and electrical conditions (applied current andthermistor). Five resistive element patterns (Table 2) and four
valve structures with different metals and Parylene thicknesses
were tested (Table 5). Both air and water were used as contact
medium. Some valves were operated under pressurized conditions (200 mmHg) to evaluate their mechanical robustness. Threebiasing modes (constant current, current ramping, and wireless)and the use of a series thermistor as an inrush current limiter,
were investigated to determine their effect on valve opening
performance. Initial experiments were first conducted on freed
valves using a custom testing fixture that facilitated control of thecontact medium and applied pressure. Then, testing of fully
packaged valves was performed.

104 | Lab Chip, 2010, 10, 101–110

Occupied Diameter (mm)Element Line Width (mm)Total Element Length (mm)Element Style
Element Corner Geometry

LayoutValve ID

Table 2 Valve design parameters

A

450
20
1312
Serpentine
SquareValve opening experiments in test fixtures
Custom testing fixtures were laser-machined (Mini/Helix 8000,
Epilog, Golden, CO) in acrylic; this fixture allowed control ofboth electrical connections and mechanical pressure loading suchthat conditions similar to those imposed on a valve in the actual
drug delivery application were easily replicated (Fig. 5). The
experimental conditions are listed in Table 5. The valve opening
process was monitored using a compound microscope andcomputer-controlled CCD camera (PL-A662, PixeLINK,
Ottawa, Canada) system. As listed in Table 5, this testing
apparatus allowed application of mechanical and electricalconditions including: (1) contact medium (water or air) on thebottom side of the valve (2) pressure (200 mmHg), and (3) biasing
using a DC power supply or a custom RF inductive powertransfer system (2 MHz) specifically developed for biomedical
implants.12 This RF system includes an external 5-turn primarycoil (20 cm diameter) and an implanted 5-turn, 1.3 cm diameter
Litz-wire secondary coil. A class E oscillator was chosen for thetransmitter because it achieves much higher efficiency thanThis journal is ª The Royal Society of Chemistry 2010

Fig. 2 Thermal modeling: (a) a schematic diagram showing a 2-D heat transfer model with arrows indicating the heat propagation routes, and (b) an
equivalent thermal resistance circuit model. Transient thermal FEM using COSMOSWorks comparing the temperature distributions of (c) a valve witha serpentine trace covering the entire valve area and (d) a valve with an U-shaped trace occupying only the perimeter of the valve. For (b), h is theconvective heat transfer coefficient, T is the temperature, L is the material thickness, and q is the heat generated by the resistive element. Only the top halfof the valve is used due to the symmetry. The step time is 133 ms for (c) and 13.3 ms for (d). The arrow on the temperature scale indicates the melting
temperature of Parylene (290  C).

Table 3 Parameters used in transient thermal FEM
Valve Properties
or Parameters
Applied ElectricalPower (mW)Material of
Resistive Element
Width ofResistive Element (mm)Geometry ofResistive Element
Trace Route ofResistive Element
Material of
Valve Membrane
Thickness ofValve Membrane (mm)Diameter of Valve
Membrane (mm)Previous Valve

Current Valve

40.5

40.5

Platinum

Platinum

20

20

Serpentine

U-shaped

Whole
Valve Area
Parylene

Valve
Perimeter Only
Parylene

10

10

500

500

Table 4 FEM thermal stress results of thermal elements having different
styles, materials, widths, and thicknesses
Element Element
Pt Thickness Max Thermal
Valve Valve ID Material Width (mm) (Å)
Stress (MPa)
1
2
3
4a
5
6
a

A
A
B
B
C
D

Pt
Pt
Pt
Ti
Pt
Pt

20
20
20
20
5
5

1000
3000
3000
3000
3000
3000

The optimal design.

This journal is ª The Royal Society of Chemistry 2010

493.0
438.0
351.2
276.2
725.0
553.6

Fig. 3 FEM thermal stress analysis of resistive elements having different
styles, materials, widths, and thicknesses as listed in Table 4. The
maximum stress sites are circled. The original corresponding patterns
were shown in Table 2.

conventional class B or C amplifiers. The biasing circuits are
summarized in Fig. 5(c) and include constant current biasing(2400 SourceMeter, Keithley Instruments Inc., Cleveland, OH)
with or without a series thermistor (SM103J1K, US Sensors,
Orange, CA), and wireless powering with or without a series
thermistor. In RF experiments, a resistor was first connected toacquire the root-mean-squared (RMS) voltage using an oscilloscope (AFG3021, Tektronix, Inc., Beaverton, OR). The associated RMS current could then be calculated.
Packaged valve opening testing
The opening performance of packaged valves connected toa drug reservoir was investigated (Table 5 and Fig. 5(b)). Here,Lab Chip, 2010, 10, 101–110 | 105

Fig. 4 Illustrated assembly process for the Parylene electrothermal
valve: (a) exploded view of components, (b) sandwiched valve withelectrical leads, (c) epoxy encapsulation, (d) addition of silicone tubing
for connection to drug pump, and (e) silicone encapsulation of entire
valve assembly. Photograph (f) showing components for the valve (left)
and a packaged valve with silicone tubing (right).

three valve structures were tested. Constant current or RF power
were applied; the thermistor was used in one experiment. The
drug reservoir was pre-pressurized with DI water to 200 mmHg.
Experiments were conducted with or without an air pocket
between the valve and fluid contained in the reservoir.

Results and discussion
Valve opening experiments in test fixtures
Four valve structures clamped in test fixtures were tested undertwenty one experimental conditions (conditions 1–21) and theresults are summarized in Table 5. Valve designs were improved
through careful design followed by experimental validation inorder to achieve the goal of low power operation of valves
exposed to pressurized liquid. First, devices having the newrefined U-shaped geometry were tested in air (Pt (2000 Å)/Parylene (2 mm) and conditions 1–4 in Table 5). As expected, valves
were opened successfully and consistent opening currents wereobtained for each of the resistive element patterns (A–D). The
typical opening time was <1 s; with the fastest at  tens of ms(Fig. 6(a)), which was in close agreement with thermal FEM
results (Fig. 2(d)). Higher applied current can decrease further
the opening times. However, sufficiently large applied currents
induced significant thermal stress in excess of the mechanicalstrength of the resistive elements. Consequently, resistive106 | Lab Chip, 2010, 10, 101–110

elements fractured instantly before complete valve opening; thiswas a common failure mode. As expected, the maximum opening
current for patterns B and D (straight) was higher than A and C(serpentine).
Successful valve opening was more difficult when water was
the bottom medium (conditions 5–20). Water was introduced
below the valve but not on the top in order to easily assess thevalve condition with a compound microscope; in select cases,
water was also introduced on the top side (conditions 14 and 18).
As expected, higher currents were necessary to open the valve inwater. In initial experiments with Pt/Parylene devices (conditions
5–8), we observed instantaneous fracture of the resistive elements
under high currents before complete valve opening. Only one
valve opened using constant current biasing of 20 mA. Therefore,
an improvement of valve design was required.
Several approaches were investigated to improve valve
performance for the intended application, with focus on theappropriate balance between low power operation, valve
mechanical strength, and optimal heat transfer. First, fracture of
the metal trace was addressed by changing the geometry of the
trace (thicker and wider) and the metal (from Pt to Ti). These
modifications were supported by thermal FEM simulations(Fig. 3). In subsequent experiments described later, delaminationof the metal trace was identified as a failure mode (Fig. 6(c),condition 9). A top layer of Parylene was added or made thicker
to better support the metal element. As reported previously,16,17
constant current biasing may also induce premature failure of themetal trace prior to valve opening due to the initial current spike.
Instead, constant rate current ramping was used to successfully
trigger valve opening in the presence of water (condition 10). Thisapproach was associated with a significant increase in opening
time and required complex circuitry. So, variable rate current
ramping was implemented by simply including a series thermistor
as an inrush current limiter (conditions 13, 14, and 16–19).
After changing the metal from Pt to Ti and increasing the
thickness, delamination in addition to fracture was observed inmetal structures following current application (Fig. 6(c),condition 9). Even when applying a constant rate current ramp,
some valves failed in the same manner (condition 10). The large
difference in the coefficients of thermal expansion between
Ti and Parylene (8.8  10 6 vs 35  10 6 K 1) may contribute to
the detachment but the cause of failure is not well understood.
However, in comparison to the thinner Pt elements, these
Ti elements survived higher currents indicating that increased
metal thickness improved fracture resistance.
A simple solution to prevent detachment was to deposit anadditional layer of Parylene (8–10 mm) on top of the otherwise
open metal pattern (conditions 11–21). Valves were successfully
opened under constant current in water (condition 11) but notwhen pressure was applied (200 mmHg, condition 12). Also,
valves with thicker Parylene did not open consistently in water(condition 15). Given the prior success with current ramping,a thermistor was connected in series to provide variable ratecurrent ramping and limit inrush current. First, thermistors were
calibrated to confirm a suitable variation of resistance over time atdifferent currents; note that in the target application, rapid
opening is required (Fig. 7). Variable rate current ramping
successfully opened valves with and without pressure that had
thicker Parylene and wider metal traces (Fig. 6(b), conditions
This journal is ª The Royal Society of Chemistry 2010

This journal is ª The Royal Society of Chemistry 2010

Lab Chip, 2010, 10, 101–110 | 107

A
E
E

24
25
26

Pt (2000 Å)Ti (7500 Å)Ti (7500 Å)Pt (2000 Å)
7 mm
10 mm
10 mm

Air
Air
Air

Air

Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Water
Air
Air
Air
Water
Air
Air
Air
Air

Top

200
200
200

200

0
0
0
0
0
0
0
0
0
0
0
200
200
200
0
0
200
200
200
200
200
200

CC
RF
RF

RF

CC
CC
CC
CC
CC
CC
CC
CC
CC
CR
CC
CC
CC
CC
CC
CC
CC
CC
RF
RF
RF
CC

No
Yes
No

No

No
No
No
No
No
No
No
No
No
No
No
No
Yes
Yes
No
Yes
Yes
Yes
Yes
No
No
No

Yes
Yes
Yes

Yes

No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Yes

2
1
4

2

9
6
6
7
2
2
4
2
6
3
4
2
3
2
5
5
7
5
1
1
1
5

0
0
1

0

8
5
5
6
0
0
1
0
0
1
4
0
0
0
1
4
3
0
0
0
1
1
or X
X
LP
O or X+/P

O

O or X
O or X
O or X
O or X
X
X
O or X
X
O or X
O, O , or X
O
X+ or X
O /P or X
X
O, O or X
O or OX
O, XP, or X
O /P or X+/P
LP
O /P
O
O or X

Typical
Number Number Valve
Applied Appliedof Valves of Valves Opening
Pressure Current
OpenedResultsc(mmHg) Modeb Thermistor Package Tested

S
N/A
S

S or BS or BS or BS or BS or B
B
B
S or BB
S or D
S or DS
S or BS or BB
S or BSB
S or BSB
N/ASS
S or B

TypicalResistive
Element
Performanced

N/A
N/A
58e

N/A

25.0–30.0
25.0–32.5
7.5–12.5
7.5–15.0
N/A
N/A
20
N/A
N/A
1.67 (CR)
30
N/A
N/A
N/A
40
35
38 and 50
N/A
N/A
N/A
39e
27.5

Valve Opening
Currents (mA)or Current
Ramping
Rate (mA/sec)a
See Table 2 for key. b CC: constant current, CR: current ramping, and RF: radio frequency power transfer. c O: valve opened successfully, O : valve opened partially, X+: small opening hole formed,
X: valve did not open, P: Parylene balloon, and LP: insufficient power to open. d B: element broke instantly, S: element broke during operation, and D: element broke instantly and detached from valve
membrane. e This power was estimated using a series resistor with the same resistance as the valve immediately before the opening experiment.

2 mm
10 mm
10 mm

2 mm

N/A

C

23

Air
Air
Air
Air
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Air
Water
& Air
PocketWater& Air
Pocket
Water
Water
Water
& Air
Pocket

N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
8 mm
8 mm
8 mm
8 mm
10 mm
10 mm
10 mm
10 mm
10 mm
10 mm
10 mm
N/A

Pt (2000 Å)Pt (2000 Å)Pt (2000 Å)Pt (2000 Å)Pt (2000 Å)Pt (2000 Å)Pt (2000 Å)Pt (2000 Å)Ti (5000 Å)Ti (5000 Å)Ti (10000 Å)Ti (10000 Å)Ti (10000 Å)Ti (10000 Å)Ti (7500 Å)Ti (7500 Å)Ti (7500 Å)Ti (7500 Å)Ti (7500 Å)Ti (7500 Å)Ti (7500 Å)Pt (2000 Å)

A
B
C
D
A
B
C
D
A
A
A
A
A
A
E
E
E
E
E
E
E
C

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
10 mm
10 mm
10 mm
10 mm
10 mm
10 mm
10 mm
2 mm

Top
Parylene Bottom

MediumExperimental Valve Bottom
Condition
IDa
Parylene Metal

Valve Structure and Material(Thickness)

Table 5 Parameters used in the investigation of the valve opening performance using a test fixture

Fig. 5 Schematics of the valve opening experimental setups: (a)universal test fixture for unpackaged valves, (b) testing setup for packaged valves (an optional air pocket may be inserted on the pressurizedside), and (c) circuit diagrams illustrating the valve electrical connections
with or without a series thermistor in both DC and RF operation modes.

16–17) but not thinner Parylene (condition 13). Interestingly,
thinner valves experienced incomplete opening due to theformation of a Parylene balloon (Fig. 6(d)). The permanent
balloons were formed as the valve membrane began to melt.
Softened Parylene inflated under pressurized conditions causing
the trace to fracture. Without Joule heating, the balloon cooled
quickly in the presence of water. Thicker valve membranes andwider traces provided sufficient mechanical support to metal
traces to allow complete opening prior to metal fracture, althoughsome Parylene balloons were still observed. In these strengthened
devices, the Ti thickness was decreased to maintain similar
resistance values.
In two experiments (conditions 14 and 18), water was introduced on both sides of the valve. Both valves experienced instanttrace fracture and Parylene balloons as failure modes due to theincreased current. Significant heat transfer from the valve to the
108 | Lab Chip, 2010, 10, 101–110

Fig. 6 Microscope images showing (a) sequential images of a valve
opening for a Pt (2000 Å)/Parylene (2 mm) valve under a constant currentof 32.5 mA in air and (b) for a Parylene (10 mm)/Ti (7500 Å)/Parylene(10 mm) valve under 50 mA in water. (c) Before and after images of a valve
element that delaminated after application of constant current. (d) Side
view of a Parylene balloon formed by cooling of the membrane due tofracture of the metal element and heat transfer into water.

surrounding medium limits valve application to cases in which
air or a small air pocket can be tolerated on one side.
Finally, RF operation of valves was examined (Fig. 5(a),conditions 19–21). Operation was successful for pressurized air(condition 21) but not water (conditions 19–20). The water
experiments were performed with (condition 19) and withouta series thermistor (condition 20). Due to the finite power
consumption of the thermistor, the wireless system was unable tosupply enough current to trigger the valve. Without theThis journal is ª The Royal Society of Chemistry 2010

Fig. 7 Thermistor calibration in which the real-time resistance drop
under different applied currents was measured. The variable resistanceallows a current ramp to be applied to the valve.

thermistor, partial valve opening and a Parylene balloon wasobserved. Thus, for practical operation with the current wirelesssystem, air pockets need to be introduced on both sides of thevalve (1–20 mL) to separate the water from contacting the valve.
However, increasing the RF power is preferred and will also be
pursued to accommodate a series thermistor.

a real-time in vivo neuroimaging application. The wireless activation of the valve was achieved using a custom RF inductive
power transfer system. This system will be further improved inorder to power a series thermistor and the valve. Then preliminary
animal implants will be performed followed by neuroimaging
experiments. The neuroimaging approach is briefly summarized
here. A pump is implanted into the animal. Following recovery,
animal behavior is monitored and when the targeted behavioral
event occurs, an RF signal is transmitted wirelessly to activate thevalve and rapidly infuse radiotracer followed by euthanasia agent.
Finally, the brain is removed, sliced, and imaged using autoradiographic processing of labeled tissue slices to obtain a 3-D
reconstructed statistical parametric mapping.
Freely-moving animals will vary their location as well as theirposition in these planned experiments. Thus, the wireless activation system must be able to activate the valve regardless ofanimal position and location within the testing cage. In ourprevious study in freely-moving rats, we demonstrated wirelessactivation of an electromagnetic valve.10–12 Therefore, RF power
coupling to different orientations and locations of the implanted
coil is expected to be adequate for our future experiments inmice.

Conclusion
Packaged valve opening testing
Completely packaged valves were evaluated and a total of threevalve layouts were tested (Table 5 and conditions 22–26). Pressurized valves with Pt and thinner Parylene did not open except
in one case which was consistent with the results for theunpackaged valves (Fig. 5(b), conditions 22–24). Valves having
thicker Parylene were operated under RF power transfer(Fig 5(b)). With a series thermistor, again the power supplied wasinsufficient to open the valve (condition 25). Successful wirelessoperation of a completed packaged valve was achieved when an
air pocket was introduced between the valve and pressurizedwater (condition 26). In total, four valves were tested wirelessly
with different RMS currents and an air pocket volume (1 or
20 mL). These RMS currents were estimated prior to valve
operation through a load resistor and the associated currents
were calculated. For a 1 mL air pocket, three valves were tested
but none of them opened. The Ti trace remained intact until thepressurized water contacted the valve at which point the valve
cooled instantly, the trace fractured, and a Parylene balloon
formed. By increasing the air pocket to 20 mL, complete valve
opening was obtained (58 mA) and the pressurized water rapidly
flowed through the valve. This air pocket volume allowed sufficient time for the valve to open before contact with pressurizedwater. However, 20 mL of air may not be tolerated by the mouse
circulatory system and an air embolus may occur. Therefore,
based on the previous results, increasing the wireless system’s
power to accommodate the thermistor will allow optimal
operational conditions for the current valve without requiring
the presence of air pockets.

We developed a low power, on demand Parylene MEMS electrothermal valve suitable for implantable drug delivery andbiological fluid handling applications. The U-shape resistive
element melts only a small portion of the valve membrane which
decreases the power consumption and increases the valve
opening speed. A metal flap was incorporated to strengthen thefolded valve membrane and prevent debris from entering theblood stream. FEM thermal modeling predicted a higher
temperature increase rate for the U-shape resistive elementcompared to a serpentine design that spanned the entire valve
area. Thermal stress FEM indicated wider and thicker traces
with rounded corners improved mechanical robustness. Valves
were fabricated using a simple process and packaged withminimal dead volume. Operation was demonstrated in both air
and water, with or without pressure. Through FEM and experiments, a robust valve was obtained and optimal operating
conditions were determined. Failure modes were identified andaddressed. A series thermistor improved the valve opening by
optimizing heat transfer. Successful wireless valve operation wasdemonstrated.

Acknowledgements
This work was surported in part by the NIH/NINDS (1 R01
NS050171). The authors would like to thank Dr Donghai Zhu,
Dr Tuan Hoang, and members of the Biomedical Microsystems
Lab at the University of Southern California for their contributions to this work.

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