JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 20, NO. 5, OCTOBER 2011

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A Modular Heat-Shrink-Packaged Check ValveWith High Pressure Shutoff
Ronalee Lo and Ellis Meng, Senior Member, IEEE

Abstract—A novel check valve featuring adhesiveless packaging
in heat-shrink tubing and dual regulation of in-plane flow ispresented. The modular design enables simple replacement ofvalve components to modify valve behavior and performance.
The specific design is intended for low-profile fluidic applications requiring flow control, such as drug delivery devices. The
heat-shrink packaging scheme is extremely robust and can withstand > 2000 mmHg (266.6 kPa) without leakage. Three different
valve geometries were investigated and evaluated with theoreticaland finite-element modeling analyses. Repeated flow regulation
experiments on a fully packaged, hydrated valve demonstrated
flow regulation between 25 and 2000 mmHg (3.33–266.6 kPa) andleak-free closure up to 500 mmHg (66.7 kPa) of reverse pressure
with no observed stiction. The valve closing time constants werealso determined.
[2011-0064]
Index Terms—Check valve, drug delivery, dual regulation,
heat-shrink tubing.

A

Fig. 1. A MEMS ocular drug delivery device, which is sutured to theeye, contains a refillable drug reservoir, contoured morphology, cannula, andmodular valve. The valve comprises four stacked disks (valve seat, valve plate,spacer plate, and pressure limiter. The cannula is inserted into the anterior orposterior segments of the eye for targeted delivery of drugs. (Modified from[4].) copyright IEEE).

Manuscript received March 3, 2011; revised July 18, 2011; accepted
July 20, 2011. Date of publication September 1, 2011; date of current version
September 30, 2011. This work was supported in part by NIH/NEI under Award
R21EY018490 and in part by a Wallace H. Coulter Foundation Early Career
Translational Research Award. Subject Editor C.-J. Kim.
R. Lo was with the Department of Biomedical Engineering, University ofSouthern California, Los Angeles, CA 90089 USA. She is now with Exponent,
Menlo Park, CA 94025 USA (e-mail: rlo@usc.edu).
E. Meng is with the Departments of Biomedical and Electrical Engineering,
University of Southern California, Los Angeles, CA 90089 USA (e-mail:ellis.meng@usc.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2011.2163301

on the cellular level [3]. Cao et al. presented a drug delivery
system having three piezoelectrically actuated pumping chambers to drive drug flow under a peristaltic operation principle.
Each pump doubled as a normally closed valve when turned off.
In particular, MEMS are well-suited for ocular drug delivery
applications. Ocular delivery constraints, such as: 1) the space
limitations of the eye and its surrounding tissues; 2) physiological barriers to the target area; and 3) precise dosing control toachieve therapeutic levels, point to MEMS-based drug delivery
components and devices as an attractive approach to overcome
the challenges associated with current treatment methods.
Chronic ocular diseases, such as glaucoma, are currently
managed by one or more of the following methods: topicallyand orally administered medications, injections (periocular
or intraocular), and biodegradable drug implants. The drug
administration mode must provide therapeutic drug levels by
overcoming physiological barriers while simultaneously being
safe and minimally invasive. Additionally, the method must
address chronic delivery for long-term treatment of incurable
diseases [4]. Topical and oral methods suffer from severe side
effects due to systemic absorption while frequent injections
may result in tissue trauma. These methods have poor patient
acceptance and compliance. Current drug delivery implants
have limited payloads. A MEMS approach to refillable oculardrug delivery provides targeted and accurate dosing to theanterior chamber of the eye while overcoming these limitations(Fig. 1).

I. I NTRODUCTION
DVANCES IN the area of microelectromechanical systems (MEMS) devices for drug delivery are of particularinterest due to their potential advantages over current state-ofthe-art technology. These devices may provide greater temporaland spatial control of drug delivery at therapeutic levels directly
to a target location [1]. Advanced drug delivery devices feature
flow regulation which is commonly achieved by inclusion ofa valve. Valves improve safety by enabling accurate control ofdosing (e.g., prevention of diffusion of the drug into the bodyand accidental dosing) and elimination of backflow of bodily
fluids into the device.
Existing MEMS-enabled drug delivery devices utilize microvalves in conjunction with other microfluidic components
such as micropumps, microreservoirs, and microchannels toform complete systems. Discrete and continuous dosing or a
combination of both is possible. Santini et al. presented a drugfilled microreservoir array in which individual reservoirs wereselectively opened to provide discrete doses [2]. Chen and Wisedeveloped a neural probe device capable of chemical delivery

1057-7157/$26.00 © 2011 IEEE

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Fig. 2. Valve operation (from left to right): initially the normally closed valve opens under forward pressure that exceeds cracking pressure, excessive pressuresclose the valve, and the valve remains closed under reverse pressure.

Presented here is a dual regulation check valve which will
be incorporated into a refillable MEMS ocular drug delivery
system. The system comprises a refillable drug reservoir, electrolysis pump, cannula, a dual regulation check valve, andrefill port. Electrolysis actuation drives drug from within thedevice reservoir into the eye. The reservoir stores sufficient drug
for delivery over several months (∼50 μL); the specific drug
regimen (dose volume and dosing frequency) varies with drug
and disease and is determined by an ophthalmologist. Once thevolume of drug has been depleted, a 30 gauge noncoring needle
can be used to refill the drug reservoir via a refill port.
The cannula and check valve can provide targeted delivery to
the anterior or posterior segment of the eye. The current target
application is glaucoma treatment which affects the tissues in
the anterior chamber of the eye. Accordingly, the cannula isinserted into the anterior chamber via a small incision in thesclera, a portion of the eye wall. The device reservoir is sutured
to the eye and completely covered by the outer portion of theeye wall (conjunctiva) to prevent infection. In the rabbit model,
the cannula extends 3 mm into the anterior chamber.
We previously reported a drug delivery system consisting ofa refillable silicone drug reservoir, flexible Parylene C cannula,and electrolysis pump [5]. Drug was pumped directly into theeye by electrolysis actuation. However, this prototype lacked aflow control valve; drugs and bodily fluids could readily diffuse
into and out of the device through the cannula. Furthermore, thedelicate, thin-walled Parylene C cannula was easily damaged
during surgical manipulation and implantation. Its rectangularprofile made it difficult to seal the incision through which it wasinserted with sutures alone; leakage paths were often present at
the interface between the cannula and incision site. Therefore,a more robust, circular cannula with an integrated check valve
was developed.
A cannula having circular cross section facilitates woundclosure with sutures and prevents leakage around the incision.
Ideally, the cannula diameter is < 1 mm; an incision of thissize in the eyewall can self-seal even without the use of sutures.
Therefore, the valve design should accommodate a circular
cannula. Additionally, the valve must be in-plane with respect
to the fluid flow. This ensures that that moving mechanicalparts of the valve do not come in contact with ocular tissues[6]. This valve orientation also minimizes dead-volume. To
prevent diffusion of drug from the device into the tissue, thevalve must be normally closed. Finally, the valve must be safeand function reliably under normal intraocular pressure (IOP)
conditions [15.5 ± 2.6 mmHg, 2.1 ± 0.35 kPa, (mean ± SD)][7], elevated conditions (glaucoma IOP > 22 mmHg, 2.9 kPa),
and transient pressures fluctuations (e.g., patient sneezing, eyerubbing, or changes in ambient pressure) [7], [8]. In summary,the specifications of the packaged valve are: 1) outer diameter< 1 mm; 2) valve cracking pressure > 22 mmHg (normal
IOP maximum value); and 3) valve closure for large andsudden pressure spikes to prevent accidental dosing. Therefore,“bandpass” regulation of flow is desirable. The precise highpressure shutoff specification is currently unknown and will be
determined in planned in vivo experiments.
Many MEMS valves have been developed and were reviewed recently by Oh and Ahn [9]. To minimize the power
requirements for the ocular drug delivery device, only passive
mechanical valve designs were considered. These valves allow
flow under forward pressure and exhibit diode-like regulationof flow. Examples include valves consisting of flow orifices
controlled by pressure-sensitive flaps, membranes, and spherical balls. Lin et al. presented a glaucoma drainage device with
Parylene C check valves (normally closed and normally open)
in series to achieve bandpass regulation. Adhesives were used tosecure the valves in a Parylene C tube [10]. Lo et al. fabricateda normally closed silicone valve within a rectangular cannula
by stacking patterned layers of silicone rubber [4]. However,current valve designs and packaging schemes are not suitablefor integration into an advanced ocular drug delivery device;
therefore, a new valve and package are necessary.
Our valve approach is modular and consists of four plates
stacked together to form a normally closed valve with a pressure
limiter feature to provide bandpass fluid regulation [4]. The four
plates are the valve seat, valve plate, spacer plate, and pressure
limiter. Each of these plates can be replaced or exchanged for adifferent plate design. The normally closed valve opens under
an applied pressure greater than the cracking pressure. At thecracking pressure, the valve plate deflects away from the valve
seat such that a flow path forms between them to allow forward
fluid flow. Valve plate deflection increases with pressure andeventually, at a pressure exceeding the closing pressure, thesoft silicone rubber valve plate seals against the rigid pressure
limiter to stop flow through the valve altogether. Thus, the“bandpass”-like behavior cuts off flow to prevent accidental
dosing when unanticipated excessive forward pressures occur(Fig. 2).
To simplify the packaging processes and to increase theyield, no adhesives were used. Instead, our approach utilizes
biocompatible heat-shrink tubing as a packaging material forour valve. Here, the heat-shrink tube also serves as the drugdelivery cannula. Heat-shrink tubing has been widely used inelectronics for applications such as electrical isolation, environmental protection, repair, and strengthening of joints. Heatshrink tubes are available in many thermoplastic materials,
such as polyolefin, fluoropolymer (fluorinated ethylene propylene (FEP), polytetrafluoroethylene, polyvinylidene fluoride),polyvinyl chloride, neoprene, and silicone elastomer. The heatshrink mechanism is achieved by first forming the material into

LO AND MENG: A MODULAR HEAT-SHRINK-PACKAGED CHECK VALVE WITH HIGH PRESSURE SHUTOFF

Fig. 3. Photo of the valve components (valve seat, valve plate, spacer plate,and pressure limiter), heat-shrink tube prior to thermally-assisted shrinking, andfully assembled valve. (Modified from [4].)Fig. 4. (a) Side view and (b) top view of the packaged valve in a FEP heatshrink tube. The valve was placed inside the tube with a custom jig. The entire
fixture was heated to 215 ◦ C at 1.5 ◦ C/min and cooled at the same rate to room
temperature [4].

its final shape. Then, ionizing radiation is used to cross-link
the material. Once cross-linked, the part is heated above themelting temperature. The part is then stretched or blown into an
expanded configuration and cooled to maintain the expanded
state. The part will then shrink when heat, near the melting
point, is again applied to the part due to the elastic natureof the cross-linked material [11]. Biocompatible heat-shrink
tubing (polyolefin) was investigated as a packaging scheme forflexible sensors [12] and glaucoma drainage implants [13]. Dueto its conformal nature, circular shape, and biocompatibility,
heat-shrink tubing was selected for packaging the valve for theocular drug delivery application.
II. D EVICE D ESIGN
A. Design
The four modular valve components are shown in Fig. 3
along with the heat-shrink tubing. The valve seat and valve plate
form the normally closed portion of the valve. The SU-8 spacer
plate defines the distance the valve plate must deflect to seal
against the pressure limiter plate. Therefore, the thickness ofthe spacer plate, in part, controls the pressure at which the valve
closes. The valve seat and pressure limiter provide the structural
support for the valve plate.
The valve plate is fabricated using medical-grade silicone.
SU-8 was chosen for the valve seat and spacer plate due to its
ease of fabrication and larger Young’s modulus compared tosilicone rubber. SU-8’s biocompatibility has been investigated,
and reduced biofouling compared to silicon was demonstrated[14]. Other medical-grade polymers can also be used to replace
the SU-8 components.
The assembled valve is packaged into a 22G FEP heat-shrink
tube (Zeus Industrial Products Inc., Orangeburg, SC) (Fig. 4).
FEP is a well-known medical material and is designated a
USP class VI biocompatible polymer. FEP is also transparent,

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Fig. 5. Photo of (a) Parylene C cannula integrated with a drug delivery pumpand (b) clogging of Parylene C cannula after ex vivo testing.

with a refractive index of 1.338 [15], thereby allowing visualinspection of the valve after packaging. FEP is resistant tomost chemicals and solvents and can withstand temperaturesin excess of 260 ◦ C, making it suitable for a wide range ofapplications.
Upon heating, the heat-shrink tube contracts around the valve
stack forming a robust package that securely holds the valve
assembly in place without any adhesives. The SU-8 plates
surrounding the silicone rubber valve plate prevent significant
radial stress on the valve plate.However, this heat-shrink packaging method does impose an axial compression on the outercircular perimeter valve plate, which translates into a compression of the components in the entire valve stack. This gives rise
to a finite cracking pressure. The previous Parylene C cannula
was prone to clogging or damage during surgical implantation(Fig. 5); therefore, a more robust cannula design was necessary.
The heat-shrink tube wall thickness (approximately 200 μm) is
greater than that of the Parylene C cannula (thickness 7.5 μm),
resulting in a more mechanically robust structure. As mentioned
previously, the circular cannula facilitates sealing of the incision, thereby minimizing leakage at the cannula and tissue
interface.
B. Theory
The valve diameter was dictated by surgical requirements; amaximum incision length of 1 mm was permitted. Therefore,
valve components were limited to 900 μm in diameter, leaving
100 μm for packaging. The dimensions and geometry of theindividual valve components were determined using theoreticalequations and finite-element modeling.
The analytical solution for large deflection of a flexible
plate of uniform thickness guided selection of the thicknessesof the spacer and valve plates [(1) and (2)]. The maximum
deflection (wmax ) of a uniform and homogenous place wascalculated from the plate thickness (t), applied pressure (p),plate radius (a), and flexural rigidity (D). Flexural rigidity isa function of Young’s modulus (E), plate thickness (t), andPoisson’s ratio (ν) [16].


2
wmax
pa4
(1)
=
wmax 1 + 0.486 2
t
64D
D=

Et3
.
12(1 − ν 2 )(2)

The outer edge (150 μm wide band) of the valve plate wasreserved for clamping by the valve seat and pressure limiter,
leaving a 600 μm diameter area for the active deflecting area of1166

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TABLE IS UMMARY OF VALUES U SED IN T HEORETICAL C ALCULATIONS OF
L ARGE D EFORMATIONS IN U NIFORM T HIN P LATESTABLE IID IMENSIONS OF VALVE C OMPONENTS , I NCLUDING THE T HREE
D IFFERENT VALVE D ESIGNS (H OLE , S TRAIGHT A RM , S -S HAPE A RM ).
VALVE M ATERIALS (MDX4-4210 AND SU-8) ARE B IOCOMPATIBLE .
A LL C OMPONENTS ARE 900 μm IN D IAMETER

Fig. 6. Three different valve plate designs (a) hole, (b) straight arm, and(c) s-shape arm ([4], copyright IEEE); and the corresponding fabricated silicone
rubber valve plates (d) hole (through holes are indicated by the arrows), (e)straight-arm, and (f) s-shaped arm.

the plate. In the analysis, applied pressure was varied between
0 and 1000 mmHg (0 and 133.3 kPa) and applied to valve
thicknesses between 0 and 150 μm. The resulting calculated
wmax was assigned as the maximum thickness of the spacer
plate. It should be noted that wmax is the deflection at the centerof the plate; sealing against the pressure limiter requires greater
deflection. The final values were chosen based on estimatedpressure operating ranges and ease of handling. The values usedin the large-deflection equations are shown in Table I. Thismodel, however, does not account for the prestress exerted by
the heat-shrink packaging nor the presence of IOP in the eye.
The IOP of the eye exerts a constant reverse pressure on thevalve plate on the order of 15.5 ± 2.6 mmHg [7].
C. Finite-Element Modeling
SolidWorks models of the valve components were created
for finite-element modeling (FEM) analyses. Stress and deformation FEM analyses determined the stress distribution andvalve behavior for forward pressure values ranging from 0 to1000 mmHg (0 to 133.3 kPa), and the reverse pressure valueof −500 mmHg (66.7 kPa). FEM results provided convenientvisualization of valve plate movements and its interaction withthe pressure limiter.
Three different valve plate designs (hole, straight arm, ands-shape arm) were investigated (Fig. 6). Each design possesseda different effective fluidic resistance and thus differing bandpass flow regulating characteristics (e.g., opening and closingpressure). FEM estimations and theoretical analyses guided theassignment of valve geometries such that the operational pressure range would be limited at the lower bound by normal IOP,
<35 mmHg (4.7 kPa), and an upper bound that was arbitrarily
chosen to be at least two orders of magnitude greater thannormal IOP values (e.g., 2000 mmHg, 266.6 kPa). However, theoperating pressure ranges of the valve are easily customized by

Fig. 7. Fabrication process for the valve seat and pressure limiter plates.
Fabrication steps are cross-section views at the A-A line. (Modified from [4].)altering the dimensions of the valve plate and spacer plate. The
dimensions of the valve components are presented in Table II.
The operating characteristics of the valve are easily modified
to adapt the valve for other uses such as alternate drug delivery applications. Theoretical equations and FEM deflection
simulations show that adjustments to the valve plate or spacer
plate dimensions will affect the bandpass region of the valve
behavior. Increasing the valve plate diameter, reducing thevalve plate thickness, creating more compliant tethers in thevalve plate, using a more flexible material for the valve plate, orusing a thinner spacer plate will narrow the bandpass region bylowering the closing pressure value. Adding a bossed structure
to the bottom of the valve plate can also narrow the bandpass
region by increasing the cracking pressure.
III. FABRICATION AND PACKAGING
A. SU-8 Valve Seat and Pressure Limiter
The SU-8 valve seat and pressure limiter shared identicaldesigns. This choice simplified fabrication and ensured interchangeability of the two parts. A two-layer SU-8 process wasused (Fig. 7). First, a soda-lime wafer (Mark Optics, Santa Ana,
CA) was dehydrated for 20 min at 120 ◦ C. Then, the wafer wastreated with Omnicoat (MicroChem, Newton, MA) to facilitate
release of the SU-8 components. Three layers of Omnicoat
were spun onto the wafer (3000 rpm, 30 s) with a bake step
(1 min at 200 ◦ C) performed after each coat [Fig. 7(a)].
Multiple Omnicoat layers reduced the time and temperature
required for the release step. SU-8 2100 (MicroChem, Newton,MA) was prespun onto the wafer (30 s, 500 rpm) to provide
an even coating. Then, 160 μm SU-8 was applied (30 s,

LO AND MENG: A MODULAR HEAT-SHRINK-PACKAGED CHECK VALVE WITH HIGH PRESSURE SHUTOFF

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Fig. 8. Fabrication process for the SU-8 spacer plate. Fabrication steps arecross-section views at the A-A line. (Modified from [4].)1750 rpm) to form the first layer of the component [Fig. 7(b)].
This layer was softbaked on a hotplate at 95 ◦ C for 2 h(temperature ramped at 3 ◦ C/min) and slowly cooled to room
temperature. The layer was then patterned (390 mJ/cm2 ); theenergy dosage was determined by using the suggested energy
level for 160 μm (260 mJ/cm2 ) and adjusting with a 1.5 multiplier for using a glass substrate instead of silicon [Fig. 7(c)].
Dicing saw tape was placed behind the wafer prior to exposure
to prevent unwanted exposure from the reflected UV fromthe aligner chuck. A postexposure bake (12 min, 95 ◦ C) wascompleted, again ramping from room temperature to 95 ◦ C at
3 ◦ C/min and slowly cooling back to room temperature. 40 μm
of SU-8 2050 (MicroChem, Newton, MA) was spin coated(30 s, 4000 rpm) to form the features in the valve seat andpressure limiter [Fig. 7(d)]. The wafer was then baked for 3 h at
95 ◦ C (with ramp up and cool down). The 40 μm was patterned(192 mJ/cm2 ; 160 mJ/cm2 times a 1.2 multiplier for a SU-8 substrate), and postexposure baked for 14 min at 95 ◦ C [Fig. 7(e)].
The components were developed using SU-8 developer(MicroChem, Newton, MA) [Fig. 7(f)].
To remove the valve seats and pressure limiters from thewafer, the wafer was immersed in Remover PG (MicroChem,
Newton, MA) [Fig. 7(g)]. The components were rinsed in isopropyl alcohol, IPA, and DI H2 O and then hardbaked at 215 ◦ C
for 1 h. The hardbake step annealed the SU-8 components
to improve thermal resistance for the subsequent heat-shrink
packaging process. Hardbaking was performed in an air environment; however, discoloration of the SU-8 parts resulted
which may be attributed to oxidation. Discoloration of the valve
seat (dark orange) resulting from hardbaking in air can be seenin Figs. 3 and 4. This was consistent with other reports inliterature in which the threshold for the discoloration reaction
occurred between 200 and 250 ◦ C when baked in air whereas
SU-8 baked in vacuum exhibited no discoloration up to 300 ◦ C[17], [18]. Thus, the hardbake process was modified and performed under vacuum to prevent discoloration of the SU-8
and reduce the residual stress in the thick film structure [17].
Finally, this separate hardbake step was removed to simplify the
fabrication process; the entire assembled valve was hardbaked
during the heat-shrink tubing process (heat-shrink packaging
was performed under vacuum at 215 ◦ C).
B. SU-8 Spacer
The spacer plate was fabricated on a dehydrated wafer(20 min, 120 ◦ C) coated with three layers of Omnicoat (asdescribed in the fabrication of the valve seat and pressure limiter) [Fig. 8(a)]. The 40 μm thick spacer plate was spin coated(SU-8 2050, 30 s, 4000 rpm) [Fig. 8(b)]. The layer was softbaked at 95 ◦ C for 1 h, ramping at 3 ◦ C/min from room temper-

Fig. 9. Fabrication process for the valve plate using an SU-8 master mold.
Fabrication steps are cross-section views at the A-A line. (Modified from [4].)ature to 95 ◦ C. Dicing saw tape was applied to the backside of
the wafer prior to exposure (240 mJ/cm2 ; 160 mJ/cm2 times a
1.5 multiplier for a glass substrate) [Fig. 8(c)]. The wafer was
immersed in SU-8 developer [Fig. 8(d)]. The spacer plates werethen released from the substrate using Remover PG and rinsed
using IPA and DI H2 O [Fig. 8(e)].
C. Silicone Valve Plate
The valve plate was fabricated by casting medical grade
silicone pre-polymer (MDX4-4210, Dow Corning, Midland,MI) on a SU-8 master mold. The SU-8 master was createdon a soda lime wafer using SU-8 2050. First, the wafer wastreated with A-174 (a silane adhesion promoter) to enhance
Parylene C adhesion to the soda lime wafer. A 4 μm layer ofParylene C (Specialty Coating Systems, Inc., Indianapolis, IN)
was vapor deposited onto the wafer to prevent the SU-8 fromdelaminating from the wafer due to mismatch of the thermal
coefficients of expansion between soda lime and SU-8 [5]. A
75 μm layer of SU-8 2050 was spin coated (30 s, 2000 rpm)
and softbaked for 90 min at 95 ◦ C [Fig. 9(a)]. This layer of
SU-8 defined the valve plate thickness. The SU-8 layer was
patterned (308 mJ/cm2 ; 208 mJ/cm2 dose times a 1.5 multiplierfor the glass substrate) and developed using SU-8 developer[Fig. 9(b) and (c)]. MDX4-4210 (10 : 1 base to curing agent
ratio), was poured onto the mold and degassed under vacuum.
Excess silicone was removed by scraping the mold with a metalsqueegee [Fig. 9(d)] [19]. Valve plates were released from themold; any excess silicone was manually removed using a finetipped blade [Fig. 9(e)].
D. Heat-Shrink Packaging
Valves were assembled by stacking individual components.
First, the components were gathered onto a silicone sheet undera stereo microscope. Silicone provided a tacky surface to holdthe stack steady while making alignment adjustments.
First, the valve seat was placed face up (sealing rings up)
on the silicone working space [Fig. 10(a)]. Next, the valve
plate was stacked on top [Fig. 10(b)]. The valve plate mustbe aligned such that none of the through holes are placed
over the valve seat opening. Misalignment may compromise
the normally closed function of the valve, or prevent the valve
from closing at elevated pressures. The spacer plate was addedon top of the valve plate, again, ensuring that the spacer plate
was aligned and not covering the valve plate through holes

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Fig. 13.
Fig. 10. Top and side views of valve assembly. (a) Valve seat. (b) Valve plate
added to valve seat. (c) Spacer plate placed on valve plate. (d) Pressure limiter
added to assembled valve.

Valve plate deflection setup.

lowered until the valve stack was securely clamped. The FEP
tubing was carefully slipped around the valve [Figs. 11(b) and12(b)]. The entire jig was then placed in a vacuum oven andheated to 215 ◦ C at a rate of 1.5 ◦ C/min, held at 215 ◦ C for atleast 30 min, and then cooled to room temperature at the samerate [Fig. 12(c)]. The baking and cooling steps were ramped tolimit the thermally induced stress on SU-8 which may lead tocracking. The jig was removed from the oven and disassembled.
Then, the packaged valve was slipped off the centering pins
[Fig. 12(d)].
IV. E XPERIMENTAL M ETHODS

Fig. 11. (a) Heat-shrink jig setup showing Teflon base and top each contain acentering pin. The top and base are aligned with machine screws. Nuts set the
separation between the top and base. (b) Close up view of an assembled valve
with heat-shrink tube surrounding the valve prior to thermal treatment.

Fig. 12. Process steps to package assembled valve. (a) FEP heat-shrink tube isplaced around bottom centering pin, assembled valve is placed on centering pin,(b) jig top is added, valve is clamped between top and bottom centering pins,
FEP tube is lifted around valve, (c) jig and valve assembly is placed in vacuum
oven, and (d) packaged valve is removed from jig. (Modified from [4].)[Fig. 10(c)]. Finally, the bottom side of the pressure limiter, theside with the sealing rings, was identified and placed face down
on top of the spacer plate [Fig. 10(d)].
The assembled valve was then packaged in 1.3 : 1 shrink
ratio FEP heat-shrink tubing with the aid of a custom Teflon
jig containing stainless steel centering pins (813 μm diameter)[Fig. 11(a) and (b)]. A 22G (inner diameter prior to shrinkage:
914 μm, maximum wall thickness: 254 μm) heat-shrink tube
was placed around the centering pin on the jig base. The heatshrink tube must be shorter than the centering pin. Next, theassembled valve (valve seat, valve plate, spacer plate, pressure
limiter) was carefully placed on the centering pin [Fig. 12(a)].
The jig top, which has a matching and adjustable stainlesssteel centering pin, was aligned and secured to the jig baseusing four machine screws. The distance between the jig baseand top was set using nuts positioned along the screws. The
two centering pins were aligned, and the top pin was slowly

A. Valve Plate Deflection
The deflection for each valve plate design (hole, straightarm, s-shaped arm) under forward pressure was measured andcompared to theoretical values for large deflection of a uniform
plate ((1) and (2)). Each plate was clamped at the periphery ina custom-made jig which allowed pressurized air to be applied
to the backside (Fig. 13). Plate deflection was measured using
a compound microscope with 1 μm resolution. The microscope
was focused on the center of the valve plate under zero appliedpressure. Pressurized nitrogen gas (0–500 mmHg, 0–66.7 kPa)
was applied to deflect valve plate. The microscope was refocused on the center of the deflected plate, and deflection was
calculated from the change in the microscope fine focus knob
position (1 μm resolution).
B. Heat-Shrink Packaging Characterization
The heat-shrink packaging method was evaluated to determine the dimensional changes during the thermal shrinking
process, the robustness of the package, and the fluidic integrity.
Two different gauges (22G and 18G) of FEP heat shrink werecharacterized. The outer diameters of preshrink and postshrunk
tubing were compared, and the percent change in outer diameter
was calculated. The fluidic integrity of each tube was quantified
by packaging a solid 200 μm thick SU-8 disk. A 900 μm
diameter disk was used in the 22G tube; the solid disk possesses
the same diameter as the individual valve components. The 18G
tube was packaged with a 1.5 mm diameter disk for comparison. The solid disks were packaged under the same conditions
as the valve (room temperature to 215 ◦ C at ≤ 1.5◦ C/min
and cooled from 215 ◦ C to room temperature at ≤ 1.5◦ C/min)(Fig. 14).
Both pressurized water and nitrogen gas (0–2000 mmHg,
0–266.6 kPa) were applied through the heat-shrink tube against

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Fig. 14. Pre- and postheat-shrink tubing. Solid disk packaged in heat-shrink
tubing to test robustness of the adhesiveless packaging method.

Fig. 15. Test setup to determine valve operating characteristics.

one side of the solid disk. A 100 μL calibrated pipette (Clay
Adams, Parsippany, NJ, USA) was placed at the outlet tomeasure leakage of water between the disk and heat-shrink
tubing. For pressurized N2 , the tubing outlet was immersed inwater to visualize any bubbles due to leakage.
C. Packaged Valve Characterization
The bandpass flow regulation behavior of a packaged valve(hole valve plate) was determined (Fig. 15). Pressurized water(0–1000 mmHg, 0–133.3 kPa) was applied in incremental steps
to the valve inlet. The flow rate from the packaged valve wasmeasured using a 100 μL calibrated pipette connected to theoutlet. The pipette was prefilled with double distilled water anda bubble was introduced between the valve outlet and pipette
inlet to facilitate flow measurement. The system was held at
each test pressure setpoint for 3 min to allow the system toequilibrate. The cracking pressure and flow rates for differentpressures were measured. Reverse pressure (0–500 mmHg,
0–66.7 kPa) was also investigated. Several flow profiles (flowrate versus pressure) for a hydrated valve (valve that was kept
in contact with water at all times) were obtained to verify
repeatability of valve operation. The hydrated valve data wasalso compared to a dry valve (valve where water was allowed
to evaporate between experiments).
The closing time constant for the valve was determined byapplying pressurized water (250, 500, and 750 mmHg; 33.3,
66.6, and 100 kPa, respectively) to the valve, shutting off thepressure with a pneumatic solenoid valve, and measuring theaccumulated volume of water exiting the valve after pressure
shut-off. A circuit controlled the pneumatic solenoid valve, anda light-emitting diode was used to indicate the solenoid valve
state (open or closed). Closing time was defined as the duration
between the elapsed time from pressure shut-off to when 63.2%of the total accumulated volume had exited the valve.
V. R ESULTS AND D ISCUSSION
A. Valve Plate Deflection
The experimentally obtained valve plate deflection for eachvalve plate design was compared to theoretical values (Fig. 16)

Fig. 16. Comparison of measured valve plate deflection to the theoreticalvalues for a flat plate [4].

and found to be in agreement. While the theoretical model
did not completely predict the behavior, it still provided useful
insight on valve design. Deviations from the measured data
could be attributed to the geometrical differences from thetheoretical thin plate geometry. The plate thickness used in theequation was 75 μm, whereas the fabricated valve plates wereslightly thicker at 84 ± 3.7 μm, 87.8 ± 5.3 μm, and 82.2 ±5.4 μm (mean ± SE, n = 4) for the hole, straight arm, ands-shaped arm valve plates, respectively. A thicker plate generally leads to less deflection. However, changing the geometry
from a flat plate to a selectively perforated plate increased
achievable vertical deflection due to increased flexibility. Thus,
the straight-arm valve plate deflected more than the hole design even though the straight-arm plates were slightly thicker.
The clearest demonstration of the impact of tether complianceon achievable deflection was the s-shaped arm valves. The
s-shaped tethers bend allowing the plate to twist upward andaway from the valve seat as pressure is applied, providing additional deflection [20]. Therefore, the s-shaped arm valve plateachieved the greatest deflection for a given applied pressure.
B. Finite-Element Analyses
Finite-element analyses of displacement and stress wereconducted on an assembled hole valve (Table III). A linear
FEM model with small-displacement conditions was used toprovide a qualitative understanding of valve behavior. The
effect of radial strain from the heat-shrink package on theflexible valve plate and the presence of IOP were not includedin the model. The 22AWG heat-shrink tubing O.D. change
at the valve was minimal (a 4.65% decrease in size). Furthermore, the 200 μm rigid SU-8 valve seat and pressure limitersupport the valve stack and prevent buckling. However, a finite
axial strain is present on the outer circular periphery of the valve
plate and expected to contribute to the finite cracking pressure
encountered in actual valve operation along with IOP.
As the applied pressure on the valve plate increased in themodel, displacement and stress values also increased. The valve

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TABLE IIIS UMMARY OF THE FEM R ESULTS FOR D ISPLACEMENT AND S TRESS ON
AN A SSEMBLED VALVE . (M ODIFIED F ROM [4].)plate deflected until it was constrained by the pressure limiterand eventually sealed. However, the center of the valve platecontinued to deflect with increasing pressure, albeit in smaller
increments. Under reverse pressure application, the valve plate
deflected < 7 μm and maintained an effective seal against thevalve seat.
The stress analysis provided guidance on the selection ofsuitable materials on the basis of mechanical robustness. At the
maximum forward applied pressure (1000 mmHg, 133.3 kPa),
stress accumulated in the bottom of the valve seat and valve
plate. The maximum stress was experienced by the valve plate(0.99 MPa) and was concentrated along the edge in contact withthe valve seat. The observed stress was < 20% of MDX4-4210
tensile strength (5 MPa) and significantly less than the tensile
strength of SU-8 (60 MPa). Reverse pressure (500 mmHg,
66.7 kPa) analysis verified that the induced stresses (0.46 MPa)
were at least an order of magnitude less than the tensile stressesof MDX4-4210 or SU-8.
C. Heat-Shrink Package Characterization
The final postshrink diameter for the 22G FEP cannula
used to package the valve was approximately 1 mm. The
outer diameter of the heat-shrink tube decreased 19.4–25%
postshrinkage in areas adjacent to the valve stack. In the valve
region, shrinkage was reduced (4.65%) and the diameter was
slightly greater than 1 mm. This slight increase in diameter does
not compromise the incision site which must be sealed aroundthe cannula alone and not the valved portion.
A segment of heat-shrink tube was packaged with a solid
SU-8 disk to determine the robustness of the packaging method
and the quality of the seal around object. For both the 18 and22G tubes, the solid disks remained in position under appliedpressure and the entire system was leak-tight up to 2000 mmHg
(266.6 kPa) of pressurized water. This value is the pressure limitof our testing apparatus and almost two orders of magnitude
greater than normal IOP values. The packaged system was alsoable to withstand up to 2000 mmHg (266.6 kPa) of pressurized
nitrogen gas as verified by the absence of bubbles while submerged under water. The maximum pressure for microfluidic

TABLE IV
S UMMARY OF H EAT-S HRINK T UBE C HARACTERIZATION R ESULTS FOR
T WO T UBE G AUGE S IZES (22 AWG AND 18 AWG)

interconnects using heat-shrink tubing also reported leak-freeconnections up to 200 kPa [13]. These results are comparable
to devices which are secured using adhesives [21]–[23]. A
summary of the results is presented in Table IV.
In Fig. 14, the packaged solid SU-8 disk was slightly rotatedwith respect to the axis of the tube. When packaging single
disks, it was difficult to perfectly align the disk. However, despite the rotation, the seal around the disk edge was leak-tight.
When packaging the full valve, the multiple layers facilitatealignment of the stack within the tube bore. In combination withthe alignment pins in the assembly fixture shown in Fig. 11,
misalignment was avoided.
For the pressures of interest for the drug delivery application,compliance of the heat-shrink tubing was not observed in theheat-shrink packaging nor the valve characterization. The FEP
heat-shrink tube is mechanically robust (elastic modulus of
0.44–0.64 GPa and Shore D hardness of 50–65 according to
manufacturer data sheets [15]).
This characterization demonstrates that this packaging technique is extremely robust and thus suitable for a wide variety
of applications. A diverse selection of materials and gauges ofheat-shrink tube are commercially available and can be selected
to match specific packaging needs.
D. Packaged Valve Characterization
The valve operating range was determined by visually observing valve operation and measuring flow rate at specific pressure setpoints. Pressurized rhodamine dye was applied to thecannula inlet and enhanced visualization of fluid flow throughthe valve. Rhodamine was observed to exit only at the pressure
limiter through hole. No leakage at the interface between theassembled valve and heat-shrink tube was observed (Fig. 17).
This demonstrates robustness of this packaging method forstacked components fabricated from different polymers.
Consistent flow rate profiles (pressure versus flow rate) wererepeatedly obtained in sequential experiments when the valve
remained hydrated between runs. All experimental trials were
performed within a 24-h period. A representative result of
the bandpass regulation with a hole valve plate is shown in
Fig. 18. The hole valve cracking pressure was approximately 25–50 mmHg (3.3–6.7 kPa) and the maximum flowrate (3.18 ± 0.18 μL/sec, mean ± SE, n = 4) occurred near

LO AND MENG: A MODULAR HEAT-SHRINK-PACKAGED CHECK VALVE WITH HIGH PRESSURE SHUTOFF

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Fig. 17. Visualization of flow rate through valve using rhodamine dye.

Fig. 19. Accumulated volume measurements to determine closing time constant for a packaged hole valve. Closing time constants were calculated bydetermining the amount of time for 63.2% of the total accumulated volume
to exit the valve.

Fig. 18. Valve performance from four runs on the same packaged hole valve.
The valve was kept hydrated in double distilled water between runs to prevent
valve from drying out. The valve remained leak-tight up to 500 mmHg ofreverse pressure (not shown on graph). Cracking pressure between 25 and
50 mmHg (3.3 and 6.7 kPa) and closing pressures between 1750 and2000 mmHg (233.3 and 266.6 kPa) were measured.
TABLE VS UMMARY OF PACKAGED VALVE (H OLE , S TRAIGHT A RM , AND
S -S HAPED VALVES ) O PERATING C HARACTERISTICS

500 mmHg (66.7 kPa) (Fig. 18). The valve closed between 1750
and 2000 mmHg (233.3 and 266.6 kPa) and remained leak-free
up to 500 mmHg (66.7 kPa) of reverse pressure, which is at least
an order of magnitude greater than normal IOP values (reverse
pressure data not shown on graph). Stiction was not observedin hydrated valves. Also, consistent performance was observed
under continuous usage.
Table V summarizes packaged valve performance for all
three valve designs. As expected, the s-shaped arm closed at

the lowest forward pressure. This agrees with the theoreticaland FEM models in which s-shaped arm valve plates achieved
greater deflection compared to the other two valve designs
for the same applied pressure. The pressure corresponding tomaximum flow rate was identical for the hole and straightarm valves since the hole and straight-arm valves have similar
deflection responses for pressures between 0 and 500 mmHg
(as shown in Fig. 16).
The averaged flow rate profile (mean ± SE, n = 4) was
compared to that of a dried valve. For dried valves, the cracking
pressure was much higher (200–300 mmHg, 26.7–40 kPa).
The increase might be attributed to stiction between the valve
plate and seat. Additionally, the dried valve had a smaller
closing pressure (1000–1250 mmHg, 133.3–166.7 kPa). Hydration causes the silicone to swell, increasing the valve
plate thickness and altering the deflection behavior; swelling
could not be quantified due to the presence of the packaging.
An implanted valve will remain hydrated from contact withthe aqueous or vitreous humors; therefore, the hydrated flow
profile is more representative of long-term valve behavior.
However, this also suggests that valves may need to be preconditioned prior to implantation. Stiction was not observedfor hydrated valves. It is possible to coat the silicone valve
plate with Parylene C to avoid swelling altogether. This barrier
layer also prevents drug diffusion through the porous silicone
plate.
The valve has a finite response time when the applied pressure is removed. The closing time constants were determined
for three different applied pressures (250, 500, and 750 mmHg;
33.3, 66.7, and 100 kPa, respectively) (Fig. 19). These values were selected based on maximal flow rate observed near
500 mmHg (66.7 kPa). An electronically controlled pneumatic
valve, with a response time of 3 ms, was used to provide a near
instantaneous application and shut-off of pressure. Dispensed
volume data was extracted from video footage of the flow

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TABLE VI
S UMMARY OF C LOSING T IME C ONSTANTS FOR THE H OLE VALVEACKNOWLEDGMENT
The authors would like to thank Dr. D. Zhu, B. Lee, and themembers of the USC Biomedical Microsystems Laboratory for
their assistance with this work.
R EFERENCES

tracked by a bubble moving in a 100 μL pipette after pressure
removal. The bubble was observed for 5 min after the pressure
was removed from the valve to ensure that the valve wascompletely closed. Accumulated volume experiments for eachpressure were repeated four times. The data are summarized inTable VI.
Closing time constants were very consistent with only aslight increase in response time at higher pressures. As pressure
increases, the valve plate deflects further from the valve seatand has a greater distance to travel when returning to theinitial position. These results are the best case scenario of thethree designs. The hole valve design is the least compliant andtherefore would have the fastest response time. The other two
designs, with tether designs, will have less restoring force toclose the valve.
One would expect the highest accumulated volume to correspond to the highest flow rate (500 mmHg, 66.7 kPa). However, 750 mmHg (100 kPa) resulted in the largest accumulated
volume (Fig. 19) and is attributed to the difference between
transient and steady state flow rates in the closing time and flowrate profile experiments, respectively.
The total accumulated dosage after the pressure is removed
from the valve depends on the applied pressure and the amountof time the valve was pressurized (i.e., if the flow rate hasreached steady state). However, the accumulated volume canaffect the final dosage, particularly for small targeted values. Therefore, shorter closing times are desirable; designs toshorten the closing time can be investigated for high precisionapplications.
VI. C ONCLUSION
An in-plane check valve capable of bandpass fluid flow regulation and packaged within biocompatible heat-shrink tubing
without the use of adhesives was designed, packaged, andtested. The valve demonstrated consistent regulation of fluid
flow with a cracking pressure of 25–50 mmHg (3.3–6.7 kPa)
and closing pressure of 1750–2000 mmHg (233.3–266.6 kPa).
Peak flow through the valve, 3.18 ± 0.18 μL/sec, was observed at 500 mmHg (66.7 kPa). The valve was able to withstand a reverse pressure of 500 mmHg (66.7 kPa) without
leaking or valve failure. The heat-shrink package is very robust and can withstand both water and N2 gas pressures inexcess of 2000 mmHg (266.6 kPa). Packaged valves were
integrated into a drug delivery device surgical model; thisdevice will be used for ex vivo and in vivo validation of valve
operation.

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LO AND MENG: A MODULAR HEAT-SHRINK-PACKAGED CHECK VALVE WITH HIGH PRESSURE SHUTOFF

Ronalee Lo received the B.S. degree in engineering from Harvey Mudd, College Claremont, CA, in
2001, and the M.S. degree in biomedical engineering(medical devices and diagnostic engineering) andthe M.S. degree in electrical engineering from the
University of Southern California, Los Angeles, CA,
in 2005 and 2009, respectively. She received the
Ph.D. degree in 2009 from the University of Southern California Department of Biomedical Engineering, where she was a member of the Biomedical
Microsystems Laboratory.
She worked at a biotechnology consulting firm, bioSTAR Group, Inc.,
Germantown, MD, prior to starting her graduate studies. Her research interests
include bioMEMS drug delivery devices and microfluidic interconnects. She iscurrently working for Exponent, an engineering and scientific consulting firm.
Dr. Lo is a member of the Biomedical Engineering Society.

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Ellis Meng (M’02–SM’09) received the B.S. degree
in engineering and applied science and the M.S.
and Ph.D. degrees in electrical engineering from the
California Institute of Technology (Caltech),
Pasadena, in 1997, 1998, and 2003, respectively.
Since 2004, she has been with the Departmentof Biomedical Engineering, University of Southern
California (USC), Los Angeles, where she is currently an Associate Professor. She holds a joint
appointment in the Ming Hsieh Department of Electrical Engineering, USC. She was the Thrust Leader
for Interface Technology and the Associate Director of Education and Student
Diversity Researcher at the NSF’s Biomimetic Microelectronic Systems Engineering Research Center, USC. She held the Viterbi Early Career Chair in
the USC Viterbi School of Engineering. Her research interests include biological microelectromechanical systems, implantable biomedical microdevices,microfluidics, multimodality integrated microsystems, and packaging.
Dr. Meng is a member of Tau Beta Pi, the Biomedical Engineering Society,
the Society of Women Engineers, and the American Society for Engineering
Education. She was a recipient of the Intel Women in Science and Engineering
Scholarship, the Caltech Alumni Association Donald S. Clark Award, andthe Caltech Special Institute Fellowship. She has also received the NSF
CAREER and Wallace H. Coulter Foundation Early Career Translational Research Awards. Recently, she was selected as an innovative young engineering
educator by the National Academy of Engineering.