Multi-Disciplinary Engineering Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: 06215
EXPANDABLE ALVEOLAR SAC FOR PIV IMAGING
Vivan Amin (Industrial Engineering)
Jorge Rivas (Mechanical Engineering)
Nathaniel Benz (Mechanical Engineering)
Jackie Russo (Mechanical Engineering)
Avery Sonnenberg (Electrical Engineering)
Jimy Pesin (Electrical Engineering)
ABSTRACT
The purpose of this project is to create a device with which the
flow fields that exist during normal breathing in alveoli can be
studied in a future thesis. Particle Image Velocimetry, or PIV,
is the method that will be used to study these flow fields. PIV
is commonly used to study particle deposition phenomena
through the mapping of particle velocity profiles.
The
experimental apparatus consists of a control box enclosure
containing a physical model of an alveolar sac with a glycerol
and water based fluid. The model is transparent and was
created based on extensive anatomical research using novel
manufacturing techniques. The model simulates breathing as it
expands and contracts by varying the pressure outside of the
model causing fluid to flow into and out of the model. A
syringe pump is used to move fluid surrounding the model and
changing its external pressure. The fluid containing particles
will flow into the model allowing the particles to interact with
the PIV laser.
sites of gas exchange are the alveoli, which are tiny airs sacs
clustered like grapes. Distal to the terminal bronchioles, alveoli
are the smallest air conduits in the lung. Understanding the
flow patterns in these tiny gas exchangers is critical to our
understanding of and protection from airborne diseases,
allergens and biological weapons, high tech-drugs administered
by aerosol, and other human factors including industrial
environmental hazards and athletic performance [1].
PIV is an optical method used to measure velocity fields. A
fluid seeded with particles is pumped through a clear model,
where a laser causes the particles to fluoresce. While the fluid
is flowing, a high-speed digital camera is used to capture
magnified images of the fluorescent particles. The digital
images taken by the camera are then processed by VisiFlow.
VisiFlow is an analysis software package that converts the
images into a vector field. From the vector fields, velocity
profiles can be derived.
NOMENCLATURE
PIV:
CFD:
Particle Imaging Velocimetry
Computational Fluid Dynamics
Proximal: structures close to the main body
Distal: structures further away from the main body
Trachea: a conduit that allows air to move from the throat to
the lungs
Generation: descriptor that indicates the number of structural
divisions that have occurred between the trachea and
the described area of the lung
Sylgard 184: Silicone Optical Elastomer
Figure 1: Diagram of the PIV setup [2].
INTRODUCTION
The respiratory sub-system of the human body functions as
both a gas exchanger and a foreign particle filter. The main
© 2006 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Engineering Design Conference
Page 2
The way PIV method works is as follows:
Illuminate
fluorescent
Particles using PIV
Laser
Images captured
by High speed
Camera
Images transferred
digitally to
computer
Figure 3: System Diagram
Model Geometry and Materials
 The geometry of the model is a justifiable
representation of a human alveolar sac while
expanding and contracting.
 To simulate breathing the model is able to expand
30% volumetrically.
 For optical visualization and use with the PIV system,
the material of the model is completely transparent.
 After being made, the model will have no impurities
that would disperse laser light from the PIV.
Path of Particles
from Images
Analyzed by
VisiFlow
Particle Path
converted to
Vector fields
Velocity profile
derived from
Vector fields
Figure 2: Generalized PIV Flow Chart Process
DESIGN OBJECTIVES
The apparatus consists of an enclosure containing a physical
transparent model of an alveolar sac. The model was created
based on extensive anatomical research using novel
manufacturing techniques. The device simulates breathing as it
expands and contracts by varying the pressure outside of the
model causing fluid to flow into and out of the model. A
syringe pump is used to move a glycerol and water based fluid
surrounding the model, changing its external pressure.
Identical fluid now containing particles would flow into the
model allowing the particles to interact with the PIV laser.
Fluid and Particles
 The fluid used has an index of refraction that matches
that of the model material (±0.02).
 Fluid transparency must be achieved in order to allow
maximum transmission of the PIV laser.
 Particles have negligible deposition such as
sedimentation, impaction, and diffusion.
 The particles are capable of being illuminated by the
PIV laser.
Enclosure
 The enclosure is built using sheets of acrylic with the
sides oriented in a way that index of refraction will not
distort the PIV laser.
 The box is leak free to ensure proper expansion when
used with the pump.
 The box is transparent for use with the PIV laser.
 Inside of the enclosure is accessible for cleaning.
 The attachment for the model is interchangeable to
allow for future models to use the same enclosure.
 The enclosure can accommodate the model’s size
through expansion and contraction while allowing
fluid to enter the model from the reservoir.
Paper Number 06215
Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference
Flow Control
 The pump used to control flow is bi-directional
allowing fluid to be withdrawn and infused from and
into the model.
 The flow rate is configurable to arbitrary waveforms
for accurate simulation breathing patterns.
 The pump can control/change flow on-the-fly or while
stopped via RS-232 port.
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After acquiring this information a model based on the
geometries from the two articles was created. The final model
is based on a 23rd generation alveolar sac with the use of the
following dimension:
Table 1: Alveolar Sac Dimensions.
Alveolar Radius
0.14 mm
MODEL GEOMETRY
Number of Alveoli
17
From research, it was found that most models of alveolus,
alveolar ducts, or alveolar sacs that were created were based on
Weibel geometry. In “Morphometrics of the lung,” Weibel
defines a radius of an alveoli, diameter and length of each
generation airway, and number of alveoli at each generation.
The geometry of the alveoli is described as truncated spheres,
with the truncation length of 5/3 of the length of the radius.
Entrance Diameter
0.25 mm
Entrance Length
0.75 mm
The model was increased in size by a factor of 75 in order to
increase the flow rate as much as possible while remaining in
the limits of the PIV system. The geometry of the alveolar sac
is shown in Fig. 5 and 6.
Figure 4: Truncated Sphere length for alveoli.
Weibel also gave dimensions for the 23rd generation. The
publication gave the alveolar radius to be 0.14mm; the airway
diameter and length to be 0.41mm and 0.5mm respectively, and
states that there are 17 alveoli on an airway at this generation
[3].
Figure 5: Cross section of model. All units in mm.
The article “Morphometry of the Human Pulmonary Acinus”
by Haefeli-Bleuer and Weibel defines updated dimensions for
the length and diameter of an alveolar sac at the 23 rd generation,
but does not give any further information about the alveoli
geometry. The new dimensions of the airway diameter and
length are given as 0.25mm and 0.75mm respectively. The
length of this airway at the 23rd generation includes the terminal
cluster of alveoli, meaning the length of the airway is measured
to the inner radius of the outermost alveoli [4].
Figure 6: Model based on Haefeli-Bleuer and Weibel
geometry. All units are in mm.
Copyright © 2006 by Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Engineering Design Conference
SELECTION OF MATERIAL, FLUID AND SEEDING
PARTICLES
Taking into account factors such as transparency, index of
refraction, elasticity, workability, and price the material chosen
to build the model was Sylgard 184 and is made by Dow
Corning. It was chosen because it is able to be molded in the
selected geometry and is able to expand and contract without
plastically deforming as flow moves in and out of it. Secondly
it is transparent so it is compatible with the PIV system.
The fluid that will carry the PIV particles and surround the
alveolar model must overcome two major limitations. Firstly,
the index of refraction needs to match that of the materials
previously discussed. In order to ensure that in fact this
property was an important constraint, Snell’s law, Eq. (1) was
used to create a projection of the effects of the index of
refraction, taking into consideration different materials,
thicknesses, and angles.
n1 sin 1  n2 sin  2
(1)
Figure 7: Snell's Law.
After studying this projection, it was determined that the
thickness of the alveolar model combined with its intricate
geometry makes it essential to match the index of refraction. If
it is not matched, large distortions will be present between what
the cameras are detecting and what is actually taking place in
the model. Therefore, a mixture of 59% glycerin and 41%
water was chosen in order to match the index of refraction of
Sylgard 184.
According to Pruyne [5] factors such as sedimentation, impact
deposition, and luminosity should be considered in selecting the
seeding particles. Due to the fluid being used and flow rate of
the fluid (discussed below) the particles selected were
Sphericel® 110P8 Hollow Glass Spheres from Potters
Industries. These particles have a diameter of 10μm and a
density of 1.1 g/cm3.
SCALING THE FLOW FIELD
In order to accurately model the flow field inside the alveolar
sac, the model needs to be geometrically similar and the
Reynolds number, Eq. (2), and Womersley number, Eq. (3)
need to be the same in order to guarantee kinematic similarity.
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Making a model that is geometrically similar was discussed
earlier in the report. The Reynolds number (Re) is a nondimensional parameter to describe fluid flows that compares
the inertial to viscous forces. The Womersley number (Wo) is
an indicator of the unsteady nature of the flow.
Re 
Wo 
LV

L
2
f 2

(2)
(3)
Where L is the characteristic length (for this model it is the
entry diameter of alveolar sac), ρ is the density of the fluid, μ is
the dynamic viscosity, and f is the frequency of the flow in
cycles/s.
Based on publications, the Re number in the 23 rd generation of
the lungs is 0.0139 and Wo number is 0.11. To match these
values using the glycerin-water mixture that was chosen, the
flow rate in the model would be 1.3 micro liters per second. A
very low flow rate would require an expensive pump,
approximately $1000, which is a third of our total budget. Also
a low flow rate would be very sensitive to vibration. Lastly,
sedimentation of the seeding particles means the particles will
have the tendency to fall or rise in the medium [5].
Sedimentation of seeding particles is a function of the particle
velocity, and thus at slower flows sedimentation adversely
affects the validity of the results.
Due to these factors it would be desirable to increase the flow
rate and without affecting to flow field. Since the Reynolds
number is much less than one, the flow is called “creeping
flow”. In 1851 G.G. Stokes showed that, if the Reynolds
number is very small the inertial forces are negligible [6].
Since the inertial forces can be ignored the flow will depend
solely on viscous forces. This means the forces acting on the
fluid are independent of the velocity of the fluid. As a result
the flow field will be similar for a Reynolds number less than
one.
When the Womersley number is less than one the flow is said
to be “quasi-steady”. Quasi-steady flow means at any time, the
instantaneous flow rate is determined by the instantaneous
pressure gradient [7]. Therefore, as long as the Womersley
number is less than one, the period of the flow will not affect
the flow field.
Using the phenomena of creeping flow and quasi-steady flow,
the flow rate can be increased and the period shortened and the
flow field will still accurately represent that of an alveolar sac.
However the percent of expansion must remain the same in
order to keep the model geometrically similar. For this
particular experiment, the flow rate of the water-glycerin
mixture is increased from 1.3 μL/s to 0.07 mL/s.
Paper Number 06215
Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference
Page 5
MANUFACTURING PROCESS
Rapid Prototyping Technique
Figure 9: Step 3 - The wax male mold is created using the
smaller of the two sets of female molds.
4) The resulting wax male mold is placed inside the original,
larger female mold. The molds are designed to create a space
of 1.5mm between the outer female mold and the inner wax
male mold. This space between the molds defines the thickness
of the model.
Figure 8: Rapid prototyping machine at RIT.
In order to create a part using a rapid prototyping machine,
shown in Fig. 8, it is first necessary to create a three
dimensional drawing of the part using CAD software. For our
parts we used SolidWorks. This drawing is then fed into the
machine using proprietary software that “cuts” the drawing into
hundreds of layers.
To ensure proper alignment of the molds, pegs were created on
both the outer female and wax male molds as shown in Fig. 10.
5) The female and wax male molds are aligned together and the
Sylgard 184 silicone is poured into the gap between them and
allowed to cure for 24 hours. 6) After the Sylgard has
completely cured, the molds, Sylgard and wax are placed into a
pot of boiling water. 7) The pot is taken off the heat when the
wax has melted out of the model and rises to the top of the
boiling water. The model stays in the water until the wax has
re-solidified on the top of the water. 8) The wax is then
removed, followed by the mold containing the model. 9) The
molds are then removed leaving behind only the silicone
model.
The machine utilizes ABS plastic, which comes in cartridges. A
nozzle deposits particles of the ABS material in 0.010 inch
layers onto a platform. The platform moves down once the
layer is completed and the nozzle proceeds to create the next
layer on top of the last. This process is repeated until the three
dimensional part is completed.
Modeling Process
The manufacturing process that was chosen for the model was
lost wax casting. The manufacturing process is made up of 9
discrete steps. This technique involves the use both an inner
piece (male) and outer 2 piece (female) mold to achieve the
desired model geometry. 1) The inner male mold is made out
of wax using two separate 4 piece female molds, totaling an 8
piece mold. These 8 pieces enclose a geometry that is slightly
smaller than the first female mold. After the two sets of 4 molds
were assembles they look like the two female molds in Fig. 9.
The reason for the 8 piece female mold was for the ease of
removing the model due to the undercut in the geometry. 2)
The 8 piece female mold was aligned with 12 alligator clamps.
3) Once aligned, wax was poured into the 8 piece female mold.
After the waxed solidified it was removed from the 8 piece
female mold and is shown by the male mold in Fig. 9.
Pegs
Figure 10: Step 4 - The outer female molds are placed
around inner wax male mold. The thin space between will
create the final model.
ENCLOSURE DESIGN
It is necessary to control the model in a way that results in
uniform expansion and contraction, while accurately
representing the system it is intended to mimic. It was decided
that the best way to accomplish this was to place the model
inside a fluid-filled container and force it to expand by
removing some of the fluid inside of the box at a controlled
rate. To return the model to its resting state, the fluid is
replaced.
The fluid is withdrawn and infused with a
bidirectional syringe pump. The set up of the enclosure is
shown in Fig. 11.
Copyright © 2006 by Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Engineering Design Conference
Page 6
Model attaches Here
Reservoir
Pump Attaches Here
Container
Therefo
re, a
mixture
of 59%
glycerin
and
41%
Figurewater
11: Fluid is withdrawn and infused from the
indicated
was pump connection, forcing expansion and
contraction
chosenof the model. Fluid is fed to the model from the
reservoir
to accommodate volumetric changes.
in order
to match
The model
the is attached to a standard 3” PVC cap, which is
screwed
intoofthe threading in the enclosure, as shown in Fig.
index
12. This
allows the model to be removed when the inside of
refractio
the enclosure
requires cleaning and allows for multiple models
n of
to be used
with
Sylgard the same system.
184.
Figure 13: New Era Pump Systems NE-500 Syringe Pump
To achieve the original flow rates and match the Reynolds
number and the Womersley number, the pump must be able to
pump accurately at a rate as low as 1.3µl/sec. It was shown
above that this rate could be increased to 70 µl/sec while
maintaining the accuracy of the system. The NE-500 can
control fluid flow at rates from 8 nl/sec to 580 µl/sec with a 60
ml syringe. This functional range fully encompasses the
desired range of operation. Flow rates can also be lowered
below 8 nl/sec if a smaller syringe size is used.
PVC Cap
Figure 14: NE-500
Communication Port
Model
Attaches Here
Figure 12: Attaching the model to a standard 3" PVC cap
allows easy removal and adaptability to multiple models.
FLOW CONTROL
Side
view
showing
RS-232
The NE-500 communicates with a PC through a RS-232 port
on the pump, shown in Fig. 14. Through a DOS-based
application, the pump can be programmed to repeat a 41-phase
cycle or changes to the pump rate can be made on-the-fly. The
standard RS-232 interface will also facilitate any desired
modifications or advancements to pump control and allows the
pump to be adapted for other uses.
NEXT GENERATION
The pump that was determined to best fit the design criteria was
the New Era Pump Systems NE-500, shown in Fig. 13. It can
accommodate syringes up to 60 ml in volume. This ensures
that the pump will be able to infuse and withdraw a large
enough volume of fluid to produce the desired amount of
expansion from rest. A withdrawal volume of at least 20 ml
would be sufficient to provide the desired expansion of at least
30% of the resting volume.
Due to time and budget constraints there are several design
concepts that were not able to be implemented. These concepts
could potentially lead to more accurate results of the fluid flows
inside the alveolar sac.
Model Geometry
While the geometry of the model is based on the currently
accepted dimension of an alveolar sac it is still an idealized
model. The dimensions laid out by Weibel and Haefeli-Bleuer
are based on averages several actual human lungs, however the
geometry of the lungs is as unique as a fingerprint.
Paper Number 06215
Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference
A technique known as Volume Imaging can provide a more
realistic model geometry. In this process a CT scan of an actual
alveolar sac is taken. The CT scan produces a series of twodimensional (2D) slices of the alveoli. A software program is
then used to “stack” these 2D images together so as to create a
three-dimensional (3D) image. This 3D image can then be
converted into a CAD file.
If this method is used one can create a model of a person’s
actual alveolar sac. However the model created would still be
representation of one single alveolar sac from one person. The
geometry of every other alveolar sac would be different.
Page 7
Since RIT does not have SLA machines the mold drawings
would need to be sent to companies who have the SLA
machines to make the molds. A price quote from American
Precision Prototyping indicated that the cost of each set of
molds would be $550. The molds made at RIT cost less than
$200 but it required some extra work in smoothing the insides
using epoxy resin and sanding for a smoother finish. It was also
realized that applying the epoxy resin and sanding is a very
time consuming process due to maintaining uniform
distribution of epoxy in the gaps of the molds, otherwise there
would be different thicknesses in each alveolus. This nonuniform distribution of thickness would result in non-uniform
expansion which would give an error in the measurements
acquired through PIV.
Another possible alternative would be to chrome plate the
molds which would result in a smooth inside finish. This price
estimate for this process would add $150 per mold to the given
cost.
Flow Control
LabVIEW implementation would be used to expand the
controllability of the pump. A more visually appealing
graphical user interface would be created for ease of use. This
would eliminate the learning curve for anybody that wanted to
operate the pump. Changing waveforms, flow rates, and
volume displacement could all be done quickly and with only
one interface.
ACKNOWLEDGMENTS
Figure 15: Top view of 3D image of a lung produced by
Volume Imaging. [8]
In order to use this process to produce model a CT machine
with a resolution of 5 microns or less should be used.
Furthermore, a software program needs to be developed that
can assemble the 2D images of an alveolar sac captured by the
CT machine. Currently there are no known software packages
that are capable of assembling images specifically for an
alveolar sac.
Improved Rapid Prototyping Models
The process we used for our project was fused deposition
modeling due to its rapid availability of the parts and cost
benefits by getting them made at RIT. Just like every other
process with decreasing cost you loose quality.
There are better rapid prototyping techniques available in the
market today. Stereo lithography (SLA) is one of the best
techniques to be used for our applications in the future. The
advantages of this RP process over Fused Deposition Modeling
are:
1.
2.
3.
Smoother finish.
Higher tolerances.
More Complex geometries.
The Expandable Alveolar Sac for PIV Measurements senior
design team would like to extend its gratitude to the numerous
individuals who offered assistance and guidance. We would
like to especially thank Dr. Debartolo, Dr. Robinson and Dr.
Day for their continued guidance and serving us as our mentors.
Dr. Carrano and Dr. Taylor in the ISE department were most
helpful with regards to model fabrication techniques. Professor
Leonard and Kevin Egan in CAST assisted us in mold
fabrication for cheaper cost at RIT. Further more, we would
like to thank Dave Hathaway and Rob Kraynik in the machine
shop for helping us setup the machines. Dr. Doolittle in
Medical Sciences was helpful in answering questions and
guiding us towards a more anatomically correct design. Also,
Dr. Borkholder in the EE department assisted us in assessing
our design objectives and provided technical guidance. All the
funding provided to us came from the Provost Learning
Initiative Grant from RIT.
REFERENCES
[1] Robinson. “Expandable Alveolar Sac for PIV.” Project
Homepage. 2 Feb. 2006.
www.rit.edu/~rjreme/senior%20design_List%20of%20Projects
_expandable%20alveoli.doc
[2] Particle Imaging Velocimetry. Optical Engineering
Laboratory Homepage. 10 May 2006.
http://www.eng.warwick.ac.uk/oel/courses/engine/piv_basics
Copyright © 2006 by Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Engineering Design Conference
[3] Weibel. Morphometrics of the Lung. Berlin: Springer, 1963.
[4] Haefeli-Bleuer, and Weibel. “Morphometry of the human
pulmonary acinus.” Anatomical Record. 220 (1998) 401-414.
[5] Pruyne. The Mapping of Velocity Profiles in a Three
Generation Lung Model Using Particle Image Velocimetry
Flow Analysis Techniques. (2004) 26-72
[6] White, F., Fluid Mechanics, 5th edition. McGraw-Hill. 1998.
[7] Loudon and Tordesillas. “The Use of the Dimensionless
Womersley Number to Characterize the Unsteady Nature of
Internal Flow.” J. Theor. Biol. 191 (1998) 63-78.
[8] The Cancer Institute Hospital of JFCR. Cancer Screening
Center. http://www.jfcr.or.jp/hospital/english/medical/
Paper Number 06215
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