A Model of Cerebrospinal Fluid Flow
BME 201
University of Wisconsin - Madison
May 5, 2004
Team:
Timothy Eng, Team Leader
Lauren Hensley, BSAC Representative
Mary Lim, BWIG Representative
April Zehm, Communications
Client:
Victor Haughton, M.D.
UW Department of Radiology
UW Hospital
Advisor:
Professor Justin Williams
Department of Biomedical Engineering
University of Wisconsin
Abstract
A physical model of the human hindbrain and upper cervical spinal canal was desired to
study the effects of varying dimensions and obstructions on pressure changes within this part of
the brain. The model will reflect the Chiari I malformation, a potentially life-threatening
neuromuscular brain deformation. The model needed to maintain oscillatory fluid flow in the
mock spinal canal, and pressure had to be quantifiable. Over the past semester, a prototype was
assembled to roughly mimic this disorder. The final design is a multi-piece module, which
houses a funnel-like cavity. The module will be used in conjunction with an electronically
controlled piston pump and pressure will be quantified using a multi-channel transducer. Future
work includes increasing the complexity of the cavity within the module by replicating CT scans
of this part of the spinal canal.
Problem Statement
The goal of this project was to create a life-size physical model of the human hindbrain
and upper cervical spinal canal, through which a substitute for cerebrospinal fluid (CSF) would
flow. This system would ultimately be used by doctors and/or researchers to study how varying
dimensions and obstructions in this part of the brain affect CSF flow in terms of pressure, and
possibly velocity. Oscillatory flow was required in the model, and pressure had to be
quantifiable.
Background Information
CSF is a clear fluid that surrounds the entire central nervous system of the body,
including the brain. It has various functions, including protecting the brain from damage,
reducing pressure at the base of the brain, excreting waste products and delivering hormones to
the brain. CSF is produced in several ventricles within the brain at a rate of 400-500 ml per day;
it pulsates throughout the brain and spinal canal, and is eventually absorbed by one’s
bloodstream (http://faculty.washington.edu/chudler/vent.html).
It is thought that circulatory and respiratory changes affect the pressure within this
system of CSF flow (http://faculty.washington.edu/chudler/vent.html). These fluctuations in
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pressure are carefully regulated; any dramatic changes can have serious consequences in terms of
a person's health.
The Chiari I malformation is one such condition; it is essentially a neuromuscular
deformity that is present at birth but does not cause noticeable symptoms until one reaches early
adulthood. Chiari I involves the deformation of the cranial vault, a funnel-like region of the
hindbrain, in which the brain rests (Mueller, 2004). As a direct result of this defect, the
brainstem and cerebellar tonsils (brain tissue) are displaced abnormally low (Figure 1), a
condition that can have two consequences. First, it obstructs normal CSF flow, and second, it
causes an increased pressure on this part of the brain.
Figure 1: A comparison of normal hindbrain structure and the Chiari I malformation. In Chiari
I, the brainstem is clearly displaced downward and can result in cysts within the spinal canal.
Image from Mueller, 2004.
Symptoms of the Chiari I malformation include headaches, pain, nausea, dysphagia
(difficulty in swallowing), ataxia (loss of balance), loss of consciousness and even paralysis in
extreme cases. Chiari I is almost always diagnosed via magnetic resonance imaging (MRI) and
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is treated surgically. During surgery, the cranial vault is physically enlarged and sometimes the
cerebellar tonsils are minimized (Mueller, 2004).
Our client works with adolescent patients who exhibit the Chiari I malformation at the
UW-Hospital’s Radiology department. He was interested in designing an anatomically correct
model of the hindbrain and upper cervical spinal canal to study the effect of the aforementioned
obstructions and malformations on pressure. The model created was intended to demonstrate
that as the inner diameter of the closed area increases, so does pressure.
Primary literature and patent searches revealed that no physical models that mimic
obstructions and measure pressure within this area of the brain exist. An electrical model has
been simulated to address this issue, but the conversion of units to represent a physical scenario
has raised some problems (Chang and Nakagawa, 2003). Other methods include mathematical
or hydrodynamic modeling, to analyze the fluid dynamics of CSF flow (Loth et al., 2001).
By designing a device that will physically mimic the Chiari I malformation, doctors and
researchers can hopefully gain a greater understanding of the physiology of such disorders. This
type of quantitative measuring model, in conjunction with MRI, may also become a useful
noninvasive diagnostic tool (Loth et al., 2001).
Design Constraints
The client is interested in a modular system, in which fluid will flow in an oscillatory
(back and forth) manner. It has to mimic the upper cervical spinal canal and be capable of
variance in dimensions and mock obstructions, to replicate real-life situations. The model also
has to be anatomically correct: inner dimensions must be around 15 mm in diameter.
Of primary concern is the model’s ability to accurately measure pressure. The method of
doing so has to be accurate and exemplify a precision of within 20%, as requested by the client.
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Since the model will be used at least once daily, it has to be able to withstand repeated
use. The durability of materials used was taken into consideration by the design team. Materials
have to also face the possibility of corrosion by the fluid used in the model, as well as possible
interaction with an MRI machine. Therefore, nonmagnetic materials have to be employed. The
complete system has to remain under the overhead cost of several hundred dollars.
While this will not be a patient-based system, the safety of the user (doctors or
researchers) is, as always, a concern. The device cannot pose a hazard to the user; any
potentially dangerous parts or system interactions, such as an electrically powered pump and
fluid, have to be clearly labeled. Neither sterility nor FDA approval is required of the model.
Piston Pump
It is important to note that the model has to be compatible with the pump currently used
by the client. The client intends to utilize a large, tabletop piston pump supplied by Quest
Image®, which will generate the desired oscillatory flow within the system (Figure 2).
Figure 2: The Quest Image® piston pump to be used by the client. An electronically controlled
piston moves back and forth, generating a pulsating fluid flow in and out of the chamber of the
module being used.
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The pump is electronically controlled and can generate various oscillatory flow patterns,
including those that follow a standard sine or cosine function. A small piston within the pump
moves back and forth, forcing fluid to flow correspondingly. The team has viewed the pump,
and our model is compatible with it. Additionally, water is a clear, non-viscous fluid comparable
to CSF and will be used as the fluid substitute within the model.
See Appendix A for a complete and detailed Product Design Specification outline.
Design Alternatives
After extensive brainstorming, the design team generated three distinct alternative
solutions to the problem. We compared the advantages and disadvantages of each, and chose
one to pursue for the remainder of the semester.
Alternative Solution 1: Exterior Pin Design
Given the design specifications, a solution that made use of a continuous, flexible plastic
tube to mimic the spinal canal was proposed (Figure 3). This model would be positioned
upright, supported by a rigid structure enclosing the tube. Holes drilled into the supportive
structure 1 cm apart along the length of the tube would allow small blunt pins to depress the
inner channel, altering its dimensions. At the top of the continuous tube would be a funnelshaped piece representing the cranial vault (Figure 3). The pump provided by the client would
induce fluid flow at the other end of the tube. Oscillatory flow will be achieved as the pump
forces the fluid up to the “cranial vault” region; reverse flow will be caused by gravity.
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Figure 3: Exterior pin design. This design consists of a vertical continuous tube whose
dimensions may be altered by applying force to the pins.
Measuring pressure caused by flowing liquid inside the mock canal is a primary concern
of this project. The most straightforward method to do this would involve inserting a number of
thin tubes at various locations along the main canal (Figure 3). Fluid flowing past one of these
points would be forced into the thin tube, rising to a certain level. If pre-calibrated, these thin
tubes would allow the pressure to be quantified. One stipulation with these tubes is that their
diameter must be large enough so as to eliminate capillary action.
The main advantage of this design is its minimization of the potential for fluid leaks
through the use of a continuous tube. On the other hand, by employing the continuous tube
along with the exterior pins, limitations arise in terms of varying the inner dimensions. For
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example, since only two pins would be found at a given location along the tube, applying force
to these pins pinches the tube in the middle, and it would lose its circular cross-sectional shape.
Alternative Solution 2: Exterior Blocks Design
Similar to Alternative Solution 1, the second design utilizes a continuous flexible tube;
however, it strays from upright positioning (Figure 4). By adopting horizontal orientation, this
model would depend less on gravity and more on suction for reverse flow. As the pump’s piston
is retracted, a vacuum would be created, pulling the fluid back towards the end representing the
cranial vault. At the other end of the model, an elbow joint would connect the main tube to a
broad funnel, which would fill and empty with each revolution of the pump (Figure 4). This
funnel must be wide enough to avoid increased fluid velocities during reverse flow that would be
observed with greater slopes.
In contrast to employing pins to alter inner tube dimensions, this design would make use
of individual blocks that snap over the tube. A number of blocks would be constructed, each one
differing in conformation and diameter, which would more thoroughly represent real-life
malformations. Because the blocks are removable, the design allows specific alterations in
dimensions to be achieved at any point along the tube. Pressure inside the mock spinal canal
would be measured with thin capillary-like tubes, in the same fashion as Solution 1.
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Figure 4: Exterior Blocks Design. Fluid flows horizontally, until reaching an elbow joint and
reservoir, which is continuously filling and emptying with each pump cycle. Blocks of differing
conformations alter the tubing externally.
This design boasts the same advantage of a continuous tube as the first solution; however,
a second meeting with the client prompted the team to reconsider this continuous feature, as he
requested a strictly piecewise design. An additional drawback to this solution occurs in the case
that a greater main tubing diameter is desired for testing. A different size would be required for
both the elbow joint and the connector to the pump. Furthermore, the new tube would need to be
punctured to accommodate the pressure-measuring tubes.
Proposed Solution: Modular Design
The proposed solution would utilize multiple pieces, or blocks to construct the mock
spinal canal. Each block would be 58 x 58 x 9.5 mm. A total of ten blocks would be connected,
for a total length of approximately 133 mm. Openings of varying inner shapes as shown in a
sequence of CT scans provided by the client would be cut into the center of each block (Figure
5).
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Figure 5: CT scan of cerebrospinal fluid flow (white area in center). A series of these images
were to be imposed on ten different blocks in two different modules, one mimicking the Chiari I
malformation and one mimicking normal patients.
The team proposed to have two working modules, one that mocks the Chiari I malformation and
one to represent a normal patient. The blocks would be made of transparent polycarbonate, a
rigid, non-corrosive thermoplastic. These blocks would simply be aligned and held together via
two nonmagnetic stainless steel screws that would penetrate through the opposite corners of each
block. Unscrewing the two screws would release the blocks from their fixed connection,
allowing them to be interchanged or altered to mimic real-life situations and obstructions. On
each end block, a small plastic adapter would be anchored in place. These adapters are
compatible with the piston pump to be used by the client; once connected to the pump's existing
tubing, the pump will push fluid through the inner shapes of the module in a manner that
replicates actual cerebrospinal fluid flow.
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Through the top of each block, a small hole will be drilled down, eventually reaching the
inner cavity. Through this hole, pressure will be measured with a probe as part of a transducer,
which will convert the energy generated by the fluid within the cavity into an electrical output.
This was a decidedly more sophisticated and accurate way of measuring pressure within the
canal than the thin tubes. Having a multiple-channel transducer would allow simultaneous
measurements to be taken, as well. When not in use, the holes could be plugged with any kind of
sealant.
This design was approved by the client and was ready to be pursued by the team after the
mid-semester proposal. However, some unexpected setbacks arose late in the semester and the
design had to be altered slightly.
Problems Encountered
A problem arose while tracing the CT images onto our polycarbonate blocks. When
aligned properly, parts of some images fell outside the blocks. We would have needed larger
blocks to correctly model the CT images.
In addition, since the CT image tracings were so intricate, it was impossible to cut these
shapes using the machines available at the shop. We had to get these blocks cut professionally to
achieve the complexity we desired. We obtained one estimate of 3-4 hours to cut the first block
at $55.00/hour. This would greatly exceed our budget since we had 20 blocks to cut.
Final Prototype
To address these problems, our proposed design was slightly modified. Rather than using
the intricate CT scans as the basis for the carved shapes on the blocks, the shapes were simplified
to circles of varying diameters, to form a funnel-like cavity within the ten blocks (Figure 6).
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Figure 6: Side view schematic of final design. Ten blocks containing holes of varying
diameters will be held together with two nonmagnetic stainless steel screws positioned in
opposite corners of the module. An inner funnel-like cavity mimics the upper cervical spinal
canal.
This was the only major change that was made in the design. Pressure will still be measured in
the same way: a probe will be inserted into the inner cavity through small holes drilled through
the top of each block. The model is still compatible with the piston pump to be used. A steel
screw will still hold the blocks together. The client approved these modifications, and prototype
construction began.
Construction
The first step of the construction process was to order materials. A 30.5 x 61.0 x 0.95 cm
polycarbonate sheet was ordered from McMaster Carr. Two stainless steel nonmagnetic screws
were purchased. The client provided two plastic adapters, compatible with his pump.
The polycarbonate sheet was cut using a band saw and milled to eliminate the rough
edges generated by the saw. Edges and corners were filed as a finishing process. We then used
a drill press with varying drill bit diameters, ranging from 1.0 cm to 2.4 cm to drill holes through
the ten blocks (Figure 7). These, too, were then filed. The milling machine was then used again
to carve two small half-circle cavities, positioned in opposite corners of all of the blocks. These
would be the “slots” that would house the two tightening screws. The aforementioned plastic
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adapters were fixed into place on each end block using super glue. The blocks were then aligned
and the screws inserted into place to hold them together.
Figure 7: Starting materials of prototype construction. Polycarbonate blocks were cut from a
large sheet, and holes of varying diameters were drilled into each block. Nonmagnetic stainless
steel screws are used to hold ten blocks together.
Prototype Testing
A peristaltic pump was loaned from the BME department to conduct initial testing of the
prototype for functionality. Flexible, thin tubing was obtained to join the module to the pump.
Thin capillary-like tubes were inserted into the small, drilled holes in the top of several blocks
and fixed into place using removable rubber cement (Figure 8). This provided a temporary way
to observe pressure changes within the inner canal, as the client would actually be using a
transducer. The change in height of the fluid in these tubes is a direct reflection of pressure
changes inside of the model.
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Figure 8: Functional prototype, which is constructed of polycarbonate plastic pieces, held into
place with screws. Plastic adapters on either end are compatible with the pump to be used by the
client. Small tubes seen inserted into various blocks are temporarily held into place with
removable rubber cement. These can be removed and replaced with an electronic probe to
measure pressure more accurately.
One major problem that the team encountered during prototype testing was that of fluid
leakage in between each block. We observed both water loss as well as unwanted air
accumulation within the supposedly airtight cavity. We learned that petroleum jelly was used in
many biomedical engineering applications, and decided to coat each block in a thin layer prior to
tightening them together with the screws. This solved the problem completely; we no longer
experienced any water loss and air bubbles were greatly minimized.
More importantly, the design appeared to work, in terms of generating pressure changes
within the model. Though the pulsating force of the loaned peristaltic pump made fluid height
changes in the thin tubes difficult to observe, the model appeared to exemplify Bernoulli's law:
as the inner diameter of the channel increased, so did pressure, as seen by an increase in fluid
height within the small, pressure-monitoring tubes.
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Advantages
This design was chosen because the client expressed a preference for a piecewise model
consisting of interchangeable parts. This model allows for a great degree of versatility in the
manipulations the user can impose to replicate obstructions. A piecewise setup also simplifies
cleaning and storage. In addition, the exact dimensions of the inner canal are known, and the
effect of these manipulations can be quantified precisely.
Since the design is comprised of only plastic and nonmagnetic steel, this design is MR
compatible, allowing for possible testing to be conducted in such a setting.
In addition, the cost of manufacture of such a prototype is extremely low. The entire
functional prototype cost the team less than fifty dollars. Exact prices of materials are listed
below.
Item
30.5 x 61.0 x 0.95 cm polycarbonate sheet
(2) 15.24 cm stainless steel screws and wingnuts
Plastic adaptors
0.64 cm diameter plastic tubing
Rubber sealant
Total
Cost
$29.76
$5.06
$0.00
$0.89
$4.39
$40.08
Figure 9: Cost analysis of final design. Total cost of functional prototype is around $40.00.
Note that the last two items will most likely not be used by the client and can therefore be
eliminated from final cost of model.
Disadvantages
It is important to note that while approved by the client, the simplification of the inner
shapes of the cavity is only a temporary solution to the problem. How accurately our funnel
shape reflects the human hindbrain and upper cervical spinal canal in either normal or Chiari I
patients is unknown. We feel that it would be more appropriate to actually replicate the CT
scans provided by the client, and include this in our future work discussion.
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Another disadvantage may be found in the model’s ease of use and assembly. Though
this module is similar to others already being used at the hospital with the piston pump, we
acknowledge the awkwardness of interchanging the blocks, a major requirement of the model.
Because fluid is encompassed by the module and the rest of the tubing of the pump with no
“shut-off” valve, un-tightening the screws will cause water to rush out, uncontrolled. A separate
holding container for this fluid will be necessary to avoid a large mess. In addition, petroleum
jelly needs to be reapplied to prevent leaking of the model. The small holes created for insertion
of the transducer probe also need to be plugged and unplugged, depending on its use.
Ethical Concerns
While there are no obvious ethical issues to discuss regarding this design project, there
are some underlying matters that should be considered. While our design isn’t primarily
intended for patient use, it could progress into a more invasive diagnostic tool using human
subjects. If this were the case, the ethics regarding such a procedure would have to be carefully
considered.
Perhaps more eminent is the current issue of intellectual property within this design
proposal. As a first generation design of its type, the design of this model must be guarded and
claimed as property of the designers for future modifications to be made. Plagiarism of ideas is
always a concern with groundbreaking design ideas such as this one.
Future Work
Future work on this design is very conceivable and the design team has discussed several
modifications and improvements that can made to the prototype. A future goal of the team as
well as the client is to implement the use of the CT scanned images in the prototype. Currently
the prototype displays a cylindrical inner-shape, but if these CT scans were used, the prototype
would more accurately resemble the flow of CSF through the spinal canal. Altering the inner
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cavity in this way would ultimately produce two prototypes: one modeling normal CT scans and
the other modeling the CT scans of patients with the Chiari I malformation. Due to the
inefficient resources here at the university, the team will present the client with alternative
locations where this small-scale task can be done for a reasonable price. Some preliminary
research indicates that the material of the blocks may need to be changed. Acrylic could be used
a polycarbonate substitute. In addition, the outer dimensions of the blocks may have to be
increased to accommodate the actual size of the CT images.
How many simultaneous pressure measurements will be taken in the module is also in
question. The team exceeded the minimum of one pressure point in the current prototype and
created points in three separate blocks to measure pressure. Measuring pressure in the remaining
blocks is simply a matter of drilling more holes into the canal, a modification that can clearly
yield more comprehensive data.
Lastly, the client has generously offered the team a chance to continue involvement in
this project by assisting with data collection and monitoring pressure changes.
Bibliography
Arnett, B. 2002. Arnold-Chiari malformation. History of Neurology: reprinted 2003.
Automation Creations, Inc. 2004. “Material Property Data.” Accessed 12 March 2004.
URL: www.matweb.com
Chang, H.S. and Nakagawa, H. 2003. Hypothesis on the pathophysiology of syringomyelia
based on simulation of cerebrospinal fluid dynamics. Journal of Neurology,
Neurosurgery and Psychiatry 74: 344.
Haughton, V. Personal Interview. January 6, 2004.
Haughton, V. Personal Interview. February 13, 2004.
Haughton, V. Personal Interview. April 14, 2004.
Haughton, V. Personal Interview. April 16, 2004.
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Loth, F., Yardimci, M.A., and Alperin, N. 2001. Hydrodynamic modeling of cerebrospinal fluid
motion within the spinal cavity. Journal of Biomechanical Engineering 123.
Mueller, D. “The adult chiari I malformation.” The Chiari Clinic. Accessed: 01 March, 2004.
URL: http://tribble.missouri.edu/ns/chiari/aboutchiarimalformation.htm
Hoffman, R.D. 2003. Piston Pump. The Internet Glossary of Pumps. Accessed: 15 February
2004. URL: http://www.animatedsoftware.com/pumpglos/pistpump.htm
The Ventricular System and CSF. Accessed 08 March 2004. URL:
http://faculty.washington.edu/chudler/vent.htmlAppendix
May 5, 2004
CSF Flow Model – Product Design Specifications
Team:
Lauren Hensley
April Zehm
Mary Lim
Tim Eng
Function: The goal of this project is to create a life-size physical model of the human hindbrain
and upper cervical spinal canal. This will be used to study how varying dimensions
affect cerebrospinal fluid flow (CSF flow) in terms of pressure. Oscillatory flow is
required in the model, and pressure must be quantifiable.
Client requirements:
 Measurable pressures at various points along the spinal canal
 Oscillatory flow that mimics real-life situations
 Interchangeable parts with varying dimensions for the spinal canal
 Replicates actual size of spinal canal and cranial vault
Design requirements:
1. Physical and operational characteristics
a. Performance requirements: The model must accurately measure pressure
exerted by fluid flow within the mock-spinal canal. It must duplicate
dimensions of the human cranial vault and spinal canal. The model must
exemplify oscillatory fluid motion. Users must be able to modify dimensions
and make adjustments as needed to mimic real-life malformations and
obstructions.
b. Safety: As a research-based system, there are no concerns regarding patient
safety. The model must not be hazardous to researchers or any other
personnel operating it. Should an electric motor be used, it should be shielded
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from interaction with any fluids used within the model. Safety warnings must
be present and clearly labeled.
c. Accuracy and reliability: Both precision and accuracy are of great
importance, as the model will be used in medical research. Pressure readings
must be accurate and repeatable, within ± 20%.
d. Life in service: The final product must withstand repeated use—at least daily.
The model will be used in multiple experiments, lasting several months or
even years.
e. Shelf life: The device will be stored in a controlled hospital environment at
normal room temperature.
f. Operating environment: The model need not be sterile, but must meet the
operating requirements for use in a laboratory or MR setting. Should an
electric motor be used, noise should remain minimal. Depending on what
materials are used, corrosion from fluids may be an issue. The device must be
able to handle external forces such as being bumped or dropped by the user.
g. Ergonomics: The device will be used on a counter or table and must be able
to be manipulated by a single user.
h. Size: The model should internally mimic the actual size of the human cranial
vault and spinal canal (approximately 15 mm in diameter). Inner dimensions
of the mock spinal canal should vary within a range of 10-25 mm. Exterior
dimensions will be 15 mm in thickness and 75 mm in both width and length
for each of 10 blocks. Overall, the device must be relatively portable and
easily accessible for everyday use, with the option of sitting on a desktop.
i. Weight: The system must be light enough to lift by a single technician.
j. Materials: The model will use water to mimic CSF. The blocks must be
made of a water-resistant, lightweight material. The system should not
contain any ferrous-based or other magnetic materials, as it will be used in
conjunction with magnetic resonance research.
k. Aesthetics, appearance, and finish: The device should resemble professional
hospital or laboratory equipment. Sharp corners of the blocks will need to be
smoothed.
2. Production characteristics:
a. Quantity: One model will be prototyped; if successful, it can be manufactured
and used for future research.
b. Target product cost: No similar products are in existence; therefore a price
comparison is difficult. The cost of the entire device should be under several
hundred dollars.
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3. Miscellaneous:
a. Standards and specifications: FDA approval is not required.
b. Customer: The client would prefer a model that incorporates piece-wise
assembly and adjustability. Use of capillary-like tubes was suggested as a
means to measure pressure. The device must be compatible with the pump
provided by the client.
c. Patient-related concerns: Not applicable.
d. Competition: There exists an electrical model as well as physical models of
the hindbrain and upper cervical canal, but none directly measure pressure.
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