final report three-point bending device for flexure
TRANSCRIPT
FINAL REPORT
THREE-POINT BENDING DEVICE FOR FLEXURE TESTING OF SOFT TISSUES
TEAM 4
Michael Harman
Minh Xuan Nguyen
Eric Sirois
CLIENT CONTACT
Wei Sun, Ph.D.
Assistant Professor
UCONN BME and ME Department
Arthur B. Bronwell Building Rm. 203
Phone: (860) 486-0369
Fax: (860) 486-5088
E-mail: [email protected]
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TABLE OF CONTENTS
ABSTRACT ........................................................................................................................... 2
1.0 INTRODUCTION .................................................................................................................. 3
1.1 Background ..................................................................................................... 3
1.2 Purpose of the project ..................................................................................... 3
1.3 Previous work done by others .......................................................................... 4
1.3.1 Products ...................................................................................................... 4
1.3.2 Patent Search Results ................................................................................. 4
1.4 Purpose of the project ..................................................................................... 5
2.0 PROJECT DESIGN ................................................................................................................. 6
2.1 Optimal Design ............................................................................................. 10
2.1.1 Objective ................................................................................................... 10
2.1.2 Subunits .................................................................................................... 11
2.1.2.1 Motor System ............................................................................. 11
2.1.2.2 Mounting Bath ........................................................................... 13
2.1.2.3 Temperature Controller .............................................................. 15
2.1.2.4 Image Acquisition ....................................................................... 18
2.1.2.5 The Program, .............................................................................. 21
2.2 Prototype ...................................................................................................... 25
3.0 REALISTIC CONSTRAINTS .......................................................................................... 39
4.0 SAFETY ISSUES .......................................................................................................... 41
5.0 IMPACT OF ENGINEERING SOLUTIONS ...................................................................... 42
6.0 LIFE-LONG LEARNING ............................................................................................... 43
7.0 BUDGET ................................................................................................................... 44
8.0 TEAM MEMBERS CONTRIBUTION TO THE PROJECT ................................................... 45
9.0 CONCLUSION ............................................................................................................ 49
10.0 REFERENCES ............................................................................................................. 50
11.0 ACKNOWLEDGEMENTS ............................................................................................. 50
12.0 APPENDIX ................................................................................................................ 51
12.1 Updated Specifications .................................................................................. 51
12.2 Other Data .................................................................................................... 52
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ABSTRACT
This 3-Point Bending Device is intended to provide the user with a novel tool used to
obtain material property information for biological tissues. Specifically, the device provides the
user with the stress-strain relationship of the tested tissue in the low-strain region (<5% strain)
and provides the location of the neutral axis. The stress-strain relationship is useful because it
allows the user to predict the response of tissue to an applied load. The location of the neutral
axis is important because it allows the user to estimate the contributions of different layers in a
multi-layered tissue specimen. No patented device or device described in the literature is
capable of carrying out both of these functions.
In addition to the primary functions of the device, secondary capabilities are included to
maximize the validity and repeatability of the data provided to the user. One secondary
function is to provide a stable, physiologically appropriate environment for testing. The other
secondary function is to allow for convenient calibration of the force-measuring system.
The 3-Point Bending Device works by submerging a tissue test specimen in into a bath
containing a phosphate-buffered normal saline solution maintained at a pH of 7.3 and a
temperature of 37° ± 1° C. The temperature is maintained by circulation of water through a
separate outer bath containing a heating element. The tissue is deformed by the application of
a small force generated by rotation of a stepper motor coupled through a stage to a bending
bar. The bending bar acts upon the tissue and the actual force applied is calculated by the
bending bar’s distance from a reference bar. The reference bar is also moved by the translation
of the stepper motor rotation, but does not interact with the tissue specimen. The change in
distance between the two bars is a direct result of the force of the tissue on the bar. Because
the bending bar is a homogeneous, linearly elastic material, this displacement can be correlated
with an applied force. The correlation is experimentally derived by the device prior to each
experiment using a convenient built-in bending bar calibration feature. On the side of the
tissue opposite from the bending bar, two stationary posts are used to hold the tissue in place
such that all deformation is due to bending. The deformation of the tissue and displacement of
the bending bar are monitored using a CCD camera mounted above the testing bath. The CCD
camera is capable of resolving markers placed on the tissue as small as 10 microns in diameters.
The 3-Point Bending Device features a custom-designed LabView program capable of
controlling system hardware, interfacing with the user, and performing the complex
calculations required to provide information about tissue mechanical properties. The hardware
controlled includes output to the stepper motor for movement, output to the heating element
for heat generation, input from the CCD camera for marker tracking (both in test mode and in
calibration mode), and input from the resistance temperature detector for control of the
heating element. Through a series of calculations, the program is able to output to the user
several data items, including force vs. displacement, stress vs. strain, stress vs. strain rate,
moment of inertia vs. time, location of the specimen’s neutral axis, and test bath temperature.
Should a need be identified, the software could also be reconfigured to include any other
combination of this data either in relation to another variable, or in relation to time elapsed.
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1.0 INTRODUCTION
1.1 Background
An understanding of the mechanical properties of soft tissues can lead to better
comprehension of tissue pathologies and how the tissue reacts, mechanically, towards an
implant. Because the mechanical properties, such as stress, of soft tissues cannot be measured
directly in vivo, finite element method will be required to accurately estimate the stress
distribution and simulate the interactions between the implant and host tissue. This requires
comprehensive and accurate quantitative information on tissue material behavior.
Experimental testing is, thus, necessary to provide data for the quantification and
characterization of soft tissues. This can usually be accomplished through tensile mechanical
testing, such as uniaxial or biaxial testing. Uniaxial testing involves loading of a tissue specimen
in one direction, whereas biaxial testing is loading of the specimen in two axes. Tensile
mechanical testing, however, is limited in that it cannot provide accurate quantification of the
mechanical behavior of soft tissues in the low strain region and with different layers of fibers.
Flexure testing, on the contrary, is an effective method of evaluating the force-deformation
relationship of different layers of soft tissues. It can complement tensile mechanical testing
with its ability to measure the mechanical behavior of soft tissues experiencing very little stress
and strain.
1.2 Purpose of the Project
The client and his research team in the Biomechanics Lab are currently
conducting studies on the mechanical properties of various soft tissues, primarily heart valves.
Their lab contains a biaxial testing machine, which is frequently used to determine the stress
and strain response of tissues. Data collected via biaxial testing are fundamental in quantifying
and validating the nonlinear elastic, anisotropic nature of the tissues. Biaxial testing, however,
is limited because it treats the test specimen as a homogeneous material. Soft tissues, such as
blood vessels and heart valves, are heterogeneous and consist of multiple layers of fibers
arranged in different networks. When biaxial testing is performed on the leaflet, the collected
data is unable to indicate how the different layers of the leaflet response to the applied load
because, as previously mentioned, the leaflet is treated as homogeneous.
Thus, the client has requested for the construction of a three-point bending device
capable of performing flexure testing on soft tissues. Flexure testing is capable of rendering the
different layers of soft tissues to deformed by different amounts in different regions, which is
essential and critical in analyzing the effect of applied loads on the different layers of the tissue.
Furthermore, since soft tissues have very low bending stiffness, flexural deformation provides a
sensitive approach to evaluating the mechanical properties of the tissue, especially in the low
strain range, which is often very difficult if using tensile mechanical testing.
Therefore, the purpose of the project was to design and construct a three-point bending
device capable of flexural testing of soft tissues. This device is able to compute the flexural
rigidity, bending stiffness, transmural strain, and transverse shear stiffness of a soft tissue
specimen. It is run and controlled by a computer program that has been written specifically for
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the device. The program allows the user to apply a load to a tissue specimen submerged in
saline solution at body temperature. The deformation of the tissue is tracked by a high
resolution camera and computed, along with the amount of load applied, by the program. The
data collected is used by the program to calculate the flexure rigidity, bending stiffness,
transmural strain, and transverse shear stiffness.
1.3 Previous Work Done by Others
There are a couple of three-point bending devices that are similar to the current
project that have been built previously by others. These devices are available in university
laboratories and are usually constructed by researchers of the labs for their research needs.
For example, in the Bioengineering Lab at the University of California in San Diego, there is a
soft tissue bending device consisting of a muscle bath, a system to apply and control force, a
force-measuring system, a deformation-measuring system, and a photographic system. A force
is applied to the tissue by a thin stainless steel wire. The wire is clamped at the top and free at
the bottom, and it is deflected when a dead load is applied at the free end. The deflection of
the wire is used to measure the force acting at the tip on the tissue. In addition, at the Tissue
Mechanics Lab at the University of Miami, there is also a three-point bending device that
utilizes a thin bar of a known, homogenous material to apply force to a soft tissue in cantilever
bending. A specimen is held in place between two stationary posts an adjustable distance
apart, and the force is applied to the center of the specimen. All tests are recorded on a high-
resolution CCD camera. These two devices are similar to the current project and many features
of their design and construction have been used towards the implementation of the project.
1.3.1 Product
A product search for three-point flexure test device did not yield many
results in the market. There is a three-point flexure test device for ampoules from Zwick Roell
Co. This universal device is able to measure the predetermined breaking strength at the break
ring of pharmaceutical ampoules. It can be adjusted to the various geometries and sizes of the
ampoules with the help of distance shims and movable bending bearings. The device is
designed for a nominal force of 500 N. Although this device is capable of performing flexure
testing, it can only test on ampoules and measure the breaking strength. The design of this
device is different from the current project as the current project requires very small, precise
bending of soft tissues and also tracking of the displacement of the tissues.
1.3.2 Patent search results
A patent search on the website http://patft.uspto.gov/ for “three-point
bending device” yielded one result that is irrelevant to the current project. A search for
“bending device” on the same website yielded 1156 results ranging from bending devices use
for solid materials to devices use for wires and a variety of other materials.
Patent number 7,283,891 invented by Werner Butscher, Friedrich Riemeier, Ru Rubbert,
Thomas Weise, and Rohit Sachdeva involves a robotic bending apparatus for bending
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orthodontic archwires and other elongate, bendable medical devices. The device consists of a
robot comprised of a six-axis bending robot with gripping tools and a movable arm that can
move about three translational axes and three rotational axes. This device is suitable for use in
a precision appliance-manufacturing center.
Patent number 7,275,406 invented by Teruaki Yogo is a bending device which includes a
fixed mount having a chuck mechanism gripping workpiece and an articulated robot which
moves a bending mechanism. The workpiece is clamped between a bending die and a clamping
die. The bending and chuck mechanisms are moved by the articulated robot to bend the
workpiece at a plurality of positions.
Although there are a variety of bending devices, none of the patented devices are
similar to the current project. Thus, it can be safely concluded there are currently no patented
three-point bending device for flexure testing of soft-tissues.
1.4 Map for the Rest of the Report
The rest of the report will focus primarily on the design of the project, the
realistic constraints and safety issues, the budget, and others. In the project design, a brief
overview will be given about what the device should at least be able to do, how the device was
implemented, what subunits were used, and how the subunits and the whole device was
tested. The project design will described succinctly each of the three alternative designs
proposed for this device and the advantages and disadvantages of each in terms of complexity,
functionalities, and cost-efficiency. From the advantages and disadvantages of each alternative
design, an optimal design was proposed which makes use of the best characteristics all three
alternative designs and minimizes their disadvantages. The project design will described the
optimal design in great detail along with the parts and subunits that will be used to implement
the design. In addition, the project design will also describe the finished prototype and its
operations.
Subsequently, the report will concentrate on constraints and safety issues associated
with the device. The constraints included environmental constraints, manufacturability,
sustainability, ethics, and health and safety. The constraints section will described in depth
each of these constraints, how they affect the overall design of the device, and how they will be
resolved. Furthermore, safety is always the main concern of any product design. Thus, this
report will also address some of the safety issues associated with the device and how the
device was implemented such that these issues are minimized.
One of the most important aspects of product design is budget. This report will clearly
address the budget allowed for this project and how the budget was allocated to each subunit.
In addition, the report will also include a timeline for the project.
The rest of the report concerns with impacts of engineering solution, life-long learning,
and contributions of each team member. Impacts of engineering solution describes how some
of the engineering features associated with the device help solve, on a smaller scale, simple,
every day problems and, on a larger scale, global and societal issues. Life-long learning explains
how working on this project help enhance the team’s understanding of engineering. Finally,
the contributions of each team member are also listed in this report to show some of the work
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each team member had done towards this project and how he or she helped in improving and
making the team stronger and better.
2.0 PROJECT DESIGN
In order to simplify the construction of the device, the device is divided into multiple
systems: the mounting bath system, the force application system, the temperature controller
system, the image acquisition system, the calibration system, and the program and interface
system. The purpose of the mounting bath system is to provide an area where flexure testing
of the tissue can occur. It is used to hold the solution, the tissue specimen, and the two posts.
The purpose of the force application system is to use a bending bar to apply a force to the
tissue to make it bends. The system consists of a bending bar, a reference bar, and a motor
unit. The motor unit is attached to the bending bar, such that when the motor rotates, the
bending bar will move linearly in one axis. The temperature controller unit is used to maintain
the 37° ± 1° C temperature of the solution. It is consisted of flow loops, a power unit, a
centrifugal pump, a heater, contact plates, and a temperature recorder. The image acquisition
system is used to track markers on the tissue while flexure testing is performed. It is consisted
of a CCD video camera and a camera mount. The CCD video camera captures images of the
tissue while it is being tested and transmits the images to the computer. The calibration system
is used to calibrate the bending bar. The calibration system consists of the CCD video camera,
known weights, and the bending bar. The known weights are attached to the bending bar, and
the displacement of the bending bar for each known weight is tracked by the camera. This will
create a calibration curve for the bending bar, such that a load can be determined from a
known bending-bar displacement using the calibration curve. Finally, the program and
interface system is used to connect all the previously-mentioned systems together. The
interface provides a display where the user can control and interact with the device. And the
program controls the functioning of all the systems according to the user’s commands. In
addition, the program also performs all the necessary calculations using the data obtained from
all the systems. Table 1 below gives a summary of all the systems, their functions, and the parts
they contained.
To construct a device that is easy to use and maintain, safe, cost-effective, and has all
the required functionalities, several ideas were proposed (see Alternative Designs Report). The
first of which consists of a movable testing platform within a mounting bath, as shown in Fig. 1.
The advantages of this design are only one motor is needed to move the platform and the
bending bar and reference bar can remain stationary. On the contrary, the disadvantages are
tracks have to be placed inside the mounting bath and wheels have to be attached to the
platform. This creates unnecessary frictional forces due to the wheels rubbing against the
tracks. Moreover, this system also requires that the flow loops have to be flexible and move
with the platform in order for the solution inside the platform to move in and out (Fig. 2). This
is also a major disadvantage of the system because movable flow loops are very inefficient and
their sustainability will be limited due to fatigue at the sections that continuously move back
and forth.
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Table 1: Summary of all the systems, their functions, and associated parts.
Systems Functions Parts
Mounting Bath Provide an area for testing
Hold the solution and the tissue specimen
Mounting bath
Two posts
Force Application Apply a force to the tissue
Bending bar
Reference bar
Motor unit
Temperature Controller Maintain the 37° ± 1° C of the solution
Flow loops
Power unit
Centrifugal pump
Heater
Contact plates
Temperature recorder
Image Acquisition
Track images of the tissue and resolve
markers on the tissue while it is being
tested
CCD video camera
Camera mount
Calibration Calibration the bending bar to create a
load-displacement calibration curve
CCD video camera
Bending bar
Known weights
Program and Interface
Provide an interface for the user
Control all the functioning of the device
Perform all necessary calculations from
collected data
Computer
LabVIEW program
Matlab program
The second design idea was to have a movable bending bar while the platform
consisting of the two posts remains fixed (Fig. 3). The advantages of this design are there is no
need for the platform inside the mounting bath – thus, saving time and money – and the flow
loops can remain stationary. The primary disadvantage of this design is, unless the CCD camera
can focus on the whole test area (i.e., the area of the maximum displacement of the tissue) and
clearly resolve each markers on the tissue, the camera will have to move as the bending bar
moves to keep the markers on the tissue in view. This will tremendously complicate the design
because a second motor will be needed to move the camera, and the movement of the camera
has to be rigorously controlled in order for it to move simultaneously and exactly with the
movement of the tissue.
The third and last design idea was to move the two posts, to which the tissue specimen
is fixated, and keep the bending bar stationary. The advantage of this design is a similar version
of it has been built and shown to work by a notable research group, see [2]. Moreover, this
design also allows the measurement of the transmural strain of the tissue without the need for
complex automation controls and image-control feedback to move the camera. A notable
downside of this design, however, is that the stationary region near the bending appears to be
a stress concentration point when a mock-up of the system is analyzed using finite element
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software (Fig. 5). Such stress concentrations are known to mask the true tissue stress-strain
response.
Fig. 1 – Cross-sectional side view of mounting bath
Fig. 2 – Side view of mounting bath
Mounting
Bath
Tissue
Bending Bar
Posts
Reference Bar
Motor Fig. 3 – Cross-sectional side view of mounting bath and motor
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Fig. 4 – Sketch of three-point bending apparatus taken from [2]
Fig. 5 – Finite element analysis of the 3-point bending system. The region of peak stresses is
highlighted, which is also the region of least tissue radial movement in this design, and thus the
region used for transmural strain measurement
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After careful deliberation of all three alternative designs, it was decided that the second
alternative design offers the best design ideas in terms of least complexity, cost-effectiveness,
and functionality. Additionally, this design can use ideas proposed in the other two alternative
designs to further improve and enhance its capabilities and functioning. Figure 6 shows a
schematic of the complete system.
2.1 Optimal Design
2.1.1 Objective
The client has requested the development of a 3-point bending device
similar to two devices described in previous literature. The current device will, however, be
entirely original and take advantages of advancements in automation and image acquisition to
provide the user with the most accurate results possible. The device must be capable of
bending a tissue specimen and outputting to the user strain measurements in the low strain
region (<5%) and the location of the neutral axis. The device must also allow for accurate and
repeatable data collection by providing a closely controlled environment for the specimen
during the testing procedure.
Force Application. The device must be capable of applying an appropriate load to cause
small bending deformation of a tissue specimen. This will require extremely small magnitude
loading. Since accurate measurement of such small loads would require costly load cell
equipment, the client has requested that measurement of applied force instead be measured
by comparing the displacement of a material of known mechanical properties. In practice, this
will mean that the bar (described below in Section 2.2) applying load to the tissue be operating
within its linear stress-strain region, and the Young’s Modulus of this region determined via a
calibration procedure. The device must be equipped to carry out this calibration procedure as
well as the actual tissue bending.
Strain Measurement. The device must be capable of imaging tissue displacement and
calculating the instantaneous effective modulus for the tissue’s stress-strain response. This
modulus should be output to the user graphically and made available numerically for data
collection purposes. The time rate of change of the modulus itself should also be calculated
and output to the user.
Neutral Axis Determination. The device should have high enough resolution to image
tissue deformation in a very small region such that local tissue strain can be observed. This will
allow for the measurement of strain as a function of position through the tissue thickness. The
device should track markers as small as 2µm and output the location of the neutral axis to the
user.
Environmental Controls. The device should allow for accurate and repeatable data
collection by providing the tissue with a physiologically appropriate environment. This
environment must include chemical and thermodynamic components. Chemically, the tissue
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must remain submerged at all times in a phosphate-buffered normal saline solution with a pH
of 7.4. Thermodynamically, the bath must be maintained at 37 0C. The device should be
designed to minimize time of testing, which will minimize inaccurate data due to tissue
degradation.
Figure 6 below shows a schematic of the overall device.
CPU
Monitor
Camera Stand
Motor
Motor Thread
CCD Camera
Motion Controller
Bending Bar
Tissue
Posts
Mouting Bath
Temp. Recorder
Flow
Centrifugal PumpHeater
Contact Plates
Power Supply*Not drawn to scale
Fig. 6 – Schematic of the overall device
2.1.2 Subunits
2.1.2.1 Motor System
The motor system consists of a National Instrument (NI) NEMA 17
stepper motor, a Thomson’s Baseline WB linear drive from Danaher Motion, a NI P70360
stepper drive, a NI PCI-7332 motion controller, and SH68-to-SH68 pin cable. Table 2 below
summarizes the parts in the motor system and their functions. Basically, the motor will be
connected to the stepper drive, and the stepper drive will be connected, via the cable, to the
motion controller, which will be installed to the CPU. The motion controller receives user
commands from the computer (i.e., the LabVIEW program) and transmits the commands to the
stepper drive. The stepper drive, in turn, translates the command signals into current that
causes rotation in the motor. The motor will also be attached to the linear drive. Rotation of
the motor will cause the stage on the linear drive to move linearly in one axis. Motion of the
linear drive translates into motion of the bending bar, resulting in movement of the bending bar
back and forth. Figure 7 below shows a general schematic of the motor system.
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Table 2: Summary of the parts in the motor system and their functions
Parts Manufacturer Images Functions
NEMA 17 Stepper
Motor NI
Receive current from
stepper drive and rotate
Cause linear drive to move
WB Linear Drive Danaher
Motion
Move linearly in one-axis
Cause bending bar to move
P70360 Stepper
Drive NI
Receive signals from
motion controller
Convert signals to current
causing motors to rotate
PCI-7332 Motion
Controller NI
Receives user commands
from computer
Transmits commands to
stepper drive
SH68-SH68 Cable NI
Connect stepper drive and
motion controller
Fig. 7 – Schematic of the motor system
The NEMA 17 stepper motor has up to 80 oz-in of holding torque and 1.8 degree of
resolution. Since the motor is only used for rotating and translating the rotation to linear
movement of the linear drive, torque is not an important aspect of the motor. The resolution
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of the motor determines how many steps per revolution the motor is able to rotate. A 1.8
degree resolution means that the motor is able to move 1.8 degrees per step, resulting in 200
steps per revolution. The specification of the linear drive includes a maximum linear speed of
250/1000 mm/s or 0.25 mm/s. The linear speed of the drive in conjunction with the resolution
of the motor determines the velocity of the linear drive per revolution of the motor. Thus, one
revolution of the motor will move the linear drive 0.25 mm/s.
The PCI-7332 is a stepper motor controller which provides fully programmable motion
control for up to two independent or coordinated axes of motion. The 7332 motion controller
can be used for point-to-point and straight-line vector moves for stepper motor applications,
which is completely suitable to perform the linear movement function required in this project.
It accepts real-time commands from the user, and converts the commands into stepper pulses
to be transmitted to the stepper drive. The stepper drive, then, accepts full step pulse
commands from the controller and inserts fine micro-steps to smooth coarse low speed motion
via current flow to the motor. The P70360 provides 2.5 A of continuous current and 3.5 A peak
current output to the motor. The advantage of using the P70360 is its capability of current
reduction. This means that if the user does not require the motor’s full torque to hold a load at
rest, the user can select the right amount of current to reduce the motor heating and power
consumption. This will tremendously increases the life of the motor system.
2.1.2.2 Mounting Bath
The mounting bath system is the central testing platform of the
entire three point bending device. All the other systems are located around and function from
the mounting bath. It is a simple, yet critical aspect of the device design that could essentially
determine the success of the overall project. Without a properly designed mounting bath
everything from the tissue fixation to the testing measurements will be at risk. The mounting
bath itself is composed of two main components.
The first constituent is the inner bath. This inner bath will hold a phosphate buffered
normal saline solution and two stationary fixation posts. The fixation posts in this design are a
specific distance apart, but if possible it would be advantageous to make this distance
adjustable for different size tissue samples. One of the fixation posts will have a removable
sleeve to attach to one end of the tissue sample to create frictionless movement under loading.
The other end of the tissue will lie against the opposite fixation post during bending. The
bending bar and reference bar will move between these fixation posts during testing while the
CCD camera is above capturing the movement. The saline solution inside of the inner bath will
be heated by the water that is being pumped around it.
The second main section is the outer bath. The outer bath will include an inlet and
outlet so that temperature controlled water may be pumped through it at a desired rate. The
outer bath will be larger so that the inner bath can rest inside of it. There will be four posts
located on the base of the outer bath for the inner bath to rest on top of, thus creating more
surface area below the inner bath for heat transfer to the saline solution.
The entire bath will be machined using sheets of Lexan. Lexan is a highly durable
polycarbonate resin thermoplastic that was chosen for its extreme resistance to corrosion and
strength. Lexan is readily available and easily machineable, therefore making it an ideal choice
for material. The exact dimensions and spe
Fig. 8
strength. Lexan is readily available and easily machineable, therefore making it an ideal choice
for material. The exact dimensions and specifications of each part can be seen in Fig. 9 below.
Fig. 8 – Top view of mounting bath with labeling
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strength. Lexan is readily available and easily machineable, therefore making it an ideal choice
cifications of each part can be seen in Fig. 9 below.
15
Fig. 9 – Schematic of mounting bath with dimensions
2.1.2.3 Temperature Control System
The principle function of the temperature control system is to
maintain the temperature of the test specimen within physiological bounds. Maintaining this
temperature range will minimize the variance of data collected using the device from one test
run to another. The optimal temperature band for living tissue has been determined to be
37+/-1 oC. This subsystem will be responsible for maintaining the water in the vicinity of the
test specimen within this temperature band.
The in direct contact with the test specimen is in fact in contact with excised tissue. This
fluid is thus potentially contaminated with hazardous or harmful microbial growth. To minimize
the spread of microbial growth, this fluid has been kept to as small a volume as possible.
Specifically, the device has been designed using a “bath within a bath” concept, where the
inner bath contains the test specimen and the outer bath does not. This system has the added
benefit of requiring fewer chemicals for maintenance of salinity and pH (Section 2.2) because
only the inner bath need be maintained as a constant environment for the tissue. This design
also prevents the water circulation pump from coming in contact with high salinity water, which
is corrosive and would inevitably lead to shortened pump life (or much more expensive
16
corrosion-resistant materials). The two baths will from this point on be referred to as the inner
and outer baths.
One downside of the two bath system is that direct mixing of fluid would provide for
much more efficient heat transfer. In this configuration, the outer bath fluid flows against the
inner bath wall and transfers heat into the wall. The heat is transferred through the wall and
into the inner bath fluid. This system is less efficient that the direct mixing alternative, but the
potential benefits of the two bath system have been deemed more significant.
To maintain inner bath temperature, heat must be added into it. Setting up a control
volume around the inner bath, all directions of heat transfer can be seen (Fig. 10). On the five
sides of the inner bath that are in contact with the fluid of the outer bath, convective heat
transfer occurs between the moving outer bath fluid and the inner bath’s outer wall. The
convective heat transfer equation is:
�� �� � ����� ����
In the above equation, �� ��is the convective heat transfer into the control volume, h is the
convective heat transfer coefficient, A is the surface area upon which the fluid is acting, TOBF is
the temperature of the outer bath fluid, and TIBOW is the temperature of the inner bath’s outer
wall. The total energy transferred into and out of the control volume is the sum of the fluxes
across each face, or:
����� � � �� ���
���� �����
The system is assumed to be in steady state, so the net flux (�����) is equal to zero. Also
acting on the control volume of the inner bath is the outbound flux, or the radiant heat transfer
of the inner bath water into the surrounding ambient air. This form of heat transfer follows the
equation:
����� � ������ ����
In the above equation, ����� is the free convective heat transfer out of the control volume, h
and A have the same meaning but potentially different values, TIBF is the temperature of the
inner bath fluid, and Tamb is the ambient air temperature.
17
Fig. 10 – Side view of the inner bath. Here the inner bath is taken as a control volume for
energy balance analysis. On each of the five faces in contact with the fluid of the outer bath
(larger blue arrows, with two sides not shown) forced convective heat transfer occurs from the
fluid of the outer bath into the wall of the inner bath. On the top surface of the inner bath, the
inner bath fluid loses heat via free convection to the ambient air. Note that other heat input
sources, such as friction from tissue movement and radiant losses, are negligible compared to
other sources and are thus not described here.
There remains one step in the transfer of heat for this particular control volume. The
heat transfer through the inner bath wall is conductive heat transfer, and follows the equation:
��� � � !��� ����" #
In the above equation, ���is the conduction heat transfer from the outer wall of the inner bath
to the inner wall of the inner bath, κ is the thermal conductivity, A is again the area of the wall
in question, TIBOW and TIBIW are the temperatures of the inner bath’s outer and inner wall
surfaces (respectively), and t is the thickness of the wall.
Combining the conduction and convection equations, it was possible to determine the
required temperature of the outer bath, as well as the flowrate in the outer bath (this goes into
the determination of h). The same process was repeated for the outer bath, taking losses to
ambient air and losses into the inner bath’s outer wall as heat sinks, and taking heat in from a
heating element as the heat source. This gave us the required capacity of the heat source,
which was determined to be (including safety margin) 1000 watts. Additionally, the required
capacity of the pump was determined from the calculated flowrate. Since it was assumed that
the pump would be at the same elevation as the bath system and the bath is open to ambient,
pump flow rates against no backpressure were used. The required pump capacity was
determined to be (including safety margin) 5 L/min.
Free Convection
Heat Transfer to air
Inner Bath
Lh
Water
Level
Forced
Convection
Forced
ConvectionForced
Convection
18
The pump has been designed to operate continuously, supplying water to the outer
bath. The pump will be powered by single phase 120v power from a standard electrical outlet.
The pump draws suction from a return line which is also connected to the outer bath. To match
pump outlet size, the inlet and outlet pipes are ½” inner diameter. The loop containing the
pump will contain three valves, including inlet and outlet pump isolation valves to allow for
pump removal or system isolation, and a drain valve to allow for efficient draining of the
system. The pump inlet and outlet lines will also be equipped with a recirculation line to allow
the pump to safely run even when the pump’s outlet valve to the outer bath is shut.
The heating element has been designed to operate intermittently. When in operation,
the heating element receives single phase 120v power from a standard electrical outlet. When
not is operation, the heating element receives no power. This operation will be carried out via
a temperature feedback loop. A resistance temperature detector (RTD) will be used to record
inner bath temperature. The output temperature from the RTD will be taken as in input into
the controlling LabView program. The program will monitor the current temperature. When
inner bath temperature falls below the acceptable operating band (36oC), the program will send
a signal to close a set of contacts, which will allow input power to reach the heating element.
When temperature reaches the high end of the operating band, the contact will be opened and
the heating element will lose power. The program will also provide high and low temperature
alarms to warn the operator that data obtained may not be reliable. A separate manual
thermometer will also monitor inner bath temperature for redundancy.
The combined temperature control system can be seen below in Fig. 11.
19
Fig. 11 – Schematic of temperature control system. Note that the walls of the inner bath have
been omitted for the sake of clarity.
2.1.2.4 Image Acquisition System
The flexibility and accuracy of the image acquisition system
represent the heart of the device’s capabilities. This system is responsible for gathering
location information for several key components: the bending bar, the reference bar, the two
fixed posts, the inner and outer edges of the specimen (for thickness), and the test specimen
itself in many specific regions. The imaging system is also responsible for bending bar
calibration, which is a crucial component in force determination (and in the subsequent stress
calculation).
A high resolution CCD camera will be used to track the positions of the desired
components in real time. The positions will be defined for the benefit of the user as marker
20
beads or ink, depending upon the application. The beads will be glued on micro-fragments of
crushed stone (Fig. 12a) and the ink will be sprayed on in microdots as small as 2 µm in
diameter (Fig. 12b). Markers will be applied to the bending bar, the reference bar, each of the
two stationary posts, and at as many points in the field of view on the test specimen surface as
possible.
Fig. 12 – Markers used for location tracking, including a) glued on micro-beads and b) sprayed
on ink microdots.
The CCD camera must be capable of resolving the smallest of microdots, which are
predicted to be 2 µm in diameter. This specification is called spatial resolution, in µm/pixel.
Additionally, the maximum previously recorded tissue displacement for this three-point
bending test was 2 mm. Allowing a safety margin, the field of view in the y-direction was thus
taken to be 4 mm. The boundary in the horizontal direction should be wide enough to
encompass the two stationary posts. If this x-direction distance was chosen to be wider, then
more representative data would be obtained using the device. As a trade-off though, a wider
view area resulted in more total pixels and thus a much more costly CCD camera. Common
pixel field values give one direction to be 2/3 of the other direction, so for the least costly
version of a CCD camera with a spatial resolution of 2 µm/pixel that would still meet device
specifications would be a 4 mm by 6 mm field. The number of pixels in each row (Nx) and
column (Ny) would thus be:
$� � % &'()*"'+, -),."�/01"'1- ()/+-2"'+,
$3 � 4 &'()*"'+, -),."�/01"'1- ()/+-2"'+,
The relationship between distance and spatial resolution can be seen below in Fig. 13. Also, the
total pixel count (PC) could be determined by the following equation:
21
PC = Nx * Ny
Fig. 13 – Visualization of field distances and spatial resolution used in pixel determination
For the above example where the x-direction was 6 mm, the y-direction 4 mm, and the spatial
resolution was 2 µm/pixel, PC was determined to be only 60 kPix. The ideal dimension for x-
direction is, however, 30 mm (giving a y-direction value of 20 mm). Using the same spatial
resolution, this would require a pixel count of 150 MPix. Clearly, the best option lies between
the two bounding scenarios. The price of the CCD camera drives the final selection though, and
exact price quotes are still pending at this time.
One additional feature of the imaging system will be its role in stress calculation. The
final objective of the device is to output the stress-strain relationship of the test specimen to
the user. The imaging system will calculate strain by tracking displacement (of markers, as
mentioned above), but it will also be largely responsible for the calculation of stress. Stress
calculation will be derived from the imaging system in two specific ways: bending bar
displacement due to the tissue’s resistance and bending bar Young’s Modulus.
The bending bar displacement will be computed by tracking the positions of the tips of
both the bending bar and the reference bar. Since their movements are coupled mechanically,
any difference in their relative distance is due solely to reactionary force of the tissue against
the bending bar. This displacement can be converted into force by using the second imaging-
force subsystem, calibration.
Before each test run (this may be extended based on the observed changes in
calibration from one run to another during pilot trials, but for now the most conservative
option is being applied) a calibration of the bending bar to be used will be carried out. The
calibration procedure consists of observing the bending bar in a zero state condition and then
applying a known load and observing bending bar displacement. By accounting for bending bar
X-direction distance
Spatial Resolution
22
cross-sectional area, the stress-strain relationship for the bar can be calculated. Since the
bending bars will be metallic, they will have a linear stress-strain relationship over the elastic
region. As long as the known load caused elastic deformation, the slope of the line drawn
between the stress-strain pairs (0,0) and (measured strain, measured stress) is the Young’s
Modulus for that specimen.
Specifically, to carry out the calibration procedure with minimal system reconfiguration,
the CCD camera will be mounted such that it can swivel 90 degrees and lock in place in either
the test position or the calibration position. The test position will be down and the calibration
position will be parallel to the table-top. The bending bar will be mounted into a clamp at a
height even with the viewing field of the CCD camera. The bar will then be loaded with the
known load, which will be a weight appropriately sized for that particular bending bar. The
weight will be tied with fishing line to the end of the bar and allowed to pull downward due to
the gravitational force. The resulting displacement will yield a Young’s Modulus as described
above. When the bending bar is loaded by the test specimen, the displacement can be
converted into strain and the strain cross-referenced to find the corresponding stress value.
This stress value can be mathematically equated back to force as long as the cross-sectional
area of the bending bar is known (and it is because it was already determined during the
calibration step).
The entire imaging system can be seen below in Fig. 14.
Fig. 14 – Schematic of camera system. Note that the walls of the inner bath have been omitted
for clarity.
2.1.2.5 The Program
A program will be specifically written to integrate all the
hardware, give the user full control over all the functioning of the hardware, and perform all
23
the necessary calculations. Writing the program will require the use of the following software:
NI LabVIEW 8.5, Measurement and Automation 4.3, Motion Assistant, Vision Assistant 8.5, and
Matlab. The Measurement and Automation (MAX) program will be used to configure all the
connected hardware. Briefly, it detects all the hardware that is connected to the computer – in
this case, the motion controller, the CCD camera, and the temperature controller. It, then,
gives each piece of hardware a unique identity which can be used by the LabVIEW program to
communicate with that specific piece of hardware. For example, the motion controller
connected to the computer will be identified by the software as having board ID number 1.
This number will be unique only to the motion controller and will not be used to identify any
other hardware. The LabVIEW program will use this board ID number 1 to communicate only
with the motion controller. MAX will be used extensively throughout this project to manage
and configure all the hardware.
National Instrument Motion Assistant is a stand-alone prototyping tool used to quickly
develop motion applications. It is used to graphically construct, preview, and test motion
applications without writing code. It can also create usable LabVIEW code that can be used to
build stand-alone applications or add to other LabVIEW applications. Motion Assistant will be
used in this project to configure, precisely, the movement of the motor such that it will move
linearly in one axis and at user-determined speed and direction.
Vision Assistant is a National Instrument image processing software. It allows the user
to quickly and easily process an image to the desired image quality and construct usable
LabVIEW code to be integrated into the LabVIEW program. Vision Assistant will be used to
process the images captured by the CCD camera in order to resolve and track the markers on
the tissue. Table 3 below shows some of the main processing functions that will be used to
process the images and track the markers on the tissue.
Table 3: Summary of processing functions
Processing Functions Purpose
Brightness Alter the brightness, contrast, and gamma of
an image
Threshold
Segment pixels in grayscale images
Isolate pixels that are of interest and set all
other pixels as background
Convert an image from grayscale to binary
Filter Smooth, sharpen, transform, and remove
noise from an image
Morphology
Prepare particles in the image for quantitative
analysis, such as finding the area, perimeter,
or position
Particle Analysis Analyze particles and display all of the
measurement results
LabVIEW is short for Laboratory Virtual Instrument Engineering Workbench. It is a
powerful and flexible graphical development environment that uses a graphical programming
24
language, known as G programming language, to create programs relying on graphical symbols
to describe programming actions. LabVIEW programs are called Virtual Instruments, or VIs.
LabVIEW consists of a front panel and block diagram of LabVIEW. The front panel window is
the interface to the VI code. The block diagram window contains program code that exists in a
graphical form. The front panel contains various types of controls and indicators, or inputs and
outputs, respectively. The block diagram contains terminals corresponding to front panel
controls and indicators, as well as constants, functions, subVIs, structures, and wires that carry
data from one object to another [6]. Figure 15 shows an example of a motion controller code
that was recently constructed in LabVIEW. The block diagram contains all the graphical symbols
and functions to execute the code; and the front panel displays the inputs for the user to enter
in and the outputs of the code after it has been executed.
Fig. 15 – Front panel and block diagram of a motion control VI
LabVIEW will be used in this project to integrate all the hardware together, perform all
necessary calculations (by implementing Matlab user-defined functions), and to provide the
user full control of the device. Codes will be written to control the movement of the motor,
process acquired images from the camera and track the positions of the markers as the tissue
deforms, and calculate the deformation of the tissue. The program will use codes generated by
Motion Assistant and Vision Assistant to simply the coding process for the motion control and
image acquisition. In order to determine the deformation of the tissue, the calculations will be
broken into two parts: 1) to identify the instantaneous effective modulus, and 2) to identify the
neutral axis location. To determine these two parts, LabVIEW will make use of the following
equations:
25
5 � 678∆
8 � �:;�<
= � |?�|·|��|·|�?|AB�|?�|C|��|C|�?|��|?�|D|��|C|�?|��|?�|C|��|D|�?|��|�?|D|?�|C|��|�E
F� � A2H�� I 1, L�)() ' � 1,2
λ� � 1 I 1� N� O� , L�)() ' � 1,2
$+(P1-'Q)& $� -+*1"'+, � �R�R
Please refer to the Proposal Report for detailed explanations of the equations. Summarily,
LabVIEW calls user-defined Matlab functions to allow its graphical programming language and
symbols to calculate the deformation of the tissue from the data collected from the image
acquisition and motion control, using all of the equations above. The overall programs consists
of nearly 100 user-defined functions, but the major functions for the flexural rigidity portion are
responsible for determination of the young’s modulus of the bending bar from the calibration
data, converting distance measurements in pixels into millimeters, calculating flexural rigidity,
and then graphically and numerically outputting flexural rigidity data to the user. The
subroutines used for the transmural strain portion include inputting point location data in the
undeformed and deformed states, correlating points from the two states such that they are in
the same order, developing a 2-D finite element triangular mesh of the undeformed points in
real time (and not requiring use of another program), using the mesh to determine finite
element strain for each of the triangles in the mesh, and then outputting the transmural strain
data both numerically and graphically to the user, including a blended multicolor gradient
display of strain in each principle direction.
2.2 Prototype
A prototype of the device has been designed to the specifications described
above, fabricated, assembled, and tested. The major subsystems of the device are the physical
components, the software to control device movement and image acquisition, and the software
to perform all necessary calculations and provide output to the user.
2.2.1 Prototype Subsystems
2.2.1.1 Physical Components
26
The three point bending device prototype carries out its principle
function of bending tissue by applying a force to the specimen via a bending bar. The actual
force applied is measured by comparing the distance between the bending bar and reference
bar at any point in time. The bending bar and reference bar mounting system can be see below
in Figure 16. During testing, the bending bar is normally attached to a linear actuator, which is
also shown in Figure 16. The bending bar mount is able to be removed though, for conducting
the calibration procedure. Additionally, the bending and reference bars may easily be removed
by loosening setscrews to allow for bars of differing diameters to be used. In this way, the user
maintains full control over bending bar stiffness and the most appropriate configuration for
each specimen can be determined.
Fig. 16 – Bending bar mount shown in the testing configuration with bending and reference
bars connected to a linear actuator (which is driven by a Stepper Motor, and controlled by the
device software)
The tissue specimen lies in a dual bath system, where the inner bath houses the tissue
along with two reinforcing posts. The inner bath is maintained in a chemically appropriate
solution for tissue testing (see above), while the outer bath is normal tap water and is used to
provide the proper thermal environment for testing. The temperature of the system is
maintained by a 1KW heater that is controlled by comparing a user-adjusted control knob value
to the reading of the capillary bulb, which is intended to be placed within the inner fluid bath.
The fluid in the outer bath is circulated by a 500 gph centrifugal pump. The components for this
bath and temperature control system can be seen below in Figure 17.
Bending Bar
Reference Bar
Linear
Actuator
Stepper Motor
Wire Harness to
Motion Controller
27
Fig. 17 – The dual bath system, shown with the individual components of the temperature
control system. Each component is labeled individually above.
The data acquisition for the device is carried out via a 5 megapixel, 12 bit monochrome
CCD camera. The camera is mounted upon a custom built stand that is capable of maneuvering
the camera either closer to or further away from the specimen via a height adjustment.
Additionally, the camera stand is capable of rotating away from the specimen by 90 degrees to
allow for imaging of the calibration procedure. Output from the camera is sent to the hardware
interface portion of the software via a USB connection to the controlling PC. The images
acquired are then processed to determine specific locations, such as marker coordinates. The
camera and camera mount can be seen below in Figure 18.
Fixation PostsOuter Bath
Mounting
Bath
Centrifugal Pump
Pump Outlet
Pump Inlet Priming Reservoir
Capillary Bulb
Thermostat Dial
Power Lead
Heater Lead
Heating Elements
Lead Housing
Heater & Power Leads
28
Fig. 18 – CCD camera and camera mount. The camera is shown in the testing configuration, but
can also be rotated 90 degrees to allow for calibration imaging.
The subunits are all mounted upon a 24”x24” table using 1/4-20 standard machine
screws. The arrangement of all subunits can be seen below in Figure 19.
CCD Camera
USB to Computer
Base
Height
Adjustment
Pin to Adjust
Camera Position
29
Fig. 19 – Assembled prototype. All component labels correspond to the descriptive table
above.
A
B
C
D
E
G
H
I
J
K
F
Label Component Features
A Thermostat SPST with dial
B Immersion Heater Copper sheath, single element
C Camera Stand Adjusts height and position of camera
D CCD Camera Image acquisition
E Linear Actuator Moves the bending bar mount
F Stepper Motor Controls the linear actuator
G Bending Bar Mount Holds the bending and reference bars
H Centrifugal Pump Regulates the temperature flow loop
I Outer Bath Used for heated water flow
J Tissue Bath Used for tissue testing, saline solution
K NI Motion Controller Integrates the stepper motor with LabVIEW
30
2.2.1.2 Hardware Interface Software
A LabVIEW program was written to provide automatic control
over all the necessary hardware. Specifically, this program was written to control the motor
and the camera. In addition, this software is the main software for the user to use. It is the
interface between the user and the device.
The program is divided into three separate individual modules. The first module
controls the motion of the motor, allowing the user to move the motor to any position. It
allows the user to move the motor away or toward the mounting bath prior to testing. Figures
20 A and B show the front panel and associated block diagram for the motor control module.
As can be seen from the front panel, there are two buttons designing the directions the motor
can go. In addition, there is a motor speed bar, which allows the user to adjust how fast the
motor moves. Finally, there is a stop motor button that stops the motor from moving when
pressed.
The second module is the image acquisition module. This module allows the user to
acquire continuous, real-time image from the camera. It allows the user to adjust the settings
on the camera such that optimal image is acquired before any testing. In addition, this module
also allows the user to process the image such that only markers of interest are tracked.
The third and final module is the testing module. Basically, this module is the
integration between the motor module and image acquisition module. The testing module
allows the user to input in all the necessary information prior to actual flexural testing, and
then presses the start button and the testing will be performed automatically through the
program. The testing module controls the movement of the motor, while simultaneously
acquires images through the camera and tracks markers on the images. In addition, while the
motor and camera are performing their functions, the testing module also output all the critical
information, such as the positions of the markers and the time. After the testing is completed,
the output data will be written into a text file. Furthermore, the output data will also be passed
a calculation subroutine that will calculate all the necessary information for the user. The
results will be outputted on the front panel of the program. Thus, to summarize, the testing
module consists of the integration between the motor, the camera, and also the calculation.
Figure 21 below shows the front panel and block diagram for the testing module.
31
Fig. 20 – A) Front panel and B) block diagram for motor control module
32
Fig. 21 – A) Front panel and B) block diagram for image acquisition module
33
Fig. 22 – A) Front panel and B) block diagram for testing module
2.2.1.3 Calculation and Output Software
To process the acquired image data and provide the desired
output to the user, a custom set of Matlab user-defined functions was developed. There are
nearly 100 of these functions, but the purpose of at least the principle subroutines will be
discussed here. First, for the flexural rigidity calculations the desired output is an effective
modulus and its relation to rate of change of the curvature. To carry out this functionality, the
34
program is capable of converting the coordinate locations from their pixel values into their
adjusted real-world equivalent values. Additionally, the modulus of the bending bar must be
determined using data provided during the calibration procedure. This modulus is then used to
determine the force applied to the tissue at any given time during testing. Also, each of the
tissue markers that were tracked by the CCD camera is used in a calculation of the current
tissue radius of curvature and location of circle center. By combining this radius of curvature
information with the force applied calculations, the effective modulus was determined at each
time instance. This information is then output to the user in real time (with a slight delay for
calculation). The LabView MathScript implementation of these Matlab functions can be seen
below in Figure 23.
Fig. 23 – LabView MathScript subVI used to call user-defined Matlab functions associated with
the flexural rigidity calculations.
In addition to the flexural rigidity calculations, a separate family of Matlab user-defined
functions was developed for processing of the transmural strain data. Based on the work by
Y.C. Fung (see references), it was determined that three independent measurements were
required to determine the transmural strain of a region. These three measurements naturally
lend themselves to building a triangular area and using the three measurements as the lengths
of the three sides of the triangle bounding the area. Turning a surface into a grid of triangles is
known as meshing. While there are many commercial programs available to perform this
meshing function, none could be performed in real time and all would require additional
(undesirable) work on the part of the user. As such, the device includes a unique real-time
meshing algorithm that converts the dataset of points into a triangular grid. Other subroutines
then calculate the transmural strain for each of these triangles, much as a commercial finite
element software package might do. The output to the user also was designed to mirror that of
35
commercial finite element displays and includes a blended color representation of strain in
each of the principle directions. The LabView Matlab Script that calls the Matlab subroutines
can be seen below in Figure 24. A Matlab Script was used vice a MathScript node to facilitate
use of higher level Matlab graphical functions, many of which are not available in LabView
alone. The downside of this approach is that the user must open Matlab prior to opening the
LabView program.
Fig. 24 – LabView Matlab Script implementation of Matlab user-defined functions associated
with the transmural strain calculations.
2.2.2 Prototype Testing
To test the functionality of the device, we carried out the calibration,
flexural rigidity, and transmural strain procedures. The calibration procedure, as mentioned
above, involved turning the camera 90 degrees and recording the deflection the bending bar
away from the reference bar as a result of the application of two different weight values. The
result was a calculated Young’s modulus for that particular bending bar, which could be used
later to correlate distance between bending and reference bars with force applied to the
bending bar. The calibration procedure can be seen below in Figure 25.
36
Fig. 25 – Calibration procedure, where a weight of known mass is hung from bending bar.
Change in distance between bending bar and reference bar is used to determine a Young’s
modulus for the bending bar.
After the calibration procedure was performed, tissue specimens were marked for both
the flexural rigidity test and the transmural strain test. The flexural rigidity marking included a
series of marking beads placed roughly along the center of the thickness of the tissue. If placed
properly, they should all lie on the perimeter of a circle as the tissue is deformed. The
Camera
Bending Bar
Calibration Force
37
transmural strain marking was performed using inked applied with a common toothbrush,
following the practice of Yu et al. Since transmural strain markings were tracked for very small
regions, the best region was chosen and imaged. The marking of both tissue specimens can be
seen below in Figure 26.
Fig. 26 – Tissue specimens marked for a) the flexural rigidity test and b) the transmural strain
test.
The tests were carried out on the specimens and the resulting image data analyzed
using the software built for the device. The Flexural rigidity data closely followed that of data
published previously. The transmural strain calculations provided numerical data as well as
graphical output to the user. The results of pilot trials both for theoretical and actual tissue
specimens can be seen below in Figure 27.
a. b.
38
Fig. 27 – Transmural strain output data for a theoretical test (left vertical panel) and for a test of
a porcine aorta (right vertical panel)
39
3.0 REALISTIC CONSTRAINTS
3.1 Engineering Standards
This device will be required to record, calculate, and output extremely small
order stresses and strains. The margin for error must then be kept appropriately small or the
data obtained with the device will have little or no value. Possible sources of error include:
• Environmental Changes – if testing is performed on the same tissue at two different
environmental conditions (temperature, for example) there will be two different
responses
• Imaging Error – The device must track the change of position of the test specimen over
time. The tissue strain calculations will be based on these measurements. The tissue
thickness and radius of curvature must also be recorded. These will be used in stress
calculations. This error occurs when the position determined by the device differs from
the actual position or when strain is measured in a non-representative region.
• Force Recording Error – The device must record the change in force applied to the tissue
over time. The tissue stress calculations will be based on this force measurement. This
error occurs when the force read by the device differs from the actual force applied. A
possible cause could be an unaccounted-for loss of force (such as friction).
• Calculation Error – This type of error could result from rounding off significant figures,
which is of particular concern for the strain values which will be of low magnitude.
To prevent excessive error, many features have been built into the device design. To
reduce environmental error, a stable, repeatable environment has been planned. This
environment will be discussed below in section 2.2. To reduce imaging error, a large number of
small markers will be used, along with a CCD camera capable of resolving very small
displacements. The displacement of the tissue will be taken as the average of the numerous
markers. Any error should be removed, or at least identifiable in the form of a standard
deviation. The force recording error has been minimized by tracking the force using
displacement of the bending bar (see mounting bath subsection of the subunits section) instead
of inputting force directly from linear actuator displacement. Using the linear actuator would
not account for any force lost to rolling friction in the wheels of the platform, or any sliding of
the test specimen. Calculation error will be reduced by carefully monitoring use of significant
digits in any calculation performed.
3.2 Environmental
For proper tissue mechanical response, the tissue must be maintained in
conditions that simulate the in vitro environment. By developing a suitable environment, the
data obtained during device operation will be closer to the in vivo response, and thus more
relevant. By creating specific bounds for the environmental conditions, the introduction of
error due to testing being conducted at different conditions is also eliminated. For example,
tissue responds differently to mechanical stimuli at higher temperatures than it does at lower
temperatures. By fixing the tissue’s environment, the device is more likely to provide
40
reproducible data. This will inevitably make the device more attractive to researchers and
industrial scientists.
It should be pointed out here that the first step in creating a stable in vitro environment
is to maintain the test specimen submerged at all times. Even short-duration exposure to open
air begins an irreversible degradation process. The next step is to select the ideal properties for
the fluid in which the tissue will be submerged. To fix the environment, we have targeted three
specific areas that will make the testing fluid closely resemble blood: salinity, pH, and
temperature. The first, salinity, will be accomplished by using a normal saline solution. Normal
saline mimics blood in many ways. The second condition, pH, will be accomplished by buffering
the normal saline solution. We have chosen to use phosphate as a buffer. Phosphates will be
added until the saline solution reaches a pH of 7.4. For the third condition, temperature, the
phosphate-buffered saline solution will be maintained at 37 ± 2°C. This will be accomplished
using a circulation system that will be described later. The resulting environment will be mildly
corrosive.
3.3 Manufacturability
Many of the parts required for system operation will be extremely small, and
thus expensive to manufacture precisely. In some cases trade-offs will have to be made to
lower manufacture costs at the expense of device accuracy. Whenever possible, parts will be
manufactured in-house (at the University of Connecticut machine shop).
3.4 Sustainability
Wherever possible, non-corrosive parts have been chosen. All potentially
corrosive parts will be periodically replaced. The method chosen for measuring tissue
displacement will be calibrated prior to each use. Any components subject to friction will be
lubricated periodically to prevent wear and unnecessary error. It is anticipated that this will
include the wheel system beneath the platform. The system will be monitored closely during
prototype development and testing to identify any additional wear surfaces. These measures
will increase the repeatability of data obtained using the system.
3.5 Ethics
The budget for this project is limited. As such, all purchased parts will be
thoroughly researched so that the most reliable, and yet most cost-effective, component is
chosen in each case. All funds spent will be used solely for the purchase of parts and services.
Whenever possible, parts will be manufactured in-house to conserve funds. Finally, the client
will be kept informed of the initially planned budget along with periodic updates and consulted
prior to any necessary deviations from the budget.
41
3.6 Health and Safety
The device will be routinely exposed to excised animal tissue. Surface materials
should be chosen such that they are smooth and capable of being sterilized after each use.
Materials chosen should also be non-toxic and non-destructive to the tested tissue. The health
and safety allowances for the device will be expanded upon in Section 3.
4.0 SAFETY ISSUES
The finished device will contain several subunits that could pose health risks if left
unchecked. Such subunits include:
• Pump – the centrifugal pump (in the temperature control loop) represents a rotating
mechanical device. If run dry, the pump could cause damage to itself, the rest of the
device, or the operator. While running, the pump also represents a hazard to the user if
skin were to come in contact with the impeller. This could result in loss of digits.
• Pump motor – the pump motor represents both an electrical and a fire hazard.
• Heating element – like the pump motor, the heating element poses both an electrical
and a fire hazard. The heating element additionally poses a burn hazard to the user.
• Motion system motor – the motion system motor also poses both an electrical and a
fire hazard. The exposed rotating shaft of the motor also poses a risk to users with long
hair or loosely worn accessories (such as necklaces).
• Fluid medium – the test fluid medium poses a potential chemical risk to the user and to
the other components.
• Test bath surface – the test bath will routinely be exposed to excised biological tissue.
This will result in a potentially biohazardous surface.
To minimize the potential negative impact of the above subunits, several measures have
been taken. For the pump, the system has been designed to have the pump hard-mounted into
the piping. This should minimize the chance of the pump impeller coming in contact with the
user. Also, the user’s manual will include warnings against running the pump in a dry tank
condition. The pump has also been selected such that the lowest allowable horsepower rating
will be used, which will minimize the potential damage caused if any adverse event were to
occur. The pump motor will be contained in an enclosed structure, and will also benefit from
the choice of a low horsepower pump. The heating element has been chosen such that it can
safely withstand being submerged in the test fluid. This should prevent any short-circuiting of
the heating element and the subsequent fire hazard that it could pose. The heating element
will also remain submerged and has been chosen to emit as little power as necessary to
maintain system temperature. These safety measures should combine to eliminate the risk of
user burn injury.
The motion system motor will be enclosed to minimize the risk of electrical shock and
fire. The motor shaft will also be enclosed to prevent personal injury due to the rotating
machinery. Users will be warned about the potential risks of operating the system with either
the motor or shaft exposed. The fluid medium has been chosen such that the chemical
composition and pH are insignificant hazards to humans. The bath remains, however, mildly
42
corrosive. A warning will be provided to the user to remind them that the use of corrosive
replacement parts should be avoided whenever possible.
The surface of the bath itself is a potential breeding ground for microbial agents. Three
measures have been taken to reduce this risk. The first is that a “bath within a bath” system
has been employed. This means that the fluid circulating through the pump and the outer bath
will never have been exposed to biological tissue. Reducing the potential surface area for
microbial growth should significantly reduce the risk to the user. The second measure taken
was the choice of material used. The bath will be fabricated from Lexan, which will form a non-
porous and sterilizable surface. The third measure, in combination with the second, is to
routinely sterilize the exposed surface. The user will be provided with instructions on sterilizing
the work surface with alcohol after each use.
5.0 IMPACT OF ENGINEERING SOLUTIONS
The chosen optimal design for the project consists of a moving bending bar with a
stationary mounting bath and image acquisition system. This design helps simplify the device
while still determining the flexural rigidity and bending stiffness of the tissue sample. By
combining techniques taken from the other alternative designs the transmural strain and
transverse stress will also be measured during testing. This device offers the capability of
measuring nearly all the mechanical properties of a given tissue sample in order to accurately
characterize the specific material behaviors of the tissue. Additionally the device will be able to
locate and quantify specific layers within the tissue that display distinct behaviors, rather than
measuring the entire sample as a single, homogenous material. This is critical to the
understanding and analysis of the soft tissue mechanical properties.
5.1 Global Impact
In general this three point bending device will assist many disciplines beyond the
field of engineering in understanding the mechanical behavior of soft tissues. It is the first
device to combine techniques for testing not only flexural rigidity and bending stiffness, but
also transmural strain and transverse stress. The comprehension of these properties is critical
to many areas of study, specifically biomechanics.
The client for this project is Dr. Wei Sun at the University of Connecticut. The focus of
much of his research is on cardiac valve replacements. In recent years there have been a
plethora of advances in the way of valve repair, but most of the cardiac valve implants have
experienced continued failure. While some valve implants are successful in the short term,
nearly all of them suffer failure within a year of implantation. This catastrophe is mainly due to
the radical difference in behavior between the valve implant and the surrounding cardiac
tissue. This three point bending device will give specific properties to the tissue so that the
valve replacements can be more precisely designed and will successfully integrate into the
functioning system.
The applications of this testing device extend beyond cardiac valve replacements and
will be optimal for studying a wide variety of soft tissues. These tests may facilitate in
furthering the significant understanding of soft tissues such as vascular, pulmonary or renal.
43
5.2 Societal Impact
As advances in medicine continue to extend the expected life span, patients are
constantly eager for further improvements in treatment. Patients will no longer accept that a
doctor is not capable of repairing, replacing or healing any disease or abnormality. The thought
of a cardiac valve replacement is no longer foreign to society and it is expected that these
advancements in technology will be readily available and completely successful. For this reason
many companies continue to develop more and more products, however many of them fail to
perform their proposed function. The further study of soft tissues using this three point
bending device will enable many products to be more accurately designed and implemented
into their environment. Not only will they perform their desired function, but they will also
remain successful once inside the anatomical setting.
6.0 LIFE-LONG LEARNING
The project has emphasized the integral objective of biomedical engineering; that is, the
improvement of health and life of mankind through the integration of research and technology.
Technology enhances, improves, and expands the capability of research, and research, in
return, expands and disseminates knowledge on how to improve the health of human. This
project was requested by Dr. Wei Sun in order to enhance his research on the biomechanical
interactions between host tissues and implants. Dr. Sun’s research area focuses on
understanding, through experimental and computational approaches, the biomechanical
interactions between percutaneous devices and surrounding body tissues for the treatment of
heart valve diseases, such as mitral regurgitation and aortic stenosis. He is currently working on
constructing finite element models of body tissues and percutaneous devices to simulate
tissue-device interactions when the device is implanted. Critical to this is a comprehensive and
accurate description of material behavior. Experimental testing is, thus, necessary to provide
data for the comprehension of the dependence of tissue stress on strain and the effect of small
deformation on the different layers of tissue. The construction of the three-point bending
device will contribute tremendously to this understanding of tissue behavior and the
improvement of implant design for optimal implant performances in vivo.
This project also underscores many important engineering principles and designing
aspects that the team will use in their future career. Such principles and aspects include taking
into consideration, during the designing process, the safety, maintainability, sustainability,
marketability, etc., of the device. These considerations can tremendously affect the success of
the device in terms of its functioning and marketability. The team has also learned that
sometimes, some designing aspects have to be sacrificed for the sake of other aspects. For
instance, in this particular project, the accuracy and repeatability of the data collected from the
device are extremely important in obtaining correct calculations. Thus, the mechanical
properties of the bending bar and the resolution of the CCD camera have to be exactly the
same as their desired specifications, even if such specifications result in having to spend more
money. Consequentially, the material chosen for the mounting bath has to be cheap at the
expense of its sustainability in order to keep the budget low for the device. The team has also
44
learned that safety should always be the primary consideration in designing any devices. Safety
should never be sacrificed for any other aspects of the design.
Additionally, the team learns that designing a device requires extensive planning,
organization, time, and cooperation in order to be successful. Planning sets the goals to be
accomplished in a designing project. Timing determines the amount of time and sets the time
required to perform and accomplish the goals. Organization makes certain that each goal is
carried out and accomplish in an orderly manner. And cooperation from each team member is
very critical in fulfilling all the objectives. Designing and constructing a device that functions
correctly requires a tremendous amount of work and contribution from each team member. If
one team member does not perform his or her duty, than the other team members have to
pick up the slack or the design will fail. Therefore, planning, organization, time, and team
cooperation are essential in design success; without them, the project will be doomed to
failure.
This project has also improved the team’s understanding of nonlinear biomechanics.
The team learned that soft tissues consist of various layers of collagen fiber and each layer
deformed differently to an applied load. Moreover, in order to characterize and quantify the
material properties of soft tissues, experimental testing to obtain material parameters is
essential. In addition, this project has also improved the team’s LabVIEW programming skill.
This project will make extensive use of the LabVIEW program, and by the completion of this
project, our knowledge of LabVIEW and programming skill will be dramatically expanded and
improved.
Furthermore, the team also learns that success is also based upon failure. In the
designing process, failure is inevitable as mistakes will be made and some goals will not be
accomplished. However, failure will not deter the team’s determination. We will accept the
failure, learn from it, and continuously try our best throughout this project to make the device
works. Ultimately, throughout this design project, we will learn valuable designing principles
which we will use and improve on through our engineering career.
7.0 BUDGET
The cost of this project was close to $3000. Our allowed budget was around $5000, but
throughout this project, we managed to keep the cost down by using materials that were
already available, recycling materials, and finding materials that are the cheapest but have all
the specifications that were needed.
The majority of the money was used to purchase the parts for the motor, the camera,
and the breadboard. The linear actuator was an expensive component of the motor system,
while the camera was the most expensive component of the whole device. The reason why we
purchased such an expensive camera was because the specifications we required for the
camera are extremely strict. We required that the camera has a large field of view while at the
same time able to resolved markers that are approximately 2 μm in size. Due to these strict
specifications, we were able to only find a few cameras that fit. These cameras, however, came
with a very expensive price tag. The camera we purchased was the cheapest of all the available
cameras.
45
Of all the components, the aluminum breadboard was a last addition. It was quite
unnecessary, actually. However, the sponsor wants the whole device to be assembled on the
breadboard such that it looks professional and organized. Thus, he allowed us to purchase the
breadboard. Without the breadboard, our expense would have been cheaper, but overall, we
were quite below our allowed budget. Table 4 shows a summary of the budget.
Table 4: Summary of Budget
ITEM / PART COST
Motor
- Linear Actuator
- NEMA 11 Motor
- MID Connector
$534
$86
$60
Camera
- Computar Lens
- Extension Tube Kit
- 5 MP 12-bit Monochrome CMOS camera
$122
$53
$749
Flow Loop
- Nylon Tube
- Pondmaster Pump
$4
$54
Heater
- Screw Plug Heater
- Thermostat
- Knob
- Housing for Thermostat
$113
$50
$5
$21
Aluminum Breadboard $537
Miscellaneous (materials, etc.) $500
TOTAL $2888
8.0 TEAM MEMBERS CONTRIBUTIONS TO THE PROJECT
Michael Harman
My responsibilities for this project were mainly focused on the mechanical
aspects of the device, while Xuan worked on the imaging and Eric processed the coding
necessary to perform the calculations for the desired mechanical properties. There were three
46
distinct areas of the project that required the majority of my time. The CCD camera, along with
the bending bar, both needed mounting systems, and the temperature control loop needed to
be constructed.
The camera mount was constructed from 3/8 inch thick aluminum plating, a 1/2
inch thick stainless steel bar and a variety of hardware. The necessary functions were that it
adequately and safely held the camera, was adjustable in height and position, could swivel from
the testing to calibration position and be cost effective in the system. The camera is supported
with a horseshoe shaped attachment to add stability. The height is adjustable using two wing
nuts in a sliding track and the entire stand can be moved using the tracks machined into the
base. The entire camera mount can swivel using a push pin system.
The bending bar mount needed to be able to attached the stage of the linear
actuator and move the bending and reference bars towards the tissue at the correct height and
location. This was again made from ¼ inch thick aluminum bars, utilizing a set screw setup to
hold the bending and reference bars. In this way the actual height of the bending bars could be
adjusted for different tissues. These bars also needed to be set on a slight angle so that the
camera could still have a good view of the testing.
For the temperature control loop I reconstructed the mounting bath several
times to accommodate new features of the device. I decided on multiple different heater
setups before I found the copper sheath immersion heater, which works perfectly in the device.
I also researched and found an appropriate centrifugal pump for a low price and constructed
the actual flow loop using ½” ID ¾” OD flexible tubing.
Minh Xuan Nguyen
My contributions to this project were primarily working on setting up the motor
system and writing the main LabVIEW program. I purchased all the necessary components for
the motor, such as the motor, the linear actuator, and the MID connector which connects the
motor to the power unit. In addition, I connected the motor to the power unit, and the linear
actuator to the motor.
The majority of my contribution was in writing the LabVIEW program. The
program was an extremely extensive program. I spent a lot of time trying to use LabVIEW to
communicate to each of the individual hardware, such as the motor and the camera. After
LabVIEW was able to communicate to the hardware, I had to try to integrate the hardware
such that they can work together and simultaneously within the program.
My other contributions included mainly administrate works. I setup the format
for the reports and maintained the team’s website. In addition, I helped updated the final
report and wrote part of the operator’s manual.
Eric Sirois
My contributions to the project include the theoretical framework for the tissue
bending process, the theoretical framework for the selection of flow loop components, the
theoretical framework for the selection of the CCD camera, the safety analysis of the device, an
analysis of the realistic constraints for the device, and implementation of the calculations
performed for both the flexural rigidity and transmural strain portions of the device.
47
The theoretical framework for the tissue bending process included a
review of the literature to determine previously employed methods and their limitations. I
determined that two groups had successfully measured tissue bending, but that neither fully
accomplished all goals for our project. I combined the methods used in the two approaches to
yield one new approach.
The theoretical framework for selection of the flow loop components
yielded a minimum heating element capacity and pump flowrate requirement. This was done
by first analyzing the heat that would be lost to ambient from the inner bath. I then calculated
the heat flux required into the inner bath from the outer bath (through the Lexan wall). This
information, combined with the heat loss of the outer bath to ambient allowed me to calculate
the amount of heat that would be required to maintain optimal system temperature.
The theoretical framework for the selection of the CCD camera involved
calculating pixel size from our required resolution and determining the number of pixels that
would be necessary to take data on the entire field (also determined from our initial
specifications). I was also responsible for the eventual purchase of the CCD camera based upon
these specifications and the wishes of the client for future flexibility in use of the camera.
Safety analysis and realistic constraint analysis are fairly self-explanatory.
I have also been responsible for implementing the software that performs the calculations and
output for both the flexural rigidity and transmural strain portions of the device. I chose to use
the Matlab programming environment for its flexibility and simplified integration with LabView,
which was used for the interface with all hardware components. The Matlab user-defined
subroutines consisted of 78 unique functions adding up to thousands of lines of custom-written
code. The major components of the software include a real-time finite element meshing
program (Fig. 28a), a real-time finite element calculation program, and a commercial equivalent
blended strain graphical output (Fig. 28b). The software is also capable of performing all
specified calculations and functionalities.
48
Figure 28 – Output from the meshing subroutine (a) and finite element strain calculation
(magnitude of strain is shown here) (b). Both subroutines are built and output to the
user in real time base on point data input from a CCD camera and processed by LabView
code.
Although I have done much work, each item represents a collaborative effort
from all team members and our client. As an example, the calculations for the flow loop
required extensive details of the dimensions of the baths and the materials to be used. Both of
these items were provided during numerous conversations with Michael Harman, who
designed the baths. The calculations to determine CCD camera requirements involved many
conversations with Xuan Nguyen about the capabilities of the automation system. I would
personally like to thank Dr. Claire Gloeckner for her insights into the flexural rigidity
calculations, Dr. Miles Li of Mightex systems for his help in selecting both a CCD camera and
lens, David Kaputa for his assistance with integration of LabView and Matlab, David Price for his
many suggestions and insights, including CCD camera selection and LabView integration, and of
course Dr. Wei Sun for his insights on finite element meshing and calculation, as well as for his
support for and direction of the project. There are many more such examples.
a.
b.
49
9.0 CONCLUSION
This 3-Point Bending Device, as designed, will be a novel tool for research scientists to
conduct mechanical analysis on soft biological tissue specimens. The device will output to the
user, in real time, the requested tissue response. The user will be able to select from an
extensive list of output variables, including stress vs. strain and location of the neutral axis. This
data will be useful because accurate stress vs. strain data in the low strain region (<5%) is not
obtainable using traditional soft testing methods such as biaxial and uniaxial mechanical
testing. Additionally, these traditional tests provide little or no insight into the contribution of
various layers of a tissue specimen to the overall response, but outputs such as the neutral axis
location provide this valuable information. The data provided by the test device will also be
accurate and repeatable due to the environmental control systems (including chemical control,
salinity control, pH control, and temperature control), calibration system, and high resolution
CCD camera.
The system functions by using metal bending bars of varying thickness and stiffness to
deform the test specimen. The force applied is measured by use of a built-in calibration and
calculation system. In this system, the effective Young’s modulus of the bending bar is
determined prior to each experiment and the displacement of the bending bar from a reference
bar is used to calculate the force that the tissue specimen exerts upon the bar (due to force
equilibrium, this is also the force that the bar exerts on the tissue).
The bar displacement and tissue displacement are tracked and reported to the
controlling software system in real time by a high-resolution CCD camera. The camera is
capable of resolving tissue markers as small as 10 microns in diameter. It also has a field of
vision large enough to encompass the undeformed test specimen, bending bar, reference bar,
both stationary posts, and the deformed test specimen.
The CCD camera gives marker position as an input to the governing LabView program.
This program uses specific marker information, such as post location, tissue thickness, tissue
edge marker location, and transmural marker location to determine the tissue dimensions and
radius of curvature. When combined with the force applied to the tissue, this information can
be used to calculate bending moment, stress, strain, strain rate, and transmural strain (for use
in determining the location of the neutral axis). The program also controls several important
hardware systems, including stepper motor rotation for bending bar movement, and heating
element operation for control of test bath temperature.
This 3-Point Bending Device represents advancement in imaging and automation
capabilities over other soft tissue bending devices that have been previously described. The
result is a device that provides the user with valuable testing information about low strain
material properties and transmural strain distribution. This combination of (relatively) large
scale measurement and micro-scale measurement was not previously possible due to the
limitations of CCD cameras and precise automation controls. The enhanced features of this
device, in combination with the secondary features added to provide reliable, repeatable data,
will make this a valuable addition to any tissue mechanics laboratory.
50
10.0 REFERENCES
1. Mirnajafi, Ali, et al. The Effects of Collagen Fiber Orientation on the Flexural
Properties of Pericardial Heterograft Biomaterials. Biomaterials 26 (2005) 795-804.
2. Yu, Qilian, et al. Neutral Axis Location in Bending and Young’s Modulus of Different
Layers of Arterial Wall. The American Physiological Society (1993) H52 –H60.
3. Gloeckner, Claire, et al. Effects of Mechanical Fatigue on the Bending Properties of
the Porcine Bioprosthetic Heart Valve. ASAIO Journal (1998).
4. Fung, Y.C., Biomechanics: Mechanical Properties of Living Tissues. 2nd ed. 1993, New
York: Springer Verlag. 568.
5. Bathe, K.J., Finite Elements Proceedures in Engineering Analysis. 1982, Englewood
Cliffs,NJ: Prentice-Hall.
6. National Instrument. [www.ni.com]
7. Danaher Motion. [www.danahermotion.com]
8. Bishop, Robert. Learning with LabVIEW 8. Pearson Education, Inc.; Upper Saddle
River, NJ, 2007.
9. Lexan. [www.polymerplastics.com/transparents_lexan.shtml]
10. Digital Camera Resolution. [http://www.howstuffworks.com/digital-camera2.htm]
11. Charge-coupled device. [http://en.wikipedia.org/wiki/Charge-coupled_device]
12. CCD Camera. [http://www.sentechamerica.com/]
13. Temperature Controller.
[http://www.omega.com/prodinfo/temperaturecontrollers.html]
14. Force Sensing Probes. [http://www.femtotools.com/force.htm]
15. Linear Actuators. [http://deltron.com/image_search/dril-down.php?g=20]
16. NEMA 11 Motor.
[http://www.linengineering.com/line/contents/stepmotors/211.aspx]
17. Micro Force Sensor. [http://sensorone.com/AE801_Home.asp]
18. FlexiForce Force Sensor. [http://www.tekscan.com/flexiforce/flexiforce.html]
19. Engineering Search Engine. [http://www.globalspec.com]
20. DL Linear Actuator. [http://deltron.com/DL_Linear_Actuators.html]
21. NEMA 11 Stepper Motor.
[http://www.linengineering.com/line/contents/stepmotors/211.aspx]
11.0 ACKNOWLEDGMENTS
The team would like to thank Dr. Wei Sun for sponsoring this project and for his
constructive feedbacks and advices.
Thanks Dr. John Enderle for guiding the team in the right direction toward engineering
design and providing very useful information.
Thanks Dave Price for providing many constructive feedbacks throughout the whole
designing process.
51
The team would also like to acknowledge the following people for their helps,
information, and suggestions: Dr. Claire Goeckner, Dave Kaputa, Thuy Pham, Dr. Miles Li
and Steve Stagon.
12.0 APPENDIX
12.1 Updated Specifications
Physical:
Tissue Mounting Size
Thickness 100-400 μm
Width 3-5 mm
Length (maximum) 30 mm
Minimum image points on tissue specimen 30
Tissue Displacement
Distance
In direction of tissue curvature 0.5 mm
Against direction of tissue curvature 2 mm
Time 5 seconds
Mounting Bath
Inner Bath
Length 85 mm
Width 85 mm
Height 20 mm
Thickness 5 mm
Outer Bath
Length 115 mm
Width 115 mm
Height 30 mm
Thickness 5 mm
Electrical:
Input voltage 1φ, 115V AC
Input current 5 A
Mechanical:
Applied Load (maximum) 10 grams
Force Measurement Accuracy 0.001 grams
Motor
Torque 0.42 lb-ft
Resolution 1.8 degree
Software:
User Interfaces Keyboard, Mouse
Hardware Interfaces Monitor, Data Acquisition,
Equipment, Camera
Resolution Capability (minimum) 640 x 480 pixels
52
Spatial Resolution 30 μm / pixel
Communication Protocols USB, PCI, PXI
Computer Requirements
Operating System XP or higher
Processor 2 GHz Intel Core 2
Memory 1 GHz
Environmental
Optimum Operating Temperature 37 ± 1°C
Optimum Operating pH 7.4
Operating Environment Indoor
12.2 Other Data
Dimensions for NEMA 11 Motor (inches)
CAD Drawings for DL 15 Linear Actuator