Multidisciplinary Senior Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: P13022
BREAKAWAY PORT FOR THE REDUCTION OF TRAUMA IN VENTRICULAR
ASSIST DEVICE DRIVELINES
Nicholas Dominesey
Mechanical Engineering
Matthew Myers
Industrial & Systems
Engineering
Michael Edson
Electrical Engineering
Jason Inman
Electrical Engineering
Elizabeth Sanford
Mechanical Engineering
Christopher Smith
Mechanical Engineering
ABSTRACT
A breakaway port was designed and created to reduce trauma and infection in ventricular assist device (VAD)
patients. The driveline is the cable that connects the internal pump to the external controller and battery supply, and
often becomes infected due to traumatic forces. A breakaway port for a left ventricular assist device (LVAD)
driveline does not currently exist on the market. The port has been designed to disconnect when a certain force is
applied. This disconnect will activate an internal control system that will keep the pump running with a small
internal battery supply until the driveline is reattached. Preliminary tests show that the force needed to disconnect
the breakaway port is 3 N, and the internal system runs for over 45 minutes, which are both within the customer
needs.
BACKGROUND
Heart failure is a major problem in the medical community due to the lack of good treatment options such as
heart transplants. In the United States, about 5.7 million people have heart failure, and it is a contributing cause to
over 280,000 deaths per year. One treatment option is a VAD, which works in conjunction with a failing heart to
pump blood throughout the body. This is considered a bridge-to-transplant implant that helps increase survival and
quality of life until a transplant is available. Due to the low availability of hearts to transplant, many VAD users
have the device for several years, leading to many complications.
One of the major complications of the VAD is infection due to the transcutaneous (travels through the skin)
power cable or driveline. This driveline site must be closely monitored and treated to keep the infection to a
minimum; otherwise it can spread along the cable causing sepsis (severe inflammatory response). The driveline
connects the pump (inside the body) to the motor controller, which is usually worn on the belt. The main
contributing factor in driveline infection is traumatic forces on the cable that break the interface between the skin
tissue and the velour coating of the driveline. This can happen easily because the cable length is short,
approximately one foot outside of the body, which is not long enough to reach the floor if the motor controller
and/or battery pack unit is dropped (see Figure 1).
To reduce these traumatic forces at the driveline, a breakaway device is proposed to help minimize infection
rates. A driveline port will be implemented at the transcutaneous site. This port connects to the motor controller unit
and battery packs through a cable that attaches to the port during normal usage. When the tension in the cable
reaches a certain limit to cause traumatic forces at the port, the cable will disconnect, preventing the transmission of
the tension. This should theoretically reduce the rate of driveline infections, which will in turn save a lot of money
due to faster recovery time after the surgery and less clinical monitoring. Since VAD patients have very weak hearts,
a backup power source and motor controller unit will need to be implemented. These devices can be implanted along
with the pump and allow for support until the cable can be reconnected. The backup support should last long enough
for the patient to seek assistance if the main support fails.
Copyright © 2013 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
Page 2
Figure 1 – Current Thoratec HeartMate II LVAD (image courtesy of thoratec.com)
PROCESS
Table 1 - Customer Needs:
CN1
Reduce the magnitude of traumatic forces on the port connection that passes through the skin.
CN2
Reduce inflammation and infection incidence at the interface between the skin and the
foreign body.
CN3
Assist device pump always has a direct connection to a power source and a motor controller
signal (an internal battery and motor controller must be implemented).
CN4
Port makes a reliable connection for the Motor Controller signal while connected and does
not disrupt the signal.
CN6
Driveline cable has degree of flexibility between the port connection and the motor
connection to reduce fatigue failure (this is the segment of the driveline that is internal).
CN7
Connecting the external driveline to the transcutaneous port is easy to make, especially for
elderly patients.
CN8
Backup power lasts long enough for the user to seek assistance.
CN9
Internal motor controller and battery are small enough to be implanted along with the pump
in one of the peritoneal cavities.
CN11
Internal motor controller and battery need to be able to handle the environment of body:
temperature (~100o).
*Note: Customer Needs 5 and 10 were removed because this alpha prototype will not be implantable.
Table 2 - Engineering Specifications
Spec ID CN ID(s) Description
S1
CN1,2,7
Force to disconnect
S2
CN6
Spring constant of cable
Marginal Value
3N
--
Ideal Value
1N
750 N/m
S3
CN8
Backup support time
15 minutes
30 minutes
S4
CN3
Power transmitted
10 Watts
8 Watts
S5
CN3,4,8
Recharge time
120 minutes
15 minutes
S6
CN9
Volume of internal unit
500 cm3
100 cm3
S7
CN3
Battery Voltage
14 Volts (DC)
12 Volts (DC)
S8
CN4
Backup power switchover time
< 3 seconds
< 1 seconds
S10
CN11
Operating Heat Flux
40 mW/cm2
0 mW/cm2
*Note: Specifications 9 and 11 were removed because this alpha prototype will not be implantable.
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Proceedings of the Multidisciplinary Senior Design Conference
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Assumptions Made
1. Device is an alpha prototype and therefore will not be fully implantable when complete.
2. Thoratec Heartmate II pump and external controller cannot be modified in any way.
3. Driveline may be modified, but must still connect to and control pump.
Functional Decomposition
Figure 2 - Functional Decomposition Diagram
Concept Selections
Figure 3 - Concept Generation and Selections
Heat Analysis
It is known that the body will adapt to some temperature range and that the heat flux into the body is a more
important factor to monitor. Based on research, the heat flux should not exceed 80mW/cm2. This is the value before
damage is done to the tissue. In order to ensure a factor of safety of 2, the team designed the device for a limit of 40
mW/cm2.
It was assumed that the body was at steady state and that the device was only an additional heat source. Thus,
the analysis doesn’t include the metabolic heat generation of the body nor does it include the perfusion rate or
circulation of heat due to the blood.
Copyright © 2013 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
Page 4
A simple one-dimensional energy balance analysis was conducted using a total temperature resistance
relationship. This was used to find the temperature on the surface of the device for a range of heat generation, and
the maximum heat generation allowed from this analysis appears to be about 0.6W.
In order to verify this analysis, an equation for the temperature distribution was found. The results show that the
device would safely be able to generate 2 W of heat energy while remaining at 26mW/cm2, well below the
constraint. In general the temperature distribution solution seems to fit a model better than the previous solutions.
The problem still remains that on the right side of the device, the initial temperature and heat generation of the
human body are not accounted for. Also, this is not the worst case scenario because if the left side of the device were
to be adiabatic, then the temperature would be unknown and thus, a solution could not be obtained. Finally, this
model neglects the heat generation in the body but assumes that the body’s energy was used to bring the device
temperature up to body temperature. It is assumed that the device reaches steady state body temperature and can
then be neglected in the solution.
Figure 4: Results of the ANSYS Analysis – Contour Plot of Heat Flux for 2 W of Heat Generation. 166.326 on the
scale corresponds to 16.6mW/cm2, which is the maximum value and occurs at the corners of the device.
In the body, the maximum heat flux that occurs naturally is about 11.2mW/cm2. The 2D analysis results show
that a much smaller heat flux can be obtained for the same amounts of heat generation than from the 1D analysis.
In the ANSYS analysis, the temperature appears to be relatively high. This is not likely to pose too much of an
issue because of the assumptions made. In theory, this analysis seems quite simple but the body is truly difficult to
model, so the result of this analysis is likely to have a large amount of error.
From the ANSYS model, it can be concluded that the device is not likely to pose a threat to the body when
generating up to 2W of heat energy. The factor of safety between the results and our limitations is about 4, which
leaves an allowance of plenty more heat generation within the internal device.
Force Pull Test
A small experiment was conducted to determine the specification for the breakaway force. Skin in the abdomen
area was pulled and measured with a force gauge. It was determined that at 1 N of force, the patient became a bit
uncomfortable, and at 3 N the force was beginning to be painful. Based on multiple iterations of this test, our
specification for breakaway force is ideally 1 N, with a marginal value of 3 N.
Electrical Programming
Internal microcontroller:
The internal microcontroller’s primary purpose is to send a periodic signal to the motor controller in the event
of a disconnect in order to run the pump. This is performed using the Timer A module available on the
microcontroller. In this setup, Timer A is run in continuous mode and counts all the way to 0xFFFF. The register
TACCR0 is used to determine the width of the actual pulse and the output is 3.3 V during the duration of 0 to
Project P13022
Proceedings of the Multidisciplinary Senior Design Conference
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TACCR0. From TACCR0 to 0xFFFF the pulse is 0. With this method, the correct duty cycle can be achieved
needed by the motor controller.
This microcontroller is also in charge of monitoring the internal battery during disconnect. This is done by
monitoring the voltage of the battery using the analog to digital converter on the microcontroller. This information is
then put in an ASCII string and sent wirelessly to the external microcontroller.
The final function is to monitor the temperature of the circuit during the charge cycle, as this produces the most
heat. This is performed using voltage division between a 10 kΩ resistor and a thermistor rated for 10 kΩ getting
power from the 5 V regulator such that the output is nominally 2.5 V. This voltage fluctuates based on the resistance
of the thermistor which changes with temperature. This voltage is also read using an analog to digital converter and
compared to an equation to determine the temperature.
External microcontroller:
The primary purpose of the external microcontroller is to receive the battery voltage data using the UART and
the receiver and use the voltage to determine an approximate lifetime remaining. The lifetime (in minutes) is then
displayed on the two 7-segment displays for the user. When there are 30 minutes remaining, one beep from the
alarm will occur. When there are 15 minutes, two beeps will be given. When the lifetime reaches zero, a solid beep
is heard until reconnected. There is also a 15 minute safety factor built in, such that when the display says that there
are zero minutes remaining there are actually 15 minutes of runtime left. This microcontroller also has access to a
relay, which is used to toggle the alarm on and off as needed.
Voltage Regulators:
Four voltage regulator circuits are being used for this project. Two of these are the 3.3 V supplies for the
microcontrollers on the internal and external boards. There is also a 5 V supply for the receiver circuit on the
external board. These three regulators are all very similar Buck converters, and use the same external component
(capacitors, inductors, and diodes) to ensure low ripple voltage and quick recovery times. The internal transmitter
receives its 5 V power from the motor controller in order to save layout space. The other regulator is a 15 V boost
circuit that converts the external battery power supply to 15 V for use in charging the battery. The external
components were chosen through several calculations guided by the datasheet for the part (LM2577SX). Testing has
shown that these regulators perform well under load conditions shown in the schematics.
Battery Selection
Batteries have been chosen for this project to minimize size and weight while also achieving the 30 minute
runtime required by the specifications. Cost could also have an impact but was not important for the design. 4.32
Whr batteries were chosen for the prototype design and have an approximate volume of 100 cm3. If the total voltage
is 12.8 V roughly and the current draw is 0.5 A, the power usage is 6.4 W, which gives a runtime of roughly 162
minutes. AA size batteries were chosen as the ideal size for the final design with a charge rating of 600mA*hr.
Port Design and Selection
Figure 5: Top 2 Port Designs (female sides pictured only) – Angled port (left) is rigidly connected to the body (flat
plate sits flat on abdomen). Free port (right) has length of wire outside body, which would be taped to abdomen at a
few locations (no flat plate was used in final build).
Assumptions for both ports: Port is aligned vertically (points straight towards ground). Wire has some degree of
stiffness (not completely flimsy). Subjects do not sleep on their stomachs.
Angled Port:
Pros: Port is easier to connect because of angle. Female end is less likely to catch on foreign objects. Port has
greater comfort and is more compact because there is less material on the outside of body.
Copyright © 2013 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
Page 6
Cons: Male end sticks out from body due to angle. Port may be harder to clean because the base is flat against
body. When bending over, the port may cause large bending moments on the driveline and cause it to disconnect.
Free Port:
Pros: Port has some degree of motion and flexibility. Port is easier to connect because it is easy to see both ends
of port. Port is easier to clean because no part is permanently attached to body. When bending over, port will move
and cause smaller bending moments on port connection. Tape relieves pressure from transcutaneous site.
Cons: Freedom of wire increases possibility of wire catching on objects. Port requires more maintenance
because of the tape coming loose. Port might be less comfortable because of the material outside of the body. Tape
may unexpectedly detach.
Port Selection:
The free port was chosen as the final design mainly because it would be easier to connect for the patient and
because it would not cause any additional infection. It will also be more comfortable to wear for the patient.
Geriatric Considerations
Less than half of one percent of all LVAD users are below the age of 50, so some considerations of the elderly
population must be accounted for in our design. Many of these patients suffer from some form of arthritis, which
will impair their ability to grasp small objects or use a large amount of pinching force. Eyesight is also a concern,
since one third of the population over 65 will lose some portion of their vision. Most commonly, blurred or distorted
vision occurs, but sometimes blind spots can form in the eye, causing more vision loss. Lastly, hearing loss is a
concern accounted for in our design.
To account for the arthritis, the team designed a port that is easy to grasp and does not take much pinching force
to disconnect. A bright stripe of color along the port and a bump on either side would help with poor eyesight, but
these details did not make it into the final design. Finally, a vibrating motor was placed in the internal device to alert
the user of the battery status when they are disconnected. This will address the hearing loss issue.
RESULTS AND DISCUSSION
Final Prototype
Internal Device Construction
Since the internal device will not be implanted, the case was made out of clear plastic because the material was
readily available. The size of the internal electrical components was reduced greatly from the initial prototype to the
final build.
Figure 6: Internal Device CAD drawing
External Device Construction
An external device case was constructed to hold the external electrical components, switch and display (shown
below). The case was made out of sheet metal into a triangular form to fit against the existing motor controller
device. The edges were welded to create a very sturdy and durable case.
Breakaway Port and Cable Construction
The free port design was constructed from an existing breakaway port. The force to disconnect the port
originally measured too high, so the port was machined to lower the contact friction until the 3 N force was
obtained. Pins were inserted into existing holes in the port connection to connect the necessary wires across the port.
10-strand cable was obtained and utilized for the connections between internal and external devices.
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Proceedings of the Multidisciplinary Senior Design Conference
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Figure 7: Device Final Build – Internal device and port connection (left) and external device (gray) attached to
existing motor controller with port connection (right).
Testing
Table 3 - Testing Plan:
Spec# Description
Testing Plan
Marginal Value Ideal Value Results
S1
Force to
disconnect
S2
Spring constant
of cable
Backup support
time
S3
S4
Power
transmitted
S5
Recharge time
S6
Volume of
internal unit
Battery voltage
S7
S8
Backup power
switchover time
S10
Operating Heat
Flux
Use force gauge to pull breakaway
cord and measure different
speeds/methods of pulling off cable
Measure deflection of cable
3N
1N
3N
--
750 N/m
25 N/m
Unplug external system and keep
track of how long internal system
lasts
Check to see if pump is running on
back-up power and under normal
conditions
Let internal system run out of power,
then plug back in and measure how
long until fully charged
Use calipers to measure dimensions
15 minutes
30 minutes
51 min*
10 Watts
8 Watts
Acceptable
120 minutes
15 minutes
23 min*
500 cm3
100 cm3
370 cm3
Check voltage with a multimeter
(see if it is 12.8 volts +/- 1.2)
Check with a stop watch to time how
long the pump is not running
14 Volts (DC)
< 3 seconds
12 Volts
(DC)
< 1 seconds
14 Volts
(DC)
Not tested
Place thermocouple on outside of
internal unit, possibly while unit is
inside simulated body tissue
40 mW/cm2
0 mW/cm2
46.2
mW/cm2
*Values were extrapolated from 18650 cell batteries to the AA size batteries chosen for the design. The 18650 batteries were
used in the final design due to recharge board soldering issues.
Internal Microcontroller:
The internal controller at its present state has the ability to power the motor controller while running on the
internal batteries. It sends out the required pulses to the motor controller to initiate and sustain the pump. Also, the
vibration function was proven to work in testing.
External Microcontroller:
Currently, the external controller was shown to be able to display numbers from a table on the LED displays.
An approximate battery lifetime data chart was placed in the code and the lifetime in minutes can be displayed to the
user. Also, the relays trigger when the countdown reaches 0, which should alert the user that they are running out of
time.
Connection with P13021:
An additional port was added to the internal case so that wireless energy transfer team could actually plug in
there device to our circuit. Testing with their equipment was successful, and the internal components successfully
powered the pump using this connection.
Copyright © 2013 Rochester Institute of Technology
Proceedings of the Multidisciplinary Senior Design Conference
Page 8
PCB Design:
The final PCB design is smaller compared to the prototype with the idea of creating a small implant. Enough
parts were ordered to make two of each circuit board as a backup.
External Device:
The external device experienced noise on the input power line. This noise resulted in the regulators on the
external board and the existing external controller malfunctioning. On the 3.3V regulator on the external board, 1.1V
was seen when in system with the other components. However, when the board was directly connected to the power
line, it performed as designed.
CONCLUSIONS AND RECOMMENDATIONS
Overall, the project was successful. As an alpha prototype, this device shows that the idea is achievable as a
proof of concept. Although not all specifications were met, the device functions properly.
Internal Microcontroller:
The programming for temperature sensing and battery life determination was not completed due to time
restraints and lack of programming knowledge. The circuitry is all present for the programming to work though. The
temperature is read as a voltage division between a resistor and a thermistor. This feeds into an ADC that can be
used in the program to stop charging or turn off the device. The same is done for the battery voltage.
With more time, the UART would have been used on both of the microcontrollers to send and receive battery
life data. The transmitter and receiver are in place on the boards and are located at the UART pins of the controllers.
In theory, all that is required is the programming for the device to operate as desired.
Batteries:
AA size batteries for the internal device were not implemented in the final design due to an unexpected shortage
of charging regulator boards. Instead, older batteries wrapped in electric tape were used as a quick fix. With more
time, another charging board could be ordered and the smaller AA size batteries could be used as well as the cradle.
Time testing would then need to be performed on these batteries to ensure they still meet the specs. It may also be
possible to decrease the size of the implant by finding smaller batteries.
External Electronics:
More testing could be done with the alarm feature using the microcontroller and the programming for receiving
the data was not completed. Once again, the UART should be usable for the programming.
Increased capacitance on the input power line may fix the power issues exhibited by the external controller
boards. Possibly the use of a wall-mounted recharging system for use on the internal implant could be implemented
in order to separate the power demand on the power device as well as prolong external battery life. Based on the
numbers found in testing, the external device battery life has been reduced by roughly 25%.
TI Motor Controller:
It is possible to reduce a large amount of internal circuitry by using the TI motor controller chip. This was
attempted at the start of the quarter, but due to lack of programming knowledge was passed up for the
microcontroller. If an EE or CE with good programming knowledge were to take part in the next iteration of the
project, it is a strong recommendation that they attempt using this device. With it, the implant could become half of
its current size.
Internal Device Case:
The device is larger than desired and retains some heat, but it is acceptable since it is the first prototype.
External Device Case:
The case was very sturdy, but ultimately it could have been more compact, less bulky, lighter, and the design
could have been laid out in a way that utilized the space more efficiently.
Breakaway Port:
The breakaway port is bulky and heavier than originally intended, so a smaller waterproof port could have been
purchased and utilized in the design. Color and raised surfaces could have been used for easier alignment, especially
for geriatric patient consideration. If a wireless signal could have been obtained, only power would need to be sent
through port, reducing wire size and possibly port size as well.
Project P13022
Proceedings of the Multidisciplinary Senior Design Conference
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REFERENCES
[1] Bergman, T L, and Incropera, F P, 2011, "Example 1.6, Example 3.12," Fundamentals of Heat and Mass
Transfer, pp. 26+.
[2] Wolf, P D, 2008, "Thermal Consideration for the Design of an Implanted Cortical Brain-Machine Interface,"
U.S. National Library of Medicine.
[3] Miller, L W, 2011, "Left Ventricular Assist Devices Are Underutilized," Circulation, pp.1552-558.
[4] Roger, V L, 2011, “AHA Statistical Update,” Circulation, 125, pp. e2-e220.
[5] Henriksen, K, 2009, “The Human Factors of Home Health Care: A Conceptual Model for Examining Safety and
Quality Concerns,” J Patient Saf, 5 (4), pp. 229-236.
[6] Quillen, D, 1999, “Common Causes of Vision Loss in Elderly Patients,” Am Fam Physician, 60 (1), pp. 99-108.
[7] McNerney, M, "Human Factors Considerations in System Design for the Elderly," San Diego State University.
[8] "Engineering ToolBox," http://www.engineeringtoolbox.com.
[9] "Thoratec - Innovative Therapies for Advanced Heart Failure," http://www.thoratec.com.
ACKNOWLEDGMENTS
Rochester Institute of Technology
Joseph Tartakoff, Graduate Consultant
Dr. Steven Day, Faculty Customer
Mr. Edward Hanzlik, Faculty Guide
Dr. Todd Massey, M.D.
Dr. Coley Duncan, M.D.
Dana Shannon, NP
Copyright © 2013 Rochester Institute of Technology