VANDERBILT UNIVERSITY
CAUTERIZATION CATHETER – AN ADVANCEMENT IN CONDUCTIVE
BIOMATERIALS AND MEDICINE
Biomedical Engineering Senior Design, 2006-2007
4/24/2007
C. BLYTH1, C. FERNANDEZ1, S. HITTINGER1, C. JONES1, B. MCGEE1
ADVISORS: B. WOOD2, MD, T. KAM2, MD
1VANDERBILT
UNIVERSITY, NASHVILLE, TN; 2NIH, BETHESDA, MD
ABSTRACT
Electrocautery is a process that utilizes high frequency waves to deliver energy in the form of
heat to surrounding tissue in order to induce coagulation safely, effectively, and efficiently.2
Increasing human tissue temperature to 42°C causes local proteins to denature leading to
completely denatured proteins at temperatures around 67-75°C. 3 Recently, radiofrequency (RF)
ablation has been used on biopsy tracts, resulting in 63% less blood loss in hepatic sites and 97%
less blood loss in renal regions of interest.11 Due to the frequent use of anticoagulants during
catheterization procedures, RF ablation is a solution to preventing internal hemorrhaging and
pseudoaneurysms after the removal of the catheter.13 This paper presents a proof of concept for
designing a catheter capable of conducting high energy radiofrequency waves constructed solely
of flexible polymers. Flexibility, biocompatibility, and efficacy of this device are crucial, yet this
device must additionally fulfill the tasks of a regular catheter; which is capable of draining fluid
from any region of interest.
Utilizing the idea of doping and its consequent increase in
conductivity of polymeric chains of conjugated bonds, the device designed will be an ideal asset
to any operating room or interventional radiology suite as they are already outfitted with RF
generators and external power sources. Heat transfer modeling equations were used to estimate
the power needed to cauterize the tissue within a 1 mm circumference around the exposed
conductive polymer. Simplifying and solving the Arrhenius Equation and Pennes Equation of
Bioheat were essential modeling components of our design process as they prove tissue damage
and perfusion results for ablation at specified radial distances. Automated Computer Design was
exploited by the group for intricate design and specific dimension modeling for machine shop
preparation. Based on the mathematical modeling, and the ability to heat tissue at specific radial
distances, there is a bright future and demand for an effective polymeric cauterizing catheter.
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CAUTERIZATION CATHETER – AN ADVANCEMENT IN CONDUCTIVE BIOMATERIALS AND
MEDICINE
C. BLYTH1, C. FERNANDEZ1, S. HITTINGER1, C. JONES1, B. MCGEE1
ADVISORS: B. WOOD2, MD, T. KAM2, MD
1VANDERBILT
UNIVERSITY, NASHVILLE, TN; 2NIH, BETHESDA, MD
INTRODUCTION
Cauterizing techniques are used in nearly every surgical procedure that involves cutting
through tissue.
Heating and destroying tissue induces coagulation which locally stops
hemorrhaging, limiting blood loss and potentially saving a limb, an organ, or even a life. The
first generation of cauterization involved applying live embers or burning logs directly to open
wounds.1 Heated metals destroyed damaged tissue, decreased the risk of infection, and limited
blood loss since bleeding would stop after application.2
Although unorthodox by health
standards today, burning logs and heating metal objects spawned the process of cauterization.
Risks inherent to these procedures have led to advancements in medical tools resulting in the
development of standard electrocautery practices implemented by many health care
professionals.
Electrocautery is a process that utilizes high frequency waves to deliver energy in the
form of heat to surrounding tissue in order to induce coagulation safely, effectively, and
efficiently.2
Microelectrodes, placed strategically at the end of any device, allow for the
transmission of radiofrequency (RF) waves to tissue. The energy carried in an RF pulse, created
by an RF generator, travels the length of the conductive medium until it reaches the electrode
where the surrounding tissue receives the RF waves. The tissue conducts the waves poorly
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resulting in an increase in the frictional movement of ions within the tissue causing an increase in
thermal energy. Increasing the tissue temperature to 42°C causes local proteins to denature
leading to completely denatured proteins at temperatures around 67-75°C.
3
This process of
desiccating tissue rapidly and specifically is an effective tool in killing tumors in various areas of
the body, and because of its overwhelming ease of use and benefits, its application is extending
to other procedures.
Biopsy needles and tumor extraction devices currently use microelectrodes to heat and
kill surrounding tissue to prevent fluid loss and to rid patients of harmful cellular bodies. The
metallic nature of needles and biopsy devices constructs a high conductivity network allowing
electricity to run freely from a generator through the needle and to the region of interest.
Invasive cauterization devices allow a biopsy to be taken from a tumorous growth enabling a
physician to quickly decide whether the sample is benign or malignant. Cancerous growths can
be destroyed by the cauterizing device with microelectrodes inserted for administration of high
frequency energy to that tissue.4 As a result of ablation, the risk of internal bleeding from an
alternative process of resection is eliminated, the fluid loss during recovery is significantly
decreased, and the recovery time is cut to a fraction of the time of other procedures. With nearly
100,000 radiofrequency ablations of tumors performed annually and radiofrequency generators
available in most operating rooms and interventional radiology suites in the United States, there
is a significant market need for cauterizing catheter systems in percutaneous image guided
therapies by interventional radiologists.5
Many previous technologies for limiting blood loss during catheter removal have been
utilized in the past. In 1995, researchers introduced fibrin glue to act as a sealant after routine
lung biopsies minimizing blood and fluid loss.6 The fibrin sealant used contains fibrin and
4 of 48
thrombin which are essential compounds in the blood clotting cascade. Clotting occurs when
fibrinogen is converted to fibrin, a thread-like molecule that begins the clotting process, by
thrombin.6,7
By injecting fibrin and thrombin into the biopsy tract, the concentration of fibrin
and thrombin is increased rapidly to make the conversion of fibrinogen to fibrin so clotting can
occur before pneumothorax forms.8 A similar fibrin sealant has been used in swine for post liver
biopsy tract sealing.7 Both groups concluded that the approach to fluid loss and pneumothorax
prevention is effective in both patients with and without anticoagulation treatment and could be
considered for testing in other percutaneous liver biopsy procedures.7
The
current
technology is not sufficient
and requires a new generation
of research because the use of
a fibrin/thrombin sealant has
proven
cumbersome
and
costly.
These attempts to
Figure 1: Traditional epoxy injection system used for delivering fibrin and thrombin
into the catheter tract.
limit fluid and blood loss
require the physician to switch between instruments in the middle of the procedure (Figure 1).
Furthermore, the physician must be capable of uniformly injecting the clotting compound along
the length of the tract while simultaneously removing the syringe.
The idea of injecting a fibrin/collagen compound, though original, has not been accepted
on a national or international scale.7 As stated previously, biopsy needles have been equipped
with microelectrodes and used as cauterization devices.9 RF ablation has opened a number of
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Figure 2: Results from an RF ablation study showing positive results for ablation and blood
clotting.
possibilities for quickly and easily eradicating a tumor10; yet although this novel idea has become
extremely useful in diagnostics and therapy, it is limited by its lack of flexibility, its large size,
and its cost.5 The current procedure for biopsy tract RF ablation returns extremely positive
results for renal and hepatic regions of interest.11 Performing RF ablation on biopsy tracts has
resulted in 63% less blood loss in hepatic sites and 97% less blood loss in renal regions of
interest (Figure 2).11
St. Jude Medical, makers of Angio-Seal Vascular Closure Devices, sells millions of
angio-seals which are dissolvable seals used to close punctures to vessels after percutaneous
access from catheterization.8
While these devices minimize blood loss, they are cumbersome
for physicians to use. Instead, a cauterizing catheter would save time and be more cost efficient
than the currently available devices.5 Insertion of islet cells into the liver demands this new
technology.
Preparation of the patient for this process involves administration of
anticoagulation drugs such that the portal vein will not clot when the islet cells are injected.12
However, a large risk of this procedure is internal bleeding when the catheter is inserted and
removed from the portal vein. Due to the presence of anticoagulants, bleeding tends to occur at
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an increased frequency.
In patients undergoing vascular catheterization, a common
anticoagulant is heparin which thins the blood and has caused severe complications including
internal hemorrhaging after a procedure is performed with a catheter.13 Many other procedures
are performed each year requiring anticoagulants and catheterization procedures. When inserting
catheters into nephrostomy and biliary tubes, the vasculature of the region of interest is highly
variable and vessel puncture is not uncommon.5 Should a vessel be struck during an operation, a
pseudoaneurysm could result.5,13,14
This paper presents a proof of concept for designing a catheter capable of conducting
high energy radiofrequency waves constructed solely of flexible polymers. This catheter, used
for electrocautery, will be manufactured without the requirement of metal devices because
conducting metals are inflexible or too brittle. Increased flexibility will allow access to more
regions of interest. Flexibility, biocompatibility, and efficacy of this device are crucial, yet this
device must additionally fulfill the tasks of a regular catheter which is capable of draining fluid
from any region of interest. Ablation to the immediate surrounding tissue must occur quickly
and uniformly, ensuring that perfusion is not cut off to more tissue than necessary.15 The
production cost of this device will remain low, and the device can be made to fit any gauge
needle for insertion.
The versatility of the catheter will not only appeal to all medical
professionals, but will also entice patients since it is safer and ensures a quick recovery. The
economic and safety concerns of the catheter will be discussed in depth later in the paper.
METHODS
Both economic and material advantages exist for using conductive polymers instead of
metallic wires. The design calls for the cylindrical tube of the catheter to have on an inner radius
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of 0.038 inches. Because the catheter must follow a non-rigid pathway while maintaining a
length many times larger than its width, biocompatible material capable of withstanding a very
narrow diameter without breaking or weakening from repeated bending must be selected. A
ductile material must be chosen that exhibits elasticity under stress.16,17 The poor elasticity of
metals could result in fracture preventing the RF signal not reaching its target tissue at the
terminal end of the catheter. In contrast, polymers react to the stress and strain of flexing with
greater elasticity in their operating regions.16 Their material properties, most notably their tensile
strength, are not significantly affected by material deformation from bending.16 By using a
conductive polymer as the signal carrying agent, the device will be more reliable and less
expensive than current alternatives.
In a conductor, the outer electrons of the atoms are loosely bound and free to move
through the material.18 There are two requirements for conducting electricity: 1) there must be
empty valence bond orbitals (Figure 3a) that can transport the electrons, and 2) you must apply
an external electric field19. Conductivity is determined by the types of atoms in a material (the
number of protons in each atom's nucleus, determining its chemical identity) and how the atoms
are linked together with one another.20 Metals tend to be good conductors because of their high
number of free valence electrons.21 Valence electrons exist in the outermost electron shell of an
atom and have the greatest freedom to move because of their distance from the positively
charged nucleus.22
Most atoms hold on to their electrons tightly and thus are insulators. In
copper, the valence electrons are essentially free and strongly repel each other.20,22 Any external
influence which moves one of them will cause a repulsion of other electrons which propagates,
in a “domino fashion” through the conductor (Figure 3b)20. An electric current is actually a
stream of electrons jumping from one atom to the next.18 The electrons of different types of
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Figure 3: Properties of a metal conductor. Figure 3a shows that an open valence electron is required for
conduction. Figure 3b shows a cross-section of a copper wire and the effects of an external electric influence.
atoms have different degrees of freedom to move around. With some types of materials, such as
metals, the outermost electrons in the atoms are so loosely bound that they chaotically move in
the space between the atoms of that material by nothing more than the influence of roomtemperature heat energy.18 Because these virtually unbound electrons are free to leave their
respective atoms and float around in the space between adjacent atoms, they are often called free
electrons.18
In the field of metal conductors, copper and silver have almost identical
conductivities yet copper is preferred over silver as it costs less.19
Conductive polymers can be used to transmit high frequency waves. In 2000 Alan J.
Heeger, Alan G. MacDiarmid, and Hideki Shirakawa earned the Nobel Prize in Chemistry for
developing a method for creating conductive polymers.23 Previous to their work polymers were
used as insulators, but these investigators found that by doping polymers they could increase
their conductivity to similar levels of many metals. For instance, by doping polymers with
9 of 48
bromine or iodine vapor, they were able to raise the conductivity of polyacetylene to nearly the
same degree as silver when polyacetylene is at room temperature or higher as depicted in Figure
4.24 Doping increases the conductivity by altering the electrical properties of conjugated double
bonds. Conjugated double bonds exist when the backbone of the polymer alternates between
single and double bonds as depicted in Figure 5.23 Sigma bonds are fixed bonds that are unable
to move and are present in single, double, and triple bonds. In contrast, pi bonds are more
dynamic and are present in only double and
triple bonds.23 Conjugated bonds cause pi
electrons to be delocalized between the
alternating bonds which causes the double
and single bonds to act differently from the
normal behavior of isolated double and
single bonds. Doping the polymer causes
more conjugated bonds to form by either
removing or adding electrons to the polymer.
The presence of free electrons results in a
much more conductive material due to the
extra “room” that becomes available with an
Figure 4: The conductivity of doped polyacetylene as a function
of temperature and compared to that of silver.
unpaired electron. By running an electrical
current through this new backbone, a free electron can move down the molecule and transmit a
current with it. Doping increases conductivity because it creates a situation where electrons have
more freedom to move, as is inherent in conjugated bonds due to the high number of pi bonds.23
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Figure 5: Polyacetylene backbone showing alternating double and triple bonds. Conjugated double
bonds exist when double and single bonds alternate.
It is vital to manufacture a catheter from a biocompatible, conductive material since it
will be in direct contact with the tissue at the terminal end of the catheter. The polymer must
also be able to carry an electrical signal without allowing the signal to dissipate as it travels
down the catheter. The insulating material that will encase the conductive polymer must also be
biocompatible as well as electrically inert to minimize the amount of RF waves transmitted
through the insulator. Direct contact, and RF transmission with the target tissue, only occurs at
the exposed conductive polymer tip located at the terminal end of the catheter.
Basic heat transfer modeling equations were used to estimate the power needed to
cauterize the tissue within a 1 mm circumference around the exposed conductive polymer.
Equation 1 was derived (see appendix) as a means to calculate the power (P) as a function of
𝑑𝑇
change in temperature( 𝑑𝑡 ), specific heat (c), tissue density (), and volume necessary for an
effective cauterization.
Using the power calculated from Equation 1 along with known
resistivities of the tissues being cauterized, appropriate conductivity ranges can be established.
Two specific polymers, polyacetylene and polythiophene, were identified as strong candidates
for conductive polymers and, moreover, have similar conductivities to that of metals such as
silver and copper (107 Siemens per meter). These materials are also biocompatible and
inexpensive, fulfilling other device constraints.
𝑃=
𝑑𝑇
𝑑𝑡
𝑐𝜌(𝜋𝑟𝑜2 𝑙 − 𝜋𝑟𝑖2 𝑙)
Equation 1
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To describe the expected degree of tissue death thermal modeling equations are also
required.
The Pennes Equation of Bioheat and the Arrhenius Equation are complimentary
equations that can be solved after using the Laplace Transform to solve for the ∇ variables. The
Pennes Equation of Bioheat (Equation 2)15 is solved for temperature (T) after the Arrhenius
Equation (Equation 3)15 is solved for temperature at various time steps.
𝑑𝑇
𝜌𝐶 𝑑𝑡 = 𝑣(𝑘∇𝑇) + 𝜎(|∇V|)2 − 𝜌𝑏 𝐶𝑏 𝛼𝜔(𝑇 − 𝑇𝑎𝑚𝑏 ) + 𝑄𝑚
𝑐(0)
Ω(𝑡) = ln ( 𝑐(𝑡) ) = 𝐴 ∫ 𝑒
(−
∆𝐸
)
𝑅𝑇(𝑡)
𝑑𝑡
Equation 2
Equation 3
These equations are solved whereby  represents density in kg/m3, C represents the heat capacity
in J/kg K, k represents the heat conduction coefficient in W/K m,  is the tissue state coefficient
supplied by Chang et al.,  is the blood perfusion coefficient in sec-1, (t) represents the degree
of tissue injury, c represents the concentrations, R is the universal gas constant, E is the
activation energy in J/mol, and A is the frequency factor for kinetic expression with respect to
the liver as well.15
The degree of tissue damage, Ω(𝑡), found in the Arrhenius Equation, is matched to a
value of the percentage of surrounding.15 When the  value equals 4.6, 99% of the surrounding
tissue has been ablated and there is zero perfusion.15 The baseline value used for comparison is
an  value of 1 which represents 63% tissue damage and the first indication of tissue
coagulation.15
RESULTS AND DESIGN
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Flexibility of an ablating tool is significant for a successful surgery. Current ablation
techniques send an RF pulse down rigid metallic guides that extend the shaft of an ablating
biopsy needle to effectively cauterize the desired tissue. Uniting the ablation techniques of a
metallic conductor and the flexibility of a plastic catheter, an ideal resolution can be achieved.
Advancements in biomaterials as conductive polymers have shown promise for finding a flexible
substitute to the current ablating needle.
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In order to transfer a current from the RF generator to the target tissue without a metallic
conduit, this proposal employs an insulated catheter, which shields a conductive polymer, and a
non-insulated, exposed tip that delivers an RF current to ablate the tissue in the immediate
surroundings. In all designs, the fundamental requirement necessitated a continuous conductor
from the RF cable to the tip of the catheter. Designs explored the advantages of several models
varying the method in which the conductive polymer extends down the shaft of the catheter. The
two most viable ideas are shown in Figure 6. The main difference between Figure 6a and 6b are
the separate channels of conductive polymers compared to a solid, continuous ring. Although
both approaches resulted in transferring the RF current to the tip of the catheter, Figure 6b was
selected as the design in order to transmit
the highest degree of power to the tip.
With more conductive material, as well as
Figure 6: Two proposed methods for sending conductive polymers
down the shaft of the catheter. Figure 6a shows separate channels
of conductive polymer and figure 6b shows a continuous ring of
conductive polymer.
an integrated, continuous ring, more
power flow to the tip will occur in the
design model of Figure 6b than Figure 6a.
Furthermore, the continuous ring of polymer will add strength to the polymer limiting any
potential plastic deformation.
The selection of the material of the outer layer of the catheter is just as important as the
inner layer since it is in direct contact with the tissue. Also seen in Figure 6 and imperative to a
functionally sound ablating catheter is an outer insulating layer. This layer is a non-conductive,
biologically safe, polymer such as polyurethane or silicone. The purpose of this outer insulation
is two-fold. It is first is to prevent ablation to parts of the tissue tract that are unintended. The
RF current must be well shielded so that ablation only occurs in the specified areas. Over
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ablation is unnecessary and potentially harmful. The power dissipation leaving the catheter walls
1
is a relationship characterized by 𝑃 ∝ 𝑑2 where P is the power and d is the distance away from
the source. Power dissipates in a spherical pattern at a rate of that inversely proportional to the
square of the radius. Because of the nature of an RF pulse, the most important characteristic of
the insulator is the heat conductivity of the material (polyurethane’s heat conductivity is 0.02
W/mK).25
The other function of the outer insulating material is to focus the power at the tip of the
catheter.4 If the entire shaft of the catheter were exposed, this would yield unequal ablation
along the tissue tract due to inhomogeneities in the tissue and because of power loss reaching the
tip of the catheter. Ideally, all resistance is focused at the tip; moreover, the smaller the area of
the tip, the larger the resistance and therefore, the greater the ablation. The exact amount of
exposed conductor will be determined upon experimentation.
A suitable range would be
between 1-5 mm.2-4,11 The balance lies in determining the greatest amount of exposed region
without sacrificing function because of the method in which the surgeon uses the ablation
catheter. In a typical procedure, the surgeon would remove the catheter at discrete intervals,
ablating along the tract until the catheter is removed; thus leaving a fully ablated canal. The
other consideration is the smaller the ablating surface, the smaller the intervals the surgeon must
make, introducing the potential for missing a region of the tract.
The final feature of Figure 4 is a hollow shaft with an inner insulating coating. It is
important to iterate that this is a catheter and so all functions of a catheter must be intact
(including, but not limited to, drainage and the ability to house a needle). One potential motive
for the additional inner insulation is to protect the conductive polymer. If liquids or other
materials are traveling down the catheter, protecting the conductive polymer is important such
15 of 48
that there is not any damage to the polymer. It is reasonable to hypothesize that contaminants to
the conductive polymer would decrease the functionality and conductivity of the material leading
to a reduced ablating power and other complications.
The design of our catheter has been modeled in CAD and is shown in Figure 7. The
basic structure of the catheter has been previously described and is shown in both Figure 7a and
7b.
Figure 7a is a top-down projection showing the inner and outer insulating material
(polyurethane) and a middle conductive polymer (polyacetylene).
Also viewable in these
schematics is the exposed conductive tip. Figure 7b is a bottom projection displaying the
layering of the conductive and non-conductive strata. The base of the catheter, too, is exposed at
the site where the RF current will be transmitted from the connection piece (Figure 7d) to the
catheter.
This design was modeled after a Torcon NB Advantage Angiographic 5F Catheter,
manufactured by Cook Inc.
From the supplied catheter, two key components have been
modified in the model and shown in Figures 7c and 7d. The first, a generic plastic cap (7c), has
been redesigned such that the catheter will fit flush against the top (extending out and through
the small opening) and may be screwed tightly to the top of the RF connection piece (7d). The
coupling of the two pieces allows for a solid union between the inner conductive polymer of the
catheter and the transmitting conductive polymer of the RF coupler. The RF coupler (7d) is also
a generic plastic housing but includes a conductive polymer core which efficiently transmits the
RF signal to the catheter by means of a flush connection established by the catheter cap (7c).
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Figure 7: CAD modeling of the conductive polymer catheter. Figure 7a shows a top-down
projection focusing on the exposed conductive tip. Figure 7b is a bottom projection showing
the opening of the catheter as well as the point at which the RF current will connect with the
catheter to deliver the RF pulse. Figure 7c is a cap that will ensure that the catheter is
connected flush with the RF coupler (7d). Both the cap and the coupler are threaded such
that a tight junction can be established.
The CAD dimensions for the catheter and connecting
components are drawn to scale and the full assembly is shown in Figure
8. The inner core has been drawn to fit a 5F needle (inner radius of
0.038 inches) and has an overall length of 7.87 inches. All three layers of the catheter have a
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standard thickness of 0.011 inches and the exposed tip extends 0.59 inches past the outer
insulated shielding. The base radius of the catheter has an outer dimension of 0.142 inches. The
coupler to the RF cable (Figure 7d) is 1.25 inches in length with an inner core opening of 0.071
inches. The threads which dovetail with the catheter cap are 0.12 inches in height with a 0.02
offset.
DATA
Table 1 (available in the appendix) shows that in order to raise the body’s temperature to
a temperature high enough for the denaturing of local proteins in a course of one to five seconds,
anywhere from 0.3 to nearly 32 Watts of power is needed. Since the rate that the catheter is
extracted is likely to vary based on the physician doing the procedure, it is difficult to predict
which combination of electrode length and time is optimal without testing in tissue. However,
since there is a range of approximately 30°C between effectively killing tissue and charring
tissue which typically occurs at 100°C and higher, there is some room for variance.26
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Power Needed as a Function of Exposed Electrode Figure
Length
8: CAD
35
Power (watts)
catheter fully
assembled.
Time in Seconds
30
time = 1
25
time = 2
20
time = 3
time = 4
15
time = 5
10
5
0
0
0.005
0.01
0.015
0.02
0.025
Exposed electrode length (m)
Figure 9: Graphical representation of Table 1 which shows a correlation between time, electrode length, and
power.
Figure 9 illustrates the data represented in Table 1 which shows a correlation between
time, electrode length, and power. If the time is limited to two to five seconds, then the power is
kept in a narrow enough range that variation in speed that the catheter is removed can be
decreased so that the primary variable to focus on adjusting is the exposed catheter length which
is far easier to control.
When solving (Equation 3)15 for temperature at various time steps, the  value was set at
4.6 to ensure that perfusion was stopped and tissue was thoroughly cooked.5
Figure 10
illustrates the results obtained from the temperature results from Equation 3 as well power
analysis from Equation 1. This depicts the relationship between the temperature and exposure
time required at each time step for an  = 4.6. These values were calculated by integrating over
times 0-1 second through 0-5 seconds. The time steps of 1s to 5s for each position that the
catheter is moved were supplied and allows for a quicker procedure and the activation energy,
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Time for 99% cell death and 0 perfusion to
immediate surrounding tissue
71
70
Temp (deg. C)
69
68
67
66
65
64
63
0
1
2
3
4
5
6
time (s)
Figure 10: Temperature required for 99% tissue damage with respect to exposure times ranging from 1-5
seconds.
too, was supplied.5
Seeing as protein denaturization begins at approximately 42°C, the
temperatures obtained are reasonable and attainable through the use of a 450 kHz pulse
sequence. Figure 11 (available in the index) shows how the temperature and tissue death vary
with radial distance from the center of the catheter. This diagram ensures that unnecessary tissue
cauterization will not occur and even as temperature increases, charring and over ablation will
most likely be avoided. Even with variable power supply and radial distances, the issue of overablation is by passed as the tissue temperature decreases with time after the tissue’s maximum
death occurs.15 Figure 11 illustrates this phenomenon and is a reproduction of the complexity
required to model this trend using FEMLAB and MATLAB.
DISCUSSION
Although it is difficult to predict the possible shortcomings of a design prior to testing,
components that will need to be optimized via experimentation can be foreseen. For instance,
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various lengths of exposed conductive polymer should be tested to determine the exposed
electrode length that results in optimal cauterization of the tissue surrounding the electrode.
Additionally, future work needs to be conducted to find an appropriate ratio between the mass of
the electrical conductor and the insulator. Ideally, the tissue should be cauterized to the point of
cell death, but not to the point that the tissue is charred.
Another possible aspect that requires experimental testing to fully optimize the device is
selecting the best doped conductive polymers. Since conductive polymers are considered an
emerging field, there are still many new materials that could have higher conductive properties.
Based on the thorough modeling and design considerations, few major changes should be needed
to fully optimize the device’s design.
Foreseen ethical issues in future testing will not limit the product’s viability.
The
immediate need is to test the device in representative tissue samples. After testing is completed,
and the instrument is optimized, the device will undergo clinical testing. This process will
require the standard precautions taken when any new device is tested in a clinical setting. Patient
care and safety are well considered in this design.
Economically the creation of a catheter using conductive polymers is relatively
inexpensive.
Current catheters are made with Polyparaxylylene (PPX) which each cost
approximately $13 and is not a conductive material.27 The majority of our proposed catheter is a
non-conductive material with similar costs compared to that of PPX. The more expensive part of
the catheter is the conductive polymer material, but when produced on an industrial scale, these
costs will decrease and will be relatively insignificant to the efficiency the product will entail.
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Figure 9 shows the power needed as a function of exposed electrode length in order to
reach a certain temperature able to ablate surrounding tissue. Prototype tip lengths would range
from 0.1 -2.0 inches which includes the tip length used to test lesion sizes in canines.4,28 Testing
on bovine livers would be performed using a 200W, 480 kHz RF generator which is standard for
current biopsies. During testing, the power will range from 5-25 watts in order to properly
model the effects of different power ranges.
This design has a high capacity for producing desired results based on the multiple
benefits it provides. There are 430,000 new cases of liver disease alone worldwide each year and
0.1 – 3.6% of cases have complications of internal bleeding (430 - 15,480 people turned down
due to risk).
FDA approval of this device as well as approval by the Human Research
Committee is the next step in providing this device to thousands of people who could benefit
from it. To be approved, this catheter must prove that the conductive polymers can carry
electrical current similar to that of electrodes in a safe, biocompatible manner as previous
technologies like biopsy needles.
The need for more adaptable and versatile catheters is becoming a reality as medicine and
surgical applications become more advanced. A European company, Sterotaxis, has just released
an FDA approved magnetically guided 8mm ablation catheter. After preliminary and clinical
trials, the catheter has shown success in complex ablations. The magnetically enabled, and
image guided catheter will be used for treating atrial cardiac arrhythmias by delivering a high
power ablation to the tissue. Dr. Carlo Pappone, Director of the Arrythmology Department said:
“The power of the 8mm catheter, combined with the safety of precise and soft contact in critical
areas of the heart, simplifies the treatment of complex atrial arrhythmias.”29
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CONCLUSION
The purpose of this paper was to present compelling evidence that a cauterizing catheter
with the conductive element being a flexible polymer, as opposed to a brittle metal, is both a
promising design to improve patient care and economically advantageous to current alternatives.
After thorough research into available conductive polymers in conjunction with heat transfer
calculations and modeling, two promising conductive polymers were chosen to construct a
prototype. Doped polyacetylene and polythiophene were chosen based on their conductivity,
biocompatibility, and ability to carry an electrical signal. The catheter was designed to minimize
the loss of an RF signal as it travels towards its target. The body of the catheter is a hollow tube
with the conductive polymer forming a hollow cylinder with a polyurethane coating acting as an
insulator on both the inside and outside of the conductive polymer with the conductive polymer
only being exposed to tissue at the terminal end. The next step for the project is to test its
efficacy in animals so that the design can be further optimized. Based on the findings discussed
in this paper, there is a bright future and demand for an effective cauterizing catheter.
23 of 48
Appendix A: Equations
Equation 1:
𝑃=
𝑑𝑇
𝑑𝑡
Derivation:
𝑐𝜌(𝜋𝑟𝑜2 𝑙 − 𝜋𝑟𝑖2 𝑙)
𝑃=
𝑑𝑇
𝑑𝑡
ℎ
ℎ = 𝑐𝜔
𝜔 = 𝜌𝑉
𝑑𝑇
𝑉 = 𝜋𝑟𝑜2 𝑙 − 𝜋𝑟𝑖2 𝑙
Equation 2:
𝜌𝐶 𝑑𝑡 = 𝑣(𝑘∇𝑇) + 𝜎(|∇V|)2 − 𝜌𝑏 𝐶𝑏 𝛼𝜔(𝑇 − 𝑇𝑎𝑚𝑏 ) + 𝑄𝑚
Equation 3:
Ω(𝑡) = ln ( 𝑐(𝑡) ) = 𝐴 ∫ 𝑒
𝑐(0)
(−
∆𝐸
)
𝑅𝑇(𝑡)
𝑑𝑡
24 of 48
Appendix B: Figures
Figure 1
Figure 11: Traditional epoxy injection system used for delivering fibrin and thrombin into the
catheter tract.
25 of 48
Figure 2
Figure 12: Results from an RF ablation study showing positive results for ablation and blood
clotting.
26 of 48
Figure 3
Figure 13: Properties of a metal conductor. Figure 3a shows that an open valence electron is
required for conduction. Figure 3b shows a cross-section of a copper wire and the effects of
an external electric influence.
27 of 48
Figure 4
Figure 14: The conductivity of doped polyacetylene as a function of temperature and
compared to that of silver.
28 of 48
Figure 5
Figure 15: Polyacetylene backbone showing alternating double and triple bonds. Conjugated
double bonds exist when double and single bonds alternate.
29 of 48
Figure 6
Figure 16: Two proposed methods for sending conductive polymers down the shaft of the
catheter. Figure 6a shows separate channels of conductive polymer and figure 6b shows a
continuous ring of conductive polymer.
30 of 48
Figure 7
Figure 17: CAD modeling of the conductive polymer catheter. Figure 7a shows a top-down
projection focusing on the exposed conductive tip. Figure 7b is a bottom projection showing
the opening of the catheter as well as the point at which the RF current will connect with the
catheter to deliver the RF pulse. Figure 7c is a cap that will ensure that the catheter is
connected flush with the RF coupler (7d). Both the cap and the coupler are threaded such
that a tight junction can be established.
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Figure 8: Figure 18: CAD catheter fully assembled.
32 of 48
Figure 9
Power Needed as a Function of Exposed Electrode Length
Power (watts)
35
Time in Seconds
30
time = 1
25
time = 2
20
time = 3
time = 4
15
time = 5
10
5
0
0
0.005
0.01
0.015
0.02
0.025
Exposed electrode length (m)
Figure 19: Graphical representation of Table 1 which shows a correlation between time,
electrode length, and power.
33 of 48
Figure 10
Time for 99% cell death and 0 perfusion to
immediate surrounding tissue
71
70
Temp (deg. C)
69
68
67
66
65
64
63
0
1
2
3
4
5
6
time (s)
Figure 20: Temperature required for 99% tissue damage with respect to exposure times
ranging from 1-5 seconds.
34 of 48
Figure 11
Temperature and tissue death occurring with a 30 Volt supply and 4mm distance radially from
the center of the catheter.
35 of 48
Figure 12
36 of 48
Appendix C: Tables
Table 1. Power needed as a function of time and exposed electrode length.
time (seconds)
exposed electrode length
(m)
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
0.011
0.012
0.013
0.014
0.015
0.016
0.017
0.018
0.019
0.02
0.021
1
1.510408
3.020816
4.531224
6.041633
7.552041
9.062449
10.57286
12.08327
13.59367
15.10408
16.61449
18.1249
19.63531
21.14571
22.65612
24.16653
25.67694
27.18735
28.69775
30.20816
31.71857
2
3
4
power (watts)
0.755204 0.503469 0.377602
1.510408 1.006939 0.755204
2.265612 1.510408 1.132806
3.020816 2.013878 1.510408
3.77602 2.517347 1.88801
4.531224 3.020816 2.265612
5.286428 3.524286 2.643214
6.041633 4.027755 3.020816
6.796837 4.531224 3.398418
7.552041 5.034694 3.77602
8.307245 5.538163 4.153622
9.062449 6.041633 4.531224
9.817653 6.545102 4.908826
10.57286 7.048571 5.286428
11.32806 7.552041 5.66403
12.08327 8.05551 6.041633
12.83847 8.558979 6.419235
13.59367 9.062449 6.796837
14.34888 9.565918 7.174439
15.10408 10.06939 7.552041
15.85929 10.57286 7.929643
37 of 48
5
0.302082
0.604163
0.906245
1.208327
1.510408
1.81249
2.114571
2.416653
2.718735
3.020816
3.322898
3.62498
3.927061
4.229143
4.531224
4.833306
5.135388
5.437469
5.739551
6.041633
6.343714
Table 2. Resistance as a function of time and exposed electrode length.
time (seconds)
exposed electrode length
(m)
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
0.011
0.012
0.013
0.014
0.015
0.016
0.017
0.018
0.019
0.02
0.021
1
1510408
3020816
4531224
6041633
7552041
9062449
10572857
12083265
13593673
15104081
16614489
18124898
19635306
21145714
22656122
24166530
25676938
27187346
28697754
30208163
31718571
2
3
4
5
resistance (ohms)
755204.1 503469.4 377602 302081.6
1510408 1006939 755204.1 604163.3
2265612 1510408 1132806 906244.9
3020816 2013878 1510408 1208327
3776020 2517347 1888010 1510408
4531224 3020816 2265612 1812490
5286428 3524286 2643214 2114571
6041633 4027755 3020816 2416653
6796837 4531224 3398418 2718735
7552041 5034694 3776020 3020816
8307245 5538163 4153622 3322898
9062449 6041633 4531224 3624980
9817653 6545102 4908826 3927061
10572857 7048571 5286428 4229143
11328061 7552041 5664030 4531224
12083265 8055510 6041633 4833306
12838469 8558979 6419235 5135388
13593673 9062449 6796837 5437469
14348877 9565918 7174439 5739551
15104081 10069388 7552041 6041633
15859285 10572857 7929643 6343714
38 of 48
Appendix D: Resources
Innovation Workbench
39 of 48
Develop Concepts
Concept/Idea #1 Conductive polymer runs through the catheter throughout the circumference and does not
break at any point. This method will be capable of delivering, ideally, a much more uniform
distribution of heat into the tissue. By having the polymer totally circumnavigating the catheter, the
power dissipation will also be uniform outside of the catheter (1/R^2). However, the modelling of this
system is much more complicated due to the increased polymer exposure. Regardless, this will
provide a much more reliable means of delivering an RF pulse down the length of the catheter.
Concept/Idea #2 Conductive polymer runs down the length of the cather in separate cylindrial tracts. Consisting
of upwards of 4 tracts evenly spaced around the catheter, these pieces will be exposed at the end to
present separate prongs that will distribute the RF pulse into the tissue. Although it will provide more
surface area through which the energy can be delivered, it results in a less uniform heat distribution.
The modelling of this system is intricate at the end of the catheter at the site of the prongs, and
requires calculation of exact patterns of tissue death as occurs around a cylinder.
Comparing these two approaches makes it clear that the former idea with uniform ablation is much
more desirable. Although the modelling is much more difficult, it provides a certain amount of energy
throughout the circumference of the catheter. In addition to this, since the conductive polymer is a
plastic, the prongs in the latter idea are much more likely to be damaged, bent, or unable to deploy
once the catheter is inserted. Therefore, the first concept, through its reliability and uniform RF
energy distribution will be used and hopefully preferred.
Evaluate Results
Reveal and prevent potential failures
Potential failure #1 -
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Too much heat delivered and charring occurs during cauterization. This problem can be
prevented through adequate testing of the device and proper modelling or thermal calculations. This
will require accurate measurements of how much tissue to ablate and how long the catheter will be
placed at each increment along the tract during removal.
Potential failure #2 The M.D. performing the procedure at location A is not the same M.D. performing the
procedure at location B and therefore, their methods of removal and their senses of feel are not the
same. If we say that the catheter must be removed at 2 inch increments with 5 seconds at each site,
the first doctor may leave it at the site for 6 seconds and remove it 1.9 inches while the other doctor
may leave it at the site for 6 seconds and remove it 1.6 inches. The overlap of energy in this situation is
not the same and excess burning may arise as a consequence. Either way, there is room for error in
this sense and must be considered. This can be goverend with dissipation of amount of energy
delivered over 5-8 second increments such that after 5 seconds, the power dies out and does not pick
up again until the catheter has reached a new location. These problems can also be prevented by using
a power that will prevent charring after x amount of seconds such that if the catheter is left in place too
long, it will not damage the tissue too dramatically.
Plan the implementation
Experimentation Cow liver testing at the NIH
Test the catheter in cow liver with various exposed tip lengths. This will allow us to hone in
on a sepecific length of exposure such that removal and cauterizaiton will be optimal for the doctor
performing the task and the denaturization of protein. The cow liver will give us a good idea of the
environment we are working with in humans and will give feedback regarding the power dissipation
through the catheter and the ratio of power dissipation to acutal heat delivery to tissue at the site of
cauterization. Through this testing we can refine the catheter and use in more specific circumstances,
clinically. In addition to this, we will be able to compare our procedure with previously used
procedures in biopsy needle cauterization.
The modelling of this system is of largest concern.
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- Electrical engineering specialists at the NIH working with Dr. Wood
Testing
- NIH facility
Prototype
- Machine molds shop in Ohio
Vascularization, need, and demand of this product
- Vanderbilt Hospital - M.D.s, Clinical Specialists, ER/Trauma center...
42 of 48
INNOVATION WORKBENCH NOTES/IDEAS
12/5/2006 12:52:42 AM
Idea:
Progress report is being completed. After obtaining AutoCAD we hope to be able to create an interactive
CAD file for our catheter. This can then be used as our model for the machine shop. Still attempting to
get in touch with Dr. Wood after several weeks of no communication via phone. Group is concerned.
01/13/2007 4:17:34 PM
Idea:
We met with professor A.B. Bonds today. He was very insightful into the workings of RF ablation and the
complexity of modeling this device. Modeling will hopefully happen down the road through help of Dr.
Wood at the NIH. We need to look at Maxwell's equations apparently.
01/15/2007 11:56:11 AM
Idea:
Is an epoxy or collagen/fibrin sealant really the best approach? Seems tedious and cumbersome,
but I guess that is why we are approaching this problem. We need to obtain specific values and
thermal properties to apply to this device. Otherwise, burning the patient and charring tissue can
occur.
01/15/2007 12:34:58 PM
Idea:
We have eliminated the possibility of using an epoxy as a sole means of sealing off a catheter
tract. However, companies such as Epotek may prove useful later in the project for catheter
connections regarding a high RF pulse.
01/20/2007 2:03:41 PM
Idea:
This is a biomaterials problem....find out what biomaterials are conductive enough to run an RF
pulse through it, generating enough energy to denature protein.
02/02/2007 5:17:29 PM
Idea:
Nobel Prize awarded in 2000 for discoveries and advancements in conductive polymers. Source of
conductivity lies in doping the polymer. Polyacetylene. Find a professor knowledgeable in the field of
biomaterials. Dr. Shastri. Dr. Wittig.
02/03/2007 12:12:13 PM
Idea:
How is the catheter going to connect to the RF generator????
Dr. Kam, working with Dr. Wood, will be aiding us in the electrical and thermal modeling for this project.
02/20/2007 12:13:54 PM
Idea:
Group meeting: we have decided to, upon completion of CAD and obtaining biomaterials, test the
catheter with polyacetylene and polythiophene. These two polymers have adequate conductivities to act
as our conductive polymers. The group has also decided that by testing a variety of exposed tip lengths,
we will be able to discover the optimal exposed tip length for best use. Testing will include exposed tip
lengths of 0.10 inches to 2.0 inches. Please see sketches and ideas on attached paper.
43 of 48
03/14/2007 1:17:05 AM
Idea:
Still learning CAD. Has proven to be a little more involved due to our design and the insulation required
for our catheter. We will need to split the group tasks into two sections - designing the catheter and
designing the RF connection....unless the RF connection is not as trivial as it seems.
3/26/2007 12:20:22 PM
Idea:
This week we met with Dr. Fleetwood to discuss mathematical modeling of our design. Unfortunately, he
informed us that modeling this type of product is extremely difficult and creates careers for people in the
research and mathematical industry. We exhausted much time attempting to tackle this aspect of the
catheter and were slightly disappointed that we received this news. However, he did inform us of crucial
power properties and dissipation tendencies for devices such as this that will be important for our proof of
concept presentation.
3/27/2007 8:42:21 PM
Idea:
Catheter CAD files are complete. Must work on connection of catheter to RF generator.
4/10/2007 9:00:58 PM
Idea:
CAD complete for connection to RF cable. Sound CAD file with correct scaling and useful for machine
shop. Will send to Dr. Wood.
4/13/2007 12:24:09 PM
Idea:
Design safe initiated and almost completed. Will refine.
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Design Safe
45 of 48
Appendix E: References
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http://science.jrank.org/pages/1285/Cauterization.html. Accessed 9 January 2007.
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Current Indications, Techniques, and Outcomes.” Springer Publishing. October 2003.
3. Schmitt, C., Deisenhofer I., and Zrenner, B. “Tumor Ablation: Principles and Practice”. SteinkopffVerlag Darmstadt, Inc. 2006.
4. Kim, EH, et al. “Electrocautery of the tract after needle biopsy of the liver to reduce blood
loss”. Journal of Investigational Radiology. Volume 28, Issue 3. Department of Radiology,
Indiana University School of Medicine, Indianapolis. 1993.
5. BRAD WOOD, M.D. National Institute of Health, Bethesda, MD. Advisor, Senior Design,
Biomedical Engineering, Vanderbilt University. 2006-2007.
6.
Petsas, Theodore et al. “Fibrin Glue for Sealing the Needle Tract in Fine-Needle
Percutaneous Lung Biopsy Using a Coaxial System: Part II – Clinical Study”. Journal of
Cardiovascular and Interventional Radiology. Volume 18; pg. 378-382. Springer-Verlag,
New York, New York, 1995.
7. Paulson, Erik et al. “Use of Fibrin Sealant as a Hemostatic Agent after Liver Biopsy in
Swine”. Journal of Interventional Radiology - Hepatic Intervention. Volume 11; pg. 905911. 2000.
8. Sherwood, Lauralee. Human Physiology: From Cells to Systems. 5th Edition. Thomson Learning,
Inc. Belmont, CA. 2004.
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9. Dasari, SD, Bashetty, NK, Prayaga, NS. “Radiofrequency Ablation of Lingual Thyroid”. Journal of
Otolaryngol Head and Neck Surgery. Volume 136; Issue 3. March 2007.
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Results in 14 Patients”. Journal of Vascular Interventional Radiology. Vol. 18; Pg. 67-72. January
2007.
11. “Pritchard, William, et al. “Radiofrequency Cauterization with Biopsy Introducer Needle”.
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March 2007.).
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1990.
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15. Chang, Isaac, Nguyen, Uyen. “Thermal Modeling of Lesion Growth with Radiofrequency
Ablation Devices”. Biomedical Engineering Online. Volume 3; Issue 27. 06 August 2004.
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Weight Distribution on Mechanical Properties of Polymers”. Polymer Engineering and Science.
Volume 22; Issue 4. 25 August 2004.
17. Callister, William Jr. Materials Science and Engineering: An Introduction. 6th Edition. John Wiley
& Sons, Inc. Hoboken, NJ. 2003.
18. “Conductors, Insulators, and Electron Flow”.
http://www.allaboutcircuits.com/vol_1/chpt_1/2.html. Accessed 04 March 2007.
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19. “Conductors and Insulators”. http://hyperphysics.phy-
astr.gsu.edu/hbase/electric/conins.html. Accessed 04 March 2007.
20. Myron, Harold Ph. D. “Metal Conductivity”.
http://www.newton.dep.anl.gov/askasci/chem99/chem99447.htm. Argonne National
Laboratory. Accessed 02 March 2007.
21. Serway, Raymond, Jewett, John. Principles of Physics: A Calculus-Based Text. 3rd Ed. Thomson
Learning, Inc. Willard, OH. 2002.
22. Bodner, George M. “The Covalent Bond.” Purdue University. Accessed 16 April 2007.
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2007.
23. “The Nobel Prize in Chemistry, 2000: Conductive Polymers.”
http://nobelprize.org/nobel_prizes/chemistry/laureates/2000/chemadv.pdf
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12 January 2007.
25. NIST http://cryogenics.nist.gov/NewFiles/Polyurethane.html
26. Tungjitkusolmun, Supan etal. “Modeling Bipolar Phase-Shifted Multielectrode Catheter
Ablation.” IEEE Transactions on Biomedical Engineering. Vol 49, No.1 pgs 10-17.
January 2002.
27. Baker, T.E., Fix, G. L., Judge, J. S. “Modified Poly-Paraxylylene Coatings and Films with Improved
Oxidation Resistance”. Journal of the Electrochemical Society. Volume 127; Issue 8. 1980.
28. FRED H.M. WITTKAMPF, Ph.D., and HIROSHI NAKAGAWA, M.D., Ph.D.. (2006) RF
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29. “Stereotaxis Announces FDA Approval of Partnered 8mm Ablation Catheter” Stereotaxis, Inc. PRNewswire Association (Accessed April, 2007) < http://www.pr-inside.com/stereotaxis-announcesfda-approval-of-r84207.htm>.
30. “Radiofrequency cauterization with introducer needle”. JOURNAL OF VASCULAR
INTERVENTIONAL RADIOLOGY. 2004. 15:183-187.
31. “Radiofrequency cauterization with introducer needle”. JOURNAL OF VASCULAR
INTERVENTIONAL RADIOLOGY. 2004. 15:183-187.
32. “Electrocautery of the Tract after Needle Biopsy of the Liver to Reduce Blood Loss”. Investigative
Radiology Volume 28. Number 3 228-230
33. Breen, Marc. “Metals as Conductors and Plastics as Insulators”.
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March 2007.
34. Laeseke, Paul, et al. “Postbiopsy Bleeding in a Porcine Model: Reduction with Radio-Frequency
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