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, MD1VANDERBILT UNIVERSITY, NASHVILLE, TN; 2NIH, BETHESDA, MD

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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, MD1VANDERBILT 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

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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

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

possibilities for quickly and easily eradicating a tumor10; yet although this novel idea has become

Figure 1: Traditional epoxy injection system used for delivering fibrin and thrombin into the catheter tract.

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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

an increased frequency. In patients undergoing vascular catheterization, a common

anticoagulant is heparin which thins the blood and has caused severe complications including

Figure 2: Results from an RF ablation study showing positive results for ablation and blood clotting.

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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

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

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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

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

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the space between the atoms of that material by nothing more than the influence of room-

temperature 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

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

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.

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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 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

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

Figure 4: The conductivity of doped polyacetylene as a function of temperature and compared to that of silver.

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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

change in temperature( dTdt ), 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=dTdt

cρ (π ro2 l−π r i

2l ) Equation 1

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.

Figure 5: Polyacetylene backbone showing alternating double and triple bonds. Conjugated double bonds exist when double and single bonds alternate.

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ρC dTdt

=v (k∇T )+σ (|∇V|)2−ρbCb αω (T−T amb )+Qm Equation 2

Ω ( t )=ln( c (0 )c ( t ) )=A∫ e

( −∆ERT (t ))dt 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

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

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

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.

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ablation is unnecessary and potentially harmful. The power dissipation leaving the catheter walls

is a relationship characterized by P∝1d2 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

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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 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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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,

too, was supplied.5 Seeing as protein denaturization begins at approximately 42°C, the

Figure 8: CAD catheter fully assembled.

0 0.005 0.01 0.015 0.02 0.0250

5

10

15

20

25

30

35

Power Needed as a Function of Exposed Electrode Length

time = 1

time = 2

time = 3

time = 4

time = 5

Exposed electrode length (m)

Pow

er (w

atts)

Time in Seconds

Figure 9: Graphical representation of Table 1 which shows a correlation between time, electrode length, and power.

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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 over-

ablation 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,

various lengths of exposed conductive polymer should be tested to determine the exposed

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.56162636465666768697071

Time for 99% cell death and 0 perfusion to immediate surrounding tissue

time (s)

Tem

p (d

eg. C

)

Figure 10: Temperature required for 99% tissue damage with respect to exposure times ranging from 1-5 seconds.

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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.

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

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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

CONCLUSION

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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.

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Appendix A: Equations

Equation 1: P=dTdt

cρ (π ro2 l−π r i

2l )

Derivation: P=dTdt

h h=cω ω=ρV V=π ro2 l−π r i

2l

Equation 2: ρC dTdt

=v (k∇T )+σ (|∇V|)2−ρbCb αω (T−T amb )+Qm

Equation 3: Ω (t )=ln( c (0 )c (t ) )=A∫ e

( −∆ERT (t ))dt

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Appendix B: Figures

Figure 1

Figure 11: Traditional epoxy injection system used for delivering fibrin and thrombin into the catheter tract.

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Figure 2

Figure 12: Results from an RF ablation study showing positive results for ablation and blood clotting.

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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.

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Figure 4

Figure 14: The conductivity of doped polyacetylene as a function of temperature and compared to that of silver.

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Figure 5

Figure 15: Polyacetylene backbone showing alternating double and triple bonds. Conjugated double bonds exist when double and single bonds alternate.

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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.

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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.

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Figure 9

0 0.005 0.01 0.015 0.02 0.0250

5

10

15

20

25

30

35

Power Needed as a Function of Exposed Electrode Length

time = 1

time = 2

time = 3

time = 4

time = 5

Exposed electrode length (m)

Pow

er (w

atts)

Time in Seconds

Figure 19: Graphical representation of Table 1 which shows a correlation between time, electrode length, and power.

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Figure 10

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.56162636465666768697071

Time for 99% cell death and 0 perfusion to immediate surrounding tissue

time (s)

Tem

p (d

eg. C

)

Figure 20: Temperature required for 99% tissue damage with respect to exposure times ranging from 1-5 seconds.

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Figure 11

Temperature and tissue death occurring with a 30 Volt supply and 4mm distance radially from

the center of the catheter.

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Figure 12

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Appendix C: Tables

Table 1. Power needed as a function of time and exposed electrode length.

time (seconds) 1 2 3 4 5exposed electrode length

(m) power (watts)

0.0011.51040

80.75520

40.50346

90.37760

20.30208

2

0.0023.02081

61.51040

81.00693

90.75520

40.60416

3

0.0034.53122

42.26561

21.51040

81.13280

60.90624

5

0.0046.04163

33.02081

62.01387

81.51040

81.20832

7

0.0057.55204

1 3.776022.51734

7 1.888011.51040

8

0.0069.06244

94.53122

43.02081

62.26561

2 1.81249

0.00710.5728

65.28642

83.52428

62.64321

42.11457

1

0.00812.0832

76.04163

34.02775

53.02081

62.41665

3

0.00913.5936

76.79683

74.53122

43.39841

82.71873

5

0.0115.1040

87.55204

15.03469

4 3.776023.02081

6

0.01116.6144

98.30724

55.53816

34.15362

23.32289

8

0.012 18.12499.06244

96.04163

34.53122

4 3.62498

0.01319.6353

19.81765

36.54510

24.90882

63.92706

1

0.01421.1457

110.5728

67.04857

15.28642

84.22914

3

0.01522.6561

211.3280

67.55204

1 5.664034.53122

4

0.01624.1665

312.0832

7 8.055516.04163

34.83330

6

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0.01725.6769

412.8384

78.55897

96.41923

55.13538

8

0.01827.1873

513.5936

79.06244

96.79683

75.43746

9

0.01928.6977

514.3488

89.56591

87.17443

95.73955

1

0.0230.2081

615.1040

810.0693

97.55204

16.04163

3

0.02131.7185

715.8592

910.5728

67.92964

36.34371

4

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Table 2. Resistance as a function of time and exposed electrode length.

time (seconds) 1 2 3 4 5exposed electrode

length (m) resistance (ohms)

0.001151040

8755204.

1503469.

4 377602302081

.6

0.002302081

6151040

8100693

9755204

.1604163

.3

0.003453122

4226561

2151040

8113280

6906244

.9

0.004604163

3302081

6201387

8151040

8120832

7

0.005755204

1377602

0251734

7188801

0151040

8

0.006906244

9453122

4302081

6226561

2181249

0

0.007105728

57528642

8352428

6264321

4211457

1

0.008120832

65604163

3402775

5302081

6241665

3

0.009135936

73679683

7453122

4339841

8271873

5

0.01151040

81755204

1503469

4377602

0302081

6

0.011166144

89830724

5553816

3415362

2332289

8

0.012181248

98906244

9604163

3453122

4362498

0

0.013196353

06981765

3654510

2490882

6392706

1

0.014211457

14105728

57704857

1528642

8422914

3

0.015226561

22113280

61755204

1566403

0453122

4

0.016241665

30120832

65805551

0604163

3483330

6

0.017256769

38128384

69855897

9641923

5513538

8

0.018271873

46135936

73906244

9679683

7543746

9

0.019286977

54143488

77956591

8717443

9573955

10.02 302081 151040 100693 755204 604163

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63 81 88 1 3

0.021317185

71158592

85105728

57792964

3634371

4

39 of 48

Appendix D: Resources

Innovation Workbench

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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...

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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.

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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

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Appendix E: References

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http://science.jrank.org/pages/1285/Cauterization.html. Accessed 9 January 2007.

2. Lee, Ellis M., Curley, Stephen A., Tannabe, Kenneth. “Radiofrequency Ablation for Caner:

Current Indications, Techniques, and Outcomes.” Springer Publishing. October 2003.

3. Schmitt, C., Deisenhofer I., and Zrenner, B. “Tumor Ablation: Principles and Practice”. Steinkopff-

Verlag 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. 905-

911. 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.

10. Gunnabushanam, G et al. “Radiofrequency Ablation of Liver Metastases from Breast Cancer:

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”.

Journal of Vascular Interventional Radiology. Volume 15; pg 183-187. 2004.).

12. Bretzel, RG et al. “Islet Cell Transplantation Today”. Langenbecks Archives of Surgery. 28

March 2007.).

13. Oweida, SW, et al. “Postcatheterization Vascular Complications Associated with Percutaneous

Transluminal Coronary Angioplasty”. Journal of Vascular Surgery. Volume 12; Issue 3. September

1990.

14. Waigand, J, et al. “Percutaneous Treatment of Pseudoaneurysms and Arteriovenous Fistulas after

Invasive Vascular Procedures”. Journal of Catheter Cardiovascular Interventions. Volume 47; Issue

2. June 1999.

15. Chang, Isaac, Nguyen, Uyen. “Thermal Modeling of Lesion Growth with Radiofrequency

Ablation Devices”. Biomedical Engineering Online. Volume 3; Issue 27. 06 August 2004.

16. Nunes, Ronald, Martin, John, Johnson, Julian. “Influence of Molecular Weight and Molecular

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.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/index.php. Accessed 16 April

2007.

23. “The Nobel Prize in Chemistry, 2000: Conductive Polymers.”

http://nobelprize.org/nobel_prizes/chemistry/laureates/2000/chemadv.pdf

24. “Conductive Polymers” http://www.hitechpolymers.com/Products/conductive.asp. Accessed

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

Catheter Ablation: Lessons on Lesions. Pacing and Clinical Electrophysiology 29:11, 1285–

1297.

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29. “Stereotaxis Announces FDA Approval of Partnered 8mm Ablation Catheter” Stereotaxis, Inc. PR-

Newswire Association (Accessed April, 2007) < http://www.pr-inside.com/stereotaxis-announces-

fda-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”.

http://www.madsci.org/posts/archives/jan2001/979088220.Eg.r.html. Center of Biomolecular

Science and Engineering, U.S. Naval Research Laboratory. 9 January 2001. Accessed 02

March 2007.

34. Laeseke, Paul, et al. “Postbiopsy Bleeding in a Porcine Model: Reduction with Radio-Frequency

Ablation – Preliminary Results”. Radiology. Volume 227. 2003.