Cardiac Lead Insulation: A Test of EnduranceTunaidi Ansari, Jun Hong, Yipeng Zhang

BIOE 498 SM

Presently, there exists much demand for implantable biomedical devices, especially those

of the cardiac variety. These devices fulfill a variety of essential functions in the body, such as

directing the pace of the heart or supporting the structural integrity of vascular systems. The

specific cardiac device we investigated in this study was insulating tubing for cardiac lead. This

tubing serves to keep biological molecules in the bodily fluid out and help uninterrupted

electrical signals to travel along the lead. The material selected to best fit the functional

requirements of this device may not be the best suited for implantation in an in vivo

environment. Therefore, there exists a need for providing a barrier or insulator against the

environments of the body that proves more chemically inert, nontoxic, sterilizable, and

biocompatible than the original native device material. After years of research, scientists have

discovered many materials—such as silicone rubber, polyimide, and polyurethane—that possess

the above characteristics. The monomer structure of

polyurethane is shown in Figure 1; polyurethane has two ester

groups and an aromatic ring flanked by two amine groups.

The tubing around the electrode of the cardiac device can be

manufactured via mixing isocyanic acid polymethylene polyphenyl ester with polyurethane resin

and casting [1]. Although water in the process of aging can deteriorate the material’s insulating

ability, its advantages such as high thermal resistance, easy manufacturability because of the low

viscosity, the low cost, and high impact strength, outweigh its disadvantages [1][2][3][4]. The

implanted cardiac leads with polyurethane tubing, however, have shown high failure rates. In

Figure 1: The monomer structure of polyurethane.

our case study, we examined the possible cause of the tubing’s failure and propose solutions to

the problem.

The results in Figure 2 shows the mechanical behavior of the as-received (right after

injection molding into flat sheet) polyurethane at room temperature. This plot illustrates that the

material is somewhat elastic, which makes it desirable for the application as insulation tubing

because high rigidity can make the material brittle. Table 1 shows how aging in the air versus in

the PBS at room temperature affect different mechanical properties of polyurethane after 36-

month period. The ultimate tensile strength and the % elongation are determined when

polyurethane failed. As shown in Table 1, the ultimate tensile strength, % elongation, and glass

transition temperature decreased with time. The presence of PBS further decreased these

properties. These changes in the properties, however, were relatively minimal as shown by large

standard deviations, and thus the material properties do not change drastically in either PBS or

air over 36-month period.

Samples Tg (°C) UTS (MPa) % ElongationAs-received -18.2 ± 0.8 65.2 ± 3.2 521.3 ± 20.436-month dry control -20.4 ± 1.4 60.4 ± 4.1 512.7 ± 17.036-month water -22.0 ± 1.6 52.8 ± 10.3 504.6 ± 22.5

Table 1: Summary of Test 1 results – the properties of polyurethane after being received, after dry aging at room temperature, and after aging in water at room temperature.

Figure 2: A uniaxial tensile test of as-received polyurethane at room temperature. The polyurethane was received after being injection molded into flat sheets.

According to the data from Table 2, the presence of glass wool leads to accelerated aging

and loss of % elongation. The previous condition was the control condition and the addition of

PBS to the test results in a slightly higher ultimate tensile strength and drastically lower %

elongation after aging. In essence, the material becomes brittle after aging in a condition of glass

wool and PBS. The addition of 200% strain alone to the control environment leads a higher

ultimate tensile strength and lower % elongation. However, while at first glance the material

properties of condition 4, which involved 200% strain, PBS, and glass wool, show that average

ultimate tensile strength was greater than control while % elongation was lessened; a closer look

reveals a more complex picture. While the average ultimate tensile strength was higher than

control, the presence of a large standard deviation of 28.9 MPa versus a mean of 53.1 MPa,

suggests that the samples exposed to the environment experienced differing degrees of

degradation. The high end confirms the analysis that while strain increase the ultimate tensile

strength, the presence of PBS degrades the material somewhat from the heights of 200% strain

and glass wool alone. The low end reveals a more troubling picture, where the drastic fall in

ultimate tensile strength reveal that the presence of strain and PBS along with glass wool

degrades the material much more than the control test. This may be due to the fact that while

strain aligns and orders the polymer chains in the polyurethane, resulting in a higher initial

ultimate tensile strength, the presence of PBS and wool contribute to the ultimate degradation of

material properties in many samples. The calculated Young’s modulus for conditions 1, 2, 3,

and 4 were approximately 7.27 MPa, 100 MPa, 60 MPa, and 35 MPa, respectively. This data

shows that aging in both PBS and high strain conditions leads to the material becoming more

brittle, which may lead to increased chances of cracking. This was confirmed in Figure 3, where

one can see an electron micrograph of two samples, one taken from condition 2, and the other

from condition 4. The condition 2 micrograph shows a smooth material surface, which

corresponds to the relative stability of the ultimate tensile strength of the material while under

the influence of PBS and glass wool. The drastic drop in % elongation due to PBS is not

observed in the micrograph due to the fact that the material was not aged under any strain,

meaning that the loss in percent elongation could not be visually confirmed at first glance. The

second micrograph was that of condition 4, which confirmed our hypothesis regarding the

degradation of the material under 200% strain, PBS, and glass wool. The micrograph showed

that the material had severe amount of pores and displayed generally poor material properties,

which corresponds to expectations of lower end samples from condition 4. The reduction of

elastic properties due to PBS can be seen clearly with the 200% strain case with the formation of

pores.

Condition UTS (MPa) % Elongation1) 0% strain, control 37.7 ± 0.7 518.5 ± 27.92) 0% strain, PBS 40.2 ± 0.9 40.2 ± 0.93) 200% strain, control 109.6 ± 5.6 182.5 ± 17.74) 200 % strain, PBA 53.1 ± 28.9 149.0 ± 14.8

Table 2: Summary of Test 2 results – the mechanical properties of polyurethane after 3 months aging in four conditions. All samples were in contact with H2O2/CoCl2-glass wool.

Figure 3: Scanning electron micrographs of two polyurethane samples after Test 2. The sample of the left was from test condition 2, while the sample on the right was from test condition 4 described above in Table 2.

In Table 3, copper contact without aqueous solution is the control. However, when this

condition is altered so that PBS is also present, there are notable differences. It is evident that

copper exposure and the presence of PBS lead to a large decrease in ultimate tensile strength and

a subsequent decrease in % elongation. As a result, this leads to worse overall material properties

and the material becomes slightly less rigid. A substantial decrease in ultimate tensile strength

makes the material far more susceptible to failure and it is obvious that flexibility is also

compromised. As pictured in Figure 4, it is safe to conclude that copper contact with

polyurethane in conjunction with PBS causes cracks to form on the insulation.

Condition UTS (MPa) % Elongation1) Control 41.4 ± 3.2 504.3 ± 25.72) PBS 18.7 ± 6.3 320.5 ± 56.1

Table 3: Summary of Test 3 results – the mechanical properties of polyurethane after 3 months aging in two conditions. Both samples were in contact with a copper film during aging.

Figure 4: Scanning electron micrographs of two polyurethane samples after Test 3. (i) a sample from aging condition 1 and (ii) a sample from aging condition 2.

The tests presented in the case study are relevant to an exploration of the in vivo

degradation of polyurethane for a variety of reasons. Oxidation has been shown to be one of the

primary reasons for the degradation of polyurethane in vivo, and H2O2/CoCl2 wool is an

excellent choice to accelerate oxidative biodegradation to explore this phenomenon [5][6]. The

presence of PBS mimics the ionic conditions of the extracellular fluids omnipresent in an in vivo

environment. The strain conditions are important due to the manufacturing of the cardiac wiring.

In a high stress, interference fit, the inner diameter of the insulation tube is smaller than that of

the outer diameter of wiring. The polyurethane tubing is placed in organic solvent as a means of

increasing its inner diameter. Afterwards, the lead wiring is inserted into the insulation tubing.

Once the solvent is evaporated off the polyurethane, the result is an extremely tight cardiac lead

[7]. Consequently, this compact fitting causes a direct transfer of stress and strain from the

wiring to the polyurethane, which places a primary importance on the testing strain conditions on

the polyurethane. This also means that strain caused by intercorporeal movements inside the

body is easily transferred along the lead. Finally, the copper contacted condition is important as

it mimics the behavior of the insulation in contact with the conductor while in a solution that

mimics the extracellular fluid.

We hypothesize that a combination of a copper contacted in vivo environment and a high

strain in vivo environment contributed to the initial failure of the device. That is because the

exposure of the material to high strain and a cellular environment, as seen in condition 4 of test

2, leads to a drop in ultimate tensile strength and % elongation in select samples as well as

formation of pores. In the case of the copper contacted material, the presence of high salinity

liquids and copper lead to cracking and loss of ultimate tensile strength and % elongation. The

electron micrographs confirm our conclusion due to the fact that copper contact and PBS induces

cracks in the material, which can be easily exacerbated by additional stress. Also, the presence

of PBS and strain along with glass wool also is demonstrated to greatly degrade the material

properties of some samples as well as generate vulnerable pores. Our results are also confirmed

by the fact that in explanted cardiac leads, the cracking occurs in the copper contacted surface

through oxidation of the ether soft segment [5]. The reason of degradation and oxidation have

been proposed to be hydrogen peroxide diffusing through the insulation to react with the wiring,

forming oxygen species that chemically degrade and embrittle the polyurethane [5].

The progression is that the degradation starts at the copper contacting portion of the

polyurethane insulation. The diffusion of oxidative elements through the polyurethane in vivo

helps with the degradation of the polyurethane material. The presence of extracellular liquids

and copper leads to formation cracks as well as a massive decrease in ultimate strength. Then,

the cracks propagate along the insulating material from the original cracked zone outwards. The

propagation of cracks also deepens at original site leading to a failure of the outer sheath. This

leads to a loss in insulation, leading to the device failure due extracellular fluid contacting the

copper wiring, which causes dispersions of electrical signal. This leads to cardiac devices such as

pacemakers and defibrillators failing to perform their designed functions, which may have life

threatening implications. Our hypothesis is supported by a case study examined by Christenson

et al, where explanted failed cardiac leads were examined [5]. The explanted leads were found

to have cracked metal contacting surfaces, and it was hypothesized that hydrogen peroxide

diffused through the outer insulation and after contact with the conductive surface, formed

oxygen species that degraded the metal contacting portion of the polyurethane implant [5]. They

found that chemical degradation resulted in a brittle surface that is more susceptible to cracking,

and that the material failed under the repeated physical strain of intercorporeal movements,

leading to electrical dysfunction of the cardiac device due to insulation failure [5].

A variety of biological factors contribute to the degradation of polyurethane in an in vivo

or simulated in vivo environment. The unique environmental factors, such as stress,

biochemicals, molecular and phase structure of the polymer, metallic ions, salts, inorganic and

organic acids and cells, work together to degrade polyurethane in a cellular environment [8].

Hydroxyl radicals are the primary species in the degradation of polyurethane via oxidative

hydrolysis of ether and urethane linkages [9]. It is highly unlikely that oxidation alone drives the

breakdown of polyurethane since hydrolytic processes catalyzed by enzymes have been shown to

be a major factor [8]. The inflammatory response to the presence of a polyurethane implant

contributes to its degradation through hydrolytic activities and the release of oxidative

compounds. This degradation is predominantly caused by white blood cells and monocyte-

derived macrophages in particular, as studies have shown these cells release highly oxidative

hydrogen peroxides and other compounds [8].

The presence of water of relatively high salinity results in higher chances of degradation

due to the fact that the higher the salinity of water, the greater absorption by the polyurethane,

which in turn leads to higher chances of activities that lead to degradation such as hydrolytic

reactions [10]. The water acts as a plasticizer, decreasing elastic modulus and tensile strength

while increasing elongation [11]. The presence of strain during the aging process leads to a

higher ultimate strength in tested components due to the fact soft-segment chains line up and

crystallize [8] [12]. The free volume decreases in the polymer as well due to the fact that the

chains line up [12]. These factors also reduce the material’s vulnerability to water and enzyme,

thereby reducing the hydrolysis of the carbonate groups [8]. Manufacturing factors may also

have contributed to the degradation of the polyurethane, as environmental stress cracking can be

induced due to polymer surface stress that was introduced during fabrication of the material and

not sufficiently reduced by thermal annealing [8].

To strengthen the material, a variety of techniques can be considered. Starting in the

manufacturing phase, one can increase the time that thermal annealing is done to the

polyurethane in the appropriate temperature range, in order to minimize the micro-fissures

caused by environmental stress cracking due to residual polymer stress. In relation to the

chemistry of the polyurethane itself, one can reduce the polyether segments of the soft-segment

portion of the polyurethane to eliminate the vulnerability to oxidation. Techniques include

induction of large hydrocarbon segments between ether groups and the incorporation of silicone

segments [8]. Another approach is to use ampiphilic copolymers to change the surface chemistry

of the polyurethane while retaining the physical properties of the bulk polymer. One can mask

the oxidation sensitive segments of the surface of the polyurethane by introducing silicone

containing oligomeric surface modifying additives, or one can reduce the extent of hydrolysis via

fluorinated surface modifying macromolecules. Anti-oxidants could also be delivered to the

surface of the polymers via oligomeric polymers to pro-actively reduce oxidation and minimize

hydrolysis [8].

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