New material for implantable cardiac leads

Introduction 

Implantable cardioverter-defibrilators (ICDs) have become the standard care for the prevention of sudden cardiac death and for the management of ventricular arrhythmias. These indications resulted in a 20-fold increase in annual device implants during the last 15 years.

The same improvement of technology was applied to ICD leads and to left ventricular leads trying to offer the best performance and the easiest technique of the implant. Leads must survive mechanical stress due to millions of cardiac contractions and allow high-voltage energy delivery for defibrillation when necessary. Nevertheless, several studies demonstrated that the Achilles' heel of the ICD system is a long-term reliability of the leads. It has been previously shown that the braided multifilament wire electrodes have a high failure rate both for sensing of spontaneous heart activity and heart stimulation.

It is understandable that with the increasing age of the leads, the risk of malfunction also increases. In fact, the reported incidence of malfunction rate of the leads reaches 40% after 8 years.1, 2

The most common abnormalities reported by the studies are the insulation defect and a mechanical failure. This mainly affects patients' safety, which depends on appropriate detection of potentially lethal ventricular arrhythmias and on successful delivery of therapy; Duray et al3 reported the lead malfunction rate of approximately 5% of the 816 patients who underwent the transvenous ICD implantation. These results are comparable to data reported by Eckstein et al.4 A rate of ∼5% per patients-years of malfunction observed by Duray and colleagues is “understandable” considering the rapid evolution of the technology during the last 10 years, but not yet acceptable considering the particular role.

Nevertheless, the failure of ICD leads may have substantial clinical consequences, including failure to sense, failure to pace, failure to defibrillate, inappropriate shocks, and even death of the patient.4 The reported ICD lead survival varies significantly between studies: 91% to 99% at 2 years, 85% to 95% at 5 years, and 60% to 72% at 8 years.5

A recent Food and Drug Administration recall of Sprint Fidelis leads, which are prone to pace-sense lead fractures,6 exemplifies the problem with current implantable lead technology.

It is therefore important to explore new technological developments in lead materials that will result in higher reliability implantable leads.

We have searched for high-strength polymer fibers as candidates for implantable leads materials. We hypothesized that the incorporation of gold into a polymer matrix of the fiber will result in a mechanically superior, highly electrically conductive fiber.

The hybrid polymer fiber should be lightweight and chemically inert and should contain a low-volume fraction of metal while retaining the metallic conductivity comparable with current state-of-the-art implantable materials such as steel alloys.

Fiber material selection 

Polymers suitable for use in implantable leads include those that may be formed into fibers by a coagulation process or those that can be swelled in a solvent to allow infiltration of the metal precursors. We investigated the following candidates: rigid-rod polymers such as poly(p-phenylene benzobisthiazole) (commercially available from SRI International, Menlo Park, CA), poly(p-phenylene benzobisoxazole) (PBO; commercially available from Toyobo under the designation Zylon, Osaka, Japan), poly(p-phenylene benzobisimidazole), ladder polymers such as poly(imidazoisoquinolines), and extended-chain polymers such as poly(p-phenylene terephthalamide) (commercially available from DuPont under the designation Aramis, Wilmington, Delaware).

We have selected the PBO (Zylon) as the best candidate for the lead material due to its superior mechanical properties and suitability for gold cladding.7 Of particular interest for implantable lead material are the mechanical properties of Zylon. Table 1 summarizes the mechanical properties of Zylon as compared with other modern fibers.

Table 1. Comparison of Zylon's mechanical properties to other fibers

		Tenacity		Modulus		Elongation (%)	Density (g/cm3)	Moisture regain (%)	LOI	Heat resistancea (C)
		cN/dtex	GPa	cN/dtex	GPa
	Zyfon AS	37	5.8	1150	180	3.5	1.54	2.0	68	650
	Zylon HM	37	5.8	1720	270	2.5	1.56	0.6	68	650
	p-Aramid (HM)	19	2.8	850	109	2.4	1.45	4.5	29	550
	m-Aramid	4.5	0.65	140	17	22	1.38	4.5	29	400
	Steel fiber	3.5	2.8	290	200	1.4	7.8	0
	HS-PE	35	3.5	1300	110	3.5	0.97	0	16.5	150
	PBI	2.7	0.4	45	5.6	30	1.4	15	41	550
	Polyester	8	1.1	125	15	25	1.38	0.4	17	260

Zylon has the highest tensile strength and tensile modulus among high-performance fibers. Zylon is more than 5 times stronger than steel fibers. LOI indicates limiting oxygen index; HS-PE, high strength PE; PBI, poly(p-phenylene benzobisimidazole.

a Melting or Decomposition Temperature.

Poly(p-phenylene benzobisoxazole) is a lyotropic liquid crystalline polymer that is soluble in strong acids such as poly-phosphoric acid (PPA) and sulfuric acid. When PBO is dissolved in such acids, the resulting concentrated solutions may be extruded and coagulated to form high-strength, high-modulus (HM) fibers (Fig. 1, Fig. 2, Fig. 3).

View Large Image Download to PowerPoint

Fig. 1. Technical parameters of Zylon AS and HM. HM is different from AS in modulus, moisture regain, and other properties.

View Large Image Download to PowerPoint

Fig. 2. Stress-strain characteristic of the PBO HM fibers as compared with other high-performance fibers.

View Large Image Download to PowerPoint

Fig. 3. Strength-twist relationship.

Properties of the PBO 

The following are properties of the PBO fibers that make them good candidates for implantable cardiac leads:

Influence of twist on tensile properties 

Twist factor is defined by the following formula:

Zylon retains an excellent strength for twist factors in the range from 0 to 30.

Creep properties 

Zylon has superior creep resistance to p-Aramid fibers. (Creep means a nonrecoverable strain after prolonged static loading.) When a certain load is applied to yarn, recoverable strain (initial strain) and nonrecoverable strain are observed. For Zylon HM, nonrecoverable strain after 100 hours under 50% of breaking load (safety factor = 2) is less than 0.03%.

Creep parameters (slope of straight line in Fig. 4) are compared with p-Aramid fibers. Zylon shows less than half of creep paremeter of p-Aramid fiber. Creep strain is measured under 50% of the breaking load for each fiber. Note that the actual load applied to Zylon is almost double that of p-Aramid fiber (Table 2).

View Large Image Download to PowerPoint

Fig. 4. Creep resistance of the Zylon fibers.

Table 2. Creep parameter under 50% of breaking strength

	Zylon AS	Zylon HM	p-Aramid	p-Aramid HM
	3.2 × 10−4	1.1 × 10−4	5.0 × 10−4	2.5 × 10−4

Moisture pickup 

The moisture regain of Zylon at 200°C, 65% relative humidity (RH) is 2.0% for as spun (AS) and 0.6% for HM. The moisture regain of Zylon HM is far less than that of p-Aramid (Fig. 5, Fig. 6, Fig. 7, Fig. 8).

View Large Image Download to PowerPoint

Fig. 5. Effect of temperature (°C) on strength and modulus.

View Large Image Download to PowerPoint

Fig. 6. Zylon resistance to strong inorganic acids as a function of time (hours).

View Large Image Download to PowerPoint

Fig. 7. Zylon resistance to alkalines.

View Large Image Download to PowerPoint

Fig. 8. Moisture pickup by Zylon fibers.

Fatigue property of Zylon 

Most important parameter of the PBO is anisotropic mechanical property, which determines the fatigue resistance of the fiber. Yamashita et al8 reported an experimental study to determine the fatigue parameters of the PBO fiber. Single-fiber tensile testing machine (KES-G1), single-fiber twisting tester (KES-GX), single-fiber transverse testing machine, and twisting tester (KES-G7) for the axial compression were used for the measurement. The measurement was done at room temperature (25°C). The single-fiber tensile measurement was 50 mm in the length of the sample, and tensile speeds 0.5 mm/s (the 1%/s strain rate). Strain (0.4%, 0.8%, 1.2%, 1.6%, 2.0%) was loaded 1000 times. The single-fiber twist measurement was done at the 2.0-mm sample length.

The fiber did not break in the twist strain test. Shear strain (2.2%, 4.4%, 8.8%, 13.2%, 17.6%, 22.0%) was taken to 1000 times. Modulus of transverse direction was measured with the single-fiber transverse compression testing machine.

The fiber axial compression sample was made as follows. The compression strain caused by the twist of the rod is loaded to the fiber. This technique is easy and is the best for the measurement of the axial compression fatigue. Fatigue was repeatedly loaded 1000 times.

Fig. 9A shows the tensile fatigue property. Fig. 9B shows the result of the fatigue property standardized by the first cycled modulus. The decrease in modulus by the cyclic fatigue does not depend on the strain, and it has very small fatigue in the tensile strain (Table 3).

View Large Image Download to PowerPoint

Fig. 9. A, Relationship between the tensile modulus and the frequency of the fatigue for different strain vales. B, Relationship between the tensile relative modulus and the frequency of the fatigue for different strain vales.

Table 3. Resistance of Zylon to organic chemicals

	Chemicals	Strength retention (%) after immersion for 500 h at room temperature
		Zylon AS	Zylon HM
	Methylethylketone	100	99
	Dimethylformamide	100	97
	Methanol	100	99

Longitudinal compression characteristic ELC 

The testing method that loaded compressive strain on the fiber was established by winding the fiber to plastic round bar at the angle of 45 degrees and by twisting it. As this measuring technique, the experiment and the fatigue measurement are easy. Fig. 10 shows the relation between the axial compression stress and strain of the PBO fiber obtained by this technique. In the tension side, PBO fiber shows the tensile modulus over the double in comparison with Kevlar fiber. However, there is hardly any difference between these fibers in the compression side.9

View Large Image Download to PowerPoint

Fig. 10. A, Longitudinal property of PBO and Kevlar. B, Relationship between torque and twist angle of the rod.

Methods 

We have constructed a prototype pair of pacing and sensing leads using PBO as a carrier. A PBO fiber was processed from a 14 wt% PPA dope at 90°C, by a dry-jet, dry-spinning method using a 0.015-in spinneret. The fiber was air cooled and collected on a take-up drum. A draw ratio of 40:50 was achieved for this fiber (defined as the ratio of the take-up speed to the extrusion rate). A 10-ft segment of the PBO fiber was coagulated in water to remove PPA and was subsequently air dried. This fiber was designated as pristine fiber. Another 10-ft segment of the PBO fiber was immersed in a 55 wt% silver nitrate solution for 10 minutes to allow formation of silver salts in the fiber. The fiber was then soaked in a 2 wt% NaBH4 solution for 10 minutes to reduce the silver salts. This fiber was designated as silver-infiltrated PBO fiber. The silver-infiltrated PBO fiber was further gold coated using an electrochemical process.

Results 

The silver-infiltrated gold-plated PBO fiber showed a slightly large diameter than the pristine fiber. The tensile strengths of the 2 fibers are identical within experimental error when the effect of fiber diameter is taken into consideration. It can be concluded that the PBO fiber did not degrade due to the infiltration and the reduction of the silver salts.

The electrical and mechanical properties of the PBO gold-clad fiber are summarized in Table 4. Poly(p-phenylene benzobisoxazole) is compared with a standard beryllium copper wire to better understand PBO's electric and mechanical properties. Scanning electron micrograph of cross-section of gold-metallized PBO is shown in Fig. 11.

Table 4. CFM fiber material specificitions⁎ and comparison

	Conductive PBO fiber material (CFM)	CFM	Beryllium copper wire
	Weight: 120-in specimen (g)	0.3179	0.4332
	Resistance (milliohm)	78.70	52.73
	Fusing time at 9 A (s)	>300	>300
	Breaking strength (lbs) requirement for 26 gauge: 21.5 lbs	60.3	29.47
	Fluid immersion	All performed similarly	All performed similarly
	Flammability	Passed	Passed
	Smoke	Passed	Passed
	Toxicity	Passed	Passed
	Dynamic cut through force (lbs)	Ambient: 43.7-114.2	Ambient: 10.5-13.7
		200°C: 22.7-49.3	200°C: 3.6-6.4
	Shielding effectiveness (dB) (0.1-10 GHz)
	100-500 MHz	32.2	35.4
	500-1000 MHz	39.5	41.1
	1-2 GHz	40.0	39.0
	2-4 GHz	42.4	41.7
	4-8 GHz	42.5	43.0
	8-10 GHz	42.2	39.8

CFM indicates conductive fiber material.

⁎ The above specifications are for 3 twisted strands of gold-clad fibers.

View Large Image Download to PowerPoint

Fig. 11. Scanning electron micrograph of the cross-section of gold-metallized PBO multiple-strand fiber.

The morphology of plated fibers was further studied by x-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, and electrochemical polarization measurements. It was found that gold was uniformly plated on the PBO fiber, and the gold-plated fibers have good corrosion resistance. The electrical conductivity of the gold-plated fibers were higher than 4 × 104 S/cm, and its tensile strengths and Young moduli were greater than 1.9 and 130 GPa, respectively, when estimated in terms of a single-fiber strand.

Preliminary dog study test results 

The experimental pacing and sensing leads comprising gold-plated Zylon fiber were constructed. The lead isolation between the pacer and the sensor tip was accomplished by Parylene C coating. The leads were implanted into a dog. Both pacing and biopotential sensing capability of the leads was tested by acquiring on electrocardiogram from the right ventricle (RV) of the dog as shown in Fig. 12.

View Large Image Download to PowerPoint

Fig. 12. Paced electrocardiogram recording obtained by a sensing PBO fiber placed in the dog's right ventricle.

We measured the RV sensor impedance of 350 ohm and observed good sensing capability of the Zylon gold-plated leads.

Conclusions 

We have constructed gold-plated PBO (Zylon) fibers and coated them with Parylene C to assemble experimental pacing and sensing implantable leads. The high strength and resistance to flexing and twisting of the Zylon fibers have a potential to eliminate the mechanical failures of the implantable leads. The gold plating resulted in excellent electric conductivity and low impedance of the pacing fiber. Due to a very small diameter of the experimental leads, it was easy to place them inside the dog's heart using a standard technique. The hybrid-metallized PBO polymer fibers have advantages over conventional metal cardiac leads in mechanical strength, durability, and tailored electrical conductivity. Therefore, the combined engineering properties of the new fiber afford implantable cardiac lead applications at reduced diameter while having higher strength. Furthermore, the new fiber can be terminated just like a regular metal wire with the choice of ultrasonic bonding, crimping, or band connection. This very preliminary study confirms our initial hypothesis that metal-plated polymers such as Zylon may be good candidates for future implantable cardiac leads designs. Future research is needed to further investigate PBO fibers mechanical properties in environments that mimic the interior of the human body.