In vitro evaluation of a new resilient, hard-carbon, thin-film coating as a bearing material for ventricular assist devices ——In Vitro Bearing Evaluation of BioMedFlex

doi:10.4236/jbise.2010.35073

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J. Biomedical Science and Engineering, 2010, 3, 525-528

doi:10.4236/jbise.2010.35073 Published Online May 2010 (http://www.SciRP.org/journal/jbise/ JBiSE

Published Online May 2010 in SciRes. http://www.scirp.org/journal/jbise

In vitro evaluation of a new resilient, hard-carbon, thin-film

coating as a bearing material for ventricular assist devices

——In Vitro Bearing Evaluation of Bi oMedFlex

Nicole A. Mielke1, Alex L. Massiello1, David J. Horvath1, Stephen M. Benefit1, Darren Burgess2,

Leonard A. R. Golding1, Kiyotaka Fukamachi1

1Department of Biomedical Engineering/ND20, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio, USA;

2BioMedFlex, LLC, Denver, North Carolina, USA.

Email: [email protected]

Received 1 December 2009; revised 14 December 2009; accepted 20 December 2009.

ABSTRACT

Our aim was to evaluate the potential use of Bio-

MedFlex® (BMF), a new resilient, hard-carbon, thin-

film coating, as a blood journal bearing material in

Cleveland Heart’s continuous-flow left and right ven-

tricular assist devices (VADs). BMF is not classified

as a diamond-like carbon (DLC) and differs from

other thin-film carbon coatings by its high flexural

strength, radiopacity, and wear resistance. A 2- to

4-μm-thick BMF adhesion layer was deposited on the

VAD journal bearing surfaces. A commercial DLC

coating used in other clinical blood pump applica-

tions was used as a control. Durability and reliability

of the BMF coating was verified in severe pump

start/stop testing using 20 BMF-coated journal bear-

ing pairs. The BMF-coated surfaces showed no coat-

ing failures, whereas 57% of the DLC bearing pairs

developed scratches through the carbon coating, do-

cumenting that BMF can provide a durable coating

in our blood journal bearing application. In conclu-

sion, BMF has shown qualities that support its sig-

nificant advantages as an alternative journal bea-

ring material in Cleveland Heart pumps. Our plan

includes biocompatibility testing with ongoing animal

studies, endurance testing with submerged pumps

running in saline, and assessment of batch coating

processing capability.

Keywords: Heart Assist Device; Diamond-Like Carbon;

Materials Testing; Wear Resistance; Journal Bearing

1. INTRODUCTION

Implantable ventricular assist devices (VADs) are a

reliable treatment option for patients with terminal-

stage heart failure who are unresponsive to conven-

tional therapies. Although current designs demonstrate

adequate performance, reliability remains a significant

issue, as these devices are implanted for extended dura-

tions (well beyond 1 year) and may likely be intention-

ally implanted in patients as permanent (“destination”)

therapy. Because components of implantable pumps

must withstand difficult in vivo environments and run-

ning conditions, carbon materials (diamond-like carbon

[DLC] and pyrolytic carbon) have emerged as promising

coating materials for pump components to enhance du-

rability and biocompatibility characteristics. Some of

these carbon coatings are chemically inert, wear and

corrosion resistant, and bio- and hemocompatible. These

coatings may also minimize platelet adhesions and acti-

vation, prevent thrombogenicity, and improve perform-

ance by decreasing power usage or increasing pump

output.

The continuous-flow VADs of Cleveland Heart (Char-

lotte, NC) have been adapted from the CorAide LVD-

4000 Left Ventricular Assist System (Arrow Interna-

tional, Reading, PA), developed at Cleveland Clinic.

Detailed descriptions of the pump have previously been

published [1,2]. The left and right ventricular assist de-

vices (LVADs and RVADs, respectively) consist of three

subassemblies: the volute housing, the rotatin g assembly

(RA), and the stator assembly (Figure 1). The RA con-

tains a cylindrical four-pole magnet and spins aroun d the

titanium stator housing post, which contains the motor

windings. The RA is supported axially by permanent

magnets and radially by a thin film of blood, forming a

patented blood journal bearing which, except for startup

and shutdown, makes no mechanical contact during use.

The blood journal bearing materials, dimensions, clear-

ances and geometry are shared by both VADs. The vo-

lute housings and RA impeller blades are optimized for

the different afterload conditio ns that occur in the RVAD

526 N. A. Mielke et al. / J. Biomedical Science and Engineering 3 (2010) 525-528

Figure 1. Depiction of an LVAD subassembly. RVADs differ

only in RA impeller and in volute housing design.

vs. LVAD.

During European clinical trials of the CorAide LVAD,

it was found that the original fluorinated ethylene propyl-

ene (FEP) coating on the journal bearings delaminated

from its titanium substrate. The authors have since sea-

rched for the best alternative bearing material to use

on both the LVAD and RVAD; this article focuses on

the preliminary in vitro evaluation of the BioMedFlex

(BMF) thin-film coating (BioMedFlex, Denver, NC).

2. MATERIAL AND METHODS

2.1. BioMedFlex Coating

The BMF coating is deposited on substrates by a pro-

prietary plasma-assisted chemical vapor deposition

process in a high-vacuum environment at temperatures

below 200℃. Because this coating is created from layers

of nano-crystalline diamond and nano-crystalline silicon

carbide in a matrix of the noncrystalline forms of both

compounds, BMF’s unique characteristics are set apart

from other thin-film carbon coatings used in biomedical

applications [3]. Its high flexural strength, its radiopacity,

and its wear resistance are surpassed only by pure dia-

mond thin films. Several other material properties of the

BMF coating that make it a good candidate for use in

blood journal bearing applications include chemical re-

sistance, dimensional stability, and low coefficient of

friction. BMF does not absorb water and does not cor-

rode in saline. These two key qualities prevent dimen-

sional changes due to absorption or corrosion in the

high-tolerance clearance and profile of the journal bear-

ing-conditions that occurred with the FEP coating in the

CorAide clinical trial. BMF is a clean, reliable, high-

technology coating process that is not line-of-sight and

does not require parts to be rotated in the chamber. The

BMF coating is 2-4 m thick and aggressively adheres

to titanium surfaces, requiring sandblasting to remove it

from the surface of coated parts. For reference, a matrix

comparing the material properties of BMF vs. titanium

nitride, a commonly used material in the field, is pro-

vided bel ow as Table 1.

2.2. Journal B ea r in g Starting Conditions

Once running, the bearing allows the RA to revolve

levitated from the stator housing post surfaces on a

film of blood, deriving lift from its own motion and

thereby eliminating any surface contact between mov-

ing parts until the pump is shut down (Figure 2).

However, there is transient (300 ms) sliding and vi-

brating contact (800-900 g) load between the RA and

stator assembly at start-up and again during stopping

(Figure 3); the only mechanical loading the bearing

undergoes is during its start/stop cycles. The worst-

case expected loading sustained by the bearing over its

lifetime was established based on the fact that, on av-

erage, the pump is started and stopped six times during

manufacturing and three times during clinical implanta-

tion. During its service life with the patient, the pump

may be started and stopped (in a worst case) eight times

per year due to external component changes, software

upgrades, or simple patient connection mistakes. These

values translate to about 50 starts and stops in a 5-year

period. Multiplying by a safety factor of 2, the number

of starts and stops that will approximately simulate the

worst-case service life is 100.

Table 1. Comparison matrix of material properties: Bio-

MedFlex vs. Titanium nitride.

Typical values BioMedFlex Titanium nitride

Inert Yes No

Flexural strength High triaxial Low single axis,

brittle

Optical transparency 50Å Yes, 2 μm No No

Mechanical wear Low High

Conformal Yes No: Line of sight

Hardness (V) 3800 2500

Young’s modulus (GPa) 130-180 600

Adhesion (PSI) > 8500 < 2000

Coefficient o f friction 0.10-0.15 0.65

Thickness (μm ) 0.1-4 0.25-12

Surface Ra (μm) 0.10 0.20

Figure 2. Journal bearing. Le ft: Inner surfaces of t he RA are

in contact with stator when pump is not running. Right: After

starting, the RA is separated from the stator and rides on a

film of blood dur ing operation. Assy, Asse mbly.

N. A. Mielke et al. / J. Biomedical Science and Engineering 3 (2010) 525-528 527

the DLC-coated parts limited the number of DLC test

pieces evaluated in this study, as the primary goal of the

project was to develop new journal bearing materials.

The success criterion was the completion of at least 100

start/stop cycles per bearing pair without damage to the

coated surfaces.

3. RESULTS

BMF-coated pumps showed no coating failures (Table

2), whereas four of seven commercial DLC bearing pairs

developed scratches through the carbon coating (Table

3), documenting that BMF can provide a durable coating

in our blood jour nal beari n g application. A representative

scratch is shown in Figure 4; scratches were present on

the stator post only, with no damage to the RA on com-

mercial DLC bearing pairs.

Figure 3. Pump vibration and motor current at start-up and

shut-down of a Cleveland Heart VAD.

2.3. Start/Stop Test

To evaluate the durability and reliability of the coating

on pump journal bearing mated surfaces, pump assem-

blies consisting of a BMF-coated stator and RA were

exposed to 100 start/stop cycles at the most challenging

bearing load conditions: maximum pump speed and

wide-open flow conditions in de-ionized water. The

pumps were disassembled, and the stator and RA were

visually inspected for scratches, delamination, or other

damage after 1, 5, 25, and 100 start/stop cycles.

4. DISCUSSION

Although titanium alloy has been the material of choice

for implantable rotary blood pumps [4], given the bear-

ing loads seen in our pumps at startup and shutdown, it

is a very poor choice as a bearing material. It has been a

priority to determine the optimum coating material and

coating parameters to obtain the bearing properties

needed for the Cleveland Heart LVAD and RVAD.

BMF’s unique thin-film carbon formulation and its lay-

ered matrix of diamond and silicon carbide have pro-

vided a new biomedical material that combines very

high hardness, aggressive substrate adhesion and much

higher flexural properties. BMF has proven to be more

wear resistant than the commercially available DLC

control.

A commercial DLC coating used in other blood pump

applications was also applied to another set of bearing

surfaces as a control. Twenty different combinations of

BMF-coated journal bearing pairs and seven different

DLC-coated pairs were exposed to start/stop testing to

compare the two coatings for durability and reliability.

The high component costs, long lead time for fabrication

and coating of parts, and the early and frequent failure of Previous studies of this bearing design have documented

Figure 4. BMF-coated RVAD RA (left) and stator assembly (middle); commercial DLC-

coated stator assembly scratches post-test (right).

Table 2. BMF bearing testing results.

BMF Coated RA

BMF Bearing Testing RA BMF-01 RA BMF-02 RA BMF-03 RA BMF-04

Stator BMF-01 Passed: No damagePassed: No damagePassed: No damage Passed: No damage

Stator BMF-02 Passed: No damagePassed: No damagePassed: No damage Passed: No damage

Stator BMF-03 Passed: No damagePassed: No damagePassed: No damage Passed: No damage

Stator BMF-04 Passed: No damagePassed: No damagePassed: No damage Passed: No damage

BMF-Coated Stator

Stator BMF-05 Passed: No damagePassed: No damagePassed: No damage Passed: No damage

BMF, BioMedFlex®; RA, Rotating Assembly.

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528 N. A. Mielke et al. / J. Biomedical Science and Engineering 3 (2010) 525-528

Table 3. Commercial DLC bearing testing results.

Commercial DLC-Coated Rotating Assembly

Commercial DLC Bearing TestingRA DLC-01 RA DLC-02 RA DLC-03 RA DLC-04 RA DLC-05

Stator DLC-01 Passed: No damagePassed: No damageFAILED FAILED -

Stator DLC-02 - - - Passed: No damage FAILED

Commercial

DLC-Coated

Stator Stator DLC-03 - - FAILED - -

DLC, diamond-like coating; RA, Rota ting Assembly.

that the blood film and journal bearing remain stable

once the start-up period is over [5]. Because of this,

damage to the bearing can occur only dur ing startin g and

stopping; it is not a matter of total running time, but the

number of start/stops that is critical. According to the

test procedure, pump disassembly and component in-

spection was performed only after the 1st, 5th, 25th and

100th start/stop cycle. Generally, bearing damage oc-

curred early on in the testing (within the first 5 start/stop

cycles) for the DLC-coated parts and was progressive

and cumulative. Representative bearing wear (Figure 4)

for the commercial DLC-coated stator is compared to

undamaged BMF-coated RA and stator parts post-bear-

ing test; bearing wear was only on the stator post of the

DLC-coated parts and not the RA.

With start/stop reliability demonstrated, endurance tests

of indefinite duration in 37℃ saline and glycerin blood

analog have been started to verify compatibility in a

saline environment. After 275 and 70 days, respectively,

there has been no bearing wear on two pump assemblies

with BMF paired journal bearings undergoing mock

circulatory in vitro endurance testing in an aggressive,

pulsatile environment.

BMF was successfully deposited onto titanium pump

surfaces and endured at least 100 start/stop cycles for

each bearing pair without damage; over 57% of the

commercial DLC-coated pairs failed at the same test

parameters. Five of the BMF stators accumulated up to

800 start/stop cycles with no wear. Neither the testing

duration nor number of start/stop cycles were increased,

as the test was deemed to be adequately aggressive given

the “failure” of the commercial DLC pairs. These tests

demonstrated that BMF can provide a durable coating in

our application, along with a viable solution to bearing

FEP coating adhesion problems seen in the initial Cor-

Aide clinical trials.

Biocompatibility is being validated with ongoing in

vivo studies of the performance of LVADs, RVADs, and

biventricular assist devices in animals [6].

5. CONCLUSIONS

BMF has many qualities that support its significant ad-

vantages as an alternative journal bearing material in

both Cleveland Heart pumps: 1) demonstrated bearing

reliability, 2) a thin-film coating that still offers a depth

of several micrometers, and 3) the capability for batch

coating processing. To further test durability, BMF-

coated pumps have been placed on mock circulatory

endurance test with the pumps running and su b merged in

a saline/glycerin blood analog fluid at body temperature.

6. ACKNOWLEDGEMENTS

This project was supported by the Global Cardiovascular Innovation

Center and the State of Ohio under the Third Frontier project. The

authors also acknowledge support from grant BRP 5 R01 HL074896

(to K.F.) from the National Heart, Lung, and Blood Institute of the

National Institutes of Health.

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