Feature Article

The Use of PEEK for Advanced Active Implants


Posted in Implantable devices by yklopping on March 1, 2010


Implantable polyetheretherketone (PEEK) is a biocompatible and stable polymer that offers a variety of advantages over other accepted implant materials such as ceramics, metals and other polymers. Its properties have been assessed to determine its potential benefits for active implantable medical devices.


A range of unique functions
The active implantable medical device (AIMD) sector is one of the most dynamic in the health care system and AIMDs are unique products. The functions they are designed to perform rely on an electrical energy or another power source not provided by the human body or gravity, and they must be designed to remain within the body following insertion by a surgeon or other medical professional. Examples include pacemakers, defibrillators and other cardiac rhythm management devices.

In addition to the robust and established cardiac market, the continuously evolving AIMD sector also encompasses the neuromodulation field. Propelled by clinical and technological advances, neuromodulation is one of the most rapidly growing areas of medicine today. Neuromodulation is defined as therapeutic alteration, stimulation, inhibition or regulation of activity in the central, peripheral or autonomic nervous systems, electrically or pharmacologically, by means of implanted devices. Applications include chronic pain management, stimulation for movement disorders, urological disorders, spasticity and gastric deficiencies (Figure 1).

FIGURE 1: Eon rechargeable spinal cord stimulation device used for the treatment of chronic pain, which has a PEEK polymer for the housing. (Image courtesy of St. Jude Medical Inc.)

Titanium (Ti) is one of the most widely used materials in the AIMD sector, in part because of its biocompatibility and mechanical performance. However, there are some disadvantages in using a metal. These include poor electric insulation and telemetry behaviour, poor magnetic resonance imaging (MRI) and lengthy battery recharging times with associated device temperature increase.

Polyetheretherketone (PEEK) polymer is a high performance biomaterial that offers interesting solutions for implant manufacturers. In the past 10 years, implantable PEEK polymer has experienced increased use in a variety of medical devices. Applications include spinal fusion, joint replacement, trauma applications, cranio-maxillofacial implants, dental implants and components such as connectors, leads and enclosures in AIMDs. The diverse nature of its implementation is due in part to the versatility of the material. Implantable PEEK has been used in conjunction with, or to replace, more traditional materials such as Ti and ceramics. It can be processed by conventional methods, for example, injection moulding and extrusion, and can be machined, which allows broad design and manufacturing flexibility. The material properties of implantable PEEK polymer have been analysed to assess its potential benefits to the AIMD sector.

TABLE I: PEEK electrical insulation and telemetry behaviour.2
Property Units PEEK
Dielectric strength KV.mm-1 1.98 × 10+02
Volume resistivity Ω.cm 4.9 × 10+16
Dielectric constant
(23 °C, 1 kHz)
3.3
Eddy current loss mW ≈ 0
Telemetry attenuation at 400 MHz ≈ 0

Sterilisation resistance. Implantable PEEK polymer can be repeatedly sterilised using conventional sterilisation methods including steam, gamma radiation and ethylene oxide (EtO) processes without the degradation of its mechanical properties or biocompatibility.

Tailored implant radiopacity. The demand for enhanced in situ implant visualisation through radiograph and MRI techniques has increased in the past years. PEEK polymer is naturally radiolucent and compatible with imaging techniques such as X-ray, MRI and computer tomography. Unlike metals, which generate imaging artefacts and scatter, PEEK image contrast grades provide the possibility of tailoring the visibility of an implant to suit a particular application (Figure 2). It is, therefore, possible to achieve an appropriate balance of implant, bone and tissue visualisation.

 

FIGURE 2: X-ray imaging of image contract grades of an implantable PEEK polymer compared with aluminium. (Image courtesy of Victoria Hospital, Blackpool, UK)
 

Mechanical performance. Even after sterilisation, implantable PEEK provides a superior combination of strength, stiffness and toughness. Furthermore, it displays extreme resistances to hydrolysis and implantable stability, as confirmed by 30-day exposure to simulated body fluid environments with no adverse influence on the material’s mechanical properties. When the mechanical properties of metals are required, PEEK polymer can be tailored by adding reinforcing fibres to substantially increase the strength of the natural unfilled polymer, while providing all the benefits of creep resistance, light weight and MRI and X-ray compatibility. The thin wall impact strength properties of implantable PEEK were found to be within the range of traditional implantable metals (Figure 3) (method reference ASTM D3763 Test Method for High Speed Puncture Properties of Plastics Using Load and Displacement Sensors). Those mechanical performance properties have supported the use of PEEK in structural applications in AIMDs such as components and enclosures.

Electrical insulation and tele-metry. The dielectric strength properties of PEEK are stable over a range of temperatures, humidities and frequencies, thus making it suitable for use in a variety of electrical insulation applications (Table I). Implantable devices with recharging batteries are prone to temperature increases because of their metal enclosures. This temperature increase is mainly a result of energy losses from generated eddy currents. As an electrical insulating material, PEEK polymer displays clear telemetry advantages over implantable metals. The material allows for radio frequency communications with no energy attenuation. Moreover, the induction charging of implantable batteries can be performed through a PEEK barrier with no heat being generated from eddy current losses.

Joining options
Research into the adhesion behaviour of implantable PEEK polymer has been pursued to provide device designers with a range of solutions for implementation in AIMDs. PEEK can be joined via fusion welding, laser welding, ultrasonic welding and adhesive joining. These processes have been proven to generate strong joints.

Tests conducted with several adhesives, for example, silicone, epoxy and acrylic, demonstrate that PEEK can be successfully joined. Furthermore, it was demonstrated that the joint strength of PEEK-to-PEEK joints using implantable silicone adhesives remained mostly unchanged before and after sterilisation by steam, EtO and gamma sterilisation.

As with most polymers, implantable PEEK has a low surface energy. Therefore surface treatments can be implemented to increase the surface energy of the material. As shown in Figure 4, PEEK joint strength was successfully enhanced with the use of surface pretreatments. Surface treatments were also observed to enhance the adhesion of PEEK to metals (Ti and cobalt chromium alloy) and to generate strong bonds.

Barrier properties
Implantable PEEK displays good resistance to gas permeation such as oxygen and helium. Furthermore, under humidity conditions the polymer shows low water absorption rates (0.5% wetwater basis). For an advanced implantable medical polymer, PEEK also displays low water vapour permeation. These barrier properties are shown in Table II.

FIGURE 3: Falling weight impact energy of PEEK and Ti films (method reference ASTM D3763).2 FIGURE 4: Impact of surface treatment in enhancing the joint strength of PEEK-to-PEEK bonded joints using implantable silicone adhesive.3

The long life expectancy of AIMDs means there is a need for long-term material hermeticity for applications such as some implanted pulse generator enclosures. One option for PEEK barrier enhancement is the metallisation or coating of its surface. It has been observed that Ti alloy coating, single sapphire layer coating and epoxy adhesive potting would allow for a substantial reduction in the water vapour permeability rate. Another alternative that medical device manufacturers have successfully followed is redesigning devices by incorporating hybrid PEEK–metal enclosures.

Substrate for implantable electrodes
PEEK polymer is receptive to being printed on by normal methods such as screen printing, transfer printing and ink jet printing. Studies conducted in collaboration with the University of Strathclyde (Glasgow, UK, www.strath.ac.uk/bioeng) evaluated PEEK performance as a substrate material for screen printing of implantable electrodes and flexible circuits. It was observed that amorphous and semicrystalline PEEK films (100-µm thick) can be used to screen print platinum and silver (Ag/AgCl) electrodes. Platinum and silver PEEK electrodes were flexible and resistant to manual bending, ultraviolet and EtO sterilisation and 72-hour saline immersion.

Impedance measurements on PEEK electrodes displayed similar behaviour to respective standard wire electrodes. PEEK electrodes showed stable impedance behaviour in physiological saline solution over 72 hours. Electrode impedance was measured with a 1260 impedance analyser (Solartron, CS international Ltd., UK, www.tics.co.uk), over a test frequency of 0.1 Hz to 10 MHz with an applied 200 mV amplitude. Also, fibroblast cells were confirmed to have grown in the presence of platinum screen printed PEEK electrodes. Therefore, platinum PEEK electrodes are found to be viable implantable solutions with no apparent change in their morphology.

An advanced solution
There has been progressive expansion in the adoption of implantable PEEK polymer for AIMDs since its introduction in 1999. Its benefits support it as the material of choice for the development of the next generation of AIMDs.

Acknowledgements
The work on PEEK substrate for implantable electrodes was conducted in collaboration with V. Raghunathan and P. Connolly at the University of Strathclyde, Glasgow, UK, www.strath.ac.uk/bioeng.

References
1. E.S. Krames at al., “What is Neuro-modulation?” Neuromodulation,Volume 1, E.S. Krames, P.H. Peckham, A. R. Rezai, Eds., Academic Press, San Diego, USA, 3–8 (2009).
2. Data on file at Invibio Ltd, Thornton Cleveleys, UK.
3. M. Tavakoli et al., “An Assessment of Adhesive Bonding of PEEK-OPTIMA for Medical Device Applications,” Proceedings of Medical Polymers 2004, 4th international conference focusing on polymers used in the medical industry, Smithers Rapra Ltd, Shrewsbury, UK (2004).
4. S. Green, “Implantable Device,” World Intellectual Property Organisation,
WO 2009/068914A1.

Nuno Sereno PhD is a Product Development Project Manager at Invibio Ltd, Technology Centre, Hillhouse International, Thornton Cleveleys FY5 4QD, UK tel. +44 1253 898 000, e-mail: nsereno@invibio.com www.invibio.com

 



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