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Dr Orhan Soykan is a facultymember at Michigan Technological

University. He is also a Principal Scientist at the Materials

and Biosciences Center ofMedtronic, Inc., the world’s leading

manufacturer of implantable medicaldevices. He is also a seniormember of the Institute of

Electrical and Electronics Engineers,Inc. (IEEE). Dr Soykan has worked

at the Center for Devices andRadiological Health in the US Foodand Drug Administration (FDA). His

areas of expertise are inimplantable electronics and their

integration with molecular medicine.He holds a PhD in Electrical

Engineering from Case WesternReserve University.

a report by

D r O r h a n S o y k a n

Adjunct Professor, Center for Biomedical Engineering, Michigan Technological University

C a t e g o r i s a t i o n o f Imp l a n t a b l e D e v i c e s

Implantable medical devices can be categorised aspassive or active devices. Most passive implants arestructural devices such as artificial joints, vascular graftsand artificial valves. On the other hand, activeimplantable devices require power to replace oraugment an organ’s function or treat associated disease.Some implants that need power to operate and theconditions they are used to treat are listed in Table 1.

Power to these devices is supplied by one of twomeans:

1. internal batteries integrated into the implanteddevice; or

2. an external power source.

Powe r S u pp l i e s I n t e r n a l t o t h eImp l a n t e d D e v i c e

The most important factor for an implantable batteryis reliability. Unlike many consumer products,batteries in implantable devices cannot be replaced.They are hard-wired at the time of manufacture,before the device is hermetically sealed. From thatpoint on, the battery is expected to power the device

during final testing at the factory, during the shelf lifeand throughout the useful life of the device while itis implanted.

In general, the power source of the implantabledevice is the only component that has a known andpredictable service life, which, in turn, determinesthe service life of the implantable device itself.

Currently, implanted batteries are required to powerthe implant for five to eight years, with minimaldrop in the output voltage and without anyundesirable effects such as swelling due to gasgeneration. Most likely, by the end of that time,advances in technology would make replacement ofthe device desirable, regardless of the condition ofthe battery.

Both surgeons and patients demand that implantabledevices, and therefore the integrated batteries, be assmall as possible. A cardiac pacemaker takes upabout 20ml of space, and an implantable defibrillatortakes up three to four times that volume. In eithercase, about half of the occupied space is consumedby the internal battery. Therefore, the energydensity (energy/volume) and specific energy(energy/mass) are important considerations forimplantable batteries.

Most implantable devices are shaped as variations oncircular or elliptical objects to avoid having sharpcorners that might penetrate the skin or damagesurrounding tissues. Therefore, the batteries in thesedevices are shaped to conform to the overall devicegeometry, and often approximate a semicircle with aradius of 3cm and a depth of 8mm.

The battery itself is hermetically sealed inside thedevice where the metal case, usually stainless steel,constitutes one of the electrodes. The other terminalof the battery is available via a metal feed-through onthe flat portion of the battery.

Different types of implantable devices may haveradically different power requirements. Devices withlow power consumption and those with infrequenthigh power usage can utilise batteries internal to the

Power Sources for Implantab le Med ica l Dev i ces

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Device Technology & Applications ELECTRONICS

Table 1: Active Implantable Devices and the Conditions they are

Used to Treat

Implantable Device Medical Condition or Disease

Cardiac pacemakers Conduction disorders (bradycardia)

Heart failure

Cardiac defibrillators Ventricular and atrial tachyarrhythmia and fibrillation

Muscle stimulators Urinary incontinence

Faecal incontinence

Gastroparesis

Neurological stimulators Essential tremor (e.g. due to Parkinson’s disease)

Cochlear implants Hearing disorders

Monitoring devices Syncope

Seizures

Drug pumps Pain caused by cancer and its treatments

Pain from injuries sustained

Diabetes (external/internal insulin pumps)

Spasticity (intrathecal baclofen pumps)

Left ventricular assist devices Heart failure – bridge to transplant or recovery

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implantable device itself. For example, a cardiacpacemaker uses half of its battery power for cardiacstimulation and the other half for housekeeping taskssuch as monitoring and data logging. None of thesetasks requires high power. Therefore, a 1 amp-hourbattery built using lithium iodine technologyprovides nominally five years of operation inaddition to approximately six months of shelf life.Compared with lead, the same volume of lithiumprovides eight times as much electricity, at one-thirtieth the weight.

Implantable defibrillators, on the other hand, arecapable of providing electrical shocks six orders ofmagnitude larger than a pacemaker’s pulses, butmuch less frequently. Since the battery alone cannotproduce the shock pulse all at once, energy isdrained from the battery for aperiod of about 20 secondsand stored in an internalcapacitor before beingdelivered to the heart.During the charging period,an implantable defibrillatordrains 1–2 amps of current,which can be supplied bylithium silver vanadiumoxide batteries.

Lithium iodine batteries usedin implantable medicaldevices are of a type withsolid electrolytes. The anode(negative electrode of thebattery) is formed by lithiumwhile the cathode is acomplex formed by iodineand a polymer such as poly-2-vinylpyridine. The solidelectrolyte between these twoelectrodes – lithium iodide –gives the battery severaladvantages including:

• no potential for leakage; • increased reliability; and • extended shelf life of the

battery itself (up to 10years).

However, the lack of liquidelectrolyte reduces themobility of the ions in theelectrolyte, limiting theoutput current of the battery.Nevertheless, the success oflithium iodine batteries in theimplantable medical deviceindustry is evidenced by the

fact that more than five million of them have beenimplanted in pacemakers since 1972.

Some devices, such as drug pumps, require morecurrent than lithium iodine batteries can deliver.Drug pumps utilise electromechanical actuators tocreate high pressure inside a chamber and push thedrug from the reservoir to its target, such as thecerebrospinal fluid surrounding the spinal cord.

This pumping action is not continuous, but isperiodic or is triggered by the patient and requiresmany milliamps of current when the pump is turnedon. In these cases, batteries with low sourceimpedance, like lithium thionyl chloride, lithiumcarbon monofluoride or lithium silver vanadiumoxide, are chosen.


Most battery manufacturers do not allow the use oftheir products in an implantable medical device,particularly if the device is life-sustaining, as in the caseof cardiac stimulators such as pacemakers and defibrillators, due to the highly litigiousenvironment of the medical industry. Therefore, fewspecialised manufacturers of batteries produce batteriesfor implantable medical devices. Also, somemanufacturers of implantable medical devices buildtheir own batteries.

Imp l a n t e d D e v i c e s U t i l i s i n g E x t e r n a lP owe r S o u r c e s

Power to some implantable devices is supplied froman externally wearable source, either via a directelectrical or pneumatic linkage or via a radiofrequency (RF) link.

A d v a n t a g e s a n d D i s a d v a n t a g e s o f

E x t e r n a l l y P o w e r e d I m p l a n t a b l e D e v i c e s

Advantages include:

• continuous availability of high levels of power tothe implant;

• ability to control and interrogate the implantabledevice with the external device using the sameRF link; and

• lifetime and shelf life of the implant are notrestricted by the battery.

Disadvantages are:

• dependence on patient compliance;

• need for back-up power;• need for accurate positioning of the external device;• maintenance of external device and batteries;• possible RF interference;• heating of the tissue by transmitted RF power;• low cosmetic acceptance; and• risk of infection.

Some devices are too small to contain batteries forlong-term usage. In the case of cochlear implants, anexternal device containing the microphone andspeech analyser provides coupling to the implant.Power and data are transmitted by generatingelectromagnetic waves in the RF range.

L e f t V e n t r i c u l a r A s s i s t D e v i c e s

Other implants such as left ventricular assist devices(LVADs) require much more power than could besupplied by implanted batteries for any length oftime. LVADs are all powered transcutaneously,either electrically or pneumatically.

Some of the electrically powered devices containimplanted rechargeable batteries that allow up to20 minutes of autonomous operation – longenough for a patient to take a shower and changeclothes. The implanted rechargeable batteries arerecharged by the external power sources that, inturn, are likely to be rechargeable themselves. Theelectrically powered LVADs also have no tubingpenetrating the skin, therefore reducing thechances of infection.

Pneumatically powered LVADs are usually smallersince they do not need power couplers, powerregulators and implanted rechargeable batteries, butthey rely on external rechargeable batteries forgeneration of the pressurised air. LVADs aredesigned as either a bridge to heart transplant or astemporary relief devices and are not intended for usefor more than six months.

L i t h i u m I o n R e c h a r g e a b l e B a t t e r i e s

Although nickel cadmium is the most commonrechargeable battery for commercial applications,nickel metal hydride and lithium ion rechargeablebatteries are also used with implantable medicaldevices due to higher specific energy (watt-hoursper kilogram) and energy density (watt-hours perlitre). Properties of each are listed in Table 2.

Just as in the case of lithium iodine batteries, lithiumion rechargeable batteries also use positive lithiumions as the charge carrier in the electrolyte. Theanode (negative electrode of the battery) can beformed by solid lithium metal or by lithiumembedded in carbonaceous materials such as

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Device Technology & Applications ELECTRONICS

Table 3: Various Compounds Used to Form the Cathode in Rechargeable

Lithium Ion Batteries

Cathode (Positive Battery Energy Density

Electrode) Material Voltage (Watt-hour/kg)

Vanadium oxide 2.4V 860

Titanium disulphide 2.6V 480

Molybdenum disulphide 1.8V 300

Manganese dioxide 2.9V 440

Lithium cobalt dioxide 3.9V 700

Lithium nickel dioxide 3.8V 830

LiMn2O4 4.0V 480

Table 2: Technical Specifications for Three Different Types of Rechargeable

Batteries

Nickel Nickel Metal Lithium

Cadmium Hydride Ion

Specific energy (watt-hour/kg) 30 50 80

Energy density (watt-hour/litre) 100 180 200

Cycle life (number of charges) 1,500 500 300–500

Nominal cell voltage 1.25V 1.25V 3.6V

Exercise requirements Every 30 days Every 90 days None



graphite. Various compounds, as listed in Table 3,may form the cathode.

Rechargeable batteries are constructed as a sandwichof five layers:

1. metal to collect the negative current;2. the lithium-based negative electrode;3. polymer electrolyte to transport lithium ions;4. the positive electrode (see Table 3); and5. the metal to collect the positive current.

Lithium ion rechargeable batteries have the highestenergy density among the commercially availabletypes. In addition, they have the lowest rate of self-discharge. Nevertheless, due to the relatively highercost in comparison with nickel cadmium,penetration of lithium ion rechargeable batteries intothe consumer market has been slow. However, withthe necessary emphasis on size and function, theimplantable medical device industry may provide anideal forum for the use of lithium ion batteries.

T h e F u t u r e

There are many exciting frontiers for implantabledevices and the batteries that power them. Advancesmade in rechargeable battery technology should

allow development of hybrid implantable devices,which are powered by internal rechargeablebatteries, working in conjunction with lithiumiodine batteries. The requirement for back-uppower will remain because of the uncertainties ofpatient compliance.

Advances in lithium polymer and thin-film batteries,with their high energy densities, also make thempotential candidates to power implantable devices inthe future. The implantable device industry continuesto grow.

Perhaps the most promising and long-awaited activeimplantable device is the artificial pancreas for thetreatment of diabetes. Outcomes from recent humanclinical studies using chronically implantable glucosesensors are very encouraging, showing up to ninemonths of operation.

It is only natural to expect that a combined glucosesensor and insulin pump will form the first trueclosed-loop diabetes management system. Given the size of the diabetes market and the complicationscaused by this disease, an artificial pancreas wouldhave a sizeable market and would make a significantaddition to the total number of active medicaldevice implants on the market. ■

Biocompatibility Services

Cytotoxicity test Haemolysis testingEvaluation of Medical Devices for Biological Hazards In-VivoBacterial Endotoxin (LAL Method)Kinetic-QCL Chromogenic LAL AssayGel-clot LAL Testing

Chemical Analysis

SpectroscopyChromatographyICH Compliant Stability StorageMineral Analysis

Microbiological Services

Bioburden TestingSterility TestingOn Site ServicesProcess monitoring and validationEnvironmental monitoringThe Services of a QualifiedMicrobiologist

With Toxicology, Biocompatibility, Microbiology, AnalyticalChemistry and Bioanalysis departments, Wickham LaboratoriesLimited has a wide range of expertise for over 25 years. Weare renowned in the industry for being fast and efficient as wellas cost effective.

Wickham Laboratories Limited, Winchester Road, Hampshire, PO17 5EUTel: 01329 832511, Fax: 01329 834262, Email: [email protected]

Website: www.wickhamlabs.co.uk

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