Medical device battery design

In the opinion of material scientists, thin-film technology is essential in the development of rechargeable hthium-based microbatteries for potential applications, such as smart cards, nonvolatile memory backup devices, MEMS sensors and actuators, and miniaturized implantable medical devices. Battery designers predict that for such applications film thickness should not exceed a few tens of micrometers or microns (10 cm). This means that the film thickness must be at least ten micrometers or 0.001 cm (0.0025 in.), which may be suitable for minimum battery... 

General Design Considerations for Medical Device Battery Petformance... 

Medical device batteries are fundamentally the same as any other battery designed for consumer electronics, military, or aerospace applications. All require the same three components to be able to function as an electrochemical power source - a negative electrode (or anode) material to supply electrons, a positive electrode (or cathode) material that takes up electrons, and an electrolyte that completes the electrical circuit through ionic conduction. The other compmients in a cell are necessary to make the cell perform efficiently, minimize its size, and make it safe and reliable. These components include one or more separators that are electrically insulating to prevent direct contact between the anode and cathode but allow ions to pass through, current collectors to convey electrons to or from the electrodes and various insulators to prevent short circuits. 

Fig. 11.1 Electrode assembly designs for prismatic medical device batteries...

Lithium batteries are used in many portable consumer electronic devices and are also widely used in industry. The most common type of lithium cell used in consumer applications comprises metallic lithium as the anode and manganese dioxide as the cathode, with a Li salt such as Li perchlorate or Li tetrafluoroborate dissolved in an organic solvent. Lithium batteries find application in many long-life, critical devices such as cardiac pacemakers and other implantable electronic medical devices. These devices use special lithium-iodide batteries designed to last 15 years or more. Lithium batteries can be used in place of ordinary alkaline cells in many devices such as clocks and cameras. Although they are more expensive, lithium cells provide a much longer life, and thereby minimize battery replacement. 

The overall performance and reliability of both implanted and external medical devices depend strongly on the battery characteristics (including chemical composition of the electrodes as well as the battery design and electrolyte). Lithium-based batteries are the current choice for pacemakers, where continuous power requirements are on the order of 10"4 W battery duration depends on demand factors and is about 7 years for continuous service (16). Other devices are being developed that have higher power needs and serve specialty markets (probably smaller than that for pacemakers). Some of these are the following ... 

The easy removeability provision is also specified in the 1996 legislation and was an early requirement in several of the state NiCd battery recycling laws. These provisions were developed at the time when many NiCd battery power tools and appliances did not provide for easy removeability, mainly as a consumer safety measure. Since the early 1990s, however, battery operated tools, appliances and other devices have been designed so that in most cases the batteries are easily removable while still ensuring consumer safety. In fact, many manufacturers of battery powered tools today market replacement battery packs, which are interchangeable in a number of different tools. Only in certain systems such as computer memory backup and medical devices are the batteries permanently installed to avoid system failure. 

Reference cells in scientific and professional equipment, and batteries and accumulators placed in medical devices designed to maintain vital functions and in heart pacemakers, where uninterrupted functioning is essential and the batteries and accumulators can be removed only by qualified personnel. 

In this entry are discussed a few of the specialized batteries for medical devices that are portable or wearable (carried with the patient, like hearing aids), or implantable (surgically placed inside the body as with neurostimulation pain management devices). There is a focus on the batteries designed for a few of the more common applications - implantable cardiac rhythm management (cardiac pacemakers and defibrillators), pain management, and hearing loss devices. 

There are several important features that battery developers must consider when designing batteries for medical devices. Many of these are also important for most other battery types, as well. 

Medical devices, especially implantable devices, often use batteries that are custom designed and built specifically for that device. Their sizes and shapes may not be standard. The electrode assemblies may consist of pellet or slug electrodes, spirally wound electrode foils, or stacked electrode plates (Fig. 11.1). 

One of the challenges of designing batteries for some medical devices is the wide-ranging power requirements to support different device functions. The energy... 

Reliability and safety are utmost concerns when designing Li ion cells for implantable medical devices. They are hermetically sealed and may contain redundant safety features in the batteries (as well as part of the charging circuitry in the device). Also, cell sizes and shapes are often specially designed for a particular device. 

Energy and power density (ratio of energy and power deliverable by a battery to its volume) of batteries determine their suitability for most electrical devices. Other important criteria for use in medical devices are safety and reliability in operation during lifetime of the device (up to 10 years or longer), predictability of battery performance characteristics, and provision of device replacement indicators based on battery depletion and mechanical design features such as shape and size to allow efficient packaging in the device and for patient comfort. 

An important area to be considered in battery selection is safety of the battery in combination with the device. Part of the safety consideration is the design of the battery compartment. In some cases, the battery compartment should be designed for maximum heat dissipation - in some cases, they should be designed for maximum electrical isolation. Electrical isolation can be important for devices which are not in waterproof containers and can result in battery short circuiting if the device is immersed in water. Small batteries such as miniatore alkaline or lithium coin cells should be enclosed in the device in a way that is difficult for small children to access the battery. Many of these small cells have been swallowed by children with the result of serious medical complications. Considerations of battery design are complex, and the size, electrode configuration, and format of the cell ate critical to the... 

High-rate designs are employed in cardiac defibrillators, while moderate rate units find applications in implantable neurosimulators and drug infusion devices. Lithium/SVO batteries are employed for biomedical applications and as such must be produced under the Good Manufacturing Practices (GMP) for medical devices of the U.S. Food and Drug Administration. 

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