Advanced lithium battery technology is powering the development of a new generation of medical devices. These products are becoming increasingly sophisticated, feature-rich, and miniaturized, necessitating the use of the lithium options. This article highlights a number of battery solutions and offers the outcome of a real world case application.
Advancements in technology have led to a new generation of lithium battery-powered medical devices, ranging from surgical drills and power tools to automatic external defibrillators (AEDs), robotic inspection systems, RFID asset tracking tags, infusion pumps, bone growth stimulators and other wearable devices, glucose monitors, blood oxygen meters, cauterizers, and remote sensors.
Primary lithium batteries are preferred for medical applications because they offer the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of any battery chemistry. Lithium cells have a nominal open circuit voltage of between 1.7 and 3.9V. Their electrolyte is also non-aqueous, permitting certain cells to operate in temperatures ranging from -55 to 125°C. In addition, specially modified bobbin-type LiSOCl2 batteries are used to monitor the transport of frozen tissue samples, pharmaceuticals, and transplant organs throughout the medical cold chain, where temperatures as low as -80°C must be maintained.
A Wide Selection of Chemistries
Among lithium primary batteries, there are numerous chemistries to choose from, each offering unique performance characteristics (Table).
For example, hand-held glucose monitors are often powered by LiMNO2 (lithium manganese dioxide) batteries that have certain limitations, including high self-discharge, low energy density, and a narrow temperature range (-10 to 60°C).
LiSO2 (lithium sulfur dioxide) batteries deliver high pulses, especially at low temperatures. They also have drawbacks, including high annual self-discharge and added bulk due to their low energy density.
Bobbin-type LiSOCl2 (lithium thionyl chloride) cells feature high energy density, high capacity, and very low self-discharge rate, permitting up to 40-year operating life for certain low power applications, including RFID asset tracking devices. Certain brands of bobbin-type LiSOCl2 cells can operate at extreme temperatures (-80 to 125°C) making them ideal for use in applications that require autoclave sterilization. A patented version of this battery, the PulsesPlus, combines a standard bobbin-type LiSOCl2 cell with a hybrid layer capacitor to deliver low background current along with periodic high pulses, which is ideal for AEDs and for powering advanced two-way communications.
Applications that require short bursts of very high power from a miniaturized power supply, such as surgical drills and power tools, can be powered by a unique type of lithium metal oxide battery, the TLM Series, which delivers high voltage, high energy density, instant activation, and exceptionally long operational life even in extreme temperatures. TLM batteries feature an open circuit voltage of 4.0V, along with the ability to deliver up to 15A pulses (5A continuous current at 3.2V). These cells can operate in -40 to 85°C temperatures.
Requirements Dictate Choice
The specific application requirements of a device are going to dictate the choice of battery technology used. A bone growth stimulator, for example, requires low continuous current to emit low-intensity, high frequency sonic pressure waves that stimulate bone growth and healing. Use of a standard LiSOCl2 battery pack, with its high energy density, enables this wearable device to be lighter and more compact, resulting in greater user comfort.
On the opposite side of the spectrum, an AED can go for years without requiring any background current but needs to deliver high pulses during a cardiac arrest. These application requirements are well suited to hybrid bobbin-type LiSOCl2 batteries, which can deliver pulses of up to 15A, feature a very low annual self-discharge rate (under 1% per year), and operate reliably in extreme temperatures.
Surgical Drills Present a Unique Challenge
Surgical suites are highly controlled environments, so introducing any new technology requires a careful vetting process to ensure product safety and efficacy. A prime example involves the development of new, battery-powered automatic torque limiting surgical screwdrivers.
Manufactured by Pro-Dex (www.pro-dex.com), these screwdrivers are used by neurosurgeons to firmly set titanium screws into thin titanium plates that cover portions of the skull removed during surgery. The drivers are equipped with an automatic torque limiting sensor to ensure that the titanium screws are properly driven to the right depth — completely flush with the top surface of the titanium plate — thus eliminating the risk of an unsightly bump that could cause unnecessary patient discomfort.
The new technology drivers combine a special high-rel motor with advanced microelectronics, proprietary algorithms, and state-of-the-art sensor technology to automatically limit the torque. Other key performance features include reversible variable speed control and touch sensors that enable left- or right-handed surgeons to experience tactile feel during screw insertion or extraction.
Since the device is completely reusable (except for the battery pack, which is discarded after a single use), all electro-mechanical components have been ruggedized to withstand the excessive heat and humidity associated with multiple autoclave sterilization cycles. To achieve such rigorous performance standards, virtually all components, except the batteries, are being manufactured at the company’s production facility in Irvine, CA.
According to Stu Gallant, vice president of product and business development at Pro-Dex, “Creating a truly surgeon-friendly power tool requires the ideal power supply, as selecting the right battery can result in a dramatic reduction in size and weight without sacrificing power or performance. Several primary lithium chemistries were considered, but we selected lithium metal oxide (TLM Series) batteries for their ability to deliver high continuous power, high pulse amplitude, and up to 4.1V continuous current per cell during active drilling cycles. Use of these small but powerful batteries helps make the new drivers handy and ergonomic tools that save time and reduce operator fatigue.”
Currently, two models have been developed — one powered by two TLM 1,550HP batteries, which delivers up to 8V of variable current to permit speeds of up to 2,200RPM; and another version powered by four TLM 1,550HP batteries, which delivers up to 16V of variable current to permit speeds of up to 4,000RPM.
A Future Option
The battery pack used to power these drivers is discarded after a single use. However, in the future, it could be possible to develop a reusable battery pack powered by TLI Series medical-grade rechargeable Lithium-ion (Li-ion) batteries.
Consumer-grade Li-ion rechargeable batteries are not manufactured to medical-grade standards, as they deliver only five years of maximum operating life, 1,000 recharge cycles, and are constructed with crimped seals that are prone to breakage and corrosion. Consumer-grade Li-ion batteries are also not designed to handle the high temperature and humidity associated with autoclave sterilization procedures.
By contrast, medical grade rechargeable TLI Series Li-ion batteries can deliver up to a 20-year operating life and 5,000 full recharge cycles, and can draw up to 15A of continuous current. These robust batteries also feature a hermetic seal for added protection against leakage and corrosion, and provide an extended temperature range to handle multiple autoclave sterilization cycles.
Exciting possibilities exist for next generation medical technology being powered by primary and rechargeable lithium batteries.
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