Powering Modern Medicine
Primary lithium batteries enable advanced medical devices to be smaller, lighter, and more feature-rich. This article showcases a number of different chemistries and features several real world applications for which these batteries are used.
The medical industry is in the midst of a technological revolution that is causing increased demand for miniaturized devices that are powered by lithium batteries.
First utilized in pacemakers during the 1960s, primary lithium batteries are currently found in a variety of medical devices, from automatic external defibrillators (AEDs) to surgical saws, drills, robotic inspection systems, RFID asset tracking tags, infusion pumps, bone growth stimulators, glucose monitors, blood oxygen meters, cauterizers, and various remote sensors.
Benefits of Lithium Chemistry
Lithium batteries are often preferred for advanced medical devices because they offer the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of any battery type. Lithium cells, all of which use a non-aqueous electrolyte, have nominal open circuit voltages between 1.7 and 3.9 V.
The absence of water also permits a wider temperature range (-55 to 125°C). Specially modified cells are able to withstand the -80°C temperatures associated with the medical cold chain, permitting around-the-clock monitoring of frozen pharmaceuticals, transplant organs, and tissue samples.
Variety of Solutions
Among lithium primary batteries, there are numerous competing chemistries, each offering certain advantages and disadvantages (see the table at the online version of this article at http://bit.ly/MDTarticles).
Li/MNO2 (lithium manganese dioxide) batteries, originally designed for consumer toys and cameras, are now used in hand-held glucose monitors. These cells feature relatively low cost and high current-pulse capabilities, but suffer from high self-discharge and low energy density, making these devices bulkier. Li/MNO2 cells also have a relatively narrow temperature range (-10 to 60ºC).
Li/SO2 (lithium sulfur dioxide) batteries, often used in automatic external defibrillators, deliver high current-pulses at low temperatures. Low energy density makes Li/SO2 cells inherently larger than other lithium chemistries, and high self-discharge reduces their potential service life.
Li/SOCL2 (lithium thionyl chloride) cells are ideal for long-term applications. These cells feature high energy density, high capacity, and a very low self-discharge rate, enabling a 25+ year service life. Certain bobbin-type lithium thionyl chloride cells can operate in temperatures ranging from -80 to 125°C.
A hybrid version of the Li/SOCL2 cell, the PulsesPlus, combines lithium thionyl chloride chemistry with a hybrid layer capacitor to deliver high current-pulses, ideal for automatic external defibrillators (AEDs) and other applications that require low background current with periodic high-current pulses (in the multi-amp range). PulsesPlus cells also offer the potential for an end-of-life indication when 90% to 95% of the cell’s capacity has been depleted.
Another specialty battery, the TLM Series, uses lithium metal oxide chemistry to deliver high cell voltage, high energy density, instant activation, and long operational life. Even in extreme temperatures, TLM Series batteries feature an open circuit voltage of 4.0 V along with the ability to deliver high current-pulses of up to 15 and 5 A continuous current at 3.2 V. Commonly used in disposable devices, such as surgical drills, power tools, and cautherizers, TLM Series batteries provide valuable ergonomic benefits by allowing hand-held devices to be smaller, lighter, and more powerful.
Case in Point
These case studies demonstrate the practical benefits of lithium chemistry when applied to medical applications.
Bone Growth Stimulator–low continuous current
Of the nearly six million bone fractures that occur annually in the U.S., 5% to 10% of all cases show delayed or impaired healing. Bone growth stimulators are usually strapped-on over the fracture site or fitted into a cast to continuously emit low-intensity, pulsed high frequency sonic pressure waves that stimulate bone growth and healing. Use of a battery pack consisting of AA-size Li/SOCL2 cells can substantially reduce the size and weight, highly beneficial since the device is worn by the patient.
AEDs–low background current; periodic low current-pulses; high current pulses when in use
Portable AEDs are used to restore normal heart rhythm to patients in cardiac arrest. They analyze the patient’s heart rhythm and then advise the rescuer as to whether a shock is needed to restore a normal heartbeat. AEDs are often located in public places, such as schools, restaurants, airports, and office buildings (public access AEDs), where hardwired AC power may not be available, so the use of rechargeable batteries is often not an option. These locations could also be exposed to extreme temperatures, which could compromise battery performance. As a result, AEDs often rely on primary lithium batteries to deliver high current-pulses even after extended periods of inactivity.
For example, a leading manufacturer of automatic external defibrillators has specified PulsesPlus high current-pulse Li/SOCL2 batteries due to their extremely long shelf life (self-discharge under 1% per year) as well as their small size and high pulse rate.
Hand-Held Surgical Drill–requires no background current; very high current-pulses
BioAccess manufactures a single-use, cordless bone drill that is powered by a battery pack, located within the handle of the saw, and consists of 15 AAA-size alkaline cells.
To offer a more powerful and ergonomic solution, an alternative version was created by substituting 6 AA-size TLM-1550HP high-energy lithium batteries, reducing the weight of the battery pack by 36%. Use of lithium metal oxide chemistry enables TLM Series batteries to deliver an open circuit voltage of 4.1 V and handle high current-pulses of 15 A with a 5 A continuous load. Further, added capacity and energy density permits faster drilling speeds, extended drilling time, and increased torque for more efficient drilling cycles. An alkaline battery pack that delivered equivalent power and performance would have required three times the weight and two times the volume (requiring 15 AA-size alkaline batteries versus 6 AA-size TLM-1550-HP batteries). (Click to read more about this application.)
Looking to the Future
Lithium batteries are also finding their way into exotic applications, such as spider-like robotic capsules that are swallowed and crawl through the GI tract to perform diagnostic and surgical procedures.
In addition, medical devices are beginning to combine telematics, GPS, and RFID tracking capabilities to monitor the whereabouts and well being of patients in hospitals, nursing homes, assisted living quarters, or remote locations via satellite.
For example, Awarepoint recently introduced an RFID asset tracking device that uses extended temperature lithium thionyl chloride batteries to withstand the high temperatures associated with autoclave sterilization cycles. Whereas medical asset tracking devices previously had to be removed prior to equipment sterilization to protect the battery from overheating, Awarepoint RFID tags can remain online during sterilization cycles, thus permitting continuous real-time monitoring and reporting capabilities. (Click to read more about this application.)
As modern medical technology continues to evolve, exciting possibilities and challenges will emerge that will be effectively addressed by long-life lithium batteries.