Technological advancements are making medical devices increasingly feature-rich and miniaturized: two performance characteristics that are inherently conflicting, thus requiring increasingly sophisticated battery power management solutions.
Battery-powered devices span the entire medical spectrum, from surgical drills and power tools, to automatic external defibrillators (AEDs), robotic inspection systems, infusion pumps, bone growth stimulators and other wearable devices, glucose monitors, blood oxygen meters, cauterizers, RFID asset tracking tags, and other remote wireless devices.
Application-specific requirements dictate the choice of power supply, including:
- Reliability: patient wellness depends on procedure outcome
- High power-to-size ratio: keeping the medical device small, lightweight and ergonomic for ease of use and accuracy
- Long shelf life: making sure the instrument in in working order even after prolonged storage without having to recharge or replace the battery
- High temperature survivability: for autoclave sterilization
- Cold temperature operability: for reliable operation in the cold chain
- Ability to supply high pulses: extra power needed to run motors and communications circuits.
Consumer or Industrial Grade?
Certain devices will continue to be powered by consumer grade alkaline and rechargeable batteries. However, industrial grade lithium primary batteries are increasingly being utilized in advanced medical equipment, as lithium chemistry offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of any available 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 extreme temperatures.
A Wide Choice of Primary Lithium Chemistries
As Table 1 shows, several primary lithium battery chemistries are available. For example, lithium manganese dioxide (LiMNO2) batteries are commonly used to power hand-held glucose monitors. These cells are inexpensive, easily replaced, and good enough for most in-home applications.
Lithium sulfur dioxide (LiSO2) batteries deliver high pulses, especially at low temperatures, but add bulk due to their low energy density. These batteries also have high annual self-discharge rates.
Bobbin-type lithium thionyl chloride (LiSOCL2) cells feature the highest energy density, highest capacity, and lowest self-discharge rate, which is ideal for use in long-life applications that require small amounts of current. Bobbin-type LiSOCL2 cells can also operate at extreme temperatures (-80°C to 125°C), making them suitable for autoclave sterilization. Specially modified bobbin-type LiSOCL2 batteries can withstand temperatures as low as -80°C (with certain cells surviving prolonged testing at -100°C), which is required for monitoring frozen tissue samples, pharmaceuticals, and transplant organs transported in the cold chain.
However, due to their low rate design, standard bobbin-type LiSOCL2 batteries cannot deliver the high pulses needed to power motorized instruments. One economical solution is to combine a standard bobbin-type LiSOCL2 battery with a patented hybrid layer capacitor (HLC) that stores the energy and delivers periodic high pulses to applications such as automatic external defibrillators (AEDs). Use of an HLC offers a far less bulky, and less complex solution than supercapacitors.
Specialized Batteries Deliver High Rate Current
Applications that require continuous high rate power, such as surgical drills and power tools, can benefit from an innovative lithium metal oxide battery that delivers high voltage, instant activation, and exceptionally long shelf life even in extreme temperatures.
Constructed with a carbon-based anode, a multi metal oxide cathode, and an organic electrolyte, these batteries can deliver up to a 20-year operating life with an annual self-discharge rate of less than 1 percent per year. These small but powerful cells feature a nominal voltage of 4V and up to 2 Wh of energy, with a discharge capacity of 135 mAh to 500 mAh, capable of handling 5A continuous loads and 15A maximum pulses. They also offer a wide temperature range (-40°C to 85°C), and a hermetic seal.
Medical Grade Rechargeable Lithium-Ion Chemistries
Consumer grade Lithium-ion (Li-ion) rechargeable batteries are not manufactured to medical grade standards, as their high self-discharge rate may result in low battery availability if the cell is not properly recharged before the procedure. Consumer rechargeable batteries also have a relatively low power-to-size ratio, thus requiring the use of very large battery packs to deliver the high pulses required to drive surgical power tools. Consumer grade Li-ion batteries also have crimped seals that may leak, and cannot handle the high temperature and humidity associated with autoclave sterilization.
A medical grade rechargeable Li-ion battery was recently developed that can operate for up to 20 years and 5,000 recharge cycles. This battery can draw up to 15A of continuous current from a small AA size cell, and has an extremely low selfdischarge rate, thus allowing the instrument to sit on the shelf for extended periods and still be ready for instantaneous use when called upon. These ruggedized batteries also feature a hermetic seal and an extended temperature range.
Application-Specific Requirements Dictate the Ideal Power Supply
Bone growth stimulator (low continuous current): A bone growth stimulator requires low continuous current to emit low-intensity, high frequency sonic pressure waves that stimulate bone growth and healing. Use of a LiSOCL2 battery pack makes this wearable device more compact and lightweight for greater user comfort.
AED (long shelf life, high pulses): An automatic external defibrillator (AED) requires a power supply that offers unparalleled shelf life, as the device can remain idle for extended periods, but then must operate reliably in the event of a cardiac arrest. Hybrid Li/SOCL2 batteries are ideal for use in AEDs because they feature very low annual self-discharge rate (less than 1 percent per year), can withstand extreme temperatures, and are able to deliver up to 15A pulses.
Hand-held surgical drills (high rate, high pulse): Surgical suites are highly controlled environments, so introducing any new technology requires a careful vetting process to ensure optimal product safety and efficacy. A prime example involves the development of new, battery-powered automatic torque limiting surgical screwdrivers.
Neurosurgeons use screwdrivers like those manufactured by Pro-Dex to firmly set titanium screws into thin titanium plates that cover portions of the skull removed during surgery. The screwdrivers 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 unsightly bumps that can cause patient discomfort. (See Figure 1)
These devices feature a special high reliability motor with advanced microelectronics, proprietary algorithms, state-of-the-art sensor technology to automatically limit the torque, reversible variable speed control, and touch sensors that provide tactile feel during screw insertion or extraction. The screwdrivers are completely reusable (except for the battery pack, which is discarded), so all electro-mechanical components had to be ruggedized to withstand multiple autoclave sterilization cycles. To meet such rigorous performance requirement, virtually all components, except the battery, 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 ideal battery can result in a dramatic reduction in size and weight without sacrificing power or performance. Several primary lithium chemistries were reviewed, and we selected lithium metal oxide batteries due to 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 incredibly powerful batteries helps make the new drivers handy and ergonomic tools that saves time and reduce operator fatigue.”
Currently, two models have been developed: one powered by two 1550HP batteries, which delivers up to 8V of variable current to permit drilling speeds of up to 2,200 RPM; and another version powered by four 1550HP lithium metal batteries, which delivers up to 16V of variable current to permit drilling speeds of up to 4,000 RPM. While these battery packs are discarded after a single use, similar applications could consider the use of industrial grade rechargeable Li-ion batteries to make the device fully reusable.
Hand-held surgical drill (smaller alternative to alkaline batteries): BioAccess, Inc., Baltimore, MD, a surgical device manufacturer, chose lithium metal oxide batteries as an optional power supply upgrade for its surgical drill, which were previously powered exclusively by alkaline battery packs. While the alkaline batteries performed well and offered excellent reliability, they added unnecessary weight. By substituting 6 AA-size 1550HP batteries for the alkaline battery pack, BioAccess explained that it achieved a 36 percent weight reduction with only 40 percent of the volume. An equivalent alkaline battery pack would have required 3x the weight and 2.5 times the volume (15 AA-size alkaline batteries vs. 6 AA-size 1550HP batteries). (See Figure 2)
Use of a lithium metal oxide battery pack also enabled the surgical drill to deliver faster drilling speeds, more active drill time (30 to 40 seconds at a time for up to 20 to 30 cycles), more instantaneous power, and greater stall torque, resulting in more efficient drilling cycles with less operator fatigue.
These case histories demonstrate how advanced lithium battery technology is enabling wireless medical devices to become miniaturized without impacting product performance.
This article was written by Louis Adams, Southwest Regional Manager, Tadiran Batteries, Lake Success, NY. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/55595-160.