Sol Jacobs, VP and General Manager, Tadiran Batteries
Medical technology is advancing rapidly, requiring increasingly sophisticated power management solutions, especially when then the device needs to be self-powered.
Battery-powered medical devices encompass a wide range of applications, including surgical drills, power tools, automatic external defibrillators (AEDs), infusion pumps, bone growth stimulators and other wearable devices, such as glucose monitors, blood oxygen meters, cauterizers, RFID asset tracking tags, as well as other remote wireless devices utilized to monitor patients, procedures, medications and equipment. Each device has unique power requirements; therefore, the choice of power supply requires careful consideration based on application-specific requirements.
Consumer versus industrial grade batteries?
Many wearable medical devices and certain equipment designed for in-home can be powered by consumer grade alkaline and rechargeable batteries, which are reasonably inexpensive and readily available. However, certain applications require the use of industrial grade lithium primary batteries, as lithium offers 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 3.9 V. Their electrolyte is also non-aqueous, permitting certain cells to operate in extreme temperatures.
The choice between a consumer battery and an industrial grade lithium battery often hinges on the following factors:
1. Reliability – patient wellness depends on procedure outcome
2. High power-to-size ratio – enables the medical device to be smaller, lighter and ergonomically designed for ease-of-use, comfort and accuracy
3. Long shelf life – ensures that the device is ready to operate even after long periods of storage without needing to recharge or replace the battery
4. High temperature survivability – if sterilization is required
5. Cold temperature operability – for items transported through the cold chain
6. Ability to supply high pulses –to run motors and communications circuits.
A wide choice of primary lithium chemistries
Numerous primary lithium battery chemistries are available for medical applications.
Lithium manganese dioxide (LiMNO2) batteries are often used to power hand-held glucose monitors. LiMNO2 cells are inexpensive, easily replaced and usually sufficient for in-home use, but suffer from a narrow temperature range and a high annual self-discharge rate.
Lithium sulfur dioxide (LiSO2) batteries can deliver high pulses, especially at low temperatures, but add bulk due to their low energy density. LiSO2 batteries also have a high annual self-discharge rate.
Bobbin-type lithium thionyl chloride (LiSOCL2) cells feature the highest energy density, highest capacity and lowest self-discharge rate of any lithium chemistry, making them ideal for use in applications that draw small amounts of current over extended periods, with certain cells able to operate maintenance-free up to 40 years.
Bobbin-type LiSOCL2 cells are also well suited to extreme temperatures (-80o° to 125o° C), including autoclave sterilization. Bobbin-type LiSOCL2 batteries can also be specially modified to withstand cold chain temperatures as low as -80° C for transporting frozen tissue samples, pharmaceuticals and transplant organs. Certain of these specially modified batteries have survived prolonged testing at -100° C.
However, due to their low rate design, standard bobbin-type LiSOCL2 batteries are not well suited for delivering the high pulses required to initiate data sampling or wireless communications. An economical solution is to combine a standard bobbin-type LiSOCL2 battery with a patented hybrid layer capacitor (HLC) that stores the energy and delivers the periodic high pulses required for applications such as AEDs. Another option is to combine a standard LiSOCL2 battery with a supercapacitor, which is a bulkier, more complex and more costly solution.
Powering devices that draw high rate current
Certain medical equipment, such as surgical drills and power tools, require continuous high rate power. To address this specialized requirement, lithium metal oxide chemistry was developed, the “TLM Series,” an innovative chemistry 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, TLM Series batteries can deliver up to 20-year operating life with an annual self-discharge rate of less than 1% per year. These powerful little cells feature a nominal voltage of 4V and up to 2 Wh of energy, with a discharge capacity of 135 to 500 mAh, capable of handling 5A continuous loads and 15A maximum pulses. These batteries also offer a reasonably wide operating temperature range (-40° to 85 °C.), and a hermetic seal to help prevent battery leakage.
Medical grade rechargeable lithium-ion chemistries
Certain medical devices, especially those designed or in-home use, can operate consumer grade Lithium-ion (Li-ion) rechargeable batteries, which are inexpensive and readily available, but have certain drawbacks, including high self-discharge rates that can result in low battery availability if the cell is not properly recharged before the procedure. Consumer grade rechargeable batteries also have a low power-to-size ratio, so large battery packs are required to deliver the high pulses and continuous energy required by surgical drills and power tools. Consumer grade Li-ion batteries also have crimped seals that may leak, and are not designed to handle high temperature autoclave sterilization.
To address these problems, an industrial grade rechargeable Li-ion battery was developed, the “TLI Series,” which can draw up to 15A of continuous current in a small AA size cell, enabling surgical power tools to be smaller, lighter and more ergonomic. Industrial grade Li-ion batteries also feature an extremely low self-discharge rate, enabling medical devices to sit on the shelf for extended periods but still provide instantaneous activation. These ruggedized rechargeable batteries also feature a hermetic seal and an extended temperature range.
Some real-life examples
- Bone growth stimulator (low continuous current): To stimulate bone growth and healing, a bone growth stimulator requires low continuous current to emit low-intensity, high frequency sonic pressure waves. Use of a standard bobbin-type LiSOCL2 battery pack makes this wearable device more compact and lightweight, resulting in greater user comfort and convenience.
- AED (long shelf life, high pulses): AEDs require a power supply that delivers extremely long shelf life, as the AED can remain idle for extended periods, but then must be able to deliver high pulses in the event of a heart attack. Combining a standard LiSOCL2 battery with an HLC provides the combined benefits of a low annual self-discharge rate (less than 1% per year) along with the ability to deliver high pulses of up to 15 A.
- Hand-held surgical drills (high rate, high pulse): Pro-Dex recently developed battery-powered automatic torque limiting surgical screwdrivers, which neurosurgeons use to insert titanium screws that secure in place titanium plates that cover portions of the skull removed during surgery. These precision 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 an unsightly bump that could cause patient discomfort.
The screwdrivers are completely reusable (except for the battery pack, which is discarded), featuring a special high-rel 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 enable left- or right-handed surgeons to experience tactile feel during screw insertion or extraction. All electro-mechanical components had to be ruggedized to withstand the excessive heat and humidity associated with multiple autoclave sterilization cycles.
“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,” said Stu Gallant, VP of product and business development at Pro-Dex. “Several primary lithium chemistries were reviewed, and we selected lithium metal oxide (TLM Series) 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.”
Two Pro-Driver models are currently available: one model that uses two TLM 1550HP batteries to deliver up to 8V of variable current to permit drilling speeds of up to 2,200 RPM; and a second model powered by four TLM 1550HP Lithium metal batteries, which delivers up to 16V of variable current to permit drilling speeds of up to 4,000 RPM. While the Pro-Dex battery packs are discarded after a single use, similar devices could be designed to be completely re-usable using industrial grade rechargeable Li-ion batteries.
Hand-held surgical drill (smaller alternative to alkaline batteries)
BioAccess, a surgical device manufacturer, chose lithium metal oxide batteries as an optional power supply upgrade for their 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 TLM-1550HP batteries for the alkaline battery pack, BioAccess achieved a 36% weight reduction with only 40% of the volume. An equivalent alkaline battery pack would have required three times the weight and two and a half times the volume (15 AA-size alkaline batteries versus 6 AA-size TLM-1550-HP batteries).
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 secs 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 illustrate how recent advanced lithium battery technology is enabling medical devices to become increasingly miniaturized while still delivering uncompromising power and performance.