Last week, Micro Power Electronics announced that it can manufacture rechargeable lithium-ion (Li-ion) battery packs that can be sterilized via hydrogen peroxide gas. In February, the company made a similar announcement about battery packs that could be sterilized by gamma rays. The questions that immediately come to mind are “why?” and “how?”
Micro Power is a good company to ask. First, it’s well known in the business of assembling battery packs, especially battery packs that use lithium cells. While lithium cells have well-known safety issues, Micro Power is equally known for minimizing those issues. Second, Micro Power product marketing engineer Robin Tichy is a regular columnist for Electronic Design. When the news came out, I spoke with Dr. Tichy about the how and why of sterilizable packs. The answers turn out to be illustrative of the convoluted ways the real world impacts how engineers approach designs.
So, why batteries? In particular, why lithium battery packs? First, if the electrical device under consideration can be run by replaceable batteries instead of the ac lines, that’s attractive from a development standpoint because it simplifies the EN-61000 safety testing that medical devices must pass. Second, lithium battery packs are much lighter than the alternatives and exhibit a relatively flat output-voltage curve as they discharge. Light weight is always helpful in the operating theater, even if your surgeon played rugby in college and still swims 50 laps every morning. An output voltage that remains consistently high means more useful life from each charge as well.
The next consideration is sterilization. Almost everybody has seen steam autoclaves in their doctor’s office. They’re satisfactory for reusable scalpels, forceps, retractors, and other simple metal tools. They’re not so good for things like electronics, electric bone saws, and battery packs, particularly lithium battery packs. Thus, medicine has developed other methods of sterilization: gamma rays, ethylene-oxide gas, and vaporized hydrogen peroxide (or a mixture of peracetic acid and hydrogen peroxide).
Choices, of course, bring up tradeoff issues, which is where engineering decisions come in. for example, the gamma ray approach requires heavy shielding and safe storage for the Cobalt-60 source. It’s satisfactory for lithium primary cells, but not for rechargeable cells that incorporate electronics for charge management or validation because of the rays’ effects on semiconductor junctions. Rechargability, of course, is the primary appeal of lithium battery packs. Ethylene oxide is toxic and carcinogenic. Through a series of tradeoffs, hydrogen peroxide thus becomes the most appealing way to sterilize all kinds of powered surgical tools, from saws and drills to endoscopes.
The challenge then is to create Li-ion battery packs that are ruggedized to tolerate the partial vacuums and overpressures that are used in HOOH sterilization and whose electrical connections are not attached by the chemical. That’s the primary objective. Then there is one interesting additional constraint for these battery packs: if someone mistakenly runs them through a sterilization cycle in a steam autoclave, although it’s expected that they will fail and cease to store energy (the internal separators will break down), they must always fail-safe and not overheat or release hydrogen.
Micro Power offers rechargeable lithium-ion battery packs capable delivering up to 24 V, 100 A, 10 Ah of capacity, fuel gauging for state-of-charge indication, and a serial communication bus for integration with surgical tools.