Lithium ion batteries come in all sizes. Some of the tiniest are used in the medical field to power bioengineering devices. Often, these devices are implanted within the body to help monitor and maintain health.
Dr. Marissa Caldwell is a battery research and technology scientist at Medtronic. Caldwell was a panelist in a session on “Batteries for Medical and Personal Electronic Devices” during The Battery Show in Novi, Michigan this week. Design News caught up with Dr. Caldwell after the session to find out more about the use of lithium ion batteries in medical devices.
Caldwell describes her day job as “identifying and developing new battery chemistries and new battery designs for products that might come out in five to ten years.” Medtronic, which is one of the world’s largest medical device companies, has its operational headquarters and does much of its research and development in Fridley, Minnesota. It is one of the few companies in the medical device industry that builds its own batteries for use in some of its cardiac and neurostimulation devices.
|Medical device maker Medtronic is unique in that it builds its own batteries. (Image source: Medtronic)|
All Lithium Ion
“All of Medtronic’s rechargeable batteries are lithium ion—we’ve been doing this since 2004,” said Caldwell during her presentation. The recharging of implanted batteries is all inductive. The recharge system is sent home with the patient and is worn over the area where the device is implanted. It recharges right through the skin.
“What do we think about when we are designing batteries?” Caldwell asked rhetorically. “For us, reliability is one of the first things that comes to mind. It really means looking at reliability from all of the different facets, including what the requirements are from the device level and circuit level to how can we anticipate people using this device. What are the different recharge modalities? What are the possible failure modes that could happen? And all of this then gets wrapped up into the testing and modeling and designing. Most batteries that we design to be implantable are designed very conservatively. In order to build in this reliability, we do give up a lot of margins for, say, energy density. We will not be the most energy-dense cell on the market. But we will last longer and be more reliable,” she explained.
Meeting regulatory requirements is also a huge part of medical device battery development. In addition to transportation and all of the other different issues that batteries themselves are required to meet, there are different medical-specific requirements from different medical working groups and the Food and Drug Administration (FDA). Much of the paperwork involved goes into proving that the batteries that are going into medical devices will actually perform in the manner claimed. According to Caldwell, “It’s really hard to make a battery that’s going to last a long time, and it’s even harder to prove that it is going to last a long time.”
One of the big driving forces in the electric vehicle battery market is the elimination of expensive cobalt from the battery chemistry. This is not necessarily the case for medical batteries. “We’re not as cost sensitive to cobalt as most people, so we don’t have as big a driver to get to cobalt-free. The drive to nickel-manganese-cobalt (NMC) and cobalt-free materials is not as relevant to us. Getting to fast-charge graphite and higher energy density, higher voltages—those are all things that we are very interested in,” said Caldwell.
That is not to imply that new technologies, such as solid state electrolytes, aren’t being actively researched. “The more we can design in safety, it’s always on our radar. Solid state electrolytes are something we are keeping a close eye on, but we are still kind of gauging its maturity. Power is a problem still for most of the solid electrolytes—and manufacturability. So if you look at, say, some of the oxides and the garnet materials and harder inorganics, how are we going to manufacture it—especially in our really little batteries? For some of the things, like polymers and soft materials, what is the tradeoff versus safety?”
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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