Medical Device Design
It is important that safety remains a top priority in device design.
Technology is ever-changing in the medical device world. The public is searching for the best device to help keep them healthy and safe. In order to fulfil the demand, small medical device manufacturers constantly have to make better products to meet the end-consumer’s needs. But before they can do this, there are several things that small medical manufacturers have to take into account, including prioritizing the end-consumer’s needs and wants. Small medical devices have different requirements based on their intended usage and also the batteries that the manufacturers decide to employ. There are three main types of batteries used in small medical devices: zinc, nickel, and lithium, each with their own benefits and limitations.
Zinc-based batteries include: Nickel-Zinc (NiZn), Carbon Zinc and Zinc Chloride. These zinc-based batteries are commonly used as general-purpose batteries. Carbon Zinc batteries are good for devices that need a long shelf life; they have relatively low and stable energy levels. This lends them well to be used in a medical device like a hearing aid, but not a medical device that requires wireless communication.1 The main advantage to zinc-based batteries for medical devices is that zinc does not interact volatilely with the environment. This means less packaging is required for it to be used safely, and therefore, the cost of producing a device using a zinc-based battery is lower.2 Yet, the issue remains that zinc-based batteries have a limited power capacity3, and they are not currently rechargeable. These are batteries that can only be used once and they will have to be thrown away.
If the end-customer is interested in a longer lasting battery, Nickel-MH (NiMH) is a great option. Nickel-based batteries are considered low risk to the end-user, are inexpensive and have been around for many years due to their reliability. There are two main kinds of Nickel-based batteries: Nickel-Cadmium (NiCd) and Nickel-Metal Hydride (NiMH). Both of these batteries have been typically used in small medical devices. NiCd batteries were the first rechargeable batteries in the nickel-based battery family, but have seen a significant reduction in use by the industry primarily due to Restriction of Hazardous Substances Directive legislation, which deemed cadmium as a restricted substance for its high toxicity.4 They also were deemed not as effective as the Nickel-Metal Hydride battery, due to lower charge capacity and memory issues. NiMH batteries are the better alternative to the original NiCd batteries. NiMH is better for the environment, as it can be easily and safely disposed of without environmental consequences, has a higher charge capacity, and better memory storage.
The lithium-based battery is the most powerful battery technology available on today’s market. They are preferred for small, lightweight devices. Lithium ion batteries are rechargeable, and have significantly less energy density than standard lithium batteries. Manufacturers often request lithium-based batteries, as they have a long life. The drawback of lithium batteries, however, is the risk of explosion or fire due to the battery overheating. This risk increases the overall cost of the device, as significant testing and increased safety circuitry is required to ensure the safety of the end-user. Additionally, recycling locations for lithium batteries are not as readily available. For medical devices, batteries are usually returned to the manufacturer, which further increases the device maintenance costs.
Batteries in Small Medical Devices
As a designer of small medical devices it is important to not only look at the current battery options on the market, but also to consider the main purpose of the device. Zinc-air and/or NiMH batteries are used in hearing aids. Why? Wouldn’t it make more sense to use a lithium battery that has a longer life, and the ability to be recharged? No. The main goal of hearing aid manufacturers is to make a small device that helps the person hear; the priority is put on size over power. The end-user would much rather have to replace the battery every few days, or even every day, versus having a larger hearing aid with a longer battery life. All lithium batteries are required to have a safety circuit that prevents the battery from overheating. This circuit must be contained within the hearing aid or part of the battery itself. When size is your main concern, there is no room for extra circuits. There are currently four different sizes of hearing aid batteries on the market; that is not predicted to change in the foreseeable future.
The consistency in the NiMH and zinc batteries is because they are inherently considered to be very safe. Non-lithium batteries under 700mAh capacity do not have to go through any safety testing in accordance with IEC/ANSI standards; there just is not enough energy capacity to justify it. Their main testing requirement is performance based, surrounding length of use. Performance testing of the battery in hearing aids can be done in real-world circumstances, but also in harsh conditions to ensure that the battery will function as planned between maintenance visits. The specific performance testing requirements for hearing aid devices are set forth by NEMA in ANSI C18.1M, Part 1-2015.
This is not the same for all small medical devices. Primary, non-rechargeable, Lithium batteries are used internally on a regular basis in small medical devices like pacemakers. These internal devices are safe and have minimal risk after proper testing is done. Even though they are small, size is not such a priority that the safety circuit is seen as taking up too much space. These devices can last anywhere from five to seven years because they deliver such a low current shock that the battery’s power has a lasting effect. The main downside to these batteries could be their end life. Because the device is internal, when the battery needs replacing, it must be done surgically, by a doctor. Then the battery has to be replaced with a new one, and the old one has to be properly disposed of.
Lithium-Ion batteries are lithium batteries that can be recharged several times before they need to be replaced. These batteries are used in devices like defibrillators or AEDs. There has been a lot of concern over the past few years about the reliability of AEDs. There have been several reports of these devices failing in the past few years. But studies have shown that it is not the fault of a bad lithium battery5, but due to improperly maintained batteries. The regular maintenance and testing of lithium-based batteries is crucial for small medical devices.6 For a small additional cost, unexpected lithium-based battery failures in small medical devices can be reduced from 28% to 7%.7
Due to the higher-risk that lithium-based batteries possess, they are subject to different forms of testing before they can be released onto the market. A basic safety standard for primary non-rechargeable lithium batteries is IEC 60086-5 or IEC 62133 for rechargeable lithium batteries, which addresses the safety of batteries with aqueous electrolytes. Lithium-based batteries require the addition of several harsh conditions tests, which can include: device and battery damage, cell exposure, and overheating. There are also mandatory transportation requirements for Lithium-based batteries found in UN 38.3, which discusses how these batteries must be tested and shipped in order to prevent a battery related explosion. However, many countries require additional country specific testing and labelling for lithium batteries.
The Ever-Changing Market
Even though there has been relatively little progression in the market in the past few years, that doesn’t mean there are not battery innovators who are looking to improve batteries. There are advances being made in the hearing aid battery realm. Chemists are currently working on creating a zinc-air battery that is rechargeable. With batteries, cost is always an issue and metal-free catalysts can reduce cost while improving performance.8 These batteries have the potential to have three times the energy density than any other battery on the market9, but without the safety testing and protocols needed for lithium based batteries. This could be a significant improvement for the small medical device community and ultimately the end consumer.
Zinc batteries are not the only batteries that are striving to make improvements; lithium battery designers are making strides in creating a new form of lithium battery: lithium air. Lithium air batteries would use oxygen as an oxidizer instead of a metal.10 Like the zinc-air batteries this would make them last longer and less expensive to manufacture. These batteries could be a big game changer, but they are still several years off from production.
It will be up to the designers to stay up to date on the newest battery advances in order to ensure that their devices will best serve the end-user. Until the market changes, it is important that safety remains a top priority in their device design. Working with both internal and external testing laboratories to guarantee products are properly tested to meet the safety requirements and functionality needs of the device is key to keeping everyone in the supply chain happy and safe.