When seeking power solutions for in-home patient monitoring devices, manufacturers have an array of technologies from which to choose. However, the specific benefits and considerations for each must be evaluated for each application. This article highlights a number of these options and looks to consumer devices as excellent examples of how power technologies can best be used.
|Energy density is better with Li-ion when measured both volumetrically and gravimetrically.|
As the population of western countries ages, there is a broad movement to enable patient care in the home. Hospital stays can be shortened and doctor visits can be augmented with home treatment and monitoring. There is increased recognition that both short term quality of life and long term outcome are improved for an ambulatory patient. Devices from dialysis machines to ventilators to nebulizers are being designed to be used in the home setting. Patient monitoring devices are following the trend of direct use by the patient. In-home monitoring devices help patients take a more active role in their health and stay connected to their healthcare providers. Patients use wireless measurement devices to take their own vital signs and respond to personalized, clinician-directed surveys. The home-use trend drives many technical requirements—most importantly, ease of use. For a simple user experience, designers must incorporate a power system that complements wireless communication and a low maintenance charging model.
Smaller Size and Lighter Weight
Portability is key to home-use success. Bulky, heavy medical equipment can quickly crowd the patient’s home. In many cases, the home units are meant to be taken from place to place. The battery for portable equipment consists of a cell or cells, power management electronics, and a plastic housing. Much like consumer electronic devices, the battery tends to be the heaviest part of a home patient monitor. Also, patient monitoring devices are used in relatively benign environmental conditions and have fairly low current draw. Therefore, designers of home monitoring systems can look to consumer electronics for the latest in battery technology. Lithium ion (Li-ion) technology offers a pronounced energy density increase over older chemistries. For their size and weight, Li-ion cells store and deliver more energy than other rechargeable batteries and the performance of these battery cells continues to improve year after year. Energy density is better with Li-ion when measured both volumetrically and gravimetrically. Also, there are an increasingly wide variety of types of Li-ion cells. Li-ion is best thought of as a family of chemistries that have the working lithium ion shuttling from anode to cathode and back in common. Several variations of Li-ion technology are on the market with various cathode and anode chemistries and assembly methods. Recently, some high power chemistry variations have gained favor in the news, but these aren’t necessary for the low current draw of patient monitor electronics.
|A digital x-ray plate, one of the first medical products to make use of the thin polymer form factor, is thin enough to fit in conventional film x-ray cassettes.|
Home use products can benefit from the small form factor afforded by Lithium polymer (Li-polymer) cells. This is the physical structure of the cell rather than fundamentally different electrically active materials. The means of electrode separation—in other words, the separator material—is one difference between conventional cells and polymer cells. The layers of separator and the active materials are laminated together. The other difference is the external cell packaging. Unlike lithium-ion cylindrical cells, which have a rigid metal case, Li-polymer cells have a flexible, foil-type case. Therefore, these cells are sometimes called laminate or pouch cells. Since individual Li-polymer cells have no strong metal casing, by themselves, they are over 20% lighter than equivalent cylindrical cells. The primary advantage of Li-polymer batteries is the variety of form factors available. They can be made very small and thin or very large and can come in custom shapes. Manufacturers of Bluetooth devices were the first to recognize the advantage of Li-polymer batteries. The availability of very thin batteries soon enabled the Motorola RAZR phone with great market success simply because it was so thin. Apple was the next company to recognize the appeal of very thin products. A digital x-ray plate, one of the first medical products to make use of the thin polymer form factor, is thin enough to fit in conventional film x-ray cassettes. Like the consumer products mentioned, patient monitors for home healthcare can also benefit from the small thin form factors afforded by Li-polymer batteries.
Permanent Battery Installation
One of the first considerations for the patient monitor should be how it will interact with the battery and how the battery and charger will interface with the functional unit, since powering and charging the device is fundamental to the patient’s use model and user experience. While some industrial and medical equipment utilize external, multi-bay chargers, most consumer electronics use charging electronics internal to the device itself. For example, the power supply on a laptop, tablet, or smart phone plugs straight into the host unit. A home patient monitor will use a similar scheme. This simplifies the number of products that the patient must use and keep track of. Another trend in consumer products is to permanently embed the battery in the host device. Battery cycle life is getting so long that the battery is unlikely to need to be replaced. Also, the increased use of more fragile Li-polymer systems increases the need for structural protection, so it makes sense to permanently protect the battery with the patient monitor’s plastic housing. This decision leads to a number of other conclusions for the electronics. A battery pack for a patient monitor needs to contain an electronic diagnostics system for state of charge and state of health, so a communications interface must be selected that is compatible with the battery and appropriate resources allocated to the host processor. However, it’s also possible to put the fuel-gauge on the patient monitor motherboard itself. This, in turn, reduces the direct connections to the battery pack and is most suitable for non-removable battery packs.
|The elimination of contacts between the charging system and battery is another option to make a patient monitor easy to use and keep clean in the home setting.|
Wireless or contact-less charging has been around for some time—most recognize its use in an electric toothbrush. Improvements in magnetics and the proliferation of hand-held devices have expanded the number of techniques and uses for contact-less charging. Today, there are multiple methods for implementation, including a new “standard” adopted by many industry players for low power wireless transfer. Although the standard describes an inter-operability technique, it does not limit innovation, and there are multiple components now available from several industry players that make adopting this technique much more cost effective and easy. The elimination of contacts between the charging system and battery is another option to make a patient monitor easy to use and keep clean in the home setting.
Robin Sarah Tichy, Ph.D. is the marketing manager at Electrochem Solutions Inc. She has developed an expertise in translating market drivers into technical solutions in the battery and charger industry. Prior to joining Electrochem, Dr. Tichy applied technical and project management skills to orchestrate and implement solutions to solve vital business problems at leading organizations like Hewlett Packard and International SEMATECH, in the semiconductor, nanotechnology and MEMS verticals.