Today’s modern medical devices are evolving at a record pace. From small wearable watches and bands worn by health enthusiasts, to portable wireless units for patients to use at home, to the larger more complex handheld devices used by healthcare professionals—there’s no question we are at the forefront of developing new ways in which to empower patients and medical professionals alike.

Fig. 1 – Super compact size, a small amount of onboard storage, and the ARM processor allow the Pebble watch to run numerous health/fitness apps. (Credit: Pebble Technology Group)
While all of this is very exciting, there are some critical manufacturing and design requirements developers must keep in mind. Wearable medical devices need to be small, constantly connected, secure, and have extended battery life. Developers must survive a highly competitive market of increasing complexity as well. To accomplish this, developers must build upon a platform that is fast, flexible, lightweight, and cost effective.

In this article we will touch on a few design considerations developers need to keep in mind before they embark on developing their next wearable or portable medical device.

How Will the Device Be Used?

One way to best guarantee the success of your medical device is to first consider its use cases, not just by the end-user, but how your device will be designed, developed, and tested.

Will your new device have communication modes or is it purely a stand-alone? Depending on its communication needs, you may have to move from bare metal to some type of operating system (OS). Are there any deterministic, real-time needs? For some portable medical devices, there is no requirement for real-time behavior. If an interrupt is serviced 100ms late, the results may be delayed by 100ms, but that’s not going to cause a failure.

Perhaps your device will be a critical piece of equipment so there is minimal sensitivity to cost. On the contrary, a device that is sold in the thousands has a high sensitivity to costs. The type of decisions you make will directly affect the need to minimize the bill of materials, which, in turn, might potentially minimize the memory you will need to effectively build the complete application with some margin.

It’s All about the Hardware

Once the use cases have been defined, it’s time to find the appropriate hardware. Wearable, and to some extent portable medical systems can be extremely small with an 8-bit microcontroller clocked at less than 25 MHz, and use only 8K of memory. Low power ARM devices such as the ARM Cortex-M and Cortex-A families are ideal for the smaller, wearable devices because of their small form factor and minimal power requirements. The Pebble watch is a good example of a wearable device running on the ARM Cortex M-3 processor. (See Figure 1)

More complex designs often include feature-rich systems on chips (SoCs) clocked in the hundreds of MHz and megabytes of memory. There are hybrid systems that have special purpose processors or digital signal processors (DSPs) to systems that include numerous multicore chips. The more complex portable devices often require the operation of a graphical user interface (GUI) and wireless connectivity to the Internet or cloud.

The Heart of the Matter: The Operating System

Wearable devices, and to a greater degree portable medical devices, are usually managed by some sort of operating system (OS). An OS can vary from a simple, in-house creation, to a more complex OS purchased from an established vendor. The general purpose operating system (GPOS), such as Linux or Android, provides a feature-rich platform for application development, but it can be overkill at times consuming more memory than necessary. A real-time operating system (RTOS) is also a good choice for modern medical devices. An RTOS is ideal when specific requirements within the system require the capabilities of a deterministic preemptive kernel and a small memory footprint.

Device Connectivity

The compelling difference between medical devices today compared to a few years ago is the ability to offer global connectivity—either directly to the Internet (the “Internet of Things”), or to a local intermediary device, such as a medical device paired to the user’s smartphone, which eventually provides a path to the Internet. This connectivity can be intermittent, using either wireless or a temporary wired connection, or it can be constant, using one of the many of wireless options available.

Wired options are the least expensive route, but offer the most limited flexibility. However, they are still a viable solution for low-cost devices. When connecting through a wired link to another device provided by the vendor of the wearable system, it’s possible to resort to extremely simple methods such as Serial Peripheral Interface (SPI) and an inter-integrated circuit (I2C). It’s quite feasible that the choice will change over the lifetime of the device, or even during the development cycle. Insulating the application level from the underlying connection method is most effectively handled by the operating environment.

The true future of wearable medical devices is, however, in the realm of wireless connectivity. While universal serial bus (USB) is a more complex protocol than SPI, varying wireless connectivity options are far more complex than USB, especially when security is involved. Wireless connection methods span the range of Near Field Radio, Bluetooth/BLE, Wi-Fi, up through mobile cellular networks. This is an area where technology, protocols, and options are changing rapidly. More important, the dynamics of the costs of these systems is such that solutions currently deemed to be too expensive today can easily become the economical standard tomorrow.