How to power on a battery-operated medical or IoT device

By Tim Resker, global business development manager, Coto Technology, Inc.

Battery-operated, wirelessly connected devices are becoming increasingly pervasive in today’s society. Driven forward by advancements in wireless and battery technologies, coupled with shrinking electronic components that consume less power and cloud-based services ready to collect, analyze, and disseminate data, these devices are commonly found in consumer, medical, and wearable devices as well as in commercial and industrial applications.

Whether the device is a wearable continuous glucose monitor (CGM), an ingestible or implantable medical device, or a smart home device, asset tracker, or environmental monitor, all share the common requirement of small size, long life, reliability, and ease of use. One of the major problems faced by designers of these products is powering on the device when needed.

Powering on an IoT device only when it is needed (or keeping it powered down before it is deployed) is vitally important because designers want to use the smallest, lowest-cost battery possible. For this reason, extending battery life is always a design goal; battery drain must be minimized during use as well as before it has been powered on.

One popular example is the CGM prescribed to a Type 1 or Type 2 diabetic. This device adheres to a patient’s body, continuously monitoring his/her glucose level. Resulting data is wirelessly transmitted to the patient, doctor, and/or insulin pump. CGMs must be very small, “waterproof,” and easy to attach, as well as have a reasonably long life before they run out of battery power.

There are three basic options for powering on these devices at the point of use or deployment. For each of these options, essential variables for consideration are battery current drain, size, ingress protection, and user-friendliness.

Fig. 1: The TMR magnetic sensor offers almost zero power consumption in an ultra-miniature package size, and its contactless “power on” capability promotes ease of use.

The first “power on” option is electromechanical, or the common “switch.” This option is the means for powering on most battery-operated electronic devices such as laptops and phones. Although switches come in many forms, (e.g., pushbutton, slider, or toggle) they operate on the same principle of opening and closing a mechanical contact to allow current to flow (when closed) or completely prevent it from flowing (when open).

With regard to the first consideration of current drain,  the electromechanical switch is highly efficient because it is a passive device that consumes no power. However, in terms of size, mechanical switches are a poor option, especially given the size constraints of many wearable, ingestible, and implantable medical devices and other small IoT devices.

In terms of ingress protection (or the need to have a device that is impermeable to water and humidity), mechanical switches are not the best option, as designing a switch that can be mechanically moved by the user into on/off positions while maintaining impermeability is challenging.

Lastly, the consideration of user-friendliness, or ease of use, rates poorly with mechanical switches for two reasons. First, because the user must actually take this step (and many need to be instructed to do so), the requirement for many devices is “out-of-the-box turn-on” — a clear conflict with manually operated switches. Secondly, a very small mechanical switch, necessitated by a very small device, could pose a problem for users’ ability to actually move the switch to the on position, thereby reducing usability. So, in summary, mechanical switches score highly in terms of current consumption but very low relative to ingress protection, size, and ease of use.

Wireless power-on is the second option to analyze. Because the devices already have wireless capabilities to transmit data, designers could technically use that same wireless capability to power on a device from a mobile phone app.

From an ingress protection standpoint, powering on wirelessly is rated very highly. And from a size standpoint, powering on wirelessly also rates highly, as there is nothing more to add to the device for this functionality.

However, from a current drain standpoint, wireless power-on scores extremely low because a wireless receiver inside the device must be powered on to receive a signal to power on. For this reason alone, wireless power-on is rarely used for devices that have stringent battery life requirements.

Fig. 2: Technology comparisons

The third option is the use of a magnetic sensor inside the device to initiate the power-on function. In this case, a magnetic field is applied to the sensor to trigger the power on. The magnetic field is typically produced by a magnet that is located within the product’s packaging or in an auxiliary component to the device (such as an applicator for a CGM). The magnetic field can also be applied by the user swiping across the device with a handheld magnet.

Recommended

How does a TMR sensor operate?

Magnetic sensing scores very highly for ingress protection (because it is a “contact-less” method). Magnetic sensing also scores very highly in ease of use — especially when the magnet can be embedded in the device packaging (enabling “out-of-the-box power-on”) or in an auxiliary component to the device (e.g., an applicator). Sometimes the device itself is designed as two components that must be connected together at the time of deployment.

In terms of current drain and size, the desirability of magnetic sensing depends entirely on the magnetic sensing technology. Older, more traditional magnetic sensing technology types were either small in size but high in power consumption (Hall-effect) or large in size with zero power consumption (reed switches).

However, many new devices are designed with a newer magnetic sensing technology called tunneling magnetoresistive (TMR), which offers both a very small size (as small as an LGA-4) and extremely low power consumption, similar to the reed switch. In effect, TMR magnetic sensors offer the “best of both worlds.”

With the current onslaught of new devices designed to make life easier, safer, contact-less, and/or operable remotely, electronic designers are having to adopt new technologies to keep up with the evolving requirements of battery-operated wearables, implantables, ingestibles, and other IoT devices. In terms of best capabilities relative to small size, lower power consumption, ingress protection, and ease of use, magnetic sensors — and TMR sensor technology, in particular — are helping to make “impossible” designs possible.

About the author

As global business development manager for Coto Technology, Tim Resker works closely with solution providers in industrial, consumer, medical, and commercial industries to address their magnetic sensing needs. Tim has also worked for Analog Devices and other embedded technology suppliers in product marketing and business development roles.