Proper ESD protection is key to ensuring reliability of medical wearables

James Colby, Littelfuse


Smaller and smarter must also be safer

It is, by no means, hyperbolic to say that we are on the verge of another revolution. A revolution of the way we leverage modern technologies to access information about our own health conditions. Until only recently, anyone interested in obtaining even the most basic information (blood glucose, heart rate, etc.) would have to visit a medical professional or use an invasive tool to get a drop of blood for use in a blood glucose meter. The costs, time, access/availability, and inconvenience have always made it very difficult to collect physiological data.

The “Quantified Self” movement promises to help us understand our health parameters at all times. Quantified Self is essentially a concept by which electronic sensors monitor a person’s physiological parameters to understand the current state of the body (heart rate, glucose, hydration, oxygen consumption, sleep patterns, calories ingested, etc.) in real time.

The main goal here is to enable people to act on their physiological information to improve their health, state of mind, etc. Unfortunately, we have always treated the human body as a “black box” that must be responded to rather than be understood in real time. But, a real-time understanding (acquired through physiological monitoring) would allow us to change behaviors to achieve a desired condition (lower blood pressure, weight loss, faster recovery from injury/surgery, etc.). Without this information on one’s current state, we would hardly be able to make plans and move to the next step.

If this information were readily available, it would encourage people to work toward improving their overall health much faster. Even making simple changes like taking the stairs instead of the elevator or drinking water instead of consuming sugary soft drinks would have a measurable and recognizable impact and thus lead to better health in general.

The explosion in wearable tech

Wearable technologies that incorporate physiological sensors are becoming increasingly available. Rather than forcing users to carry blood glucose meters with them, the next generation of monitoring devices will actually be worn on the body itself (similar to the device in Figure 1). This nearly transparent incorporation of these medical sensors will enable people to monitor their condition in near-real time and allow them to monitor considerably more data points over the course of a day.

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Figure 1. The next generation of wearable monitoring devices has already started transforming the way people capture and record physiological data

Initial examples of this novel approach such as wristbands that are capable of measuring how far a person has walked, pulse, etc. are already on the market. Unobtrusive undergarments (undershirts, bras, etc.) designed for use during fitness training allow for data to be collected on key parameters such as pulse, breathing rate, posture, and even distance travelled.

But as beneficial as these monitoring options are, the biggest breakthroughs are yet to come (Figure 2). Just imagine for a moment if people with diabetes no longer had to prick their fingers several times a day to measure their blood glucose in order to adjust their insulin dose. This would not only make it much easier to collect this vital data more frequently but also eliminate the pain.. This in turn would help people with diabetes to actually control their blood glucose levels more effectively over the long term and postpone or even prevent the most serious consequences of this increasingly common disease.

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Figure 2. Wristbands will soon be able to communicate important information on blood sugar levels, blood pressure, cholesterol, heart rate, nutrition, pulse oximetry, sleep, and other relevant health matters to a user’s smartphone for easy transfer to a doctor

Researchers in Germany have even developed a method that uses infrared laser light and a technique called photo-acoustic spectroscopy to determine blood glucose levels without having to penetrate through the skin. The advantage here is that this method is non-invasive and could one day revolutionize the diagnosis, monitoring, and treatment of diabetes.

Interestingly enough, the fact that these systems touch skin is not only their greatest strength, but also a potential weakness. After all, they will inevitably be exposed to user-generated static electricity, which can render them inoperable without the proper protection. After all, a simple human touch can be all it takes to initiate an electrostatic discharge (ESD) transient. The reason is that any of the sensor circuits, buttons, battery-charging interfaces, or data I/Os could provide a path for ESD to enter the device.

Manufacturers of semiconductor-based ESD protection components are working hard to improve the capabilities of these solutions. Littelfuse, Inc., for instance, continually invests in developing new processes that enhance their protection products.

Recent component innovations include:

Lower clamping voltage to protect even the most sensitive circuits: During an ESD event, the main job of the ESD protector is to divert and dissipate as much of the ESD transient as possible. This characteristic is improved by reducing the on-state resistance (often called “dynamic resistance”). By reducing the dynamic resistance, the ESD protector carries significantly more of the surge current than the circuit being protected. In doing so, it reduces the electrical stress on the integrated circuit and ensures that it survives. The SP3014 Series TVS Diode Array from Littelfuse, for example, has a dynamic resistance value of less than 0.1Ω to provide best-in-class performance.

Lower capacitance to avoid interfering with high speed data transfer: Although circuit protection is vital to an ESD protection device’s purpose, it is important to keep in mind that it must perform this role without interfering with the day-to-day functioning of the circuit it protects. For example, on an RF interface (Bluetooth, ZigBee, etc.) or wired port like USB 2.0, the ESD protector must be prevented from causing distortion or loss of strength of the data signals. To ensure signal integrity, the capacitance of the ESD protector must be minimized without compromising protection levels. The SP3022 Series TVS Diode from Littelfuse features a capacitance value of 0.35pF, ensuring that it will remain “invisible” to high-speed signals.

Smaller form factors to fit the limited board space available in the wearable devices: No matter how well a protection device performs, it won’t be particularly useful if it can’t fit into the application it’s meant to protect. Wearable medical devices will gradually get thinner and smaller (watches, wristbands, chest bands) or be incorporated directly into clothing, so that the circuit boards will have minimal space available for the ESD protection solutions. Discrete diodes are ideal for giving design engineers exceptional board layout flexibility; and SP1020 (30pF) and SP1021 (6pF) Series are packaged in the 01005 package outline to minimize the amount of space they take up. In addition, the SP1012 Series (see Figure 3) packs five bi-directional channels of protection in a compact 0.94mm x 0.61mm package outline for applications that demand reducing part counts and protection device footprint.

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Figure 3. SP1012 Series five-channel bidirectional TVS Diode Arrays provide reliable 

Although it is obvious that upcoming wearable technologies will help advance users’ quality of life, they will continue to pose challenges for the designers. Making sure that they also provide long-lasting reliability is therefore extremely important. In fact, they must allow for accurate measurements to be made no matter how active the lifestyle or how often they are subjected to potentially damaging ESD events. Manufacturers of ESD protection devices are equally committed to working as hard as wearable device designers to provide protection for these devices while not interfering with their core functionality.