Protecting Wearable Technology in the Medical Market

How well do we really know ourselves? Consider that the typical modern automobile provides far more real-time feedback on its operating status than we know about the health of our own bodies. My personal vehicle is loaded with sensors and indicators that monitor and report on everything from engine temperature to fuel efficiency, and from driver and passenger seatbelt use to ambient light and temperature conditions. In sharp contrast, humans are limited to relatively few (albeit unpleasant) indicators of problems. Sure, anyone can recognize the signs of a fever, a cough, a sneeze, or some level of pain. Although equipment manufacturers have constantly incorporated higher levels of self-diagnostic and preventive maintenance routines, we humans lag far behind in terms of monitoring our bodies’ performance.

Fig. 1 – The next generation of wearable monitoring devices has already begun to transform the way physiological data is captured and recorded.

Clearly, this situation is far from ideal for optimizing one’s quality of life. Recognizing that a parameter (blood glucose, heart rate, etc.) was “out of bounds” in near-real time would go a long way toward minimizing the impact of major health issues. Having this information would allow us to take proactive steps to bring a critical parameter back under control. So, how could we get this information without spending our days constantly connected to diagnostic equipment? Wouldn’t this level of monitoring be time-consuming and invasive?

Not too long ago, the answer to these questions would have been yes. Anyone interested in gathering even the most basic information would have to visit a medical professional or use an invasive tool (like a lancet to get a drop of blood for use in a blood glucose meter). The associated costs, time, access/availability, and inconvenience have long complicated gathering physiological data.

Building Better Health Monitors

What if there were a better way? As it turns out, we are on the verge of a health monitoring revolution. The “Quantified Self” movement promises to help those interested in “getting under the hood” and understanding their health parameters to do so continually and conveniently. In short, Quantified Self is a concept in which electronic sensors monitor one’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.

Fig. 2 – Soon, wristbands will be able to communicate information on blood sugar levels, blood pressure, cholesterol, heart rate, nutrition, pulse oximetry, sleep, and other health matters to a user’s smartphone for easy transfer to a healthcare professional.

The ultimate goal is to allow people to act on their physiological information to improve their health, state of mind, etc. In the past, we’ve treated the human body as a “black box” that must be responded to rather than being understood in real time. A real-time understanding (acquired through physiological monitoring) would allow us to change behaviors to achieve a desired state (lower blood pressure, weight loss, faster recovery from injury/surgery, etc.). Without this information on one’s current state, it is difficult to make plans and move to the next step. However, if this information were readily available, it would encourage people to move toward their goals faster. Even simple steps like taking the stairs rather than the elevator or drinking water instead of sugary soft drinks would have a measurable and recognizable impact and lead to better health.

Fortunately, wearable technologies that incorporate physiological sensors are becoming increasingly available. Rather than forcing users to carry devices like blood glucose meters with them in a pocket, purse, or backpack, the next generation of monitoring devices will be worn on the body itself (See Figure 1). This nearly transparent incorporation of these medical sensors will allow people to monitor their condition in near-real time and let them amass a far larger number of data points over the course of the day.

Various examples of this approach are already on the market today, including wristbands designed to measure how far someone has walked, their pulse, etc. Unobtrusive undergarments (undershirts, bras, etc.) designed for use during fitness training, allow gathering data on parameters such as pulse, breathing rate, posture, and distance travelled.

As useful as these monitoring options are, the biggest breakthroughs are still to come (See Figure 2). Imagine if people with diabetes no longer had to prick their fingers multiple times over the course of the day to measure their blood glucose in order to adjust their insulin dose. This would not only eliminate the pain associated with making this measurement but would make it easier to collect this important data more frequently. This could allow people with diabetes to control their blood glucose levels more effectively over the long term and postpone or prevent the most serious consequences of the disease.

Fig. 3 – The bidirectional configuration of TVS Diode Arrays provides symmetrical protection when AC signals are present for robust protection against destructive electrostatic discharges.

Researchers in Germany have developed a method that uses infrared laser light and a technique called photo-acoustic spectroscopy to determine blood glucose levels through the skin. Along similar lines, researchers in Israel and the Netherlands are working on developing wearable devices that employ laser light, a magnet, and a camera to measure blood glucose concentration using the “speckle” effect, the grainy interference patterns that are produced on images when laser light reflects from an uneven surface or scatters from an opaque material. Both of these methods are non-invasive and may one day revolutionize the diagnosis, monitoring, and treatment of diabetes.

Avoiding Damaging Discharges

Clearly, the development and introduction of these new technologies hold the promise of improving data gathering and, ultimately, users’ health. However, the fact that these systems are ultimately intended to be worn next to the skin is both their greatest strength and a potential weakness because they will invariably be exposed to user-generated static electricity, which can render them inoperable without the proper protection. Unfortunately, a simple human touch can be all it takes to initiate an electrostatic discharge (ESD) transient. Any of the sensor circuits, buttons, battery-charging interfaces, or data I/Os could provide a path for ESD to enter the wearable device.

Fortunately for wearable device manufacturers, suppliers of semiconductor-based ESD protection components are on the job and working to improve the capabilities of these solutions. Electronics companies continually invest 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 primary 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 substantially more of the surge current than the circuit being protected. In doing this, it reduces the electrical stress on the integrated circuit and ensures that it survives. For example, TVS Diode Arrays are available with a dynamic resistance value of less than 0.1 to provide best-in-class performance.

Lower capacitance to avoid interfering with high speed data transfer: Although circuit protection is central to an ESD protection device’s purpose, it must perform this role without interfering with the day-to-day functioning of the circuit being protected. For example, on a radio frequency interface (Bluetooth^®, ZigBee^®, etc.) or wired port like USB 2.0, the ESD protector must not cause 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.

Smaller form factors to fit the limited board space available in the wearable devices: No matter how well a protection device performs, it’s not particularly useful if it can’t fit into the application it’s meant to protect. Wearable medical devices will continually get thinner and smaller (watches, wristbands, chest bands, etc.) or be incorporated directly into clothing, so 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. For instance, the diodes shown in Figure 3 can pack five bi-directional channels of protection into a space-saving 0.94mm × 0.61mm package outline for applications that demand reducing part counts and the protection device footprint.

So, although it is clear that upcoming wearable technologies will help advance users’ quality of life, they will challenge the designers who have to make sure that they not only do their jobs but also provide long-lasting reliability. They must allow making accurate measurements no matter how active the lifestyle or how often they are subjected to potentially damaging ESD events. Manufacturers of ESD protection devices continue to work just as hard as wearable device designers to provide protection for these devices while not interfering with their core functionality.

This article was written by James Colby, Manager, Business and Technology Development for the Semiconductor Business Unit of Littelfuse, Inc., Chicago, IL. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/49749-164.