The flexible circuit market has been growing steadily for the last three decades. This trend will continue into the foreseeable future as flexible circuitry supports many of the same industries and many of the same applications that have been around for more than 30 years. Past and current industries include military and avionics with most of these applications being high layer count, high-density rigid flex, and also consumer electronics, telecom, and automotive applications with flex circuit designs that are typically less complex than those of mil/aero. Medical diagnostic applications will continue to grow as new equipment is developed and older equipment is refurbished or redesigned. But if I had to sum up an answer to the question “where is flex going in the near future?” my answer would be simply “on you.”
The wearable electronics market has absolutely exploded in the last few years with new applications emerging almost daily. If an electronic device is going to be worn on the body comfortably, it has to be flexible. So what better way to provide interconnects for these types of devices than a flex circuit? Here are just a few of the current and emerging wearable products that contain flexible circuitry.
Wrist-Worn Activity and Body Function Monitors: Electronic watches were some of the first wearable electronics, so it was just a natural progression to include more advanced functionality than just time keeping. Wrist-worn activity monitors are light weight and use multiple axis accelerometers and other sensors to detect motion and body functions. They can capture and record daily activity levels as well as sleep cycles. This data is stored in on-board memory in the device until it can be downloaded to the user’s mobile phone. Since the human hand is larger than the wrist, these monitoring bands need to be able to expand when the user is putting it on or taking it off. Flexible circuits allow the band to flex while maintaining connectivity across flexing sections.
Foot-Worn Sensors: I have seen a lot of applications recently for electronics that are worn on the foot or inside the shoe. Foot-worn electronics monitor everything from steps taken when running or walking to stride irregularities that can contribute to back problems. These sensors need to be very thin in order to be comfortable and also very robust to survive in what I would consider a pretty hostile environment. Flexible circuitry is thin enough to lay on the sole of a shoe and be almost undetectable to the wearer.
Wearable Baby Monitors: Baby monitors are one of the newer products in the wearable electronics market. New parents no longer have to rely on a simple walkie talkie system to keep tabs on their little ones while they sleep. These monitors can be worn on the baby’s leg or in their clothing and can keep track of breathing, heartbeat, body temperature, etc. If the device senses that there is a problem, an alert is sent to the parents phone to wake them. It is almost like having a private nurse watching the child all night long.
Medical Sensors: This is an area that has been growing rapidly, and I predict that the trend will continue at an accelerated rate. With today’s push to get patients out of the hospital as quickly as possible, electronic home monitoring of the patient is going to be necessary. There are currently sensors that can be worn by the patient for several days at a time, while keeping tabs on heart functions continuously during this time. Just like the baby monitor referred to earlier, these devices can send alerts to the patient’s physician if any abnormalities are detected. These devices will allow a patient to recover from heart attack or surgery in the comfort of their own home while still having continuous monitoring of their state of health.
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Pet Monitors: Even Rover gets to wear electronics these days. Training collars have been around for a while, but now thanks to shrinking electronics there are collars that contain GPS and mobile phone capabilities. Today a lost pet can use the GPS to figure out where he is and call his owner for a ride home! Not really, but if your pet is wearing one of these devices he is never truly lost. The mobile phone module is used to transmit the GPS coordinates to tracking service, where the owner can log on and track the pet’s location to within a few feet.
Clothing Worn Electronics: This is an area that is just starting to emerge, and new technology is being developed to support these applications. Standard flex circuitry is constructed from a combination of polyimide film, thermo-setting film adhesive, and copper foil. Unfortunately, flex circuits fabricated with these materials will not survive the crumpling that they would be exposed to in a washing machine. I have seen several applications where flex has been incorporated into clothing that does not need to be machine washed (e.g., flexible heaters in winter gloves). The key to making this type of wearable application machine washable is to make the flex circuit not only flexible, but also stretchable. This means that both conductors and dielectrics must be developed that will allow the finished product to stretch and still maintain electrical continuity. This technology is not mainstream yet, but it is on its way.
These examples are just a small sampling of the applications that are currently on the market, and there are many others in development. As more and more of these applications emerge, flexible circuitry will continue to be the interconnect method of choice.
This article appears in Circuit Cellar 296 (March 2015).
Mark Finstad is a Senior Application Engineer at Flexible Circuit Technologies in Minneapolis, MN. He is a nationally recognized expert in the design, fabrication, and test of flexible and rigid flex printed circuits with more than 30 years of experience in the flexible PCB industry.
