How to Bring Flexibility to Wearable Design
SOLVING THE INTERCONNECT CHALLENGE: How to Bring Flexibility to Wearable Design
Micro-Electro-Mechanical Systems (MEMS) are more prevalent than ever, especially in the popular configurations known today as “wearables.” Leading MEMS and sensors in the multitude of wearable technologies include accelerometers, gyros, magnetometers, microphones, UV sensors, glucose monitors, barometers, humidity detectors, and heart rate instruments.
Wearable systems include microcontrollers that detect, interpret, and communicate multiple signals. The combined MEMS, sensors, and hub electronics within wearables communicate via interconnects.
AMEMS devices on a silicon wafer. (Credit: Veryst)
Future categories of wearable devices may not fit this archetype, such as a MEMS and sensor array covering an entire jacket. Would the designers of such a jacket really want a bulky array of devices sewn into the fabric that require different batteries and wireless antennae for power and communications? How can we best connect arrays of MEMS that monitor and transfer data over a large range of surface areas?
The challenge of wearable interconnects is to combine sensor devices with miniaturized conductive grid systems that provide low signal loss and increased mechanical flexibility. The requirement for flexibility in wearable devices necessitates the development of novel technologies for device manufacturing and processing. As an example, new manufacturing methods include roll-to-roll technology, which produces bendable interconnect systems.
Some of the most popular signals to monitor in the world of wearables are movement, direction, sound, light, and pressure. BioMEMS and biosensors provide health data by monitoring heart rate, pulse rate, oxygen in blood, blood pressure, respiration, hydration, and chemistry of perspiration. Interconnects enable power and communication between sensor signals and hub electronics (such as microcontrollers).
Additionally, the devices bring wireless signals to host devices.
The market size of MEMS (Figures 1 and 2) and sensor systems for wearables grows daily. In fact, research firm IHS Technology predicts over 450 million MEMS and sensors shipments for wearables by 2019. In this article, we will review the MEMS devices in today's wearables, and demonstrate how a flexible interconnect capability can better support the increasingly popular MEMS-powered technologies.Accelerometers
One of most popular wearable technologies today is the fitness and health monitor. Pedometers, wristband trackers (Figure 3), and portable blood pressure monitors commonly contain accelerometers. Accelerometers are MEMS devices that detect acceleration, vibration, shock, and tilt. When coupled with MEMS gyroscopes, rotation can also be sensed, providing additional accuracy in movement. Current products include multi-axis accelerometers and multi-axis gyros in one package.
Most MEMS accelerometers today use a differential capacitive measurement system which calculates g force (acceleration). Upon experiencing an acceleration, a MEMS mass attached to a spring will move in distance — the moveable mass is one plate of a capacitor. An adjacent fixed mass is the other plate of the capacitor. The relative change in displacement between the two masses results in a change in capacitance. The signal is typically converted into volts subsequently sensed by a data acquisition system. A microprocessor then performs the proper interpretation of the data through algorithms that determine the output of the accelerometer.Gyros
Optical image of a MEMS resonator. (Credit: Veryst)
A MEMS gyroscope measures rotation; the rotational speed is also known as angular velocity. The gyroscope therefore provides movement information in the absence of any linear acceleration. An on-axis gyro will measure a single axis rotation, while a three-axis gyro will measure rotation around all three axes. Since the gyro measures in units of degrees/second, small and slow movements are detectable with a MEMS gyroscope, through a small resonating MEMS mass that shifts as angular velocity changes. The output is amplified and fed into a microcontroller for interpretation.Magnetometers
Detection of the Earth's magnetic field using MEMS magnetometers has the benefit of not requiring metallic materials. The coupling of a MEMS magnetometer with an accelerometer in the wearable technology allows for tilt compensation, so that pitch and roll can be accurately calculated. The magnetic sensor requires calibration to convert raw data to a heading calculation (also known as yaw). Coupling a MEMS magnetometer with a MEMS accelerometer results in a 6-axis device; the combination uses approximately 50% less power and can boost magnetic signal by 30%.
Researchers are developing methods to use a magnetometer and the human body in wireless communication between two wearable devices. Gesture recognition, a promising feature for many MEMS devices, can be performed with magnetometer device technology.The Interconnect Challenge
We have demonstrated above that multiple MEMS devices have been combined in one package for improved functionality. To enable light and small wearable sensor systems, sensor device sizes are being reduced using very small packaging. Compact packaging approaches include wafer-level, chip-scale packaging and 3D packaging with interposers or fan-out technology. The latter allows MEMS and microcontrollers to be stacked and arranged as one unit. Accommodation of small sensor packaging in wearable interconnects, with a form factor that moves and flexes with ease, is needed for further advancement of the technology.
A fitness wristband wearable. (Credit: Veryst)
Most wearable technologies today monitor human action. Unfortunately, today's rigid circuits threaten to hinder the ease and comfort of the consumer's movement. Wearable electronics worn in shirts, pants, and jackets will cover a wide surface area and must not be readily felt during the normal motions of the user. Typical human movements such as walking, running, bending, and twisting cannot be hindered by tight wiring designs — this is why interconnect flexibility will be most important in smart clothing applications.
To this end, let's identify today's less flexible technologies, followed by emerging manufacturing techniques for bendable electronics. The flexible manufacturing methods have the potential to disrupt the current barriers that limit wearables to predominantly watch-sized electronics seen in the present market.