The benefits of wearable devices are threefold, according to PA’s report: they can act as a ‘digital lifestyle coach’; provide unobtrusive monitoring of patient data; and drive efficiencies in the delivery of treatments.
According to Mike Salas, vp marketing and strategy, Ambiq Micro, “People are voluntarily embracing consumer products ranging from heart rate and sleep monitors to pedometers.
“Unobtrusive sensors, when combined with the connectivity enabled by the Internet of Things, are making it possible to deliver on-going care, as well as allowing clinicians to collect long-term data and make more informed decisions as a result.”
The potential of wearable devices in healthcare has been clear for some time, suggests Salas. “The advent of wearable appliances and ubiquitous connectivity could provide the impetus needed to finally make such initiatives a reality.”
A ‘wearable’ can be defined as a product that is worn by the user for an extended period of time and which enhances their experience as a result of the product being worn. But add connectivity and independent data processing capabilities, and you have a ‘smart’ wearable device.
Bio-stats, for example, are vital signs that measure the human body’s basic functions and can be used to indicate an individual’s state of health. These can include body temperature, pulse/heart rate, respiratory rate and blood pressure.
Traditional patient monitoring has usually required a trip to the doctor or hospital; wearable solutions, by contrast, can offer an efficient and inexpensive alternative enabling these stats to be measured in the home or at work. As a result lifestyle and behaviour modifications could be suggested and made in real time.
Semiconductor manufacturer ams has developed a new optical heart rate sensor for use in wrist wearables. The device, the AS7000, has been designed to measure a person’s heart rate by shining light into blood vessels, using a technique known as photoplethysmography (PPG), which works by analysing scattered reflections.
“The device includes two green LEDs and a photosensing signal processing IC based around an ARM Cortex-M0,” explains Peter Trattler, a senior product manager with ams. “The module has been paired with an external accelerometer which allows internal algorithms to handle several potential causes of interference and distortion.”
The main challenges for measuring PPG on a wrist-worn device are the impact of ambient light, cross talk and motor-generated artefacts.
“The light from the sun is easy to cancel out,” suggests Jan-Hein Broeders, Analog Devices’ healthcare business development manager Europe, “but light from fluorescent and energy saving lamps carry frequency components that can cause AC errors.”
Analog Devices, which has developed the ADPD142 optical module, uses two structures to reject this type of interference. After the analogue signal conditioning, a 14bit, successive approximation A/D converter digitises the signal, which is transmitted via an I2C interface to a microcontroller for final post processing.
The device includes a synchronised transmit path that is integrated in parallel with the optical receiver. Its independent current sources can drive two separate LEDs with current levels programmable up to 250mA. The LED currents are pulsed, their lengths being in the microsecond range, so the average power dissipation is kept low.
A practical solution
Sensor technology and falling device costs means that wearable technology is becoming increasingly practical, whether as simple ‘single vital sign’ units that can be attached to the body, such as the AS7000, or in more sophisticated full body sensor filled exoskeletons.
According to Steve Knoth, senior product marketing engineer, power products, for Linear Technology: “The core architecture of a smart wearable has to be a combination of parts such as a microprocessor or microcontroller; some sort of micro-electromechanical sensors (MEMS); mechanical actuators; Bluetooth/cellular/Wi-Fi connectivity to collect/process and synchronise data; imaging electronics, LEDs; computing resources; a battery pack and support electronics.
“What is crucial is a compact form factor, low weight and ultra-low energy consumption. But engineers need to be aware that powering these devices efficiently and accurately with a minimal current draw isn’t simple.”
According to Knoth, smart wearables need:
* ultra-low quiescent current, both in operating mode and shutdown
* wide input voltage range to accommodate a variety of power sources
* the ability to efficiently power system rails (some with voltages of more than 5V)
* The ability to count Coulombs accurately without affecting IC quiescent current (battery consumption) current in order to determine battery run time
* small, light weight and low profile solution footprints
* advanced packaging for improved thermal performance and space efficiency.
“We have sought to address these market requirements with a variety of products, from ultra-low buck regulators to nanopower devices and energy harvesting regulators,” Knoth explains.
“For example, the LTC3388 is an ultra-low quiescent current synchronous buck converter capable of delivering up to 50mA of continuous output current from a 2.7 to 20V input supply. It has a no-load operating current of only 720nA which makes it suitable for battery-powered and low quiescent power applications.
“The LTC3335 is a nanopower high efficiency synchronous buck-boost converter with an onboard precision Coulomb counter that can deliver up to 50mA of continuous output current. With only 680nA of quiescent current and programmable peak input currents from as low as 5mA up to 250mA, we are using it to target wearable and IoT devices.”
ADI’s Broeders believes the market for wearable medical devices is evolving to not only provide health monitoring, but also more pro-active healthcare. But he accepts that power will always be a critical factor.
“Wearable designs require high performance, small size and low power, but integrating everything, including the battery, in a small package is still a challenge,” he says. Despite new battery technologies that are capable of bringing more capacity per unit volume, the battery ‘is still large compared to the electronics’.
Energy harvesting can reduce battery size and extend battery life and a number of technologies to harvest energy, including thermoelectric, piezoelectric, electromagnetic, and photovoltaic are available.
“Light and heat tend to be the most appropriate for wearable devices,” Broeders suggests. “The sensors usually don’t provide a lot of output power, so every Joule generated should be caught and used.”
Linear has developed an energy harvesting device – the LTC3331 – that is capable of delivering up to 50mA of continuous output current to extend battery life when harvestable energy is available.
“It integrates a high voltage energy harvesting power supply, a battery charger, and a synchronous buck-boost DC/DC converter, creating a single continuous regulated output for energy harvesting applications such as those in wireless sensor networks,” according to Knoth.
The wearable devices market is a fast changing one and developments are continuing at a pace but the biggest challenges will be, for the time being at least, how to minimise size and maximise battery lifetime.
