Medical wearables: powered by energy harvesting

01 November 2017

Credit: Shutterstock
Credit: Shutterstock

The total medical electronics market in 2015 was valued at ~$3B and is expected to grow at an annual compound growth rate of 5.4% – reaching $4.41B by 2022. Here, Linear Technology explains that it should therefore come as no surprise that some of the key factors driving this growth are ageing population and growing lifestyle diseases; and the ever-increasing adoption of wearable medical electronics.

This piece originally appeared in the September 2017 issue of Electronic Product Design & Test; to view the digital edition, click here – and to register to receive your own printed copy, click here.

At the same time, the costs associated with keeping patients in hospital beds for prolonged periods of time are becoming economically unsustainable – both for the institution itself, and for patients.

As a result, hospitals are looking for ways to reduce these expenses by getting patients well – and autonomous – as quickly as possible, without compromising a complete recovery. One way of achieving this is by discharging the patient, having provided them with remote monitoring and diagnostic devices. These remote patient monitoring functions might typically include heart rate, blood pressure, breathing rate, sleep apnea, blood glucose levels and body temperature.

This bolsters, moreover, the premise that one of the current trends fuelling the growth of portable and wireless medical instrumentation is outpatient care. Consequently, many of these portable electronic monitoring systems must incorporate RF transmitters, so that any data gathered from the remote patient monitoring systems (RPMSs) can readily be sent back to the hospital’s relevant supervisory system – where it can later be reviewed and analysed by the governing physician.

Low power, precision components have enabled the rapid growth of portable and wireless medical instruments. However, unlike many other applications, these types of medical products typically have much higher standards for reliability, runtime and robustness. Much of this burden falls on the power system and its components. Medical products must operate reliably and switch seamlessly between a variety of power sources, such as an AC mains outlet, battery backup, and even harvested ambient energy sources. Furthermore, great lengths must be taken to protect against, as well as tolerate, various fault conditions; maximise operating time when the RPMS is powered from batteries; and ensure that normal system operation is reliable whenever a valid power source is present.

Potential solutions for patient monitoring systems

Given the above scenario, it is reasonable to believe that the cost of supplying the patient with the appropriate medical instrumentation for home use is more than offset by the money saved by not keeping them in the hospital for the same purposes. Nevertheless, it is vital that the equipment used by the patient is not only reliable – but patient proof! As a result, the manufacturers and designers of these products must ensure that they can run seamlessly from multiple power sources (including backup sources), whilst maintaining a highly reliable record of the data collected from the patient, as well as 99.999% integrity of the wireless data transmission.

This requires that the system designer ensures that the power management architecture in question is not only robust and flexible, but also compact and efficient. This way, the needs of the hospital and those of the patient are mutually satisfied. As there are many applications in medical electronic systems that require continuous power, even when the mains supply is interrupted, a key requirement is low quiescent current – to extend battery life.

Accordingly, switching regulators with standby quiescent current less than 9 microamps are usually needed. Some of the new systems, in fact, that run on a combination of battery power and energy harvesting as their main power sources require their quiescent currents to be in the single digit micro-amps range – or in some cases – even nanoamps. This is a necessary prerequisite for adoption in such 'in-home use' patient medical electronic systems.

Although switching regulators generate more noise than linear regulators, their efficiency is far superior. Noise and EMI levels have proven to be manageable in many sensitive applications as long as the switcher behaves predictably. If a switching regulator switches at a constant frequency in normal mode, and the switching edges are clean and predictable with no overshoot or high frequency ringing, then EMI is minimised. A compact package size and high operating frequency can provide a small, tight layout, which reduces EMI radiation.

Furthermore, if the regulator can be used with low ESR ceramic capacitors, both input and output voltage ripple can be minimised, which are potentially additional sources of noise in the system.

While the number of power rails in today’s feature-rich patient monitoring medical devices has increased, the opposite applies to operating voltages. Nevertheless, many of these systems still require 3V, 3.3V or 3.6V rails for powering low power sensors, memory, microcontroller cores, I/O and logic circuitry. Furthermore, since their operation is sometimes critical, many of them have a battery backup system – should the main power supply to the unit fail.

Their voltage rails, traditionally, have been supplied by step-down switching regulators or low-dropout regulators. However, these types of ICs do not capitalise on the battery cell’s full operating range, thereby shortening the device’s potential battery run time.

Therefore, when a buck-boost converter is used (it can step voltages up or down), it allows the battery’s full operating range to be utilised. This increases the operating margin and extends the battery run time, as more of the battery’s life is usable – especially as it nears the lower end of its discharge profile.

Energy harvesting as a power source

Recently, there has been a great deal of innovation in the area of energy harvesting; in particular, the use of a person’s own body heat as a potential energy source to power electronic monitoring systems (or recharge the batteries of such devices).

Such advances enable modifications to the size and shape of medical electronics components, so as to accommodate a milliwatt and/or microwatt power range. This means that many complex electronic systems and devices, such as wearable medical and autonomous devices, can now consume power in the range of less than 250µW.

Furthermore, wireless sensor networks, with power levels in the range of µWs to 100mWs, are routinely operated from battery power. However, due to the intrinsic limitations of battery power (such as the longevity of charge – and where applicable – the need for periodic recharging), few possibilities to use ambient energy sources (such as heat or vibration for the periodic recharging of a ‘rechargeable’ battery) have presented themselves. That is, until now. Linear Technology has been manufacturing energy harvesting ICs for almost a decade – the first product introduced being the LTC3108 in December of 2009.

The LTC3108 is an ultralow voltage DC/DC converter and power manager that is designed specifically to collect and dispense surplus energy, creating extremely low voltages from heat sources. This can be from hot to hotter or cold to colder, since all that is needed is a temperature gradient of 1°C or more.

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