Wearable, scalable and accessible smart healthcare solutions

background

The overall medical electronics market was valued at approximately $3 billion in 2015 and is expected to continue growing at a CAGR of 5.4% to reach a market size of $4.41 billion by 2022. [Source: Marketsandmarkets.com] It is no surprise, then, that some of the key drivers of this growth are: rising aging populations and increasing lifestyle diseases; the growing demand for personalized, easy-to-use and advanced healthcare devices; and the increasing use of wearable medical electronics.

At the same time, the costs of keeping patients in hospital beds for extended periods of time for treatment and recovery are becoming increasingly unsustainable financially, both for the healthcare facilities themselves and the patients. As a result, hospitals are looking for ways to reduce these cost burdens and get patients back to health and autonomy as quickly as possible without sacrificing full recovery. One way to achieve this is to free patients with remote monitoring and diagnostic devices so they can return to their own homes to recuperate. These remote patient monitoring capabilities typically include heart rate, blood pressure, respiratory rate, sleep apnea, blood sugar levels, and body temperature. This, therefore, supports the assumption that one of the real trends driving the growth of portable and wireless medical instrumentation is outpatient care. As a result, many of these portable electronic monitoring systems must have built-in RF transmitters so that any data collected from the patient monitoring system can be easily sent directly back to the monitoring system within the hospital, where it can be reviewed and analyzed later by the attending physician.

Low-power precision components have enabled the rapid growth of portable and wireless medical instruments. However, unlike many other applications, such medical products typically have much higher standards for reliability, operating time, and ruggedness. Much of this burden falls on the power system and its components. Medical products must operate correctly and switch seamlessly between multiple power sources such as AC outlets, backup batteries, and even harvested ambient energy sources. In addition, every effort must be made to provide protection and tolerance against a variety of different fault conditions, maximize operating time when powered by batteries, and ensure that normal system operation is reliable whenever some valid power source is connected.

Potential Solutions for Patient Monitoring Systems

Given the above, it is reasonable to assume that "it would cost far less to provide patients with appropriate medical instrumentation at home than it would to keep them in the hospital for the same purpose". However, it is crucial that the equipment used by patients is not only reliable, but also protective! Therefore, manufacturers and designers of such products must ensure that they can operate seamlessly on multiple power sources (including backup energy sources), collect data from patients with high reliability, and achieve 99.999% integrity of wireless data transmission. This requires system designers to ensure that the power management architecture to be adopted is not only rugged and flexible, but also compact and efficient. In this way, the needs of hospitals and patients are met.

Fortunately, several analog companies such as Linear Technology are committed to providing solutions to the above problems by introducing innovative products. Since there are many applications in medical electronic systems that require continuous operating power even when the AC power is interrupted, a key requirement is to achieve low quiescent current to extend battery life. Accordingly, switching regulators with a standby quiescent current of less than 9mA are often required by users. In fact, some new systems that rely on a combination of batteries and energy harvesting as their main power supply require quiescent currents in the single digit microamperes, or even nanoamperes in some cases. This is a prerequisite for adoption in such "home use" patient medical electronic systems.

Although switching regulators generate more noise than linear regulators, their efficiency levels are much better than the latter. As long as the switching power supply operates in a predictable manner, the noise and EMI levels have proven to be manageable in many sensitive applications. If the switching regulator performs switching operations at a constant frequency in normal mode, and the switching edges are clean and predictable (no overshoot or high-frequency ringing), EMI is minimized. Small package size and high operating frequency can provide a compact layout, which greatly reduces EMI radiation. In addition, if the regulator can use low-ESR ceramic capacitors, the input and output voltage ripples, which are additional noise sources in the system, can be minimized.

The number of power rails in today's multi-function patient monitoring medical devices has increased, while operating voltages continue to decrease. Even so, many of these systems still require 3V, 3.3V or 3.6V rails to power low-power sensors, memory, microcontroller cores, I/O and logic circuits. In addition, because their operations are sometimes life-threatening, many of them are equipped with a battery backup system in case the device's main power supply fails.

Traditionally, their voltage rails have been provided by step-down switching regulators or low-dropout regulators. However, such ICs do not utilize the full operating range of the battery, thus shortening the potential battery run time of the device. Therefore, when a buck-boost converter (which can either step up or step down the voltage) is used, it enables the full operating range of the battery to be utilized. This increases the operating margin and extends the battery run time because more of the battery life is available, especially when it is near the low end of its discharge curve.

Energy harvesting as a power source

There has been a lot of innovation recently in the area of ​​energy harvesting; specifically, using one’s own body heat as a potential energy source to power electronic monitoring systems or to recharge the batteries that power these systems. These advances have enabled the size and shape of medical electronic components to change to fit into the 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 less than approximately 250µW of power.

Furthermore, wireless sensor networks with power levels in the range of a few µW to hundreds of mW typically operate from battery power. However, due to the inherent limitations of battery power (e.g., charge storage life and the need for periodic recharging where applicable), the possibility of using ambient energy sources such as heat or vibration to periodically recharge "rechargeable" batteries has emerged. Now is the time to introduce it.

Linear Technology has been manufacturing energy harvesting ICs for almost 10 years; the first product was the LTC3108, which was introduced in December 2009. The LTC3108 is an ultra low voltage DC/DC converter and power manager specifically designed to harvest and distribute surplus energy, generating very low voltages from thermal sources. This can be from "hot" to "hotter" or from "cold" to "colder", as the only thing required is a temperature gradient of 1°C or more.

However, a more recent product is the LTC3107, a highly integrated DC/DC converter designed to extend the life of the main battery in low power wireless systems by harvesting and managing surplus energy from very low input voltage sources such as TEGs (thermoelectric generators) and thermopiles.

With the LTC3107, a point-of-load energy harvester takes up very little space, just enough to fit the LTC3107's 3mm x 3mm DFN package and a few external components. By generating an output voltage that tracks the voltage of an existing main battery, the LTC3107 can be seamlessly adopted to bring the cost savings of free thermal energy harvesting to new and existing battery-powered designs. In addition, the LTC3107 can extend battery life (in some cases up to the shelf life of the battery) together with a small thermal energy source, thereby reducing the recurring maintenance costs associated with battery replacement. The LTC3107 is designed to augment the battery or even completely power the load, depending on the load conditions and available harvested energy. Figure 1 shows how the LTC3107 can easily harvest thermal energy to power a wireless sensor node (WSN) and seamlessly switch to battery power when ambient energy sources are not available.

 Figure 1: The LTC3107 harvests thermal energy to power a WSN and/or charge a battery

In addition, the LTC3331 is a versatile ambient energy harvester that forms a complete energy harvesting regulation solution, providing up to 50mA of continuous output current to extend battery life when harvestable energy is available, see Figure 2. When providing regulated power to the load with harvested energy, the device requires no supply current from the battery and only requires 950nA of operating current when powered by the battery under no-load conditions. The LTC3331 integrates a high voltage energy harvesting power supply and a synchronous buck-boost DC/DC converter (which is powered by a rechargeable primary battery) to provide an uninterrupted output for energy harvesting applications such as WSN and Internet of Things (IoT) devices. 

Figure 2: The LTC3331 can convert multiple energy sources and use a rechargeable primary battery.

The LTC3331's energy harvesting power supply consists of a full-wave bridge rectifier suitable for AC or DC input and a high efficiency synchronous buck converter to harvest energy from piezoelectric (AC), solar (DC) or magnetic component (AC) sources. A 10mA shunt enables easy battery charging with harvested energy, while a low battery disconnect function protects the battery from deep discharge. The rechargeable battery powers a synchronous buck-boost converter that operates from an input range of 1.8V to 5.5V and is used to regulate the output when harvested energy is not available, regardless of whether the input is above, below or equal to the output. When dealing with micropower supplies, the LTC3331 battery charger has very important and cannot be ignored power management functions. The LTC3331 incorporates logic control of the battery charger so that the battery is charged only when the energy harvesting power supply has excess energy. Without this logic control function, the energy harvesting power supply will be stuck at a non-optimal operating point at startup and will not be able to complete startup and power the target application. The LTC3331 automatically switches to the battery when the harvested energy source is no longer available. This has the added benefit of allowing battery-powered WSNs to extend their operating lifetime from 10 years to over 20 years if a suitable energy harvesting source is available at least half the time, or even longer if ambient energy sources become more ubiquitous.

in conclusion

The market for smart medical wearables is ready to take off. Driven by the high cost of hospital patient care and the exploding aging population, this new wave of products includes healthcare wearables that use sensors to monitor key biometrics, such as heart rate and blood pressure outside of the hospital, creating opportunities for more active, healthier lifestyles. The core architecture of a smart wearable depends on the product type, but essentially consists of a microcontroller, MEMS sensors, wireless connectivity circuitry, battery, and supporting electronics.

Therefore, having the current wave of versatile energy harvesting and/or IoT solutions that can harness multiple forms of ambient energy to power human health monitoring devices could allow patients to return to their homes for treatment and recuperation sooner without sacrificing full recovery.