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Self-powered sensors implanted inside a human body can perform lifetime monitoring.Portable healthcare is experiencing exponential growth. Looking ahead, the technology will likely become even more integrated and located more conveniently (i.e., implanted). However, powering these devices for constant use will be challenging. This article looks at using energy harvesting technology to achieve this goal.

Homo sapiens are once again at a crucial juncture in their evolution timeline that started almost three million years ago. Evidence of the next phase is already here, but no one knows for sure how or where we will end up. But a term coined in the 1960s offers a clue: cyborg. It is an organism that shows enhanced – sometimes super – abilities due to technology. Could it be that one day, we all might evolve into a creature like a cyborg? No one knows – but it is certainly imaginable. What will morph us into that?

To search for answers, we need to accept first and foremost that in the future, Homo sapiens are very likely to have within them, or be part of, an external network or networks that are connected in some way. Some already call this the Internet of Things. Others call it the Sensors Revolution, or Augmented Reality. Regardless of what you call it, it is clear that some form of enhanced sensory feedback will be an integral part of our future. Our sensory world will no longer be limited to the five traditional senses. In the future, you may be able to get instant feedback or advance warnings from vital organs in your body.

Such networks are referred to as body area networks (BAN), or body sensor networks. They have the potential for making lasting impacts on the human race – literally. For example, future cardiac patients may gain the sensory ability to instantly know if the heart is functioning as expected. Or perhaps, one could gain the ability to regularly monitor the cerebral pressure of a patient showing symptoms of Parkinson’s or Alzheimer’s disease.

Paralyzed patients may be able to move using tiny electrical devices and sensors placed inside their bodies. Or cochlear implant patients may be able to gather sound using tiny sound-collecting electrical sensors placed inside their ears. So, it is not hard to imagine that the body of the future could be augmented by a vast array of sensors and tiny electrical gadgets placed inside the body to enhance form and function. It is also not difficult to imagine that such an electric body can perform life-time monitoring of vital statistics and gather other data that may be used in future medical research.

Depending on which research report is referenced, billions or even trillions of such sensors are predicted to come online within the next decade. In other words, some predict that the electric body will be a reality within the foreseeable future. No one knows for sure the exact timeframe. But one thing is for certain – these sensors will have a real need to be powered.

Resolving the Power Question
Where will this power come from? The current batch of sensors is powered by batteries. Most are designed for three to five years of useful life. The pre-determined lifetime is often dictated by the longevity of the power source – the battery that runs them. For ex ample, the batteries in a human implantable cardioverter defibrillator (ICD), also referred to as a pacemaker, last anywhere from four to ten years.

A block diagram representing a typical energy harvesting system used in self-powered sensors.

In many drug delivery systems, such as implantable infusion pumps, the battery life is even less due to the complex dosing protocols. Some pumps may last from a few weeks to a few months, thus requiring frequent replacements. Such replacements are not only inconvenient and time consuming, but can also be downright costly. With the large numbers of sensors expected to come online, implant surgeries just to replace batteries in an implanted device would soon become impractical. So how could one replenish the energy source for these sensors?

Fortunately, advances in semiconductor technology provide the hope of perpetually-powered electronic implants. New integrated semiconductor circuit technology will play a critical role in energizing these sensors and implantable devices. After all, the body itself is a vast pool of energy. Movement or motion, glucose, and heat are some of the potential energy sources within the human body. New ICs have the capability to extract tiny amounts of ambient energy from sources such as heat, light, vibration, and even radio-frequency energy.

But how will it all work? The power extracted is likely to be very small. Almost all nano-generators are scavengers of energy from minute movements of fluids, heartbeats, diaphragm, and respiration activities. Others also scavenge from motion of the limbs. Because of the minute amounts of energy scavenged, this energy will need to be boosted to make any difference. This is where new IC technology plays a critical role. Ultra-low power boost converters and chargers (energy management ICs) can play a huge role in the boosting process. Advances in semiconductor technologies have led to a whole new category of ultra-low power semiconductor ICs, including ultra-low power microprocessors, energy management ICs, and RF transceivers (for the radio link in case wireless data capability is desired).

Energy Extracting IC
One example of an IC that can extract energy from heat, vibration, and RF waves, is the Texas Instruments (TI) bq25570 ultra-low power energy management integrated circuit. This IC can extract, condition, and manage the small amount of power produced by nano-generators, or harvesters as they are sometimes called. The extracted energy can be stored in a storage element (battery) for later use. This energy is usually in the order of microwatts to milliwatts, but still enough to power ultra-low power ICs such as an ultra-low power MSP430G2553 microcontroller (MCU), or a SimpleLink CC2541 Bluetooth low energy wireless MCU from TI. The bq25570 IC also allows an innovative method to recharge implantable devices using body heat.

Heat produced in the body is a particularly appealing energy source. It is always present. Could there be a way to utilize the heat from the human body itself to power medical devices? It would require a way to first convert the body heat into usable electrical energy. Thermoelectric generators – devices that convert a temperature gradient into electrical voltages – are often used in industrial applications. A small-sized generator could be placed subcutaneously to take advantage of the temperature gradient between the internal tissue and the skin to be able to produce small amounts of power that can be harvested to recharge small batteries, thus extending the run-time. Designed correctly, such a system can provide perpetual power to small devices such as infusion pumps and other similar devices.

Case in Point: Indus Instruments
Other promising technologies for energizing the electric body are wireless charging, and wireless RF energy harvesting. Their contactless nature is well suited for medical applications. Innovative companies are using these technologies to differentiate themselves. For example, Indus Instruments is a company that specializes in turnkey design, development, and manufacture of custom electronic devices and computer systems for the medical, scientific, and industrial market sectors. The company’s MouseMonitor telemetry and rhythm products are preclinical research devices that can be implanted in small animals such as mice for vital signs monitoring and pacing of the heart. Unlike standard human pacemakers that rely on relatively large, non-rechargeable batteries, these devices use radio frequency energy harvesting to operate ultra-low power electronics and to recharge a small, on-board battery.

“Without energy harvesting and battery management, such as the bq25570 [see sidebar], it would be extremely difficult to meet the size and weight challenges posed by working with mice whose total body weight is often under 20 grams,” said Sridhar Madala, Founder & President of Indus Instruments.

Conclusion
We are closing in on human implantable devices that will integrate some kind of body sensor to give live feedback to the user, doctors, and medical practitioners. Additionally, external, wearable gadgets like smart watches, bracelets, smart eye wear, and other personal fitness devices could be powered by ambient energy sources in the future. This technology also could be adopted in the use of lifetime animal care and monitoring. As technology improves, devices will get smaller, allowing widespread use, and ultimately creating sensor networks that will need to be powered perpetually and often remotely. Energy harvesting will begin to play a pivotal role in the development of these self-powered body networks.

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