Alternative Energy Harvesting Techniques Emerge for Sensor Networks

It's clear that wireless sensor networks and mobile sensor systems are proliferating everywhere. On one hand, there are sensor networks made up of a large number of sensor nodes employed in vast inaccessible areas. At the other extreme, there are applications where compact mobile sensor systems are necessary with a wireless communication system.

Whether it is a large-scale or small-scale system, these implementations face the challenge of supplying enough energy for the proper long-term functioning of devices. Such sensor systems are critical in a wide range of applications, including biomedical engineering, environmental monitoring, controlling industrial processes, quality assessment of organic materials, and many more.

 

There's a natural synergy between wearable sensors and energy harvesting. The chart shows types of self-powering smart wearable sensors. Images used courtesy of Rong et al

 

In general, these sensor-based systems are battery-powered. However, in many applications, sensor networks are deployed in inaccessible areas or as a biomedical devices implanted in a human body. In those cases, replacing or recharging depleted batteries is not practically realizable.

In this article, we'll round-up research from three universities making strides in wearable, energy harvesting technologies. 

 

Battery Fueled by Human Sweat

One solution to that challenge is to design a power supply to ensure enough energy is supplied to the device for its complete lifecycle. The problem is that this type of approach demands higher capacity bulkier batteries which, in turn, increases the device's size. To address this issue, researchers are now focusing on flexible energy storage devices, such as flexible capacitors and batteries that are lightweight, safe, stretchable, and can be easily integrated.

Along just those lines, researchers from NTU have developed a soft and stretchable battery powered by human sweat. According to the researchers, human perspiration consists of lactic acid, glucose, and ions, making it suitable for the realization of wearable fuel cells, capacitors, and batteries. The battery is attached to a sweat-absorbent textile that can be attached to wearable devices. It measures 2 cm x 2 cm with the thickness of a paper bandage.

 

This stretchable battery is attached to a sweat-absorbent textile. The textile can be attached to a wearable device. Image courtesy of Nanyang Technological University

 

Wearable devices have benefited from advancements in the fabrication of stretchable conductive wires. However, the conductors can corrode by the effect of sweat. In their device, the NTU researchers fabricated conductive silver flakes and an elastic binder directly on the textile. In the presence of sweat, the chlorine ions increase the conductivity of silver.

From trial tests, the team reported an output power of 3.9 mW (4.2 V) generated by individuals cycling for 30 minutes with the wearable battery around their wrists. 

 

Energy Harvesters for Low-power Printed Electronic Devices

Another path for avoiding bulky batteries is to lower the power consumption of the systems themselves. Recent developments in ultra-low-power circuits have reduced the power consumption of mobile devices significantly. In standby mode, power consumption is getting down as low as mere picowatts, while active mode power consumption has shrunk down to just nanowatts.

These breakthroughs allow devices to self-sustain themselves using energy harvesters. The energy harvesters harvest energy needed to power the system from natural sources such as sunlight and thermal difference or ambient vibrations, radiofrequency, static electricity, etc.

There are some tricky issues. The energy produced from different sources has different power levels. Meanwhile, some sources like solar energy are discontinuous, meaning that power levels differ throughout the day. For such power sources, energy storage solutions are necessary.

With all that in mind, a number of choices have to be considered when choosing an energy harvesting system for a device. This includes:

• Availability of the energy sources in its environment
• Feasibility of fabrication
• Energy storage solutions

 

Self-Powered Sensors Using Piezoelectricity

Along the theme of energy sources in the sensor's environment, researchers from Tampere University have proposed a technique that leverages the properties of human skin. Their proposal is a fabrication process for a self-powered, bio-compatible, ultrathin, and transparent printed pressure sensor applicable in biosignal monitoring.

Human skin can transmit and generate biochemical signals that can provide useful information regarding respiration rate, heart rate, and body motion. Pressure sensors based on piezoelectricity can detect these signals, and that too without any external supply. Piezoelectric elements allow for self-powered operation.

 

Tampere University researchers developed a fabrication process with piezoelectric elements that enable self-powered operation. Image used courtesy of Tampere University

 

One hurdle is that fabricating a piezoelectric sensor on an ultrathin biocompatible device is very difficult because it consists of lead. In their research, the scientists proposed a two-step printing process that uses biocompatible polymer-based materials for substrate, electroactive layer, and interdigitated electrodes. Researchers used the sensor to monitor arterial blood pulse waves, which can provide various vital information on health status like blood pressure.

 

Energy Harvesting on a Glass Substrate

Another team of researchers from RISE Research Institutes of Sweden AB (RISE) reported the development of a battery-less electronic circuit printed on a glass substrate. Organic-based materials have gained much popularity in 3D-printed electronics because they can be easily fabricated on flexible surfaces.

However, these materials are sensitive to their surroundings. Organic materials can degrade with fluctuations in temperature and humidity. Therefore, they added the electronic components on glass by the pick-and-place assembly.

 

A schematic of the device's printed layers (a). Shown here is the continuous voltage outport level when the system is activated through the NFC interface of a mobile phone (b). Image (modified) used courtesy of RISE Research Institutes of Sweden

 

The system the team developed is comprised of several elements:

• A sensor capable of detecting water
• An electrochromic display
• Conductors
• A power management chip managing the power supply through energy harvesting of electromagnetic radiation
• A microcontroller responsible for monitoring the sensor status

Importantly, the power management chip manages the power supply by the energy harvesting of electromagnetic radiation.

 

A Battery-less Future for Mobile Sensor Systems?

The developments discussed here indicate that ultra-low-power devices and 3D printing could help pave the road for the next generation of mobile sensor systems. These new techniques will enable self-powering, eco-friendly systems that will not rely on batteries.