Charging Implantable Biomedical Devices Using Ultrasonic Waves
Researchers create efficient triboelectric receivers for ultrasonic energy transfer to wirelessly charge batteries underwater or in body-implanted electronic devices.
Recent advances in ultra-low-power circuits have opened the path for new portable electronics devices and low-power wireless nodes that can be employed in remote places or implanted in the human body. Portable, life-saving medical implants such as pacemakers and defibrillators face a massive drawback as the batteries eventually run out. Moreover, the energy harvesters that harvest energy from the mechanical movements of the heart or thermal energy of the body are not reliable sources to recharge pacemaker batteries.
Therefore, for such applications, there is a need for a wireless energy transfer mechanism to recharge batteries efficiently. Researchers at the Korea Institute of Science and Technology introduced a new wireless charging mechanism using ultrasonic waves to charge the batteries of implanted medical devices and underwater devices, such as sensors to monitor the condition of submarine cables.
The researchers considered ultrasonic waves as a medium to transmit energy over EM waves or magnetic fields since EM waves cannot propagate through a dense medium, and it generates significant heat during charging, which is harmful in the case of implant devices. The magnetic resonance methods require large coils for higher efficiencies, and the magnetic waves can easily get interfered with or blocked. Ultrasound waves, on the other hand, are more suitable for transferring energy through dense matters.
Talking about receivers, piezoelectric materials are a common choice for receiving energy through acoustic waves. These materials produce electricity when stressed by mechanical waves. However, piezoelectric receivers can harvest efficiently at a particular frequency, and the material needs to be designed to resonate at the frequency of the incident acoustic waves. In contrast, triboelectric generators are a potential energy receiving mechanism as the surface charges generated due to stress are proportional to the area, eliminating the need for matching resonance. Due to this fact, the KIST researchers chose triboelectric generators for ultrasound energy transfer in their study.
Triboelectric Nanogenerators - An Overview
The triboelectric effect is electrification or charge accumulation on the surface of certain materials after they are separated from which they were in contact, causing a potential difference to form.
A triboelectric nanogenerator consists of two different materials and two electrodes placed at the back of each material. With the stress from acoustic waves, the materials make contact and generate friction, producing surface charges. During this event, one of the materials loses electrons and becomes positively charged, while the other material gains electrons and becomes negatively charged. After the materials separate, an internal potential is established, causing the current to flow. When the stress is reapplied, the materials move towards each other, and the charges flow back, causing an opposite current to flow. As soon as they make contact and there is friction, the new cycle begins.
However, the efficiencies associated with these generators are extremely low, and the power density achievable is also very low.
Ferroelectrically Boosted Triboelectric Nanogenerator
In their study, researchers maximized the surface electrification by introducing a ferroelectric crystal with high remnant polarization under the triboelectric layer. The ferroelectrically boosted nanogenerator consists of a flexible electrode, a fixed component, a triboelectric layer, a ferroelectric layer, and a bottom electrode.
The researchers chose indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrate as the bottom electrode and polytetrafluoroethylene (PTFE) and thin aluminum foil for the triboelectric and top electrodes. The ferroelectric layer is placed between the base and the triboelectric layer, which provides the electric field due to remnant polarization. The polarization shifts the energy configuration of the triboelectric layer, changing the magnitude and direction of electrification depending on the polarization.
Schematic of the underwater acoustic energy transfer system using the ferroelectrically boosted triboelectric energy receiver. Image used courtesy of KIST
With the addition of the ferroelectric layer, the ultrasound energy transfer efficiency improved from less than 1% to more than 4%. Upon testing, the researchers observed that the nanogenerator can produce 8mW of power at a distance of 6cm from the source, which is sufficient to operate 200 LEDs or to communicate Bluetooth sensor data. Moreover, it generated minimal heat.
Ferroelectrically boosted nanogenerator with a rectifying circuit and Bluetooth wireless sensor. Image used courtesy of KIST
Future Scope of Energy Harvesting Systems
Recently, low-power energy harvesting systems technology has attracted attention as the enabling technology that expands the opportunities of low-power sensor networks. With the advancement of associated converging technologies, the harvesters are getting more efficient and reliable. With this trend, they are slowly becoming indispensable for sensing the environment or human bodies.