by Michael D’Agati ’18
Some devices that aid human function, such as cochlear or real-time biosensors, only function inside the body. However, it is not practical to power them with batteries because of their potential toxicity and relatively short lifetimes. Because of these reasons, inductive coupling has been studied for wireless power transfer to these embedded biosensors. In other words, an external power source can power coils of wire outside the body, generating a magnetic field that powers coils on the receiving end inside the body. The two coils, separated by the skin and muscle layers, would never physically touch. Currently, two-coil inductive coupling, which only utilizes two wire coils, uses magnetism to power radio frequency identification (RFID) tags and commercial medical implantable devices. Researchers are trying to increase the voltage sent to the receiving end to drive circuits requiring more power.
One particular research group from Texas Tech University led by Professor Tam Nguyen has published research that utilizes a four-coil design for powering mini biosensors. The four-coil design powers the miniature device at a frequency of 5.8 GHz, which is higher than previous wireless power transfer (WPT) designs with two coils. This higher frequency of operation allows for a smaller implanted coil inside the body. In this case, the receiver coil is only 110 µm x 110 µm, compared to a previous size of 2 mm x 2 mm.
In Nguyen’s project, two different devices were built and tested at the new 5.8 GHz frequency: a two-coil system and four-coil system. In the two-coil system, the receiver coil was 110 µm in size with two loops, otherwise known as turns, and the transmitter coil was 3 mm in size with one turn. The two coils were spaced apart at 1 mm and the coupling power ended up at -26.29 dB. This result means that only about 26 percent of the power was transmitted. In the four-coil system, the same transmitter and receiver coils were used. The spacing between the coils was also only 1 mm, just like in the two-coil system. However, the other two coils, also called relay coils, were 5 mm in size and each had one turn. This time, the coupling power was 20.19 dB. Approximately 36 percent of the power was transferred – a 10 percent increase from the previously accepted two-coil WPT system.
The four-coil system developed in this project advances the field of wireless power transfer, specifically for powering implantable biosensors. Creative ways, such as this system, are important for monitoring health. Future alternatives to batteries for powering systems in the body will be extremely beneficial, since they will hopefully be safer, more practical, and much more convenient.
- B.T. Nukala, et al., Efficient near-field inductive wireless power transfer for miniature implanted devices using strongly coupled magnetic resonance at 5.8GHz. IEEE Xplore, (2016). doi: 10.1109/WMCaS.2016.7577481.
- Image retrieved from: https://commons.wikimedia.org/wiki/File:VFPt_quadrupole_coils_2.svg.