Abstract
In this paper we study the read range of implanted sensors based on batteryless, Near-Field Communication (NFC) integrated circuits (IC) using an NFC-equipped smartphone as a reader. The most important challenges are the low coupling between loops of different sizes, the limited quality factor imposed by the bandwidth communication, the effects of the body on propagation, and the detuning of the antennas. Two systems are analyzed: a conventional system based on resonant coupling between two coils; and a system based on resonant coupling between three coils. With the latter, a relay antenna is attached to a patch, which is adhered to the skin. Simulations and measurements show that the quality factor of both antennas can be improved by including a spacer made of low-permittivity material. A circuit model is proposed for the implanted and relay antenna, which simplifies its usage in circuit simulators. Some implanted and relay antenna prototypes are analyzed and a system model that includes a nonlinear model of the tag is used to analyze the maximum depth at which the implant can be read. Our experimental results show that the system based on three coils performs much better performance at longer distances and is more robust to misalignments between coils. A 15×15 mm-implanted tag with commercial NFC IC and energy harvesting can be read using commercial smartphones. It can feed sensors at a distance of up to 16 mm inside the body and at a distance of 3 cm from the skin. Our results also show that data previously stored in the IC memory can be transferred to the reader located at distances of up to 2 cm and 3.8 cm for the 2-coil and 3-coil systems, respectively. This study demonstrates the potential of batteryless NFC sensors for biomedical and wearable applications using mobile phones as readers.
Highlights
Implantable devices are enjoying increasingly widespread acceptance and becoming ever smaller and more powerful [1], [2]
The aim of this paper is to study the possibility of working with conventional Near-Field Communication (NFC) integrated circuits with energy harvesting that are available on the market [27] in implanted devices using the smartphone as a reader
Using standardized NFC readers and devices rather than custom-designed wireless power transfer (WPT) systems presents a number of challenges such as the power and the system’s overall quality factor and detuning effects
Summary
Implantable devices are enjoying increasingly widespread acceptance and becoming ever smaller and more powerful [1], [2]. Such devices include implantable glucose and oxygen sensors for diabetics [3]–[5], implantable chemical devices [6], [7], neural implants [8], [9], cochlear implants [10], ocular implants [11]–[13] and cardiovascular devices [14]. One of the most critical of these challenges is the power source needed by electronic devices. The readrange and size constraints imposed by the limited space available inside the body are fundamental challenges
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