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In this letter, the in-body and off-body channel models at the frequency of 2.4 GHz are studied for development of multi-node leadless capsule pacemaker technology based on experiments in homogeneous liquid phantom model of human heart and living animal experiments. For conducting the experiments, we design a battery-operated self-contained transmitter capsule consisting of a small antenna and transmitter printed-circuit board, sub-cutaneous implant and the off-body antennas. The in-body path-loss model obtained from the phantom experiment is a linear function of distance, whereas the off-body path-loss model between the implant and the off-body antenna is a logarithmic function of distance comparable to the free-space path-loss model. The phantom experiment study shows that coupling between implants decreases linearly at the rate of 3.6 dB/cm for cardiac implants and by 4.1 dB/cm for cardiac to sub-cutaneous implant at 2.4 GHz. The animal experiment results are in good accordance with the phantom results.

Next generation of cardiac pacemakers are expected to be completely wireless bringing along new security threats. Thus, it is critical to secure the pacemaker transmissions between legitimate nodes from a third party or an eavesdropper. This work explores the potential of securing leadless cardiac pacemaker by using physical layer security methods. In this work, we perform phantom experiments to replicate the dielectric properties of human heart and measure the path loss models for in body to in body scenario and in body to off body scenario. These scenarios reflects the channel between legitimate nodes and that of channel between legitimate node and eavesdropper in frequency band ranging between 1.7-2.5 GHz. In our case, legitimate nodes are leadless cardiac pacemaker implanted in right ventricle of human heart transmitting to a legitimate receiver, which is subcutaneous implant beneath the collar bone under the skin, whereas third party outside the body trying to eavesdrop the communication. By using these models, potential of positive secrecy capacity has been shown along with its probability by varying eavesdropper distances. It has been seen that even if eavesdropper is at the same distance as of legitimate node, still we have about 45% probability of positive secrecy rate. The randomness in channel measurements is observed because of measurements at different angles from source, both for legitimate link and as well as eavesdropper link, which can reflect to propagation through different parts/organs of human body.

Multi-node leadless pacemaker system overcomes the main limitations related to lead complications of the conventional cardiac pacemaker and will thus replace them in the near future. The multiple nodes of the technology require the development of low-power, low data-rate and energy-efficient communication framework for device synchronization and bi-directional communication between them. Moreover, the nodes need to communicate with the outer peripheral devices for data telemetry, control and remote monitoring. This paper focuses on evaluation of different energy-efficient modulation schemes at 433 MHz for bi-directional communication between the nodes using homogeneous liquid phantom model of human heart and living animal experiments. In this paper, we have analyzed three simple, low-budget modulation schemes-On Off-Keying (OOK), Frequency Shift-Keying (FSK), and Gaussian Frequency Shift-Keying (GFSK). The analysis is done based on the total transmitter power required to achieve a reliable communication indicated by the minimum threshold values of bit-error rate and packet-error rate. The experiments have been conducted for three common implant communication scenarios-in-body to in-body, in-body to on-body and in-body to off-body links. For conducting the experiments, we have designed the experimental setup with electronic components and fabricated antennas. The results have shown that GFSK has the best performance among the other modulation techniques based on the total transmitter power. We also investigated higher order of the same modulation schemes-4-FSK and 4-GFSK. The results showed that GFSK performed much better than 4-FSK and 4-GFSK. This research will be carried forward to build the entire radio frequency communication framework for the multi-node pacemaker technology.