# An NFC on Two-Coil WPT Link for Implantable Biomedical Sensors under Ultra-Weak Coupling
In this paper, we propose a near-field communication (NFC) attached to an inductive WPT link, tailored to an implantable sensor system for glaucoma treatment. The intraocular sensor device will be implanted in patients' eyeball to monitor and regulate the intraocular pressure (IOP), as seen Figure 1. The IOP is measured by a micro-hydrostatic sensor. The uplink sends the data of the pressure for monitoring the IOP and the data of the peak voltage on the load for adaptively adjusting the transmitted power of WPT. When the measured IOP is above the normal IOP, the downlink sends the commands to control an actuator (micro-pump) to regulate the IOP. High IOP is considered a leading risk factor for glaucoma, and lowering IOP continues to be the only evidence-based treatment to prevent its development or to reduce the progression rate [12; 13]. Due to the limits of the weight and size in the eyeball, the battery cannot be integrated, and only one coil could be implanted for NFC and WPT. In a word, the wireless link of this device needs to transmit power and data simultaneously through the same coil pair under weak coupling. The proposed NFC uses two carriers through the same coil pair. One carrier is the power carrier for the power and downlink data transmission, while the other is the uplink carrier for the uplink data transmission. BPSK modulation is used in the downlink to minimize the amplitude ripple of the power carrier. BPSK is also utilized in the uplink to be able to demodulate the weak signal under strong interference. In the inductive link circuit, dedicated passive components are introduced and optimized to minimize the power transfer efficiency (PTE) decrease caused by adding the data links. For the uplink, we firstly explore the constraint of avoiding the out-body receiver from being saturated by the power carrier and the limit of uplink receiver sensitivity. Then, we optimize the circuit parameters to get the maximal PTE and at the same time keep the received uplink signal power to be sufficient for reliable demodulation. For the downlink, a shaping filter is introduced in the downlink modulator to reduce the impact of BPSK modulation on the WPT power amplifier (Class E PA). Finally, a prototype was implemented. Our proposed NFC overcomes the strong interference from the power carrier and achieves reliable communication under ultra-weak coupling. At the same time, our NFC causes less PTE loss and negligible amplitude ripple on the WPT power carrier, compared with the traditional NFC using LSK. This paper focuses on the design and optimization of the physical link layer, as it is the key to achieve the simultaneous power and data transmission. The rest of this paper is arranged as follows. Section 2 presents the overview to introduce the system structure and considerations of this dual-carrier NFC scheme. Section 3 presents the equivalent model and analysis of the inductive  
Figure 1: Inductive wireless link of the implantable sensor system for glaucoma treatment. link circuit. Section 4 discusses the minimization of the interactions between the NFC and the WPT. Section 5 presents the prototype implementation and the measured results. Section 6 is the conclusion.