Wireless power transfer systems, particularly those based on inductive coupling, provide an increasingly attractive method to safely deliver power to biomedical implants. Although there exists a large body of literature describing the design of inductive links, it generally focuses on single aspects of the design process. There is a variety of approaches, some analytic, some numerical, each with benefits and drawbacks. As a result, undertaking a link design can be a difficult task, particularly for a newcomer to the subject. This tutorial paper reviews and collects the methods and equations that are required to design an inductive link for biomedical wireless power transfer, with a focus on practicality. It introduces and explains the published methods and principles relevant to all aspects of inductive link design, such that no specific prior knowledge of inductive link design is required. These methods are also combined into a software package (the Coupled Coil Configurator), to further simplify the design process. This software is demonstrated with a design example, to serve as a practical illustration.

This paper presents a passive phase-shift keying (PPSK) modulator for uplink data transmission for biomedical implants with simultaneous power and data transmission over a single 13.56 MHz inductive link. The PPSK modulator provides a data rate up to 1.35 Mbps with a modulation index between 3% and 38% for a variation of the coupling coefficient between 0.05 and 0.26. This modulation scheme is particularly suited for biomedical implants that have high power demand and low coupling coefficients. The PPSK modulator operates in conjunction with on-off-keying downlink communication. The same inductive link is used to provide up to 100 mW of power to a multi-channel stimulator. The majority of the system on the implant side was implemented as an application specific integrated circuit (ASIC), fabricated in 0.6-μm high voltage CMOS technology. The theory of PPSK modulation, simulated and measured performance evaluation, and comparison with other state-of-the-art impedance modulation techniques is presented. The measured bit error rate around critical coupling at 1.35 Mbps is below 6 × 10 ^-8 .

Inductive powering for implanted medical devices, such as implantable biosensors, is a safe and effective technique that allows power to be delivered to implants wirelessly, avoiding the use of transcutaneous wires or implanted batteries. Wireless powering is very sensitive to a number of link parameters, including coil distance, alignment, shape, and load conditions. The optimum drive frequency of an inductive link varies depending on the coil spacing and load. This paper presents an optimum frequency tracking (OFT) method, in which an inductive power link is driven at a frequency that is maintained at an optimum value to ensure that the link is working at resonance, and the output voltage is maximised. The method is shown to provide significant improvements in maintained secondary voltage and system efficiency for a range of loads when the link is overcoupled. The OFT method does not require the use of variable capacitors or inductors. When tested at frequencies around a nominal frequency of 5 MHz, the OFT method provides up to a twofold efficiency improvement compared to a fixed frequency drive. The system can be readily interfaced with passive implants or implantable biosensors, and lends itself to interfacing with designs such as distributed implanted sensor networks, where each implant is operating at a different frequency.

Wireless biosensors are playing a pivotal role in health monitoring, disease detection and management. The development of wireless biosensor nodes and networks strongly relies on the design of novel low-power, low-cost and flexible CMOS sensor readouts. This paper presents a CMOS potentiostat that integrates a control amplifier, a dual-slope ADC and a wireless unit on the same chip. It implements a novel time-based readout scheme, whereby the counter of the dual-slope ADC is moved to the receiver and the sensor current is encoded in the timing between two wireless pulses transmitted via pulse-harmonic modulation across an inductive link. Measured results show that the potentiostat chip can resolve a minimum input current of 10pA at a sampling frequency of 125 Hz and a power consumption of 12 μW.

Wireless data telemetry for implantable medical devices (IMDs) has, in general, been limited to a few Mbps, and used for applications such as transmitting recordings from an implanted monitoring device, or uploading commands to an implanted stimulator. However, modern neural interfaces need to record high resolution potentials from hundreds of neurons; this requires much higher data rates. While fast wireless communication is possible using existing standards such as WiFi, power consumption demands are far too high for IMDs. Short range inductive link based telemetry, in particular impulse-based systems such as pulse-harmonic modulation (PHM), have demonstrated transfer speeds of up to 20 Mbps with a small power budget. However, these systems require complex and precise circuits, making them potentially susceptible to inter-symbol-interference. This work presents a new method named Short-range Quality-factor Modulation (SQuirM), which retains the low power consumption and high data rate of PHM, while improving the resilience of the system and simplifying the circuit design. Transmitter and receiver circuits were fabricated using 0.35 ${\mu }m$ CMOS. The circuits were capable of reliably transceiving data at speeds of up to 50.4 Mbps, with a BER of $< 4.5 \times 10^{-10}$ , and a transmitter energy consumption of 8.11 pJ/b.

Wireless sensing systems are becoming popular in a range of applications, particularly in the case of biomedical circuits and food monitoring systems. A typical wireless sensing system, however, may require considerable complexity to perform the necessary analog to digital conversion and subsequent wireless transmission. Alternatively, in the case of inductive link based systems, large, manually operated impedance analyzers are required. Based on a detailed analysis of the link impedance, this paper proposes a simple method for wireless capacitive sensing through an inductive link that uses a self-oscillator and a frequency counter. The method enables changes in capacitance to be sensed and wirelessly transmitted simultaneously. In order to test the effectiveness of the method, a self-oscillating circuit was designed and fabricated in 0.18 μm CMOS, and combined with an on-chip humidity sensing capacitor. The system was tested in a humidity chamber across a range of 20-90%rh. Measured results from the system demonstrate that capacitive changes as small as 28 fF, translating to <2%rh, can be resolved, with a power consumption of 1.44 mW.