Gurkan Yilmaz

An efficient wireless power link for implanted biomedical devices via resonant inductive coupling

This study presents a high efficiency inductive power link aiming for the implanted biomedical devices. Improving the efficiency of the inductive link is critical, since the overall efficiency of a remote powering system is dominated by the inductive link. Therefore, the proposed system utilizes the resonance phenomenon which enables maximum energy transfer between objects which are at resonance. In this study, the resonance is created between 4 spiral coils which are implemented on the internal and external units; each formed by two concentric coils. The system operates with 72% power efficiency at 10-12 MHz band at 10 mm distance which is a reasonable distance for powering implanted devices. Moreover, the characterization of the resonant inductive link with respect to distance and frequency is performed.

Wireless data and power transmission aiming intracranial epilepsy monitoring

This study presents a wireless power and data transmission system to overcome the problems in intracranial epilepsy monitoring associated with transcutaneous wires. Firstly, a wireless power transfer link based on inductive coupling is implemented and a power management unit for the implant is designed. 4-coil resonant inductive link scheme is exploited since it exhibits a high efficiency and optimal load flexibility. Power management unit consists of an active rectifier, a low drop-out voltage regulator which is biased internally with a supply independent current source; all implemented as integrated circuit. Wireless power link provides 10 mW under 1.8 V dc to the load, more specifically the electrodes and read-out electronics. Wireless data communication is realized using the same frequency, 8.4 MHz, as the power link. Load shift keying is performed for uplink (from implant to external) communication by switching an integrated modulator which, in fact, detunes the resonance. Modulated signal is recovered on the external device by means of an integrated self-referenced ASK demodulator. Data rate is adapted for a fast ripple ( < 500 Hz) detection system which requires 300 kbps communication. The measurements show that the system works at 36% power transfer efficiency without communication link and the efficiency drops to 33% with 300 kbps uplink data transfer. Finally, in-vitro tests that emulate the real operation scenario are performed thanks to the two-polymer packaging and almost the same power transfer efficiency is achieved under same operation conditions.

An implantable system for intracranial neural recording applications

This article presents integration of a wireless power transmission and bidirectional data communication system for implantable neural recording applications. Wireless power transfer is realized by means of magnetic coupling at 10 MHz achieving 36% efficiency. Downlink communication at 1 Mbps is realized on the same frequency as the wireless power transmission by changing the amplitude of the source signal. Uplink communication at 1.8 Mbps is performed at MICS band by means of an integrated transmitter and a discrete receiver. Design and implementation of the entire system has been explained and practical integration issues have been discussed.

Wireless Power Transfer

This chapter introduces the fundamentals of wireless power transfer with an emphasis on implant powering . Firstly, an overview of implant powering solutions is introduced, and then, the decision of using wireless power transfer is justified. Next, among possible wireless power transfer methods, it is explained that why magnetic coupling befits the best for the target application and its specifications. In this book, a recent inductive link topology which employs 4 coils at resonance is employed in order to realize magnetic coupling . This approach has brought two main advantages: higher power transfer efficiency and less dependence of power transfer efficiency on load impedance. Detailed explanation of the 4-coil resonant inductive link is followed by the design of electronic circuits utilized to create a reliable power supply in the implant. This unit consists of an active half-wave rectifier and a low drop-out voltage regulator .

Wireless communication and power transfer system for intracranial neural recording applications

This study presents a system which provides a wireless solution to implant powering and bidirectional communication between a neural implant and an external base station. Wireless power transfer (WPT) is realized by using a 4-coil resonant inductive link at 8.6 MHz at a separation distance of 10 mm, which is the average human scalp thickness. Downlink communication at 10 kbps is performed on the same frequency as the WPT by ASK modulation. Uplink communication at 1.8 Mbps which carries digitized neural data is realized by using an integrated transmitter and a discrete receiver in 426-432 MHz band. The transmitter is composed of a cross-coupled symmetrical LC voltage controlled oscillator which is brought to resonance with a loop antenna instead of an integrated inductor. The loop antenna serves also for radiation.

System-Level Experiments and Results

This chapter presents the system-level integration of the proposed wireless power transfer and data communication system for the intracranial neural recording system. In the previous chapters, design and implementation of all the subblocks have been explained in detail. This chapter mainly focuses on the issues encountered during the integration and corresponding solutions while giving extensive experimental results. Experimental results include various combinations of functional blocks, namely wireless power transfer link, uplink communication, and data communication. Furthermore, it is worth nothing that the system has been experimentally tested in air, in vitro in a solution that mimics the cerebrospinal fluid (CSF), and in vivo on the cortex of a rat. More explicitly, the system has been completely characterized in air and in vitro ; however, only the power management circuits have been employed for initial in vivo experiments.

Wireless Data Communication

Risk factors associated with the transcutaneous wires employed for data transmission can be reduced by means of wireless data communication between external base station and the implant unit. It is worth noting that bidirectional communication is indispensable for the majority of the neural monitoring applications. For the sake of definition used throughout this book, transferring data from the external base station to the implant is called downlink, while data transmission in the reverse direction is called uplink communication. Downlink communication is commonly utilized to reprogram the implant chip. Possible benefits of using a configurable implant chip can be enlisted as choosing the recording channels, modification of sampling parameters, and parameters associated with data compression or signal processing. Uplink communication, on the other hand, carries the processed information acquired by the MEAs. Moreover, it may contain additional information related to power feedback to ensure maximum power transfer efficiency all the time. Note that the number of recording channels directly affects the power demand of the implant; therefore, the transmitted power should be arranged accordingly to minimize power dissipation in the implant. Possible schemes to realize bidirectional communication is to use a half-duplex communication on a single channel with the help of a multiplexing method or to realize full-duplex communication. Considering the current requirements of the neural implant applications, half-duplex communication which allows communication in only one direction instantaneously is sufficient. However, this does not imply that power and data transfer should be performed at a single frequency. Depending on the data rate requirements, for instance in the case of multi-unit activity recording, a second frequency could be utilized to increase the data rate and allow full-duplex communication.

Packaging of the Implant

Any kind of passive or active device which is to be implanted inside the body must satisfy certain biocompatibility and biosafety requirements and standards. It is worth noting that not harming the patient (primum non nocere) is one of the fundamentals of the medicine and medical sciences. These requirements have been established in years in order to prevent inflicting any damage to the human body. More explicitly, the implant may contain a toxic material which causes a direct damage to the tissues, or the body may refuse the implant even if the implant itself is not toxic, which causes an indirect damage. This refusal is called as foreign body reaction [1], and as a result, the immune system attacks the implant which results in an inflammation or swelling in return. In such cases, the implant should be removed immediately. Therefore, a set of preliminary experiments have to be conducted in order to get an approval. This chapter introduces the regulations and how these regulations are addressed within the frame of this work including a polymer-based packaging, and its modeling in terms of hermetical sealing is presented. Proposed packaging has been tested only for in vitro conditions and requires a detailed characterization to fulfill the requirements of biocompatibility before in vivo experiments.

20–300 MHz frequency generator with −70 dBc reference spur for low jitter serial applications

This paper presents a frequency synthesizer based on a phase locked loop (PLL) targeting low jitter applications. The frequency generator covers a range wider than a decade, more explicitly from 20 to 300 MHz. Thanks to a 4-stage ring oscillator, it can provide quadrature signals and exhibit a −117 dBc/Hz phase noise at an offset of 1 MHz from the carrier. Settling time simulations on MATLAB and Cadence Spectre match with measurement results, which yield maximum 1.3 μs (or 26 reference cycles) for N to N+1 switching, while N denotes the integer division ratio. PLL core draws a current less than 5 mA when it is supplied from a 1.8 V dc supply. PLL is driven by a 20 MHz clock source having 0.67 ps rms jitter. At 200 MHz, the circuit provides a differential output with 2.05 ps rms jitter (equivalent to 0.41 mUI) within an integration window from 10 Hz to 40 MHz and creates a reference spur lower than −70 dBc.

Optimization of the data rate of an OOK CMOS medical transmitter based on LC oscillators

This paper presents techniques for increasing communication data rates for OOK modulated signals in the case the carrier frequency is generated by a cross-coupled pair LC oscillator. The proposed circuitry completely turns off the transmitter during the transmission of “0” bit and oscillation occurs only when the data bit is “1” to reduce power consumption. The data bit controls the steady state bias current and the operation of the oscillator. The communication data rate is limited by the turn-on and turn-off time of the LC oscillator. In order to speed up the turn-on time of the oscillator, in addition to bias current, an extra current source is used during the build-up of the oscillator. The decay time of the oscillator when the data is switched from “1” to “0” is accelerated by shortening the inductor connections. The concept is demonstrated through the design of an LC VCO in 0.18 μm CMOS technology. The LC oscillator is designed to oscillate in MedRadio band at 416 MHz frequency. Simulation results show that the proposed architecture shortens the turn-on time from 86.5 ns to 24 ns and the turn-off time from 101 ns to 5.93 ns. With the additional techniques, the maximum achievable communication data rate is increased by more than 6 times.