Jiwoong Park

Silicon-Integrated High-Density Electrocortical Interfaces

Recent demand and initiatives in brain research have driven significant interest toward developing chronically implantable neural interface systems with high spatiotemporal resolution and spatial coverage extending to the whole brain. Electroencephalography-based systems are noninvasive and cost efficient in monitoring neural activity across the brain, but suffer from fundamental limitations in spatiotemporal resolution. On the other hand, neural spike and local field potential (LFP) monitoring with penetrating electrodes offer higher resolution, but are highly invasive and inadequate for long-term use in humans due to unreliability in long-term data recording and risk for infection and inflammation. Alternatively, electrocorticography (ECoG) promises a minimally invasive, chronically implantable neural interface with resolution and spatial coverage capabilities that, with future technology scaling, may meet the needs of recently proposed brain initiatives. In this paper, we discuss the challenges and state-of-the-art technologies that are enabling next-generation fully implantable high-density ECoG interfaces, including details on electrodes, data acquisition front-ends, stimulation drivers, and circuits and antennas for wireless communications and power delivery. Along with state-of-the-art implantable ECoG interface systems, we introduce a modular ECoG system concept based on a fully encapsulated neural interfacing acquisition chip (ENIAC). Multiple ENIACs can be placed across the cortical surface, enabling dense coverage over wide area with high spatiotemporal resolution. The circuit and system level details of ENIAC are presented, along with measurement results.

Magnetic human body communication

This paper presents a new human body communication (HBC) technique that employs magnetic resonance for data transfer in wireless body-area networks (BANs). Unlike electric field HBC (eHBC) links, which do not necessarily travel well through many biological tissues, the proposed magnetic HBC (mHBC) link easily travels through tissue, offering significantly reduced path loss and, as a result, reduced transceiver power consumption. In this paper the proposed mHBC concept is validated via finite element method simulations and measurements. It is demonstrated that path loss across the body under various postures varies from 10-20 dB, which is significantly lower than alternative BAN techniques.

A 144MHz integrated resonant regulating rectifier with hybrid pulse modulation

This paper presents a CMOS fully-integrated resonant regulating rectifier (IR³) for inductive power telemetry in implantable devices. Employing PWM and PFM feedback, the IR³ achieves 1.87% of ΔV_DD/V_DD ratio despite a tenfold change in load with a 1nF decoupling capacitor. At 1V regulation of a 100μW load from a 144MHz RF input, the measured voltage conversion efficiency is greater than 92% at under 5.2mV_pp ripple and 54% power conversion efficiency. Implemented in 180nm SOI CMOS, the IR³ circuit occupies 0.078mm² active area.

A miniaturized ultrasonic power delivery system

A pair of 4.4mm diameter lead zirconium titanate (PZT) discs were employed for ultrasonic power delivery across biological tissue. The overall system, including the biological tissue and matching layers, was analyzed and modeled as a two-port network with an associated scattering matrix. The matrix coefficients were obtained experimentally in order to determine the maximum available gain (MAG) and optimal operating frequency of the system. The results were validated against finite element analysis simulations, and together they suggest that the miniaturized ultrasonic power delivery system has higher power transfer efficiency than a comparably-sized inductively coupled design at coupling distances greater than 9.5-15.5mm depending on the medium.

A fully integrated 144 MHz wireless-power-receiver-on-chip with an adaptive buck-boost regulating rectifier and low-loss H-Tree signal distribution

An adaptive buck-boost resonant regulating rectifier (B²R³) with an integrated on-chip coil and low-loss H-Tree power/signal distribution is presented for efficient and robust wireless power transfer (WPT) over a wide range of input and load conditions. The B²R³ integrated on a 9 mm² chip powers integrated neural interfacing circuits as a load, with a TX-load power conversion efficiency of 2.64 % at 10 mm distance, resulting in a WPT system efficiency FoM of 102.

Design of miniaturized wireless power receivers for mm-sized implants

Advances in free-floating miniature medical implants promise to offer greater effectiveness, safety, endurance, and robustness than todays prevailing medical implants. Wireless power transfer (WPT) is key to miniaturized implants by eliminating the need for bulky batteries. This paper reviews design strategies for WPT with mm-sized implants focusing on resonant electromagnetic and ultrasonic transmission. While ultrasonic WPT offers shorter wavelengths for sub-mm implants, electromagnetic WPT above 100 MHz offers superior power transfer and conversion efficiency owing to better impedance matching through inhomogeneous tissue. Electromagnetic WPT also allows for fully integrating the entire wireless power receiver system with an on-chip coil. Attaining high power transfer efficiency requires careful design of the integrated coil geometry for high quality factor as well as loop-free power and signal distribution routing to avoid eddy currents. Regulating rectifiers have improved power and voltage conversion efficiency by combining the two RF to DC conversion steps into a single process. Example designs of regulating rectifiers for fully integrated wireless power receivers are presented.

A 5.5 nW battery-powered wireless ion sensing system

This paper presents a battery-connected wireless ion sensing system comprising a Na+-selective electrode, a 406 pW Potentiometrie front end, a 780 pW reference-free asynchronous SAR ADC, a 2.4 GHz power oscillator-based transmitter that consumes 2.4 nW when transmitting 100 bps, a 485 pW quiescent power 1.8-to-0.6 V switched-capacitor DC-DC converter with 96.8% peak efficiency, and two temperature-stabilized relaxation oscillators that serve as clocks for the DC-DC converter, ADC, and power gating switches, consuming 140 pW each. The system was tested in-vitro, where measurement results exhibit a linear, near-Nertian response to varying Na+ concentration with a slope of 71 mV/Log10[Na+]. With all blocks operating, the entire system consumes only 5.5 nW.

A 200-kHz/6.78-MHz wireless power transmitter featuring concurrent dual-band operation

A wireless power transfer (WPT) transmitter is presented that can simultaneously power devices operating at 200 kHz and 6.78 MHz, enabling a true multi-standard WPT transmitter. To achieve this, the proposed design utilizes two coils, each optimized for efficient power transfer when operating alone. By placing the coils co-axially on a single charging stand, concurrent power transfer is possible. However, nominally doing this invokes significant eddy current losses, degrading WPT efficiency. To combat this, an eddy-current filter is included in the 200 kHz path. The proposed design is built into a 12.5 × 8.9 cm2 charging pad area, and at 25 mm separation, the system is able to concurrently power two smartphone-sized receiving devices at 25 mm separation. The system achieves a total power delivery of 9 W and 7.4 W with efficiencies of 78% and 70.6% at 6.78 MHz and 200 kHz, respectively.