Dongjin Seo

Model validation of untethered, ultrasonic neural dust motes for cortical recording

A major hurdle in brain-machine interfaces (BMI) is the lack of an implantable neural interface system that remains viable for a substantial fraction of the users lifetime. Recently, sub-mm implantable, wireless electromagnetic (EM) neural interfaces have been demonstrated in an effort to extend system longevity. However, EM systems do not scale down in size well due to the severe inefficiency of coupling radio-waves at those scales within tissue. This paper explores fundamental system design trade-offs as well as size, power, and bandwidth scaling limits of neural recording systems built from low-power electronics coupled with ultrasonic power delivery and backscatter communication. Such systems will require two fundamental technology innovations: (1) 10-100 μm scale, free-floating, independent sensor nodes, or neural dust, that detect and report local extracellular electrophysiological data via ultrasonic backscattering and (2) a sub-cranial ultrasonic interrogator that establishes power and communication links with the neural dust. We provide experimental verification that the predicted scaling effects follow theory; (127 μm)(3) neural dust motes immersed in water 3 cm from the interrogator couple with 0.002064% power transfer efficiency and 0.04246 ppm backscatter, resulting in a maximum received power of ∼0.5 μW with ∼1 nW of change in backscatter power with neural activity. The high efficiency of ultrasonic transmission can enable the scaling of the sensing nodes down to 10s of micrometer. We conclude with a brief discussion of the application of neural dust for both central and peripheral nervous system recordings, and perspectives on future research directions.

A 50mW-TX 65mW-RX 60GHz 4-element phased-array transceiver with integrated antennas in 65nm CMOS

The 60GHz band has gained great interest as an enabler for multi-Gb/s wireless links. Recent efforts [1-4] have focused on reducing transceiver power to drive adoption of 60GHz in mobile devices. To further accelerate this adoption, the cost of current mm-Wave solutions should also be reduced. Especially for short range designs (<;1m), overall cost may be dominated by packaging and testing. This paper therefore presents a low-power 60GHz CMOS 4-element phased-array QPSK transceiver with integrated slot-loop antennas. Utilizing such antennas as well as circuit stacking techniques, the transceiver achieves 10.4Gb/s with a range of >40cm in all directions while consuming only 115mW (TX+RX).

Ultrasonic beamforming system for interrogating multiple implantable sensors.

In this paper, we present an ultrasonic beamforming system capable of interrogating individual implantable sensors via backscatter in a distributed, ultrasound-based recording platform known as Neural Dust [1]. A custom ASIC drives a 7 × 2 PZT transducer array with 3 cycles of 32V square wave with a specific programmable time delay to focus the beam at the 800mm neural dust mote placed 50mm away. The measured acoustic-to-electrical conversion efficiency of the receive mote in water is 0.12% and the overall system delivers 26.3% of the power from the 1.8V power supply to the transducer drive output, consumes 0.75μJ in each transmit phase, and has a 0.5% change in the backscatter per volt applied to the input of the backscatter circuit. Further miniaturization of both the transmit array and the receive mote can pave the way for a wearable, chronic sensing and neuromodulation system.

Miniaturizing Ultrasonic System for Portable Health Care and Fitness

We present a miniaturized portable ultrasonic imager that uses a custom ASIC and a piezoelectric transducer array to transmit and capture 2-D sonographs. The ASIC, fabricated in 0.18 μm 32 V CMOS process, contains 7 identical channels, each with high-voltage level-shifters, high-voltage DC-DC converters, digital TX beamformer, and RX front-end. The chip is powered by a single 1.8 V supply and generates 5 V and 32 V internally using on-chip charge pumps with an efficiency of 33% to provide 32 V pulses for driving a bulk piezoelectric transducer array. The assembled prototype can operate up to 40 MHz, with beamformer delay resolution of 5 ns, and has a measured sensitivity of 225 nV/Pa , minimum detectable signal of 622 Pa assuming 12 dB SNR ( 4σ larger than the noise level), and data acquisition time of 21.3 ms. The system can image human tissue as deep as 5 cm while consuming less than 16.5 μJ per pulse-echo measurement. The high energy efficiency of the imager can enable a number of consumer applications.

Beamforming approaches for untethered, ultrasonic neural dust motes for cortical recording: A simulation study

In this paper, we examine the use of beamforming techniques to interrogate a multitude of neural implants in a distributed, ultrasound-based intra-cortical recording platform known as Neural Dust [1]. We propose a general framework to analyze system design tradeoffs in the ultrasonic beamformer that extracts neural signals from modulated ultrasound waves that are backscattered by free-floating neural dust (ND) motes. Simulations indicate that high-resolution linearly-constrained minimum variance beamforming sufficiently suppresses interference from unselected ND motes and can be incorporated into the ND-based cortical recording system.

Blind parallel interrogation of ultrasonic neural dust motes based on canonical polyadic decomposition: A simulation study

Neural dust (ND) is a wireless ultrasonic backscatter system for communicating with implanted sensor devices, re-ferred to as ND motes (NDMs). Due to its scalability, ND could allow to chronically record electro-physiological signals in the brain cortex at a micro-scale pitch. The free-floating NDMs are read out by an array of ultrasonic (US) transducers through passive backscattering, by sequentially steering a US beam to the target NDM. In order to perform such beam steering, the NDM positions or the channels between the NDMs and the US transducers have to be estimated, which is a non-trivial task. Furthermore, such a sequential beam steering approach is too slow to sample a dense ND grid with a sufficiently high sampling rate. In this paper, we propose a new ND interrogation scheme which is fast enough to completely sample the entire ND grid, and which does not need any information on the NDM positions or the per-NDM channel characteristics. For each sample time, the US transducers transmit only a few grid-wide US beams to the entire ND grid, in which case the reflected beams will consist of mixtures of multiple NDM signals. We arrange the demodulated backscattered signals in a 3-way tensor, and then use a canonical polyadic decomposition (CPD) to blindly estimate the neural signals from each underlying NDM. Based on a validated simulation model, we demonstrate that this new CPD-based interrogation scheme allows to reconstruct the neural signals from the entire ND grid with a sufficiently high accuracy, even at relatively low SNR regimes.

Miniature ultrasonic imager for personal fitness tracking

A highly integrated low-power ultrasonic imager tracks changes of personal fitness by measuring the thickness of subcutaneous muscle and fat layers. The system combines a transducer fabricated from off-the-shelf PZT with a custom integrated circuit that implements all interface functions including high-voltage generation, beam forming, and and a low noise receiver for sensing and amplifying received echo. A novel transmit driver architecture is key to reducing the power dissipation of the high voltage circuits and enable battery operation. The system is capable of recording up to a depth of 6 cm in human tissue with 2mm depth resolution. Beam forming is used to improve signal strength and image quality in situations where the sensor and target are not parallel.