SpaceMan: Wireless SoC for concurrent potentiometry and amperometry

This work describes the implementation of SPACEMan, a wireless electrochemical system with concurrent potentiometric and amperometric sensing that can be utilised for saliva, sweat or point of care diagnostics. This system is designed with the vision of simpler interfaces for biofluid analysis. With a complete system-on-chip including electrochemical sensing, power management and data transmission, conventional interfaces like wirebonds will no longer be required in post-processing steps. The proposed architecture consists of a sensor front-end with four electrodes for concurrent amperometric and potentiometric sensing. This front-end outputs square wave signals mixed together with varying frequencies dependent on the sensed input, with the output type switchable with a state machine. A power management system consisting of a low dropout regulator (LDO) band gap reference (BGR), and a rectifier bridge is utilised for supplying power from an inductive link at 433MHz. Sensor data is transmitted wirelessly to a base station using LSK (Load-Shift Keying). The sensor front-end consumes 18µW, which the power management system more than adequately provides. The core area of the electronics without the coil is a conservative size of 0.41mm 2

Ultra-thin chips with ISFET array for continuous monitoring of body fluids pH

This paper presents ISFET array based pH-sensing system-on-ultra-thin-chip (SoUTC) designed and fabricated in 350 nm CMOS technology. The SoUTC with the proposed current-mode active-pixel ISFET circuit array is desined to operate at 2V and consumes 6.28 W per-pixel. The presented SoUTC exhibits low sensitivity to process, voltage, temperature and strain-induced (PVTS) variations. The silicon area occupancy of each active-pixel is 44.9x33.5 m2 with an ion-sensing area of 576 m2. The design of presented ISFET device is analysed with finite element modeling in COMSOL Multiphysics using compact model parameters of MOSFET in 350 nm CMOS technology. Owing to thin (~30m) Si-substrate the presented SoUTC can conform to curvilinear surfaces, allowing intimate contact necessary for reliable data for monitoring of analytes in body fluids such as sweat. Further, it can operate either in a rolling shutter fashion or in a pseudo-random pixel selection mode allowing the simultaneous detection of pH from different skin regions. Finally, the circuits have been tested in aqueous Dulbeccos Modified Eagle Medium (DMEM) culture media with 5-9 pH values, which mimics cellular environments, to demonstrate their potential use for continuous monitoring of body-fluids pH

Wearable, low-power CMOS ISFETs and compensation circuits for on-body sweat analysis

Complementary metal-oxide-semiconductor (CMOS) technology has been a key driver behind the trend of reduced power consumption and increased integration of electronics in consumer devices and sensors. In the late 1990s, the integration of ion-sensitive field-effect transistors (ISFETs) into unmodified CMOS helped to create advancements in lab-on-chip technology through highly parallelised and low-cost designs. Using CMOS techniques to reduce power and size in chemical sensing applications has already aided the realisation of portable, battery-powered analysis platforms, however the possibility of integrating these sensors into wearable devices has until recently remained unexplored. This thesis investigates the use of CMOS ISFETs as wearable electrochemical sensors, specifically for on-body sweat analysis. The investigation begins by evaluating the ISFET sensor for wearable applications, identifying the key advantages and challenges that arise in this pursuit. A key requirement for wearable devices is a low power consumption, to enable a suitable operational life and small form factor. From this perspective, ISFETs are investigated for low power operation, to determine the limitations when trying to push down the consumption of individual sensors. Batteryless ISFET operation is explored through the design and implementation of a 0.35 \si{\micro\metre} CMOS ISFET sensing array, operating in weak-inversion and consuming 6 \si{\micro\watt}. Using this application-specific integrated circuit (ASIC), the first ISFET array powered by body heat is demonstrated and the feasibility of using near-field communication (NFC) for wireless powering and data transfer is shown. The thesis also presents circuits and systems for combatting three key non-ideal effects experienced by CMOS ISFETs, namely temperature variation, threshold voltage offset and drift. An improvement in temperature sensitivity by a factor of three compared to an uncompensated design is shown through measured results, while adding less than 70 \si{\nano\watt} to the design. A method of automatically biasing the sensors is presented and an approach to using spatial separation of sensors in arrays in applications with flowing fluids is proposed for distinguishing between signal and sensor drift. A wearable device using the ISFET-based system is designed and tested with both artificial and natural sweat, identifying the remaining challenges that exist with both the sensors themselves and accompanying components such as microfluidics and reference electrode. A new ASIC is designed based on the discoveries of this work and aimed at detecting multiple analytes on a single chip. %Removed In the latter half of the thesis, Finally, the future directions of wearable electrochemical sensors is discussed with a look towards embedded machine learning to aid the interpretation of complex fluid with time-domain sensor arrays. The contributions of this thesis aim to form a foundation for the use of ISFETs in wearable devices to enable non-invasive physiological monitoring.Open Acces

A Flexible, Highly Integrated, Low Power pH Readout

Medical devices are widely employed in everyday life as wearable and implantable technologies make more and more technological breakthroughs. Implantable biosensors can be implanted into the human body for monitoring of relevant physiological parameters, such as pH value, glucose, lactate, CO2 [carbon dioxide], etc. For these applications the implantable unit needs a whole functional set of blocks such as micro- or nano-sensors, sensor signal processing and data generation units, wireless data transmitters etc., which require a well-designed implantable unit.Microelectronics technology with biosensors has caused more and more interest from both academic and industrial areas. With the advancement of microelectronics and microfabrication, it makes possible to fabricate a complete solution on an integrated chip with miniaturized size and low power consumption.This work presents a monolithic pH measurement system with power conditioning system for supply power derived from harvested energy. The proposed system includes a low-power, high linearity pH readout circuits with wide pH values (0-14) and a power conditioning unit based on low drop-out (LDO) voltage regulator. The readout circuit provides square-wave output with frequency being highly linear corresponding to the input pH values. To overcome the process variations, a simple calibration method is employed in the design which makes the output frequency stay constant over process, supply voltage and temperature variations. The prototype circuit is designed and fabricated in a standard 0.13-μm [micro-meter] CMOS process and shows good linearity to cover the entire pH value range from 0-14 while the voltage regulator provides a stable supply voltage for the system

Integrated circuit & system design for concurrent amperometric and potentiometric wireless electrochemical sensing

Complementary Metal-Oxide-Semiconductor (CMOS) biosensor platforms have steadily grown in healthcare and commerial applications. This technology has shown potential in the field of commercial wearable technology, where CMOS sensors aid the development of miniaturised sensors for an improved cost of production and response time. The possibility of utilising wireless power and data transmission techniques for CMOS also allows for the monolithic integration of the communication, power and sensing onto a single chip, which greatly simplifies the post-processing and improves the efficiency of data collection. The ability to concurrently utilise potentiometry and amperometry as an electrochemical technique is explored in this thesis. Potentiometry and amperometry are two of the most common transduction mechanisms for electrochemistry, with their own advantages and disadvantages. Concurrently applying both techniques will allow for real-time calibration of background pH and for improved accuracy of readings. To date, developing circuits for concurrently sensing potentiometry and amperometry has not been explored in the literature. This thesis investigates the possibility of utilising CMOS sensors for wireless potentiometric and amperometric electrochemical sensing. To start with, a review of potentiometry and amperometry is evaluated to understand the key factors behind their operation. A new configuration is proposed whereby the reference electrode for both electrochemistry techniques are shared. This configuration is then compared to both the original configurations to determine any differences in the sensing accuracy through a novel experiment that utilises hydrogen peroxide as a measurement analyte. The feasibility of the configuration with the shared reference electrode is proven and utilised as the basis of the electrochemical configuration for the front end circuits. A unique front-end circuit named DAPPER is developed for the shared reference electrode topology. A review of existing architectures for potentiometry and amperometry is evaluated, with a specific focus on low power consumption for wireless applications. In addition, both the electrochemical sensing outputs are mixed into a single output data channel for use with a near-field communication (NFC). This mixing technique is also further analysed in this thesis to understand the errors arising due to various factors. The system is fabricated on TSMC 180nm technology and consumes 28µW. It measures a linear input current range from 250pA - 0.1µW, and an input voltage range of 0.4V - 1V. This circuit is tested and verified for both electrical and electrochemical tests to showcase its feasibility for concurrent measurements. This thesis then provides the integration of wireless blocks into the system for wireless powering and data transmission. This is done through the design of a circuit named SPACEMAN that consists of the concurrent sensing front-end, wireless power blocks, data transmission, as well as a state machine that allows for the circuit to switch between modes: potentiometry only, amperometry only, concurrent sensing and none. The states are switched through re-booting the circuit. The core size of the electronics is 0.41mm² without the coil. The circuit’s wireless powering and data transmission is tested and verified through the use of an external transmitter and a connected printed circuit board (PCB) coil. Finally, the future direction for ongoing work to proceed towards a fully monolithic electrochemical technique is discussed through the next development of a fully integrated coil-on-CMOS system, on-chip electrodes with the electroplating and microfludics, the development of an external transmitter for powering the device and a test platform. The contributions of this thesis aim to formulate a use for wireless electrochemical sensors capable of concurrent measurements for use in wearable devices.Open Acces

A thermally powered ISFET array for on-body pH measurement

Recent advances in electronics and electrochemical sensors have led to an emerging class of next generation wearables, detecting analytes in biofluids such as perspiration. Most of these devices utilize ion-selective electrodes (ISEs) as a detection method; however, ion-sensitive field-effect transistors (ISFETs) offer a solution with improved integration and a low power consumption. This work presents a wearable, thermoelectrically powered system composed of an application-specific integrated circuit (ASIC), two commercial power management integrated circuits and a network of commercial thermoelectric generators (TEGs). The ASIC is fabricated in 0.35 μm CMOS and contains an ISFET array designed to read pH as a current, a processing module which averages the signal to reduce noise and encodes it into a frequency, and a transmitter. The output frequency has a measured sensitivity of 6 to 8 kHz/pH for a pH range of 7-5. It is shown that the sensing array and processing module has a power consumption 6 μW and, therefore, can be entirely powered by body heat using a TEG. Array averaging is shown to reduce noise at these low power levels to 104 μV (input referred integrated noise), reducing the minimum detectable limit of the ASIC to 0.008 pH units. The work forms the foundation and proves the feasibility of battery-less, on-body electrochemical for perspiration analysis in sports science and healthcare applications