Matthew Douthwaite

Bio: Matthew Douthwaite is an academic researcher from Imperial College London. The author has contributed to research in topics: ISFET & CMOS. The author has an hindex of 3, co-authored 10 publications receiving 47 citations.

A Thermally Powered ISFET Array for On-Body pH Measurement

Abstract: 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 $\mu$ m 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 $\mu$ W 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 $\mu$ V (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.

A Time-Domain Current-Mode MAC Engine for Analogue Neural Networks in Flexible Electronics

Abstract: Flexible electronics is becoming more prevalent in a wide range of applications, particularly wearable biomedical devices. These devices would greatly benefit from in-built intelligence allowing them to process data and identify features, in order to reduce transmission and power requirements. In this work, we present a novel time-domain multiply-accumulate (MAC) engine architecture that can act as the basic block of an artificial analogue neural network. The design does not require analogue voltage buffers, making them easier to realise in flexible technologies and consumes less power than conventional methods. The research could be used in future to construct a low power classifier for a low cost, flexible wearable biomedical sensor.

CMOS ISFET Arrays for Integrated Electrochemical Sensing and Imaging Applications: A Tutorial

Abstract: The ion-sensitive field-effect transistor (ISFET) is a type of electrochemical sensor with a wide range of applications. They offer advantages of being compatible with standard CMOS technology, a miniaturised form-factor, robustness, scalability and low power requirements to name a few. There are now many architectures and design strategies to construct ISFET-based systems in CMOS, and the appropriate choice of these depends heavily on the specifications of the intended application. This tutorial aims to give a designer the knowledge needed to make the best decisions for the required specifications by providing a background in theory and an overview of design trade-offs and existing approaches. Example designs which maximise performance in particular applications are discussed and practical considerations for simulation, layout and implementation are presented.

A Digital ISFET Sensor with In-Pixel ADC

Abstract: This paper presents a novel ISFET architecture with in-pixel ADC for large-scale integration and on-chip computational capabilities. Each pixel is composed by a comparator connected to a set of memory elements, storing in-pixel each conversion. Using this sensing scheme, the entire frame of ISFET pixels is acquired and stored in one single ADC cycle, eliminating the need for global or column-level ADCs and making this architecture scalable to larger arrays without instrumentation overhead. To enable compensation of ISFET non-idealities such as trapped charge, a novel gate-bootstrapping mechanism is introduced, enhancing the ADC dynamic range without additional circuitry and accommodating a trapped charge compensation range of 4.45 V. Fabricated in standard 180nm CMOS technology, the system is composed by a 16x16 ISFET array, a global DAC and a Digital Control Unit that enables operation and off-chip communication. The entire frame is acquired in 51.2 μs with a pixel power consumption of 10.15 μW, while storing the result in-pixel and eliminating the need for off-chip memories.

A CMOS ISFET array for wearable thermoelectrically powered perspiration analysis

Abstract: Recent advances in micro-electronics and electro-chemical sensors has led to an emerging class of next generation wearables, detecting analytes in bio-fluids such as perspiration. Most of these devices utilise Ion Selective Electrodes (ISEs) as a detection method, however Ion Sensitive Field Effect Transistors (ISFETs) provide a solution with better integration and low-power consumption. This work presents a system consisting of an ISFET array designed to read the pH of perspiration as a current, average the signal to reduce noise and modulate the frequency of a transmittable pulse. The input referred noise of the array is shown to be reduced by a factor of 3, compared to a single sensor of the same overall power consumption. The sensing and processing system consumes less than 10μW making it feasible to be supplied by a thermoelectric generator.

A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications

Abstract: In this article, an overview of recent advances in the field of battery-less near-field communication (NFC) sensors is provided, along with a brief comparison of other short-range radio-frequency identification (RFID) technologies. After reviewing power transfer using NFC, recommendations are made for the practical design of NFC-based tags and NFC readers. A list of commercial NFC integrated circuits with energy-harvesting capabilities is also provided. Finally, a survey of the state of the art in NFC-based sensors is presented, which demonstrates that a wide range of sensors (both chemical and physical) can be used with this technology. Particular interest arose in wearable sensors and cold-chain traceability applications. The availability of low-cost devices and the incorporation of NFC readers into most current mobile phones make NFC technology key to the development of green Internet of Things (IoT) applications.

Sweat Biomarker Sensor Incorporating Picowatt, Three-Dimensional Extended Metal Gate Ion Sensitive Field Effect Transistors

Abstract: Ion sensitive field effect transistors (ISFETs) form a very attractive solution for wearable sensors due to their capacity for ultra-miniaturization, low power operation, and very high sensitivity, supported by complementary metal oxide semiconductor (CMOS) integration. This paper reports for the first time, a multianalyte sensing platform that incorporates high performance, high yield, high robustness, three-dimensional-extended-metal-gate ISFETs (3D-EMG-ISFETs) realized by the postprocessing of a conventional 0.18 μm CMOS technology node. The detection of four analytes (pH, Na+, K+, and Ca2+) is reported with excellent sensitivities (58 mV/pH, -57 mV/dec(Na+), -48 mV/dec(K+), and -26 mV/dec(Ca2+)) close to the Nernstian limit, and high selectivity, achieved by the use of highly selective ion selective membranes based on postprocessing integration steps aimed at eliminating any significant sensor hysteresis and parasitics. We are reporting simultaneous time-dependent recording of multiple analytes, with high selectivities. In vitro real sweat tests are carried out to prove the validity of our sensors. The reported sensors have the lowest reported power consumption, being capable of operation down to 2 pW/sensor. Due to the ultralow power consumption of our ISFETs, we achieve and report a final four-analyte passive system demonstrator including the readout interface and the remote powering of the ISFET sensors, all powered by an radio frequency (RF) signal.