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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.

The ion-sensitive field effect transistors (ISFETs), proposed little over 50 years ago, today make the most promising devices for lab-on-a-chip, implantable, and point-of-care (POC) diagnostics. Their compatibility with CMOS (Complementary Metal Oxide Semiconductor) technology and the low cost through mass production have been the driving factors so far. Nowadays, they are also being developed in flexible form factors for new applications such as wearables and to improve the effective usage in existing applications such as implantable systems. In this regard, the CMOS ultra-thin chip (UTC) technology and the bonding by printing are the noteworthy advances. This paper comprehensively reviews such new developments in the CMOS-compatible ISFETs, along with their theory, readout circuitries, circuit-based techniques for compensation of the ISFET's instabilities, such as the offset, flicker noise, and drift. The sensing mechanisms and the properties of interface between the electrolyte under test and the metal-oxide based ion-sensitive electrodes have been discussed along with a brief overview of the metal-oxide based pH sensors. An overview of the reported mechanically flexible pH sensors, including ISFETs, is provided and the history of ISFET applications are also covered. Finally, established models that can be used to design flexible circuits are presented, and possible opportunities to use circuit techniques to compensate for mechanical deformation are discussed.

Flexible multifunctional sensors on skin or wearables are considered highly suitable for next-generation noninvasive health care devices. In this regard, the field-effect transistor (FET)-based chemical sensors such as ion-sensitive FETs (ISFETs) are attractive as, with the ultrathin complementary metal oxide semiconductor technology, they can enable a flexible or bendable sensor system. However, the bending-related stress or strain could change the output of devices on ultrathin chips (UTCs), and this has been argued as a major challenge hindering the advancement and use of this technology in applications such as wearables. This may not be always true, as with drift-free ISFETs, we show that bending could also enhance the performance of UTCs. Through fine control of bending radius in the micrometer scale, the mechanically flexible RuO2-based ISFETs on UTCs (44.76 μm thickness) are shown to reproducibly enhance the performance even after 1000 bending cycles. The 1.3 orders of magnitude improved stability (the drift rate changed from −557 nA/min to −28 ± 0.16 nA/min) is observed over a time period of 417.3 s (∼7 min) at fixed biasing and temperature conditions and under different pH conditions. Finally, a compact macromodel is developed to capture the bending-induced improvements in flexible ISFETs. The performance enhancement by controlled bending of devices could generally benefit the rapidly growing field of flexible electronics.

With rising hazardous organic vapours in the environment, the detection of volatile organic vapour compounds (VOCs) is becoming important. To this end, this paper presents a conductive droplet-based disposable sensor. Unlike conventional sensors, the droplet system is easily replaceable and is capable of detecting multiple vapours based on surface tension gradient. The chemiresistive sensor presented here is fabricated on $2.5~\mu \text{m}$ thick ultra-flexible graphene oxide-chitosan (GOC) with Pt electrodes having $60~\mu \text{m}$ gap. The electrostatic interaction and strong hydrogen bonds between GO and polysaccharide groups in chitosan provide tunable hydrophobicity and stability to the droplet. With a conductive droplet of $\sim 10~\mu \text{L}$ of aq. NaCl as an active sensing material dispensed between the Pt electrodes, it was observed that the droplet showed 14-21% change in resistance in the presence of VOCs. A read-out circuit was also designed to get the data from the droplet sensor. The response time for the presented sensor (3-4 seconds) is significantly better than its solid-state sensor counterparts. With attractive features such as disposability, affordability and fast response the presented sensor will find applications in industrial environments, laboratories, health centres, and biomedical devices.

This paper presents a flexible ultraviolet (UV)\ud photodetector (PD) system based on zinc oxide (ZnO) nanowires\ud (NWs) for wearable UV dosimetry. High-crystal quality ZnO NWs\ud have been synthesized by chemical vapour transport (CVT)\ud technique on c-plane sapphire substrates, and thereafter,\ud transferred and aligned at pre-defined locations on a flexible\ud substrate using dielectrophoresis (DEP). The accurate control\ud over DEP parameters permitted the fabrication of large-area\ud (wafer scale) arrays of ZnO NWs based UV PDs. Resulting PDs\ud showed photocurrent-to-dark current ratios above 103%, fast\ud response times (

This paper presents printed thick film Ag|AgCl|KCl reference electrodes (RE) for electrochemical sensors. The screen-printed REs with 10- $\mu \text{m}$ thick glass-KCl salt matrix layer exhibit a stable potential of 5 mV. Cyclic voltammetric analysis shows that the anodic and cathodic peak current of the RE increases with the scan rate in the range of 25–150 mVs−1. The analytical performance of the REs shows a stable open circuit potential for the NaCl concentrations in the range of 30–100 mM. Testing the presented REs for electrochemical pH sensor application (with RuO2-based sensitive electrode) the sensitivity of 55 mV/pH was noted in the pH range of 4.5–9. Evaluating the effect of temperature on the performance of REs, a potential variation of −3.8 mV/ °C was observed. Finally, a LabVIEW interface was developed to store, analyze, and calculate the sensitivity of the sensor under different temperature conditions. The LabVIEW interface can also be used to calculate the pH-value/temperature of unknown solutions under known temperature/pH conditions.