There is growing attention and rapid development on flexible electronic devices with electronic materials and sensing technology innovations. In particular, strain sensors with high elasticity and stretchability are needed for several potential applications including human entertainment technology, human–machine interface, personal healthcare, and sports performance monitoring, etc. This article presents recent advancements in the development of polydimethylsiloxane (PDMS)-based flexible resistive strain sensors for wearable applications. First of all, the article shows that PDMS-based stretchable resistive strain sensors are successfully fabricated by different methods, such as the filtration method, printing technology, micromolding method, coating techniques, and liquid phase mixing. Next, strain sensing performances including stretchability, gauge factor, linearity, and durability are comprehensively demonstrated and compared. Finally, potential applications of PDMS-based flexible resistive strain sensors are also discussed. This review indicates that the era of wearable intelligent electronic systems has arrived.

Non-invasive fetal electrocardiograms (FECGs) are an alternative method to standard means of fetal monitoring which permit long-term continual monitoring. However, in abdominal recording, the FECG amplitude is weak in the temporal domain and overlaps with the maternal electrocardiogram (MECG) in the spectral domain. Research in the area of non-invasive separations of FECG from abdominal electrocardiograms (AECGs) is in its infancy and several studies are currently focusing on this area. An adaptive noise canceller (ANC) is commonly used for cancelling interference in cases where the reference signal only correlates with an interference signal, and not with a signal of interest. However, results from some existing studies suggest that propagation of electrocardiogram (ECG) signals from the maternal heart to the abdomen is nonlinear, hence the adaptive filter approach may fail if the thoracic and abdominal MECG lack strict waveform similarity. In this study, singular value decomposition (SVD) and smooth window (SW) techniques are combined to build a reference signal in an ANC. This is to avoid the limitation that thoracic MECGs recorded separately must be similar to abdominal MECGs in waveform. Validation of the proposed method with r01 and r07 signals from a public dataset, and a self-recorded private dataset showed that the proposed method achieved F1 scores of 99.61%, 99.28% and 98.58%, respectively for the detection of fetal QRS. Compared with four other single-channel methods, the proposed method also achieved higher accuracy values of 99.22%, 98.57% and 97.21%, respectively. The findings from this study suggest that the proposed method could potentially aid accurate extraction of FECG from MECG recordings in both clinical and commercial applications.

This paper proposes a compact, high-linearity, and reconfigurable continuous-time filter with a wide frequency-tuning capability for biopotential conditioning. It uses an active filter topology and a new operational-transconductance-amplifier (OTA)-based current-steering (CS) integrator. Consequently, a large time constant $\tau$ , good linearity, and linear bandwidth tuning could be achieved in the presented filter with a small silicon area. The proposed filter has a reconfigurable structure that can be operated as a low-pass filter (LPF) or a notch filter (NF) for different purposes. Based on the novel topology, the filter can be readily implemented monolithically and a prototype circuit was fabricated in the 0.18 μm standard complementary-metal–oxide–semiconductor (CMOS) process. It occupied a small area of 0.068 mm² and consumed 25 μW from a 1.8 V supply. Measurement results show that the cutoff frequency of the LPF could be linearly tuned from 0.05 Hz to 300 Hz and the total-harmonic-distortion (THD) was less than −76 dB for a 2 Hz, 200 mVpp sine input. The input-referred noises were 5.5 μVrms and 6.4 μVrms for the LPF and NF, respectively. A comparison with conventional designs reveals that the proposed design achieved the lowest harmonic distortion and smallest on-chip capacitor. Moreover, its ultra-low cutoff frequency and relatively linear frequency tuning capability make it an attractive solution as an analog front-end for biopotential acquisitions.

This paper describes a low-offset analogue front-end (AFE) integrated circuit (IC) for multi-channel physiological signal acquisitions. The mixed signal IC consists of low-offset gain programmable instrumentation amplifiers (GPIAs), high sensitive current-to-voltage converters (I-V converters), reference and an 8-bit analogue-to-digital converter (ADC). The IC offered adjustable gains that were elaborated for various physiological signal acquisitions. The conditional signals were quantized by ADC that offered an optimal solution for portable applications. The circuit was fabricated in SMIC 0.18-mum mixed-signal CMOS technology, and core area of the whole IC measured 1.36 mm². The post-annotated simulations suggested that the system achieved a common-mode-rejection-ration (CMRR) of 142 dB, the adjustable gain from 31.6 dB to 76.5 dB, and the offset voltage less than 80 muV. The 8-bit ADC exhibited less than 0.8 LSB DNL and 1.1 LSB INL. Power dissipation of each channel and the ADC were approximately 348 muW and 1.65 mW under a 1.8 V single supply voltage, respectively. It is suitable for a wide range of high precision biomedical applications.

A novel continuous-time notch filter based on current steering technique was described in this paper. The second-order notch filter consisted of two integrators, one unity-gain inverter and two alpha blocks that were fully integrated onto a silicon chip. The circuit was implemented in SMIC mixed-signal 0.18nm 1P6M process with the die area of 0.06 mm². Post-annotated simulation indicated the continuous-time notch filter was capable of providing 55.4dB attenuation for the 50 Hz power line interference and its center frequency was tunable against process variations.