Personalized Machine Learning-Coupled Nanopillar Triboelectric Pulse Sensor for Cuffless Blood Pressure Continuous Monitoring.

A wearable system that can continuously track the fluctuation of blood pressure (BP) based on pulse signals is highly desirable for the treatments of cardiovascular diseases, yet the sensitivity, reliability, and accuracy remain challenging. Since the correlations of pulse waveforms to BP are highly individualized due to the diversity of the patients' physiological characteristics, wearable sensors based on universal designs and algorithms often fail to derive BP accurately when applied on individual patients. Herein, a wearable triboelectric pulse sensor based on a biomimetic nanopillar layer was developed and coupled with Personalized Machine Learning (ML) to provide accurate and continuous monitoring of BP. Flexible conductive nanopillars as the triboelectric layer were fabricated through soft lithography replication of a cicada wing, which could effectively enhance the sensor's output performance to detect weak signal characteristics of pulse waveform for BP derivation. The sensors were coupled with a personalized Partial Least-Squares Regression (PLSR) ML to derive unknown BP based on individual pulse characteristics with reasonable accuracy, avoiding the issue of individual variability that was encountered by General PLSR ML or formula algorithms. The cuffless and intelligent design endow this ML-sensor as a highly promising platform for the care and treatments of hypertensive patients.

A 3D-printed microneedle extraction system integrated with patterned electrodes for minimally invasive transdermal detection.

Point-of-Care-Testing (POCT) is a convenient and timely clinical analysis method, leading the development trend of advanced biosensors. The development of POCT equipment that can achieve minimally invasive percutaneous monitoring can avoid the pain felt by the subjects and achieve in vivo and efficient measurement. Here, we reported the development of a microneedle (MN) extraction system based on patterned electrodes, which could provide convenient and minimally invasive detection of bio-analytes (including glucose, pH, and H2O2). The 3D-printed hollow MN array was used as a painless transdermal tool, while the interstitial fluid was extracted under negative-pressure conditions. The patterned electrodes could improve the electrochemical performance of the sensor, with the synergistic effect of the micropillar structure to increase the enzyme coating surface area and the nanomaterial electron layer. The patterned electrodes were placed on the back of the MN arrays for electrochemical detection. In vitro and in vivo studies showed that the MN-extraction system could detect the corresponding bio-analytes in a minimally invasive manner and it did not cause significant tissue damage. The system developed in this work will provide promising technology to expand the application of POCT for minimal tests on interstitial fluids.

Capacitive-piezoresistive hybrid flexible pressure sensor based on conductive micropillar arrays with high sensitivity over a wide dynamic range.

Flexible pressure sensors are the foundation of wearable/implantable biosensing and human-machine interfaces, and mainly comprise piezoresistive-, capacitive-, piezoelectric-, and triboelectric-type sensors. As each type of sensor exhibits different electro-mechanical behaviors, it is challenging to detect various physiological mechanical signals that cover a large pressure range using a given sensor configuration, or even a single type of sensor. Here, we report a capacitive-piezoresistive hybrid flexible pressure sensor based on face-to-face-mounted conductive micropillar arrays as a solution to this challenge. The sensor exhibited high sensitivity over a wide dynamic range of five orders of magnitude, which covers almost the full range of physiological mechanical signals. A process for fabricating large-scale and morphologically homogeneous conductive micropillar arrays was first developed and refined. This track-etched-membrane-based process provides a facile, cost-effective, and highly flexible way to precisely adjust the morphology, modulus, and conductivity of the micropillars according to the application requirements. Subsequently, conductive-micropillar-array-based pressure sensors (MAPS) were developed and optimized to attain all-round sensing performance. The pillar contact behaviors generated significant variations in both the capacitance and resistance of the MAPS in the low-pressure regime (10-4-0.2 kPa), providing high sensitivity in both the capacitive and piezoresistive working modes. The vertical contact, bending and thickening of the pillars under medium pressure (0.2-16 kPa) led to a continuous linear response in both modes. Configuration and optimization enabled the MAPS to detect acoustic pressure (<1 Pa), milligram weights, soft touch (<1 kPa), arterial pulses (1-16 kPa preload), joint motions and plantar pressure (∼100 kPa), and the hybrid sensing mode allowed the MAPS to work in a desirable way. In this work, the piezoresistive mode was mainly employed for a higher accuracy and sampling rate, and can apparently simplify IC design for wearable applications. The circuit converts the resistive variations into electrical signals via the voltage division method and directly reads out the signals after further amplification, filtering and transmission. The improved facile and highly adjustable fabrication process, as well as the flexible hybrid sensing strategy, will benefit the unified design, batch production, quantifiable optimization, and functional diversity of wearable/implantable bioelectronics.

Progress of flexible strain sensors for physiological signal monitoring.

Flexible strain sensors, as a key component of the cutting-edge wearable and implantable electronics, have facilitated applications pertaining to human health monitoring and diagnosis. To fulfill the increasing requirements of sensing performance and broadening the application scope, novel materials and device design strategies have been continuously developed over the past decade. Herein, the recent progresses of flexible strain sensors developed for monitoring the physiological signals are selectively reviewed, from the perspective of the possible correlation between the device microstructure and their corresponding applications. Firstly, representative strain sensors developed based on four fundamental working mechanisms: piezoresistance, capacitance, piezoelectricity and triboelectricity are respectively introduced, subclassified by the type of active material or the similarity in microstructure. Next, a number of biomedical applications of flexible strain sensors are highlighted, including the detections of different types of physiological signals using specific microstructured strain sensors. Lastly, the role of the transduction mechanism and the device microstructure in the sensing characteristic are comprehensively discussed, and prospective forms of flexible strain sensors to meet the existing and future challenges in wearable/implantable electronics are summarized.

Flexible Tongue Electrode Array System for In Vivo Mapping of Electrical Signals of Taste Sensation.

Tongue is a unique organ that senses tastes, and the scientific puzzle about whether electricity can evoke taste sensations and how the sensations have been distributed on the tongue has not been solved. Investigations on tongue stimulation by electricity might benefit the developments of techniques for clinical neuromodulation, tissue activation, and a brain-tongue-machine interface. To solve the scientific puzzle of whether electrical stimulation induces taste-related sensations, a portable flexible tongue electrode array system (FTEAS) was developed, which can synchronously provide electrical stimulation and signal mapping at each zone of the tongue. Utilizing the FTEAS to perform tests on the rat tongue in vivo, specific electrical signals were observed to be evoked by chemical and electrical stimulations. The features and distributions of the electric signals evoked during the rat tongue tests were systematically studied and comprehensively analyzed. The results show that an appropriate electrical stimulation can induce multiple sensations simultaneously, while the distribution of each sensation was not significantly distinguished among different zones of the tongue, and at the same time, this taste-related electrical signal can be recorded by the FTEAS. This work establishes a promising platform to solve the scientific puzzle of how sensations are activated chemically and electrically on the tongue and may provide advanced noninvasive oral-electrotherapy and a brain-tongue-machine interface.

All-Optical Tuning of Light in WSe2-Coated Microfiber.

The tungsten diselenide (WSe2) has attracted considerable interest owing to their versatile applications, such as p-n junctions, transistors, fiber lasers, spintronics, and conversion of solar energy into electricity. We demonstrate all-optical tuning of light in WSe2-coated microfiber (MF) using WSe2's broad absorption bandwidth and thermo-optic effect. The transmitted optical power (TOP) can be tuned using external incidence pump lasers (405, 532, and 660 nm). The sensitivity under 405-nm pump light excitation is 0.30 dB/mW. A rise/fall time of ~ 15.3/16.9 ms is achieved under 532-nm pump light excitation. Theoretical simulations are performed to investigate the tuning mechanism of TOP. The advantages of this device are easy fabrication, all-optical control, high sensitivity, and fast response. The proposed all-optical tunable device has potential applications in all-optical circuitry, all-optical modulator, and multi-dimensionally tunable optical devices, etc.

Broadband all-light-control with WS2 coated microfibers.

All-optical light amplitude tuning functionality is demonstrated in a layered tungsten disulfide (WS2) nanosheets coated microfiber (MF) structure. Due to the strong light-matter interactions between WS2 nanosheets and the evanescent field around the MF, a large variation in the transmitted power can be observed under both external and internal pump light excitations over a broadband spectrum (~100 nm). A power variation rate of ~0.3744 dB/mW is obtained under external violet pump light excitation, whereas the power variation rate of similar devices in the state of the art are usually less than 0.3 dB/mW. In terms of the response time, a moderate rise/fall time of ∼20.5/19.6 ms is achieved, which is mainly limited by the employed structure fabrication methods. These results indicate that the optical transmitted power of the WS2 coated MF can be modulated by different pump light with the power in the order of mW, thus the proposed device might have potential applications in all optical controllable devices and sensors, etc.