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Biophysical Sensors for Biomedical/Health Monitoring Applications
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Biophysical Sensors for Biomedical/Health Monitoring Applications

A special issue of Biosensors (ISSN 2079-6374). This special issue belongs to the section "Biosensors and Healthcare".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 43682

Special Issue Editor

Dr. Sang Min Won

E-Mail Website
Guest Editor
Department of Electrical and Computer Engineering, Sungkyunkwan University, 16419 Seoul, Republic of Korea
Interests: biophysical sensors; flexible microsystems; biomedical device; implantable electronics; bio-integrated electronics

Special Issue Information

Dear Colleagues,

Engineering control over materials forms and structures provides tissue-compliant, flexible, and stretchable classes of biophysical sensors and microsystems that facilitate both fundamental biological research and biomedical diagnosis. Examples of recent advanced systems combine theoretical and experimental efforts in materials (e.g., metal, semiconductor, carbon, liquid metal) and sensory transduction (e.g., piezoresistive, acoustic waves, optical, capacitive) for monitoring diverse biophysical signals (e.g., strain, pressure, ultrasound, temperature, vibration). The complete systems are well configured for a range of applications, such as human health monitoring, robotic prosthesis control, acoustic-based care systems, and measuring activities inside the body. This Special Issue aims to highlight recent advanced biophysical sensors in such biocompatible configurations.

The scope of the Special Issue includes:
• Unconventional biophysical sensor geometry and configuration;
• New methodology and concept for sensing biophysical signals (strain, pressure, ultrasound, temperature, etc.);
• Theoretical study of sensor mechanics;
• Biophysical sensors and systems for biomedical applications;
• New methodology to process the monitored biophysical data set.

Dr. Sang Min Won
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Biosensors is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • biophysical sensor
  • flexible device
  • biointegrated electronics
  • stretchable microsystem
  • implantable electronics
  • biomedical device
  • health monitoring

Published Papers (15 papers)

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Research

Jump to: Review

13 pages, 5389 KiB  
Article
Arterial Pulse Wave Velocity Signal Reconstruction Using Low Sampling Rates
by Sungcheol Hong and Gerard Coté
Biosensors 2024, 14(2), 92; https://0-doi-org.brum.beds.ac.uk/10.3390/bios14020092 - 08 Feb 2024
Viewed by 1202
Abstract
Pulse Wave Velocity (PWV) analysis is valuable for assessing arterial stiffness and cardiovascular health and potentially for estimating blood pressure cufflessly. However, conventional PWV analysis from two transducers spaced closely poses challenges in data management, battery life, and developing the device for continuous [...] Read more.
Pulse Wave Velocity (PWV) analysis is valuable for assessing arterial stiffness and cardiovascular health and potentially for estimating blood pressure cufflessly. However, conventional PWV analysis from two transducers spaced closely poses challenges in data management, battery life, and developing the device for continuous real-time applications together along an artery, which typically need data to be recorded at high sampling rates. Specifically, although a pulse signal consists of low-frequency components when used for applications such as determining heart rate, the pulse transit time for transducers near each other along an artery takes place in the millisecond range, typically needing a high sampling rate. To overcome this issue, in this study, we present a novel approach that leverages the Nyquist–Shannon sampling theorem and reconstruction techniques for signals produced by bioimpedance transducers closely spaced along a radial artery. Specifically, we recorded bioimpedance artery pulse signals at a low sampling rate, reducing the data size and subsequently algorithmically reconstructing these signals at a higher sampling rate. We were able to retain vital transit time information and achieved enhanced precision that is comparable to the traditional high-rate sampling method. Our research demonstrates the viability of the algorithmic method for enabling PWV analysis from low-sampling-rate data, overcoming the constraints of conventional approaches. This technique has the potential to contribute to the development of cardiovascular health monitoring and diagnosis using closely spaced wearable devices for real-time and low-resource PWV assessment, enhancing patient care and cardiovascular disease management. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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13 pages, 2986 KiB  
Article
Early Blood Clot Detection Using Forward Scattering Light Measurements Is Not Superior to Delta Pressure Measurements
by Anna Fischbach, Michael Lamberti, Julia Alexandra Simons, Erik Wrede, Alexander Theißen, Patrick Winnersbach, Rolf Rossaint, André Stollenwerk and Christian Bleilevens
Biosensors 2023, 13(12), 1012; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13121012 - 04 Dec 2023
Viewed by 1613
Abstract
The occurrence of thrombus formation within an extracorporeal membrane oxygenator is a common complication during extracorporeal membrane oxygenation therapy and can rapidly result in a life-threatening situation due to arterial thromboembolism, causing stroke, pulmonary embolism, and limb ischemia in the patient. The standard [...] Read more.
The occurrence of thrombus formation within an extracorporeal membrane oxygenator is a common complication during extracorporeal membrane oxygenation therapy and can rapidly result in a life-threatening situation due to arterial thromboembolism, causing stroke, pulmonary embolism, and limb ischemia in the patient. The standard clinical practice is to monitor the pressure at the inlet and outlet of oxygenators, indicating fulminant, obstructive clot formation indicated by an increasing pressure difference (ΔP). However, smaller blood clots at early stages are not detectable. Therefore, there is an unmet need for sensors that can detect blood clots at an early stage to minimize the associated thromboembolic risks for patients. This study aimed to evaluate if forward scattered light (FSL) measurements can be used for early blood clot detection and if it is superior to the current clinical gold standard (pressure measurements). A miniaturized in vitro test circuit, including a custom-made test chamber, was used. Heparinized human whole blood was circulated through the test circuit until clot formation occurred. Four LEDs and four photodiodes were placed along the sidewall of the test chamber in different positions for FSL measurements. The pressure monitor was connected to the inlet and the outlet to detect changes in ΔP across the test chamber. Despite several modifications in the LED positions on the test chamber, the FSL measurements could not reliably detect a blood clot within the in vitro test circuit, although the pressure measurements used as the current clinical gold standard detected fulminant clot formation in 11 independent experiments. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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13 pages, 3872 KiB  
Article
Development of a Direct Non-Puncture Device for Measuring Portal Venous Pressure during Liver Transplantation—A Swine Model
by Kung-Chen Ho, Tun-Sung Huang, Jiunn-Chang Lin and Huihua Kenny Chiang
Biosensors 2023, 13(12), 1007; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13121007 - 30 Nov 2023
Viewed by 1303
Abstract
Portal hypertension-related complications pose a significant risk for liver failure post-transplantation. Thus, accurate monitoring of intraoperative portal venous pressure (PVP) is crucial. However, current PVP monitoring techniques requiring direct percutaneous puncture carry the risk of graft damage. In this study, we present an [...] Read more.
Portal hypertension-related complications pose a significant risk for liver failure post-transplantation. Thus, accurate monitoring of intraoperative portal venous pressure (PVP) is crucial. However, current PVP monitoring techniques requiring direct percutaneous puncture carry the risk of graft damage. In this study, we present an innovative non-puncture PVP monitoring device (PVPMD) using a 3D-printed prototype. PVPMD design is inspired by the sphygmomanometer principle, and strategically encompasses the portal vein and enables precise PVP measurement through blood flow ultrasonography after temporary occlusion. By a series of mini-pig experiments, the prototype PVPMD demonstrated a strong correlation with invasive catheter measurements in the main trunk of the portal vein (rs = 0.923, p = 0.000). There was a significant repeatability and reproducibility between the prototype PVPMD- and invasive catheter-measured PVP. This indicates that the PVPMD holds immense potential for direct application in liver transplantation and surgery. Moreover, it has the potential to replace catheter-based central venous pressure (CVP) measurements, thereby mitigating catheter-related complications during many surgeries. In conclusion, our innovative device represents a significant advancement in PVP monitoring during liver transplantation, with comprehensive validation from principle exploration to successful animal experiments. We anticipate that this groundbreaking PVPMD will attract the attention of researchers and clinicians, propelling the noninvasive measurement of PVP or other venous/arterial pressures into a new era of clinical practice. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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16 pages, 3474 KiB  
Article
Reflection-Boosted Wearable Ring-Type Pulse Oximeters for SpO2 Measurement with High Sensitivity and Low Power Consumption
by Min Gyu Joo, Dae Hyeong Lim, Kyu-Kwan Park, Jiwon Baek, Jong Min Choi and Hyoung Won Baac
Biosensors 2023, 13(7), 711; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13070711 - 05 Jul 2023
Viewed by 1994
Abstract
In this study, we demonstrated a Monte Carlo simulation to model a finger structure and to calculate the intensity of photons passing through tissues, in order to determine optimal angular separation between a photodetector (PD) and a light-emitting diode (LED), to detect SpO [...] Read more.
In this study, we demonstrated a Monte Carlo simulation to model a finger structure and to calculate the intensity of photons passing through tissues, in order to determine optimal angular separation between a photodetector (PD) and a light-emitting diode (LED), to detect SpO2. Furthermore, our model was used to suggest a mirror-coated ring-type pulse oximeter to improve the sensitivity by up to 80% and improve power consumption by up to 65% compared to the mirror-uncoated structure. A ring-type pulse oximeter (RPO) is widely used to detect photoplethysmography (PPG) signals for SpO2 measurement during sleep and health-status monitoring. Device sensitivity and the power consumption of an RPO, which are key performance indicators, vary greatly with the geometrical arrangement of PD and LED within the inner surface of an RPO. We propose a reflection-boosted design of an RPO to achieve both high sensitivity and low power consumption, and determine an optimal configuration of a PD and LED by performing a 3D Monte Carlo simulation and confirming its agreement with experimental measurement. In order to confirm the reflection-boosted performance in terms of signal-to-noise ratio, R ratio, and perfusion index (PI), RPOs were fabricated with and without a highly reflective coating, and then used for SpO2 measurement from eight participants. Our simulation allows the numerical calculation of the intensity of photon passing and scattering through finger tissues. The reflection-boosted RPO enables reliable measurement with high sensitivity, resulting in less power consumption for the LED and longer device usage than conventional RPOs without any reflective coating, in order to maintain the same level of SNR and PI. Compared to the non-reflective reference RPO, the reflection-boosted RPO design greatly enhanced both detected light intensity (67% in dc and 322% in ac signals at a wavelength λ1 = 660 nm, and also 81% and 375% at λ2 = 940 nm, respectively) and PI (23.3% at λ1 and 25.5% at λ2). Thus, the reflection-boosted design not only enhanced measurement reliability but also significantly improved power consumption, i.e., by requiring only 36% and 30% power to drive the LED sources with λ1 and λ2, respectively, to produce the device performance of a non-reflective RPO reference. It is expected that our proposed RPO provides long-term monitoring capability with low power consumption and an enhanced PI for SpO2 measurement. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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13 pages, 2614 KiB  
Article
Morphic Sensors for Respiratory Parameters Estimation: Validation against Overnight Polysomnography
by Ganesh R. Naik, Paul P. Breen, Titus Jayarathna, Benjamin K. Tong, Danny J. Eckert and Gaetano D. Gargiulo
Biosensors 2023, 13(7), 703; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13070703 - 03 Jul 2023
Cited by 1 | Viewed by 1391
Abstract
Effective monitoring of respiratory disturbances during sleep requires a sensor capable of accurately capturing chest movements or airflow displacement. Gold-standard monitoring of sleep and breathing through polysomnography achieves this task through dedicated chest/abdomen bands, thermistors, and nasal flow sensors, and more detailed physiology, [...] Read more.
Effective monitoring of respiratory disturbances during sleep requires a sensor capable of accurately capturing chest movements or airflow displacement. Gold-standard monitoring of sleep and breathing through polysomnography achieves this task through dedicated chest/abdomen bands, thermistors, and nasal flow sensors, and more detailed physiology, evaluations via a nasal mask, pneumotachograph, and airway pressure sensors. However, these measurement approaches can be invasive and time-consuming to perform and analyze. This work compares the performance of a non-invasive wearable stretchable morphic sensor, which does not require direct skin contact, embedded in a t-shirt worn by 32 volunteer participants (26 males, 6 females) with sleep-disordered breathing who performed a detailed, overnight in-laboratory sleep study. Direct comparison of computed respiratory parameters from morphic sensors versus traditional polysomnography had approximately 95% (95 ± 0.7) accuracy. These findings confirm that novel wearable morphic sensors provide a viable alternative to non-invasively and simultaneously capture respiratory rate and chest and abdominal motions. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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16 pages, 5174 KiB  
Article
Bioimpedance Analysis as Early Predictor for Clot Formation Inside a Blood-Perfused Test Chamber: Proof of Concept Using an In Vitro Test-Circuit
by Muhammet Türkmen, Tobias Lauwigi, Tamara Fechter, Fabienne Gries, Anna Fischbach, Thomas Gries, Rolf Rossaint, Christian Bleilevens and Patrick Winnersbach
Biosensors 2023, 13(3), 394; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13030394 - 17 Mar 2023
Cited by 1 | Viewed by 3679
Abstract
Clot formation inside a membrane oxygenator (MO) due to blood-to-foreign surface interaction represents a frequent complication during extracorporeal membrane oxygenation. Since current standard monitoring methods of coagulation status inside the MO fail to detect clot formation at an early stage, reliable sensors for [...] Read more.
Clot formation inside a membrane oxygenator (MO) due to blood-to-foreign surface interaction represents a frequent complication during extracorporeal membrane oxygenation. Since current standard monitoring methods of coagulation status inside the MO fail to detect clot formation at an early stage, reliable sensors for early clot detection are in demand to reduce associated complications and adverse events. Bioimpedance analysis offers a monitoring concept by integrating sensor fibers into the MO. Herein, the feasibility of clot detection via bioimpedance analysis is evaluated. A custom-made test chamber with integrated titanium fibers acting as sensors was perfused with heparinized human whole blood in an in vitro test circuit until clot formation occurred. The clot detection capability of bioimpedance analysis was directly compared to the pressure difference across the test chamber (ΔP-TC), analogous to the measurement across MOs (ΔP-MO), the clinical gold standard for clot detection. We found that bioimpedance measurement increased significantly 8 min prior to a significant increase in ΔP-TC, indicating fulminant clot formation. Experiments without clot formation resulted in a lack of increase in bioimpedance or ΔP-TC. This study shows that clot detection via bioimpedance analysis under flow conditions in a blood-perfused test chamber is generally feasible, thus paving the way for further investigation. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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13 pages, 2060 KiB  
Article
One-Step Fabrication of Paper-Based Inkjet-Printed Graphene for Breath Monitor Sensors
by Wei Yin Lim, Choon-Hian Goh, Keenan Zhihong Yap and Narayanan Ramakrishnan
Biosensors 2023, 13(2), 209; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13020209 - 30 Jan 2023
Cited by 9 | Viewed by 2153
Abstract
Irregularities in breathing patterns can be detected using breath monitor sensors, and this help clinicians to predict health disorders ranging from sleep disorders to heart failures. Variations in humidity during the inhalation and exhalation of breath have been utilized as a marker to [...] Read more.
Irregularities in breathing patterns can be detected using breath monitor sensors, and this help clinicians to predict health disorders ranging from sleep disorders to heart failures. Variations in humidity during the inhalation and exhalation of breath have been utilized as a marker to detect breath patterns, and graphene-based devices are the favored sensing media for relative humidity (RH). In general, most graphene-based RH sensors have been used to explore resistance change as a measurement parameter to calibrate against the RH value, and they are prone to noise interference. Here, we fabricated RH sensors using graphene ink as a sensing medium and printed them in the shape of interdigital electrodes on glossy paper using an office inkjet printer. Further, we investigated the capacitance change in the sensor for the RH changes in the range of 10–70%. It exhibited excellent sensitivity with 0.03 pF/% RH, good stability, and high intraday and interday repeatability, with relative standard deviations of 1.2% and 2.2%, respectively. Finally, the sensor was embedded into a face mask and interfaced with a microcontroller, and capacitance change was measured under three different breathing situations: normal breathing, deep breathing, and coughing. The result show that the dominant frequency for normal breath is 0.22 Hz, for deep breath, it is 0.11 Hz, and there was no significant dominant cough frequency due to persistent coughing and inconsistent patterns. Moreover, the sensor exhibited a short response and recovery time (<5 s) during inhalation and exhalation. Thus, the proposed paper-based RH sensor is promising wearable and disposable healthcare technology for clinical and home care health applications. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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13 pages, 5513 KiB  
Article
Evaluation of the Information Content for Determining the Vascular Tone Type of the Lower Extremities in Varicose Veins: A Case Study
by Ahmad Hammoud, Alexey Tikhomirov, Andrey Briko, Alexander Volkov, Aida Karapetyan and Sergey Shchukin
Biosensors 2023, 13(1), 96; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13010096 - 06 Jan 2023
Cited by 5 | Viewed by 1500
Abstract
The incidence of cardiovascular diseases is continuously increasing around the world. Therefore, the study of new methods for diagnosing cardiovascular diseases is very important. Early diagnosis and evaluation of the effectiveness of treatments are among the most important tasks. In this work, we [...] Read more.
The incidence of cardiovascular diseases is continuously increasing around the world. Therefore, the study of new methods for diagnosing cardiovascular diseases is very important. Early diagnosis and evaluation of the effectiveness of treatments are among the most important tasks. In this work, we study changes in vascular compliance and vascular tone of the lower extremities in a patient diagnosed with an early stage of varicose veins. The study is based on recording the bioimpedance signals of the lower extremities and their parts using the Rheo-32 multichannel device. Registration in the monitoring system takes place in two stages: the first in a state of relaxation, and the second after applying a local massage on one of the legs for five minutes. The results indicate a change in the type of vascular tone of the lower extremities after the massage, while the type of vascular tone changes and shifts on average towards the normotonic type. The method proposed in this study makes it possible to quantitatively and qualitatively assess changes in the tone of the vessels of the extremities. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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15 pages, 2782 KiB  
Article
Synergistic Effect of Composite Nickel Phosphide Nanoparticles and Carbon Fiber on the Enhancement of Salivary Enzyme-Free Glucose Sensing
by Tania P. Brito, Nicole Butto-Miranda, Andrónico Neira-Carrillo, Soledad Bollo and Domingo Ruíz-León
Biosensors 2023, 13(1), 49; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13010049 - 29 Dec 2022
Cited by 2 | Viewed by 1434
Abstract
An electrospinning method was used for the preparation of an in situ composite based on Ni2P nanoparticles and carbon fiber (FC). The material was tested for the first time against direct glucose oxidation reaction. The Ni2P nanoparticles were distributed [...] Read more.
An electrospinning method was used for the preparation of an in situ composite based on Ni2P nanoparticles and carbon fiber (FC). The material was tested for the first time against direct glucose oxidation reaction. The Ni2P nanoparticles were distributed homogeneously throughout the carbon fibers with a composition determined by thermogravimetric analysis (TGA) of 40 wt% Ni2P and 60 wt% carbon fiber without impurities in the sample. The electrochemical measurement results indicate that the GCE/FC/Ni2P in situ sensor exhibits excellent catalytic activity compared to the GCE/Ni2P and GCE/FC/Ni2P ex situ electrodes. The GCE/FC/Ni2P in situ sensor presents a sensitivity of 1050 µAmM−1cm−2 in the range of 5–208 µM and a detection limit of 0.25 µM. The sensor was applied for glucose detection in artificial saliva, with a low interference observed from normally coexisting electroactive species. In conclusion, our sensor represents a novel and analytical competitive alternative for the development of non-enzymatic glucose sensors in the future. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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21 pages, 29482 KiB  
Article
Multifrequency Microwave Radiometry for Characterizing the Internal Temperature of Biological Tissues
by Enrique Villa, Beatriz Aja, Luisa de la Fuente, Eduardo Artal, Natalia Arteaga-Marrero, Gara Ramos and Juan Ruiz-Alzola
Biosensors 2023, 13(1), 25; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13010025 - 26 Dec 2022
Viewed by 4063
Abstract
The analysis of near-field radiometry is described for characterizing the internal temperature of biological tissues, for which a system based on multifrequency pseudo-correlation-type radiometers is proposed. The approach consists of a new topology with multiple output devices that enables real-time calibration and performance [...] Read more.
The analysis of near-field radiometry is described for characterizing the internal temperature of biological tissues, for which a system based on multifrequency pseudo-correlation-type radiometers is proposed. The approach consists of a new topology with multiple output devices that enables real-time calibration and performance assessment, recalibrating the receiver through simultaneous measurable outputs. Experimental characterization of the prototypes includes a well-defined calibration procedure, which is described and demonstrated, as well as DC conversion from the microwave input power. Regarding performance, high sensitivity is provided in all the bands with noise temperatures around 100 K, reducing the impact of the receiver on the measurements and improving its sensitivity. Calibrated temperature retrievals exhibit outstanding results for several noise sources, for which temperature deviations are lower than 0.1% with regard to the expected temperature. Furthermore, a temperature recovery test for biological tissues, such as a human forearm, provides temperature values on the order of 310 K. In summary, the radiometers design, calibration method and temperature retrieval demonstrated significant results in all bands, validating their use for biomedical applications. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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20 pages, 5318 KiB  
Article
Cuff-Less Blood Pressure Prediction from ECG and PPG Signals Using Fourier Transformation and Amplitude Randomization Preprocessing for Context Aggregation Network Training
by Treesukon Treebupachatsakul, Apivitch Boosamalee, Siratchakrit Shinnakerdchoke, Suejit Pechprasarn and Nuntachai Thongpance
Biosensors 2022, 12(3), 159; https://0-doi-org.brum.beds.ac.uk/10.3390/bios12030159 - 04 Mar 2022
Cited by 6 | Viewed by 4419
Abstract
This research proposes an algorithm to preprocess photoplethysmography (PPG) and electrocardiogram (ECG) signals and apply the processed signals to the context aggregation network-based deep learning to achieve higher accuracy of continuous systolic and diastolic blood pressure monitoring than other reported algorithms. The preprocessing [...] Read more.
This research proposes an algorithm to preprocess photoplethysmography (PPG) and electrocardiogram (ECG) signals and apply the processed signals to the context aggregation network-based deep learning to achieve higher accuracy of continuous systolic and diastolic blood pressure monitoring than other reported algorithms. The preprocessing method consists of the following steps: (1) acquiring the PPG and ECG signals for a two second window at a sampling rate of 125 Hz; (2) separating the signals into an array of 250 data points corresponding to a 2 s data window; (3) randomizing the amplitude of the PPG and ECG signals by multiplying the 2 s frames by a random amplitude constant to ensure that the neural network can only learn from the frequency information accommodating the signal fluctuation due to instrument attachment and installation; (4) Fourier transforming the windowed PPG and ECG signals obtaining both amplitude and phase data; (5) normalizing both the amplitude and the phase of PPG and ECG signals using z-score normalization; and (6) training the neural network using four input channels (the amplitude and the phase of PPG and the amplitude and the phase of ECG), and arterial blood pressure signal in time-domain as the label for supervised learning. As a result, the network can achieve a high continuous blood pressure monitoring accuracy, with the systolic blood pressure root mean square error of 7 mmHg and the diastolic root mean square error of 6 mmHg. These values are within the error range reported in the literature. Note that other methods rely only on mathematical models for the systolic and diastolic values, whereas the proposed method can predict the continuous signal without degrading the measurement performance and relying on a mathematical model. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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Review

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28 pages, 4842 KiB  
Review
Biophysical Sensors Based on Triboelectric Nanogenerators
by Zimeng Ma, Xia Cao and Ning Wang
Biosensors 2023, 13(4), 423; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13040423 - 27 Mar 2023
Cited by 5 | Viewed by 2435
Abstract
Triboelectric nanogenerators (TENGs) can not only collect mechanical energy around or inside the human body and convert it into electricity but also help monitor our body and the world by providing interpretable electrical signals during energy conversion, thus emerging as an innovative medical [...] Read more.
Triboelectric nanogenerators (TENGs) can not only collect mechanical energy around or inside the human body and convert it into electricity but also help monitor our body and the world by providing interpretable electrical signals during energy conversion, thus emerging as an innovative medical solution for both daily health monitoring and clinical treatment and bringing great convenience. This review tries to introduce the latest technological progress of TENGs for applications in biophysical sensors, where a TENG functions as a either a sensor or a power source, and in some cases, as both parts of a self-powered sensor system. From this perspective, this review begins from the fundamental working principles and then concisely illustrates the recent progress of TENGs given structural design, surface modification, and materials selection toward output enhancement and medical application flexibility. After this, the medical applications of TENGs in respiratory status, cardiovascular disease, and human rehabilitation are covered in detail, in the form of either textile or implantable parts for pacemakers, nerve stimulators, and nerve prostheses. In addition, the application of TENGs in driving third-party medical treatment systems is introduced. Finally, shortcomings and challenges in TENG-based biophysical sensors are highlighted, aiming to provide deeper insight into TENG-based medical solutions for the development of TENG-based self-powered electronics with higher performance for practical applications. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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41 pages, 8056 KiB  
Review
Recent Progress in Long-Term Sleep Monitoring Technology
by Jiaju Yin, Jiandong Xu and Tian-Ling Ren
Biosensors 2023, 13(3), 395; https://0-doi-org.brum.beds.ac.uk/10.3390/bios13030395 - 17 Mar 2023
Cited by 3 | Viewed by 4635
Abstract
Sleep is an essential physiological activity, accounting for about one-third of our lives, which significantly impacts our memory, mood, health, and children’s growth. Especially after the COVID-19 epidemic, sleep health issues have attracted more attention. In recent years, with the development of wearable [...] Read more.
Sleep is an essential physiological activity, accounting for about one-third of our lives, which significantly impacts our memory, mood, health, and children’s growth. Especially after the COVID-19 epidemic, sleep health issues have attracted more attention. In recent years, with the development of wearable electronic devices, there have been more and more studies, products, or solutions related to sleep monitoring. Many mature technologies, such as polysomnography, have been applied to clinical practice. However, it is urgent to develop wearable or non-contacting electronic devices suitable for household continuous sleep monitoring. This paper first introduces the basic knowledge of sleep and the significance of sleep monitoring. Then, according to the types of physiological signals monitored, this paper describes the research progress of bioelectrical signals, biomechanical signals, and biochemical signals used for sleep monitoring. However, it is not ideal to monitor the sleep quality for the whole night based on only one signal. Therefore, this paper reviews the research on multi-signal monitoring and introduces systematic sleep monitoring schemes. Finally, a conclusion and discussion of sleep monitoring are presented to propose potential future directions and prospects for sleep monitoring. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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45 pages, 6926 KiB  
Review
Advances in Materials, Sensors, and Integrated Systems for Monitoring Eye Movements
by Seunghyeb Ban, Yoon Jae Lee, Ka Ram Kim, Jong-Hoon Kim and Woon-Hong Yeo
Biosensors 2022, 12(11), 1039; https://0-doi-org.brum.beds.ac.uk/10.3390/bios12111039 - 17 Nov 2022
Cited by 6 | Viewed by 5528
Abstract
Eye movements show primary responses that reflect humans’ voluntary intention and conscious selection. Because visual perception is one of the fundamental sensory interactions in the brain, eye movements contain critical information regarding physical/psychological health, perception, intention, and preference. With the advancement of wearable [...] Read more.
Eye movements show primary responses that reflect humans’ voluntary intention and conscious selection. Because visual perception is one of the fundamental sensory interactions in the brain, eye movements contain critical information regarding physical/psychological health, perception, intention, and preference. With the advancement of wearable device technologies, the performance of monitoring eye tracking has been significantly improved. It also has led to myriad applications for assisting and augmenting human activities. Among them, electrooculograms, measured by skin-mounted electrodes, have been widely used to track eye motions accurately. In addition, eye trackers that detect reflected optical signals offer alternative ways without using wearable sensors. This paper outlines a systematic summary of the latest research on various materials, sensors, and integrated systems for monitoring eye movements and enabling human-machine interfaces. Specifically, we summarize recent developments in soft materials, biocompatible materials, manufacturing methods, sensor functions, systems’ performances, and their applications in eye tracking. Finally, we discuss the remaining challenges and suggest research directions for future studies. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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30 pages, 3457 KiB  
Review
Wearables for Engagement Detection in Learning Environments: A Review
by Maritza Bustos-López, Nicandro Cruz-Ramírez, Alejandro Guerra-Hernández, Laura Nely Sánchez-Morales, Nancy Aracely Cruz-Ramos and Giner Alor-Hernández
Biosensors 2022, 12(7), 509; https://0-doi-org.brum.beds.ac.uk/10.3390/bios12070509 - 11 Jul 2022
Cited by 13 | Viewed by 3802
Abstract
Appropriate teaching–learning strategies lead to student engagement during learning activities. Scientific progress and modern technology have made it possible to measure engagement in educational settings by reading and analyzing student physiological signals through sensors attached to wearables. This work is a review of [...] Read more.
Appropriate teaching–learning strategies lead to student engagement during learning activities. Scientific progress and modern technology have made it possible to measure engagement in educational settings by reading and analyzing student physiological signals through sensors attached to wearables. This work is a review of current student engagement detection initiatives in the educational domain. The review highlights existing commercial and non-commercial wearables for student engagement monitoring and identifies key physiological signals involved in engagement detection. Our findings reveal that common physiological signals used to measure student engagement include heart rate, skin temperature, respiratory rate, oxygen saturation, blood pressure, and electrocardiogram (ECG) data. Similarly, stress and surprise are key features of student engagement. Full article
(This article belongs to the Special Issue Biophysical Sensors for Biomedical/Health Monitoring Applications)
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