https://www.mdu.se/

Accurate bladder monitoring is critical in the management of conditions such as urinary incontinence, voiding dysfunction, and spinal cord injuries. Electrical bioimpedance (EBI) has emerged as a cost-effective and non-invasive approach to monitoring bladder activity in daily life, with particular relevance to patient groups who require measurement of bladder urine volume (BUV) to prevent urinary leakage. However, the impact of activities in daily living (ADLs) on EBI measurements remains incompletely characterized. In this study, we investigated the impact of normal ADLs such as sitting, standing, and walking on EBI measurements using the MAX30009evkit system with four electrodes placed on the lower abdominal area. We developed an algorithm to identify artifacts caused by the different activities from the EBI signals. Our findings demonstrate that various physical activities clearly affected the EBI measurements, indicating the necessity of considering them during bladder monitoring with EBI technology performed during physical activity (or normal ADLs). We also observed that several specific activities could be distinguished based on their impedance values and waveform shapes. Thus, our results provide a better understanding of the impact of physical activity on EBI measurements and highlight the importance of considering such physical activities during EBI measurements in order to enhance the reliability and effectiveness of EBI technology for bladder monitoring.

Bladder monitoring, including urinary incontinence management and bladder urinary volume monitoring, is a vital part of urological care. Urinary incontinence is a common medical condition affecting the quality of life of more than 420 million people worldwide, and bladder urinary volume is an important indicator to evaluate the function and health of the bladder. Previous studies on non-invasive techniques for urinary incontinence management technology, bladder activity and bladder urine volume monitoring have been conducted. This scoping review outlines the prevalence of bladder monitoring with a focus on recent developments in smart incontinence care wearable devices and the latest technologies for non-invasive bladder urine volume monitoring using ultrasound, optical and electrical bioimpedance techniques. The results found are promising and their application will improve the well-being of the population suffering from neurogenic dysfunction of the bladder and the management of urinary incontinence. The latest research advances in bladder urinary volume monitoring and urinary incontinence management have significantly improved existing market products and solutions and will enable the development of more effective future solutions.

Cardiovascular diseases (CVDs) are a leading cause of death worldwide, and hypertension is a major risk factor for acquiring CVDs. Early detection and treatment of hypertension can significantly reduce the risk of developing CVDs and related complications. In this study, a linear SVM machine learning model was used to classify subjects as normal or at different stages of hypertension. The features combined statistical parameters derived from the acceleration plethysmography waveforms and clinical parameters extracted from a publicly available dataset. The model achieved an overall accuracy of 87.50% on the validation dataset and 95.35% on the test dataset. The model's true positive rate and positive predictivity was high in all classes, indicating a high accuracy, and precision. This study represents the first attempt to classify cardiovascular conditions using a combination of acceleration photoplethysmogram (APG) features and clinical parameters The study demonstrates the potential of APG analysis as a valuable tool for early detection of hypertension.

The extraction of relevant features from the photoplethysmography signal for estimating certain physiological parameters is a challenging task. Various feature extraction methods have been proposed in the literature. In this study, we present a novel fiducial point extraction algorithm to detect c and d points from the acceleration photoplethysmogram (APG), namely “CnD”. The algorithm allows for the application of various pre-processing techniques, such as filtering, smoothing, and removing baseline drift; the possibility of calculating first, second, and third photoplethysmography derivatives; and the implementation of algorithms for detecting and highlighting APG fiducial points. An evaluation of the CnD indicated a high level of accuracy in the algorithm’s ability to identify fiducial points. Out of 438 APG fiducial c and d points, the algorithm accurately identified 434 points, resulting in an accuracy rate of 99%. This level of accuracy was consistent across all the test cases, with low error rates. These findings indicate that the algorithm has a high potential for use in practical applications as a reliable method for detecting fiducial points. Thereby, it provides a valuable new resource for researchers and healthcare professionals working in the analysis of photoplethysmography signals.

Photoplethysmography is a non-invasive technique used for measuring several vital signs and for the identification of individuals with an increased disease risk. Its principle of work is based on detecting changes in blood volume in the microvasculature of the skin through the absorption of light. The extraction of relevant features from the photoplethysmography signal for estimating certain physiological parameters is a challenging task, where various feature extraction methods have been proposed in the literature. In this work, we present PPGFeat, a novel MATLAB toolbox supporting the analysis of raw photoplethysmography waveform data. PPGFeat allows for the application of various preprocessing techniques, such as filtering, smoothing, and removal of baseline drift; the calculation of photoplethysmography derivatives; and the implementation of algorithms for detecting and highlighting photoplethysmography fiducial points. PPGFeat includes a graphical user interface allowing users to perform various operations on photoplethysmography signals and to identify, and if required also adjust, the fiducial points. Evaluating the PPGFeat’s performance in identifying the fiducial points present in the publicly available PPG-BP dataset, resulted in an overall accuracy of 99% and 3038/3066 fiducial points were correctly identified. PPGFeat significantly reduces the risk of errors in identifying inaccurate fiducial points. Thereby, it is providing a valuable new resource for researchers for the analysis of photoplethysmography signals.

Today, in our daily life we are subjected to a wide range of pressures. When the pressures exceed the extent that we are able to deal with then stress is trigged. High level of stress may cause serious health problems i.e. it reduces awareness of bodily symptoms. So, people may first notice it weeks or months later meanwhile the stress could cause more serious effect in the body and health. A difficult issue in stress management is to use biomedical sensor signals in the diagnosis and treatment of stress. This paper presents a case-based system that assists a clinician in diagnosis and treatment of stress. The system uses a finger temperature sensor and the variation in the finger temperature is one of the key features in the system. Several artificial intelligence techniques such as textual information retrieval, rule-based reasoning (RBR), and fuzzy logic have been combined together with case-based reasoning to enable more reliable and efficient diagnosis and treatment of stress. The performance has been validated implementing a research prototype and close collaboration with experts.

The Health Sciences are, nowadays, one of the major application areas for case-based reasoning (CBR). The paper presents a survey of recent medical CBR systems based on a literature review and an e-mail questionnaire sent to the corresponding authors of the papers where these systems are presented. Some clear trends have been identified, such as multipurpose systems: more than half of the current medical CBR systems address more than one task. Research on CBR in the area is growing, but most of the systems are still prototypes and not available on the market as commercial products. However, many of the projects/systems are intended to be commercialized.

Increased exposure to stress may cause serious health problems leading to long term sick leave if undiagnosed and untreated. The practice amongst clinicians' to use a standardized procedure measuring blood pressure, ECG, finger temperature, breathing speed etc. to make a reliable diagnosis of stress and stress sensitivity is increasing. But even with these measurements it is still difficult to diagnose due to large individual variations. A computer-based system as a second option for the assessment and diagnosis of individual stress level is valuable in this domain.

A combined approach based on a calibration phase and case-based reasoning is proposed exploiting data from finger temperature sensor readings from 24 individuals. In calibration phase, a standard clinical procedure with six different steps helps to establish a person's stress profile and set up a number of individual parameters. When acquiring a new case, patients are also asked to provide a fuzzy evaluation on how reliable was the procedure to define the case itself. Such a reliability "level" could be used to further discriminate among similar cases. The system extracts key features from the signal and classifies individual sensitivity to stress. These features are stored into a case library and similarity measurements are taken to assess the degrees of matching and create a ranked list containing the most similar cases retrieved by using the nearest-neighbor algorithm.

A current case (CC) is compared with two other stored cases (C_92 and C_115) in the case library. The global similarity between the case CC and case C_92 is 67% and case CC and case C_115 is 80% shown by the system. So the case C_115 has ranked higher than the case C_92 and is more similar to current case CC. If necessary, the solution for the best matching case can be revised by the clinician to fit the new patient. The current problem with confirmed solution is then retained as a new case and added to the case library for future use.

The system allows us to utilize previous experience and at the same time diagnose stress along with a stress sensitivity profile. This information enables the clinician to make a more informed decision of treatment plan for the patients. Such a system may also be used to actively notify a person's stress levels even in the home environment.

Today, everyday life for many people contain many situations that may trigger stress or result in an individual living on an increased stress level under long time. High level of stress may cause serious health problems. It is known that respiratory rate is an important factor and can be used in diagnosis and biofeedback training, but available measurement of respiratory rate are not especially suitable for home and office use. The aim of this project is to develop a portable sensor system that can measure the stress level, during everyday situations e.g. at home and in work environment and can help the person to change the behaviour and decrease the stress level. The sensor explored is a finger temperature sensor. Clinical studies show that finger temperature, in general, decreases with stress; however this change pattern shows large individual variations. Diagnosing stress level from the finger temperature is difficult even for clinical experts. Therefore a computer-based stress diagnosis system is important. In this system, case-based reasoning and fuzzy logic have been applied to assists in stress diagnosis and biofeedback treatment utilizing the finger temperature sensor signal. An evaluation of the system with an expert in stress diagnosis shows promising result.

The aim of this work has been to develop a technical support enabling home-based motor training after stroke. The basis for the work plan has been to develop an interactive technical solution supporting three different groups of stroke patients: (1) patients with stroke discharged from hospital with support from neuro team; (2) patients with stroke whose support from neuro team will be phased out and (3) patients living with impaired motor functions long-term. The technology has been developed in close collaboration with end-users using a method earlier evaluated and described [12]. This paper describes the main functions of the developed technology. Further, results from early user-tests with end-users, performed to identify needs for improvements to be carried out during further technical development. The developed technology will be tested further in a pilot study of the safety and, usefulness of the technology when applied as a support for motor training in three different phases of the post-stroke rehabilitation process. 

Daily moderate activity reduces markedly several population diseases. Still many persons do not perform any regular physical activity. Many persons quit because they exercise at too low intensity that does not give result, or at too high intensity that is not comfortable. By exercising at an established adequate workload, optimal for the individual, the person can see results of the training and the quitting behavior can be changed. In sport at top level, the lactate threshold is used as a guideline for adequate workload, and to see changes in fitness. The lactate threshold is generally established by blood tests. 

The lactate threshold is used by athletes to optimise the intensity during exercise. It is of interest to measure the threshold on the very day and during the present sport activity. Steady state ergometer tests have been performed on 40 individuals to compare the threshold found by an electro acoustic sensor system to the lactate threshold established by blood analyses evaluated with the Dmax method. The correlation coefficient between the threshold found by the sensor system and the one established by blood analyses regarding workload (Watt), heart rate (beats/min), and lactate level (mmol lactate/l blood) at the thresholds were 0.87 (p < 0.001), 0.74 (p < 0.001), and 0.65 (p < 0.001), respectively. The findings in this study indicates that the thresholds of individuals measured by the sensor system show good correlations to the threshold established with the Dmax method from lactate levels in blood samples. 

Koldioxid är en central parameter för metabolismen i alla levande varelser. Hos människan regleras koldioxidhalten främst genom andningen. Därför är det viktigt att kunna mäta och övervaka koldioxidhalten i fysiologiska applikationer så väl inom sportmedicin som inom vården, till exempel på intensivvårdsavdelningar. Dessa mätningar kräver adekvat mätutrustning.

Syftet med denna avhandling är att kritiskt granska föreslagna metoder för övervakning av andningsaktiviteten, att föreslå och utveckla förbättringar, samt att föreslå och utveckla nya applikationer för mätning av koldioxidhalten i utandningsluften med hjälp av en elektroakustisk sensor.

Mätning av koldioxid i utandningsluften har den fördelen över andra föreslagna metoder för andningsövervakning att den även ger information om förhållandet i arteriellt blod då partialtrycket för koldioxid i slutet av ett andetag är lika stort som i artärblod hos lungfriska personer.

Den föreslagna tekniken bygger på en elektroakustisk sensor som kan registrera och mäta koldioxidhalten i utandningsluften om fukt- och temperaturvariationer utjämnas med hjälp av filter. Den elektroakustiska sensorn består av en ultraljudssändare och en reflektor placerade i varsin ände av ett perforerat rör. Den elektriska impedansen i sensorn representerar molekylvikten inne i sensorkaviteten. Molekylmassan har ett linjärt förhållande till koldioxidhalten så länge övriga gaser hålls konstanta.

Det faktum att partialtrycket av koldioxid i slutet av ett andetag kan mätas med den elektroakustiska sensorn gör den användbar i flera kliniska situationer om andra gaser inte kan påverka mätningarna.

Partialtrycket av koldioxid i slutet av ett andetag har visat sig vara användbar som en indikator för mjölksyratröskeln då personens individuella mjölksyratröskel infaller då partialtrycket av koldioxid i slutet av ett andetag börjar minska. Det ger en ny applikation inom sportmedicin. På det sättet är det möjligt för en idrottare att mäta variationerna i sin mjölksyratröskel, i sin egen sport på daglig basis, vilket kan användas för att optimera arbetsintensiteten under träning och tävling. Trots att det elektroakustiska sensorsystemet inte är selektivt för koldioxid och därför påverkas även av förändringar i syre i utandningsluften har det visat sig fungera i denna applikation.

Respiratory failure can be difficult to predict. It can develop into a life-threatening condition in just a few minutes, or it can build up more slowly. Thus continuous monitoring of respiratory activity should be mandatory in clinical, high-risk situations, and appropriate monitoring equipment could be life-saving. The review considers non-invasive methods and devices claimed to provide information about respiratory rate or depth, or gas exchange. Methods are categorised into those responding to movement, volume and tissue composition detection; air flow, and blood gas concentration. The merits and limitations of the methods and devices are analysed, considering information. content and their ability to minimise the rate of false alarms and false non-alarms. It is concluded that the field of non-invasive respiratory monitoring is still in an exploratory phase, with numerous reports on specific device solutions but less work on evaluation and adaptation to clinical requirements. Convincing evidence of the clinical usefulness of respiratory monitors is still lacking. Devices responding only to respiratory rate, and lacking information about actual gas exchange, will have limited clinical value. Furthermore, enhancement in specificity and sensitivity to avoid false alarms and non-alarms will be necessary to meet clinical requirements. Miniature CO2 sensors are identified as one route towards substantial improvement.

Objective. The aim of this study was to compare clinically relevant performance of: 1) a prototype respiratory sensor based on capnometry with two alternative signal receptor fixations, 2) a fiberoptic humidity sensor and 3) human visual observation. Comparative provocation tests were performed on volunteers at the Post-Anesthesia Care Unit at Västerås Central Hospital. Methods. The experimental tests involved 10 healthy, voluntary test subjects, instructed to intersperse normal breathing with protocol provocations of breath holding, limb and head movements, and nasal oxygen supplement. The signal outputs from the three respiratory monitoring methods were recorded on a personal computer. The signal analysis included visual categorising of the signals and counting breath events. Recognising that none of the methods could act as reference, events were classified as "unanimous," "majority" or "minority" events depending on how many of the three methods that detected a breath. Results. The average total recording time was 37 minutes per subject. The respiratory rates varied from 6.5 to 19 breaths per minute, with a mean value of 11.4 breaths/minute. The breath hold duration ranged from 18 to 50 seconds. Discrepancies between the three methods were found in more than 20% of the marked events. The most frequent majority events were due to events not recorded by the observer who, on the other hand, contributed the least to minority events. The provocations made by the subjects during the measurement did not increase the rates of majority and minority events, compared to periods of no provocation. The fiberoptic device exhibited a large count of minority events but a smaller contribution to majority events than the capnometry prototype. Conclusions. The capnometry and fiberoptic sensors exhibit differences in responses that may be understood from basic principles. The importance of the physical application of the sensor to the patient was clearly observed. The optimum design remains to be found.

End tidal carbon dioxide measurements with an electro acoustic capnograph prototype have been demonstrated. The aim of this study was to verify that it is possible to obtain an adequate capnogram using the prototype and to investigate the influence of ambient temperature and humidity variations. By simultaneous measurements with a reference capnograph, on subjects performing exercise, hypo- and hyperventilation, PETCO2 readings from the reference were compared with the output signal from the prototype. The capnogram from the prototype correlated well with the reference in terms of breath time. The first parts of the expiration and inspiration phases were steeper for the reference than the prototype. The output signal from the prototype correlated well with the reference PETCO2 readings with a correlation coefficient of 0.93 at varied temperature and relative humidity. 

End tidal carbon dioxide measurement with an electro-acoustic sensor is demonstrated. The sensor consists of an acoustic resonator coupled to a low cost electro-acoustic element. By simultaneous measurements with a reference sensor, the new device was tested on subjects performing exercise, hypo- and hyperventilation whereby the CO₂ concentration ranged from 2.1 to 7.0 kPa. The output from the experimental device correlated well with the reference CO₂ readings with a correlation coefficient of 0.976. Response time for expiration less than 0.8 seconds was noted. The new device could be useful in situations where selectivity to other gases is not important.

Today, the health care sector has no test for early age-related deterioration in physical ability. The aim of this study was to evaluate questionnaires, videos and physical tests whose task will be to identify early markers related to an increased fall risk in middle-aged people. If the person is aware of deficits in physical ability related to fall risk, the person can then use that knowledge to perform relevant training that can strengthen the physical ability related to fall risk. Self-efficacy for balance and strength, physical ability related to fall risk and body composition were measured for 36 middle-aged test participants. This study shows that the tested physical exercises were useful for self-assessment of physical ability. Impairment in physical ability could not be identified solely with measurement of body composition, walking speed, questions, videos that show adjustments that are common in people with impaired balance, or an extended version of the Short version of Activities-specific Balance Confidence scale. This study indicates that a combination of questionnaires, videos and physical exercises can evaluate physical ability and act as a method to identify early markers related to increased fall risk. The questionnaire, videos and physical exercises can be implemented in a web tool that could make persons aware that they have decreased physical ability regarding fall risk or that they needlessly make physical compensations when performing daily activities and thus are missing opportunities to strengthen their physical ability every day.

Whiplash Associated Disorders (WAD) are several remaining symptoms after an acceleration-/deceleration injury of the neck, often due to a road accident. Common symptoms are neck pain, headache, stiffness, loss of sensation, memory impairment and concentration difficulties. The whiplash-related injuries were estimated to cost Sweden more than SEK 4 billion 2005, the main part of these costs takes the form of compensation for loss of income, as a result of incapacity for work. The aim of this project has been to develop a training and rehabilitation system for patients suffering from WAD. The portable system is based on a 2-axis gyroscopic sensor with a computer interface. The sensor system is placed on the head of the patient and movements of the head are mirrored on the computer screen. The patient is supposed to follow a visible track on the screen. This enables interactive training facilities for patients, who can use the system unsupervised in their home environment.

A new configuration for a gas sensor is demonstrated. The configuration consists of an electroacoustic element coupled to an acoustic resonator, such as Kundt's tube, exhibiting a resonance frequency that is related to the velocity of sound, which, in turn is a function of the molecular mass of the gas within the resonator. Electrical impedance measurements were performed, whereby a resonance peak attributable to the resonator was identified. Contributing effects to the quality factor, Q, of the resonance, was analyzed. Predictable shifts of the resonance frequency were observed when adding CO₂ and He to air, and when varying the resonator length. Linearity within the experimental accuracy was confirmed. The new sensor configuration offers the potential advantages of smaller size, improved dynamic response, and lower cost. © 2001 Elsevier Science B.V. All rights reserved.

A theoretical model of an electroacoustic gas sensor has been designed and compared with experimental data. The sensor includes a piezoelectric element coupled to an acoustic resonator depending on the average molecular weight of the gas with respect to resonance frequency. The model is based on a transmission line coupled to a Butterworth van Dyke model of the piezoelectric element. The frequency characteristics of the sensor impedance was both simulated and measured on prototype sensors. The results indicate that the model provides an adequate description of overall sensor behaviour, and will constitute a useful design tool. Due to interdependencies of the model parameters, the precise quantification of sensor characteristics will either require highly accurate parameter data, or a considerably more complex model.

We are facing an aging population¹ and the need for health and welfare technology to meet the users’ ‘need is a fact. To meet this challenge, the Arena for Health and Welfare Technology (the Arena) was initiated at Mälardalen University in 2014, as a multi-professional and interdisciplinary research initiative². The aim of the Arena is to increase the collaboration of the researchers from the University and the external actors to create benefit and value for the future health and welfare. The Arena organizes events promoting interaction and interdisciplinary research projects. At a thematic day in May 2016, a working process to effectively initiate interdisciplinary research based on the users ‘needs, was introduced. Method To increase the participation for the thematic day and for future research projects within health and welfare technology, a call offering a grand from the Arena fond was created for participants that attended the thematic day. This call was aimed at stimulate the creation of interdisciplinary research projects within health and welfare technology, with the purpose to write proposals for external research funding. Speakers from the region were invited, representing health and social care, companies within the health and welfare technology field and regional R&D units. The speakers were presenting real needs based from the view of the clients, patients, relatives, staffs and organizations, which possibly could be solved by using health and welfare technology. After the user need presentations, a working process was followed, led by the company Konkret Utveckling AB. The goal of the working process was to identify prioritized needs for the day, and in the continuation create project groups to be the basis for future interdisciplinary research projects. After identifying the prioritized user needs, project groups started work to specify the framework for the planned research project, and then make a short oral presentation to share and discuss their research ideas. Results & Discussion The theme day attracted more than 50 interested participants. At the end of the thematic day, four interdisciplinary groups, including new constellations of researchers exists. The aims of the four groups were based on prioritized user needs, presented by regional health and welfare representatives. Involved group participants, researchers and company representatives, represented different disciplines, for example engineering, sensor technology, robotics, pedagogics, physiotherapy, nursing, social work and economy. The working method used during the thematic day was successful, and can be used in developing initiatives for interdisciplinary health and welfare technology research projects based on the needs of the end users.

The aim of this study was to evaluate an integrated probe using LDF and multiple PPG, for the long-term aspects of skin temperature and blood flow variations at different tissue depths, and especially to investigate whether the presence of the probe affects the temperature. Measurements of temperature and blood flow were performed over 60 min on the lower back of ten subjects, lying on a mattress. The surface temperature of the skin was also measured before and after the 60 min period, and repeated with three probe configurations with the probe switched on, turned off and in the absence of a probe. A general increase in the blood flow was found to occur during the 60 min interval at all depths reached by this probe, but with variations over time. No difference was found in temperatures recorded for the different probe configurations. According to our measurements, the presence of the probe does not affect the skin surface temperature at 60 min. Our investigation shows that skin temperature can be expected to increase and approach the body core temperature by just lying in supine position on the mattress. The increase in temperature and blood flow that is known to occur, possibly attributable to pressure-induced vasodilatation, must be taken into consideration when performing these kinds of measurements.

Detecting reduced circulation, which is a major factor in the development of pressure ulcers, can be done using optical methods. PPG and LDF can be combined and used to evaluate blood flow at different depths. In this study the use of a probe combining PPG and LDF to monitor multiple tissue depths is evaluated. The effects on blood flow and temperature without additional provocation was examined. Measurements were performed during 60 min and the use of an active probe was compared with the use of a semi-active probe turned off a major part of the time. Changes in temperature and blood flow using these probe configurations (active and semi-active probe) are compared; four different 5 min segments during a 60 min measurement. A general increase in both temperature and blood flow is found but this increase could not be concluded to occur due to the light sources of the probe.

Detecting reduced circulation, which is a major factor in the development of pressure ulcers, can be done using optical methods. PPG and LDF can be combined and used to evaluate blood flow at different depths. In this study the use of a probe combining PPG and LDF to monitor multiple tissue depths is evaluated. The effects on blood flow and temperature without additional provocation was examined. Measurements were performed during 60 min and the use of an active probe was compared with the use of a semi-active probe turned off a major part of the time. Changes in temperature and blood flow using these probe configurations (active and semi-active probe) are compared; four different 5 min segments during a 60 min measurement. A general increase in both temperature and blood flow is found but this increase could not be concluded to occur due to the light sources of the probe.

För att utvärdera blodflöde kan optiska metoder användas. Denna studie har genomförts för att undersöka eventuella temperatureffekter pga ljuskällorna i en optisk prob som baseras på PPG och LDF. Temperaturförändringar vid två olika mätscenarion jämfördes. Skillnader i temperaturökning kan ses, men resultatet indikerar att påverkan från ljuset endast utgör en liten del av den totala ökningen.

The aim of this study was to validate a non-invasive optical probe for simultaneous blood flow measurement at different vascular depths combining three photoplethysmography (PPG) channels and laser Doppler flowmeter (LDF). Wavelengths of the PPG were near-infrared 810 nm with source-to-detector separation of 10 and 25 mm, and green 560 nm with source-to-detector separation of 4 mm. The probe is intended for clinical studies of pressure ulcer aetiology. The probe was placed over the trapezius muscle, and depths from the skin to the trapezius muscle were measured using ultrasound and varied between 3.8 and 23 mm in the 11 subjects included. A provocation procedure inducing a local enhancement of blood flow in the trapezius muscle was used. Blood flows at rest and post-exercise were compared. It can be concluded that this probe is useful as a tool for discriminating between blood flows at different vascular tissue depths. The vascular depths reached for the different channels in this study were at least 23 mm for the near-infrared PPG channel (source-to-detector separation 25 mm), 10-15 mm for the near-infrared PPG channel (separation 10 mm), and shallower than 4 mm for both the green PPG channel (separation 4 mm) and LDF.