The aim of this review paper is to summarize recent developments in the field of wearable sensors and systems that are relevant to the field of rehabilitation. The growing body of work focused on the application of wearable technology to monitor older adults and subjects with chronic conditions in the home and community settings justifies the emphasis of this review paper on summarizing clinical applications of wearable technology currently undergoing assessment rather than describing the development of new wearable sensors and systems. A short description of key enabling technologies (i.e. sensor technology, communication technology, and data analysis techniques) that have allowed researchers to implement wearable systems is followed by a detailed description of major areas of application of wearable technology. Applications described in this review paper include those that focus on health and wellness, safety, home rehabilitation, assessment of treatment efficacy, and early detection of disorders. The integration of wearable and ambient sensors is discussed in the context of achieving home monitoring of older adults and subjects with chronic conditions. Future work required to advance the field toward clinical deployment of wearable sensors and systems is discussed.

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A review of wearable sensors and systems with application in rehabilitation.

An increase in world population along with a significant aging portion is forcing rapid rises in healthcare costs. The healthcare system is going through a transformation in which continuous monitoring of inhabitants is possible even without hospitalization. The advancement of sensing technologies, embedded systems, wireless communication technologies, nano technologies, and miniaturization makes it possible to develop smart systems to monitor activities of human beings continuously. Wearable sensors detect abnormal and/or unforeseen situations by monitoring physiological parameters along with other symptoms. Therefore, necessary help can be provided in times of dire need. This paper reviews the latest reported systems on activity monitoring of humans based on wearable sensors and issues to be addressed to tackle the challenges.

Wearable Sensors for Human Activity Monitoring: A Review

Photoplethysmography (PPG) is used to estimate the skin blood flow using infrared light. Researchers from different domains of science have become increasingly interested in PPG because of its advantages as non-invasive, inexpensive, and convenient diagnostic tool. Traditionally, it measures the oxygen saturation, blood pressure, cardiac output, and for assessing autonomic functions. Moreover, PPG is a promising technique for early screening of various atherosclerotic pathologies and could be helpful for regular GP-assessment but a full understanding of the diagnostic value of the different features is still lacking. Recent studies emphasise the potential information embedded in the PPG waveform signal and it deserves further attention for its possible applications beyond pulse oximetry and heart-rate calculation. Therefore, this overview discusses different types of artifact added to PPG signal, characteristic features of PPG waveform, and existing indexes to evaluate for diagnoses.

On the Analysis of Fingertip Photoplethysmogram Signals

Ubiquitous blood pressure (BP) monitoring is needed to improve hypertension detection and control and is becoming feasible due to recent technological advances such as in wearable sensing. Pulse transit time (PTT) represents a well-known potential approach for ubiquitous BP monitoring. The goal of this review is to facilitate the achievement of reliable ubiquitous BP monitoring via PTT. We explain the conventional BP measurement methods and their limitations; present models to summarize the theory of the PTT-BP relationship; outline the approach while pinpointing the key challenges; overview the previous work toward putting the theory to practice; make suggestions for best practice and future research; and discuss realistic expectations for the approach.

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Toward Ubiquitous Blood Pressure Monitoring via Pulse Transit Time: Theory and Practice.

A medical sensor may be adapted to account for factors that cause irregularities in pulse oximetry measurements or other spectrophotemetric measurements. Sensors are provided with surface features that reduce the amount of outside light or shunted light that impinge the detecting elements of the sensor. The sensor is adapted to reduce the effect of outside light or shunted light on pulse oximetry measurements.

Medical sensor and technique for using the same

The photoplethysmogram is a noninvasive circulatory signal related to the pulsatile volume of blood in tissue and is displayed by many pulse oximeters and bedside monitors, along with the computed arterial oxygen saturation. The photoplethysmogram is similar in appearance to an arterial blood pressure waveform. Because the former is noninvasive and nearly ubiquitous in hospitals whereas the latter requires invasive measurement, the extraction of circulatory information from the photoplethysmogram has been a popular subject of contemporary research. The photoplethysmogram is a function of the underlying circulation, but the relation is complicated by optical, biomechanical, and physiologic covariates that affect the appearance of the photoplethysmogram. Overall, the photoplethysmogram provides a wealth of circulatory information, but its complex etiology may be a limitation in some novel applications.

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Utility of the photoplethysmogram in circulatory monitoring.

An apparatus and methods for performing a circulatory measurement on an extremity, such as a hand, of a subject. The circulatory measurement results in the derivation of an output circulatory metric that may encompass blood pressure or various other circulatory metrics. An indicator of an input circulatory metric at a locus on the extremity is measured, such as a pulse transit time. To determine the pulse transit time, a first plethysmographic signal may be obtained at a first position on the extremity, while a second plethysmographic signal may be obtained at a second position on the extremity of the subject. A transit time characterizing a circulatory pressure wave is calculated based on the first and second plethysmographic signals, leading to derivation of a wave speed. A calibration is then applied to provide the circulatory measurement based at least on the derived wave speed and a measured indicator of a hydrostatic component of blood pressure. Calibration is provided, in certain described embodiments, by derivation of two calibration parameters, a gain and a pulse transit time at zero pressure. Methods for deriving the calibration parameters include performing measurements under distinct hydrostatic pressure conditions, and based upon a measured derivative with respect to pressure of the pulse wave velocity.

Wearable Pulse Wave Velocity Blood Pressure Sensor and Methods of Calibration Thereof

A ring plethysmograph having a pressure adjustment for locally pressurizing one side of a finger thereby biasing the pressure on an artery wall so that the plethysmograph is optimally sensitive without interfering with blood flow. An auxiliary photodetector, possibly with a second light source, is disposed on the low-pressure side of the finger for two purposes: providing a noise reference for canceling noise on the plethysmograph signal, and also for providing a separate motion signal for monitoring the activity level of a patient.

Photoplethysmograph signal-to-noise line enhancement

Methods and apparatus for measuring arterial blood pressure at an extremity of a subject. Arterial blood pressure is derived from a circulatory measurement performed on an extremity of a subject and the circulatory measurement is normalized to account for the instantaneous vertical displacement of the extremity. The vertical displacement of the extremity relative to the heart of the subject is obtained using the angular orientation of the subject's extremity. An improved photoplethysmograph can discriminate light traversing the extremity from ambient light on the basis of differential response. The apparatus may have a conducting polymer actuator for applying pressure to the extremity of the subject. A pulsatile waveform from the photoplethysmographic signal may be obtained at a plurality of externally applied pressures to calibrate the photoplethysmograph.

Wearable blood pressure sensor and method of calibration

A method and an apparatus for distinguishing concentrations of blood constituents among distinct vascular components in situ. The method has steps of inducing periodic vibration, characterized by a frequency, in a limb of a person in such as manner as to selectively excite a resonant response in a specified blood vessel of the person, an artery or a vein, illuminating the limb of the person with a light source, and synchronously detecting a plethysmographic signal for discriminating response attributable to the specified blood vessel.

Phillip Shaltis

Papers

Utility of the photoplethysmogram in circulatory monitoring.

Wearable Pulse Wave Velocity Blood Pressure Sensor and Methods of Calibration Thereof

Photoplethysmograph signal-to-noise line enhancement

Wearable blood pressure sensor and method of calibration

Vibratory venous and arterial oximetry sensor