With the increasing prevalence of the growing population, aging and chronic diseases continuously rising healthcare costs, the integration of wearable devices in health services has gained significant attention since the beginning of the 21st century.
Wearable sensors are becoming increasingly common in healthcare and biomedical monitoring systems, allowing for continuous measurement of critical biomarkers to monitor diseased state and health, medical diagnostics, and evaluation in biological fluids such as saliva, blood, and sweat.
Wearable devices can be mainly classified as wearable bands (such as watches, gloves), wearable textiles (t-shirt, socks, shoes), wearables gears (glasses and helmets), and sensory devices for health monitoring.
1. Wrist-mounted wearable
Wrist-Wearable Devices (WWD) are worn on the wrist, as the name suggests. WWDs have been developed for monitoring physiological parameters, with the benefit of miniaturization and increased battery life to convert raw signals into real-time interpretable data. Recently, wrist-mounted wearables such as smartwatches or ﬁtness bands have moved from basic accelerometer-based to biometric sensing (like a pedometer). Commercially existing wrist-worn devices are wristbands or smartwatches used as non-invasive human monitoring devices.
Wristbands and watches have some similarities, but wristbands are specifically designed to track human health and fitness activities and are commonly referred to as wrist-worn wearable devices. In a typical design, a wristband does not have a display screen for notiﬁcations or limited features over the smartwatches aiming to replace the conventional watches. For example, Jawbone made the UP4 band, which works on bio-impedance sensors to monitor walking and tracking to record the sleeping cycle. It can also capture signals such as heart rate, body temperature, and galvanic skin response (GSR) using various sensors (bio-impedance,tri-axis) located on the band’s inner side.
However, because the UP4 does not have a screen display, the data can only be read using a smartphone app. Other than UP4, there are other brands such as Fitbit and Huawei Talk band B3. Based on current market trends, the market for wristbands is expanding rapidly, and there is an increasing interest in healthcare monitoring and well-being. In 2016, approximately40 a million device sales were predicted, which was after the smartwatches.
b. Wrist Watches
In today’s world, smartwatches are one of the most important wearable device types. With 50 million units sold in 2016, Gartner reported that smartwatches were the second most popular product in the wearables market, trailing only smart devices. Usually, the smartwatch monitors speciﬁc human physiological signals. Therefore, it acts as a ﬁtness tracking device that helps us to log their daily activities, such as automatically recording workouts, tracking heart rate, step counts, and calories burned. Smartwatches collect data with the help of internal and external sensors integrated with a lithium-ion battery and then send it to a cloud server or smartphone for analytics and readability.
The ﬁrst commercially available non-invasive glucose monitor approved by the Food and Drug Administration (FDA) is owned by GlucoWathcﬁ biographer (Cygnus Inc., Redwood City, CA, USA). In this system, an electrochemical signal corresponding to the glucose concentration is extracted from skin interstitial ﬂuid by reverse ionophoresis. Researchers introduced a smartwatch system, including ﬂuid and storage systems, to monitor sweat sodium (Na+) content. The device can also track daily gestures, motion, and patient monitoring. Monitoring high blood pressure (BP), also known as hypertension, is one of the most important and crucial risk factors to consider when assessing the health status of patients with cardiovascular diseases (CVDs).
Meanwhile, monitoring arterial blood pressure (ABP) is promisingly an efﬁcient way to monitor and control the prevalence of CVD patients. Therefore, monitoring BP is one of the most important physiological parameters in the ambulatory setting and monitoring an individual’s health status. In conventional pulse wave sensors, a cuff system is used to non-invasively monitor BP with optical, pressure, and electrocardiogram (ECG) sensors. These sensors encounter the limitations of large size, difﬁculty in handling, and inaccurate mobile position measurement, limiting their wider utility.
To overcome the above constraints, researchers developed a wearable system with a Hall device that can detect the changes in the magnetic ﬁeld of the permanent magnet and record the pulse wave data. This is a wristwatch that functions as a pulsometer without using a cuff. Researchers have also developed a skin-surface-coupled personal wearable device that captures waveforms of high-fidelity BP in real-time and communicates with a wireless system like smartphones and laptops. Researchers have developed a heart rate sensor based on photoelectron imaging (PPG) that detects changes in heart rate and recognizes the possibility of overcoming motion artifacts in daily life.
In Parkinson’s disease (PD) patients, a smartwatch with a gyroscope/accelerometer function can be useful for monitoring and analyzing tremor and balance dysfunction. They tested a smartwatch’s ability to quantify tremor in Parkinson’s patients, as well as its clinical correlation, acceptability, and reliability as a monitoring tool. It was later discovered to be promising. Additionally, researchers designed smart devices and developed an algorithm to detect atrial ﬁbrillation (AF) from the heart data with the rate measured utilizing a sensor and accelerometer.
The wrist-worn wearables are the main contributors to the mainstreaming of wearables. Smartwatches and wristbands are the two main sub-categories of wrist-worn(wearable devices worn on the wrist) devices that currently address two different user needs. Replacement of traditional wristwatches and their use as a smartphone extension device vs. precise and specialized tracking of various fitness activities overlap in basic fitness tracking functions. For day-to-day fitness tracking, these two types of products are likely to merge in the future. Nonetheless, more sophisticated ﬁtness tracking wristbands will likely continue to exist for users who need advanced analytics.
c. Wrist Patches
Researchers developed a ﬂexible and microﬂuidic-based patch system for real-time analysis of sweat samples. This sensor is constructed on a ﬂexible plastic substrate integrated with a special spiral-channel microﬂuidic embedded with ion-selective sensors. This system interfaces the sensing component and can analyze sweat with a printed circuit board (PCB) technology. The sensor could potentially monitor ion concentrations (H+, Na+, K+, Cl) and sweat rate, making sweat parameters more useful for monitoring human physiological and clinical conditions. Furthermore, there is still room to improve the temporal resolution of the sensors, which could make fabrication easier and faster.
Considering the requirement of soft and ﬂexible WBs, which can imitate the skin surface, a wearable lab-on-patch platform constructed utilizing polydimethylsiloxane (PDMS) with an integrated microﬂuidic collection system was developed by researchers. In this design, antibodies speciﬁc to cortisol (MX210Ab) were immobilized on a stretchable and conformable nanostructured surface with impedimetric-based detection. Under optimized antibody concentration level, the patch offers a detection limit of 1.0 pg mL−1with a detection range of up to 1µg mL−1.
The 3-DAu-nanostructure as a working electrode enables higher sensitivity, even though the sensor has the Ag–Ab complex instability limitation with no reproducibility. Researchers reported an artiﬁcial molecularly imprinted polymer (MIP)synthesized from copolymerization reaction for cortisol screening was reported by researchers. MIPs have better selectivity against cortisol as a template and greater reversibility, robustness, and reproducibility. The same group of researchers created “SKINTRONICS,” a device that uses electrodermal sensing of galvanic skin response to determine stress levels. This is a multilayer device with a 7-hour wear time and flexible hybrid skin-conformant features that capture data in real-time. Various skin-interfaced wearable-patch or sensing platforms are developing, indicating a shift toward flexible sensing.
2. Head-Mounted Devices
Visual devices with hands-free capabilities mounted to the user’s head are head-mounted tools. This wearables category has the most research types, such as helmets, glasses, and caps. These devices are currently used in surgery, imaging, and simulation; however, compared to wrist-worn wearables, commercial head-mounted wearables do not appear to be as mature. Several head-mounted display devices are currently utilized in surgery, imaging, simulation, education, and navigation tools.
Wearable systems (WSs), also known as smart glasses, are a type of head-mounted computer with a display. Researchers, for example, developed pulse-sensing smart glasses with a photo-plethysmography (PPG) sensor on the nose pad that continuously monitors heart rate. The pulse-glass sensor was also compared with a participant’s laboratory ECG system during various physical activities to cross-validate the heart rate data. Eyeglasses with a lactate biosensor integrated into the nose pad were developed to track sweat lactate and potassium levels. An interchangeable sensor is a built-in feature of these eyeglasses. They have many amperometry and potentiometric sensor stickers for the nose bridge.
For example, in sweat glucose monitoring, the lactate bridge-pad sensor can be swapped out for a glucose bridge-pad sensor. These fully integrated wireless “Lab-on-a-Glass” multiplexed eyeglasses sensing platforms can be expanded to simultaneously monitor electrolytes and metabolites in sweat fluid. Other human actions, such as barometers, accelerometers, gyroscopes, altimeters, and GPSs, can be measured using smart eyeglasses. Recon Jet, for example, is a sophisticated smart glass that displays information on the display to capture health status while running or riding a bicycle. Tear biosensing to detect vitamins and minerals, computational eyeglasses for sensing fatigue and drowsiness, medical use and health monitoring, EOG (electrooculography)-based human–wheelchair interface, sweat lactate biosensor enzymatic Gel-Membrane using eyeglasses are just a few of the smart eyeglasses that have been developed.
Wearable sensors from Cavitas are attached to body cavities like contact lenses and mouthguards. The etymological origin of “cavity” is “Cavitas” in Latin. These sensors collect data from the biological fluid contained within a body cavity. Several cavitas sensors for monitoring biomolecules in tear fluid and transcutaneous gases at the eyelid mucosa have been reported. Mouthguard sensors have also been looked into for real-time monitoring of chemicals in saliva. Researchers fabricated glucose oxidase using a mouthguard glucose sensor based on MEMS (microelectrochemical system) with enzyme membrane immobilization. Using a telemetry system, this sensor could detect glucose in artificial saliva over a range of 5–1000mol L1glucose with a stable and long-term real-time monitoring of more than 5 hr.
Similarly, researchers demonstrated an enzyme-based biosensor integrated mouthguard for detecting salivary uric and lactate provided high selectivity and sensitivity. In neonates, monitoring vital signs and symptoms and developing portable and non-invasive health monitoring devices is of great interest as infants cannot speak about the discomfort or health complaints. A paciﬁer biosensor operating as a wireless device for non-invasive chemical monitoring in the infant’s saliva was developed by researchers to monitor glucose levels. Furthermore, saliva provides new potential for monitoring of metabolites in infants and neonates non-invasively
A group of Danish researchers developed a helmet that can be used to treat depression by reactivating body parts involved in depression and allowing patients to recover quickly by sending weak electrical pulses to the brain. The helmet has been approved by the Food and Drug Administration (FDA) for the treatment of depression using weak electrical impulses transmitted to a brain part centered on depression. It has also been reported that the design of two heads-up display-based systems can mitigate physiological conditions such as nausea, seizures, and body posture.
3. E-Textiles/Smart Clothing
Smart clothing (E-Textiles) comprises intelligent or smart materials that can detect and respond to stimuli such as thermal, chemical, or mechanical changes in the environment. At the ﬁrst time, the concept “E-textiles” was deﬁned in Japan in 1989. E-Textiles are an emerging interdisciplinary ﬁeld of wearables with potential applications in healthcare, ﬁtness, health, and safety. Worldwide, various material scientist groups are developing conductive fabrics with embedded sensors on fabrics that are not the focus of the present review. These are fibers and laments made up of conductive devices and clothing material interacting with the environment/human body. Sensors allow the nervous system to detect signals, and E-textiles incorporate sensors such as electrodes woven into the fabric, which are used to analyze biofluids.
E-textiles combine a high level of intelligence and are divided into three types: (a) Passive E-textiles: Which can sense the environment/user based on sensors integrated(b)Active E-textiles: Reactive nature and can sense external stimuli from the environment, integrated with an actuator function and a sensing device(c)Very smart E-textiles: Ability to sense, react, and change under given circumstances. Usually, E-textiles possess three components; a sensor, an actuator, and a controlling unit and monitor human physiological signals, biomechanics, and physical activities. Researchers fabricated an enzyme-based detection system to detect glucose and lactate by integrating the glucose and lactate-oxidase enzyme coupled electrodes into the fabric. The same group of researchers also developed a living material and a glove integrated on the hydrogel-elastomer hybrids with genetically engineered bacteria, including genetic circuits to provide a desirable function to the material. The chemically-induced bacterial cell strains were encapsulated in a hydrogel chamber.
Bacterial strain and environment interact via a diffusion process. An inducer (IPTG, Rham) contact with the bacterial sensor programmed on ﬂuorescence response is activated. The biosensor constructed utilizing synthetic biology has promising potential in healthcare and the environment due to its mechanical ﬂexibility and low cost. E-textiles also monitor physiological signals such as heart rate (HR), temperature, and breathing rate. Researchers designed a wearable gloved-based electrochemical biosensor on the stretchable printable enzyme-based electrode, which can detect the organophosphate (OP) nerve-agent compounds. Stress-enduring inks are used for printing the electrode system. A long serpentine connection was used to connect the wireless electronic surface. Glove design consists of a typical three-electrode system with a carbon-based counter electrode at an index ﬁnger, working electrodes, reference Ag/AgCl-based electrode, and a thumb-printed carbon pad.
Herein, the index ﬁnger contains an organophosphorus hydrolase layer and acts as a sensing ﬁnger, and the thumb is a sample collector/sampling ﬁnger. Later, the practical utility of lab-on-a-glove was demonstrated in defense and food security applications. Researchers developed ahexoskin wearable vest capable of monitoring HR and BR during daily activity. On the other hand, an electronic shoe has been developed to measure the walking ability and gesture to measure lateral plantar pressure, heel strike, and toe pressure. This helps to record essential information to distinguish among gait phases. A conductive-textile-based wearable biosensor for BR sensing based on the capacitive sensing approach was developed by researchers.
A t-shirt is usually worn at the abdomen or chest position. The respiration cycle is measured by the capacitance of two electrodes placed on the inner anterior and posterior sides. Researchers reported the generation of wearable thermoelectric generators (TEG) for human body heat harvesting. TEG was used to harvest electrical energy from human body heat that further powered wearable electronics in a typical design. A smart shirt-based biosensor was designed to measure electrocardiogram (ECG)and acceleration signals for continuous and real-time health monitoring. A typical design consists of a sensor for real-time monitoring of the health data and a conductive fabric as electrodes to obtain the body signal. These wearable sensors are designed so that they ﬁt well into the shirt with small size and low power consumption to reduce the battery size. To cancel the artifact noise from the electrodes made up of conducting ﬁbers, an adaptive ﬁltering method in the designed shirt was designed and tested to get a clear electrocardiogram signal while running or performing physical exercise.