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Wearable Photonic devices technology and Flexible stretchable Optical sensors

Currently, old-style personal Medicare techniques rely mostly on traditional methods, such
as cumbersome tools and complicated processes, which can be time-consuming and inconvenient
in some circumstances. Furthermore, such old methods need the use of heavy equipment, blood
draws, and traditional bench-top testing procedures. Invasive ways of acquiring test samples can
potentially cause patient discomfort and anguish. Wearable sensors, on the other hand, may be
attached to numerous body areas to capture diverse biochemical and physiological characteristics as a developing analytical tool.


“This enables light-emitting and photo-detecting devices to make very good conformal contacts on soft human tissue, an important requirement for photon-based bio-signal sensing with a high signal-to-noise ratio.”


Physical, chemical, and biological data transferred via the skin are used to monitor health in various circumstances. Wearable sensors can assess the aberrant conditions of the physical or chemical components of the human body in real time, exposing the body state in time, thanks to unintrusive sampling and high accuracy.


Optical Sensors

Wearable health and fitness has particularly benefited from the adoption of optical sensor technology.   Optical biosensors have been embraced by the healthcare industry because they offer many advantages over conventional analytical techniques. For instance, this technology has appeared in applications including glucose sensing, laminate cure analysis, protein analysis, dosage form analysis, and many more.


Using these sensors, medical device manufacturers are able to offer real-time, label-free detection of biological and chemical substances. Label-free detection entails that a “detected signal is generated directly by the interaction of the analyzed material with the transducer.” Furthermore, designers benefit from the high sensitivity, small size, and low price offered by optical technologies.


Wearable optical sensors can track multiple vital signs such as heart rate; optical chemical sensors can now measure chemical compositions of body fluids noninvasively (for example, determining blood glucose levels by analyzing human sweat). The functionality and sophistication of heart-rate monitors will continue to increase — for example, monitoring oxygen levels and blood pressure. Other health applications for wearables include UV detection and measuring pollutant levels, both indoors and outdoors.


In recent years, several platforms have been used to produce optical sensors for refractive index sensing applications. Optical sensors are distinct in that they are resistant to electromagnetic radiation, can probe nanoscale volumes, permit unintrusive examination of biological substance at comparatively deep penetration depths, and frequently use low-cost, water-, and corrosion-resistant sensing components. These resources have been used to detect and quantify ion, protein, and viral concentration, as well as pulse, blood pressure (BP), blood oxygenation, abdominal and thoracic respiration rate, targeted localized bending, and movement.


Optical sensors, like all other sensing devices, must handle the magnified problems of proper signal-to-noise ratio (SNR), restricted dynamic range, signal specificity, and user variability in the setting of wearable devices. Furthermore, there is the issue of surrounding light interfering with signal readings, as well as poor light penetration into the skin and other bio-fluids, which is unique to optical sensing devices. New optical sensing elements and integration techniques, such as photonic textiles, innovative colorimetric and fluorometric materials, and flexile
photonics, are currently being researched to solve these difficulties.



Principle of Unintrusive Optical Sensing Devices

Unintrusive biomedical measures are often conducted optically, with a light source of a certain wavelength being revealed to the area of the skin where the evaluation is desired. The sensing device detects reflected and absorbed light, as well as refracted light, and then characterizes and quantifies the biological data (identical sensations as employed by spectrophotometer). When transmitting an optical signal through the skin, the wavelength is the most important component, since it controls how far the light can penetrate.

Depending on the required penetration depth and substantial absorption peak for the relevant sensing application, the wavelength of these light sources can range from UV to deep IR.
The detectors range from broadband photodiodes (PDs) to avalanche photodetectors and
photomultiplier tubes. Several illustrations of related passive devices for light capture,  wavelength selection, and light steering are integrated optics, diffraction gratings, narrowband optical filters, and bulk lenses.


Optical sensing devices are now commercially available. The most popular optical sensing devices used in wearable electronics are for detecting blood oxygenation and tracking pulse. The absorption and scattering characteristics of light concerning the location of the body describe each clinical or biological event when diagnosing it. The oximetry technique, which makes use of variations in the optical characteristics of hemoglobin in its deoxygenated and oxygenated state, is the most visible example . The estimation of critical physiological characteristics, such as pulse inconstancy or variability using photoplethysmography (PPG) and oxygen saturation in arterial blood using pulse oximetry, is possible by utilizing analysis of the pulsatile component of the bloodstream


Researchers at the University of Texas at Dallas (Richardson, TX) and EnLiSense (Allen, TX) have developed a sensor that can be worn on your wrist or another area of the skin to monitor your glucose, cortisol, and a number of other stress and health indicators. These devices depend on Spectroscopy performed (noninvasively) on your skin rather than pricking yourself to draw blood and test your glucose levels. The device uses zinc-oxide (ZnO) thin films on nanoporous polyamide substrates to determine protein and cortisol levels in human sweat that dictate certain health conditions. As confirmed by Fourier-transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS) optical methods, the sensor is suitable for consumer applications.


The skin may also be employed as a window to see how the hidden organs are doing physiologically. The use of functional near-IR spectroscopy (fNIRS) to study oxygenation
variations in the human brain is one such approach


Also specific to blood glucose sensing, researchers at the University of Frankfurt in Germany are using photoacoustic spectroscopy (PAS)—a combination of a windowless ultrasound between 50 and 60 kHz and an external-cavity tunable quantum-cascade laser that spans 1000 to 1245 cm-1—to obtain a mid-infrared spectrum when absorption of glucose molecules creates a sound signature that records sugar levels in skin cells.


All of these measurements depend on networks of sensors that detect and measure a multitude of physiological and electrophysiological signals. “Varying optical wavelengths are used with sensors that have algorithms for extracting useful data for accurate measurements,” says David Simpson, director of business development for mobile sensing for Integrated Device Technology in San Jose, California. Data can then be stored in skin-based non-volatile memory devices (resistive random access memory or silicon-based flash memory) or wirelessly transmitted to external smart devices (smartphones or tablet computers) via an integrated Bluetooth unit.


Recent advancements in manufacturing and packaging processes have supported the
low-cost embedding of many OE devices and sensing devices on a chip. Furthermore, these
chips are extensively employed in many kinds of physiological, biomechanical, and biological
sensing, examining and gathering biomedical data remotely, thanks to a combination
of wireless 3G and 4G technologies.


Through the implementation of optical wearable technologies, such as heart rate, blood pressure, and glucose monitors, among others, individuals are becoming more empowered to generate a wealth of rich, multifaceted physiological and environmental data, making personalized medicine a reality.


Heart Rate Monitors (HRMs)

PPG-based wearable optical heart rate monitors (HRMs) have grown extremely popular, with a slew of tech businesses and products being developed and marketed. Sony, Microsoft, Apple, Motorola, FitBit, MioGlobal, and Masimo, among others, have all developed optical PPG sensing devices that may be worn on the wrist, around the chest, or even in-ear designs using headphone-based optical sensing devices that operate in reflection mode.


Optical HRM functions by beaming the light into the body and detecting how light is scattered from blood flow. The technique works best in parts of the body where physiological factors that are not connected to blood flow restrict the quantity of light that is scattered or absorbed. When collecting data from parts of the body with a variety of tissues, such as bone, muscle, tendons, and so on, the accuracy may be reduced. Parts of the body undergo greater movement when the body is in motion, such as the wrists and ankles, which has a detrimental influence. There are different spots on the body where wearables can be worn to collect the best pulse.



Continuous Glucose Monitoring (CGM)

Diabetes is a disorder that affects around 415 million people worldwide. The most prevalent kind of diabetes is type 1 diabetes, which occurs when the body attacks the cells in the pancreas, preventing it from producing insulin.


CGM, allows patients to monitor their blood sugar levels at all times using a wearable device that gathers data on the body’s blood sugar levels. CGM devices are available 24 h a day and incorporate alerts that sound when a patient’s blood sugar levels are abnormally high or low. Finger pricking is no longer required or is much reduced. This is beneficial to the patient’s health and decreases the possibility of overestimation. Furthermore, CGM enables individuals to analyze data to spot trends and patterns, which may then be shared with their Medicare providers.

Optical sensing technologies can circumvent many of the limitations associated with electrochemical sensing devices, such as sensing device sensitivity and stability being dependent on the enzyme utilized and interference with active substances (acetaminophen, ascorbate). Numerous optical recognition approaches have been proposed in the literature, including near-IR detection and Raman spectroscopy for unintrusive detection and fluorescence-based sensing devices for implanted systems.


Senseonics (Senseonics, Inc., Germantown, MD, USA) has recently successfully applied this last category of optical sensing technologies to develop the Eversense sensor, a fully implanted CGM system  that provides real-time blood sugar measurements through an external coupled transmitter for an expected lifetime of 6 months.


Flexible and soft materials

The development of flexible and soft materials is essential for wearable electronics because of their unique chemical, electrical, and mechanical properties. Traditional materials for wearable electronics are mostly metals and semiconductors with relatively poor mechanical flexibility and stretchability. Recently, organic or polymeric materials are gaining more attention from the community due to their superior mechanical flexibility.


“However, ultra-flexible and stretchable electronic devices utilize the low system modulus and the intrinsic system-level softness to solve these issues,” says Dae-Hyeong Kim, an associate professor of engineering at Seoul National University in South Korea.


Stretchable Semiconductors Detect Ultralow Light reported in Dec 2021

A stretchable semiconductor material developed by researchers at Georgia Institute of Technology (Georgia Tech) is able to act like a second layer of skin and detect ultralow levels of light. The researchers believe that their soft flexible photodetectors could enhance the utility of medical wearable sensors and implantable devices and support additional applications.


Since conventional flexible semiconductors break under a few percentages of strain, the work offers a significant improvement, according to Oliver Pierron, a professor in the George W. Woodruff School of Mechanical Engineering.

“Think of a rubber band or something that’s soft and stretchable like human skin, yet has similar electrical semiconducting properties of solid or rigid semiconductors,” said Canek Fuentes-Hernandez, a co-principal investigator formerly in the School of Electrical and Computer Engineering and now an associate professor at Northeastern University. “We’ve shown that you can build ‘stretchability’ into semiconductors that retains the electrical performance needed to detect light levels that are around hundred million times fainter than produced by a light bulb used for indoor illumination.”


Though the material has been integrated into a photodetector and tested for electrical functionality, more testing and optimization is needed to show the material’s “stretchability” under multimodal loads and its shelf stability, the researchers said.

“What’s exciting is what these materials and the devices will enable us to develop — namely, the concept of intelligence systems,” said Samuel Graham, former chair of the Woodruff School of Mechanical Engineering and now dean of engineering at the University of Maryland. “You have functional surfaces that combine sensors that monitor all kinds of physical properties.”



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