Physiological Sensors

Table of Contents

• Use Cases in Health Care
• Glucose Monitoring
• Insulin Pumps
• Chest Patches for Cardiac Monitoring
• Watches for Cardiac Monitoring
• Implanted Cardiac Monitors
• In-Hospital Monitoring
• Sleep Monitors
• Hearables
• Stimulation to Treat Disease or Pain
• EEG Monitoring
• Magnetic Field Devices for Migraines
• Augmented Reality and Virtual Reality Glasses
  

• Physiological Sensors Used in Wearable Devices

• Body Temperature Measurement
• Motion Measurement
• Heart Rate Measurement
• Blood Oxygen Measurement
• ECG, EEG, and EMG Measurement
• Respiration Rate Measurement
• Blood Pressure Measurement
• Blood Glucose Measurement
• Best Sensor Placements on the Body
• Adhesives for Attachment

DOI: 10.4324/9781003304036-14

Use Cases in Health Care

Glucose Monitoring

There are now patches that monitor glucose continuously. They are called Continuous Glucose Monitors, or CGM. They are small enough to not inter­fere with daily activities, and they can be worn in the shower. These devices are more comfortable and less intrusive than collecting blood from the finger several times a day. Many young people are uncomfortable having their friends see them doing a glucose test, because it makes them seem different. This makes a CGM device attractive. In the future, calibration may not be required, making it even more convenient. They have a single needle that penetrates the skin to sample blood, although they are comfortable and can be worn up to two weeks. It is necessary to calibrate the device when it is first installed. The patient pricks a finger and gets a reading with a standard glucose measurement device, which is entered into the CGM device, typically using a smartphone. Two leading players in this market are Abbott Diabetes and Dexcom.

There is a glucose monitor that fts into the eye. It has appeared in many reports, but it is only a laboratory study device. It has not been made into a commercial device. The accuracy has not been reported. It is unlikely that it meets FDA requirements for accuracy.

Insulin Pumps

Wearable insulin pumps have existed for many years. People in the diabetes community have nefariously hacked them to integrate with continuous glu­cose monitors, creating a complete closed loop wearable pancreas replace­ment. Notably, manufacturers of insulin pumps and continuous glucose monitors have reduced their efforts to make their devices resistant to hack­ing. There seems to be an acceptance by these companies and the FDA that it is ok for hackers to create their own devices, as long as they don’t sell them. There is now a closed loop system that uses a CGM device from one company and an insulin pump from another. This is a novel example of the FDA approving a combination of two devices from two different companies. See Chapter 1 for further information.

Chest Patches for Cardiac Monitoring

In recent years, several companies have developed patches that adhere to the chest for cardiac monitoring. The patches can be worn for a week at a Physiological Sensors

time, even in the shower. This gives a more complete picture of a person’s cardiac condition, particularly for events that don’t happen regularly, such as atrial fibrillation. They can collect and transmit data wirelessly. They are replacing Holter monitors for collection of cardiac data, because they are more convenient, lower-cost, and they can provide several days of data. It is common to send data wirelessly, allowing these devices to be used for real-time monitoring. What is holding back adoption is the lack of artificial intelligence software to monitor the data and detect anomalies that should be reviewed by healthcare professionals. With continuous monitoring, there is more data collected than anyone can effectively review. This limits the usefulness for real-time monitoring.

Cardiac monitors record ECG, but they may also monitor respiration, tem­perature, and blood oxygen.

Watches for Cardiac Monitoring

According to HealthTech Magazine*, smartwatches are now helping health­care providers collect and analyze a wider swath of data from patients between their appointments or after surgery. This data provides crucial and very valuable insights that can help identify possible and proper treatment.

[*https://healthtechmagazine.net/article/2020/01/smartwatch-where-will­it-go-2020]

The smartwatch trend, which is continuously growing sales every year, has inspired organizations such as Ochsner Health System in New Orleans. In 2015, Ochsner launched a pioneer program to better track uncontrolled hypertension among its patients. Stanford University’s study in 2019 revealed that the Apple Watch could identify heart rhythm irregu­larities, such as atrial fibrillation, a leading stroke risk, which can be detected with 84 percent accuracy. And to utilize this innovation, Ochsner now also utilizes the Apple Watch, which has benefted the doctors. The smart wearable device will send alerts about a patient’s declining condi­tion and send data to the healthcare in-charge’s wrist, even if they are wearing gloves.

Samsung Electronics Co. Ltd launched the Galaxy Watch 3 in August 2020. The Galaxy Watch3 features a PPG sensor to monitor SpO2 levels. Also, with its enhanced accelerometer, the Watch3 smartwatch automatically detects hard falls. This smartwatch also records REM cycles, deep sleep, and total sleep time to score and help improve the quality of sleep. 

These sensors are becoming popular in other wearable devices. Although watches are intended for personal health and not as medical devices, the Apple Watch did get FDA approval for detecting atrial fibrillation.

Implanted Cardiac Monitors

Implanted devices are a separate class of wearable device. They require a medical procedure to install them, but they can provide better data, because they can be placed where they work best. Implanted pacemakers and def­brillators have existed for many years. More recently, tiny cardiac monitors have been developed. Medtronic makes the Reveal LINQ. It is only 1.2 cubic centimeters in volume, and its battery lasts for three years. At that time, it needs to be removed. It has a Bluetooth wireless connection. It is intended to monitor patients for various heart conditions that do not show symptoms for long periods of time. For many disease states, however, an external monitor can be used, reducing the invasive risk and cost.

In-Hospital Monitoring

Instead of having a nurse collect vital health data, hospitals are beginning to use a patch that is applied when the patient checks in. The patches can col­lect data much more often and at lower cost. The data can be sent wirelessly to the patient health record. These monitors can measure temperature, heart rate, ECG, breathing rate, blood oxygen, and blood pressure. As is described here, blood pressure measurement without a cuff is only just becoming available, so it is currently not generally used for patient monitoring. There is work being done to have artificial intelligence (AI) software evaluate the data and predict adverse events, sending help to the person hours before the event can happen.

Sleep Monitors

Sleep monitoring is a rapidly growing area for wearable devices. There are many consumer products that monitor sleep, but there are also medi­cal devices. A medical sleep monitor is typically a patch that adheres to the abdomen. Wearable devices can monitor sleep better than going to a sleep lab, as it may be difficult for a person to sleep normally in an unusual environment. Wearable patches are normally wireless, so the per­son’s movement is unconstrained. A wearable sleep monitor can be worn Physiological Sensors 

for several nights and collect more complete sleep information compared to one night at a sleep lab, and the cost is lower than a sleep lab. They typically monitor ECG, breathing rate, SpO2, motion, and temperature. Sometimes skin impedance, also called galvanic skin response or GSR, is also measured.

Hearables

There has been development recently around hearables—devices in the ear for health monitoring. The ear is an excellent location for detecting heart rate, SpO2, and motion. With millions of people already wearing earbuds to listen to music, it seems natural to add sensors to the ear buds. This has not taken off, perhaps because the elderly who most need medical attention are less likely to want to use earbuds, or they have conficts with their hearing aid devices.

Stimulation to Treat Disease or Pain

TENS, or Transcutaneous Electrical Nerve Stimulation, has been used for years to treat pain, such as back pain. An electrical signal is applied between two electrodes. Certain frequencies have been found to have thera­peutic value.

More recently, other wearable stimulation devices have appeared. There are devices that are implanted in the spine with an external control to treat intractable pain. This has become a major medical device area with several large competitors.

There is now a device worn on the wrist that treats essential tremors. Made by Cala Health, the device is turned on and provides stimulation that is noticeable but not painful for approximately 45 minutes. Relief is said to last for many hours after treatment.

EEG Monitoring

EEG monitoring has been changing. New devices are getting useful informa­tion without a large number of leads placed all over the head and requiring shaving of hair. Ceribell makes a head band for monitoring the brain during a seizure. It has 10 electrodes around the band. No hair needs to be shaved. The EEG data is run through an algorithm to identify if the patient is having a seizure.

Magnetic Field Devices for Migraines

Magnetic fields are being used to stop or reduce migraines with a device that mounts on the head. Due to the power consumption, these devices may need to plug into power or use large batteries, so they may not be consid­ered wearable devices.

Augmented Reality and Virtual Reality Glasses

Augmented reality glasses allow you to see your surroundings. The added content appears in front of the real world. You can get instructions or access patient records without touching anything. When fully implemented, this could really help in surgery and ER wards.

Virtual reality glasses display an entirely virtual world, and the wearer does not see the real world at all. This is primarily useful for training, where you may have a virtual hospital and virtual patients. A computer can control the pace of learning.

Training of healthcare professionals is becoming more important as tech­nology keeps changing the way they perform their jobs. Some training is best done without a live patient. Other training requires a live patient. Both training approaches are benefiting from the use of augmented reality glasses and virtual reality glasses.

Physiological Sensors Used in Wearable Devices

Body Temperature Measurement

Temperature sensors are low-cost and there are many types available. However, measuring core body temperature, which is usually desired, is not simple. Skin temperature is often lower than the core body temperature, especially at the extremities. Many wearable devices are on the wrist, which is not a good place to take this measurement. The forehead, under the arms, and in the ears are good places, but most wearable devices are located elsewhere.

Hearables, wearable devices in the ears, now exist. Many people wear earbuds for long periods to listen to music. Sensors can be placed in these devices without making them much bigger. Besides tempera­ture, the ear is a good place to measure heart rate and blood oxygen saturation. Physiological Sensors

Software can combine data from multiple sensors, “sensor fusion,” to determine when skin temperature is likely to be near the core body tem­perature. If the skin is wet, the person may be perspiring or in a shower. In either case the skin temperature is not a good indicator of core body tem­perature. If the person has been outside in cold weather, the skin is likely to be cold. If the room temperature is moderate and the person has been mod­erately active, the skin is likely to be close to the core body temperature.

Wet skin is likely to be colder than the core body temperature. Moisture on the skin can be measured using galvanic skin response or GSR. The electrical impedance of the skin is measured with electrodes. The ambient temperature can be measured with a temperature sensor that is kept away from the skin. Activity can be measured with a motion sensor. These sensors can be integrated with software to produce sensor fusion.

Motion Measurement

Motion of the body has been measured for decades. Step counters were originally mechanical devices used to estimate the distance walked or run. Now, low-cost motion sensors have replaced the mechanical devices. They are very small and consume very little battery power, so they are used in many devices as auxiliary sensors, sometimes for sensor fusion.

Step counting is a good measure of activity and is used in many con­sumer devices. The manufacturers of the motion sensors have developed advanced software that is able to measure step counts when mounted on the wrist or other places on the body. This is quite a feat, although it is not perfectly accurate. The software can work with the motion sensor on the wrist, ankle, or torso. The software algorithms are even able to determine with reasonable accuracy that a person is walking, standing, or sitting.

Motion sensors are also used to measure motion during sleep. With soft­ware interpreting the data, it is possible to measure the stage of sleep with good accuracy. This is important for sleep analysis, as well as for consumer products.

Motion sensors can measure gait, which can be used to indicate several conditions, such as dementia and Parkinson’s Disease. The specificity of the indication is only moderate, but it is good enough to refer people for further diagnosis by a healthcare professional.

Another use is for dead reckoning—tracking someone’s motion. Motion sensors are only accurate for a few minutes. They accumulate errors over time, but because of their low power they can be used as a substitute for.

GPS, which is moderately power-hungry. The GPS can be turned on only every few minutes to save battery power, and the motion sensor can track the position while the GPS is off.

Heart Rate Measurement

There are several ways to measure heart rate. ECG electrodes may be used. Two electrodes located on most parts of the body can pick up a good enough signal to measure heart rate, even where the signal is not sufficient for an ECG measurement. A pulse plethysmograph (PPG) sensor can be used to measure heart rate. It was originally used to measure blood oxygen, but the heart rate is a stronger signal that needs to be removed to sense oxygen. These sensors work quite well, even on the wrist.

It is also possible, but infrequently done, to measure heart rate with a pressure sensor located over an artery. The pressure pulse can be sensed just as a person can feel the heart rate by placing fingers on the inside of the wrist.

For either an ECG electrode or a PPG sensor, it is important that there be good contact with the skin. On a wrist device this may be uncomfortable, presenting a design challenge. In many cases heart rate is not needed con­tinuously, and software can determine when the signal is good. When it is not good, the heart rate can be ignored.

Blood Oxygen Measurement

Oxygen saturation or SpO2 is only measured with a pulse plethysmograph (PPG) sensor. A pulse oximeter uses a PPG sensor. Originally, they were clipped onto a fnger. Light of at least two wavelengths is passed through the fnger. Both wavelengths are sensitive to the pulse. One wavelength is absorbed by hemoglobin. One signal is subtracted from the signal of the other wavelength to remove the pulse. The result is an accurate measure­ment of SpO2. This is a transmissive pulse oximeter where the light passes through the body.

The transmissive PPG measurement is only possible on the finger or ear, where light can pass through. For other locations, a reflective PPG measure­ment is used. The measurement uses similar wavelengths of light and sub­tracts the pulse from the signal. The reflected signal is much weaker than the transmitted signal, so the measurement is more difficult. More signal Physiological Sensors processing is required. It does not work where the body does not have good blood perfusion, but good results have been achieved on the wrist.

ECG, EEG, and EMG Measurement

All of these signals are voltages generated by the body. The sensor consists of electrodes to pick up the voltage and an amplifier to measure the tiny signal.

ECG (electrocardiogram) has been measured for a long time with labo­ratory equipment that typically uses 12 leads and wires to the equipment. Signal processing has improved to the point that single lead ECG measure­ments are almost as good as 12-lead measurements. Wearable devices almost always use two contacts (which is called single lead). The contacts are usu­ally dry for convenience, although it is easier to get a good signal with wet electrodes. The contacts need to be spaced at least four cm apart to get a good signal. ECG cannot be measured on the legs or arms. On the head it would be obscured by the EEG signal. Successful measurements have been made in pants, where the electrodes are on the lower abdomen. There are implanted ECG monitors, but they are not as widely used as non-invasive wearable devices.

EEG (electroencephalogram) has been measured with electrodes placed on shaved areas of the scalp with wires to the equipment. Many non-critical EEG applications, such as for consumer products, use only two electrodes. The electrodes can be on the temples where the elec­trodes may be attached to glasses or a headband. Successful measure­ments have been made with a helmet that has electrodes at the end of projectiles that reach the scalp without shaving any hair. This is critical, as a wearable device that requires preparation, such as shaving, is incon­venient. The measurement can only be done on the head. The signal elsewhere is too small.

EMG (electromyogram) is the measurement of the signal that activates muscles. It is not a common medical test. It can be used to sense what muscles are moving, or in the case of people with stroke, the muscles that the brain has directed to move, even if they did not move. The electrodes need to be carefully placed. On the forearm, for example, you can sense the individual muscles moving the fingers, but they are only a few millimeters apart, and the arm does not have a good reference for accurately positioning electrodes. 

Respiration Rate Measurement

The number of breaths per minute can be measured with several tech­niques. An old and still viable way is with a chest strap. A sensor measures the change in length of the flexible strap as the chest moves. This is fine for a shirt with sensors. Most wearable devices are small or not located on the chest, and this technique is not suitable in those cases.

A nasal cannula can be used in a hospital. It is not convenient or com­fortable for a wearable device.

Thoracic impedance is an accurate technique. The electrical impedance of the chest varies as the chest expands and contracts. A sensor like the GSR sensor measures the impedance. It is important to measure deep in the tissue and not the skin at the surface, so that changes in skin conduc­tivity do not interfere. This sensor works well on the chest, but it has not worked on the wrist, although efforts have been made on this. The arm, being narrower than the chest, has a much higher impedance. Thus, the overall impedance is dominated by the arm, and the signal is too small to measure accurately.

Respiration rate can also be measured directly from the ECG signal after filtering out the heart rate. If the breathing rate is well below the heart rate, this can work, as the heart is affected by the movement of the chest during breathing.

Blood Pressure Measurement

Up until 2021, the only way to get medically accurate blood pressure mea­surements without calibration on each person was with a cuff. This has changed. It is now possible to do this with at least two technologies.

A PPG sensor with advanced software can measure blood pressure. This was a big challenge, partly because the measurement is sensitive to skin color and motion. The different wavelengths of light pass through the skin differently. A clinical study was done by Valencell, Inc, that achieved medi­cally accurate blood pressure measurement without calibration*. Now a small wearable device can measure blood pressure. The study used a sensor on the fnger. They are working to achieve this result on other parts of the body.

[*https://valencell.com/featured/valencells-cuffess-calibration-free-blood­pressure-monitoring-technology-selected-to-present-at-american-college-of­cardiology-annual-scientifc-session/]

Another technique was just disclosed by PyrAmes Inc. They detect the pressure of the pulse passing through the arteries. The pressure causes movement which is detected by a capacitive sensor**. Software needs to not only calculate the pressure but determine when the signal is not accurate. This was not a clinical study, so it still needs work before being used in a medical device.

[**in the journal Sensors, published by MDPI, Basel, Switzerland, June 2021]

Another technique, Pulse Transit Time, has been used, but has not achieved medical accuracy without calibration. It measures the time differ­ence between the beat of the heart (measured with ECG electrodes) and the arrival of the pulse at an extremity (often measured with a PPG sensor). The time difference is proportional to the blood pressure. Although it is not medically accurate, it can be used in applications where the change in blood pressure is important, and the absolute value is not. Detecting a change is often important to indicate the need to take measures, such as visiting a healthcare provider.

Blood pressure has also been measured in a laboratory setting using the ECG signal and advanced neural network software. It is not medically accu­rate, however.

Blood Glucose Measurement

The standard for blood glucose measurement is a sample of blood from the finger placed into a sensor. A very large amount of money has been expended to find a non-invasive glucose measurement technique. Today this has been partially achieved, although not fully, with continuous glucose monitors (CGM).

Abbott Diabetes has a small patch that is worn on the arm held in place with adhesive. It has a needle that passes through the skin to reach the blood, so it is technically invasive, but it is comfortable to wear for more than a week at a time. It requires calibration with a finger prick each time the disposable device is replaced. However, this is much more convenient than pricking your finger several times a day.

Another technique is a patch with microneedles. The microneedles only pass through the outer layer of the skin. They don’t look or feel like needles. The texture is similar to sandpaper. It is challenging to get good results with­out intimate contact with the blood. This device also requires calibration.

Both sensors must be placed where there is good blood perfusion. They do not work on the wrist. 

There are also implanted glucose monitors. However, most people prefer a less invasive device.

Best Sensor Placements on the Body

Many wearable devices are designed to go on the wrist for convenience, but that is a poor place for most measurements.

ECG measurement is generally only done on the torso. The chest is clearly the best place because of proximity to the heart. Successful measure­ment in pants has been done using dry electrodes even when the person is moving. The Apple watch and other wrist-worn devices can only measure ECG when the opposite hand touches the watch, providing a signal that is from one arm to the other. This works well, but only while the watch is being touched.

PPG sensors for blood oxygen measurement requires good blood perfu­sion. The wrist is challenging, but it has been used successfully. Getting a pulse measurement from a PPG sensor is easier and can work nearly any­where on the body.

Temperature measurement is best done under the arm, on the forehead, or in the ear. In general skin temperature is not reliably at the core body temperature, except at these locations. On the other hand, it can be useful to measure the temperature at extremities, not core body temperature, to detect problems with circulation and certain diseases.

Figure 11.1 PPG Sensor Placement. Physiological Sensors

Adhesives for Attachment

Since the torso is the best place for most measurements, there are many devices that attach to the torso. They usually use an adhesive patch. This is convenient, but there are limitations. A patch can only be applied to the skin for one or two weeks before the skin becomes irritated and begins to break down. For most applications, the adhesive needs to perform well while bath­ing or showering.

An alternative is to have sensors in a shirt. When the shirt is put on, the sensors begin recording and transmitting data. Shirts have limitations, too. They must be washed, and it is challenging to have electronics survive many times in a washing machine. Typically, devices are tested for 50 washes, but some clothing is washed more than that during its lifetime.

Sensors in a shirt need to be flexible, and ideally stretchable. Flexible circuits have been around for decades, but they need to be protected from moisture. Good sealing methods are more recent. Stretchable circuits are barely out of the laboratory. They are not yet widely used in wearable devices.