Closing the Gap with Wearables offers a wealth of knowledge and practical insights for those interested in the technological advancements and applications of wearable devices in healthcare.
To purchase the book with a 20% discount, click on the button above and be sure to use discount code AFL04.
Batteries
Nearly all wearable devices use batteries, and batteries are not improving very fast. If batteries had improved as fast as semiconductors over the last 50 years, you would be able to buy a battery the size of the head of a pin that would power your car and cost one cent!
This will never happen. Batteries are approaching the limit of chemical energy density. Figure 13.1 shows the relative capacity of several battery types compared to gasoline, which is nearly the highest energy density of any material. The horizontal scale is Megajoules per Liter. One Joule is the energy of one Watt for one second; barely enough to detect the temperature change at your fingertip. Notice that the chart is logarithmic, so the difference between the best battery and gasoline is more than 10 times. Gasoline, however, has serious safety issues. There is a trade-off between energy density and safety.
Figure 13.1 Relative Capacity of Several Battery Types.
Source: Voler Systems
Lithium batteries are about as far as we are generally willing to go in sacrificing safety. There have been many news stories about lithium batteries catching fire. Lithium batteries require a safety circuit to protect them from catching free or exploding. Even with the safety circuit, though, there have been cases where the batteries caught fire.
The one technology that has a higher energy density than chemical is nuclear energy, but that is not going to be used in wearable devices in the foreseeable future!
Since batteries are not improving, we must look at other ways to extend the time between recharging. There are five parts of a wearable device that can be major power consumers:
The power usage of wireless communication is discussed in the chapter on Wireless Communication.
Display Power
The power used by displays varies widely from grayscale LCD displays, like those on wristwatches, to color LED displays. See Figure 13.2. 
Figure 13.2 Display Power vs Cost.
Source: Voler Systems
Only grayscale LCD and digital paper are in the microwatt range, and they have limitations that often make them unsuitable. Digital paper is a
rather new technology that consumes virtually no power unless the display is changing. It is ideal for mostly static displays like digital books. It is notable to display video. Digital paper also has only limited ability to display color.
Smartwatches usually use OLED technology, which is fairly energy-efficient, but it still can be a major power consumer in a wearable device. Like LEDs and color LCDs, OLED displays can have vivid colors and display video. The power consumption of these display technologies is a major reason displays are not used much in wearable devices.
Sensor Power
The power used by sensors varies widely, as shown in Figure 13.3.
Cameras are power hogs, and even more so if they need illumination for use at night or inside dark places. They are widely used by police to record activity, but they cannot run continuously for an eight-hour shift without a really big battery. In addition, if the video is to be transmitted wirelessly, it requires a power-hungry technology such as Wi-Fi or cellular.
GPS uses a moderate amount of power, as does a pulse oximeter. Pressure sensors can be quite low. Measuring ECG or EKG (electrocardiogram) is very low, as is a 9-axis motion sensor. Microphones and light intensity sensors can be even lower.
The lowest power is a 3-axis motion sensor. At just a few microwatts they are often left on, while other things are put in sleep mode to conserve
Figure 13.3 Power Used by Sensors.
Source: Voler Systems
power. The 3-axis motion sensor can activate the processor when a preset level of motion is detected. Then the processor can check the sensors, see
what is going on and decide if it needs to do anything. If not, it can quickly go back to sleep.
Because of their low power, low cost, and small size, motion sensors are put into all kinds of devices.
Microprocessor Power
Microprocessor power consumption varies widely depending on the function of the processor. The processor in a Windows PC may consume 50 watts or more, but it can process real-time video. The processors used in wearable devices typically consume just a few milliwatts, and new models are being introduced that consume only microwatts of power. They are fine for most of the things that need to be done in a wearable device.
In order to limit the power consumed by microprocessors, energy-intensive things like video processing and advanced encryption are usually not done in wearable devices.
Software Power
Although software is not a physical device that consumes power, it runs in a processor and determines how much power the processor uses. Functions such as encrypting data can consume a lot of power while reading data from a sensor and storing it in memory consumes very little.
Software has another important role. It determines when devices are used. Sensors, for example, do not need to run all the time. Many physiological parameters, such as temperature and blood oxygen, change slowly. A measurement every minute or slower is usually sufficient. The rest of the time, the software can turn off the sensor. The software also directs the processor when to sleep and when to send data wirelessly. Thus, the software is critical to controlling the use of power.
Turning off the wireless communication has a trade-off called latency. If data is transmitted only once a minute, it takes a minute to get an update. This is a latency of one minute. If a person is waiting to see the data, one minute is a very long time. If a wearable device receives instructions wirelessly (for example, instructions to send data), the wireless receiver must not be sleeping, or it may miss the request. If it is sleeping, the request must be sent over and over until the device wakes up. If it wakes up once a minute, it also has a latency of one minute.
The solution to long latency is running the processor or wireless communication more, which consumes more power. When listening for a signal, the wireless communication only needs to be in receive mode, which consumes low power. Thus, the latency when waiting for a device to respond can be short without consuming a lot of power.
An alternative to batteries is to use energy harvesting. Power can be picked up from motion, temperature differences, chemical reactions, radio signals, and sunlight. The problem is that most sources of energy harvesting generate microwatts of power, while most wearable devices consume milliwatts of power. For this reason, most wearable devices do not use energy harvesting, although work is being done to improve energy harvesting.
Photocells for energy harvesting are well-known. They are used to power houses and calculators. You can get a lot of power, but there are some issues. To get milliwatts of power requires rather large cells. A photocell one square centimeter in size will generate about two milliwatts of power in direct sunlight perpendicular to the plane of the photocell. Also, wearable devices are used indoors or under clothes, where there is little light. For these reasons, they are rarely used on wearable devices.
Motion is a good source of energy. Wearable devices are often in motion. The motion can be converted to energy by crystals that output a voltage when under pressure. A small mass, such as a piece of steel placed next to the crystal, will alternately apply pressure or remove it when the device moves back and forth. One such device outputs up to 18 mW at 0.5 g of acceleration, which is an aggressive motion. At 0.1 g it only outputs 1 mW. It is 52 millimeters in diameter and 24 millimeters thick, and it costs about $50 in small quantities. The price is far more than the price of a battery, and the size is bigger than many batteries, making it unattractive in most applications.
Temperature differences generate energy when there are dissimilar metals exposed to the temperature difference. This is the Seebeck effect, which makes thermocouples work to measure temperature. For small temperature changes the output is quite small, so they are not used in wearable devices.
Radio signals contain power that can be picked up and converted to electrical energy. Wireless charging can transmit many watts of power over a few centimeters. It’s difficult to transmit power very far, because the energy drops off as the square of the distance. This means that 10 watts at a distance of 1 cm would become 10 milliwatts at 10 meters. Wearable devices may not stay within 10 meters of a transmitter. Installing a wireless power transmitter is inconvenient, too. Power can be transmitted directionally at high frequencies, but the high frequencies are harmful to animals and people at useful power levels, thus this technology is not used much. It is possible to detect when an animal or person passes into the beam of the energy, but it must be ultra-reliable. Failure to turn off the beam could cause serious injury.
Chemical energy is sometimes used in implanted devices. The body is a great chemical factory and a source of power. The right materials can tap into the power. Although the power is quite limited, implanted devices are usually very low-power. For externally worn wearable devices, chemical energy is not used for energy harvesting.