Eye on Innovation: Wearable Sensors Use Body Heat

The advantages of using wireless body sensors in healthcare are obvious: more autonomy for the patient, an improved quality of life, and better signal quality due to lack of bulky cables. The sensor nodes envisaged consist of a power source (eg, a battery), a sensor function, a processor for data processing, and a radio link to transfer the data wirelessly from the patient’s body to the doctor’s computer or patient’s personal health device. To achieve widespread use and acceptance by patients, the sensor nodes should be wearable (small and wireless) and maintenance-free.

Small size

The size of the sensor nodes is intrinsically related to power consumption, as the battery makes up most of the size of the sensor system. For this reason, researchers are developing ultra-low-power electronics that comprise the radio, the processor, and the sensor function. Such an ultra-low-power sensor chip was developed by the Belgian IMEC research institute and the Dutch Holst Centre. An interesting feature of this chip is that it can be adjusted to enable it to read diff erent biopotential signals, including EEG (brain activity), ECG (heart activity), EMG (muscle activity), and EOG (eye movement) signals.

Maintenance-free

Patient acceptance will also depend on the maintenance load. Researchers are looking for ways to use ‘green’ energy to power the sensor nodes. Photovoltaic cells (often referred to as solar cells) are a well-known alternative power source that can also be used for medical sensors. Although the sensors are often used in indoor conditions, the photovoltaic cells will generate enough energy to power the ultra-low-power electronics. Another, more obvious energy source for body-worn sensors is thermal energy. Th e heat dissipated from the body can be transformed into electrical energy using thermoelectric generator (TEG) components. Of course, the TEG components should be comfortable for their wearer. Small wrist devices that resemble a watch seem the most convenient for users. However, nighttime power generation by such devices can be interrupted, for example by putting the hand under a blanket while sleeping. Th e head is another interesting heat-generating body part with potential in this area. Harnessing heat generated from the head can provide continuous performance—even during sleep because the brain temperature is wellmaintained. It is expected that future systems will use a combination of energy harvesters (eg, photovoltaic cells and thermoelectric generators), as this will make the complete power scavenger system more robust to external and environmental perturbations.

Prototypes

Prototypes of wireless sensor nodes diff er by their form-factor, their autonomy, their building blocks, and their portability. Holst Centre and IMEC also developed prototypes based on the ultra-low-power EEG/ECG/ EMG/EOG chip and solar and thermal energy harvesting. A glimpse of the possibilities:

a. Wireless autonomous pulse-oxymeter, powered by heat from the inside of the wrist. Data are wirelessly transmitted to the doctor’s laptop.

b. EEG system in stretchable headband, including thermoelectric generators that are placed on the forehead, powering the EEG sensor node at the back of the head.

c. Diadem with integrated EEG electrodes, powered by solar energy and the heat dissipated from the person’s temples. Th is kind of EEG monitoring can be used to detect imbalance between the two halves of the brain, to detect certain kinds of brain trauma, and to monitor brain activity.

d. Wireless body area network for sleep staging, consisting of three wireless sensor nodes collecting data from 2-channel EEG, 2-channel EOG, and 1-channel EMG. Data are sent wirelessly to a computer in the patient’s room.

e. Wireless ECG patch, integrated on a fl exible substrate. It measures a bipolar ECG signal between two electrodes separated by 3cm to 4cm. Data are wirelessly transmitted to the patient’s smart health card or to the doctor’s computer.

The technology required to utilize wireless, self-powered body sensors is advancing steadily. These devices hold immense potential, with future applications limited only by the ingenuity and creativity of the researchers and engineers involved in their design. As the need for more accurate, continuous health monitoring becomes greater, wireless sensors are poised to play a key role in the evolution of healthcare.

Els Parton is a scientific editor at IMEC, Europe’s leading independent research center in the field of micro- and nanoelectronics, nanotechnology, enabling design methods, and technologies for information and communications technology systems.Julien Penders is the Program Manager at the Holst Centre, an independent centre for open innovation that develops generic technologies and technology platforms for wireless autonomous transducer solutions.