Wearable Flexible Sensors: A Review
The recent large scale and growing usage of sensors has drastically changed human life for the better. From being used for environmental purposes, to measuring intricate details of the human body, sensors have taken up an important role in the society. Wearable sensors are a step ahead, by sensing instant data about a person and sending it directly to the cloud through a wireless gateway. Wearable Flexible Sensors (WFSs) are ones which are lighter in weight, and have better thermal and mechanical properties.These are produced by laser cutting, and inkjet – printing, among other processes. Electrochemical sensing is one of the areas of extensive usage of the WFSs, especially for the monitoring of glucose, cholesterol, and pH values from 5 to 9. A prominent example is the detection of Sodium and Potassium ions, using a sensor with a Cu/PI flexible electronic layer connected to an antenna for transmission to an android smartphone.
The data accumulated at the sensor needs to be sent to the monitoring unit for which the sensor network plays a crucial role. The sensor network is selected based upon the cost for setup, power consumption and others. Bluetooth is the preferred one in general, being cheaper in cost, having lesser hardware and better compatibility. Some other examples include SHIMMER, which uses a Chipcon radio transceiver and 2.4 GHz Rufa™antenna and, Telos with an IEEE 802.15.4 complaint radio.
One of the most interesting strides in this direction is the advent of Body Area Network (BAN) in which the low powered devices are mounted on the clothing surface in a fixed position. But self – powered systems do have the advantage of not needing the battery or power unit of the wireless system to be charged every time the charging-discharging cycle is over.
Usage of WFSs, for motion like walking and running, would require the sensor patches to be bigger and more flexible, whereas for the heart rate and respiration measurements, the sensors would need to be subtle and sensitive. Scientists have gone so far as to develop wearable sensors known as electronic skins to determine the changes in temperature, pressure or even our health conditions. Some practical applications of WFSs include Carbon monoxide, and Carbon dioxide gas sensors fitted on the boots of firefighters for safety. Oxygen sensing systems on wrists are instrumental in determining the level of Oxygen flowing through the blood during respiration.
Scientific endeavours like VTAMN (France), Life Shirt (USA) and Wearable Health Care Systems (WEALTHY) (Europe) are ongoing with fiber based sensor systems for medicine and health. A Drug Delivery Pump (DDP) was developed recently to be used as a smart bandage to detect the small changes in the human body while doing physical activities.
Sensors work on gathering of data, which needs to be analysed as well as stored, with the proper security to prevent misuse. The growth of NEMS and MEMS technology would definitely help us to reduce the cost of fabrication. Untiring efforts from the scientific fraternity, and companies in general would lead to a wider reach in the near future.