Wearable sensors for monitoring physiological parameters
Recent developments in the fields of nanotechnology and MEMS (Micro-Electro-Mechanical Systems) systems have enabled the production of sensors that, through their size, mass and reduced energy consumption, allow integration into portable or wearable systems. The miniaturization of the sensors is part of a larger picture, together with the reduction of the geometric dimensions and the energy efficiency of the information storage systems, the signal processing systems, the interface and communication mode.
Systems that integrate miniaturized sensors have become portable due to the development of complementary technologies: smart textiles, flexible electronic components and high capacity accumulators.
Figure 1. Wearable sensors
Of the multitude of sensors present in the market only a small part meets the conditions of being portable. The conditions that must be met by these devices to operate under maximum performance are summarized in the following lines.
From the point of view of the physical attributes, the shape and mass of the portable device must be correlated with the user's physiognomy. An important role is played by the aesthetic aspect, which acquires other dimensions when used for military purposes, where it is not only necessary to solve keeping in line with the current fashion, in this case of being in line with the military outfit, but also those related to camouflage or masking. Ergonomics is another aspect that must be considered as they are worn in the long term under the same conditions as clothes. From a functional point of view, the wearable device must always be available in order to be able to use it when needed, without knowing that moment, it can be easily configured for various situations and allow the operation with a enough data, so that the information obtained is relevant.
The classification of wearable sensors can be done according to several criteria, the most used being those related to the way they were built, the field of use, the way they are implemented in the detection systems, the way they communicate with each other or within the systems. of which they belong, as well as the functions they offer.
Biometric sensors are detectors that transform different signals from humans into electrical signals. The most common physical measurements measured are:
- the light;
- electrical capacity.
Each biometric measurement requires a certain type of sensor.
Sensors for monitoring the physiological parameters
The physiological parameters that require permanent monitoring to ensure the integrity of the user's health are those related to the activity of the heart and circulatory system:
- electrocardiogram (ECG);
- blood pressure;
- the level of saturation of oxygen in the blood;
- body temperature;
- ambient temperature;
- skin moisture (perspiration level).
* vital signs
Monitoring of cardiac activity
A portable device called a Holter is used to monitor the user's heart activity. He performs the electrocardiogram in a similar way to the fixed ECG installations.
For military applications, a special interest is the capacitive coupled electrode. The figure below shows the equivalent electrical diagram used to measure the activity of the heart using this type of electrode. To measure the signals coming from the electrode, an operational amplifier with a preamplifier role is used, which has an input resistance and an input capacity, between the preamplifier and the electrode an electrical capacity appears and also an electrical capacity between the amplifier and the human body at closing. of the earth circuit. Upon repeated touch of the electrode clothing static electricity is accumulated which can lead to saturation of the operational amplifier. To avoid such a situation, a resistor between the electrode and the earth can be inserted. Another important aspect is the minimum electrical capacity that must be between the electrode and the skin so that the useful signal is not lost in noise.
Figure 2 Capacitive coupled electrode
The electrocardiogram obtained with a single probe, according to the diagram in the figure, shows a high noise level compared to those obtained with two or three probes, but the useful signal can be identified reasonably. By adjusting the textile material having a higher dielectric constant, the shortcomings of the air pillow between the electrode and the skin can be compensated.
Blood pressure monitoring
Blood pressure in the arteries varies periodically in response to heartbeat. The importance of measuring this parameter is also stated by the fact that it is one of the four vital signs for keeping the user alive. Measurement can be done directly by inserting a catheter into an artery, or indirectly through a series of methods developed over time.
Pulse monitoring and blood oxygen saturation (pulse oximetry)
At the contraction of the ventricle of the heart, there is a pressure wave that is compounded by its reflection in the peripheral system and generates a pulse that circulates through the arteries about ten times faster than the blood.
Oxygen saturation in the blood is determined by examining hemoglobin (a protein that has an atom in either center) - the oxygen-bearing pigment of red blood cells. Hemoglobin comes in two forms: oxyhemoglobin (HbO2) - oxygen loaded hemoglobin and deoxyhemoglobin (Hb) - low oxygen hemoglobin. Oxygen saturation in the blood is the ratio of oxyhemoglobin to sum of oxyhemoglobin and deoxyhemoglobin. The amount of oxygen saturation in the blood is expressed as a percentage.
Pulse and blood oxygen saturation are measured using the pulse oximeter. Deoxyhemoglobin absorbs more visible light in red and oxyhemoglobin absorbs more light in infrared. Oxygen saturation in blood is determined by comparing oxyhemoglobin and deoxyhemoglobin ratios. Two methods can be used to determine these proportions: measuring the amount of light transmitted through tissue - transmissive oximetry and measuring the amount of light reflected by the tissue - reflecting oximetry.
Monitoring of body temperature
The temperature of the human body is one of the four vital signs. Temperature sensors can be classified into two broad categories: contact and non-contact. The latter have as mode of operation the measurement of infrared light emitted by the surface of the human body on the principle of radiation of the black body. The accuracy of the results obtained by this method is quite low, so it is not a viable option for real-time monitoring of body temperature. Contact temperature sensors can provide viable information, among the most used in this field being thermistors.