Printing Technique
The printing technique is the most popular technique for manufacturing these sensors. The printing may be screen printing also known as analog printing or maybe digital printing also known as inkjet printing.
In screen printing, a mask is made and is fed into the printer and is used to print the required pattern. Almost all the earlier sensors and electronic circuits were made by this screen-printing technique.
Nowadays, digital printing has arrived, in which no mask making is required. In digital printing, the pattern to be printed is digitally fed into the computer and is automatically printed by a computer command.
Although digital printing is simple, easy to use and requires fewer manual efforts, the ink used for this technique must meet some specific requirements in terms of viscosity and surface tension.
On the other hand, in the electrospinning technique, a polymeric solution is prepared and is fed into the syringe which is driven up to the metallic needle using a syringe pump.
A high electric voltage is applied along the needle and is used to eject the polymer solution by breaking the surface tension of the fluid. During the ejection process, the polymer-solvent volatizes, and the stable polymer material is deposited in a spiral manner and the desired fiber product is obtained.
Similarly, in the pattern transfer technique, a pattern is printed on the rigid surface using a mask via ordinary printing technique and is later transferred onto the flexible substrate.
A special care is needed while the pattern transfer process, as the made patterns are delicate and may break down if not handled properly. Additive manufacturing also known as 3D printing is the latest printing technique used to make complicated electronic devices or electrical circuit designs.
In this technique, the printing is performed in a layer-by-layer manner and the patterns are printed one over another on the flexible substrate. Using this technique, electronic devices with complex nano architecting or design can be fabricated effectively.
Flexible & wearable sensors have many general and healthcare applications. The deployment of a particular sensor in a particular utility relies upon the type of measurement or track to be accomplished.
The typically applied sensors are electrochemical sensors, pressure or strain sensors, temperature sensors, and so on. A few examples of body-worn sensors are proven in figure 1.
All those sensors have an active layer that measures the particular target quantity and converts that quantity into the corresponding electric signal. There are many sorts of electrochemical sensors to be had in healthcare tracking which include glucose, sweat, saliva, pH, drug transport cholesterol measurement, etc.
The underlying principle of the electrochemical sensors is that the chemical response between the sensing material and the target substance changes the electric residences of the sensor, and health tracking is accomplished in this way.
Pressure Sensor
The pressure sensor or force sensor is an important category of sensors that are used to measure many crucial health parameters such as pulse rate, blood pressure, heartbeat, etc.
These sensors detect mechanical force in the form of tension, stress, strain, and torque, and convert them into an electrical signal. There are many varieties of strain sensors to be had in healthcare such as resistive sensors, capacitive sensors, and piezoelectric sensors.
Resistive sensor
In a resistive sensor, the resistance of the sensing fabric modifications at the detection of a mechanical signal, and the extrude in resistance is pondered within side the shape of an extrade within side the electric signal.
Similarly, in a capacitive sensor, the capacitance of the sensor changes with the change in mechanical force or pressure and is reflected in the form of an electrical signal.
Piezoelectric sensor
A piezoelectric sensor is a sensor that develops an electrical voltage across its terminals on detection of the mechanical force or pressure. Many lead-based ceramic materials and polymers exhibit piezoelectric properties and are directly employed in such sensors.
According to the latest research on pressure sensors, porous structures or nano-architecture designs manufactured through additive manufacturing can be used to enhance the output or the sensitivity of these sensors.
Temperature Sensor
Another category of wearable sensors is temperature sensors. These sensors detect the change in the body temperature and reflect the output in the form of an electrical signal.
There are mainly two types of temperature sensors namely resistive sensors and pyroelectric sensors. In the resistive temperature sensor, the resistance of the deployed material changes with the change in temperature.
Hence the output electrical signal varies accordingly. Metal oxides, CNTs, graphene, and polymer composites are commonly used materials for making resistive temperature sensors.
In the case of pyroelectric sensors, the polarization of the material changes with the change in the temperature. The change in polarization is further used for the generation of the electrical signal that is calibrated in terms of temperature
