Vol. 05,Special Issue 01, (ICOSD-2020) January 2020, Available Online: www.ajeee.co.in/index.php/AJEEE POLYMER BASED SENSORS FOR FLEXIBLE APPLICATIONS: A REVIEW
Priya Gupta1, Megha Gupta2, Ranjeet Kumar Gupta3
1,2Department of Electrical and Electronics Engineering, Lakshmi Narain College of Technology and Science (R.I.T), Indore (M.P)
3School of Engineering, Robert Gordon University, Aberdeen AB10 7GE, UK
Abstract:- Polymers are now being widely tested and employed in sensor fabrication.
Particularly, they can be used for flexible electronics, sensitive skin, stress/strain measurements, controlling chemical reaction, identifying gases, etc. The list of applications is long, though developments are undergoing to make their usage reliable for various areas.
In this article various types of sensors and relevant applications are reviewed. Further to it, the application of strain sensors is elaborated with its technique, application and limitations discussed in detail. Strain sensors help enumerate structural changes, by undergoing changes in their electrical properties under mechanical deformations. Potential use of this readings is their application in flexible electric and electronic devices that take readings from human body and motion detection systems for computer entertainment programs where motion of human body works as an input to a system. Moreover, strain sensors are widely used beyond that: manufacturing, machinery, airplanes, aerospace and medicine are some examples of application of them. This paper discusses the concept, trend and applications of polymer-based flexible sensors.
Keywords:- Composite; Thermoplastic; Flexible sensor; Strain.
1. INTRODUCTION
A sensor is a device that detects and responds to any type of input from the surrounding environment. The specific input could be light, heat, motion, moisture, pressure, or any one of other environmental phenomena. Sensors are used in everyday objects in human life, and beyond that there are innumerable applications of them in manufacturing, machinery, airplanes, aerospace, medicine etc. [1] Development of technologies in last years caused rapid integration of sensors to our life. The demand for flexible and wearable electronic devices are increasing due to their facile interaction with human body.
Flexible, stretchable and wearable sensors can be easily attached onto the body and detect, for example motion and heart rate through a measurement of strain. With the varying physical and chemical properties, polymers can adapt to varied applications. There has been an immense demand of polymeric materials that can reversibly or irreversibly change their chemical and physical properties upon subjected to external stimuli viz.
temperature, irradiation, pH, mechanical deformations, electric or magnetic field, ions, bioactive molecules, etc.
The polymers for such applications are developed in either of the forms of liquid, sol- gel, foams, coated nanoparticles, films or solid lumps. Currently, combination of polymer with nanoparticles are used to get the best of the properties from both, like the polymer gives structural flexibility and the used nanoparticle gives the desired property. Such combinations are called as polymer nano-composites. Researchers have already tested various applications of such specially designed polymer nano-composites for controlled drug delivery, sensing and imaging, self-healing materials, fire retardation, etc. [1-4]
2. VARIOUS POLYMER SENSORS DEVELOPED 2.1 Gas Sensors
The ability to measure potential changes at electrode can be utilized to identify presence of gas or gas leakage. Varieties of gas can be identified based on the filler nanoparticles used in the prepared polymer nano-composite. The presence of active functional groups in the base polymer is a vital condition. Variety of polymers used for this application are poly (vinyl chloride), Polystyrene and poly isoprene, copolymers of butyl and methacrylate, hydrophobic polymers, poly aniline, polyester [5-10].
They also can be used as moisture sensors when made with specific fillers, and applicable widely in areas of medicine, household and industrial setup. In this case, hydrophilic properties of the polymer or its nano-composites and the ability to change conduction due to water vapor adsorption are used. The tested varieties include 2-
Vol. 05,Special Issue 01, (ICOSD-2020) January 2020, Available Online: www.ajeee.co.in/index.php/AJEEE acrylamido-2-methylpropane modified with tetraethyl orthosilicate, poly-pyrrole nanoparticles with iron oxide and poly(vinyl alcohol) [10-12].
2.2 pH Sensors
For various industrial applications of chemistry and even in environmental sciences and biochemistry, both pH measurement and control are vital. The state of the environment wherein the chemical reaction is undergone, plays an essential role and dictates the final result product. Herein, amino fluoresce in that is covalently attached to fibre surface acts as a pH sensor in photochemical polymerizable copolymers prepared by combining acryl- amide and methylene-bis-acrylamide [13].
However, poly aniline (PANI) is suggested to work best with even aqueous media [14- 16]. Amino-ethyl cellulose strands reacted with 1-hydroxypyrrole-3,6,8-trisulphochlorane gives environment change sensitive coating, and are used as commercial blood pH sensors [17]. The treated strands joined with polyester films and deposited on ion-permeable polyurethane, gave measurable change in its conductivity, hence facilitating pH measurements.
2.3 Ion selective sensors
Here, the polymer/nano-composite is included as in an electrically conductive system or directly as a conductive component responsible for sensing. When brought in contact with an analyte, ion exchange happens which registers a proportionate electrical signal. Various researchers have reported use of silicone rubber and polyurethane/ poly (vinyl chloride) copolymer as membranes for ion-selection [18]. Silicone rubber is used for Na+ ions and poly aniline-based films for Ca+ ions. The film was prepared with a conductive polymer base having organic phosphate derivatives, like bis[4-(1,1,3,3-tetramethylbutyl)]phenyl phosphoric acid (DTMBPPO4H), as a lipophilic additive. Poly aniline film converts the ionic interaction into an appropriate electrical signal picked by the detector.
2.4 Temperature Sensors
Polarizable fluorescent dyes and aggregating polymers that respond to even a slight environmental change are used for temperature measurements. Fluorescent thermometers made up from polymer were first tested in 2003 [19]. Fabricating polymer thermometers have involved various types of homo-polymers or copolymers and their combinations with multiple compounds, including N-propylacrylamide (NNPAM), N-isopropylacrylamide (NIPAM), N-tert-butylacrylamide (NTBAM), and N-isopropylmethacrylamide (NIPMAM). And the varieties of pigments used include Benzoxadiazole-containing compounds, such as 4-N- (2-acryloyloxyethyl)-N-methylamino-7-(N,N-dimethylamino) sulfonyl-2,1,3-benzoxadiazole (DBDAE) [20].
Interesting properties observed by the dyes used for fabricating fluorescent thermometers show dual phenomenon: in hydrophobic domain, enhanced fluorescence and in hydrophilic media they show poor fluorescence. Here the aggregate formation at reduced polarity when the lower critical solubility temperature (LCST) of the polymer is attained, gives the indication of temperature variation. This is due to the reason that the dye gets copolymerized with the base polymer, resulting in a material which shows enhanced fluorescent emission at temperatures above the LCST. The conversion in the dye’s microenvironment from hydrophobic to hydrophilic and alongside causing increase in the quantum yield limit of covalently deposited dyes, is brought about by the disintegration followed by aggregation of polymer chains caused by thermal phase transition [20].
2.5 Stress/Strain Sensors
Stress/Strain sensor respond to any mechanical deformations by generating a proportional electrical response. Their working principle is based on a fact that when material is subjected to some mechanical stress such as tensile load, its resistance increases due to stretching. Therefore, good flexibility is one of the main requirements in manufacturing of strain sensors. These required properties can be ensured by suitable selection of the base polymer, based on its crystallinity. The effect of crystallinity on the physical properties is summarized in Table 1.
Vol. 05,Special Issue 01, (ICOSD-2020) January 2020, Available Online: www.ajeee.co.in/index.php/AJEEE Table 1. Effects on properties caused by increased crystallinity [1].
There are many types of materials from which strain sensors can be made of. Mainly, materials based on polymer such as polyethylene (PE) are widely used in manufacturing of flexible electronic devices. Especially, low density polyethylene is most widely used material for preparation of flexible electronic devices and sensors among other types. The reason for that is its unique properties such as flexibility, high impact strength, toughness and excellent electrical characteristics. However, excellent electrical properties just from low density polyethylene is not enough to make electronic devices or sensors.
In manufacturing of these kind of devices polymers are doped with filler nanomaterials that have unique electrical properties. These polymer nano-composites represent a relatively new group of intelligent materials. Some literatures have also been published showing a working model of stress/strain measurement based on photoluminescence phenomenon, wherein the colour changes or fluorescence emissions [21,22] is corresponding to the types of applied forces on the material viz. stretching, bending or shearing. Reviewing existing literatures, it can be summarized that the principle of strain sensors is based on the fact that resistance of the material changes when it is subjected to deformation [23].
And many requirements are needed to make high-performance strain sensors [24].
Sensitivity, response speed, fabrication cost and stability are the key requirements of strain sensors along with excellent flexible characteristics. Existing strain sensors may require low fabrication cost, however poor performance of stretching and sensitivity characteristics makes them not appropriate for use in several applications [25]. The demand in flexible and stretchable sensors is increasing due to increase in demand for flexible and wearable electronic devices.
The potential application in personal health monitoring, sports performance monitoring, human motion capturing for entertainment systems and mass measurement have leaded the development of flexible and sensitive strain sensors. Various conductive materials used as nano-inclusions include carbon-based nanomaterials (graphite, graphene(GE), carbon fibre(CF), carbon black(CB) and carbon nanotubes(CNT)), nano- metals (nano-wires and nanoparticles) []. Figure 1 represents testing example of a film- based sensor.
Fig.1. Film based strain sensor tested under tensile load [25].
Recent study undertaken by our group has identified an innovative method of manufacturing effective polymer sensors, by adding piezoelectric nanoparticles into the base PE polymer material so that final material will have combined properties of both:
flexibility and tuned electrical characteristics. And its behavior as a strain sensor will be changing its resistance in response to change in strain. Piezoelectric materials are ceramic based, and cannot be flexible, however, above mentioned method will allow to prepare flexible sensors. Discovery of the piezoelectric materials made huge impact on development of sensor systems and ultrasonic technology. Identifying sensitivity characteristic of a
Vol. 05,Special Issue 01, (ICOSD-2020) January 2020, Available Online: www.ajeee.co.in/index.php/AJEEE material includes measuring strain and resulting impedance characteristics of it. In this measurement, it is important to understand how this new material reacts to various forces and changes in its impedance. By analyzing impedance, deformation of material can be obtained as a function of strain.
3. CONCLUSIONS
Various polymer-based sensors are discussed herein in gist and give a unanimous idea of the versatility of their application areas. Specifically, the stress/strain sensor is discussed in detail for emerging flexible applications of health diagnosis, electronic skin, wearable electronics and human motion detection. Smart strain sensors demand excellent flexibility, superior strechability, high sensitivity and wide sensing range.
The polymer nano-composite method of sensor manufacturing ensures these features and is a unified and effective method of sensor preparation with multifunctional capabilities, which can be used for innumerable advancing applications. Demanding flexibility and unified accurate response under varying loading conditions. This is a novel method for facilitating rapid manufacturing of flexible stress/strain sensor manufacturing.
Particularly, the barium titanate nanoparticles-based polymer nano-composite sensor seems promising as per the literatures and will be studied further in detail with experimental findings by the group.
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