Although the force/pressure applied onto a textile substrate through a uniaxial compression is constant and independent of the yarn direction, it should be noted that such mechanical action causes a geometric change in the substrate, which can be identified by the reduction in its lateral thickness. Therefore, researchers investigate the influence of the fabric orientation on both knitted and woven pressure sensors, in order to generate knowledge for a better design process during textile piezoresistive sensor development.
In addition to the role as a social communication tool [1
] and the function of protection against environmental factors [2
], textiles, by wrapping the body, can be the “territory” to detect internal (corporeal) and external (environmental) stimuli. This is the concept of Smart Textiles, in other words, fabrics that can interact with the environment/user by detecting and, sometimes, reacting and adapting to mechanical, thermal, chemical or electrical stimuli [3
]. One of these stimuli is the pressure that can be exerted by the body itself, through the bones and blood pressure, or through external objects, such as the use of wearable medical devices. In turn, the measurement and action upon said body pressure can, in some cases, prevent the development of skin diseases, like pressure ulcers [4
], and cardiovascular diseases, through pulse pressure monitoring [5
], as well as being inserted into haptic feedback systems to generate tactile information for amputees [6
Given their wide applicability, pressure sensors in a wearable textile form can, non-invasively, measure mechanical action on the body that can be used, e.g., to quantify performance or identify possible harm. To meet this sensing requirement, flexible sensors seem promising, as they can mechanically deform, granting them the ability to sense the stimuli at its origin, where the signal is most accurate [7
]. Regarding the perspective of comfort, they can also meet the requirements of negligible weight, tailorability to regular daily garments and conformability.
The transduction mechanisms that present electrical responses from mechanical deformations are piezoresistive, piezoelectric, capacitive, triboelectric and transistive [8
]. These transduction methods, in general, rely on an active material whose electrical property changes upon mechanical stress. The presence of two electrodes captures this property on the said active material. Among the mechanoelectrical ones, piezoresistive sensors are the most prevalent in the literature to be applied as electronic skin for their simple mechanism, compact structure, low cost, energy consumption and ease of signal acquisition [9
Sensitivity and detection range are two important parameters to evaluate pressure sensors’ performance. In this context, sensitivity is defined as the rate of change of the electrical property with the change of the measured variable. In bulk piezoresistive sensors, the sensitivity is, generally, measured by the polymer matrix compression modulus and the filler direction change due to decreased percolation pathways caused by disconnections and the change in the compatibility between the conductive materials and the matrix. Furthermore, it should be noted that under uniaxial compression conditions, the load applied to the material will be uniformly distributed over it. However, when referring to an intrinsically anisotropic material, such as a textile substrate, the contact between the fibres in different directions can lead to peculiar deformation performance and, consequently, alter the path of current conduction. In line with this, a recent study carried out by Xie et al. indicated that pressure sensors made of knitted fabric have different sensitivities in the walewise and coursewise directions [10
]. This study emphasized that the way the yarns are in contact with each other will influence their compressibility property and, therefore, the electrical response of the sensor. The mechanical performance of a textile substrate will also be influenced by the yarn properties (such as the diameter, the coefficient of friction and the initial Young’s modulus) [11
] and by its weave or knit structure.
2. Piezoresistive Behaviour in Polymer Composites
The term piezo comes from the Greek word “piezen”, which means to press and to compress. Through this etymological meaning, it is understood that a piezoresistive sensor transduces an applied pressure into an electrical resistance variation [12
A simple piezoresistive sensor model consists of an active part positioned between two electrodes. The resistance value of the sensor is defined by being the resistance of the electrodes (Re) added to the resistance of the active material (Ra). It is noteworthy that the change in Ra is the reason for the piezoresistive behaviour, considering that Re is a constant value. Through this discursive reasoning, it is worth mentioning the material resistance equation:
where is the material resistivity, L is the length and A is the cross-sectional area.
According to the equation above, and in line with previous studies, the resistance variation in piezoresistive sensors is influenced by two main factors: geometric deformation of the elastomeric composite, where the parameters L and A change according to the material deformation, and the resistivity of fillers in band structure and interparticle separation [9
]. The conductive polymer composites, generally composed of conductive particles, i.e., fillers, dispersed in an insulating matrix, rely on the percolation theory. From phase a, known for presenting very low electrical conductivity values, up to zone c, where the filler particles come into direct contact to form perfect networks, the conductivity values increase significantly. At some point, though, the percolation threshold is exceeded, and conductivity does not increase substantially. Hereupon, particle geometries and properties, insulating matrix properties and polymer–particle interaction will directly influence the tunnelling conduction mechanism under a mechanical deformation [9
3. Materials (Active, Electrodes and Substrates)
Given the above, the materials that compose a sensor have a direct relationship with its piezoresistive response. In its simplest form, the sensor is composed of the sensing material, also known as active material, signal transfer components and a flexible substrate. Each component comprises different requirements.
Active materials are those responsible for detecting the mechanical stimulus. In this sense, they need to possess reliable electrical conduction paths, exceptional chemical stability, good mechanical compliance and compatibility with large-area processing techniques [9
]. Semiconducting polymers such as polydimethylsiloxane (PDMS) [14
], polypropylene (PP) [15
], semiconducting nanowires and carbon-based materials are some of these. However, carbon and its allotropes, such as graphene and nanotubes, are the most used for application in polymer-based composites. In addition, graphene, the single sp2
-hybridized carbon atom-thick 2D material, has shown to be highly promising due to its excellent mechanical, thermal and electrical properties provided by its internal honeycomb-like structure [16
]. Among its variations, graphene nanoplatelets (GNPs) have gained evidence due to their ease of scalability production and low price when compared to others [17
]. Furthermore, GNPs have few graphite layers, are lightweight, and have a higher aspect ratio with a planar shape [18
]. However, the key challenge in the synthesis and processing of bulk-quantity graphene sheets is aggregation, which tends to occur through van der Waals interactions. In this sense, many synthesis and modification methods have been used, which involve either preparing “amphiphilic” graphene nanoplatelets (a strategy that involves exfoliation followed by Graphene oxide (GO) in situ reductions [19
]) or a dry-low-cost method for obtaining N-rich graphene, through gamma irradiation with the use of ethylamine [20
]. Furthermore, when dispersed in biopolymers and doped onto textile substrates, such as linen, GNPs’ ecocomposites may have their properties potentiated, namely, improved hydrophobic capacity and UV protection, in addition to their piezoresistive response [21
Therefore, graphene nanoplatelets were chosen in this study due to their aforementioned superior mechanical, thermal, electrical and scalability properties, beyond the already proven applicability as a filler in polymer matrices with a piezoresistive response.
Regarding signal transfer materials, better known as electrodes, it is expected that they are stretchable materials that can maintain high conductivity under large strains and possess excellent stability [9
]. For this purpose, metal electrode materials with a specific design are suitable for stretchable electronics, such as Au, Ag or Cu [22
]. Furthermore, in this sense, silver ink was chosen since it has an appropriate electrical conductivity value for electrodes.
For flexible sensors, the substrate can be understood as the base, known as the insulating matrix, in which the active material will be dispersed. To this end, the substrate should have excellent chemical stability, low surface roughness and flexible mechanical properties [7
]. Elastomers, self-healing materials, polyurethane (PU) and textile fabrics are presented as some of the materials that meet these requirements.
This entry is adapted from 10.3390/ma15155185