# Conductive Textiles for Signal Sensing and Technical Applications
## 4 Testing for Conductive Textiles
### Conductivity Testing  
The conductivity of textile substrates is mostly measured under a DC power source. Nevertheless, it was inspected that PPy-coated poly-aramid fabrics showed increased conductivity under an AC power source due to an electron hopping charge transport mechanism rather than electron conduction [274, 275, 276]. Given this, it is evident that the electrical characteristics may be different for AC-based applications.  
The conductivity of polymer coatings is generally determined using the four-probe technique according to the ASTM D-4496 standard. According to the method, four electrical contact terminals are placed in a line at an equal distance on a polymer film. The polymer film is set up on an insulating surface. In this process, a constant current is employed between two outer terminals from a current source, and the voltage is measured between two inner terminals. Conductivity in this method can be calculated from constant current (\(I\), ampere) applied to the outer terminals and voltage drop across the inner points (\(V\), volt). With sample thickness (\(t\)), we can measure conductivity following Equation (1). [134].  
\[\text{Conductivity},\;\sigma=\frac{ln2}{\pi t}\frac{I}{V} \tag{1}\]  
The resistance of a fiber or yarn of a specific length can be measured using the four-probe method, two-probe method, multimeters, or by applying the principle of division measurement with a known resistance and unknown resistance (sample). The resistivity value is generally expressed as the resistance per unit length--often in \(\Omega\)cm\({}^{-1}\) or \(\Omega\)m\({}^{-1}\).  
According to the two-probe method, two contact points are used. The resistance is calculated by the measured voltage and measured current. The current applied is constant, but a voltage drop over the ammeter limits the measurement's accuracy. In 2010, it was observed that the four-probe technique is suitable for inspecting intrinsic electronic transport properties (behavior of electron transport such as hopping or conduction based on material type), such as metallic nanowires, given that these nanowires may have contact resistance, which cannot be avoided in the two-probe method [277]. Moreover, the four-probe technique is more accurate when the resistance value is small as contact resistance is eradicated.  
Surface resistivity, often expressed as square resistance, is the resistance between two points of a fabric or coated surface. It is written as \(\Omega\) per square (\(\Omega\)/sq), and the value is not dependent on the distance between contact points [134]. The AATCC (American Association of Textile Chemists and Colorists) test method 76 is generally exercised to measure the surface resistivity of conductive fabric or surface. The BS 6254:1984 and ASTMD257-07 test methods can also determine sheet resistance. According to the AATCC test 76, two rectangular-shaped copper plates are placed on a fabric specimen. The resistance (\(R\), resistance) between two copper electrodes is measured, or the current (\(I\), ampere) is determined by applying DC voltage (\(V\), volts). The width (\(W\)) of the sample and the distance between electrodes (\(D\)) are also measured. The surface resistivity can be calculated using Equation (2) [134].  
\[Rs(\Omega/m)=\frac{W}{D}\cdot R=\frac{W}{D}\frac{V}{I} \tag{2}\]  
The instruments used for these electrical measurements are usually multimeters, voltmeters, and picometers.