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Textiles today are not just comfortable and stylish, but can also be smart. Known as electronic textiles, or e-textiles, these are fabrics into which electronic instrumentation is built. The purpose of these integrated devices is to act as biosensors, transmit power, or enable wireless communications. At the same time, the textiles must still act and feel like ordinary cloth
Incorporating Electronics into Textiles
There are two general categories of e-textiles. One is comprised of fabrics that have conventional electronic devices attached to the surface, such as an ordinary battery or LED. These e-textiles come at the cost of losing the feel of a textile.
The second category is made up of materials with the devices woven into the fabric itself, allowing the textile to ‘breathe,’ and be flexible and lightweight, as standard textiles are. The devices may be pure metal wires or the electronic components themselves, like solar cells or diodes.
Many common textiles, both natural and synthetic, are electrical insulators. This interferes with the electricity conduction needed for signal transmitting to or from a textile. Threads, therefore, need to allow electromagnetic interference (EMI) shielding to form an electronic device integrated with the textile.
Most conductive threads are made of solid metals, copper or stainless steel. These are woven into the cloth in the form of braided strands or a solid core. However, being stiff and brittle, they cause problems during weaving and also when being worn or used in the final application. When subjected to repeated bending, they tend to break down, which creates high resistance within the wire. Moreover, if the conductive threads are required to be present at a high density in the fabric, they can make it much heavier than usual.
A solution is that a thin-film coating with metal can be used to confer conductivity to conventional threads, whether nylon, yarns or polyester. Different types of coating techniques are in use, including sputtering, electroless plating and dip coating.
Why are Nanowires Preferred?
The characteristics of nanowires are their cylindrical shape with a diameter of 100 nm or less, but a length measured in hundreds of nanometers, thus making it a unidimensional nanostructure. There are several types, such as semiconducting, superconducting, metallic, and insulating.
In contrast to the dramatic increase in electrical resistance with stretching, nanowires have a high aspect ratio, which allows excellent conductivity while preserving mechanical flexibility.
Nanowires can be produced using top-down methods to reduce the material from macroscale dimensions to the nanoscale, or by using bottom-up approaches to create the right dimension through the self-assembly of atoms.
Silver is commonly preferred for conductive threads due to the high conductivity, cost-effectiveness compared to other noble metals, and due to its air stability. The strength and Young’s modulus actually increases with silver nanowires compared to bulk silver.
These nanowires are advantageous as they lower particle density and reduce the number of junctions. The metal junctions can also be sintered to reduce junction resistance further.
Moreover, a mesh can be adopted rather than a film. This reduces metal use and therefore lowers the cost, cuts the weight and enables the use of thinner threads, as well as increased flexibility and better mechanical properties. The deposition is simple, as the nanowire coating can be used similar to a dye, avoiding the need for complex processes.
For these reasons, silver nanowire circuits on stretchable substrates are being investigated.
Methods of Nanowire Incorporation
Due to the potential usefulness of silver nanowire for smart textiles, various approaches to incorporate them have been researched.
One method is using deposition techniques on a very soft substrate, but this could lead to thin film behavior of the silver nanowire layer at high areal density. The result could be the formation of cracks and detachment of the film from the substrate, reducing the number of silver nanowires that can be used.
Another method is to embed silver nanowires on the surface of poly(dimethylsiloxane) or PDMS. This forms a strong bond, but the conductors become electrically unstable when stretched, being very vulnerable to damage in this situation.
Another convenient way to achieve electrical stability with mechanical strength for wearable electronic circuits is to use the pre-straining and post-embedding (PSPE) process. Using this, nanocomposites made from silver nanowires and PDMS can be embedded into the textile.
The high stretchability of PDMS combined with its waterproof nature allows the substrate to be washable with water in a washing machine, without harming the circuits. This approach integrates the benefits of both pre-staining and embedding technology. Demonstrations of this technique have been provided by manufacturing radio frequency identification (RFID) tags using the PSPE process, to create smart clothes or clothing accessories.
What is the Pre-Straining and Post-Embedding (PSPE) Fabrication Process?
In this method, a PDMS film is cured and acts as the substrate. Applying a pre-strain, the suspension of silver nanowires in ethanol is sprayed on the PDMS after 80% pre-straining, which is the maximum deformation possible with stretching for PDMS. The result is a silver network with uniform conductivity. Liquid PDMS is then cast over these layers and allowed to seep into the network of silver nanowires fully.
Later the pre-strain is relieved, which causes buckling of the silver nanowire network to a wavy form. Following curing for half an hour, the network is peeled off from the substrate to yield a PSPE circuit. By regulating the amount of liquid PDMS, the upper surface of the network is exposed. The content of silver nanowires is also described by the weight of nanowire per unit area of the circuit, called the areal density.
Unlike embedded circuits, where a permanent and significant increase in resistance occurs with each stretching, the PSPE circuit shows a minimal increase in resistance at 80% pre-strain, and full recovery as well. Even after 50 stretching cycles, the resistance has increased by less than one ohm (48%) compared to 3.4 ohms (300%) for embedded circuits.
The use of PSPE to create as-prepared electronic circuits based on silver nanowire-PDMS nanocomposites has proved ideal for washable and stretchable electronics. It has the advantages of both pre-straining and embedding processes but retains good electrical stability (in the form of high conductivity and small resistance fluctuations) under high stretch. It also preserves circuit adhesion to the substrate. These textiles can be washed with water in washing machines without any deterioration in its performance. This profile has made PSPE a favored approach for research into the development of smart textiles.
Sources and Further Reading