The Three Categories of E-Textile R&D
Go deep into the technology behind smart clothing.
In 2018, the smart clothing/e-textiles industry seems to be stratified in three main tiers, based on the complexity of the technology.
Low Complexity/Simple Devices: Patches and Appliques
These devices have single-layer circuits with simple +/- connectivity or electrolytic/electrochemical operations. They have simple designs and fairly low critical tolerances in terms of deposition thickness or trace width. In many cases, they are deposition only, with basic shapes and sometimes no traces.
The circuits are typically not printed directly on a garment. Instead, they are printed on a cloth or nonwoven elastomer, or printed as a patch or attachable part. The complexity with these types of projects is less about the printing and more about how the components will be designed and converted into a functioning, marketable product.
Medium Complexity/Printed Circuit Trace Patterns: Wearables with Smart Properties
In these devices, complete circuits are printed on the cloth, but the cloth is not yet part of the circuit.
Some involve multiple layering (either through printing or stack-up construction) and contact pads, and require moderate precision in the traces. The substrate is very critical because it has a bearing on the precision of the traces. The printed stack-ups use printed dielectrics and conductives. The flexibility of the final product is pivotal to the design. Lifespan, washability, and connections are also critical factors.
High Precision/Advanced Design: Smart Textiles
This category includes fabrics that have been woven with conductive threads or threads with chemically reactive or light/energy reactive (solar) coatings. These products can actually be classified as “smart textiles” and not simple wearable electronic devices. They are process-intensive, custom substrates with tight tolerances. These substrates provide in-thread or in-cloth connectivity for devices before a circuit is ever printed. The cloth is part of the device.
In some cases, these are multilayer fabrics. The threads at different levels have different functions and potentials. Some threads are simply conductive, some are capacitive, some are heat-sensitive, and some are electrolytically reactive. Some smart textiles can block electromagnetic signals (Faraday cage), enhance electromagnetic signals (antenna), transmit electromagnetic signals (RFID), or change potential under heat and pressure.
Before the cloth is woven, conductive threads can be made by filling the polymers with metal before the polymers are spun into filaments. Other processes used to make conductive threads include: electroless plating of various metal types, chemical vapor deposition, or chemical etching and deposition processes.
In some cases, whole woven cloth made of dissimilar threads can be electroless plated. A specific angstrom/micron thickness of metal is plated within a given plating dosage period. So, in one process step, you might get a uniformly woven cloth made with several different types of thread with very different resistances or levels of connectivity. Some threads may have no plating at all. Interspersing threads with no connectivity between connective threads can create a spacer layer, just like that on a membrane switch circuit.
Manufactured smart textiles represent a whole new arena of substrates. Printing conductive circuits on cloth woven with conductive threads delivers a much wider range of possible applications. Plating and chemical or vapor deposition processes are commonly used in manufacturing rigid circuit boards and semiconductors. They have high overhead costs because they require chemical networks or clean rooms.
The R&D expenses related to advanced design applications are high, so researchers are targeting applications where a high cost per unit is tolerated. This includes military/aerospace products, medical diagnostic devices, and automotive or industrial process controls.
Hundreds of organizations are working on high precision/advanced applications of e-textiles. But few are talking publicly about it.