Conductive Threads: Flexible Electrical Pathways for Smart Textiles

Electronics keep advancing, and one area that is still evolving is smart textiles and wearable electronics. Embedding flexible electronics into garments, beddings, and other fabrics continues to unlock a whole set of possibilities, especially in health applications. We will analyze conductive threads in detail, including their types and applications, in the article below. Let's get right into it!

What is a Conductive Thread?

Conductive threads are fibers with conductive materials that can transmit electricity. These fibers are thin and flexible, like ordinary cotton or nylon threads, and can take several shapes to connect several electronic components in a circuit.

These threads are ideal for building sewn circuits in e-textiles/soft circuits and wearable electronics. For instance, you can use them to attach sewable LEDs, microcontrollers, sensors, and battery holders to clothes.

A wearable device with a sewable LED

And you can also combine them with paper and conductive tapes. This application enables you to create interactive components like switches and tilt sensors. Such projects partner well with paper circuits.

Types of Conductive Threads

These are the four most typical conductive thread types.

Conductive Polymers

Some polymers can transmit electricity intrinsically. And since they are flexible, you can process them in solutions to make bending electrical pathways. So you can use such materials to coat conventional silk or cotton threads. This coating gives the yarn electrical conductivity while maintaining the strength and flexibility of the underlying material.

A prototype of a wearable device with a home-made sewable microcontroller

Additionally, you can mix the conductive polymers with dopants and other materials for optimization to suit specific applications. You can make them into semiconductors, as well.

However, polymers have one primary drawback. Their conductive properties are much lower than those of metals. But sufficient doping and alignment can increase their conductivity. For instance, stretch-oriented polyacetylene is more conductive than stainless steel fibers.

Metal-Coated Textile Threads

Coating a textile or flexible polymer thread with metal is another way of making conductive hybrid threads. Metals like silver and nickel can make yarns have highly conductive outer skins while being flexible and sturdy.

Additionally, the outer metal skin enables soldering to other metal parts in the circuit to create reliable electrical connections.

A dress with LED lights

Although metallic, these threads might not be as durable. The outer metal layer can debond and peel from the underlying fabric substrate. So metal-coated textile threads might not be ideal for wearable clothing. The long-term wear and washing will degrade the yarn.

Stainless Steel Thread

Stainless steel is durable enough to form thin, flexible threads without snapping. The material also provides better conductivity than coating textiles with metal.

Pure copper and silver strands might have better conductivity, but they are not as durable as stainless steel. Additionally, the material is more resistant to oxidation than silver and withstands washing better than the silver-coated threads.

The downsides? Stainless steel threads are heavy and stiff.

Carbon Nanotube Yarns (CNT)

The three threads above have disadvantages relating to mechanical or electrical properties. But carbon nanotube yarns give the best of both worlds.

Mechanically, the yarns closely resemble textile fibers more than metal ones. So they are flexible, have superb tensile strength, can withstand fatigue, and are lightweight. And the material is more resistant to oxidation than stainless steel, meaning CNT will withstand washing and daily use better.

A 3D rendering of carbon nanotubes

Electrically, carbon nanotubes conduct like metals and even exceed the conductivity of stainless steel. Nickel and silver are more conductive than CNT, but their incorporation as thin coats above an insulating filament core lowers the thread conductivity.

It is possible to create a hybrid CNT thread instead of pure CNT. You can achieve this by depositing a CNT layer above other fibers to increase conductivity. Such a thread is ideal if your goal is to match the properties of the underlying fabric substrate.

Also, it is possible to coat CNT yarns with metal to boost their conductivity and enable soldering.

A carbon nanotube

But CNTs also have a disadvantage. They are significantly more expensive than the alternatives. So might not be as competitive when considering conductive textile applications.

How To Sew Using Conductive Threads

Sewing using conductive threads does not need special equipment. Regular needles, sewing machines, scissors, and others will do.

But keep these facts in mind. First, the conductive yarn ends can fray quickly. Beeswax can help if you're finding it problematic to keep things together.

Second, the threads don't have any insulation. If they touch, they will conduct. So try to prevent yarns from touching if they carry separate signals. We recommend insulating crossing conductive thread traces.

You can use electrical tape, thick fabric, fabric paint, or other similar materials to insulate the traces.

Fabric paint is used to decorate canvas sneakers.

Alternatively, you can wax the entire thread before sewing. In addition to providing an insulating layer, the wax smoothens stray fibers.

How To Use Conductive Threads With Sewable Components

E-textile electronic components like Flora lines, Lily Pads, and Aniomagic contain PCBs with large holes at each pin. Each hole and the silver pad around it is conductive, which gives enough room to needle the thread through the hole multiple times. So the thread yarn will have a wide surface to make solid, reliable contacts with the board.

If sewing your components down to a fabric surface, ensure each stitch wraps securely around the component edge. And it should hold the part down tightly to keep solid contact with the hole's interior and surrounding silver pad.

The general rule of thumb is to use at least three stitches in each hole before moving to the next.

Applications of Conductive Threads

Passive Components and Antennas with Textile Features

Implementing antennas and passive components like inductors, resistors, and capacitors on textile substrates using conductive threads is a straightforward process. These components will be stretchable, flexible, and wash-resistant. And you can form more complex electronic modules by joining several embroidered yarn components.

Textile Buttons

You can create functional textile buttons on textile substrates directly using these threads. For instance, you can create capacitive textile buttons for switching LEDs connected to pads. And it is possible to solder hybrid conductive yarns. So you can solder the LEDs to the textile pads of gloves and other wearables.

LED light suites

Sensors

You can build different sensor layouts or types using these hybrid yarns. For instance, a single chrome-nickel steel wire can create a temperature sensor. And you can modify the sensing area to measure broader sections.

This thread has a low heat capacity, meaning its time response to sudden temperature changes is quick. And the sensor features high mechanical resistance, including that of the washing/drying process (30+ cycles).

So you can build intelligent textiles that monitor a patient's temperature or give early warnings in case of burns.

Textile Electrodes

Textile electrodes or tetrodes are suitable for integration on and into t-shirts. These commercial evaluation electronics connect via snap fasteners, allowing ECG monitoring and heart rate measurement.

The electrodes don't prevent normal user movement when wearing the t-shirt and provide complete textile integration. So you won't need external electrodes.

Stretchable Conductive Textile Buttons

Stretchable conductive ribbons can extend up to 70% of their length. They can contain varying numbers of conductive lines with different conductive thread numbers. So their conductive properties remain constant. The electrical resistance also remains stable when stretching.

Dancers in LED light suites

But they have some limits because they can withstand the following.

• 25,000+ cycles with 30% stretching
• 10,000+ cycles with 50% stretching
• 9,000+ cycles with 70% stretching
• 50+ cycles of washing resistance at 400 RPM and 40°C

Smart Bed Sheets

Intelligent bed sheets contain conductive threads for sensing humidity, temperature, and body presence. Also, they can have heating elements. These features enable them to monitor the following.

• Excessive sweating and body-fluid leaks
• Detecting patient presence/absence in bed
• Sleep quality based on movements

Smart Socks

These garments contain textile strain sensors to detect limb swelling. The sensors consist of stapled stainless steel fibers to measure electrical resistance. This electrical property changes when stretching, allowing long-term swelling measurement and monitoring its development over time.

Wrap Up

In conclusion, the flexibility factor of conductive threads gives them a unique set of applications as sensors or simple electrical pathways. And as you can see above, they have several applications. You can try out any of these use cases in your project and contact us if you have any questions.