What You Should Know About Printed Electronics for Wearables

Just because the word “printed” is part of the phrase “printed electronics,” it does not mean you can quickly add printed electronics to your digital printing business. This is especially true for wearables. Printing safe, conformable, and comfortable electronics for textiles and apparel is different than printing RFID tags for labels or packaging.

While analysts are optimistic about the potential growth of the wearables market, a lot of R&D is still underway to develop combinations of cost-effective materials and manufacturing methods for products that will appeal to consumers.

Here are some facts that can help cut through some of the fog surrounding printed electronics for wearables.

A Broad Category

In the technology industry, the term wearable refers to any wireless technology worn on the body. The technology market research firm IDTechEx develops forecasts for 52 types of products, including:

• Hearables, such as cochlear implants, hearing aids, and headphones
• Skin patches for cardiac telemetry, diabetes management, remote patient monitoring, and other applications
• E-textiles with integrated electronics for biometric monitoring, heated clothing, illuminated apparel, and other applications
• AR/VR/MR devices for education, training, gaming, and entertainment
• Other products, such as smartwatches, fitness trackers, wearable cameras, and personal protective equipment for military personnel.

Each product category is at a different stage of commercial development. The production processes for consumer wearables such as hearing aids, headphones, and smart watches are fairly well established. But researchers are still exploring different methods of making e-textiles, smart clothing, and skin patches more comfortable and/or more durable.

Manufacturers can use Liquid X conductive inks to print circuitry directly onto fabric, mount components onto the printed traces, and then apply an encapsulant. | Credit: Liquid X

IDTechEX forecasts the market for e-textiles will grow from about $400 million annually in 2020, to about $1 billion by 2025. This is just a fraction of the market for skin patches, which IDTechEX expects to reach $25 billion by 2025. The skin patch category includes popular continuous glucose monitoring devices for diabetes management.

“Printed electronics are ideally suited for wearable applications, given their flexibility, low processing temperatures, and compatibility with stretchable substrates and textiles” says IDTechEx Technology Analyst Dr. Matt Dyson.

Different Than Printing Colored Inks

Most conductive inks contain particles of silver, copper, carbon, graphite, or other materials that perform electrical functions on a printed substrate. The efficiency, reliability, and durability of the patterns and layers printed with conductive inks matter more than the ability to produce vivid colors and attention-getting designs.

Conductive inks are formulated for various deposition methods, including screen, flexographic, inkjet, gravure printing, or aerosol jetting.

Screen printing is the most commonly used process to make electronics for wearables because it can lay down different viscosities and formulations of conductive pastes and inks in different thicknesses to achieve specified properties. Many screen-printing companies are also accustomed to working with garments and textiles.

NovaCentrix printed this electronic game on a T-shirt in cooperation with the EURECAT Center of Technology in Catalonia, Spain. | Credit: NovaCentrix

Higher-volume inkjet-printed electronics were not practical until NovaCentrix in Austin, Texas, discovered how to use nanomaterials to make conductive inks that would not clog the printheads or interfere with the ink delivery systems on printers built for graphics printing. Because inkjet printing is a digitally controlled, non-contact process, it can be used for printing on sensitive or delicate materials, or to perform jobs that require variations from piece to piece

“One of the first materials we saw a market for was nano silver powder,” explains NovaCentrix CMO Stan Farnsworth. After learning how to formulate dispersions for nano-silver powders, NovaCentrix developed its Metalon line of conductive inks. “The silver nano-particles in conductive inks are still 10 times heavier than water, so they tend to sink like a heavy rock,” says Farnsworth. So they used chemical additives to keep the particles afloat.

Developing conductive inks for inkjet printers created post-print challenges too. For example, the tiny, discrete silver particles in the fluid must be thermally joined (“sintered”) into solid, unbroken printed lines that allow electricity to flow freely.

The inks also must be quickly dried so they do not smear, and all additives must be removed to optimize the conductivity. The ink drying, chemical removal, and sintering processes all require heat, which can be problematic when manufacturing electronics on heat-sensitive films.

So, NovaCentrix developed PulseForge photonic curing tools. Photonic curing uses short pulses of light from a flash lamp. Because each light pulse lasts less than a millisecond, the ink is quickly heated but the substrate remains cool. Photonic curing can potentially make inkjet printing suitable for higher volume jobs that require faster throughput.

To make inkjet printing accessible to smaller research labs and start-up prototype developers, NovaCentrix engineered an integrated roll-to-roll inkjet printing system for printed electronics on flexible materials. The system features NovaCentrix’s proprietary ink delivery system, as well as grayscale capabilities that can apply multiple conductive ink thicknesses in a single print pass. In-line PulseForge tools cure and sinter the inks as the films come off the printer.

Textiles, Membranes, and More

Every new idea for a wearable will have a different set of requirements, and may involve integrating various parts, such as sensors, electrodes, antennas, capacitors, batteries, resistors, or photovoltaics. When connecting printed circuity to mounted components, traditional soldering can ruin the fabric or flexible film.

Conductive adhesives can be used to bond parts together. But many adhesives become brittle or decrease the conductivity of the component because of poor contact between the inks and the rest of the components.

More than a million parents have used the Owlet smart sock to monitor the oxygen levels and breathing rates of sleeping infants. Parents receive an alert on their smartphone if the readings fall outside of a preset safe level. | Credit: Owlet

Just as important, electronics applied to garments must be safe, flexible, washable, and comfortable. Some devices must withstand exposure to sweat, friction, or extreme temperatures. Many garments require stretchable materials and inks that can be elongated, compressed, or twisted without compromising the performance of the circuits. Some medical monitoring devices are designed to be used for less than two weeks. Other wearables must be able to withstand multiple wash cycles.

Because of these design and integration challenges, expertise in engineering and advanced materials is critical to the development of printed electronics for wearables. Sometimes, specialized software must be developed to gather and process the transmitted data.

“It’s not quite as simple as printing some conductive layers,” Dyson says. “To make something that works, you need someone with an electrical engineering background.”

A membrane switch, on the other hand, is an electrical control for turning a circuit on and off. Each switch consists of several layers of printed circuits on film and saves space when designing keypads and controls for everything from home appliances and computers, to industrial and health care products. Companies with experience in screen printing films and overlays for membrane switches include Butler Technologies, GM Nameplate, ECI Screenprint, and GGI Solutions.

Butler Technologies Inc. (BTI) was founded 30 years ago to make membrane switches, labels, and nameplates for industrial products. Today, its in-house team of engineers and materials experts help brands and start-up companies turn ideas for e-textiles and medical skin patches into functioning products. Some BTI designs and prototypes are now in commercial production.

BTI can design, create, prototype, and manufacture wearable/stretchable circuits for e-textiles, in-mold printed electronics, force-sensing resistors, capacitive touch circuits, Human/Machine Interfaces (HMI), printed sensors, antennas, and heaters for the Internet of Things (IoT). They also make circuits and assemblies for FHE (flexible hybrid electronics), and can provide unique lighting solutions.

According to Jamie Orlando, BTI’s director of sales and marketing, the company hired skilled electrical engineers and other talented professionals before expanding into printed electronics. BTI focused first on wearables to help athletes track their performance, then evolved into biosensors and skin patches for the health care industry.

As a pioneer in e-textiles, BTI worked with DuPont Advanced Materials to develop and test the stretchable conductive ink materials that are marketed as DuPont Intexar smart-clothing technology.

BTI used DuPont Intexar inks to print the thin-film circuitry used to make self-heating ski jackets that Ralph Lauren designed for athletes in the 2018 Winter Olympics. BTI used standard heat transfer methods for garment decorating to apply the encapsulated electronics to the lining of the coats.

Constantly Evolving

NovaCentrix shares its expertise in inks, materials, and curing with entrepreneurs, artists, and brands who have creative ideas for new types of wearables but do not know how to produce them. “Most start-up companies we work with use screen-printing systems,” said Farnsworth. “Screen-printing is great for wearables because they typically don’t require ultra-fine features. With smart clothing and e-textiles, the challenge is less about making micro-miniature components and more about durability, comfort, stretchability, and aesthetics.”

But printing is not the only materials deposition method available for making wearables.

This scanning electron microscope image at 250 times magnification shows a woven polyester fabric coated with Liquid X particle-free ink. Because it uniformly coats each thread, the ink can be used to create smooth, flexible, highly conductive e-textiles. | Credit: Liquid X

Liquid Wire in Portland, Ore., produces conformal and pliable electronic circuits that use a patented class of non-toxic printable liquid metals. The liquid-metal circuits are bonded directly to plastics and textiles, and provide low-resistance conduction without the need for a curing step.

Liquid X has developed a particle-free, inkjet-compatible conductive ink, as well as a process for coating the individual fibers in a textile to create an e-textile. The company recently earned OEKO-TEX certification for inks that can be safely worn next to the skin.

Less expensive, non-silver ink formulations are also being developed with materials such as graphene or carbon nanotubes.

As wearable devices become more functional and complex to design, a combination of analog and digital printing methods might be required to make products. The use of 3D printing is also being evaluated for making electronic components.

The markets for e-textiles and smart clothing still are not clearly defined, however. “One of the biggest challenges is to figure out what people want,” says Farnsworth. “Lots of people have clever ideas, but what are people willing to spend money on?”

During a webinar about IDTechEX’s post-COVID revised 2020 forecast for wearables, analyst James Hayward noted that some e-textile companies have shifted to producing face masks and other personal protective gear. Researchers at the Stanford School of Medicine have begun looking at smartwatches as a tool to detect and monitor symptoms of COVID-19. The growth of tele-health services may lead to a demand for more remote patient monitoring devices. Intelligent wearables that can detect fevers might also gain traction.

Getting into the Business

BTI’s Orlando advises companies who want to get into the business to find good people with the right skills to make it happen. Then, make a commitment to it.

In past years, you could attend events such as the IDTechEX Expo and meet experts from companies involved in research, prototyping, and producing different types of wearables. Now, you can attend webinars or subscribe to newsletters to keep abreast of new developments.

Dyson suggests the best way to learn more about the field is to partner with engineers, ink chemists, software developers, or curing and soldering experts who are already in the business. He advises screen-printing companies to partner with smaller companies: “They may not know much about large-volume manufacturing, but they do know how to design things, optimize the properties of materials, and design experiments to see what works best.”

The IDTechEx report on “E-Textiles and Smart Clothing: 2020-2030: Technologies, Markets, and Players” features a database of more than 200 companies that are either doing primary research or are developing prototypes for new types of wearables.

“We follow small start-ups and profile them, even those that only have two to three people working on an idea,” Dyson says. “Sometimes, they have fantastic technology, but may not have the money yet to scale up. So larger companies can benefit from building a partnership with them.”

Printing electronics for wearables may never become a viable, outsourced service because it is not simply about depositing inks on substrates. Most of the value in printed electronics comes from the ability to design, create, produce, and update reliable, integrated systems for different applications and requirements.

Dyson believes that the market for e-textiles, skin patches, and smart clothing is poised to grow rapidly once some important technical barriers are overcome. As more investment dollars continue to support research into e-textiles and skin patches, he believes the need to mount uncomfortable boxes of electronics onto flexible fabrics with printed circuity will go away. Eventually e-textile and smart clothing technology will evolve so that all electronic components are flexible, stretchable, and conformable.

The hype about wearable electronics over the past few years has made more innovators aware of what is possible.