by Scott R. Sabreen, The Sabreen Group and Dene Taylor, Ph.D., SPF-Inc.
The terms “wearables,” “wearable devices” and “wearable technology” all refer to electronic technologies or computers that can be worn by a consumer and often include tracking information related to health and fitness. Wristwear, headgear, glasses, armwear, legwear, footwear, skin patches, exoskeletons and e-textiles are involved, and the device business is enormous. Industrial inkjet is the one digital technology that enables direct to product functional and nonfunctional custom printing.
Demand for customization
The demand for customization drives sales, whether it’s health and fitness performance products, smartwatches or the hundreds of alternative interactive band products. IDTechEx stated in a recent article that the wearable electronics business powers from $20 billion in 2015 to almost $70 billion in 2025.1 The dominant sector will remain the health care sector, which merges medical, fitness and wellness. It has the largest number of big names, such as Apple, Accenture, Adidas, Fujitsu, Nike, Philips, Reebok, Samsung, SAP and Roche, behind the most promising new developments. According to Bain & Company, product customization helps brands boost sales and the opportunity is significant.2 Its estimated that at least 25-30 percent of shoppers are interested in customization options. For select markets, this would equate to $2 billion per year per sector. Two important trends have emerged: first, today’s consumers are more expressive; and second, customization is the new loyalty.
Inkjet printing onto wearables
The focus in this article is “customization” using inkjet printing onto molded wearables – from interactive electronic devices and watches to athletic footwear and garments to 3D printed products. One of the first and most recognizable wearables is Disney’s wrist band, also called the “MagicBand.” Each band contains an RFID and a radio like those in a 2.4GHz cordless phone. The band connects visitors to a vast and powerful system of sensors and mobile apps within the park, serving as a form of payment, park admission pass, room key and more. The majority of bands are solid colors (as molded). The underside of each band is laser marked, containing identification information (See above. Photo source: Disney FCC Filing ID Q3E-MB-R1G1). Pre-designed decorated bands are sold at retail outlets. However, for the unique one-of-a-kind experience, enthusiasts hand-craft bands using labor-intensive multi-step appliques, decals, temporary tattoos, cover bands and paints available in graphic arts stores.
Inkjet printing is driving advanced manufacturing processes due to its unique capability for the deposition of functional fluids across a broad range of applications, in unique or complicated patterns, or to novel substrates. For example, conductive inks for all technologies are breaking ground in many areas and especially for flexible substrates. Inks of silver, organic conductive polymers, graphene and carbon nanotubes are being used for circuitry and connectors throughout flexible electronics. With inkjet printing, they become useful as connectors between printed buttons (thin film touch buttons) on garments (example: ski jackets) to mobile devices, passive or active RFID tags or to LED illumination. Organic light-emitting diode (OLED) technology not only is flexible, but with inkjet also can be tailored for identification, safety or decoration on outer clothing. Additionally with inkjet, multiple layers of different materials are being printed to make constructions of six or more distinct layers.
Truly elastic conductors are rare for any technology. Purdue University announced a new class of inkjet-printable liquid metal conductors, demonstrated as suitable for stretchable and rubbery materials.3 They could make possible a new class of pliable robots and stretchable garments that people might wear to interact with computers or for therapeutic purposes.
Adhesives are a cornerstone of wearables and often are required for complex geometries (for example, athletic shoes.) Robotically controlled large droplet-sized print heads apply adhesives to an increasing range of irregular structures, giving full coverage without excess, while also accommodating short runs and frequent changes. Latex inkjet adhesives can match traditional formulations. Ultraviolet (UV)- and electron beam (EB)-cured inkjet adhesives increasingly are available, meeting the demand for immediate cure while bonding to disparate materials.
UV cure inkjet is popular with outerwear, badges, reflective gear, athletic equipment and athletic footwear. Most printing is performed on items arranged precisely with templates on trays or forms passing under reciprocating printheads. For high productivity, single-pass printers with full-width, stationary printheads apply images to items being carried on a belt at speeds to 60fpm or faster. The belt often has trays so items always are in a fixed position, or the printer may rely on cameras to locate item position and orientation and jet the image accordingly. For UV cure, there is a lamp after the print heads. LED lamps are becoming increasingly popular for long life, consistent output, limited ozone generation and lower energy use. For wearable items, the greatest value is that they are cool, with the substrate getting very little heat. Heat-sensitive plastics, fabrics and films can be printed without fear of shrinkage or distortion. A caution for users of energy-cure inks – as the liquid inks are hazardous, they always must be properly cured for safety and durability.
UV-curable inkjet inks
Inkjet printing is far more complex and delicate than analog printing. Inkjet requires the nozzles to fire precisely sized drops with exact accuracy. High-quality inkjet printing systems must simultaneously integrate printheads, fluids, electronic controllers, pretreatment and cure. All of these items must work together to produce the intended results.
UV-curable inkjet inks dry instantly. UV ink printing differs from other types because the polymer is formed during curing by chain reaction of monomers and oligomers. Monomers are low-viscosity liquids, so they also function as the liquid ink carrier and eliminate the need for water or solvent – that is why UV cure inks are 100-percent solids and ideal solvent ink alternatives. Oligomers (larger reactive molecules) have multiple chemical functionalities and are critical to properly building the binder.
Polymerization is initiated by a short exposure to UV light, which is electromagnetic radiation with a shorter wavelength and higher energy than visible light. Near UV (390–200 nanometer wavelength) is used for most UV curing. It further is refined into UV-A (390–320 nanometers) or long wave, UV-B (320–280 nanometers) or medium wave and UV-C (280–200 nanometers) or short wave. Besides Near UV, other UV categories include Far UV (200–10 nanometers) and Deep UV (31–1 nanometers). The most common peak wavelength for conventional mercury UV curing lamps is 365 nanometers, also referred to as the mercury I-line (light spectrum).
Printed wearables are held in hand by consumers and subject to very close visual inspection. Alphanumeric text may be six points or smaller. To avoid visible artifacts and ensure readability, high-resolution printing is required. High-resolution inkjet printing has two primary components – spacing of dots on the substrate and the size (volume) of the droplets making up the dots. High resolution is obtained with close droplet placing (now up to 1,800 x 1,800dpi), which takes multiple passes and tiny steps on the typical oscillating head printer.
The UV-reactive components are photoinitiators – compounds that absorb the energy and split to produce highly reactive chemical species, free-radical or cationic depending on type. Each can bond to one part of a monomer or oligomer, which transfers the activity to another part, which in turn can react with another monomer and so on to build the polymer. UV inks use pigment colorants. Formulating and ink-making developments enable manufacturers to offer white (W), which especially is useful as a base on dark-colored substrates or as a background on clear. Inkjet primary colors – cyan, magenta, yellow and black (CMYK) together – offer a large color gamut.
Fading in sunlight can be a major print durability concern. As UV inks use pigments, not dyes, they inherently are less sensitive to sunlight than desktop inks. Very good fade resistance is obtained when the inks are made with pigments from the automotive paint industry. While more costly, they are used in premium inks. Sunlight also can degrade the polymer, producing chalking and brittleness. Five-year continuous outdoor light exposure is a commonly met expectation.
Additives are essential in all ink formulations. Inks for printing plastics commonly contain adhesion promoters. Jetting and droplet formation, obviously central to inkjet, require careful balances of viscosity modifiers and surfactants. If not correct, the ink can mist or wet out on the head, neither of which are satisfactory. These same compounds also control ink wetting and flow on the substrate – i.e., adhesion and dot gain. They generally are optimized for a particular surface chemistry, but there are limitations. For example, an ink that will wet a low surface energy substrate will leak from the printhead. For that reason, many surfaces must be treated to put them in the range of ink functionality.
Inkjet inks have low viscosity and low surface tension, which create adhesion bonding challenges on many polymeric substrates, such as acetals, polyolefins and polyurethanes. These types of chemically inert plastics are hydrophobic and not naturally wettable. As a general rule, acceptable bonding adhesion is achieved when the surface energy of a substrate is approximately 8 to 10 dynes/cm greater than the surface tension of the liquid.
The speed of the printing press also can impact ink’s effective surface tension. An ink that statically measures 25 dynes of surface tension could behave dynamically like an ink with 40 dynes of surface tension on a high-speed press. The actions of inkjet print heads and print systems on the fluids they dispense significantly can impact the way fluid components realign during dispensing and interaction with the print surface and other ink or coating layers.
The degree or quality of treatment is affected by the cleanliness of the plastic surface. The surface must be clean to achieve optimal pretreatment and subsequent ink adhesion. Surface contamination, such as silicone mold release, dirt, dust, grease, oils and fingerprints, inhibit treatment. Material purity also is an important factor.
Conclusion: Total solutions methodology
A total solutions methodology is essential for developing polymeric materials and inkjet printing onto wearables. As consumer requirements and functionality develop, so do the demands on the materials designated for long-term skin contact. After FitBit recalled a million wearable devices in 2014, suppliers are certifying materials to skin-contact standards. Inkjet printing process controls are equally paramount. Important factors include the following:
- Material formulations (including leather, rubber, thermoplastic polyurethane, copolyester elastomers and additives) that will not irritate the skin, such as PPD or p-Phenylenediamine.
- Material requirements including durability, chemical resistance, UV resistance and scratch resistance.
- International Compliance Standards on toxicity such as ISO 10993-10:2010 – Biological evaluation of medical devices – Part 10.
- Excellent ink adhesion. Minimize low molecular weight compounds that migrate to the surface.
- Ink fluid chemistry and jetting performance.
- Robust printer systems that set up at kiosks, as well as manufacturing sites.
Scott R. Sabreen is founder and president of The Sabreen Group, Inc., an engineering company specializing in secondary plastics manufacturing processes – laser marking, surface pretreatments, bonding, decorating and finishing and product security. Sabreen has been developing pioneering technologies and solving manufacturing problems for over 30 years. He can be contacted at 972.820.6777 or by visiting www.sabreen.com or www.plasticslasermarking.com.
Dene Taylor, Ph.D., founded SPF-Inc. in 2000 to serve the printing and packaging industries, with a focus on adapting digital printing for new markets and applications. A nano-chemist by training, about half of his 25 US patents are related to digital printing. He is a member of SGIA, TAPPI and RadTech. He can be reached at email@example.com.