Let There Be Light – Masterclass of the POLEOT project

Printing of light emitting devices on textiles was the subject of a masterclass held by ESMA on 23rd June 2015 in Düsseldorf where German and Belgian researchers presented the results of a collaborative research project.

by Marianne Curtis, World Textile Information Network

Prof. Wim Deferme POLEOT project leaderProf. Wim Deferme POLEOT project leaderThe research, introduced by Prof. Wim Deferme of Hasselt University, Belgium, looked at three main technologies – preparation of textile and/or conductive coatings; ink formulation and printing of light emitting diodes (LEDs); and encapsulation of applications/devices.

For wearable textiles organic LEDs (OLEDs) were used. High barrier properties are required and carbon nanotubes were used to coat the textiles to maintain flexibility. For non-wearable textiles e.g. lamp shades and wallpaper, electroluminescence (EL) can be used. Less strong barrier properties are necessary and conductive yarns e.g. silver, are appropriate.

Recent developments in LEDs permit them to be used in environmental and task lighting. Advantages over incandescent light sources include lower energy consumption, longer lifetime, improved physical robustness, smaller size and faster switching. LEDs are finding applications in e.g. aviation lighting, automotive headlamps, advertising, general lighting, traffic signals and camera flashes. However, LEDs powerful enough for room lighting are still relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output. Textiles provide a flexible medium for such applications.

Smart textiles

Masterclass of the POLEOT projectMasterclass of the POLEOT projectViktorija Mecnika, a researcher at the Institut für Textiltechnik der RWTH Aachen, discussed smart textiles, defining them as “structures that sense and react to environmental conditions or stimuli such as those from mechanical, thermal, chemical, electrical, magnetic or other sources”.

Smart textiles consist of six components: sensors, actuators, external communication, internal data transfer, data processing and an energy source. Applications include clothing which can communicate e.g. for fire fighters, clothing which can monitor biological parameters and haute couture.

Looking more closely at medical applications, Ms. Mecnika said smart implants for surgical procedures and smart bandages containing miniature fibre-based biosensors that monitor or improve the healing process were being researched. Heating blankets for patients undergoing surgery were another potential application. Smart textiles could also be used for photo (light) and photodynamic therapy to treat conditions such as neonatal jaundice and for drug delivery via the skin, she added. Other potential applications include healthcare biomonitoring e.g. cardiovascular, luminous fashions, wallpaper and protective clothing.

CNT printing

Printing and coating with carbon nanotubes (CNT) was the subject of a paper presented by Dr Filip Govaert of Centexbel, the Belgian textile company membership organisation that carries out testing and research. When compared with silver, carbon nanotubes offer advantages in terms of cost and flexibility as a conductive back electrode on textiles, explained Dr Govaert. However, carbon nanotubes are prone to high levels of agglomeration and poor deentanglement in dispersion, posing challenges when formulating inks and coatings.

Dr Govaert’s research led to development of a coating formulation containing CNT and preparation method resulting in highly dispersed CNT and good electrical conductivity.

The CNT coating formulation involved mixing together the binder, surfactant and defoamer before slowly adding the CNT dispersion. Water was then added in small amounts to control the viscosity. The components were mixed for 30 minutes at 5,00010,000 rpm. They then underwent a vacuum treatment for 15 minutes before being subjected to an ultrasonic treatment for five minutes (for a 200g formulation). The thickener was slowly added followed by stirring at a slow speed.

Electrical conductivity of a CNT coating depends upon CNT concentration, layer thickness and degree of dispersion. Application methods included direct and transfer coating. “CNT inks are not suitable for large surfaces,” said Dr Govaert. “Also CNTs are always black so we have done some work to improve reflectivity by including aluminium flakes with the formulation.”

Potential applications for the technology include introducing lighting into wall paper – currently this can only be done by means of expensive panels, said Dr Govaert. “But we are working to produce it in rolls at a reasonable price.” It is likely that the contract market will be the first to take up these developments, with demand expected to come from hotels and theatres.

Challenges

Bulb less lamp LE printed directly onto textileBulb less lamp LE printed directly onto textileThe project highlighted some issues associated with electroluminescent printing. With respect to direct printing shrinkage can occur and surface roughness is critical, explained Myrene Vanderhoeven of the University of Ghent. Mirroring the design and adhesion problems were associated with transfer printing where there were no problems, however, with absorption, she added.

Ms. Vanderhoeven described various demonstrators developed in the project with commercial partners. These included a lighting wallpaper with Fibertex, Quad and Waelkens; a light emitting lampshade with Multiplot, Triton and Eckart; a ‘bed of the future’ which had lighting incorporated in the surround rather than a conventional bed side lamp with Deslee Clama, Quad and Waelkens.

For publicity purposes, the researchers developed ‘the eye of the tiger’ in partnership with Wollux. Here electroluminescent blue ink was printed for the eyes of an illustration of a tiger on an industrial screen printing installation using a nonwoven substrate. PET-based OLEDs were incorporated into elastic belts which formed part of the design of an illuminated dress with Uhasselt, Elasta and Neffa.

Work continues to develop improved barrier methods to protect OLEDs from the effects of water vapour and oxygen. They require a considerably higher level of protection against these substances than offered by conventional plastic food and drink packaging materials.

Dr Andreas Schulz described a gradient layer technique where a top inorganic layer, followed by an organic layer then another inorganic layer on top of the OLED could reduce the effect of ‘pin holes’ – tiny holes which allow oxygen and water vapour to pass through which are currently an issue. Developments concerning light emitting devices on textiles are likely to come fastest in the screen printing arena.

Currently it is difficult to develop conducting inks with the required viscosity for digital textile printing. However, researchers and delegates believe this area will develop in the future.

Article reproduced courtesy of WTiN

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