Research

INNO-KOM 49MF200091

Abstract
Textile solutions are needed to meet the increasing demands on printed electronic products, such as flexibili-ty, breathability and physiological clothing properties, e.g. for sensors in the sports and leisure sector or antennas for data transmission. The result of the ElsaTT project is a technology for producing highly conductive, printed structures directly on textiles using thermal transfer printing without the need for intermediate substrates such as films. The challenge in the project was to transfer highly conductive thermal transfer ribbons based on aluminum to the textile substrates in the printing process in a flat and consistently conductive manner. The structures created in the project are suitable for short-range RFID applications, such as reading out data or electronic access control. The quality of the antennas still needs to be significantly improved for the construction of sensor networks.

Problem:
The aim of the project was to investigate the factors that influence electrically conductive thermal transfer printing on textiles and to adapt them in such a way that this cost-effective process can be used for the production of electronic textiles. To validate the feasibility on textile base materials, selected textile substrates (e.g., woven labels) are evaluated for printability with highly electrically conductive structures in thermal transfer printing. Suitable pretreatment of the textile substrates is necessary to achieve the application and adhesion of the printed structures. To this end, it is particularly important to create a textile surface that is as flat and adhesive as possible. This surface can also be microstructured to optimize adhesion. Furthermore, it has been investigated whether a protective layer needs to be applied to the conductively printed structures in order to achieve the necessary mechanical resistance of the structure for its intended use. In addition, the electrical properties of the printed structures and their behavior under typical textile stresses have been investigated.

Solution
Thanks to their properties such as malleability and breathability, textiles are ideal for integration into flexible electronic systems. The processes used to date to produce highly conductive printed structures on textile surfaces, such as screen or chromojet printing, require wet chemical paste formulations with cost-intensive conductive particles. This, together with the necessary stencils, machine conversions, or additional units, means that conventional processes are only suitable for industrial implementation to a limited extent. In this project, the technology of thermal transfer printing of highly electrically conductive structures has therefore been transferred to textile substrates in order to be able to economically implement suitable structures for the permanent contacting of SMD components, even with batch sizes of 1. In contrast to conventional printing techniques, no complex wet chemical paste formulations with conductive particles are necessary.

Results and applications
Various textile substrates were selected and examined during the project work. A nonwoven structure proved to be particularly suitable for the parameters defined in the requirements profile. Various pre-treatment tests were carried out to ensure optimum adhesion of subsequent layers. For this purpose, simple print structures (layout test structures) tailored to the tests for evaluating the pre-treatment were developed. These test structures were printed on substrates that had been pretreated in different ways and examined in terms of their adhesion and electrical conductivity. Furthermore, suitable printing parameters were developed for the thermal transfer printing of highly conductive electronic structures for RFID applications. A layout for a suitable antenna structure was developed for the demonstrator. This antenna structure was continuously printed onto a specially pretreated textile nonwoven material using thermal transfer printing, see Figure 1. Electronic components in SMD (surface mount device) technology were then applied to the electronic structures produced in this way and connected by soldering. A protective layer applied using laminating technology ensures the functional properties of the structures produced. As shown in Figure 2, values collected by sensors can be transmitted to a reader using RFID technology.

Figure 1: Conductive thermal transfer printing on nonwoven material

Figure 2: Functional prototype on model with RFID reader

We would like to thank the Federal Ministry for Economic Affairs and Climate Protection for its financial support of the research project 49MF200091 “Elsa TT,” which was funded by the federal budget.