Flexible printing compositions as basic element of future electronics
Printed electronics is aimed at minimizing material and production costs, allowing to significantly reduce the cost of electronic products, increase their production efficiency, create flexible devices with improved performance, increased reliability and environmental safety. This is especially important for large-sized electronics, such as displays , photoelectric converters  and large area sensors . Similar results can be achieved by the joint use of new functional inks and modern platforms of gravure, flexographic and inkjet printing, as well as engraving. Various types of inks with the properties of conductors, insulators and semiconductors based on inorganic and/or organic materials for printing transistors  and light-emitting diodes  have already been developed and are being actively implemented.
In 2016, the smart clothing segment accounted for about 1% of the wearable device market, that is, a total of 1.3 million devices were produced. According to IDC Mobile Device Trackers , in the coming years, this segment will show an increase of 9.4% to 22.3 million smart clothings by 2021. Researchers of the smart clothing market from Tractica  predict that in 2022 its production volume will reach 26.9 million units, and the cumulative average annual growth rate will be approximately 76.6% from 2016 to 2021. It is projected to be the fastest growing segment in the entire wearable device market.
The main requirements for devices of this kind are high flexibility/tensile properties, durability, biocompatibility, hypoallergenicity and low weight. Based on this, when creating components and materials of wearable devices, it is extremely important to consider the following key points:
• use of non-toxic, highly soluble, chemically stable, low-temperature inks for high print quality;
• high resolution and uniformity of printing to increase the conductive properties and the degree of integration of elements;
• use of flexible/stretchable substrates for electronic devices that must be worn or integrated with the human body;
• adaptation of the structure to prevent cracking and sliding, which will ensure high durability of the device.
Thus, the control of the printing process and the correct choice of materials will allow to produce high-performance wear-resistant electronics .
In modern industry as a whole, and in the production of clothing in particular, a huge number of elastic materials are used. All of them have an extensive range of properties, but from the point of view of the printing process they can be divided into two categories:
• dense, impermeable (nonabsorbent) materials and composites, such as rubber, latex, tarpaulin;
• textile fabrics obtained from individual yarns and fibers, having a porous, permeable to the printing composition structure.
Accordingly, the principles underlying the creation of ink for rubber and textiles will have dramatic differences.
FOR IMPERMEABLE (NON-ABSORBENT) SUBSTRATES
When designing inks for rubber and other impermeable substrates, we were guided by an approach that worked well for creating printing technology for flexible substrates. The main components of such inks are conductive nanoparticles and polymer binder – adhesive. The main requirements for the binder are strength, high adhesion to the substrate, the proximity of the mechanical properties of the binder and the substrate.
To improve the elastic conductive inks, in particular, to improve their mechanical and electrically conductive properties, the principle of self-organization of the structure of the conductive layer was applied. This principle is based on the formation of complex ordered structures in the one-stage process due to the interaction of components having similar properties. Such methods are successfully used, for example, in the creation of organic semiconductor channels .
In our case, using a two-phase solvent system and a surfactant, it was achieved the formation of a near-surface conductive zone enriched with silver nanoparticles inside the printed layer. Thus, the conductivity was significantly increased without increasing in the proportion of the conductive component. Another positive aspect is the plasticization and increased elasticity of the printed path due to the formation of a layer of almost pure elastomer.
The tests showed that the samples of the stretchable paste developed by us are comparable with the best world analogues. The surface resistance of a layer with a thickness of 25 μm was as follows: without stretching – 40–80 mΩ/square; at a stretching of 50% – less than 300 mΩ/square. These parameters remained unchanged after 300 cycles of stretching.
Creating conductive tracks on woven textiles requires a different approach, since the creation of a monolithic organized layer of conductive particles is difficult due to the permeability and absorbent properties of textile fibers. In addition, the monolithic printed layer formed by the elastomer differs in its elastic properties from the porous structure of textiles, which leads to cracking of the conductive structure and sharp jumps in resistance during mechanical deformations. Another difficulty is the binding of individual fibers and the violation of their mobility relative to each other, which leads to a partial loss by the fabric of its natural elasticity and a change in texture.
Common ways to create a conductive structure on the fabric are the integration (sewing in and interweaving) of metallic conductive filaments into textiles at the fabric creation stage . These filaments increase stiffness and reduce the elasticity of the material, providing stable electrical characteristics at large strains. The production process of such fabrics is environmentally friendly and compatible with conventional manufacturing equipment used in the textile industry. However, this approach can be implemented only at the stage of manufacturing a woven fabric and, in contrast to printing technology, it is not applicable to finished garments.
When creating a printing composition for textiles, we took as a basis the idea of introducing conductive filaments into the fabric structure. To implement this approach, the natural permeability of the fabric was used, and the ability of the fibers to absorb solutions and dispersions of solids. The main difference from printing on rubber and other impermeable and non-absorbent materials is that we create not a monolithic conductive path, but separate conductive layers on the surface and inside the fibers using a special high-penetrating solvent and a two-stage drying mode, which in total provides deep impregnation of individual structural elements of tissue without binding them and filling cavities.
This technology has a number of advantages. Firstly, it is similar to the introduction of metallic conductive filaments into the woven structure of the cloth, but it is applicable to finished garments and is compatible with modern screen printing and the R2R process. Secondly, due to the preservation of the natural woven structure of textiles, its mechanical properties in the printing area, in particular elasticity and stretchability, are reduced only slightly. Thirdly, the individual threads in the fabric composition have a zigzag shape, which is repeated by the printed layer sorbed on them, therefore most of the mechanical deformations are directed at an angle to the fiber axis. Composite printing layer at the same time is experiencing significantly less stress and strain, which reduces the changes of resistance of the printed path as a whole.
The individual filaments impregnated with a conductive composite have a relatively low conductivity, but usually several tens to hundreds of such filaments are included in the printed path. Under mechanical loads associated with tension and torsion, the threads are lengthened to varying degrees, which leads to averaging of the strain. In sum, this makes it possible to achieve stable conductivity comparable to the results of printing on flexible substrates.
On the basis of the above approaches, a conducting paste for printing on textiles was created in our laboratory. This composition contains 55% silver nanoparticles and has a viscosity of 55 Pa·s. When printing on lycra, it was possible to achieve the following values of the surface resistance of a layer with a thickness of 25 microns: without stretching – 70–77 mΩ/square; at a stretching of 50% – less than 420 mΩ/square. According to the specified indicators, the developed paste is comparable with the known analogues , but at the same time surpasses them in mechanical and operational properties. This printing composition allows to create EMG electrodes, RFID tags and antennas, integrated directly into the elements of clothing. ■