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Woven and 3D-Printed Thermoelectric Textiles

Periodic Reporting for period 4 - ThermoTex (Woven and 3D-Printed Thermoelectric Textiles)

Reporting period: 2019-12-01 to 2020-05-31

Imagine a world, in which countless embedded microelectronic components continuously monitor our health and allow us to seamlessly interact with our digital environment. One particularly promising platform for the realisation of this concept is based on wearable electronic textiles. In order for this technology to become truly pervasive, a myriad of devices will have to operate autonomously over an extended period of time without the need for additional maintenance, repair or battery replacement. The goal of this research programme is to realise textile-based thermoelectric generators that without additional cost can power builtin
electronics by harvesting one of the most ubiquitous energy sources available to us: our body heat.

Current thermoelectric technologies rely on toxic inorganic materials that are both expensive to produce and fragile by design, which renders them unsuitable especially for wearable applications. Instead, in this programme we will use polymer semiconductors and nanocomposites. Initially, we will focus on the preparation of materials with a thermoelectric performance significantly beyond the state-of-the-art. Then, we will exploit the ease of shaping polymers into light-weight and flexible articles such as fibres, yarns and fabrics. We will explore both, traditional weaving methods as well as emerging 3D-printing techniques, in order to realise low-cost thermoelectric textiles.

Finally, within the scope of this programme we will demonstrate the ability of prototype thermoelectric textiles to harvest a small fraction of the wearer’s body heat under realistic conditions. We will achieve this through integration into clothing to power off-the-shelf sensors for health care and security applications. Eventually, it can be anticipated that the here interrogated thermoelectric design paradigms will be of significant benefit to the European textile and health care sector as well as society in general.
The aim of the project was the development of materials and processing routines to fabricate flexible thermoelectric devices with organic semiconductors. The project was organized in 4 work packages (WPs) that had the following sub-goals:

WP1 – development of materials. The focus of this WP was to develop materials with improved thermoelectric and mechanical properties as well as long-term stability. A major achievement was the introduction of a new class of p- and n-type conjugated polymers to the field, which carry oligoethylene glycol side chains. This new class of polymers has led to major improvements in terms of doping efficiency as well as thermal stability. A second focus of WP1 has been to establish structure-property relationships of thermoelectric materials such as the importance of choice of dopant, crystallinity, anisotropy, porosity and nanofillers.

WP2 – manufacture of fibers/yarns and use of textile manufacturing techniques. A major achievement has been the development of wash and wear resistant p-type yarns based on silk coated with a conjugated polymer. The wash-ability in a domestic-purpose washing machine could be shown. Further, WP2 demonstrated that these yarns can be used for large scale textile manufacturing, including machine weaving and embroidery.

WP3 – use of 3D printing to pattern thermoelectric devices. Compounding schemes for melt-processable carbon nanotube composites have been developed and where successfully used for 3D printing. Further, a combined synthetic and compounding approach was developed that enables the manufacture of conducting bulk architectures. These materials were extruded into filaments, which could be used for fused-filament-fabrication (FFF), one of the most common 3D printing techniques. These materials could be used to 3D print thermoelectric modules with 100 legs onto textile substrates.

WP4 – fabrication of prototype thermoelectric generators. The yarns from WP2 were used for the fabrication of thermoelectric textiles. In plane prototypes were fabricated by hand, using stitching and embroidery. Since the thermal gradient in clothes is experienced perpendicular to the textile surface, the second part of the project focused on out-of-plane devices. Embroidery by hand allowed the fabrication of out-of-plane thermoelectric textile modules. One important result was that the thickness of the module must be on the order of millimeters in order to account for the sizable thermal contact resistance that is encountered in wearable applications. One prototype design produced a useful power in the microwatt range. The second generation of out-of-plane devices involved machine sewing of conducting cellulose yarns, which resulted in out-of-plane devices that incorporated 40 thermocouples.
The main methodologies that the Thermotex team has developed deal with the design, processing and chemical doping of conjugated polymers, which allowed to improve their thermoelectric performance and stability. Further, the project has focused on the creation of millimeter-thick bulk materials and conducting yarns, which are needed to construct thermoelectric textiles. This approach is different to the rest of the field, which focuses on sub-micrometer thin films. The following methodologies are most noteworthy:

The project has introduced conjugated polymers with oligoethylene glycol side chains to the field of organic thermoelectrics (Adv. Mater. 2017, ACS Energy Lett. 2018). Many groups around the world now follow this polymer design. The use of these polymers has led to several conceptual breakthroughs in the project, which can be anticipated to greatly benefit the further development of organic thermoelectrics: (1) double doping (Nature Mater. 2019), and (2) all-polymer ground state charge transfer (Nature Mater. 2020).

A strategy was developed that enables fibre-spinning and 3D-printing of all-polymer conducting materials (ACS Appl. Mater. Interfaces 2020). It can be anticipated that these stable and biocompatible bulk materials will lead to new opportunities for wearable electronics, as indicated by 3D-printed thermoelectric devices on a textile substrate.

The realization of truly functional electronic textiles is held back by the lack of materials that are wash and wear resistant. Within the project a number of washing machine-proof yarns have been developed based on silk or cellulose coated with a conjugate polymer (ACS Appl. Mater. Interfaces 2017), or a composite coating of a conjugated polymer and silver nanowires (ACS Appl. Mater. Interfaces 2020). These materials can be processed in commercial textile manufacturing equipment (Adv. Mater. Technol. 2018). These yarns have been used to manufacture prototype textile devices that display a state-of-the-art thermoelectric performance.

The project has resulted in a method that allows the deposition of an electronic ink whose electronic character, i.e. p-type or n-type, can be switched through illumination with UV light. This method was used to ease the manufacture of thermoelectric devices through patterning of p-type and n-type thermocouple legs with selective UV illumination (Adv. Mater. 2016). This method has been patented.
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