The successful commercialisation of wearable textile electronic products requires flexible, stretchable, lightweight and washable components. The most challenging part of meeting these requirements is the data processing unit, including printed circuit boards (PCBs), LEDs, solar cells, transistors, capacitors, batteries and displays. The UK's National Physical Laboratory (NPL) developed a new technique for directly printing circuits onto fabrics to create robust, functional wearable electronics. This technique can be applied directly to finished garments with nano-silver bonding and encapsulating fibres as thin as 20 nm in diameter, which could be used for wearable sensors and antennas [104]. The properties of the printing ink determine the result and the capabilities of the printable circuits and electronic components. An ink that can be used robustly for almost any substrate would be desirable. Haydale, which is based in Ammanford, UK, is developing a metal-free graphene ink (HDPlasTM Graphene Ink Sc213) that can be applied to substrates via screen printing, flexographic techniques or gravure printing. This ink is not as conductive as silver, but it is cheaper and less volatile. Graphene ink is resistant to cracking; thus, this ink is suitable for flexible electronics and for large area prints to be used in chemical sensor electrodes.
For example, other research areas include flexible sensors, displays, thin-film photovoltaics, energy storage, transparent electrodes and catalytic devices. [77, 106] Peratech of Richmond and the Centre for Process Innovation (CPI) in County Durham have developed printing inks that can be used for pressure-sensitive switches and sensors. [79] The quantum tunnelling composite (QTC) material can be applied to textiles by using flexographic printing processes. This material also readily withstands washing. The QTC material can be used to print RFID tags on paper or plastic. Peratech is researching ways to print its QTC e-nose sensor, which can detect volatile organic compounds (VOCs), onto fabric. Certain VOCs can be used as early indicators of health issues. [79,108] Chinese researchers at Fudan University introduced a stretchable high-performance supercapacitor, which is often used for static random access memory (SRAM). The components of this supercapacitor are fibre- shaped and based on carbon nanotubes (CNTs). The elastic fibre is coated with an electrolyte gel and a thin layer of CNTs. This layer is followed by a second layer of electrolyte gel and another layer of CNT, which is covered by a final electrolyte layer. [56] Prototyping is an essential part of research and development. Georgia Institute of Technology (GT) in Atlanta, USA, the University of Tokyo, Japan and Microsoft Research in Redmond, Washington, USA have in collaboration managed to inject silver nanoparticle ink into an empty cartridge from an ink-jet printer to produce an instant ink-jet circuit for prototyping. This
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approach allows the printing of arbitrary-shaped conductors onto both rigid and flexible materials. In addition to circuit boards (CBs), this method can be used to make sensors, such as capacitive touch sensors, and antennas with little cost. [56] The development of flexible, lightweight displays is essential to body-monitoring applications for which a smart phone or tablet is not appropriate. In collaboration, Plastic Logic from Cambridge, UK and Novaled from Dresden, Germany are developing fully organic, plastic, flexible and unbreakable AMOLED displays, which consist of organic thin-film transistor (OTFT) and OLED materials. [31, 113, 114]
The researchers at the Fraunhofer Institute and the University of Heidelberg, Germany developed a stretchable polyurethane circuit board plaster, which will be used to test kidney function. With the plaster, a blue light-emitting diode (LED) and a detector, the doctor can monitor the test continually. In the traditional approach, a substance that only the kidney is able to break down is injected, and blood samples are collected every 30 min. In the plaster system, the injected substance is an organic colorant, and the blue LED causes the colorant to fluoresce. As the natural colorant is broken down by the kidney, the fluorescence also decreases. [115] The development of wireless body area network systems could lead to improvements in mobile health-monitoring applications. One solution is the use of ‘Zenneck surface waves,’ which are used as Radar systems to visualize the curvature of the Earth. Roke Manor Research in Romsey, UK developed a dielectric-coated conducting fabric, which enables worn devices to communicate wirelessly in a personal network. This material could enable the propagation of surface waves around the body without the need for repeaters, high powers or high-gain antennas. [31]
The most wearable and garment-like approach is to integrate electronics directly into the fibre or the yarn. Researchers at North Carolina State University (NCSU) in Raleigh, USA created metal-filled polymer wires that can be stretched up to eight times their original length while still functioning [117]. Miniature-sized electronics, such as thin-film temperature sensors, accelerometers and circuits, can be integrated into fabric by using plastic strips as a platform for components. The strips can be wrapped around the fibre or woven into or embroidered onto the textile surface. This method enables non-fibre based components to be integrated into the textile structure. [118] Forster Rohner Textile Innovations of St. Gallen, Switzerland developed E-broidery technology for the industrial-scale production of fabric embedded LEDs. The embroidery technology also enables the interconnection of sensors to be incorporated into the fabric. [66]
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Instead of using mobile devices as a display, other types of solutions are also being investigated. Tactile displays (see Figure 7) and LEDs, OLEDs and AMOLEDs are also interesting ways to implement a flexible lightweight display for a wearable solution; this application is predicted to replace many traditional LED functions. The skin’s sensitivity enables the use of wearable tactile displays for medium-level communication, such as for a navigational aid [119].
Figure 7. Application of a tactile UI. [119]