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Posted: Nov 21, 2006

Textile transistors to create truly wearable electronics

(Nanowerk Spotlight) If current research is an indicator, wearable electronics will go far beyond just very small electronic devices. Not only will such devices be embedded on textile substrates, but an electronics device or system could become the fabric itself. Electronics textiles will allow the design and production of a new generation of garments with distributed sensors and electronic functions. Such e-textiles will have the revolutionary ability to sense, act, store, emit, and move (think biomedical monitoring functions or new man-machine interfaces) while leveraging an existing low-cost textile manufacturing infrastructure. Today, only a few steps towards new architectural possibilities of realizing circuit topologies that can be implemented with textile technique have been made: one an example of nonplanar devices and one of textile based devices. Researchers in Italy have now developed an organic field effect transistor (OFET) fully compatible with textile processing techniques.
Dr. Annalisa Bonfiglio's lab at the University of Cagliari in Italy is working on the assembly of electronic devices and circuits on textile substrates. This can be done by following two alternative approaches: a "top down" one, consisting of assembling devices to transfer on textile substrates and a "bottom up" approach consisting of assembling an electronic fabric starting from electronically functionalized textile basic components.
Top down approach
Organic semiconductors (polymers and oligomers), having the electrical properties of semiconductors and the mechanical properties of plastics, are good candidates for realizing flexible transistors, suitable to be transferred on unconventional substrates as textiles.
Fiber integration issues, however, are very challenging. Patterning in particular, is a significant concern. While fiber transistors could be fabricated using conventional lithography, these would have limited scalability to large volume textile processing. What is required is an e-textile technology that facilitates the fabrication of fiber transistors in a textile-compatible, highly scalable manner.
Previously, based on a completely flexible and transparent polyester film, Bonfiglio's lab has studied a "transistor in a fiber" realized by glueing this film on a textile ribbon, in order to obtain a flexible yarn that could be employed in a textile process (for more details see "Organic Field Effect Transistors for Textile Applications"). Here the polyester film is the insulator layer of the field-effect transistor (FET) structure as well as the mechanical support of the whole structure.
Bottom up approach
In recent work ("Towards the textile transistor: Assembly and characterization of an organic field effect transistor with a cylindrical geometry"), Bonfiglio and her colleagues focus on the possibility of building an organic field effect transistor with a non planar geometry.
"In particular, we have demonstrated the possibility of obtaining a cylindrical organic thin film transistor that, due to the employed materials and the dimensions, can be used in a textile process such as weaving or knitting" she says.
(a) Structure of the cylindrical OFET. (b) Optical microscope image of the channel area. (Reprinted with permission from the American Institute of Physics)
The cylindrical OFETs have been obtained starting from a metallic fiber used in textile processes. The metal core of the yarn, covered with a thin polyimide layer, is the gate of the structure. A top-contact device was obtained by depositing a layer of organic semiconductor followed by the deposition of source and drain top contacts, made by metals or conductive polymers, deposited by evaporation or soft lithography. This transistor has shown very interesting performances, with typical values of the electronic parameters mobility, threshold, Ion / Ioff ratio very similar to those of planar devices.
Bonfiglio explains the potential of this research to Nanowerk: "More than solving a problem, the possibility of making a transistor in yarn form paves the way to an entire new group of applications and offers the possibility to leverage existing textile technology for building electronic circuits. For example, with weaving technology (the simplest way to make a fabric), one can build a matrix of crossing yarns: if each of these yarn bares a series of transistors, each node of the textile matrix can be singularly addressed. If each node bares a sensor or a led, then it is possible to read or write the matrix one 'node-pixel' per time. In this way, a flexible and wearable electronic platform can be produced."
Bonfiglio is currently coordinating an EU-funded 6th framework project called ProeTEX that focuses on textile-based MicroNano technologies to develop textile and fiber based integrated smart wearables for emergency disaster intervention personnel with a goal of improving their safety, coordination and efficiency and additional systems for injured civilians aimed at optimising their survival management.
Previously, Bonfiglio's Eolab was involved in two projects that explored the feasibility of textile transistors:
A 2003 EU-funded IST-FET (Information Societies - Future and Emerging Technologies) program called ARIANNE - Feasibility study of yARns and fabrIcs with ANNexed Electronic functions; and
FIRB, funded by the Italian Bureau of Scientific Research, a project for the development of technologies for implementation of electronic components and devices on textile substrates.
By . Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology (RSC Nanoscience & Nanotechnology) and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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