Thermal Molding of Organic Thin-Film Transistor Arrays on Curved Surfaces
© The Author(s) 2017
Received: 31 January 2017
Accepted: 26 April 2017
Published: 12 May 2017
In this work, a thermal molding technique is proposed for the fabrication of plastic electronics on curved surfaces, enabling the preparation of plastic films with freely designed shapes. The induced strain distribution observed in poly(ethylene naphthalate) films when planar sheets were deformed into hemispherical surfaces clearly indicated that natural thermal contraction played an important role in the formation of the curved surface. A fingertip-shaped organic thin-film transistor array molded from a real human finger was fabricated, and slight deformation induced by touching an object was detected from the drain current response. This type of device will lead to the development of robot fingers equipped with a sensitive tactile sense for precision work such as palpation or surgery.
KeywordsFlexible electronics Organic semiconductor Strain Curved surface Artificial tactile sense C 8-BTBT Thin-film transistor
Plastic and printed electronics have recently attracted extensive interest for flexible and stretchable device applications. However, research on device fabrication technologies for curved surfaces is not as active even though many modern commodities consist of smoothly curved plastics such as poly(ethylene terephthalate) bottles, blister packs, medical tubes, and connectors. Although the adaptability of stretchable electronics to spherical surfaces has been demonstrated , stretchable devices generally cannot maintain their self-standing shape on their own. Therefore, not only flexible or stretchable devices but also curved-surface devices are needed for a wide range of applications. For example, if robot fingertips could be equipped with sensitive thermal and tactile sense using curved-surface electronic devices, more precise surgical operation could be performed using medical robots.
In this paper, we present a novel technique to fabricate an electronic device array on a freely designed curved surface via thermal molding of plastic sheets and direct melting of organic semiconductors with subsequent recrystallization, which is expected to enable the preparation of 3D plastic electronics. We fabricated a fingertip-shaped organic thin-film transistor (OTFT) array as a curved surface device test case and succeeded in detecting slight deformation induced by a soft touch from an object.
Results and Discussion
The carrier mobility and threshold voltage map presented in Fig. 4 b, c summarizes the properties of the 88 TFTs. The working OTFTs are colored along the effective field-effect mobility. A group of working OTFTs are observed. However, the thicknesses of the OTFTs were not sufficiently uniform because of the limitation in mechanical precision of the present mold. Therefore, the distributions of the observed field-effect hole mobility and threshold voltage were wide at present.
The fabrication of plastic electronics on curved surfaces using thermal molding was demonstrated in this work. The induced strain distribution of PEN films when planar sheets were deformed into spherical surfaces clearly indicated that natural thermal contraction played an important role in the formation of the curved surface to avoid excess tensile strain, which could result in fracture of the electrode pattern. The OTFT characteristics of a fingertip-shaped OTFT array molded from a real human finger were also examined. The drain current response to slight deformation induced by touching an object clearly increased with increasing internal strain. By scanning the OTFTs, it would be possible to detect the hardness or edge of an object based on the internal strain distribution. This type of device array fabricated on a curved surface will enable the development of robot fingers equipped with a sensitive tactile sense, which is sufficient for precision work such as palpation or surgery.
The authors acknowledge Jun-rou Hayashi and Yushi Sasaki for their experimental help. The authors would like to thank Nippon Pneumatic Mfg. Co., Ltd., Japan for providing the pulverized organic semiconductor material prepared by supersonic jet milling. The authors would also like to thank Teijin Ltd., Japan, for providing the PEN films. This work was financially supported by a research grant from the CASIO Science Promotion Foundation.
This work was financially supported by a research grant from the CASIO Science Promotion Foundation.
MS is the corresponding author and principal investigator of this research work. KW and HI are graduate school students who carried out the experiments. YO and HY are lab members and provided the experimental and technical help. YS provided the organic semiconductor material. KK is the professor and director of this lab and provided a useful advice. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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