All-polymer hair structure with embedded three-dimensional piezoresistive force sensors

https://doi.org/10.1016/j.sna.2012.03.045Get rights and content

Abstract

This paper presents the design, fabrication, and experimental demonstration of an artificial hair sensor made of polymer materials. Long polymer hair structures embedded with flexible three-dimensional piezoresistive sensors were formed on a flexible substrate in two-dimensional arrays to detect an applied external force. The flexible piezoresistor was implemented using a carbon nanotube-polydimethylsiloxane (CNT-PDMS) composite prepared by the mixing of multi-wall carbon nanotubes with PDMS. A CNT-PDMS composite with high conductivity and a high gauge factor was obtained by increasing the CNT concentration in PDMS. In this study, we used a 13% CNT-PDMS composite for fabricating the sensing element that was embedded in the proposed hair sensor. The hair sensor is 4 mm in height and 700 μm in diameter. The implemented device shows a linear and sensitive response in the on-axis mode with a sensitivity of 2900 ppm/μm.

Introduction

The piezoresistive effect, which is the change in the electrical resistance of a material due to an applied mechanical stress, has been widely utilized in many microelectromechanical system (MEMS) devices such as pressure sensors and accelerometers. Metals and semiconductor materials such as silicon have been widely used as piezoresistive material because they are easy to process and can be integrated into an electronic circuit. However, the gauge factor (GF) of a metal is so low that a sophisticated sensor design and a readout circuit are required to increase the sensor sensitivity [1]. Also, thick-film resistors (TFRs) have been widely used as piezoresistive sensors [2], [3], [4]. Most of TFRs are based on ruthenates or RuO2 with low temperature coefficient of resistance. The typical gauge factor of TFR is between 3 and 15. In contrast, doped silicon has a high gauge factor of up to 200 because the resistance change in the semiconductor is caused by not only the geometrical change but also the resistivity change [5], [6]. However, silicon is a hard material and is suitable for restricted applications in which the deformation or displacement is so small that non-linear operation and structural failure such as breakage are avoided. With advancements in information and robot technology, artificial skin is being researched as an important component of robot systems to facilitate the effective and safe interaction of robots with humans and the environment. A recent research reported the use of a piezoresistive sensor as an intelligent sensing element for artificial skin. Because the sensor needs to adapt to a curved surface and would be exposed to repeated bending and stretching, a completely flexible sensor design would prove to be advantageous for artificial skin applications. Engel et al. reported an all-polymer artificial hair cell sensor that was fabricated using carbon-impregnated polyurethane force-sensitive resistors. Though they first introduced the three-dimensional hair sensor using microfabrication technology, the sensor was patterned on a two-dimensional surface by using micro-molding technique [7].

Conductive nanoparticles, such as carbon black (CB), or carbon nanotubes (CNT) are an interesting material for strain-sensing applications because they provide excellent flexibility and possess a piezoresistive property by forming a network linkage in films or nanocomposite polymer forms [8], [9], [10], [11], [12], [13]. Liu and Choi and Jung et al. introduced a patterning of a CNT film on a flexible substrate by using a microcontact printing method and demonstrated a resistance change according to the applied strain [10], [11]. However, this technology is suitable only for a planar sensor system, and it is difficult to apply it to three-dimensional devices including the hair sensor. In order to address this issue, research has been conducted on the use of a CNT nanocomposite polymer as a piezoresistive sensing material. Polydimethylsiloxane (PDMS) has been widely used as a base polymer material because it has superior mechanical elasticity, for example, a maximum tensile strain of 100% without any cracks. In addition, PDMS is easily processed by a simple micromolding technique. Recently, a micromolding technique was used to pattern a CNT-PDMS composite as a sensor material [8], [12], [13]. To ensure that the initial resistance of CNT-PDMS is low, the CNT concentration in PDMS should be high; however, this high concentration results in high viscosity, which makes it difficult to process CNT-PDMS.

In this paper, we propose a high-concentration nanocomposite polymer as a piezoresistive sensor material and present the fabrication of a three-dimensional microstructure using this material. Furthermore, we present the design, fabrication, and experimental demonstration of an artificial hair sensor made from the proposed CNT-PDMS piezoresistor.

Section snippets

Experimental

The dispersion of nanoparticles is an important factor in the fabrication of conductive composites, especially to obtain a high concentration nanocomposite polymer, because the viscosity of the polymer remarkably increases with increased CNT concentration, which makes it difficult to achieve uniform dispersion and implement the fabrication processes. To solve this problem, in this work, we gradually mixed the base solutions, including PDMS prepolymers, and the diluting solvent. In addition,

Design

Fig. 3 shows the concept of the proposed artificial hair cell. The hair structure is mounted on a flexible printed circuit board (FPCB). The piezoresistive sensing element made of the CNT-PDMS composite is embedded in the bottom region of the hair structure because the maximum stress and, consequently, the maximum strain is applied on the region where the structure is bent by an external force. The horizontal and vertical cross section of sensing element are depicted in Fig. 3(b) and (c),

Conclusions

In this study, we have characterized the various electrical properties of a CNT-PDMS composite in comparison with a CB-PDMS composite and developed fabrication processes to implement three-dimensional microsensing elements having high CNT concentrations. The conductivity and gauge factor were highly dependent on the concentration of CNTs. As an application of a piezoresistive CNT-PDMS composite, we designed and implemented the artificial hair sensor to measure the direction and magnitude of the

Acknowledgements

This work was partially supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010-0019453) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) [2009-0076641].

Ji-Eun Han received his B.S. and M.S. degrees in Electronic Engineering from Sogang University, Korea in 2009 and 2011, respectively. She is currently working at Samsung Electronics.

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Ji-Eun Han received his B.S. and M.S. degrees in Electronic Engineering from Sogang University, Korea in 2009 and 2011, respectively. She is currently working at Samsung Electronics.

Dongil Kim received the B.S. and M.S. degrees in Electronic Engineering from Sogang University, Korea in 2007 and 2009, respectively. He is currently a Ph.D. candidate in Electronic Engineering at Sogang University. His current research area includes MEMS and energy harvesting devices.

Kwang-Seok Yun received his B.S. degree in Electronics Engineering from Kyungpook National University in 1996, M.S. and Ph.D. degrees in Electronics Engineering from Korea Advanced Institute of Science and Technology (KAIST) in 1997 and 2002, respectively. He was a post-doctoral researcher at University of California, Los Angeles from 2005 to 2007. He joined the Department of electronic Engineering at Sogang University, Korea in 2007, where he is now an Associate Professor. His current research area includes MEMS, integrated microsystems, micro total analysis systems, and micro sensors and actuators.

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