Anomalous pH Actuation of a Chitosan/SWNT Microfiber Hydrogel with Improved Mechanical Property
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0915-R01-05
Anomalous pH Actuation of a Chitosan/SWNT Microfiber Hydrogel with Improved Mechanical Property Su Ryon Shin, Sang Jun Park, Seong Gil Yoon, Chang Kee Lee, Kwang Min Shin, Bon Kang Gu, Min Kyoon Shin, Min Sup Kim, Yu Jin Kim, and Seon Jeong Kim Hanyang University, Seoul, Korea, Republic of
ABSTRACT Composite fibers composed of chitosan and single-wall carbon nanotubes (CNTs) have been fabricated using a wet spinning method. The dispersion was improved by the sonic agitation of the CNTs in a chitosan solution followed by centrifugation to remove tube aggregates and any residual catalyst. The mechanical behavior was investigated using a dynamic mechanical analyzer (DMA). The mechanical tests showed a dramatic increase in Young’s modulus for the chitosan/CNT composite fibers fabricated using the improved dispersion method. The strain on the microfibers was determined from tensile load measurements during pH switching in acidic or basic electrolyte solutions. The microfibers showed a general actuation behavior of expanding at pH = 2 and contracting at pH = 7 under low tensile loads. However, a reverse of this actuation behavior was exhibited under high tensile loads. This anomalous pH actuation is both new and surprising. It was explained from an analysis of the differences in sample stiffness and Poisson’s ratio under tensile load in electrolyte solutions with different pH values.
INTRODUCTION There is a growing interest in developing engineered actuation systems, such as hydrogels, that have properties similar to soft biological materials [1]. In an aqueous environment, stimuli-sensitive hydrogels undergo a reversible change in volume that results in a dramatic dimensional swelling and shrinking on exposure to, and removal of, an external stimulus, such as pH [2], electric field [3], or temperature [4]. The ability to directly convert chemical energy into mechanical work makes stimuli-sensitive hydrogels attractive, and their force generation per unit mass has been reported to be of the same order of magnitude as that of human muscle [5]. Recently, many researchers have studied soft actuators using pH-sensitive hydrogels [6–8]. Spinks et al. reported on a peculiar observation, in which chitosan/poly(vinyl alcohol) gels, which normally expand under compressive load, shrank due to a change in the pH of an electrolyte solution, and these were suggested as being potential actuator materials [9]. This pH sensitivity of the hydrogels is governed by the diffusion of the electrolyte solution, and the response is achieved by a reduction in size, making these hydrogels attractive materials for microscale actuation. We have studied the mechanical properties of chitosan microfiber hydrogels, which can be improved by using single-wall carbon nanotubes (CNTs) for reinforcement. The strength of chitosan/CNT microfiber hydrogels compares favorably with that of natural muscle, and therefore these chitosan/CNT composite microfibers may be useful as bioactuators and artificial muscle materials. In this work, we prepared c
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