A study on chitosan-based hydrogels : towards the development of an artificial muscle

Abstract

This research on the artificial actuator for biomedical applications is motivated by the needs in rehabilitation engineering, particularly in prosthetic and orthotic bioengineering and a challenge of the present study. The long-term purpose of this research is to develop an electroactive hydrogel as an artificial actuator for external prostheses. People suffering from physical disabilities due to inherent physical impairments or injuries often need functional replacements, including muscle-like artificial actuators for their activities of daily living. However, the lack of actuators that are large in excursion, high in force, fast in response, small in volume, light in weight and low in cost are limiting developments in prosthetics and orthotics, particularly in the area of finger prostheses where fast and precise dexterity are required. The short-term purpose of this research is to prepare and characterize a chitosan-based hydrogel for its potential application as a prosthetic actuator. Muscles can be considered as actuators in nature. Similar to the real muscle, an actuator material should be able to undergo reversible motion and deformation in response to the control stimuli, such as electric pulses. Chitosan/poly(ethylene glycol) composite hydrogels prepared in this study were shown to possess such properties. This thesis is focused on the research of the chitosan/PEG hydrogels. Such hydrogels in the form of fiber and membrane were prepared and characterized. Under the stimulation of electric fields, their electrochemomechanical (ECM) behaviors and associated mechanisms were systematically investigated in various aqueous environments . The reversibility of the ECM behaviors was also studied in response to cyclic external stimuli. A theoretical model based on the triphasic theory for a fully hydrated polyelectrolyte matrix was applied to simulate the bending behaviors of the chitosan/PEG hydrogel. The parameters necessary to drive the model were assessed experimentally. Experimental results showed that the deformation of the chitosan/PEG hydrogel depended significantly on its composition, geometric size, crosslink density as well as other external factors, such as the applied electric potential, the pH and ionic strength of the bath medium. While maintaining adequate mechanical properties, the rate of deformation could be improved by adjusting the above factors. Within the range of parameters studied, the bending curvature was found to be proportional to the intensity of the applied electric potential. These experimental observations were interpreted in terms of fiber stiffness, fixed charge density and osmotic swelling, which depended on the equilibrium states in different pH and ionic environments. Electrochemical kinetics was involved in the transient processes. Within the ranges of crosslink density, pH and ionic strength examined in this dissertation research, an optimal condition was obtained for reversible bending behavior under an applied alternating electric field. The experimental results suggest that 0.02 M epichlorohydrin (ECH), 0.015 M Na2SO4 and 0.05% HC1 may offer an optimal condition for these ECM responses and for the mechanical properties of these chitosan/PEG fibers. The results of the theoretical simulations showed that the triphasic theory could well describe the bending behavior of chitosan-based polyelectrolyte hydrogels. For the HCl bath medium, the theoretical results apparently agreed with the experimental results. In addition, this theoretical model could also provide analyses of other information associated with the bending behavior, such as the fixed charge density, and the pressure within the gel-like mixture etc. This research is significant not only as a systematic exploration of a new actuator material, but also with regard to its many potential applications, such as in artificial muscles, robotics actuators, smart skins, drug release systems, ophthalmologic materials and electro-optical switches.