Smart Actuation from Coupling between Active Polymer Gels and Fibrous Structures

Abstract

Active polymer gels have been around now for nearly thirty years [1–2]. Their unique swelling and de-swelling characteristics in the present of solvents, combined with the possibility of triggering the volumetric expansion or contraction by a number of physical and chemical means, make them particularly interesting for actuation. Changes in temperature, pH or ionic strength are some of the “stimuli” which can be used to get to gels to respond by expanding or contracting in volume [3–6]. Electro-active polymer gels, responding to electrical fields, are particularly suited for actuation because of the ease of controlling the stimulus [7–9]. A great deal of work has already been done in characterising the physico-chemical aspects of their behaviour, quantifying the magnitude of the effects involved and in trying to develop actuators which mimic to some extent the behaviour of muscles [10–12]. By their very nature, actuators based on active polymer gels are better suited to applications where large displacements and low forces are useful, such as perhaps in the medical field. The large volumetric swelling capacity, often with swelling ratios of 10 or more, provides the means of achieving large deformations. The intrinsic low elastic modulus of a swollen gel, typically of the order of 10–100 kPa, limits very significantly the magnitude of the forces which can be generated [13–14]. However, there are in nature several examples of biological actuation besides muscular action, which can provide some solution to the problem of limited force generation. Turgid plant cells, for example, depend for their actuation function on mechanisms which are conceptually similar to those which govern the behaviour of active polymer gels but which are capable of generating large forces. They rely on partial confinement of water within a flexible fibrous container, the cell wall, and on a semi-permeable membrane, the lipid bi-layer, with a difference of chemical potential across the membrane leading to osmotic processes [15]. In this paper the potential across the membrane leading to osmotic processes [15]. In this paper the concept of integrating active polymer gels with fibrous structures will be explored, discussing the advantages and limitations of this approach. The potential benefits of such systems for large-displacement low-force or small-displacement large-force actuation will be illustrated using numerical simulations based on measured properties.