Polymer Gel Actuators: Fundamentals

Abstract

One view of materials development is to search for what materials correspond to empty spaces on a hypothetical multi-dimensional map of the properties of available materials. Following a biomimetic philosophy, for instance, it can be seen that tough ceramics and moldable short fiber composites with high moduli are possible but absent from the list of available synthetic materials. Likewise artificial muscle is missing, where the properties are defined as a developed stress of over 300 kPa, a linear contraction of 25 % and a response time of below one second. Currently dielectric elastomers come closest but have disadvantages [1,2].The actin-myosin muscle system provides the performance target for electroactive polymer actuators [3,4]. The process is driven chemically by the energy change from hydrolysis of the polyphosphate bond as ATP (adenosine triphosphate) binds to myosin and is converted to ADP (adenosine diphosphate) and phosphate. A simple chemical analogy suggests that muscle-like gels should be feasible but it is now clear that the task is much harder than it seems.Early work on gel actuation by Katchalsky-Katzir demonstrated that engines could be built using the chemical energy in diluting a lithium bromide solution to drive contraction and expansion of a collagen belt [5]. In essence, the collagen expands to take up the salt solution or contract to exclude the water. This change is both a molecular conformation change and a volume change. Other chemically driven gels, for instance polyacrylic acid fibers [6] which respond to a pH change, also rely on a solubility change giving rise to a volume change.

Actuating individual electrospun hydrogel nanofibres

“…The dashed lines shown in Fig. 7 are leastsquares linear fits to the experimental data highlighting the linear dependence of the force change with the amount of pre-stretch and in agreement with eqn (1). Further, the slopes of the linear fits correlate closely to the difference in nanofibre stiffness (S 0 -S) as measured experimentally and as expected from eqn (5).…”

Synthesis of a directionally-conductive polyaniline hydrogel composite by vapor deposition.

“…In building a gel-based actuator, it is important that swelling behavior occurs only over a range where surface properties are constant, meaning that sample size, diffusion rate, and response time are coupled [2]. The self-diffusion rate of water at 20°C is approximately = 2 × 10 -5 cm 2 /s [39], and diffusion times can estimated as (17) where is the thickness of the sample.…”

“…Most of the synthetic gels under investigation are formed by the free radical polymerization of acrylate monomers. These monomers tend to polymerize atactically, making the formation of strengthening microstructures limited [2]. Additionally, since oxygen is a free-radical scavenger [40], control of the polymerization can be difficult and can lead to irreproducible results, especially in small scale syntheses.…”

“…For one-phase gels involving a small volume change, diffusion behavior can be adequately modeled. For two-phase gels or those involving large volume changes, the kinetics of diffusion are complex [2].…”

Responsive hydrogels – structurally and dimensionally optimized smart frameworks for applications in catalysis, micro-system technology and material science

“…170 Hydrogels have long been proposed and investigated as artificial muscle materials. 171 The principal problem with hydrogel actuators is their prohibitively slow actuation rate. Hydrogel volume transitions triggered by changes in temperature, pH and ionic strength involve the mass transport of solvent (water) into and out of the hydrogel.…”