Abstract
Materials that are capable of actuation in response to a variety of external stimuli are of significant interest for applications in sensors, soft robotics, and biomedical devices. Here, we present a class of actuators using composites based on a polymer of intrinsic microporosity (PIM). By adding an activated carbon (AX21) filler to a PIM, the composite exhibits repeatable actuation upon solvent evaporation and wetting and it is possible to achieve highly controlled three-dimensional actuation. Curled composite actuators are shown to open upon exposure to a solvent and close as a result of solvent evaporation. The degree of curling and actuation is controlled by adjusting the amount of filler and evaporation rate of the solvent casting process, while the actuation speed is controlled by adjusting the type of solvent. The range of forces and actuation speed produced by the composite is demonstrated using acetone, ethanol, and dimethyl sulfoxide as the solvent. The maximum contractile stress produced upon solvent desorption in the pure PIM polymer reached 12 MPa, with an ultimate force over 20 000 times the weight of a sample. This form of the composite actuator is insensitive to humidity and water, which makes it applicable in an aqueous environment, and can survive a wide range of temperatures. These characteristics make it a promising actuator for the diverse range of operating conditions in robotic and medical applications. The mechanism of actuation is discussed, which is based on the asymmetric distribution of the carbon filler particles that leads to a bilayer structure and the individual layers expand and contract differently in response to solvent wetting and evaporation, respectively. Finally, we demonstrate the application of the actuator as a potential drug delivery vehicle, with capacity for encapsulating two kinds of drugs and reduced drug leakage in comparison to existing technologies.
Highlights
Polymers that are able to react to external stimuli by changing their properties have recently attracted interest due to their applications in biomedicine, mechanical actuation, sensing, soft robotics, and self-healing surfaces
While the intrinsic microporosity of these materials has attracted significant interest for applications related to separation membranes[20] and gas storage,[21−23] we demonstrate in this paper that these intriguing materials can be successfully used as actuators and drug delivery vehicles
We propose that the lower carbon-rich layer swells and shrinks to a lesser extent than the upper polymer of intrinsic microporosity (PIM)-1-rich layer (Figure 1), and this is in agreement with the observation that the lower smooth layer would always form the outer surface of the tube and the upper layer would form the inner layer of the tube
Summary
Polymers that are able to react to external stimuli by changing their properties have recently attracted interest due to their applications in biomedicine, mechanical actuation, sensing, soft robotics, and self-healing surfaces. Materials designed to change their shape in a controllable and reversible manner have been considered in the production of drug delivery systems,[1] gene delivery vehicles,[2] precision actuators and switches,[3−6] artificial muscles,[7] and walking devices.[8] Such adaptive soft matter can be classified depending on the triggering mechanism that initiates the shape-changing behavior; this can include temperature,[9] electric charge,[3,7,10] humidity,[6,11−14] pH, solvents,[13−16] or UV light.[17] The development of such new materials is often inspired by the strategies of living organisms to sense the environment or achieve actuation. Similar mechanisms have been employed in hydrogels to achieve controlled and reversible changes in shape under conditions of changing humidity; their response time is usually slow (up to several hours) and the generated stress is low due to high water content.[18]
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