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Development for refreshable braille devices has recently shifted to electroactive polymers (EAP). This paradigm benefits from greater precision, smaller size, and lower cost associated with modern electronics, opening the door for higher resolution and less expensive devices. Displays with resolution finer than required to display braille characters will enable representation of non-text information such as figures, tables, or diagrams. However, adoption of EAPs for responsive displays has encountered problems such as high required field strength (kV/cm), insufficient pressure (2 kPa generated), and poor durability. This work presents a new type of electroactive polymers, based on poly ionic complexes. In contrast to previously reported electroactive polymers, these gels do not rely on a solvent bath, respond to a field that is on the order of V/cm, and expand in a direction parallel to the applied electric field.
Ultimately, the expansion is driven by electrostatic interactions: The polyionic gels are overall charge neutral, however, when an electric field of sufficient strength is applied, the ionic bonds are broken and the polycation is drawn towards the plate. This reveals the charged backbone of both the polyanions and the polycations, which causes the repeat units to repel one another and the gel to expand. This mechanism is confirmed using cyclic voltammetry and impedance spectroscopy.
With the understanding of the mechanism, the gels are made to expand rapidly and tolerate 100s of cycles. These properties are controlled through the identity of the ionic repeat units and further tuning the crosslinking and processing of the gels. The ionic crosslinks within the gel impart desirable qualities such as self-healing and high toughness a through a cooperative effect, yielding durable gels that are the much stronger than their component parts.
The low power requirements and resilient nature of these EAPs make them attractive for use as refreshable braille displays. This methodology could be adapted to other actuators such as soft robotic joints or grippers.
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