Newswise — WASHINGTON, March 31, 2020 — Refreshable braille displays translate information from computer screens into raised characters, often along the bottom of a keyboard. But this technology can cost thousands of dollars and is limited, typically displaying a string of characters much shorter than most sentences. Researchers now report an improved material that could take these displays to the next level, allowing those who are blind or who have low vision to more easily understand text and images, while lowering cost.
“With more development, we think this new material’s properties could make it possible to create much higher resolution devices, perhaps even those capable of displaying information other than text, such as diagrams or maps,” says Julia R. Greer, Ph.D., the project’s principal investigator.
Braille displays currently on the market rely on the piezoelectric effect: A small crystal expands when voltage is applied to it, pushing a pin upward to create a dot. A single character, such as a letter, is encoded by up to eight such dots. Devices on the market typically display at most 80 characters at a time, or a fraction of a sentence or tweet.
Researchers have recently turned their attention to electroactive polymers (EAPs) as a type of material that could improve these displays. EAPs could display much more information than conventional devices, as well as a greater diversity of it. What’s more, the devices could be easier and cheaper to manufacture. That promise has yet to materialize, however, and EAP-based displays have encountered many issues, including the need for high voltage to operate and poor durability.
Greer’s team at the California Institute of Technology (Caltech) conceived of an entirely new type of EAP based on polyionic complexes, and in the summer of 2019, the group began working on a way to synthesize the material. An improved EAP could help braille technology catch up to that used by those with sight, says Rob Learsch, who was a graduate student in the lab at the time. “Braille technology hasn’t changed much since the 1980s,” he notes. “I think it would be remarkable to allow everyone to benefit from the revolution in miniaturization and computation that has occurred.”
Whereas conventional EAPs rely on electrical charge accumulating on electrodes, the new material contains positively and negatively charged polymers combined into a random network of chains connected at nodes. The negatively charged polymers form a solid scaffold to which the positive ones bind, acting like rubber bands that pull everything together. Applying an electrical field unravels these connections, as if cutting the rubber bands, and causes the material to expand outward. The polyionic EAP requires much less voltage, and is more efficient and resilient, than conventional EAPs.
Learsch has since joined the lab of Julia Kornfield, Ph.D., at Caltech, where he and others are continuing to study the material’s properties and develop it to the point where it can be used within braille displays and, perhaps, offer new functionality for these devices. Because the material can act like a capacitor, generating an electrical signal when pressure is applied, it could be used to build braille displays that respond to touch, much like the screen of a smart phone or tablet.
The material also could be used for other applications. If controlled by precise electrical fields, Learsch says he could foresee it opening and closing a robotic joint or gripper. “There is a lot of research to be done to get us from where we are now to these types of products, but that is all part of our long-term vision,” Learsch says.
The researchers acknowledge support from the National Science Foundation Graduate Research Fellowship Program and the Department of Defense Vannevar Bush Faculty Fellowship.
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Durable, low-voltage electroactive polymers formed from polyionic complexes
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.