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Illumination & Displays

Electroactive polymers for rigid-to-rigid actuation and Braille e-books

Bistable electroactive polymers comprise a new category of smart materials that can achieve electrically induced rigid deformation.
17 February 2010, SPIE Newsroom. DOI: 10.1117/2.1201002.002632

Impaired vision greatly limits one's ability to communicate, receive education, and travel to new locations. Braille paper has been in use for over a century and is still the main communication tool to teach blind children literacy. Text-to-speech technology does not teach literacy and might be responsible for its decline in blind children in the US. There are few refreshable Braille products on the market, and most use individual electromechanical cells made from piezoelectric bimorph actuators. Their complicated mechanical structure limits the number of Braille cells that can be packaged onto a display panel and imposes a prohibitively high price. A large number of other technologies have been experimented with or are being developed, including the use of electroactive polymers that could produce a large stroke without requiring strain amplification.1,2

Among the electroactive polymers studied for Braille applications, dielectric elastomers can be actuated to greater than 100% strain with a high specific-energy density.3–5 However, this high strain sacrifices mechanical stiffness: the gel-like elastomers generally have an elastic modulus on the order of or less than 10MPa, compared to approximately 1GPa for most commercially available polymers (such as polystyrene and plexiglas). These materials are not very stretchable, however. We have developed a new bistable electroactive polymer (BSEP) that is rigid, can be actuated reversibly and repeatedly to greater than 100% strain, and has a rigid and stable deformed shape.6 Our BSEP is used to develop Braille versions of electronic books.

The first BSEP that we demonstrated experimentally is based on poly(tert-butylacrylate) (PTBA), a thermoplastic polymer that is rigid in ambient conditions. It has a storage modulus of 1.5GPa at 30°C, which decreases to 0.4MPa through a glass transition at approximately 50°C. At this elevated temperature, the polymer behaves like a rubber, with a measured dielectric constant and dielectric strength of 5.4 and 260MV/m, respectively. A thin PTBA film can be actuated electrically to strains as high as 335% (in area) in a diaphragm actuator configuration (see Figure 1). The calculated maximum actuation pressure is 3.2MPa.

Figure 1. Diaphragm actuator. (a) Initial device. (b) Actuated to a deformed state. (c) Original shape recovered.

The actuation strain can be controlled (from 0 to 300%) by the applied electric field. The deformation is bistable through temperature control across the PTBA glass transition. The thermoplastic polymer possesses several properties that are important for BSEPs, including a fairly narrow temperature range inducing the transition between glassy and rubbery states, low elastic modulus and high dielectric properties in the rubbery state, and high strain fixity and recovery rates (shape-memory properties). The dielectrically driven actuation is instantaneous. However, the temperature management slows down the overall actuation speed. BSEP actuators are suitable for open-loom, long-duty-cycle, and intermittent actuation.

Our BSEP could be useful for controlled shape changes of rigid morphing structures. The resulting structures can support high mechanical loads, and actuation to large-strain deformations is feasible. To demonstrate the material's applicability for Braille e-books, we fabricated a six-dot diaphragm actuator using a 30μm-thick PTBA film. The latter was coated with a thin layer of single-wall carbon nanotubes (SWNT) as compliant electrode. We taped the PTBA material to a Kapton® film with six openings arrayed in a Braille-cell pattern. We then attached the bilayer sheet to a diaphragm chamber and applied an AC to the SWNT electrode to heat the polymer. We achieved dielectric actuation with high-voltage DC or pulse current. Upon actuation, the six dots were raised to half-dome shapes. The domes flattened after we reapplied the current to warm the polymer. The raised dots can each support up to 0.6N force without significant deformation. We also made a slightly larger version, which we actuated to display ‘UCLA’ in Braille letters (see Figure 2).

Figure 2. Six-dot diaphragm actuator displaying ‘UCLA’ in Braille letters.

In summary, BSEPs comprise a new category of smart materials for rigid-to-rigid, large-strain actuation. The electrically induced actuation can be programmed by a central computing unit or other electronic device for easy control. The material and processing are both scalable for low-cost production. We are currently optimizing processing and patterning approaches for refreshable Braille displays. Our goal is to demonstrate a page-size electronic reader for people with severe vision impairment to read and communicate using the Internet.

Qibing Pei, Zhibin Yu, Xiaofan Niu, Paul Brochu
University of California, Los Angeles
Los Angeles, CA