This Is Shaping Up Nicely

These days, most technological advances in digital displays involve packing in ever more pixels into smaller and smaller spaces or increasing the rate at which those pixels are refreshed. Advances such as these certainly make the images these displays produce clearer and more realistic, but they also leave out something important, especially when it comes to touchscreens — tactile feedback. Shape-changing displays are an emerging technology that has not really arrived yet. This is not because there is little demand for it, but rather because existing solutions are too cumbersome and bulky to be practical for real-world use.

When technology finally does catch up with our dreams, shape-changing displays could be used to stimulate the sense of touch in virtual worlds or even to create a dynamic braille display for the visually impaired. Before that can happen, a number of problems must first be overcome. The most promising shape-changing displays of today are actuated pneumatically or hydraulically. But these systems require bulky components like pumps and valves that make them unsuitable for use in most real-world applications, especially in portable use cases.

There may be a better path forward, however, says a team led by researchers at Tsinghua University. They have developed a flexible, shape-changing actuator called FlexEOP that leverages embedded electroosmotic pumps. The unique design of these actuators makes them fully self-contained — no external pumps or reservoirs are required for operation. Accordingly, FlexEOP is practical for integration into small and portable shape-changing displays.

Each FlexEOP unit, known as a "shape-changing unit," is composed of several components. At the core of each unit are flexible silicone reservoirs that contain and control the pumping fluid, allowing for distinct deformation modes. There are two main reservoir types: one for vertical expansion, which uses a sealed top membrane that protrudes when fluid flows in, and another for horizontal expansion, which features internal partitions that bend the material side-to-side when pressurized.

The design includes a flexible printed circuit for the electrode layer, which significantly reduces the unit's thickness and enhances flexibility. Electrodes apply an electric field to initiate fluid movement within the electroosmotic pump. A layer of pressure-sensitive adhesive binds the structural components together, providing insulation and adhesion without the rigidity of traditional spacers, boosting flexibility. Glass fiber filters are used as pump membranes, while propylene carbonate serves as the pumping fluid.

The system operates by applying voltages of +250V, 0V, or -250V, controlled through an Arduino and relays, to direct fluid movement within each shape-changing unit.

To test FlexEOP, a number of shape-changing devices were built. In one experiment, a flexible strip display, with nine independent shape-changing units in a linear sequence, was created to act as a one-dimensional tactile display that can wrap around surfaces, such as a wrist, or conform to other curved objects. In another demonstration, a flexible panel display was developed with a 3x3 matrix of shape-changing units, designed as a two-dimensional tactile display that could be used on skin. A similar device, with smaller actuators, was created in another experiment that can serve as a two-character dynamic braille display.

Looking ahead, the team is planning to further explore just how much flexing the displays can take before they crack, and also how that flexing impacts electroosmotic flows. They also hope to conduct some studies that will inform the design of future wearable devices that users will find acceptable. This work should go a long way toward making shape-changing displays more practical for real-world use.