As 3D printing has become a mainstream technology, studies have investigated printable structures that will fold themselves into useful three-dimensional shapes when heated or immersed in water. A new printable structure, however, was developed that begins to fold itself up as soon as it is peeled off the printing platform.
One of the advantages of devices that self-fold without any outside stimulus is that a wider range of materials and more delicate structures can be used to fabricate them. For printed electronics, organic materials generally are used. These materials are often very sensitive to moisture and temperature. To initiate folds in these electronics, they cannot be immersed in water or subjected to heat, both of which will degrade the electronics.
A prototype self-folding printable device was created that includes electrical leads and a polymer “pixel” that changes from transparent to opaque when a voltage is applied. The device begins looking somewhat like the letter “H,” but each of the legs of the H folds itself in two different directions, producing a tabletop shape (see figure).
Several different versions of the same basic hinge design were fabricated, demonstrating that the precise angle at which a joint folds can be controlled. In tests, the hinges were forcibly straightened by attaching them to a weight, but when the weight was removed, the hinges resumed their original folds.
In the short term, the technique could enable the custom manufacture of sensors, displays, or antennas whose functionality depends on their three-dimensional shape.
The key to the design is a new printer-ink material that expands after it solidifies; most printer-ink materials contract slightly as they solidify, a technical limitation that designers frequently have to work around. Printed devices are built up in layers. In their prototypes, the researchers deposited the expanding material at precise locations in either the top or bottom few layers. The bottom layer adheres slightly to the printer platform, and that adhesion is enough to hold the device flat as the layers are built up. But as soon as the finished device is peeled off the platform, the joints made from the new material begin to expand, bending the device in the opposite direction.
The ink that produces the most forceful expansion includes several long molecular chains and one much shorter chain made up of the monomer isooctyl acrylate. When a layer of the ink is exposed to ultraviolet light — or “cured,” a process commonly used in 3D printing to harden materials deposited as liquids — the long chains connect to each other, producing a rigid thicket of tangled molecules. When another layer of the material is deposited on top of the first, the small chains of isooctyl acrylate in the top liquid layer sink down into the lower, more rigid layer. There, they interact with the longer chains to exert an expansive force, which the adhesion to the printing platform temporarily resists.
The research provides a route to create electronics using more conventional planar techniques on a 2D surface, and then transform them into a 3D shape while retaining the function of the electronics.