POLYMERS AND MATERIALS 

Polymers Insert Tab A into Slot B

by Nancy McGuire

January 18, 2016

Imagine what you could do with polymers that fold themselves into complex shapes. Furniture and toys come home from the store as flat sheets and assemble themselves with a blast from your hair dryer. Surgeons implant tightly rolled-up medical devices through tiny incisions and unfold them using a laser—or your body’s own chemistry. Robots walk and pick things up using “muscles” that flex and bend in response to light or temperature changes.

Researchers are bringing these things closer to reality, making origami shapes from polymers with different thermal expansion coefficients, absorptivity, and cross-linking behavior in response to heat, light, and immersion in liquids.

Box remembers its own assembly instructions

Martin L. Dunn at the Singapore University of Technology and Design, H. Jerry Qi at Georgia Tech (Atlanta), and colleagues made tiny “shipping boxes” (See Figure 1) that assembled themselves in a carefully choreographed folding sequence that the authors planned by using finite element simulations and reduced-order modeling. The boxes, 15 mm × 3 mm × 0.6 mm in their flattened state, had flexible hinges that connected rigid flat panels. 

Figure 1
Georgia Tech

The authors used a 3-D printer to make right-angle hinges from the shape-memory polymers VeroWhite (a rigid plastic at room temperature) and Tangoblack (a rubbery polymer). The bending rates of the hinges were fine-tuned by varying the ratios of the two components.

The researchers flattened their boxes at 90 ºC and set the flattened shapes at 10 ºC. To reconstruct the boxes, they heated the flat sheets in a water bath at 90 ºC (see video).

The time it takes each hinge to recover its 90º bend depends on the glass-transition temperature of the polymer mixture from which the hinge is made. This feature allowed the authors to fold and lock the sides in sequence, in much the same way that a person folds and locks panels to assemble a shipping box. (Sci. Rep.DOI: 10.1038/srep13616)

Feeling the heat

Ata Sina at the University of British Columbia (Vancouver) applied a similar concept on a larger scale (up to 173 × 91 cm). He foresees that his method will be used to make insulation materials, toys, and furniture that can be shipped flat and assembled by using a hair dryer or toaster oven.

Sina uses a programmable robotic device to cut and crease sheets of paper and a thermoplastic polymer that shrinks when heated (polystyrene or low-density polyethylene, for example). Then he aligns the plastic pieces with the paper pieces and attaches them. Sina heats the coated paper for ≈10-20 s at 110 ºC to fold it (see video).

He has made 3-D snowflakes and stars, pop-up greeting cards, lampshades (Figure 2), and sound-insulation panels. One of his 3-D tessellations, which consists of alternating raised and depressed cube shapes, stood up under a 6.8-kg weight. In contrast, a hand-folded paper counterpart without polymers withstood only 2.3 kg. (Master’s Thesis, University of British Columbia, Vancouver. https://open.library.ubc.ca/cIRcle/collections/ubctheses/24/items/1.0166103)

Figure 2
University of British Columbia

Michael D. Dickey, Jan Genzer, and colleagues at North Carolina State University (Raleigh) took a different approach to heat-folded origami. They used an inkjet printer to imprint a hinge pattern made from light-absorbing ink onto a prestrained polymer sheet. Exposing the sheet to light heats the ink, and the resulting strain causes the hinges to bend. The speed and angle of folding are controlled by varying the nature of the light source, the shape and size of the ink patterns, and the properties of the ink. (American Physical Society March Meeting 2015, Abstract #A44.010. http://adsabs.harvard.edu/abs/2015APS..MARA44010L)

Just add water (and light)

Ryan C. Hayward and colleagues at the University of Massachusetts Amherst, Lang Origami (Alamo, CA), and Western New England University (Springfield, MA) made reversibly self-folding origami shapes by using a hydrogel layer sandwiched between two thin rigid polymer layers. When the sandwich structure is placed in an aqueous buffer, the photo-cross-linkable hydrogel swells in response to light.

Etching micrometer-scale gaps into the rigid polymer layers allows light to reach the hydrogel layer, forming hinges. The authors state that their hinges are two orders of magnitude smaller than hinges made using other methods. The width of the exposed hydrogel layer determines the folding angle; wide gaps allow bending by as much as 180º. Raising the temperature causes the hydrogel layer to shrink and causes the sheet to flatten out again.

The authors folded an origami bird from a square sheet, 0.8 × 0.8 mm, at 22 ºC and flattened it again at 55 ºC. The process could be cycled several times. They also made a complex origami tessellation, 1.33mm on a side, with an octahedron–tetrahedron truss and 198 crease segments (Figure 3). They note, however, that this method does not permit folding in a controlled sequence—their origami shapes fold using the “collapse” method, in which all the folds are actuated at once. (Adv, Mater. DOI: 10.1002/adma.201403510)

Figure 3
University of Massachusetts Amherst

Other researchers are exploring graphene-based “centipedes” that walk and turn corners, self-folding “dumplings” that encase droplets of liquid, and polymer fiber mats that fold when water is used to draw hinge lines on them. With any luck, do-it-yourself kits that require pages of incomprehensible assembly instructions and six kinds of fasteners will someday be relegated to museum displays.