Shrinky-Dink Science

Title: Self-folding of polymer sheets using local light absorption

Authors: Ying Liu, Julie K. Boyles, Jan Genzer and Michael D. Dickey

Journal: Soft Matter

Affiliation: Department of Chemical and Biomolecular Engineering, North Carolina State University

As a kid you may have played with Shrinky-Dinks.  I remember drawing, coloring and cutting up the thin flexible sheets of plastics and having my mom stick them in a hot oven.  I then sat and watched in wonder as these sheets curled and collapsed into something very different.  My drawings were now much smaller, harder, and thicker – ready to be used in other art projects.  Researchers from North Carolina State University have taken Shrinky-Dinks (sheets of pre-strained polystyrene) and moved far beyond the application of my childhood past.  By inking “hinges” onto Shrinky-Dinks they have been able to easily convert the two dimensional polymer sheet into a three dimensional shape.

Turning 2D patterns into 3D objects has always been an interesting problem for scientists and engineers.  In this paper the authors investigate a new form of self-folding – a process defined as “a deterministic assembly process that causes a pre-defined 2D template to fold into a desired 3D structure with high fidelity.”  Although many techniques are available, they are often cumbersome in terms of the materials and time required.  In this paper, the authors take advantage of commercially available polymers and ink to create a self-folding structure.

The authors use black ink (from a desktop printer) to define hinges on the sheets of polystyrene and by placing the sheet under an IR light bulb they are able to make the sheet fold.  The polymer underneath the ink is heated far faster than the rest of the polymer, because the ink absorbs energy from the IR light.  This part of the sheet then reaches the polymer’s glass transition temperature long before the rest of the sheet causing the folding.  Because the ink can be placed on either side of the polymer sheet bidirectional folding is possible.

They found that samples folded in a matter of seconds of being placed under the light.  The researchers found they were able to control the angle at which the sheet folded by changing the length of time the polymer sheet was exposed to the IR light and by changing the shape, size, and pattern of the black ink on the sheet.  The researchers were able to form many 3D structures including rectangular prisms and pyramids.

In this paper, the authors were able to develop a simple self-folding strategy for commercially available polymer sheets.  In the future they hope to investigate the use of different light intensities and patterns on self-folding.

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