MIT and Amherst material science researchers have published a paper in ACS Applied Material & Interfaces that describes an untouched-by-human-hands method for making self-folding circuits with a 3D printer; the materials are laid down precisely so that as it cools, differential rates of contraction cause it to bend into dimensional forms that are ready for use in a finished device.
The approach is a significant improvement over other "self-folding" approaches that require the addition of liquids, or mechanical assists, in order to effect the 3D shaping.
At the heart of this self-folding scheme is the swelling of the cross-linked polymer in its starting ink components, and the ability to deposit multiple materials. To verify that the layer-by-layer process that facilitates swelling is indeed responsible for the introduction of the residual stress, the ink is poured in a mold and cured in a single step using a high-intensity (14 W cm–2 at 365 nm, UV fusion) broadband UV source. Upon release from the mold, the cured piece does not expand but instead shrinks slightly as a result of the formation of cross-links. To quantify the swelling tendency of the cross-linked polymer in the ink components, the samples (cured in a single step) are allowed to swell by immersion in each of the individual ink components. The swelling results (Figure S1) show that SR440 (isooctyl acrylate) plays a dominant role (causing ∼10% expansion in each dimension of a 1 mm thick slab, after 1 h), whereas all of the other individual components lead to negligible swelling over the same time. This is in part due to the nature of the side chains of the acrylate components. SR440, SR504, and SR313B include an isooctyl group (C8H17 chain), ethoxylated nonyl-phenol group (C23H39O4 group, which includes a phenol group), and a dodecyl group (C12H25 chain; methacrylate), respectively. It is expected that SR440 diffuses the most due to its low molecular weight and shortest side chain, although multiple other factors may be relevant. The expected diffusion times are explained further in Figure S1. It is important to note that during the actual printing process, the uppermost printed layers are exposed to one pass of the 0.9 W cm–2 UV source before exposure to the fresh ink. Therefore, the diffusion is expected to be further enhanced in the printing process compared with the single-step fully cross-linked samples. After small molecules diffuse into the underlying layers, they are cross-linked into the existing network with the subsequent UV-curing passes.
[Subramanian Sundaram, David S. Kim, Marc A. Baldo, Ryan C. Hayward and Wojciech Matusik/ACS Applied Material & Interfaces]
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