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Engineers Show How to Make 3D Neural Circuits from Simple 2D Surfaces

Elaborate architectures that just 'pop out.'
​Image: Rogers et al

Imagine trying to spray-paint faces onto miniature figurines. It might work for some giant life-sized doll, but for small stuff, the paint just bleeds into a single smudge.

3D printing has limits, hype notwithstanding. One of those is size, or at least size as determined by precision. At a certain point, what we know as 3D printing—a computer-controlled additive process where objects are constructed in layers—becomes too low-resolution for supertiny tasks. At nanometer resolutions, we just get smudges.

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A new technique, however, offers an interesting twist to the problem: 3D construction in 2D. A single flat layer designed to "pop out" detailed nanoscale structures.

The technique comes courtesy of an international team of researchers drawn from the United States, China, and South Korea, who describe their work in the current issue of Science. The idea might feel familiar, recalling the 3D paper cutouts a lot of us probably made in grade school. The difference, however, is in scale and bottomless complexity.

In kindergarten I might have made some cool 3D paper snowflakes, but here we're talking about building brain-like electronic networks, biomedical devices, self-assembling nanomachines, and that's just the beginning.

The basic idea is that a 2D surface is printed using similar lithographic techniques to those used in existing but highly-advanced semiconductor manufacturing. This substrate is so carefully defined such that when stress is applied in just the right way, a 3D structure pops out of it.

"The resulting processes of controlled, compressive buckling induce rapid, large-area geometric extension into the third dimension," the researchers write, "capable of transforming the most advanced functional materials and planar microsystems into mechanically tunable 3D forms with broad geometric diversity."

The technique is known as "residual stress-induced bending." Different objects and shapes are etched into some flat material in just such a way that when the material is twisted or bent just a bit, the object pops out. This can be extremely elaborate.

Image: Rogers et al

"Forces above a certain threshold initiate a controlled buckling process that lifts the weakly bonded regions of the [objects] out of contact with the substrate surface and, at the same time, induces spatially dependent deformations (in terms of twisting and bending) and in- and out-of-plane translations," John A. Rogers, the study's lead author, and his team explain. The resulting structures are the product of a careful balance between the substrate's adhesive properties and the applied force.

The etched shape isn't uniformly connected to its substrate. At various points, it's firmly attached to it using chemical bonds. So, some points remain fixed as the material buckles, while others are free to leave the flat plane. The video makes it seem simpler than it is, but it's easy to see how things could quickly become very detailed and very sophisticated: cytoskeletal webs, neural circuits, vasculature networks, etc.

"We have senior co-authors on this paper who have developed really quantitatively precise models of how the mechanics works," ​Rogers told Physics World. "We are just beginning to explore those models as design tools to investigate what range of topologies we can access in this way."