Know when to fold 'em: Advances in self-assembly techniques
By David Szondy
July 17, 2013
Sure, flat-pack furniture is inexpensive and easy to transport, but when you open the box the first question almost everyone asks is, “Wouldn't it be great if it would assemble itself?” You could get a robot to help, but engineers at the Harvard Microrobotics Laboratory are working on ways to get objects to assemble themselves ... and they might give 3D printing a run for its money at the same time.
3D printing is seen as the next big thing. That’s not surprising for a technology that offers versatile manufacturing without much capital or technical expertise. You can take a simple CAD file, switch on the printer, and out come solid objects that only a few years ago would have taken a skilled craftsman to produce. However, it does have its limitations as to what it can make, how quickly, and where it can make them.
The Harvard Robotics Laboratory team have been developing an alternative to 3D printing based on flat objects that fold themselves like origami into 3D objects. According to the team, the technique uses less material than printing and is faster and less expensive because it can be done with laser cutting and lithography. It’s also logistically simpler to use and can form complex objects that are stronger than their printed counterparts because the sheets can be formed into beams and other structures.
The self-assembling 3D objects start as a series of flat sheets joined at their edges with hinges. The hinges are made of memory materials, which return to their original shape when properly stimulated. In this case, the hinge is made of such a material in a composite of several layers. In addition to the memory material, such as a polymer or metal, there’s also an inert substrate. When stimulated, the memory material softens and the substrate causes the composite sheet to bend and the whole acts as a hinge that folds under its own power.
The stimulus that causes the folding can be any of a number of things, depending on the material in question. It can be heat, magnetic fields, electric fields, liquids, visible or UV light, lasers or radiation. This removes the assembly problem by making it possible for the object to assemble itself. With the right material responding to the right stimuli, the object can be made to assemble itself remotely either automatically or on command. A global stimulus, such as temperature, can assemble simple shapes, but it doesn’t allow for complicated forms because the parts will simply jam together. On the other hand, specific stimuli, such as an electric current, can be applied in sequence, so complex objects can assemble themselves – even to the point of negotiating tab and slot fasteners.
The team’s idea was to develop an approach that is inexpensive to manufacture, functional, and where each hinge can be activated individually. It uses a shape memory polymer, called polyolen, that is bonded to a sheet of paper. The polymer is heated electrically, which turns it from a glassy to a rubber phase. As it alters, the polymer contracts and the paper base causes the composite sheet to bend, forming a hinge.
According to the team, the composite is inexpensive to make using laser cutting and chemical etching. The electric heating elements are bonded into the composite, with each layer bonded by silicone tape. The composite is then bonded to the hard sheets that make up the body of the object. This design is combined with control and computation algorithms that trigger the hinges simultaneously or in sequence to make the object assemble itself, even if it’s relatively complex.
The approach is designed to allow for quick manufacture. One version used in the experiments, the crane, took only an hour to construct using a laser cutter, a solid ink printer, and a ferric chloride etch tank.
Once developed, the technology has a number of applications that go beyond the potential for self-assembling boxes and flat-packed furniture. Satellites that assemble themselves in space could reduce payload in rockets and the military could drop flat-pack structures that can set themselves up without a team of skilled engineers. Importantly, all of this can be done without humans having to put themselves at risk in hazardous environments.
The current technique being developed on the micro-scale has its limitations, such as limited composite thickness, hinge torque, maximum fold angle, and material resolution. It will require some refinement of the materials to improve the technique and make it practical.
The findings are published in the journal Soft Matter.
The video below shows various experimental objects assembling themselves.
Source: Harvard Microrobotics Laboratory
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