Things we do with electronics nowadays are amazing. Processors in computers contain billions of transistors, each measuring a few millionth of mm in size. Airbags sensors in cars save lives every day, measuring the sudden deceleration of vehicles and ordering the deployment of pillows at great speed.
Techniques we use to construct these components show an important limitation, however -- all fabrication processes are inherently two-dimensional. Micro-technology is tied to photo-lithography which consists on applying a mask on a planar surface, silicon wafers and such, through UV illumination. Machining in the three dimensions is nearly impossible, or limited to expensive techniques. And yet, possibilities of 3D electronics are huge. When transistors approach the size of an atom, it will be soon impossible to shrink them further. Computers won't get more powerful and hard drives capacities will not increase anymore. Moving to the third dimension can help overcome upcoming difficulties.
Imagine an organized network of transistors or magnetic storage cells. In a two-dimensional flat and organized configuration, one unit can be linked to only four neighbors. Within a cube the number of possible connexions with neighbors increase to six. When considering millions of units the difference in interconnections is big.
Benefits of micrometer three-dimensional structures is not limited to electronics. Miniaturization of surgical tools would allow painless biopsies; drugs could be precisely delivered by deployable micro-cargos.
Origami at all scales: (a) Origami inspired solar array under development. When deployed, it will expand ten times its stored size. (b) Origami robot. Initially flat, the robot uses shape memory polymers that contract like muscles when heated. Science. (c) Artistic origami in its traditional meaning. Designed and folded by Antzpantz. (d) Capillary origami.. This PDMS sheet spontaneously folds when in contact with water.
New fabrication paradigms are necessary to overcome these limits. It is unrealistic to move away from existing micro-fabrication techniques however. Installations are here, techniques are well-controlled. That's where origami comes into play: to bring planar two-dimensional structures toward the third dimension. Mathematicians have recently shown that countless of shapes can be obtained by bending only . Origami is increasingly used in industry. Many metal or plastic objects are folded out of flat material plates. Compact storage before deployment is of interest in the space industry. Examples abound at the macro-scale, but are limited for small objects. Why not use the Origami concept with micro-technologies? We make amazing two-dimensional chips, let's fold them up!
Of course, manually folding is impossible at such small scales. Tricks are required. Other forces, predominant at small scales, can be used to trigger the bending. We then talk about self-folding: smart material are designed to assemble into a predefined shape through the application of an external stimulus. Scientists have successfully demonstrated the use of pH or temperature gradients to fold tiny bio-compatible structures for instance . We, at the University of Twente in the Netherlands, have used capillary forces to assemble micro-machined structures -- the so-called capillary origami technique.
Findings and Outlooks
In our experiments, flexible micro-structures are fabricated using traditional fabrication techniques. Objects are made of silicon nitride -- a type of ceramic broadly used as an insulator in electronics -- and are fully compatible with any micro-machining process. Droplets of water are then applied to the foldable objects. Through evaporation, the liquid tends to reduce its surface in contact with air, always keeping its spherical shape, and provokes bending. The moving parts meet during folding and adhere thanks to strong forces at this scale. Wet the structures, let the water dry and voilà, you get a 3D structure the size of a grain of salt!
Self-folded micro-structures by capillary forces, false colours. Scale bars are 50 um.
Many challenges are ahead for this technique to become a viable industrial option in the future. Mass production, freedom of designs, dynamic reorganization, electrical capabilities, high yield, are desired features of such micro-structures.
With this goal in mind, we have made a few advances with capillary origami. We designed structures with a central tube through which we could provide water be a pump and trigger the folding, as shown in this short video . This technique is not an option for mass production at the moment, but is a first step into that direction. So far, only passive mechanical objects could be folded by capillary origami. When correctly designed, we showed that conducting metal lines running on top of the hinges can be bent. Thousands of thin ribbons were also folded in one go after immersion in water.
Left: micro-structures with central tube, see video. Right: folded tetrahedron. Folding is stopped thanks to smart hinges.
Up to now, the techniques lack applications. The prospects are encouraging though, and we believe that Origami is a promising option to reach the third dimension in the micro world.
 For more insights see the interesting TED talk of Robert Lang
 The Gracias laboratory at John Hopkins University, USA, is very active on this field