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A Method for Building Self-folding Machines

S.Felton, M.Tolley, E.Demaine, D.Rus, R.Wood, Science.,2014,Vol. 345 no. 6197 pp. 644-646 DOI: 10.1126/science.1252610

Fig 1: A bimorph actuator formed by a combination of (A) two outer stretched polystyrene layers (PSPS) , two inner paper substrate layers, and a polyimide bearing copper circuit (PBC). (B) The cuts in the paper and PSPS layers allow for simultaneous contraction and expansion of the fold shown in (C). The fold uses an active hinge designed to move freely and repeatedly (D,E). The examples are seen on the actual robot (F,G,H).

Self-assembling mechanisms are incredibly beneficial as they create efficiencies in logistics, manufacturing and other applications. Recently, newer methods have improved these advantages by introducing a simple bimorph actuator to create a self-folding robot using a soft material found commonly in plastic cups. Felton and his team at Harvard University in collaboration with the Massachusetts Institute of Technology demonstrate this actuator, including a stretched form of the common polymer Polystyrene, through “origami robots.” This medium has shown potential as an effective soft material in shape memory composites that is malleable but yet sturdy enough to support itself and the robot. The actuator allows for a paper origami type folding mechanism that can be less bulky than typical mechanisms. These actuators simultaneously self-assemble the robots parts from a flat shape to a completely three-dimensional shape to produce the same effect as any self-assembling operation in a more energy efficient manner.
In order to perform the complex folds carried out by this robot, a shape memory composite was made out of two outer contractile layers of stretched Polystyrene (PSPS), two layers of paper substrate, and a polyimide bearing copper circuit (PBC). In essence, this layering results in a bimorph actuator (Figure 1). When a current is passed through the PBC, it heats the contractile layer to approximately 100, exerting a tension on the substrate, causing a complex fold. In operation, the tension of one layer of the PSPS contracts, while the other expands, causing a bending displacement.
The group attempted to build three different robots. Out of the three, one had the capability to move about only using its folds for three dimensional transformation and movement. The results offer potential hinge and composite layering designs that can be manipulated and improved to suit the needs of a specific self-assembly process. Sam Felton and his team have demonstrated that the new technology can be applied in future remote autonomous assembly in logistics, for example, being able to transport a large number of flat products that would assemble themselves at arrival and manufacturing through inexpensive planar fabrication techniques.

The full article can be found here where you can also watch a video of the robot in action.


- Mike Facio


Nanosheets and Stacked Nanodisks

“Supramolecular precursors for the synthesis of anisotropic nanocrystals“ Whitney Bryks, Melissa Wette, Nathan Velez, Su-Wen Hsu and Andrea R. Tao. J. Am. Chem. Soc., 2014, 136 (17), pp 6175

Electron microscope Images a) and b) show Cu2S in stacked nanodisk formation originating from CuSC16H25 alkanethiolates.

In the past, copper sulfide nanomaterials have been used in photovoltaics, battery electrodes and electrochemical sensors. Recently, colloidal chalcocite (Cu2S) nanocrystals have drawn attention for their ability to support the excitation of localized surface plasmon resonance. However, a large barrier in the application of Cu2S as a plasmonic material stems from the difficulty of assembling Cu2S anisotropic structures consisting of either shells, rods, wires or disks. Andrea R. Tao and her team at University of California, San Diego have developed a method to create nanosheets and stacked nanodisks of Cu2S via solventless thermolysis, which they hope can be applied to work as plasmonic materials.
To control the shape that was formed with the Cu2S nanocrystals, Cu thiolates that adopt lamellar, micellar and isotropic phases in various thermolysis reactions were used to template the nucleation and growth of solid-state Cu2S nanocrystals. Cu2S was formed by melting various copper alkanethiolates, CuSC12H25, CuSC4H9 and CuSC16H33. The copper alkanethiolates entered a mesogenic phase due to their hydrophobic interactions between neighboring alkane chains and strong metal-sulfur coordination resulting in the Cu2S. Afterwards, thermolysis occurred at temperatures much higher than the melting points. It was found that each Cu2S precursor resulted in a different Cu2S nanocrystal structure. The initial phase of the alkanethiolates determined the final anisotropic structure of the nanocrystals into either nanosheets or stacked nanodisks, as seen in Fig 1.
The chain length of the copper alkanethiolates directly affected the Cu2S nanocrystal template. The longer chained alkanethiolates CuSC12H25 and CuSC16H25 formed nanodisks due to micellar columns forming at 140C. The short-chained alkanethiolates CuSC4H9 formed the nanosheets during a smectic-like lamellar phase carried out at 160C. Both morphologies can be seen in Fig 1. These findings can lead to synthetically generated nanocrystal morphologies that can be applied to plasmonic nanoelectronic and optoelectric devices.
The full article can be found here:

- Marcus Rice

Induction of Defect Loops in Nematic Fields

Smalykuh, Ivan I, Angel Martinez, Miha Ravnik, Bruce Lucero, Rayshan Visvanathan and Slobodan Zumer. Nature Materials, 2014

Fig 2a) Images taken from a scanning electron micrograph showing a single knot of T(3,2) i.e. 3 loops, 2 revolutions. Fig 2b) Scanning electron micrograph of a 4x4 array of knots T(5,3). Above both images are the corresponding 3D models.

Designing and assembling three-dimensional (3D) structures of low-symmetry colloidal particles is challenged by the lack of systems and techniques that allow for controlling their spatial arrangements. When considering the nanoscale confinement and mesoscale self-assembly of nanoparticles in liquid crystal, the types and the arrangements of the spatial defects are both important to keep in mind upon the construction of 3D patterns. Ivan I. Smalyukh and his team in the University of Colorado, Boulder have developed a system that enables the generation and control of 3D patterns found in knotted nematic colloids. This work can be used to predict configurations of looped line defects and the interplay of topologies of knotted surfaces, fields and defects.

Rigid particles were obtained by using a two-photon photopolymerization with spatially patterned pulsed femtosecond laser light. This method allowed the team to construct particles with the topology of colloidal knots. Consisting of polymeric tubes, the particles are looped p times with q revolutions, T(p,q), about the colloidal rotational symmetry axis. Examples of colloidal knots can be seen in Fig. 2. Various configurations of knots even became mutually tangled when constructed in large quantities. The particles’ molecular orientations and points of incompatibility with the nematic liquid crystal resulted in point defects called “boojums”.

The team found that boojums around different knots could be specifically characterized both by the number of times the director rotates as one circumnavigates the defect core and by the bulk topological charge. The overall interplay of these particle topologies with the liquid crystal was controlled by the varying surface boundary conditions. This approach to a predictable way of experimenting with knotted colloids can lead to uses involving self-assembly of metal and semiconductor nanoparticles possibly applied to information displays, metamaterials and data storage.

The full article can be found at the Nature Materials website.

- Michael Lane




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