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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


Failed Escape: Solid Surfaces Prevent Tumbling of Escherichia coli

Physical Review Letters 113.6 (2014)

Improving our understanding of bacterial motility is crucial to developing the best and safest applications in bioengineering and medicine. Specifically, understanding interactions of bacteria close to surfaces will have an influence in development of sterile materials for medical equipment in addition to methods for creating biofilms and bacterial formations. Using Digital Holographic Microscopy (DHM), Medhi Molaei in the Mechanical Engineering Department of Texas Tech University in collaboration with MIT –Ralph M. Parson’s Laboratory- were able to observe the movement (Run & Tumble) of wild-type E. Coli when in the presence of a close surface. Their research has shown that: 1) DHM is a useful technique for observing small bacteria, which has mainly been used to observe bigger microorganisms, but importantly 2) Demonstrate how hydrodynamic forces have an effect on E. Coli flagella by reducing tumbling by 50% within 20µm of a surface. Understanding these mechanisms will have a positive effect on how further biomedical research and engineering of materials prone to biofouling will be done.
Tumbling is essentially an adaptive method used by bacteria with flagellum that allows them to reorient themselves by abruptly changing the direction of the flagellum’s original spin. When the tumbling is accomplished and the flagellum spins normally it produces torque in order to push forward in a straight “run.” Using a microfluidic channel, the wild-type E. Coli were observed using DHM’s 3-D imaging to measure concentration and motion throughout the channel. This contributed to data corresponding to reorientation of angle change and overall run and tumble time. This data was compared to other E. Coli in the same channel that was at least 20u away from a surface. Figure 1 provide a visual of different swimming trajectories and the percentage of bacteria following those movements when in bulk (>20 µm) or in a near surface region (<20 µm).
The wild-type E. Coli has adapted other means of reacting to environmental cues by using its flagella to reorient and move around through a series of run and tumbles. Molaei’s team have demonstrated that the effects on bacterial tumbling can be immensely hindered by the simple presence of a surface within a range of 20µm by 50%, and even in the case that tumbling does occur, the reorientation is fairly minimal. Biomedical research and engineering can greatly benefit from this effect as it can prevent infections in patients that are in need of implants or be used to produce specific bacterial formations. In addition, the research group speculate on similar results with other bacteria with or without flagella.

Read the full article here

- Mike Facio

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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