The web's foremost resource on soft condensed matter.

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

Flocking at a distance in active granular matter

Nitin Kumar, Harsh Soni, Sriram Ramaswamy & A. K. Sood. “Flocking at a Distance inActive Granular Matter.” Nature Communications 5, 4688 (2014)

Figure 1: A monolayer of beads and rods on top of a flower shaped vibrational bed demonstrating the process of spontaneous flocking.

Emergent order is a unique concept that describes a process in which complex patterns and structured phenomena occur from smaller entities that don’t individually exhibit these complex traits. One example of such a process is flocking, a collection of objects moving in a synchronized manner. A team lead by Ajay Sood at the Indian Institute of Science in collaboration with the Tata Institute of Fundamental Research, has used an interesting experimental technique to recreate flocking in a granular material. Sood and his team did this by using millimeter-sized tapered rods in a medium of spherical beads and an underlying vibrating surface (Figure 1). Using a simulation model they also constructed an analytical theory for the spontaneous phase change in which tapered rods transition from a disordered form to an ordered one. In addition, they describe experimental methods in which they were able to increase or decrease flocking by changing rod/bead concentrations. The “flocking” of small particulates created by their model has potential to be used as a transport mechanism for particulate or even cellular matter as a new form of active matter.
The actual mechanism of motion for the tapered step wise rods has been described in other studies cited in the article; what the group focuses on now is why and how these rods find a complex flocking order through the vibrational energy provided by the surface. Using a magnetic shaker the research group moves the amorphous monolayer of beads and rods within a flower-shaped sample cell (preventing particle accumulation on boundaries) and captured image data using a high speed camera (Figure 1).
The results show spontaneous emergent order in the form of flocking for the bead-rod system and demonstrate for the first time, the formation of a true flock in a collection of dry grains. Based on the hydrodynamic data of a 2-dimensional fluid of beads,the authors were able to develop an analytical theory for their system. Their data found a positive correlation between flocking and higher concentrations of both beads and rods.

Further research in this field could lead to development of active matter as a form of particulate transport, and potentially even be used in cellular matter in bioengineering applications.

You can read the full paper here.

And watch a video of the flocking in action featured in Science Magazine.

- Mike Facio




Content for class "clear" Goes Here