A group of researchers from Caltech and Harvard has developed a chain of parts built from soft materials which can transmit mechanical signals. These flexible, mechanical logic gates represent the next step in making soft robotics a reality.

Although mechanical signals experience significant attenuation when going through a soft material, the invention can successfully transmit the signal across long distances. The flexible device developed by Harvard and the California Institute of Technology has applications in soft robotics and lightweight aircraft built from pliable matter.


Soft Robotics

Soft robotics, which is a relatively new field, has many potential applications and can revolutionize the way humans interact with machines. Unlike a conventional robot, a soft robot can handle objects with more degrees of freedom. More importantly, such a robot can manipulate delicate objects much more successfully due to being built from soft materials. For example, a soft robot can lift a victim of injury and carry them to safety without causing them further harm.

Recently, there have been some interesting studies about soft robots. Researchers at the OCTOPUS Integrating Project have developed a soft robot which mimics the movements of an octopus as it moves through water, grabs objects, and walks along the floor of a pool.

Octobot, developed by a team of researchers at the Harvard University, is another interesting example of soft robots. Octobot, which is built by embedded 3D printing and soft lithography, is operated by a chemical reaction. In other words, instead of using conventional electrical circuits and batteries, the robot utilizes a microfluidic logic circuit to direct the flow of fuel and perform movements.


The adaptable structure of an octopus is the main reason why researchers are interested in this sea creature. Inspired by the dexterous movements of an octopus, researchers hope that they may arrive at some practical solutions for soft robotics.


The Challenge of Mechanical Signal Dampening

The recent study by the researchers from Caltech and Harvard aims to solve one of the main challenges faced by soft robotics: transmitting a mechanical signal through a soft material— to make a robot “muscle” move, for example.

Soft matter attenuates a mechanical signal. This can be intuitively understood by comparing a soft punching bag with a firm one. When we hit a soft punching bag, the flexible surface of the bag deforms and the bag experiences smaller swings compared to a firm bag. Therefore, it is not easy to transmit the signal across a long distance and at a constant speed through a soft material.

The new method prevents the signal from being attenuated or scattered by employing a nonlinear unstable system to circumvent this phenomenon. Although a nonlinear unstable system has many applications in engineering, many people prefer to use a linear stable system because of its simple analysis and design.

Going against this tradition, researchers from Caltech and Harvard have resorted to a nonlinear unstable mechanical system in order to compensate for the energy that is absorbed by soft material.


Employing Both the Linear and Nonlinear Systems

A spring is an example of a linear stable system. When we push a spring, it responds by pushing back with a force which is linearly proportional to the one that we have applied.

In contrast, consider a flexible arch which is fixed to a frame at the two ends. Built from elastic material, the arch can have two stable states; the concave and the convex states. If you apply enough force to the arch, it can go from one of these states to another one. Unlike a spring, if the force that you apply exceeds a threshold, the arch will change its state from convex to concave or vice versa. The moment this transition happens, the arch itself releases some energy and applies a force in the direction of our force.

This is an example of a nonlinear mechanical system because the direction of the force that the arch applies changes as it goes from one state to another.

Since the arch has two stable states, it is considered as a bistable system.

The research uses a chain of these bistable elements connected to one another by springs. When one of these arches goes from the concave to the convex state, its spring applies a force to the arch that is next downstream in the chain. Consequently, the next arch changes state and this goes on like dominos.


The bistable elements and their linking springs in two different stable states. Image courtesy of Kurzweil AI.


The energy released by the popping of the arch balances the energy that is absorbed by the soft material. As a result, the signal travels at a constant speed without experiencing attenuation.

According to SEAS's Katia Bertoldi, the John L. Loeb Associate Professor of the Natural Sciences and senior author of the paper, the new method not only prevents the signal from being attenuated but also eliminates the distortion of the signal. Hence, with this method, the signal strength and clarity are both maintained.

Neel Nadkarni, a Caltech aerospace graduate student involved in the research, says that the mathematical equations of these systems, which are based on phase transformations, are strongly nonlinear and not very well understood.


Creating Soft Mechanical Logic Gates

The research prototype, which is based on 3D printed elements, is described in the August 8th, 2016 publication of Proceedings of the National Academy of Sciences. This paper, entitled "Stable propagation of mechanical signals in soft media using stored elastic energy” presents mechanical AND and OR logic gates based on the new technique. This paper is the third in the series of publications introducing new methods to transmit signals.


The mechanical AND and OR gates designed based on the new method. Image courtesy of Kurzweil AI.


These logic gates are designed by the appropriate choice of the parameter $$d_{out}$$, as shown in the above figure.

By choosing a small $$d_{out}$$, a stronger force is required to enable the output and push it out. Therefore, a small $$d_{out}$$ will lead to an AND gate because the energy barrier will be higher and both the inputs must be pushed to enable the output.

However, a large $$d_{out}$$ will reduce the barrier energy which leads to an OR gate. This is due to the fact that either of the inputs being pushed is able to enable the output.

The first paper in the series, published in the journal Physical Review B, discussed the mathematical base for the introduced technique. 

The second paper, which was published in Physical Review Letters, presented the first experimental model for the system. This paper also described that it is possible to use magnets, rather than springs, to connect the bistable elements to one another. This holds the potential to reset the system to its original state by changing the polarity of the magnets.


Future Steps: Nanoscale Applications?

One of the lead researchers of the project is Dennis Kochmann, an Assistant Professor of Aerospace in Caltech's Division of Engineering and Applied Sciences. He believes that the introduced system has numerous potential applications but also that it shows similar behavior in materials at the atomic scale. Considering the fact that it is difficult to examine such atomic scale behavior either experimentally or computationally, the new system provides a macroscale valuable analog for this behavior.