TU Delft Examines Innovative 3D Printable Metamaterials with Applications in Soft Robotics

A group of materials scientists from the Delft University of Technology (TU Delft) have been working on a project that introduces the interesting concept of 3D printable “action-at-a-distance” metamaterials, which could one day be used in wearable soft robotics.

Mechanical metamaterials are a sub-category of designer materials, where the synthetic material’s geometry at the small scale is designed to introduce unusual properties and functionalities, such as shape memory behavior.

“It is a remarkable property of these metamaterials that they create very complex actuation patterns with only one actuation force,” explained Professor Amir Zadpoor, who works with the Additive Manufacturing Laboratory in the university’s Department of Biomechanical Engineering. “These materials are also easy to fabricate, as a low-cost 3D printer and a two-component elastomer is all you need to make these cellular structures.”

Professor Amir Zadpoor

A specific pattern of local actuation is programmed right into the design of soft cellular materials to 3D print these action-at-a-distance metamaterials, meaning that a single force could create the necessary local actuation. According to Dr. Reza Hedayati, another TU Delft researcher, local actuation typically needs a network of wired-together, inter-connected actuators that are externally controlled to create the activation pattern.

Dr. Reza Hedayati

“In our material, the actuation pattern is programmed into the geometry of the cellular structures,” Dr. Hedayati said. “There is therefore no need for a network of local actuators and their associated wiring and controllers. This makes these actuators much cheaper and easier to make.”

The team recently published a paper on their work, titled “Action-at-a-distance metamaterials: Distributed local actuation through far-field global forces,” in the APL Materials journal; co-authors include Dr. Hedayati, Dr. M. J. Mirzaali and L. Vergani with the Politecnico di Milano, and Dr. Zadpoor.

The abstract reads, “Here, we propose the concept of ‘action-at-a-distance’ metamaterials where a specific pattern of local deformation is programmed into the fabric of (cellular) materials. The desired pattern of local actuation could then be achieved simply through the application of one single global and far-field force. We proposed graded designs of auxetic and conventional unit cells with changing Poisson’s ratios as a way of making ‘action-at-a-distance’ metamaterials. We explored five types of graded designs including linear, two types of radial gradients, checkered, and striped. Specimens were fabricated with indirect additive manufacturing and tested under compression, tension, and shear. Full-field strain maps measured with digital image correlation confirmed different patterns of local actuation under similar far-field strains. These materials have potential applications in soft (wearable) robotics and exosuits.”

Undeformed and deformed combined structures having barrel-shaped (left), narrow-waisted (middle), and vase-shaped (right) final geometries.

Soft robotics has been attracting a lot of attention, especially in the 3D printing world, due to high flexibility and added safety for human operators. But, one of the major challenges of the field is the creation of complex actuation patterns, to accomplish tasks like grasping delicate objects or helping patients move more easily. That’s why TU Delft’s “action-at-a-distance” metamaterials are so important – they could make it easier, as well as less expensive and time-consuming, to make these actuators.

“Perhaps a more important aspect of this research is proposing a new design paradigm for soft actuators,” said Zadpoor. “Instead of using a network of interconnected actuators, you can use the geometry and program the actuation pattern into the fabric of a soft material. This could open up a lot of other opportunities to use geometry for actuation.”

The team showed how a combination of 3D printed cellular structures in honeycomb (with a positive Poisson’s ratio) and bow-tie (negative Poisson’s ratio) patterns could be utilized to make soft actuators with these complex actuation patterns.

“The basic principle in our geometrical designs is to gradually change the number of honeycomb and bow-tie cells throughout the cellular structure to create a specific actuation pattern,” explained Dr. Mirzaali, who shares first authorship of the paper with Dr. Hedayati. “These cellular structures are sometimes called graded structures because of this gradual change.”

This gradual change could be designed in a number of patterns, such as striped, radial, linear, or similar to a checkerboard. Depending on the type of change, the actuation pattern will then be different.

Dr. Hedayati said, “The other advantage of these geometrical designs is that the resulting actuators are lightweight and unwired and could be easily worn. This could be the basis for wearable soft robotics such as soft exosuits that would help patients with limited mobility.”

The name action-at-a-distance, in case you were wondering, is because a single force applied at a large distance from the actuators is all that’s necessary to activate them.

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