An automated system designs and 3D-prints complex robotic parts that are optimized according to an enormous number of specifications. The system fabricates actuators — devices that mechanically control robotic systems in response to electrical signals — that show different black-and-white images at different angles. The researchers 3D-printed floating water lilies with petals equipped with arrays of actuators and hinges that fold up in response to magnetic fields run through conductive fluids (see figure).
The actuators are made from a patchwork of three different materials, each with a different light or dark color and a property — such as flexibility and magnetization — that controls the actuator's angle in response to a control signal. Software breaks down the actuator design into millions of three-dimensional pixels (voxels) that can each be filled with any of the materials. Then, it runs millions of simulations, filling different voxels with different materials. Finally, it lands on the optimal placement of each material in each voxel to generate two different images at two different angles. A custom 3D printer fabricates the actuator by dropping the right material into the right voxel, layer by layer.
Actuators optimized for appearance and function could also be used for biomimicry in robotics; for example, other researchers are designing underwater robotic skins with actuator arrays meant to mimic denticles on shark skin. Denticles collectively deform to decrease drag for faster, quieter swimming. Underwater robots could have arrays of actuators coating the surface of their skins that can be optimized for drag and turning efficiently.
Robotic actuators today are becoming increasingly complex. Depending on the application, they must be optimized for weight, efficiency, appearance, flexibility, power consumption, and various other functions and performance metrics. Generally, experts manually calculate those parameters to find an optimal design. In addition, new 3D-printing techniques can use multiple materials to create one product. That means the design's dimensionality becomes incredibly high, creating so many combinations of materials and properties that there is no chance to evaluate every combination to create an optimal structure.
The researchers first customized three polymer materials with specific properties needed to build the actuators: color, magnetization, and rigidity. They produced a near-transparent rigid material, an opaque flexible material used as a hinge, and a brown nanoparticle material that responds to a magnetic signal. The characterization data was fed into a property library. The system takes as input grayscale image examples and executes a complex form of trial and error — around 5.5 million voxels are iteratively reconfigured to match an image and meet a measured angle.
To fabricate the actuators, the researchers built a custom 3D printer that uses a technique called drop-on-demand. Tubs of the three materials are connected to print heads with hundreds of nozzles that can be individually controlled. The printer fires a 30-micron-sized droplet of the designated material into its respective voxel location. Once the droplet lands on the substrate, it's solidified. In that way, the printer builds an object, layer by layer.
The technique could be used as a stepping stone for designing larger structures such as airplane wings.