Three viable actuator morphologies composed entirely from soft silicone rubber are explored, and these morphologies are differentiated by their internal channel structure, namely, ribbed, cylindrical, and pleated.
Additionally, three distinct casting-based fabrication processes are explored: lamination-based casting, retractable-pin-based casting, and lost-wax-based casting. Furthermore, two ways of fabricating a multiple DOF robot are explored: casting the complete robot as a whole and casting single degree of freedom (DOF) segments with subsequent concatenation.
Fabrication The vast majority of soft elastomer robots rely on the processes of soft lithography and/or shape deposition manufacturing. Two elastomer layers are molded through a casting process using pourable silicone rubber. The mold used for the outer layer contains a model of the desired channel structure. When cast, the outer layer contains a negative of this channel structure.
The mold used for the constraint layer may contain fiber, paper, or a plastic film to produce the inextensibility property required for actuation. When the elastomer is poured, this material is effectively embedded within the constraint layer. The two layers are cured, removed from their molds, and their joining faces are dipped in a thin layer of uncured elastomer. Lastly, the two layers are joined and cured together.
Lamination casting with heterogeneous embeddings
Lamination-based casting with heterogeneous embeddings is a fabrication technique that extends current soft lithography casting processes. The outer layers of a soft robot are often cast separately using soft lithography techniques to inlay channel structures. Then, these layers are laminated together with a constraint layer to form the actuator.
To power actuation, supply lines are pierced through the actuator's side wall and run external to the mechanism. This approach can be prohibitive in that it creates an unreliable pneumatic interface between supply lines and actuated channels, and also these external supply lines can inhibit the robot's movement or otherwise obstruct it from completing its intended function.
By embedding heterogeneous components within the elastomer layers as they are cast, we address both of these challenges. In this section, we show how the idea of soft lithography can be combined with embedding heterogeneous components and that it is well-suited for realizing the ribbed body segment morphology. Specifically, we illustrate this fabrication process in the context of creating both a soft ribbed manipulator and soft ribbed fish robot.
Retractable pin casting
Retractable pin casting allows the relatively simple channel structure of the cylindrical body segment to be cast without lamination. This fabrication process is advantageous because it eliminates the rupture-prone seems between the channels and constraint layer seem in the ribbed morphology fabricated through lamination-based casting.
Additionally, retractable pin casting is well-suited for the modular fabrication of multibody soft robots. Here, segments are individually cast and then concatenated together to form the robot. Specifically, in this section we demonstrate retractable pin casting in the context of fabricating a cylindrical manipulator.
A cylindrical manipulator is fabricated through a retractable-pin casting using pourable silicone rubber and 3D-printed molds. First, each body segment is independently fabricated in steps 1–3, and later these segments are joined serially to form the arm in steps 4 and 5. To start, a four-piece mold is printed.
The mold is then poured in two steps. In step 1, a low elastic modulus rubber is mixed, degassed in a vacuum, and poured to form the body segment's soft outer layer, shown in white. The mold's outer piece—one half of it is shown in green—functions to form the segment's exterior. Metal rods shown in pink are inserted into the mold and are held in place by the orange bottom piece of the mold.
These rods will form the cavities for the segment's two lateral fluidic actuation channels. After the outer layer has cured, the red rigid sleeve is removed in step 2 from the extruded feature of the orange bottom piece of the mold. This produces a cavity into which a slightly stiffer rubber is poured, forming the segment's partially constraining inner layer, shown in cyan.
The extruded feature of the orange bottom piece, shown by its orange end tip, functions to produce the segment's hollow interior core. In step 3, the body segments are removed from their molds and joined to rubber endplates, shown in cyan, using silicone adhesive. The small yellow channel inlets were added on one side of the pink metal pins during step 1. In step 4, soft silicone tubes are joined to each embedded channel's inlet. The resulting bundle of tubes is passed through each segment's hollow interior. Lastly, in step 5, multiple body segments are attached at their endplates using the same adhesive.
Lost Wax Casting
As mentioned, existing soft robots are often produced through a multistep lamination process, which produces seams and is prone to delamination. By abandoning the need for lamination, the retractable pin fabrication process enables seamless channel structures; however, the channel structures are limited to a relatively simple shape. For these reasons, we introduce lost-wax casting as part of the fabrication process for soft actuators.
With this, arbitrarily shaped internal channels can be achieved to enable a wider range of applications. As examples, in this section we fabricate a pleated unidirectional gripper and a ribbed soft fish tail using the lost-wax approach. The complete fabrication process for a pleated actuator consists of eight steps. In step (A), harder silicone rubber is poured into a mold, which contains a 3D-printed model of the wax core. In preparation for step (B), the model is removed and the rubber mold is left inside the outer mold.
Next, a rigid rod or tube, for example, made of carbon fiber, is used as a supportive inlay for the wax core. The rod is laid into the cavity of the rubber mold, supported on both ends by the outer mold. This ensures that the wax core does not break when removed from the rubber mold. Mold release spray is applied to the silicone rubber mold to ease the wax core removal process. The wax is heated up until it becomes fully liquefied.
The assembly of the rubber mold and the outer mold is heated up for a few minutes to the same temperature as the wax. Using a syringe, the liquid wax is injected into the assembly. Within a few minutes, the injected wax will start to solidify and significantly shrink in volume; this is counteracted by injecting more hot wax into the solidifying wax core during the cool-down period. In step (B), the wax core is first allowed to completely cool down, then it is released from the mold.
In step (C), the cooled down wax core is assembled together with the bottom mold, which defines the pleated structure of the actuator. The mold assembly is aligned with a top mold using pins. This top mold provides additional volume to cover the wax core.
In step (D), low elastic modulus rubber is mixed, degassed in a vacuum, and poured to form the pleats and allowed to cure. In step (E), stiffer rubber is poured on top of the cured pleats to form a constraint layer. In step (F), the cured actuator is removed from the mold.
In step (G), most of the wax core is melted out by placing the cured actuator into an oven in an upright position. After this, remaining wax residues are cooked out in a boiling water bath.
Finally, in step (H) a silicone tube and a piece of silicone cord get covered with silicone adhesive and are inserted into the front and back holes, respectively.
The actuator can be used as a unidirectional gripper or as one agonist actuated segment within a multiple body manipulator. The actuated body of the hydraulic fish is also produced via lost-wax casting. In step (A), the rubber mold is poured and cured inside an assembly consisting of an outer mold with lid and a model for the core inside of it.
In preparation for step (B), the lid and the model core are removed and the rubber mold is left inside the outer mold. The rubber mold receives a small carbon fiber tube as an inlay in its center cavity. This ensures that the wax core does not break when being removed from the rubber mold. Mold release spray is applied to the silicone rubber mold to ease the wax core removal process. The wax is heated up until it becomes fully liquefied.
The assembly of rubber mold and outer mold is heated up for a few minutes to the same temperature as the wax. Using a syringe, the liquid wax is injected into the assembly. Within a few minutes, the injected wax will start to solidify and significantly shrink in volume; this is counteracted by injecting more hot wax into the solidifying wax core during the cool down. In step (B), the wax core is first allowed to completely cool down, then it is released from the mold.
In step (C), a head constraint, a center constraint, and two wax cores are assembled together inside the tail mold halves using spacers, positioning pins and screws. In step (D), a mix of silicone rubber with glass bubbles is poured into the tail assembly and allowed to cure. In step (E), most of the wax core is melted out by placing the fish tail in an upright position into an oven. Finally, in step (F) the remaining wax residues are cooked out in a boiling water bath.