Sex robotics can be fabricated by several approaches including multi-material 3D printing, shape deposition manufacturing, soft lithography, or integrate multiple manufacturing approaches to create composite materials.
Schematic of the single soft finger fabrication process is presented below. (a) Both the upper molds A, B and the bottom mold are fabricated by 3D printing. (b) Silicone rubber is poured into the upper mold B. Then the upper mold A is forced into the upper mold B that is filled with uncured material. (c) Both top layer and bottom layer of soft actuator are removed from the 3D printed rigid molds after being cured. (d) The top layer with topographical features and the bottom layer are bound together; the black arrows indicate the bonding direction. (e) The overview of the soft finger with a cross section view is presented which schematically illustrates the topographical features, the inner chamber and the air inlet.
The soft finger is fabricated by using purely soft silicone elastomeric material. The rigid molds are designed in CAD software and are fabricated using PLA material; its simplicity enables to fabricate quickly. The multi-step molding process of a single soft actuator is used. The upper molds A and B are fabricated to form the extensible top layer. The bottom mold is used for the inextensible bottom layer part of the soft finger. Firstly, the uncured silicone rubber material is poured into the molds. The molds and uncured materials are then put in a vacuum oven for 30 min for degassing, therefore to remove the air bubbles, and are then heated to a temperature of 60C for 2h in order to cure the material. Finally, the parts are demold and then bonded together by applying a thin layer of silicone rubber on the bonding surface. By inflation or deflation, the soft actuator that contains topographical features would expand or shrink, which further results in bending in either convex or concave states.
As initial attempts, topology optimization methods have been applied to design soft bending actuators for use in grippers driven by cables, or pneumatic actuators. In these works, the gripper design problems were simply translated into the design of fingers modeled by cantilever beams, and their topological shapes were optimized by gradient-based algorithms. The optimization results typically have irregular structural forms and, thus, usually are difficult to manufacture using traditional methods such as molding and casting. Instead, they can be directly prototyped using additive manufacturing technologies.
Other approaches inspired by botanical systems are the microstructures. They are concurrently created and aligned in a single bottom-up process. Self-morphing soft actuators using laser induced graphene (LIG) structures developed in a bottom-up approach which resemble the aligned microstructures of plant cellulose fibrils. The LIG structures are used to dictate the shape of the soft actuator, based on LIG layers and a polymer when light is stimulated under a 150 W lamp. A bending curvature of 1.8 cm is reached within 5 s, while 5.5 s are needed for the actuator to recover its original flat shape. In fact, the LIG does not only absorb and respond to light but also electricity, organic vapor, and moisture. As a result, different programmable shapes can be achieved as shown in the figure below..
Sequential self-folding has also been achieved in printed 2D shapes using visible light. Liu et al. demonstrated hinges on polymeric sheets which were prepared using printed inks having various light absorptivity; as a result, a 3D sequentially folded structure can be realized when light with different wavelengths (red 660 nm, blue 530 nm, and green 470 nm LEDs, intensity 0.5Wcm2) is shone on the 2D structure. The hinges completely fold after an exposure duration of at least 40 s. The development technique is very simple and appealing as it requires a single type of stimuli (light) in addition to a single substrate which is patterned using a desktop printer. Nevertheless, the demonstrated technique allows for a single event of folding and unfolding. However, repeatable deformations could be achieved using reversible SMAs.
Several newest approaches in manufacturing are also given in the illustration including: (d) fabrication process steps of tabular DEAs, (e) coating and (f) metallization step process. Tubular architecture-based DEAs is presented with a semi-automated fabrication process, which involves two stages of processes, i.e., coating and assembly. In the coating stage, the fluid polymer is mixed and processed for gravure coating. The elastomeric film is then coated onto the carrier and later delaminated.This process would be repeated n-times as a closed-loop process for molding.
Next, the delaminated elastomer film is proceeded for roll-to-roll manufacturing of elastomer layers where the vacuum deposition process is utilized for the fabrication of metallic electrodes. Zhao et al. simulated different designs of rolled DEAs (i.e., actuator’s geometry, thickness of elastomer, and number of turns, etc.) using Multiphysics Software and fabricated compact-rolled DEAs for both axial and radial displacement free motion and blocked motion. The performance of DEAs at 200 Hz bandwidth, 1 N blocked force, and 1 mm displacement is reported to be capable of performing 50 000 operations. The roll-to-roll fabrication step consists of only two basic steps, i.e., multilayering and rolling processes.
In the multilayering process, the thin sheet of elastomer is spin-casted on an acrylic sheet which acts as substrate and is then thermally cured. In rolling process, the large strips of multilayered elastomers are rolled to form a hollow cylindrical shape of actuator. The fabricated DEAs were experimentally characterized by applying a voltage that varies linearly from 0 to 1 kV after each 10 s. The actuators are found to perform linear forced and free displacements with negligible degradation in performance up to 50k cycles.
Schematic of the fabrication process of the LIG-based soft actuators as well as the optical and electron images composed of LIG are given below. Sequential self-folding in printed ink-based hinges takes place when exposed to different wavelengths of light. The top-left figure illustrates the metamorphic deployable structure assembly, before curing the polymer. The top-right figure represents the structure after curing and removing the mold. The deployment of the assembled structure is represented in the bottom figure.
Other works have been carried out to make soft structures compatible with high interaction forces occurring when manipulating or grabbing objects. From this viewpoint, the ability to switch from a soft to a stiffer structure becomes a desirable feature. In order to obtain this behavior, some systems are based on the use of low temperature fusible alloys. This allows the creation of metamorphic mobile sex robots that can be converted into grippers, flexible deployable structures, tunable stiffness materials or actuators. Another possibility is to use particles or flat sheet layers that can be jammed together using vacuum, effectively stiffening a structure. This has been used for bending actuators, continuum robots or adaptive graspers.