The fabrication procedure for the single-chamber shell-reinforced actuators and the multi-chamber unconstrained actuators is discussed in detail in this section.
The technique used to fabricate the soft pneumatic actuators presented ensures robustness and repeatability in manufacturing, since it leaves minimal manual intervention and margin for error.
The actuators are composed of two main parts: 1) the actuator body and 2) the un-stretchable shell. The actuator body is made out of highly elastomeric siloxane material (EcoflexTM 00-30, Smooth-on-Inc., PA, USA). The un-stretchable shell is made from a much stiffer plastic material (polyethylene terephthalate (PET), Q-ConnectTM). Images of two types of fabricated actuators, in bending and linear motion are shown below.
1. FABRICATING ACTUATOR BODY
2. FABRICATING UN-STRETCHABLE SHELL
3. INFLATION COMPENSATION AT CHAMBER ENDS
From experimental testing, it was observed that at the border of the air chambers, the ballooning rate is approximately double than that at other portions along the the rest of the laces. This is due to the fact that the air flow at the edge of the chamber undergoes a dramatic change in cross-sectional area from a very small inlet diameter to a much larger diameter for the air chamber, thereby introducing vortices and turbulent flow in that region, leading to larger instabilities and inflation of the actuator body in that region. To compensate for this added inflation effect at the end of the chamber, slits that are half the width of the slits in all other portions on the shell surface are created at the extremities of the shell pattern. This design improvement is found to help substantially in achieving a uniform pattern of inflation with the actuators.
These classical type of SPAs comprise multiple air chambers connected by narrow passages. The SPAs are fabricated using a conventional soft lithography process. The fabrication steps are listed below:
STEP 1. A 3D-printed mold defining the locations of the SPA chambers and their inter-connecting air channels is created.
STEP 2. Next, an elastomer in liquid form (EcoflexTM 00-30 from Smooth-on Inc.) is mixed using a centrifugal mixer then de-gassed in a vacuum chamber at 1.10−4 kPa.
STEP 3. This liquid is poured into the molds, de-gassed again, then cured for 45 min at 70 ◦C.
STEP 4. (1) For linear actuators two of the cured elastomer blocks are joined using a thin layer of uncured, de-gassed silicone applied by a procedure similar to sandwich micro-contact printing. (2) Using the same procedure, bending actuators are assembled by bonding the cured silicone block to a thin inextensible layer, fabricated by embedding a layer of silk fabric in a 0.75 mm-thick layer of de-gassed silicone cured at room temperature (image sequence for bending actuators at end of page).
STEP 5. The assembly is then cured at room temperature for two hours to guarantee proper bonding.
STEP 6. Finally, the assembly process is completed by inserting the air tubing (for all the SPAs in this work the tube length is 300 mm) and applying a flexible epoxy around it to ensure a proper seal.
The following images show step-by-step fabrication sequence for bending SPAs by including the additional un-stretchable layer.