Elastomeric Origami: Programmable Paper-Elastomer Composites as Pneumatic Actuators

TL;DR: In this article, the development of soft pneumatic actuators based on composites consisting of elastomers with embedded sheet or fiber structures (e.g., paper or fabric) that are flexible but not extensible is described.

Abstract: The development of soft pneumatic actuators based on composites consisting of elastomers with embedded sheet or fiber structures (e.g., paper or fabric) that are flexible but not extensible is described. On pneumatic inflation, these actuators move anisotropically, based on the motions accessible by their composite structures. They are inexpensive, simple to fabricate, light in weight, and easy to actuate. This class of structure is versatile: the same principles of design lead to actuators that respond to pressurization with a wide range of motions (bending, extension, contraction, twisting, and others). Paper, when used to introduce anisotropy into elastomers, can be readily folded into 3D structures following the principles of origami; these folded structures increase the stiffness and anisotropy of the elastomeric actuators, while being light in weight. These soft actuators can manipulate objects with moderate performance; for example, they can lift loads up to 120 times their weight. They can also be combined with other components, for example, electrical components, to increase their functionality.

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Figure 8. Bellows actuators, fabricated by limiting the elongation of their pleats. a) A bellows actuator that bends in a U-shape. b) A bellows actuator with two bending modes. c) A bellows actuator with a strip (in red) limiting the expansion of the pleats. The strip was built on the outside of the device for the sake of clarity.

Figure 9. A twisting actuator with a helical patterned paper strip wrapped around a cylindrical pneumatic channel. a) The three-dimensional pattern of the paper. b) The fabrication process is similar to the one shown in Figure 4: the paper with the pattern shown in a) is first inserted into a cylindrical mold. An elastomer pre-mixtured is then poured into the mold, and cured with the patterned paper embedded. Finally, sealing the top and bottom completes the pneumatic channel. c), d), and e) show the resting and actuated states of such a device under an applied pressure of P1= 50 mbar and P2= 120 mbar respectively. f) and g) Pressure dependence of the twisting angle and the elongation after 50 pressurization/depressurization cycles.

Figure 4. A contracting actuator that is composed of longitudinally patterned paper stripes rolled around a cylindrical pneumatic channel. a) The two-dimensional pattern of the paper. b) Schematic representation of the fabrication process: the paper with the pattern shown in a) is first rolled into a cylinder, and inserted into a cylindrical mold. A pre-mixed elastomer is then poured into the mold, and cured with the patterned paper embedded. The pneumatic chamber is completed by sealing the top and bottom against two circular pieces of paper embedded in elastomer. c), d), and e) show the resting and actuated states of such a device under atmospheric pressure (Patm), P1= 80 mbar, and P2= 200 mbar, respectively. f) Dependence on pressure of the contraction factor (Eq. 1) after 50 pressurization/depressurization cycles. The error bars show the standard deviation from the mean values.

Figure 5. An elongation actuator with paper folded into a bellows-like pattern around a cylindrical pneumatic channel. a) The three-dimensional bellows-like pattern of the paper. b) The fabrication process is similar to the one shown in Figure 3: the paper with the pattern shown in a) is first inserted into a cylindrical mold. An elastomer pre-mixture is then poured into the mold, and cured with the patterned paper embedded. Finally, sealing the top and bottom completes the pneumatic channel. c) Pressure dependence of the extension (relative to its original length in the resting state) after 50 pressurization/depressurization cycles. d), e), and f) show the resting and actuated states of such a device under atmospheric pressure (Patm), P1= 50 mbar, and P2= 170 mbar, respectively.

Figure 1. Schematic diagram outlining the fabrication of pneumatic soft actuators based on programmable paper-elastomer composites. a) Fabrication. First, an elastomer pre-mixture is poured over a mold with features designed to form the pneumatic channels. After curing, it is peeled off the mold, and placed in contact with a piece of paper soaked with uncured elastomer pre-mixture. Finally, the assembly is thermally cured to generate a sealed pneumatic channel. The final device is unsymmetrical in its mechanical response, because the top and bottom layers (elastomer and paper soaked with elastomer, respectively) have very different mechanical properties.

Figure 6. Origami extension actuator lifting a standard weight of 1 kg. A transparent tube was used to constrain the weight and origami actuator to a common vertical axis during the lifting. The pressure inside the pneumatic chamber of the actuator required to lift the weight to maximum extension of the actuator was 238 mbar. The actuator weighed 8.3 g. The arrows indicate the polyethylene tubes used to supply compressed air for actuation.

Figure 2. Comparison between the actuation of a pneumatic channel in a homogeneous elastomeric matrix, and in a paper-elastomer composite, under 100 mbar of pressure (1,013.25 mbar = 1 atm). a), b) Finite element analysis, and real device, showing the symmetrical deformation of an actuator fabricated in a single material. c), d) Finite-element analysis, and real device, showing the asymmetrical deformation of a paper-elastomer composite actuator upon pressurization. The schematics are sections along the center of the long axis of the structures. In this example, the higher tensile strength of the paper limits the extension of the elastomeric matrix, and results in bending of the device on pressurization.

Figure 10. A contraction actuator that couples inflation of an Ecoflex cylindrical shell reinforced with aluminum foil to emission of blue light upon pressurization. a) Twodimensional pattern of the composite of paper and aluminum foil. b) Folded structure rolled into a cylinder to fabricate the actuator. c) Aluminum foil was used (composed with paper and Ecoflex) to fabricate a bendable structure that allows electrical conduction to a blue LED housed in the internal chamber from the outside. The anode of the LED is connected to the top aluminum cap (electrically connected through the aluminum cylinder). The cathode of the LED is connected to the bottom aluminum cap (isolated from the anode with Ecoflex). d), e), and f) Resting and actuated states of such a device with the LED on. Patm= atmospheric pressure, P1= 100 mbar, and P2= 285 mbar, respectively. The exposure time was longer for e) and f) than d). g) Dependence of the contraction with the applied pressure after 50 pressurization/depressurization cycles h) Dependence of the normalized luminous intensity (in the far field) with the internal pressurization of the device.

Figure 7. A biased bellows actuator, fabricated by modifying the elongation actuator in Figure 5. a) The three-dimensional bellows-like pattern of the paper, the same design in Figure 5. b) The fabrication process is similar to the one shown in Figure 5 except that the right edges of the paper structure were glued together. The elongation is possible from the left side of the device. c), d), and e) show the resting and actuated states of such a device under P1= 60 mbar, P2= 125 mbar, P3= 175 mbar, and P4= 250 mbar, respectively. f) Dependence of the angle θ on pressure after 50 cycles of pressurization/depressurization. The small standard deviation (error bars) indicates that θ does not change significantly as a function of the number of cycles.

Figure 3. A contracting actuator consisting of multiple stages of bending actuators sharing a single pneumatic channel. a) The schematics of the design showing the layout of the pneumatic channel and the paper. b), c), and d) show the resting state (Patm) and two different actuated states (P1 and P2) of such a device, respectively. Left inset in b) correspond to the optical image of the lateral cross section of the device. The right inset in b) corresponds to the SEM image of the perpendicular cross section of the device. P1= 70 mbar P2= 220 mbar. The arrows indicate the polyethylene tubes used to supply compressed air for actuation.