Additive Manufacturing

JUL 2018

ADDITIVE MANUFACTURING is the magazine devoted to industrial applications of 3D printing and digital layering technology. We cover the promise and the challenges of this technology for making functional tooling and end-use production parts.

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Page 37 of 60

Materials Challenge in Soft Robotics AM / Soft Robotics 35 Top hydrogel electrode Bottom hydrogel electrode Bottom copper contact lead Top copper contact lead Active DE layer DE offset infill Passive layer Schematic of a dielectric elastomer (DE) actuator's constituent ma- terial layers. While these materials have been researched and devel- oped for years, never before have they been subject to 3D printing for fabricating functional actuators, largely because of the postpro- cessing necessary to bond the hydrogel inks (which are hydrophilic, i.e. can be mixed with or dissolved by water) with silicone-based materials (which are hydrophobic, i.e. repel water). When layered in this manner, this hybrid material system results in a structure that replicates the skin's dermal and epidermal layers. Haghiashtiani's team conducted performance tests across several conditions to measure the responsiveness to electrical stimuli, including step changes in applied voltage to the electrical leads. polymerization and chemical bonding between the hydrogel and elastomer surfaces. With this bonding between previously unprintable layers now possible, the research team was able to insert copper tape pieces as electrical contact leads into the de- vice structure between layers as they were printed. The bottom electrical lead was inserted before deposition of the hydrogel layer, while the top electrical lead was placed on top of the hydrogel layer after the printing was complete. None of this would have been achievable, however, without solving the other constraint to the printability of these materi- als: viscosity. (Or, to be more exact, the rheological properties of the materials, of which viscosity is a prime characteristic.) The extrusion-based printing process used for this study, direct ink writing, necessitated material characteristics that would decrease the viscosity of the inks as they flow through the deposition nozzle and become subject to higher pressures. This property is known as shear-thinning behavior. Through manipulation of the composition of both the hydrogels and the elastomer inks, Haghiashtiani's team was able to optimize shear thinning behaviors specifically for their printer's nozzle tip diameter, extrusion pressure and printing speed. The printers themselves are custom-built devices that employ a gantry system along with a deposition-based robotic fluid dispenser made by Fisnar. The machines were originally housed at Princeton University, where this study's principal investigator, Prof. Michael McAlpine (now at UofM) performed extensive research in 3D printing active materials. For Haghiashtiani's study, the finished test device consisted of several layers, including, from bottom to top, a passive layer with a bottom electrode placed on top, followed by the dielec- tric elastomer, topped by another electrode. On this device, the team conducted performance tests across several conditions to measure the responsiveness to electrical stimuli, including step changes in applied voltage to the electrical leads (see lower left). "In a fundamental DEA configuration, when you apply electric voltage," Haghiashtiani told me, "you are going to see that the dielectric elastomer expands in area, while its overall thickness shrinks. In the bending DEA, the stiffer passive layer imposes a restriction on the structure, resulting in translating the in-plane deformation to out of plane bending motion." The team added barium titanate nanoparticles to the silicone layer to enhance the overall electromechanical sensitivity of the device, and tested different applied voltages to measure the DEA perfor- mance in lifting small payload masses placed at the device's tip. The results of these tests led Haghiashtiani to conclude that not only are 3D-printed soft dielectric elastomer actuators capable of generating movement and motion, but that the path exists for someday 3D printing self-sensing, soft robotic systems that mimic the abilities of natural muscle, including neurons that "provide a sense of spatial position and motion." Partly because this research was conducted with the U.S. Army Research Lab, media attention paid to Haghiashtiani's article was almost entirely focused on the potential military applica- tions that this technology promises. But Haghiashtiani sees other, perhaps shorter-term appli- cations in the field of smart prosthetics. "Right now the level of voltage required to activate these devices is probably too high," she says. "More work needs to be built upon this to improve performance and develop them into more complex structures that can be functional right off the printer."

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