Abstract
Shape memory polymers (SMPs) are attractive materials due to their unique mechanical properties, including high deformation capacity and shape recovery. SMPs are easier to process, lightweight, and inexpensive compared to their metallic counterparts, shape memory alloys. However, SMPs are limited to relatively small form factors due to their low recovery stresses. Lightweight, micro-architected composite SMPs may overcome these size limitations and offer the ability to combine functional properties (e.g., electrical conductivity) with shape memory behavior. Fabrication of 3D SMP thermoset structures via traditional manufacturing methods is challenging, especially for designs that are composed of multiple materials within porous microarchitectures designed for specific shape change strategies, e.g. sequential shape recovery. We report thermoset SMP composite inks containing some materials from renewable resources that can be 3D printed into complex, multi-material architectures that exhibit programmable shape changes with temperature and time. Through addition of fiber-based fillers, we demonstrate printing of electrically conductive SMPs where multiple shape states may induce functional changes in a device and that shape changes can be actuated via heating of printed composites. The ability of SMPs to recover their original shapes will be advantageous for a broad range of applications, including medical, aerospace, and robotic devices.
Highlights
SMP hinges in a foldable elastomeric structure using an ultraviolet (UV) photocurable thermoset resin
The first task for this fabrication strategy is the development of room temperature printable SMP composite ink by tailoring the composition and rheology required for reliable flow through fine nozzles and shape retention after deposition
epoxidized soybean oil (ESBO) is a commercially available material often used as a plasticizer for polyvinyl chloride (PVC)
Summary
SMP hinges in a foldable elastomeric structure using an ultraviolet (UV) photocurable thermoset resin. An alternative method is an extrusion-based 3D filamentary printing technique, known as direct ink writing (DIW), which employs highly concentrated, viscoelastic “inks” to assemble 3D structures by robotically extruding a continuous ink filament through a micro-nozzle at room temperature in a layer-by-layer scheme[28] The prerequisite for this method is the necessity to design a gel-based viscoelastic ink possessing both shear thinning behavior to facilitate extrusion flow under pressure and a rapid pseudoplastic to dilatant recovery resulting in shape retention after deposition[29,30,31,32]. Combining 3D printing with origami provides a potential route for the creation of much higher complex 3D structures with defined material distributions and, multi-stage thermal shape recovery profiles, which could enable SMP-based devices with unprecedented multifunctional performance. This approach is being referred to as “4D printing” due to the extra dimension of time (or shape change) that such programmable materials add to conventional 3D printing and origami processes
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