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In this study, we design, fabricate and characterize a novel unimorph-like structure in which a shape memory polymer (SMP) is actuated using a magneto-active elastomer (MAE), resulting in contactless and large deformation shape change. The constituent materials are: a two-part elastomer, iron oxide microparticles, and a rapid cure epoxy. Combining the elastomer with the iron oxide particles results in an MAE actuator, such that application of an external magnetic field induces dipoles in the iron oxide particles, resulting in the actuation of the MAE. The shape memory epoxy polymers are deformed at an elevated temperature and cooled to lock-in the temporary programmed shape. Subsequent exposure to elevated temperature can then recover a permanent geometry. In a first step, the layered unimorph structure is fabricated using casting techniques in which the SMP serves as the ‘passive’ substrate, and the MAE is cast on top of this substrate or joined post-processing using adhesives. To demonstrate shape change, the unimorph is heated beyond the transition temperature of the epoxy, and a magnetic field is applied to deform the structure. Upon cooling, the shape change is locked into place. This configuration serves as proof-of-concept that the proposed magnetically-responsive polymers are capable of eliciting the shape change of an SMP through bending in response to an external magnetic stimulus. After demonstrating feasibility, the next step is to use additive manufacturing (AM) to produce both the SMP and MAE by developing and tuning MAE and SMP resin formulations for printability. Printability is assessed by characterizing the viscosities, effective yield stress, gelation times, and extent of cure of the material formulations. The bending response of the layered structure is characterized as a function of magnetic field, material composition, geometric parameters, and AM process settings. The outcome of this research is to enable AM of smart materials and devices that can monitor and adapt to their environment. The medical device industry in particular stands to benefit from customizable devices that adapt their shape for particular patients, diseases, and/or stage of healing.

Additive manufacturing (AM) of high-performance composites has gained increasing interest over the last few years. Commercially available AM technologies often use thermoplastics as they are easy to process, i.e., to melt and re-solidify. However, thermosetting polymers generally achieve superior mechanical properties and thermostability. This study investigates reactive extrusion additive manufacturing (REAM) of a thermosetting polymer reinforced with carbon fibers. The process utilizes highly exothermic and fast curing resin/catalyst systems, eliminating the need for post-curing. The rheological properties of the liquid resin are first tuned for REAM using ~2wt.% fumed silica and ~10vol.% milled carbon fibers. Then, a robotic arm is used to print the composite samples. The coupons’ longitudinal and transverse tensile properties are measured and correlated with the degree of cure, porosity, fiber length distribution, and fiber orientation distribution. The incorporation of milled carbon fibers, 50-200 m long, primarily affects the stiffness. Compared to neat polymer parts, carbon fiber reinforced composites are 51% stiffer and 8% stronger. In addition, polymeric crosslinking between part layers resulted in strong inter-layer bonding. Short fibers were also randomly oriented within parts due to the nozzle size and shape, resulting in nearly isotropic parts. The results presented here pave the road for fast and low-energy AM of high-performance composites.

Magnetoactive elastomers (MAE) are capable of large deformation, shape programming, and moderately large actuation forces when driven by an external magnetic field. These capabilities enable applications such as soft grippers, biomedical devices, and actuators. To facilitate complex shape deformation and enhanced range of motion, a unimorph can be designed with varying geometries, behave spatially varying multi-material properties, and be actuated with a non-uniform external magnetic field. To predict actuation performance under these complex conditions, an analytical model of a segmented MAE unimorph is developed based on beam theory with large deformation. The effect of the spatially-varying magnetic field is approximated using a segment-wise effective torque. The model accommodates spatially varying concentrations of magnetic particles and differentiates between the actuation mechanisms of hard and soft magnetic particles by accommodating different assumptions concerning the magnitude and direction of induced magnetization under a magnetic field. To validate the accuracy of the model predictions, four case studies are considered with various magnetic particles and matrix materials. Actuation performance is measured experimentally to validate the model for the case studies. The results show good agreement between experimental measurements and model predictions. A further parametric study is conducted to investigate the effects of the magnetic properties of particles and external magnetic fields on the free deflection. In addition, complex shape programming of the unimorph actuator is demonstrated by locally altering the geometric and material properties.