Controlled Sequential Shape Changing Components by 3D Printing of Shape Memory Polymer Multimaterials

Stimuli-responsive (active) materials undergo large-scale shape or property changes in response to an external stimulus such as stress, temperature, light or pH. Technological uses range from durable, shape-recovery eye-glass frames, to temperature-sensitive switches, to the generation of stress to induce mechanical motion. Here, we demonstrate that the uniform dispersion of 1-5 vol.% of carbon nanotubes in a thermoplastic elastomer yields nanocomposites that can store and subsequently release, through remote means, up to 50% more recovery stress than the pristine resin. The anisotropic nanotubes increase the rubbery modulus by a factor of 2 to 5 (for 1-5 vol.%) and improve shape fixity by enhancing strain-induced crystallization. Non-radiative decay of infrared photons absorbed by the nanotubes raises the internal temperature, melting strain-induced polymer crystallites (which act as physical crosslinks that secure the deformed shape) and remotely trigger the release of the stored strain energy. Comparable effects occur for electrically induced actuation associated with Joule heating of the matrix when a current is passed through the conductive percolative network of the nanotubes within the resin. This unique combination of properties, directly arising from the nanocomposite morphology, demonstrates new opportunities for the design and fabrication of stimuli-responsive polymers, which are otherwise not available in one material system.

Chemically cross-linked polymers are inherently limited by stresses that are introduced by post-gelation volume changes during polymerization. It is also difficult to change a cross-linked polymer's shape without a corresponding loss of material properties or substantial stress development. We demonstrate a cross-linked polymer that, upon exposure to light, exhibits stress and/or strain relaxation without any concomitant change in material properties. This result is achieved by introducing radicals via photocleavage of residual photoinitiator in the polymer matrix, which then diffuse via addition-fragmentation chain transfer of midchain functional groups. These processes lead to photoinduced plasticity, actuation, and equilibrium shape changes without residual stress. Such polymeric materials are critical to the development of microdevices, biomaterials, and polymeric coatings.

The aortic valve exhibits complex three-dimensional (3D) anatomy and heterogeneity essential for the long-term efficient biomechanical function. These are, however, challenging to mimic in de novo engineered living tissue valve strategies. We present a novel simultaneous 3D printing/photocrosslinking technique for rapidly engineering complex, heterogeneous aortic valve scaffolds. Native anatomic and axisymmetric aortic valve geometries (root wall and tri-leaflets) with 12-22 mm inner diameters (ID) were 3D printed with poly-ethylene glycol-diacrylate (PEG-DA) hydrogels (700 or 8000 MW) supplemented with alginate. 3D printing geometric accuracy was quantified and compared using Micro-CT. Porcine aortic valve interstitial cells (PAVIC) seeded scaffolds were cultured for up to 21 days. Results showed that blended PEG-DA scaffolds could achieve over tenfold range in elastic modulus (5.3±0.9 to 74.6±1.5 kPa). 3D printing times for valve conduits with mechanically contrasting hydrogels were optimized to 14 to 45 min, increasing linearly with conduit diameter. Larger printed valves had greater shape fidelity (93.3±2.6, 85.1±2.0 and 73.3±5.2% for 22, 17 and 12 mm ID porcine valves; 89.1±4.0, 84.1±5.6 and 66.6±5.2% for simplified valves). PAVIC seeded scaffolds maintained near 100% viability over 21 days. These results demonstrate that 3D hydrogel printing with controlled photocrosslinking can rapidly fabricate anatomical heterogeneous valve conduits that support cell engraftment.