Biomimetic 4D Printing

Abstract

Advances in the design of adaptive matter capable of programmable, environmentally-responsive changes in shape would enable myriad applications including smart textiles, scaffolds for tissue engineering, and smart machines. 4D printing is an emerging approach in which 3D objects are produced whose shape changes over time. Initial demonstrations have relied on commercial 3D printers and proprietary materials, which limits both the tunability and mechanisms that can be incorporated into the printed architectures.

My Ph.D. thesis focuses on a new 4D printing method, which is inspired by the movements or natural plants. Specifically, we encode swelling and elastic anisotropy in printed hydrogel composites through the alignment of stiff cellulose fibrils on-the-fly during printing. Filler alignment parallel to the print path leads to enhanced stiffness in that direction; hence, upon immersion in water, the printed filaments expand preferentially in the direction orthogonal to the printing path. When structures are patterned with broken-symmetry, i.e., as bilayers, their anisotropic swelling leads to programmable out-of-plane deformation, determined by the orientation of printed filaments. We have demonstrated complex changes in curvature including bending, twisting, ruffling, conical defects, and more, all using a single hydrogel-based ink printed in a single step. We have demonstrated the ability to precisely control curvature by varying the actual and the effective thickness, the latter of which is governed by the interfilament spacing within the printed architectures. With collaborators, a model has been developed for solving both the forward and inverse design problems, based on an adaptation of the classic Timoshenko bending theory, allowing us to create nearly arbitrary structures.

Our filled hydrogel ink is modular, allowing a broad range of hydrogel chemistries and anisotropic filler compositions to be explored. For example, both reversible and non-reversible hydrogels were explored; namely poly(N-isopropyl acrylamide) (PNIPAm) and poly(N,N-dimethylacrylamide) (PDMAm), respectively. Additionally, light-absorbing carbon microfibers were incorporated to demonstrate reversible, multi-stimuli responsive 4D printing. In this case, reversible shape changes were encoded via 4D printing and then triggered either by heating PNIPAm or illuminating the printed architectures with a near IR laser.

In summary, this biomimetic 4D printing platform enables the design and fabrication of complex, reversible shape changing architectures printed with one composite hydrogel ink in a single step. These biocompatible shape-shifting architectures with interesting mechanical and photothermal properties may find applications in smart textiles, tissue microgrippers or scaffolds, or as actuators and sensors in soft machines.