Taking cues from biological systems, we are interested in understanding the design rules employed by Nature and applying these strategies to the development of mechanically-enhanced and tunable materials. Of particular interest is: 1) the layered configuration and synergistic mechanics of nacre, 2) the dynamics and interfacial assembly of the extracellular matrix (ECM), and 3) the composite structure and reinforcement of wood. With this bio-inspired mechanical framework, our research projects in responsive composites and dynamic, polymer-gel assemblies will be discussed.

Motivated by plant muses, we have explored the fabrication of responsive composite systems utilizing high modulus, electrospun and low molecular gelators (LMWGs) as fillers. Here, we discuss new insights into hygromorphic (e.g. hydration/humidity) response in composites utilizing concepts of interfacial assembly, transport, bias, and orientation learned from plant muses. We have fabricated a strategically interfaced hygromorphic composite utilizing an active electrospun filler and a passive, low molecular weight gelator layer in an elastomeric matrix. The impact of material parameters on water front progression and actuation were probed theoretically and experimentally in their design. Via this approach, preferential coiling was observed. However, two challenges were encountered due to the isotropic nature of the electrospun mat: (1) slow response times, and (2) non-uniformity in hydration-induced response. To overcome these limitations, we explored the impact of the alignment of the electrospun fibers as a handle to control rate of hydration and program shape change. These engineered hygromorphic composites exhibited predictable curvature, and much faster response times (2-3 min). It is anticipated that these water-responsive systems may have unique applications in therapeutic delivery and chemical/biological protection.

Inspired by the responsive behavior of the fibrous, ECM architecture, we have expanded our interest in LMWGs to the design of dynamic gel assemblies that utilize high molecular weight polymers as reinforcing additives. Via this approach, we demonstrate modular gel mechanics tailored by the nucleation behavior of the self-assembled, molecular gel. Guiding principles of the structural interplay dictated by component interactions, polymer chain conformation, and concentration were revealed. Our ultimate goal is the design of reconfigurable and responsive composites derived from LMWG fiber assemblies, transitioning from gel-state fundamentals to solid-state composites. Toward that end, we explored how the molecular gel network influences the mechanics of the polymer matrix. Insight into restrictions on chain mobility, and decoupled gel network and matrix stability are highlighted, moving closer to the realization of solid-state switchability. New thrust areas in light guide assemblies, and therapeutic immobilization and tailored release are envisioned utilizing these polymer-gel systems.