Session M06: Designing Soft Responsive Polymer Networks: Recent Advances and Challenges

Responsive biomimicking materials

The complex behaviors of living systems stem from their ability to sense and respond to their surrounding environment. Active materials equipped with such sensory mechanisms and shape-morphing capabilities can allow us to create devices that are inherently smart. One such class of materials is hydrogels functionalized with active components such as spiropyran hydrogels with embedded magnetic nanowires which interact with both light and magnetic fields [1]. Our continuum models quantitatively study these interactions and allow us to create controllable soft robots capable of walking and swimming. The interplay between the photochemistry and magnetoelasticity of the hydrogel material, and its hydrodynamic interaction with the surrounding fluid impart phototactic properties to these swimmers. Furthermore, we design autonomous devices by utilizing materials that convert chemical into mechanical energy and vice versa, [2]. Our models describe systems where the chemical reaction, the hydrogel mechanics, and the solvent diffusion inside the material are integrated and provide guidelines to develop multisensory, synthetic materials.   

Chemical Design of Stimuli Responsive Microgels: Recent Developments and Trends

Soft colloids with programmable chemical functionalities, size and shape are important building blocks for the fabrication of catalyst systems and adaptive biomaterials for tissue engineering. However, the development of the easy up-scalable and template-free synthesis methods to obtain such colloids lack of understanding of molecular interactions that occur during the synthesis step. We demonstrate that the use of “programmable” monomers able to undergo specific non-covalent interactions (electrostatic, hydrophobic or π-π-stacking) in aqueous solutions is a key to bottom-up synthesis of anisotropic microgels with controlled size, shape and chemical structure. 

A new synthesis approach based on the controlled self-assembly of growing polyelectrolyte microgel precursors during the precipitation polymerization process leading to the formation of polyampholyte Janus-like microgels was developed.<font size="1"> We confirmed the morphology of polyampholyte Janus-like microgels using super-resolution optical microscopy and demonstrated that they are capable of responding to changes in both pH and temperature in aqueous solutions. In another study, we first performed computer simulations for a series of pyrazole-modified monomers with different numbers of pyrazole groups, different length and polarity of spacers between pyrazole groups and the polymerizable group. Based on simulations, we synthesized monomers able to undergo π-π-stacking and guide the formation of supramolecular bonds between polymer segments and used them in precipitation polymerization to synthesize anisotropic microgels. We demonstrate that microgel morphologies can be tuned from spherical, raspberry-like to dumbbell-like by the increase of the pyrazole-modified monomer loading, which is concentrated at periphery of growing microgels.   

Emergent collective behavior of platelets in blood clotting: lessons for designing active polymeric networks

Blood clots play a critical role in restoring hemostasis and regulating thrombosis in the body. Upon vascular injury, a cascade of events culminates in the formation of a soft plug of fibrin fibers and small anucleate blood cells called platelets. Platelets become activated and undergo actomyosin-based contraction with fibrin fibers to shrink the overall clot size, modify clot structure, and mechanically stabilize the clot. We developed an experimentally informed mesoscale computational model of fibrin- platelet blood clots to gain insights into the contraction mechanics of clots and to examine the interactions among different clot elements. The active micromechanics of contracting clots exhibits a remarkable example of design principles that can be adapted for developing active materials. The simulations reveal that platelets utilize a new emergent behavior, triggered by the heterogeneity in the timing of platelet activation, to enhance volumetric material contraction and to magnify contractile forces. We demonstrate the connection between the forces produced by individual platelets nested within fibrin mesh and the macroscopic forces generated by the clot and show how the clot forces depend on network properties. The ability of contracting colts to entrap and retain red blood cells provides a means for controlling clot internal structure and mechanics. While we are still far from being able to synthesize materials that can match the amazing complexity of biological systems, our results provide valuable guidelines for developing advanced synthetic and hybrid materials with platelet- inspired distributed actuation.   

Amphiphilic Polymer Conetworks: Experiment and Theory

Amphiphilic polymer conetworks (APCN) [1] comprise covalently cross-linked hydrophilic and hydrophobic segments, and are consequently capable of microphase separating upon transfer into a selective solvent like water [2].  Thus, these materials represent the network analogs to surfactants.  While currently evaluated in various emerging applications in biomedicine and technology, with a notable example in the latter area that of their use as matrices for gel or solid polymer electrolytes in re-chargeable batteries, APCNs are credited with a major real-market application, with global annual sales totaling 10 billion US dollars.  This is their use as silicone hydrogel soft contact lenses, in which the carefully selected hydrophilic and hydrophobic segments, as well as their microphase separated morphology, impart to the lens important properties related to eye comfort and oxygenation [3].

Our latest APCNs are carefully formed by end-linking well-defined polymer segments involving, in most cases, an amphiphilic block copolymer or a hydrophobic homopolymer, and a hydrophilic four-armed linker [4,5].  Dynamic covalent acylhydrazone links are employed so as to allow APCN self-healing and recycling, thereby extending materials service life and reducing environmental footprint [6].  Characterization of the self-assembled structures, via scattering, and the mechanical properties in water identifies the APCNs with the largest domain sizes, longest-range ordering and greatest extensibility, providing various choices to the applications engineer to select among materials with a broad spectrum of attributes.  These experimental results on morphology and mechanical properties are complemented by and compared to the results of thermodynamic modeling and dissipative particle dynamics simulations [7].  We expect that our work will lay the foundation for the design and development of next-generation APCNs.   

Emergent Mechanics of Dynamic Polymer Network: Viscoelasticity, Damage, and Remodeling.

A large majority of soft biological materials are made of elastic molecular networks with changing topology, allowing to accommodate growth, remodeling, and self-healing over time. Such behaviors can now be replicated in man-made polymers through the synthesis of networks whose chains are connected by weak molecular bonds, which under thermal fluctuations can permanently associate and dissociate. In contrast to their elastic counterparts, these networks therefore exhibit a myriad of new physics (flow, elastic deformation, self-healing, programmability, actuation, ...) that can be controlled by network topology, bond dynamics, and deformation rate. With these opportunities come challenges, related to a steep increase in the design space, but also is our ability to understand how these spatio-temporal network give rise to more and more complex macroscopic response. The presentation will tackle this challenge by providing an overview of the inner workings of these networks together with ways to characterize their response using concepts in statistical mechanics and the dynamics of complex systems. We will first discuss how a simple dynamic network can transition from a viscous fluid to an elastic solid and are able to remodel their structure and relax stresses over time. Based on this, we will turn to the behavior of these networks under damage, fracture and self-healing with a particular attention to the development of cavitation. These concepts will be used to illustrate the puzzling response of various dynamics networks, ranging from the nonlinear rheological behavior of fire-ant aggregations to the transient fracture of vitrimers, a new class of dynamic polymers. We will finally take inspiration from biological dynamic networks to explore how a new generation of living polymers can be developed to create well-controlled active and smart soft materials