Water‐Rich Biomimetic Composites with Abiotic Self‐Organizing Nanofiber Network

Abstract

Reinforcement with clay nanosheets, [12] biominerals, [13] cellulose, [14,15] or polymeric fibers [16] enhances tensile stiffness of the hydrogels. Noncovalent interactions, i.e., ionic, [17][18][19] hydrophobic interactions, [20] and hydrogen bonding, [20,21] increase the compression stiffness or viscoelasticity of the materials. Nevertheless, imparting multiple and mutually contradictory properties, as exemplified by high stiffness, toughness, and water content, results in materials that compromise one essential property at the expense of another. Replicating the combination of physical metrics of natural soft tissues remains challenging. For instance, hydrogels with covalently crosslinked nanoreinforcement have a high tensile modulus of ≈2 MPa at low strains, [12] but they experience fracture or catastrophic softening under tensile strains above a few percent due to the limited deformability of the nanofillers. Hydrogels with multiple polymer networks can be stretched by ≈17 times even with a notch in the sample, [10] but the softness of the constituent polymers result in a tensile modulus of only ≈30 kPa, which is two to four orders of magnitude lower than those of cartilage, ligaments, or tendons. Increasing the stiffness of these hydrogels would otherwise be associated with the sacrifice of water content that is essential for cellular viability in biomedical applications. [11,13] Dense nacre-like composites based on stiff inorganic and soft organic components can exhibit high stiffness and toughness, [22,23] but similar mechanics are difficult to achieve in water-rich (e.g., >70 wt%) embodiments due to the chemical and physical limitations of the mineral constituents. Replicating the physical behaviors of load-bearing soft tissues would require a new class of stiff nanoscale components that are capable of forming porous and reconfigurable networks.We recently reported that para-aramid, commonly known as Kevlar, can form a nanofibrous dispersion in dimethyl sulfoxide (DMSO). [24] Solution-processable nanoscale versions of Kevlar, i.e., aramid nanofibers (ANFs), retain the high mechanical properties of their macroscale parent, and these materials have served as the building blocks for high-strength flexible conductors and battery separators. [24,25] In the context of this study, a key fact is that ANFs with diameters of 5-30 nm and lengths of 3-10 µm form networked structures when DMSO is exchanged with water. [25] Their structural similarity to biological nanofibers, such as those from collagen and those that display extensive branching, [26] inspired us to explore ANFs as the stiff Load-bearing soft tissues, e.g., cartilage, ligaments, and blood vessels, are made predominantly from water (65-90%) which is essential for nutrient transport to cells. Yet, they display amazing stiffness, toughness, strength, and deformability attributed to the reconfigurable 3D network from stiff collagen nanofibers and flexible proteoglycans. Existing hydrogels and composites partially achieve some of the mechanical propertie...

Interfacial Engineering of Reduced Graphene Oxide for Aramid Nanofiber‐Enabled Structural Supercapacitors

“…Desired amounts of ANF/DMSO and f‐GO/DMSO dispersions were mixed and stirred together, followed by the addition of water and vacuum filtration. The addition of water serves to reprotonate the ANF amide bonds, leading to extensive hydrogen bonding with the oxygen‐containing and/or the amine functional groups on f‐GO . Moreover, ANFs and f‐GO interact with each other through π ‐ π interactions between ANF's aromatic rings and f‐GO graphitic basal planes…”

“…As shown in Figure S7, f‐rGO/ANF films exhibited strong strain rate dependency. At lower strain rates there is more time for the f‐rGO sheets to slide and to reform hydrogen bonds, leading to lower stiffness and higher energy absorption …”

“…The improvement in toughness can be attributed to two main mechanisms. On the one hand, energy is dissipated in order to break the extensive hydrogen bonding between the ANFs and the f‐rGOs at their interfaces, and some hydrogen bonds reform which then facilitate increased extensibility but continued increase in load bearing capacity . Overall, the addition of ANFs and the amine (−NH 2 ) functionalization of the graphene sheets (rGO vs .…”

“…The addition of water serves to reprotonate the ANF amide bonds, leading to extensive hydrogen bonding with the oxygen-containing and/or the amine functional groups on f-GO. [24] Moreover, ANFs and f-GO interact with each other through p-p interactions between ANF's aromatic rings and f-GO graphitic basal planes. [25] The as-prepared f-GO/ANF composite films were then reduced thermally at 200 8C under vacuum for 2 h to yield functionalized reduced graphene oxide (f-rGO).…”

CaCO3–Polymer Nanocomposite Prepared with Supercritical CO2

“…The production of aragonite was attributed to the templating effect of the hydroxyl groups of the PVA [16]. That is, the aragonite polymorph formed because the hydroxyl groups bonded to Ca 2+ via PAA, and the arrangement of the hydroxyl groups was very similar to the arrangement of the atoms of aragonite.…”