Supramolecular polymer networks: hydrogels and bulk materials.

Supramolecular polymer networks contain non-covalent cross-links that enable access to broadly tunable mechanical properties and stimuli-responsive behaviors; the incorporation of multiple unique non-covalent cross-links within such materials further expands their mechanical responses and functionality. To date, however, the design of such materials has been accomplished through discrete combinations of distinct interaction types in series, limiting materials design logic. Here we introduce the concept of leveraging “nested” supramolecular crosslinks, wherein two distinct types of non-covalent interactions exist in parallel, to control bulk material functions. To demonstrate this concept, we use polymer-linked Pd2L4 metal–organic cage (polyMOC) gels that form hollow metal–organic cage junctions through metal–ligand coordination and can exhibit well-defined host-guest binding within their cavity. In these “nested” supramolecular network junctions, the thermodynamics of host-guest interactions within the junctions affect the metal–ligand interactions that form those junctions, ultimately translating to substantial guest-dependent changes in bulk material properties that could not be achieved in traditional supramolecular networks with multiple interactions in series.

Supramolecular polymer networks are non-covalently crosslinked soft materials that exhibit unique mechanical features such as self-healing, high toughness and stretchability. Previous studies have focused on optimizing such properties using fast-dissociative crosslinks (that is, for an aqueous system, dissociation rate constant kd > 10 s-1). Herein, we describe non-covalent crosslinkers with slow, tuneable dissociation kinetics (kd < 1 s-1) that enable high compressibility to supramolecular polymer networks. The resultant glass-like supramolecular networks have compressive strengths up to 100 MPa with no fracture, even when compressed at 93% strain over 12 cycles of compression and relaxation. Notably, these networks show a fast, room-temperature self-recovery (< 120 s), which may be useful for the design of high-performance soft materials. Retarding the dissociation kinetics of non-covalent crosslinks through structural control enables access of such glass-like supramolecular materials, holding substantial promise in applications including soft robotics, tissue engineering and wearable bioelectronics.

Viscoelasticity,stiffness, and degradation of tissue matricesregulate cell behavior, yet predictive synergistic tuning of theseproperties in synthetic cellular niches remains elusive. We hypothesizethat reversible physical cross-linking can be quantitatively introducedto synthetic hydrogels to accelerate stress relaxation and enhancenetwork stiffness, while strategic placement of isolated labile linkagesnear cross-linking sites can predict hydrogel degradation, both ofwhich are essential for creating adaptive cellular niches. To testthese hypotheses, chondrocytes were encapsulated in hydrogels formedby biorthogonal covalent and noncovalent physical cross-linking ofa pair of hydrophilic building blocks. The stiffer and more viscoelastichydrogels with DBCO–DBCO physical cross-links facilitated proliferationand chondrogenic ECM deposition of encapsulated cells by dissipatingstress imposed by expanding cell mass/ECM via dynamic disruption/reformationof physical cross-links. Degradation of labile linkages near covalentcross-linkers further facilitated cell proliferation and timed cellrelease while maintaining chondrogenic phenotype. This work presentsnew chemical tools for engineering permissive synthetic niches forcell encapsulation, 3D expansion, and release.

Supramolecular polymer networks are transiently cross-linked soft materials that have exceptional material properties such as high toughness, contractility, and recyclability. Previous studies have focused on optimizing their uniaxial mechanical features but did not manage to impart them with biaxial ductility. Here, we develop a set of biaxially ductile supramolecular polymer networks constructed by slow-dissociative, noncovalent cross-linkers. Retarding cross-link dissociation kinetics enables access of glass-like networks, exhibiting both enhanced compressibility and biaxial stretchability. Dynamic dissociation/reassociation of transient cross-linking junctions benefits to address biaxial deformation locking, thus achieving a compressive strength of more than 160 megapascals and a large areal strain up to 10,000%. A universal method of quantifying biaxial mechanical properties for supramolecular networks is also reported. This work enlightens the design and construction of biaxially ductile polymeric materials, holding substantial promise in applications including bioelectronic interfaces, soft robots, and tissue implants.

We present the first neutron spin echo (NSE) measurements of self-assembling peptide hydrogel networks to study the fibril dynamics on the nanometer and nanosecond length and time scales. MAX1 and MAX8 are synthetic beta-hairpin peptides that undergo triggered self-assembly at the nanoscale to form a physically cross-linked network of fibrils with a defined cross-section. When subjected to physiological pH and ionic strength (pH 7.4, 150 mM NaCl), the soluble peptides fold into a beta-hairpin and, subsequently, self-assemble to form a structurally rigid hydrogel stabilized by noncovalent cross-links. The sequence of MAX8 is identical to MAX1 with the exception of one single amino acid substitution that reduces the net charge on the peptide. As a result, faster folding and self-assembly kinetics are observed for MAX8 at the same peptide concentration and identical buffer conditions, and gels with a larger storage modulus are formed. NSE measurements of the peptide hydrogels demonstrate that the self-assembled peptide fibrils can be described as semiflexible chains on nanolength and time scales. Alteration of the peptide sequence affected the nanoscale dynamics of the hydrogels but not to an extent comparable to the large difference observed in the bulk viscoelasticity. Small angle neutron scattering (SANS) of the hydrogels reveals increased scattering for MAX8 at low wavevectors, an indication of a heterogeneous network with a tighter mesh size. Therefore, we conjecture that the difference in elastic modulus arises from differences in assembly kinetics that result in increased fibrillar branching and physical cross-links rather than a change in the fibril nanostructure or persistence length.

A dynamic diffraction grating (DDG) can realize on-demand in-situ regulation of light diffraction and is highly necessary in the next-generation optical devices; however, its fabrication remains challenging because of difficulties in generating a responsive pattern in the bulk material or on the surface. In this study, a facile and general approach to construct DDG is developed based on the near-infrared radiation (NIR)-driven responsive 2D ordered surface pattern, which is fabricated by light-direct writing of wrinkles on a thin film of a photosensitive supramolecular polymer network comprised of a copolymer containing amino groups, anthracene carboxylic acid (ANA), and carbon nanotubes. The resulting surface pattern, which exhibits a height dependence on the NIR irradiation, can be used as the DDG whose optical diffraction can be regulated in-situ by NIR. Furthermore, because of the presence of a reversible ANA photodimer and hydrogen bonding in the supramolecular network, the patterned surface can be erased and rewritten by UV light with different wavelengths and self-healed by NIR irradiation. The results demonstrate a simple strategy for DDGs that will have a broad application in the fields of optics, electronics, and intelligent sensing.

Review anti-swelling hydrogels from natural polysaccharide polymers: Emerging core materials for underwater wearable motion sensors.

Silicone elastomers are normally thermoset materials. While their inherent properties make them highly valuable, it would be of interest to develop stimuli-responsive silicones whose properties could be reversibly tuned at will. In the case of silicone polymers, a particularly interesting trigger is light, since silicone elastomers can readily be formulated to be transparent. We describe the utilization of coumarin-modified silicones for this purpose. On their own, the presence of coumarin groups converts silicone oils into thermoplastic elastomers through physical (noncovalent) cross-linking. UV-irradiation permits covalent cross-linking through [2 + 2] cycloadditions and is accompanied by loss of most physical cross-links. Higher energy photons permit, in part, photoinitiated retro-cycloaddition and a subsequent decrease in covalent cross-link density. It is thus possible to tailor the physical properties of the elastomer to increase and/or decrease the modulus of the elastomer using light and to conver...

The biocompatible, injectable and high water-swollen nature of hydrogels makes them a popular candidate to imitate the extracellular matrix (ECM) for tissue engineering both in vitro and in vivo. However, commonly used covalently cross-linked hydrogels, despite their stability and tunability, are elastic and deteriorate as bulk material degrades which would impair proper cell function. To improve these deficiencies, here, we present a self-recovering cross-linked hydrogel formed instantaneously with functionalized poly(ethylene glycol) as a basis. We combine covalent cross-links introduced via a strain-promoted azide-alkyne cycloaddition (SPAAC) click reaction and non-covalent links between phosphonate groups and calcium ions. By adjusting the ratios of non-covalent and covalent cross-links, we synthesized these dual cross-linked (DC) hydrogels that displayed storage moduli below ∼2000 Pa and relaxation times from seconds to minutes. The gels recovered to 41-96% of their initial mechanical properties after two subsequent strain failures. Cryo-scanning electron microscopy revealed that DC hydrogels containing approximately equal amounts of covalent and non-covalent cross-links displayed phase separation. Finally, we functionalized the DC hydrogels by incorporating an integrin binding motif, RGDS, to provide a biocompatible environment for human mesenchymal stem cells (HMSCs) by facilitating adhesion inside the gel network. Inside these DC gels HSMCs displayed a viability up to 73% after five days of cell culture.

Supramolecular polymer networks (SPNs) demonstrate great potential in the development of smart materials owing to their attractive dynamic properties. However, as they suffer from the inherent weak bonding of most noncovalent cross-links, it remains a significant challenge to construct SPNs with outstanding mechanical performance. Herein, we exploit the cryptand/paraquat host-guest recognition motifs as cross-links to prepare a class of highly strong and tough SPNs. Unlike those supramolecular cross-links with relatively weak binding abilities, the cryptand-based host-guest interactions have a high association constant and steady complexing structure, which effectively stabilizes the network and resists mechanical deformation under external force. Such favorable structural stability endows our SPNs with greatly enhanced mechanical performance, compared with the control-1 cross-linked by the weakly complexed crown ether/secondary ammonium salt motif (tensile strength: 21.1±0.5 vs 2.8±0.1 MPa; Young's modulus: 102.6±4.8 vs 2.1±0.3 MPa; toughness: 90.4±2.0 vs 10.8±0.6 MJ m-3 ). Moreover, our SPNs also retain abundant dynamic properties including good abilities in energy dissipation, reprocessability, and stimuli-responsiveness. These findings provide novel insights into the preparation of SPNs with enhanced mechanical properties, and promote the development of high-performance intelligent supramolecular materials.

Development of closed-loop chemically recyclable plastics (CCRPs) that can be widely used in daily life can be a fundamental solution to the global plastic waste crisis. Hence, it is of great significance to develop easy-to-recycle CCRPs that possess superior or comparable material properties to the commodity plastics. Here, a novel dual crosslinked CCRP, namely, supramolecular covalent adaptable networks (supra-CANs), is reported, which not only displays mechanical properties higher than the strong and tough commodity polycarbonate, but also exhibits excellent solvent resistance as thermosets. The supra-CANs are constructed by introducing reversible noncovalent crosslinks into the dynamic covalent polymer networks, resulting in highly stiff and strong thermosets that also exhibit thermoplastic-like ductile and tough behaviors as well as reprocessability and rehealability. In great contrast, the analogs that do not have noncovalent crosslinks (CANs) show elastomeric properties with significantly decreased mechanical strength. Importantly, the developed supra-CANs and CANs can be converted back into the initial monomers in high yields and purity at room temperature, even with additives, which enables the sustainable polymer-monomer-polymer circulation. This work provides new design principles for high-performance chemically recyclable polymers as sustainable substitutes for the conventional plastics.

Supramolecular hydrogel networks formed by molecular recognition of collagen and a peptide grafted to hyaluronic acid

Constitutive equations are derived for the time‐dependent behavior of semicrystalline polymers at isothermal loading with small strains. A semicrystalline polymer at temperatures above the glass‐transition point for its amorphous phase is thought of as a network of macromolecules bridged by junctions (physical crosslinks, entanglements, and crystalline lamellae) that can slide with respect to their reference positions in the bulk material under straining. The network is assumed to be highly inhomogeneous, and it is modeled as an ensemble of mesoregions (MRs) with various strengths of interchain interaction. Two types of MRs are distinguished: passive, where these interactions prevent detachment of strands from junctions; and active, where active strands separate from junctions and dangling strands merge with the network at random times as they are thermally agitated. The viscoelastic response of a semicrystalline polymer reflects reformation of strands in active MRs, whereas its viscoplastic behavior is associated with sliding of junctions. Stress–strain relations for uniaxial deformation are developed by using the laws of thermodynamics. Adjustable parameters in the constitutive equations are found by fitting experimental data for isotactic polypropylene in a tensile test with a constant strain rate and in tensile relaxation tests at various strains. Fair agreement is demonstrated between the observations and the results of numerical simulation. It is revealed that the viscoplastic flow of junctions strongly affects the rearrangement process in active MRs, whose rate reaches a threshold value in the vicinity of the apparent yield point. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1438–1450, 2003

The effect of annealing on the time-dependent behavior of isotactic polypropylene at finite strains

We report the investigation of rubbery double networks that are based on poly(n-butyl acrylate) (BA) backbones and feature either non-covalent or covalent cross-links. The polymers for the supramolecular network were prepared by reversible addition−fragmentation chain-transfer polymerization of n-butyl acrylate and up to 10 mol% of the self-complementary hydrogen bonding motif 2-ureido-4[1H]pyrimidinone (UPy), which dimerizes into supramolecular cross-links. The covalent polymer network was synthesized by UV-initiated free-radical polymerization of BA and up to 5 mol% of the difunctional cross-linker 1,4-butanediol diacrylate, either separately, or after imbibing films of the supramolecular network with the reaction mixture for the covalent network. The combination of the two network types affords materials that show features of both single networks. The co-existence of a static and a dynamic network affords materials that owe their high thermal stability and creep resistance to the covalent network, and their high extensibility and healability to the supramolecular network.