Structure-properties relationships of defined CNF single-networks crosslinked by telechelic PEGs

The high structural anisotropy and colloidal stability of cellulose nanofibrils' enable the creation of self-standing fibrillar hydrogel networks at very low solid contents. Adding methacrylate moieties on the surface of TEMPO oxidized CNFs allows the formation of more robust covalently crosslinked networks by free radical polymerization of acrylic monomers, exploiting the mechanical properties of these networks more efficiently. This technique yields strong and elastic networks but with an undefined network structure. In this work, we use acrylate-capped telechelic polymers derived from the step-growth polymerization of PEG diacrylate and dithiothreitol to crosslink methacrylated TEMPO-oxidized cellulose nanofibrils (MATO CNF). This combination resulted in flexible and strong hydrogels, as observed through rheological studies, compression and tensile loading. The structure and mechanical properties of these hydrogel networks were found to depend on the dimensions of the CNFs and polymer crosslinkers. The structure of the networks and the role of individual components were evaluated with SAXS (Small-Angle X-ray Scattering) and photo-rheology. A thorough understanding of hybrid CNF/polymer networks and how to best exploit the capacity of these networks enable further advancement of cellulose-based materials for applications in packaging, soft robotics, and biomedical engineering.

Polysaccharide Composite Alginate-Pectin Hydrogels as a Basis for Developing Wound Healing Materials

This article presents materials that highlight the bioengineering potential of polymeric systems of natural origin based on biodegradable polysaccharides, with applications in creating modern products for localized wound healing. Exploring the unique biological and physicochemical properties of polysaccharides offers a promising avenue for the atraumatic, controlled restoration of damaged tissues in extensive wounds. The study focused on alginate, pectin, and a hydrogel composed of their mixture in a 1:1 ratio. Atomic force microscopy data revealed that the two-component gel exhibits greater cohesion and is characterized by the presence of filament-like elements. The dynamic light scattering method indicated that this structural change results in a reduction in the damping of acoustic modes in the gel mixture compared to the component gels. Raman spectroscopy research on these gels revealed the emergence of new bonds between the components' molecules, contributing to the observed effects. The biocompatibility of the gels was evaluated using dental pulp stem cells, demonstrating that all the gels exhibit biocompatibility.

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

Structural and dynamic heterogeneity in associative networks formed by artificially engineered protein polymers

This work investigates static gel structure and cooperative multi-chain motion in associative networks using a well-defined model system composed of artificial coiled-coil proteins. The combination of small-angle and ultra-small-angle neutron scattering provides evidence for three static length scales irrespective of protein gel design which are attributed to correlations arising from the blob length, inter-junction spacing, and multi-chain density fluctuations. Self-diffusion measurements using forced Rayleigh scattering demonstrate an apparent superdiffusive regime in all gels studied, reflecting a transition between distinct "slow" and "fast" diffusive species. The interconversion between the two diffusive modes occurs on a length scale on the order of the largest correlation length observed by neutron scattering, suggesting a possible caging effect. Comparison of the self-diffusive behavior with characteristic molecular length scales and the single-sticker dissociation time inferred from tracer diffusion measurements supports the primarily single-chain mechanisms of self-diffusion as previously conceptualized. The step size of the slow mode is comparable to the root-mean-square length of the midblock strands, consistent with a single-chain walking mode rather than collective motion of multi-chain aggregates. The transition to the fast mode occurs on a timescale 10-1000 times the single-sticker dissociation time, which is consistent with the onset of single-molecule hopping. Finally, the terminal diffusivity depends exponentially on the number of stickers per chain, further suggesting that long-range diffusion occurs by molecular hopping rather than sticky Rouse motion of larger assemblies. Collectively, the results suggest that diffusion of multi-chain clusters is dominated by the single-chain pictures proposed in previous coarse-grained modeling.

Relatively homogeneous network structures of temperature-responsive gels synthesized via atom transfer radical polymerization

The network structures of poly(N-isopropylacrylamide) (PNIPAAm) gels prepared by atom transfer radical polymerization (ATRP) were compared with those prepared by free radical polymerization (FRP), as a conventional radical polymerization. Temperature-responsive shrinkage was observed in the PNIPAAm gels prepared by ATRP and FRP (ATRP and FRP gels), which depended on the cross-linker content. From the light-scattered intensities, 〈I〉_T, measured at the different sample positions, we used the partial heterodyne method to determine the dynamic fluctuation, 〈I〉_F, spatial component, 〈I〉_C, and correlation length, ξ, of the ATRP and FRP gels, as a function of the cross-linker content and temperature. While there is little difference in 〈I〉_F and ξ between the ATRP and FRP gels, 〈I〉_C of the ATRP gel was smaller than that of the FRP gel. In addition, we calculated the standard deviation of 〈I〉_T for the ATRP and FRP gels, as a function of temperature to quantify the inhomogeneity of the gel networks. The standard deviation revealed that increasing cross-linker content and temperature makes the gel networks more inhomogeneous. The dynamic light scattering (DLS) measurement used to characterize the gel network revealed that ATRP suppresses inhomogeneity more effectively than FRP. The standard deviation of the scattered intensity is used in this study to quantify the inhomogeneity of the network structures. Quantitative evaluations of the inhomogeneity of the network structures by the standard deviation of the scattered intensity are useful in the investigation of the structure-property relationships of gels.

Analyses of hierarchical structures in vulcanized SBR rubber by using contrast-variation USANS and SANS

The hierarchical structures of poly(styrene-ran-butadiene) (SBR) rubber vulcanized with sulfur in a swollen state were investigated by using the contrast-variation ultra-small-angle neutron scattering (USANS) and small-angle neutron scattering (SANS) techniques. The following three levels of hierarchical structure were found: (i) ZnO clusters surrounded by dense SBR networks of the order of 1000 Å in size, (ii) dense networks of SBR in the size range of 70–100 Å and (iii) a mesh size of the network of the order of 10 Å. In addition to the three kinds of structure, dense networks without ZnO of the order of 1000 Å were also observed. These last networks were formed by ZnO's reaction with sulfur. However, the ZnO clusters disappeared, which is associated with the diffusion of Zn.

Donnan Contribution and Specific Ion Effects in Swelling of Cationic Hydrogels are Additive: Combined High-Resolution Experiments and Finite Element Modeling

Finite element modeling applied to analyze experimentally determined hydrogel swelling data provides quantitative description of the hydrogel in the aqueous solutions with well-defined ionic content and environmental parameters. In the present study, we expand this strategy to analysis of swelling of hydrogels over an extended concentration of salt where the Donnan contribution and specific ion effects are dominating at different regimes. Dynamics and equilibrium swelling were determined for acrylamide and cationic acrylamide-based hydrogels by high-resolution interferometry technique for step-wise increase in NaCl and NaBr concentration up to 2 M. Although increased hydrogel swelling volume with increasing salt concentration was the dominant trend for the uncharged hydrogel, the weakly charged cationic hydrogel was observed to shrink for increasing salt concentration up to 0.1 M, followed by swelling at higher salt concentrations. The initial shrinking is due to the ionic equilibration accounted for by a Donnan term. Comparison of the swelling responses at high NaCl and NaBr concentrations between the uncharged and the cationic hydrogel showed similar specific ion effects. This indicates that the ion non-specific Donnan contribution and specific ion effects are additive in the case where they are occurring in well separated ranges of salt concentration. We develop a novel finite element model including both these mechanisms to account for the observed swelling in aqueous salt solution. In particular, a salt-specific, concentration-dependent Flory-Huggins parameter was introduced for the specific ion effects. This is the first report on finite element modeling of hydrogels including specific ionic effects and underpins improvement of the mechanistic insight of hydrogel swelling that can be used to predict its response to environmental change.

Covalently and ionically, dually crosslinked chitosan nanoparticles block quorum sensing and affect bacterial cell growth on a cell-density dependent manner

In our efforts to improve the quality and stability of chitosan nanoparticles (NPs), we describe here a new type of chitosan NPs dually crosslinked with genipin and sodium tripolyphosphate (TPP) that display quorum quenching activity. These NPs were created using a simplified and robust procedure that resulted in improved physicochemical properties and enhanced stability. This procedure involves the covalent crosslinking of chitosan with genipin, followed by the formation of chitosan NPs by ionic gelation with TPP. We have optimized the conditions to obtain genipin pre-crosslinked nanoparticles (PC-NPs) with positive ς-potential (~ +30 mV), small diameter (~130 nm), and low size distributions (PdI = 0.1-0.2). PC-NPs present physicochemical properties that are comparable to those of other dually crosslinked chitosan NPs fabricated with different protocols. In contrast to previously characterized NPs, however, we found that PC-NPs strongly reduce the acyl homoserine lactone (AHL)-mediated quorum sensing response of an Escherichia coli fluorescent biosensor. Thus, PC-NPs combine, in a single design, the stability of dually crosslinked chitosan NPs and the quorum quenching activity of ionically crosslinked NPs. Similar to other chitosan NPs, the mode of action of PC-NPs is consistent with the existence of a "stoichiometric ratio" of NP/bacterium, at which the positive charge of the NPs counteracts the negative ς-potential of the bacterial envelope. Notably, we found that the time of the establishment of the "stoichiometric ratio" is a function of the NP concentration, implying that these NPs could be ideal for applications aiming to target of bacterial populations at specific cell densities. We are confident that our PC-NPs are up-and-coming candidates for the design of efficient anti-quorum sensing and a new generation antimicrobial strategies.

Toehold Length of Target ssDNA Affects Its Reaction-Diffusion Behavior in DNA-Responsive DNA-co-Acrylamide Hydrogels

In the present study, we expand on the understanding of hydrogels with embedded deoxyribonucleic acid (DNA) cross-links, from the overall swelling to characterization of processes that precede the swelling. The hydrogels respond to target DNA strands because of a toehold-mediated strand displacement reaction in which the target strand binds to and opens the dsDNA cross-link. The spatiotemporal evolution of the diffusing target ssDNA was determined using confocal laser scanning microscopy (CLSM). The concentration profiles revealed diverse partitioning of the target DNA inside the hydrogel as compared with the immersing solution: excluding a nonbinding DNA, while accumulating a binding target. The data show that a longer toehold results in faster cross-link opening but reduced diffusion of the target, thus resulting in only a moderate increase in the overall swelling rate. The parameters obtained by fitting the data using a reaction-diffusion model were discussed in view of the molecular parameters of the target ssDNA and hydrogels.

Comparison of Dynamic Light Scattering and Rheometrical Methods to Determine the Gel Point of a Radically Polymerized Hydrogel under Mechanical Shear

The phase transition of nanocomposite hydrogels made of N-isopropylacrylamide (NIPAm) and clay (Laponite^® XLS) was investigated under mechanical shear influencing the gelation. The hydrogels were synthesized by free radical polymerization. For the processing of cross-linked gels, the phase transition (liquid–solid) and its dependence on mechanical stress are of paramount importance. On the one hand, the determination of the gel point (tg) is possible with rheometry and, on the other hand, with dynamic light scattering (DLS). With rotational rheometry, by identifying the abrupt increase of viscosity, the gel point is evaluated. The DSL is an alternative method to rheometry, to investigate hydrogels under the action of the shear flow, to make results comparable to the rheometric investigations, with and without shear. Experimental parameters were chosen based on preparatory work to obtain comparable results regarding the determination of the gel point of a radically polymerized NIPAm hydrogel.

Microscopic Structure of Swollen Hydrogels by Scanning Electron and Light Microscopies: Artifacts and Reality

The exact knowledge of hydrogel microstructure, mainly its pore topology, is a key issue in hydrogel engineering. For visualization of the swollen hydrogels, the cryogenic or high vacuum scanning electron microscopies (cryo-SEM or HVSEM) are frequently used while the possibility of artifact-biased images is frequently underestimated. The major cause of artifacts is the formation of ice crystals upon freezing of the hydrated gel. Some porous hydrogels can be visualized with SEM without the danger of artifacts because the growing crystals are accommodated within already existing primary pores of the gel. In some non-porous hydrogels the secondary pores will also not be formed due to rigid network structure of gels that counteracts the crystal nucleation and growth. We have tested the limits of true reproduction of the hydrogel morphology imposed by the swelling degree and mechanical strength of gels by investigating a series of methacrylate hydrogels made by crosslinking polymerization of glycerol monomethacrylate and 2-hydroxyethyl methacrylate including their interpenetrating networks. The hydrogel morphology was studied using cryo-SEM, HVSEM, environmental scanning electron microscopy (ESEM), laser scanning confocal microscopy (LSCM) and classical wide-field light microscopy (LM). The cryo-SEM and HVSEM yielded artifact-free micrographs for limited range of non-porous hydrogels and for macroporous gels. A true non-porous structure was observed free of artifacts only for hydrogels exhibiting relatively low swelling and high elastic modulus above 0.5 MPa, whereas for highly swollen and/or mechanically weak hydrogels the cryo-SEM/HVSEM experiments resulted in secondary porosity. In this contribution we present several cases of severe artifact formation in PHEMA and PGMA hydrogels during their visualization by cryo-SEM and HVSEM. We also put forward empirical correlation between hydrogel morphological and mechanical parameters and the occurrence and intensity of artifacts.

A comparison of the network structure and inner dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels with high crosslinker content

Poly(N-isopropylacrylamide) microgel particles were prepared via a "classical" surfactant-free precipitation polymerization and a continuous monomer feeding approach. It is anticipated that this yields microgel particles with different internal structures, namely a dense core with a fluffy shell for the classical approach and a more even crosslink distribution in the case of the continuous monomer feeding approach. A thorough structural investigation of the resulting microgels with dynamic light scattering, atomic force microscopy and small angle neutron scattering was conducted and related to neutron spin echo spectroscopy data. In this way a link between structural and dynamic features of the internal polymer network was made.

A dynamic-coupling-reaction-based autonomous self-healing hydrogel with ultra-high stretching and adhesion properties

We synthesized a dynamic coupling-reaction based hydrogel that showed excellent mechanical and adhesion properties, super-high self-healing properties and good biocompatibility.

Double network hydrogels based on semi-rigid polyelectrolyte physical networks

Double network hydrogels are formulated from polyacrylamide hydrogels containing semi-rigid polyelectrolytes through post-polymerization immersion in multi-valent ion solutions.

A body temperature and water-induced shape memory hydrogel with excellent mechanical properties

A body temperature and water-induced shape memory hydrogel with excellent mechanical properties was prepared by crosslinking dopamine-terminated tetra-poly(ethylene glycol) with an oxidation reaction.

Inorganic/Organic Double-Network Ion Gels with Partially Developed Silica-Particle Network

Tough inorganic/organic composite network gels consisting of a partially developed silica-particle network and a large amount of an ionic liquid, named micro-double-network (μ-DN) ion gel, are fabricated via two methods. One is a one-pot/one-step process conducted using a simultaneous network formation via sol-gel reaction of tetraethyl orthosilicate and free radical polymerization of N, N-dimethylacrylamide in an ionic liquid. When the network formation rates of the inorganic and organic networks are almost the same, the μ-DN structure is formed. The second method is simpler and involved the use of silica nanoparticles as the starting material. By controlling the dispersion state of the silica nanoparticles in an ionic liquid, the μ-DN structure is formed. In both μ-DN ion gels, silica nanoparticles partially aggregate and form network-like clusters. When a large deformation is induced in the μ-DN ion gels, the silica-particle clusters rupture and dissipate the loaded energy. The fracture stress and Young's modulus of the μ-DN ion gel increase as the size of the silica nanoparticles decreases. The increment in the mechanical strength would have been caused by the increase in the total van der Waals attraction forces and the total number of hydrogen bonding in the silica-particle networks.

Dynamics-based assessment of nanoscopic polymer-network mesh structures and their defects

Polymer-network gels often exhibit complex nanoscopic architectures. First, the polymer-network mesh topology on scales of 1-10 nm is usually not uniform and regular, but disordered and irregular. Second, on top of that, many swollen polymer networks display spatial inhomogeneity of their polymer segmental density and crosslinking density on scales of 10-100 nm. This multi-scale structural complexity affects the permeability, mechanical strength, and optical clarity of the polymer gels, which is of central relevance for their performance in popular applications. As a result, there is a need to characterize the polymer network structures on multiple scales. On the scale of the spatial inhomogeneity of crosslinking, 10-100 nm, scattering of neutrons, X-rays, and light has extraordinary utility and is well established. On the scale of the mesh topology, 1-10 nm, in contrast, experimental techniques are less established. This review intends to close this gap by reviewing two intrinsically dynamic methods that yield information on polymer network mesh structures. First, NMR-based assessment of residual dipolar proton-spin couplings, which arise upon the introduction of crosslinks into a liquidlike polymer system to impart partial solidlike characteristics, is suitable to quantitatively assess network meshes and local network defects. Second, diffusive penetration of molecular, macromolecular, and mesoscopic colloidal probes through a polymer gel provides insight into its obstructing network mesh structure and its potential irregularity. Either method is highly synergistic to scattering-based assessment of the network structures on larger scales, and in concert, a rich picture on the nano- and mesoscopic gel topology is obtained.

PBTTT-C16 sol-gel transition by hierarchical colloidal bridging

A versatile conjugated polymer, poly(2,5-bis(3-hexadecyllthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C₁₆, with M_w = 61 309 g mol^-1), in a relatively good solvent (chlorobenzene, CB) medium is shown to produce gels through hierarchical colloidal bridging. Multiscale static/dynamic light and X-ray scattering analysis schemes along with complementary microscopy imaging techniques clearly reveal that upon cooling from the solution state at 80 °C to various gelation temperatures (5, 10, and 15 °C), rod-like colloidal pBTTT-C₁₆ aggregates morph into spherical ones, triggering hierarchical colloid formation and bridging that eventually turn the solution into a gel after about one-day aging. A certain fraction of primal packing units-spherical gelators (∼1 nm in mean radius)-constitute the spherical building particles (∼10 nm) noted above, which in turn constitute loose-packing aggregate clusters (∼300 nm) in the sol state. As gelation proceeds, the aggregate cluster interiors tighten substantially, and micrometer-sized clusters (∼3 μm) formed by them begin to take shape and further interconnect to form the gel network (mean porosity size ∼240 nm and spatial inhomogeneity length ∼20 μm). Rheological measurements and kinetic analysis reveal that the gelation temperature can also have a notable impact on gel microstructure, gelation rate, and mechanical strength, resulting in, for instance, a prominently nonergodic and porous structure for the soft gel incubated at a higher temperature T = 15 °C. The ac conductivity exhibits a notable upturn near pBTTT-C₁₆/CB gelation, well above those achieved by the counterpart pBTTT-C₁₄ solutions, which, in interesting contrast, cannot be brought to the gel phase under similar experimental conditions.

Effect of Rubber Polarity on Cluster Formation in Rubbers Cross-Linked with Diels-Alder Chemistry

Diels–Alder chemistry has been used for the thermoreversible cross-linking of furan-functionalized ethylene/propylene (EPM) and ethylene/vinyl acetate (EVM) rubbers. Both furan-functionalized elastomers were successfully cross-linked with bismaleimide to yield products with a similar cross-link density. NMR relaxometry and SAXS measurements both show that the apolar EPM-g-furan precursor contains phase-separated polar clusters and that cross-linking with polar bismaleimide occurs in these clusters. The heterogeneously cross-linked network of EPM-g-furan contrasts with the homogeneous network in the polar EVM-g-furan. The heterogeneous character of the cross-links in EPM-g-furan results in a relatively high Young’s modulus, whereas the more uniform cross-linking in EVM-g-furan results in a higher tensile strength and elongation at break.

Hyperelastic Tough Gels through Macrocross-Linking

The wet and soft nature of hydrogels makes them useful as a mimic for biological tissues, and in uses such as actuators and drug delivery vehicles. For many applications the mechanical performance of the gel is critical, but gels are notoriously weak and prone to fracture. Free radical polymerization is a very powerful technique allowing for fine spatial and temporal control of polymerization, but also allows for the use of a wide range of monomers and mixtures. In this work, it is demonstrated that extremely tough and extensible hydrogels can be readily produced through simple radical polymerization of acrylamide or acrylic acid with a poly(ethylene oxide) macrocross-linker. These gels, with a water content of 85%, are extremely elastic with an extension much more than 15 000% at 9 MPa true stress. They can be compressed over 98% at a stress of 17 MPa. They are notch-insensitive, and the usual trouser tear test does not work because the tear simply does not propagate. This highly extensible nature seems to be related to very long chain lengths between cross-links and efficient incorporation of chains into the network.

Hybrid Microgels with Thermo-Tunable Elasticity for Controllable Cell Confinement

Stimuli-responsive hydrogels are able to change their physical properties such as their elastic moduli in response to changes in their environment. If biocompatible polymers are used to prepare such materials and if living cells are encapsulated within these networks, their switchability allows the cell-matrix interactions to be investigated with unprecedented consistency. In this paper, thermo-responsive macro- and microscopic hydrogels are presented based on azide-functionalized copolymers of poly(N-(2-hydroxypropyl)-methacrylamide) and poly(hydroxyethyl methacrylate) grafted with poly(N-isopropylacrylamide) side chains. Crosslinking of these comb polymers is realized by bio-orthogonal strain-promoted azide-alkyne cycloaddition with cyclooctyne-functionalized poly(ethylene glycol). The resulting hybrid hydrogels exhibit thermo-tunable elasticity tailored by the polymer chain length and grafting density. This bio-orthogonal polymer crosslinking strategy is combined with droplet-based microfluidics to encapsulate living cells into stimuli-responsive microgels, proving them to be a suitable platform for future systematic stem-cell research.

Macroscopic and Microscopic Elasticity of Heterogeneous Polymer Gels

Polymer-network gels often exhibit local defects and spatial heterogeneity of their cross-linking density, which may differently affect their elasticity on microscopic and macroscopic scales. To appraise this effect, we prepare polymeric gels with defined extents of nanostructural heterogeneity and use atomic force microscopy to probe their local microscopic Young's moduli in comparison to their macroscopic elastic moduli measured by shear rheology. In this comparison, the moduli of the heterogeneous gels are found to be progressively smaller if the length scale of the probed gel region exceeds the size of the purposely imparted polymer-network heterogeneities. This finding can be explained with a conceptual picture of nonaffine deformation of the densely cross-linked polymer network domains in the heterogeneous gels.

Nanostructural heterogeneity in polymer networks and gels

Many polymer gels display network defects and crosslinking inhomogeneity. This review reflects and interrelates investigations on the characterization of such polymer-network heterogeneity and on its impact on the swelling, elasticity, and permeability of polymer gels.