Sequence and gelation in supramolecular polymers

Supramolecular polymer networks exhibit unique and tunable thermodynamic and dynamic properties that are attractive for a wide array of applications, such as adhesives, rheology modifiers, and compatibilizers. Coherent states (CS) field theories have emerged as a powerful approach for describing the possibly infinite reaction products that result from associating polymers. Up to this point, CS theories have focused on relatively simple polymer architectures. In this work, we develop an extension of the CS framework to study polymers with reversible bonds distributed along the polymer backbone, opening a broad array of new materials that can be studied with theoretical methods. We use this framework to discern the role of reactive site placement on sol-gel phase behavior, including the prediction of a microstructured gel phase that has not been reported for neutral polymer gels. Our results highlight the subtleties of thermodynamics in supramolecular polymers and the necessity for theories that capture them.

Surface Topology of Redox- and Thermoresponsive Nanogel Droplets

Hydrogels are usually depicted as a homogenous polymer block with a distinct surface. While defects in the polymer structure are looked into frequently, structural irregularities on the hydrogel surface are often neglected. In this work, thin hydrogel layers of ≈100 nm thickness (nanogels) are synthesized and characterized for their structural irregularities, as they represent the surface of macrogels. The nanogels contain a main-chain responsiveness (thermo responsive) and a responsiveness in the cross-linking points (redox responsive). By combining data from ellipsometry using box-model and two-segment-model analysis, as well as atomic force microscopy, a more defined model of the nanogel surface can be developed. Starting with a more densely cross-linked network at the silica wafer surface, the density of cross-linking gradually decreases toward the hydrogel-solvent interface. Thermo-responsive behavior of the main chain affects the entire network equally as all chain segments change solubility. Cross-linker-based redox-responsiveness, on the other hand, is only governed by the inner, more cross-linked layers of the network. Such dual responsive nanogels hence allow for developing a more detailed model of a hydrogel surface from free radical polymerization. It provides a better understanding of structural defects in hydrogels and how they are affected by responsive functionalities.

Effect of Cross-Link Homogeneity on the High-Strain Behavior of Elastic Polymer Networks

Cross-link heterogeneity and topological defects have been shown to affect the moduli of polymer networks in the low-strain regime. Probing their role in the high-strain regime, however, has been difficult because of premature network fracture. Here, we address this problem by using a double-network approach to investigate the high-strain behavior of both randomly and regularly cross-linked networks with the same backbone chemistry. Randomly cross-linked poly(n-butyl acrylate) networks with target molecular weights between cross-links of 5-30 kg/mol were synthesized via free-radical polymerization, while regularly cross-linked poly(n-butyl acrylate) networks with molecular weights between cross-links of 7-38 kg/mol were synthesized via cross-linking of tetrafunctional star polymers. Both types of networks were then swollen in a monomer/cross-linker mixture, polymerized to form double networks, and characterized via uniaxial tensile testing. The onset of strain stiffening was found to occur later in regular networks than in random networks with the same modulus but was well-predicted by the target molecular weight between cross-links of each sample. These results indicate that the low- and high-strain behavior of polymer networks result from different molecular-scale features of the material and suggest that controlling network architecture offers new opportunities to both further fundamental understanding of architecture-property relationships and design materials with independently controlled moduli and strain stiffening responses.

Measuring mechanical cues for modeling the stromal matrix in 3D cell cultures

A breast-cancer tumor develops within a stroma, a tissue where a complex extracellular matrix surrounds cells, mediating the cancer progression through biomechanical and -chemical cues. Current materials partially mimic the stromal matrix in 3D cell cultures but methods for measuring the mechanical properties of the matrix at cell-relevant-length scales and stromal-stiffness levels are lacking. Here, to address this gap, we developed a characterization approach that employs probe-based microrheometry and Bayesian modeling to quantify length-scale-dependent mechanics and mechanical heterogeneity as in the stromal matrix. We examined the interpenetrating network (IPN) composed of alginate scaffolds (for adjusting mechanics) and type-1 collagen (a stromal-matrix constituent). We analyzed viscoelasticity: absolute-shear moduli (stiffness/elasticity) and phase angles (viscous and elastic characteristics). We determined the relationship between microrheometry and rheometry information. Microrheometry reveals lower stiffness at cell-relevant scales, compared to macroscale rheometry, with dependency on the length scale (10 to 100 μm). These data show increasing IPN stiffness with crosslinking until saturation (≃15 mM of Ca2+). Furthermore, we report that IPN stiffness can be adjusted by modulating collagen concentration and interconnectivity (by polymerization temperature). The IPNs are heterogeneous structurally (in SEM) and mechanically. Interestingly, increased alginate crosslinking changes IPN heterogeneity in stiffness but not in phase angle, until the saturation. In contrast, such changes are undetectable in alginate scaffolds. Our nonlinear viscoelasticity analysis at tumor-cell-exerted strains shows that only the softer IPNs stiffen with strain, like the stromal-collagen constituent. In summary, our approach can quantify the stromal-matrix-related viscoelasticity and is likely applicable to other materials in 3D culture.

Structural heterogeneity in tetra-armed gels revealed by computer simulation: Evidence from a graph theory assisted characterization

Designing homogeneous networks is considered one typical strategy for solving the problem of strength and toughness conflict of polymer network materials. Experimentalists have proposed the hypothesis of obtaining a structurally homogeneous hydrogel by crosslinking tetra-armed polymers, whose homogeneity was claimed to be verified by scattering characterization and other methods. Nevertheless, it is highly desirable to further evaluate this issue from other perspectives. In this study, a coarse-grained molecular dynamics simulation coupled with a stochastic reaction model is applied to reveal the topological structure of a polymer network synthesized by tetra-armed monomers as precursors. Two different scenarios, distinguished by whether internal cross-linking is allowed, are considered. We introduce the Dijkstra algorithm from graph theory to precisely characterize the network structure. The microscopic features of the network structure, e.g., loop size, dispersity, and size distribution, are obtained via the Dijkstra algorithm. By comparing the two reaction scenarios, Scenario II exhibits an overall more idealized structure. Our results demonstrate the feasibility of the Dijkstra algorithm for precisely characterizing the polymer network structure. We expect this work will provide a new insight for the evaluation and description of gel networks and further help to reveal the dynamic process of network formation.

Disulfide-Cross-Linked Tetra-PEG Gels

The preparation of polymer gels via cross-linking of four-arm star-shaped poly(ethylene glycol) (Tetra-PEG) precursors is an attractive strategy to prepare networks with relatively well-defined topologies. Typically, Tetra-PEG gels are obtained by cross-linking heterocomplementary reactive Tetra-PEG precursors. This study, in contrast, explores the cross-linking of self-reactive, thiol-end functional Tetra-PEG macromers to form disulfide-cross-linked gels. The structure of the disulfide-cross-linked Tetra-PEG gels was studied with multiple-quantum NMR (MQ-NMR) spectroscopy and small-angle neutron scattering (SANS) experiments. In line with earlier simulation studies, these experiments showed a strong dependence of the relative fractions of the different network connectivities on the concentration of the thiol-end functional Tetra-PEG macromer that was used for the synthesis of the networks. Disulfide-cross-linked Tetra-PEG gels prepared at macromer concentrations below the overlap concentration (c = 0.66c*) primarily feature defect connectivity motifs, such as primary loops and dangling ends. For networks prepared at macromer concentrations above the overlap concentration, the fraction of single-link connectivities was found to be similar to that in amide-cross-linked Tetra-PEG gels obtained by heterocomplementary cross-linking of N-hydroxysuccinimide ester and amine functional Tetra-PEG macromers. Since disulfide bonds are susceptible to reductive cleavage, these disulfide-cross-linked gels are of interest, e.g., as reduction-sensitive hydrogels for a variety of biomedical applications.

Tough double network hydrogels with rapid self-reinforcement and low hysteresis based on highly entangled networks

Most tough hydrogels are reinforced by introducing energy dissipation mechanisms, but simultaneously realizing a high toughness and low hysteresis is challenging because the energy dissipation structure cannot recover rapidly. In this work, high mechanical performance highly entangled double network hydrogels without energy dissipation structure are fabricated, in which physical entanglements act as the primary effective crosslinking in the first network. This sliding entanglement structure allows the hydrogel network to form a highly uniform oriented structure during stretching, resulting in a high tensile strength of ~3 MPa, a fracture energy of 8340 J m-2 and a strain-stiffening capability of 47.5 in 90% water content. Moreover, almost 100% reversibility is obtained in this hydrogel via energy storage based on entropy loss. The highly entangled double network structure not only overcomes the typical trade-off between the high toughness and low hysteresis of hydrogels, but more importantly, it provides an insight into the application of entanglement structures in high-performance hydrogels.

Comparison of gels synthesized by controlled radical copolymerization and free radical copolymerization: molecular dynamics simulation

The structures of gels synthesized by controlled radical copolymerization (CRP) and conventional free radical copolymerization (FRP) were studied by a coarse-grained molecular dynamics simulation. It was confirmed that the CRP gel has a larger number of elastically effective chains and fewer cyclic structures and entanglements than the FRP gel, i.e., the network structure of the CRP gel is more uniform than that of the FRP gel. However, the difference in the shear modulus between the two gels was small due to the opposing changes in the number of elastically effective chains and that of entanglements. The relatively uniform structure of the CRP gel is attributed to the suppression of intramolecular cross-linking by the fast initiation and slow propagation, and the development of cross-linked structures in the post-gel region due to the limited termination. The effects of these CRP characteristics were studied in detail. From the results, it was found that all of these characteristics of CRP cooperatively act to improve the homogeneity of the structure of the CRP gel.

Varying the Stiffness and Diffusivity of Rod-Shaped Microgels Independently through Their Molecular Building Blocks

Microgels are water-swollen, crosslinked polymers that are widely used as colloidal building blocks in scaffold materials for tissue engineering and regenerative medicine. Microgels can be controlled in their stiffness, degree of swelling, and mesh size depending on their polymer architecture, crosslink density, and fabrication method-all of which influence their function and interaction with the environment. Currently, there is a lack of understanding of how the polymer composition influences the internal structure of soft microgels and how this morphology affects specific biomedical applications. In this report, we systematically vary the architecture and molar mass of polyethylene glycol-acrylate (PEG-Ac) precursors, as well as their concentration and combination, to gain insight in the different parameters that affect the internal structure of rod-shaped microgels. We characterize the mechanical properties and diffusivity, as well as the conversion of acrylate groups during photopolymerization, in both bulk hydrogels and microgels produced from the PEG-Ac precursors. Furthermore, we investigate cell-microgel interaction, and we observe improved cell spreading on microgels with more accessible RGD peptide and with a stiffness in a range of 20 kPa to 50 kPa lead to better cell growth.

Fractographic mirror law for brittle fracture of nonlinear elastic soft materials

Mirror radius analysis of fractured surfaces is a powerful fractographic method for determining the cause of failure in linear elastic hard materials because it does not require prior loading information. However, mirror analysis for soft materials is lacking. In this study, we established a general mirror radius law for nonlinear elastic soft materials using highly deformable brittle hydrogels. The fracture stress and mirror radius follow a -1 power law, which differs from the -0.5 power law for linear elastic hard materials. The constant in the power law is related to the fracture energy of the material. This discovery provides insights into fracture mechanisms and leads the way for applying fractography to soft materials.

Highly resilient and fatigue-resistant poly(4-methyl-ε-caprolactone) porous scaffold fabricated via thiol-yne photo-crosslinking/salt-templating for soft tissue regeneration

Elastomeric scaffolds, individually customized to mimic the structural and mechanical properties of natural tissues have been used for tissue regeneration. In this regard, polyester elastic scaffolds with tunable mechanical properties and exceptional biological properties have been reported to provide mechanical support and structural integrity for tissue repair. Herein, poly(4-methyl-ε-caprolactone) (PMCL) was first double-terminated by alkynylation (PMCL-DY) as a liquid precursor at room temperature. Subsequently, three-dimensional porous scaffolds with custom shapes were fabricated from PMCL-DY via thiol-yne photocrosslinking using a practical salt template method. By manipulating the M_n of the precursor, the modulus of compression of the scaffold was easily adjusted. As evidenced by the complete recovery from 90% compression, the rapid recovery rate of >500 mm min^-1, the extremely low energy loss coefficient of <0.1, and the superior fatigue resistance, the PMCL20-DY porous scaffold was confirmed to harbor excellent elastic properties. In addition, the high resilience of the scaffold was confirmed to endow it with a minimally invasive application potential. In vitro testing revealed that the 3D porous scaffold was biocompatible with rat bone marrow stromal cells (BMSCs), inducing BMSCs to differentiate into chondrogenic cells. In addition, the elastic porous scaffold demonstrated good regenerative efficiency in a 12-week rabbit cartilage defect model. Thus, the novel polyester scaffold with adaptable mechanical properties may have extensive applications in soft tissue regeneration.

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.

Geometric rules for the annihilation dynamics of step lines on fracture fronts

The roughness of a fracture surface records a crack's complex path through a material and can affect the resultant frictional or fluid transport properties of the broken medium. For brittle fractures, some of the most prominent surface features are long, step-like discontinuities called step lines. In heterogeneous materials, the mean crack surface roughness created by these step lines is well captured by a simple, one-dimensional ballistic annihilation model, which assumes the creation of these steps is a random processes with a single probability that depends on the heterogeneity of the material, and that their destruction occurs via pairwise interactions. Here, through an exhaustive study of experimentally generated crack surfaces in brittle hydrogels, we examine step interactions and show that interaction outcomes depend on the geometry of the incoming steps. The rules that govern step interactions can be categorized into three unique classes and are fully described, providing a complete framework for predicting fracture roughness.

Gelation in Photoinduced ATRP with Tuned Dispersity of the Primary Chains

We investigated gelation in photoinduced atom transfer radical polymerization (ATRP) as a function of Cu catalyst loading and thus primary chain dispersity. Using parallel polymerizations of methyl acrylate with and without the addition of a divinyl crosslinker (1,6-hexanediol diacrylate), the approximate values of molecular weights and dispersities of the primary chains at incipient gelation were obtained. In accordance with the Flory-Stockmayer theory, experimental gelation occurred at gradually lower conversions when the dispersity of the primary chains increased while maintaining a constant monomer/initiator/crosslinker ratio. Theoretical gel points were then calculated using the measured experimental values of dispersity and initiation efficiency. An empirical modification to the Flory-Stockmayer equation for ATRP was implemented, resulting in more accurate predictions of the gel point. Increasing the dispersity of the primary chains was found not to affect the distance between the theoretical and experimental gel points and hence the extent of intramolecular cyclization. Furthermore, the mechanical properties of the networks, such as equilibrium swelling ratio and shear storage modulus showed little variation with catalyst loading and depended primarily on the crosslinking density.

Covalent Mechanochemistry and Contemporary Polymer Network Chemistry: A Marriage in the Making

Over the past 20 years, the field of polymer mechanochemistry has amassed a toolbox of mechanophores that translate mechanical energy into a variety of functional responses ranging from color change to small-molecule release. These productive chemical changes typically occur at the length scale of a few covalent bonds (Å) but require large energy inputs and strains on the micro-to-macro scale in order to achieve even low levels of mechanophore activation. The minimal activation hinders the translation of the available chemical responses into materials and device applications. The mechanophore activation challenge inspires core questions at yet another length scale of chemical control, namely: What are the molecular-scale features of a polymeric material that determine the extent of mechanophore activation? Further, how do we marry advances in the chemistry of polymer networks with the chemistry of mechanophores to create stress-responsive materials that are well suited for an intended application? In this Perspective, we speculate as to the potential match between covalent polymer mechanochemistry and recent advances in polymer network chemistry, specifically, topologically controlled networks and the hierarchical material responses enabled by multi-network architectures and mechanically interlocked polymers. Both fundamental and applied opportunities unique to the union of these two fields are discussed.

Nanoscale TEM Imaging of Hydrogel Network Architecture

In this work, the authors succeed in direct visualization of the network structure of synthetic hydrogels with transmission electron microscopy (TEM) by developing a novel staining and network fixation method. Such a direct visualization is not carried out because sample preparation and obtaining sufficient contrast are challenging for these soft materials. TEM images reveal robust heterogeneous network architectures at mesh size scale and defects at micro-scale. TEM images also reveal the presence of abundant dangling chains on the surface of the hydrogel network. The real space structural information provides a comprehensive perspective that links bulk properties with a nanoscale network structure, including fracture, adhesion, sliding friction, and lubrication. The presented method has the potential to advance the field.

How Material Heterogeneity Creates Rough Fractures

Fractures are a critical process in how materials wear, weaken, and fail, whose unpredictable behavior can have dire consequences. While the behavior of smooth cracks in ideal materials is well understood, it is assumed that for real, heterogeneous systems, fracture propagation is complex, generating rough fracture surfaces that are highly sensitive to specific details of the medium. Here we show how fracture roughness and material heterogeneity are inextricably connected via a simple framework. Studying hydraulic fractures in brittle hydrogels that have been supplemented with microbeads or glycerol to create controlled material heterogeneity, we show that the morphology of the crack surface depends solely on one parameter: the probability to perturb the front above a critical size to produce a steplike instability. This probability scales linearly with the number density, and with heterogeneity size to the 5/2 power. The ensuing behavior is universal and is captured by the 1D ballistic propagation and annihilation of steps along the singular fracture front.

Shrinking rates of polymer gels composed of star-shaped polymers of N-isopropylacrylamide and dimethylacrylamide copolymers: the effect of dimethylacrylamide on the crosslinking network

Thermoresponsive polymer gels can be applied as culture beds for cell sheets, drug release agents for drug delivery, and sensing materials. In general, the shrinkage behavior of thermoresponsive polymer gels is complex, and they may require much longer times than swelling to reach thermodynamically stable shrinkage states. This slow volume change during shrinkage is often a drawback in using reversible changes in polymer gel volumes with changing temperature for applications such as those described above, and attempts have been made to improve the shrinkage rates of polymer gels. However, using the conventional method results in a low density of the three-dimensional crosslinked network comprising the polymer gel, which weakens the mechanical properties of the polymer gel. In this study, we investigated the effects of monomer arrangement and composition for star-shaped polymers composed of N-isopropylacrylamide and N,N-dimethylacrylamide on the shrinkage behavior of gels comprising star-shaped polymers with the aim of increasing their shrinkage rates without reducing the network densities of the temperature-responsive polymer gels. Based on selective network decomposition by methanolysis and SAXS measurements, the network structures of the obtained spherical gels were found to be more homogeneous than those of polymer gels obtained by conventional free radical polymerization. These gels exhibited reversible volume changes in water, with low-temperature swelling and high-temperature shrinkage. The rates of volume changes from a high temperature shrunken state to a low temperature swollen one were almost the same for all gels. However, the rates of volume changes from low-temperature swollen states to high-temperature shrunken states varied greatly depending on the compositions and sequences of monomers that made up the polymer networks. We confirmed that the introduction of more than 20% DMA as a block copolymer in the network suppressed phase separation and formation of a skin layer and the water inside the polymer gel drained smoothly to the outside, which resulted in an increase in the shrinkage speed.

Linking cavitation and fracture to molecular scale structural damage of model networks

Rapid expansion of soft solids subjected to a negative hydrostatic stress can occur through cavitation or fracture. Understanding how these two mechanisms relate to a material's molecular structure is important to applications in materials characterization, adhesive design, and tissue damage. Here, a recently improved needle-induced cavitation (NIC) protocol is applied to a set of model end-linked PEG gels with quantitatively linked elastic and fracture properties. This quantitative link between molecular scale structure and macroscopic properties is exploited to experimentally probe the relationship between cavitation, fracture, and molecular scale damage. This work indicates that rational tuning of the elastofracture length relative to the crack geometry can be used to alter the expansion mechanism from cavitation to fracture during NIC.

Thermoresponsive Smart Gating Wood Membranes

Smart membranes that can open and/or close their pores in a controlled manner by external stimuli possess potential in various applications, such as water flow manipulation, indoor climate regulation, and sensing. The design of smart gating membranes with high flux, immediate response, and mechanical robustness is still an open challenge, limiting their versatility and practical applicability. Inspired by the controlled opening and closure of plant stomata, we have developed a smart gating wood membrane, taking advantage of the unique wood scaffold with its hierarchical porous structure to carry thermoresponsive hydrogel gates. Laser drilling was applied to cut channels in the wood scaffold with well-aligned pores to incorporate the smart gating membranes. In situ polymerization of poly(N-isopropylacrylamide) above its lower critical solution temperature inside the channels resulted in a hydrogel with a heterogeneous microstructure acting as a thermoresponsive gate. The wood-based smart gating membranes exhibited reversible and stable pore opening/closing under heating/cooling stimuli. The achieved rapid response and feasibility of scale-up open the venue for various practical applications. In this work, we demonstrated their potential for indoor light regulation and as a water flow manipulator.

Next Evolution in Organ-Scale Biofabrication: Bioresin Design for Rapid High-Resolution Vat Polymerization

The field of bioprinting has made significant advancements in recent years and allowed for the precise deposition of biomaterials and cells. However, within this field lies a major challenge, which is developing high resolution constructs, with complex architectures. In an effort to overcome these challenges a biofabrication technique known as vat polymerization is being increasingly investigated due to its high fabrication accuracy and control of resolution (µm scale). Despite the progress made in developing hydrogel precursors for bioprinting techniques, such as extrusion-based bioprinting, there is a major lack in developing hydrogel precursor bioresins for vat polymerization. This is due to the specific unique properties and characteristics required for vat polymerization, from lithography to the latest volumetric printing. This is of major concern as the shortage of bioresins available has a significant impact on progressing this technology and exploring its full potential, including speed, resolution, and scale. Therefore, this review discusses the key requirements that need to be addressed in successfully developing a bioresin. The influence of monomer architecture and bioresin composition on printability is described, along with key fundamental parameters that can be altered to increase printing accuracy. Finally, recent advancements in bioresins are discussed together with future directions.

Fracture of model end-linked networks

Fracture of polymer networks is molecular in nature as chains must rupture for a crack to propagate. Predicting macroscopically measured fracture properties from molecular parameters a priori has remained elusive, even for well-defined networks. This work shows that a newly developed theory, which accounts for network defects, quantitatively describes both experimental measurements and molecular dynamics (MD) simulation data for fracture of a polymer network with independently defined or measured parameters. This advance provides a missing link between chemistry and materials science and engineering for polymer networks.

Advances in polymer chemistry over the last decade have enabled the synthesis of molecularly precise polymer networks that exhibit homogeneous structure. These precise polymer gels create the opportunity to establish true multiscale, molecular to macroscopic, relationships that define their elastic and failure properties. In this work, a theory of network fracture that accounts for loop defects is developed by drawing on recent advances in network elasticity. This loop-modified Lake–Thomas theory is tested against both molecular dynamics (MD) simulations and experimental fracture measurements on model gels, and good agreement between theory, which does not use an enhancement factor, and measurement is observed. Insight into the local and global contributions to energy dissipated during network failure and their relation to the bond dissociation energy is also provided. These findings enable a priori estimates of fracture energy in swollen gels where chain scission becomes an important failure mechanism.

Defect-controlled softness, diffusive permeability, and mesh-topology of metallo-supramolecular hydrogels

Hydrogels are polymer networks swollen in water; they are suitable materials for biomedical applications such as tissue engineering and drug delivery. In the latter, the controlled diffusion of small diffusants inside the network is essential, as it determines the release mechanism of the drug. In general, the diffusion inside a polymer network is controlled by its mesh-size. Here, we actively control the diffusivity and also the softness of metallo-supramolecular hydrogels via the network mesh-topology by introducing connectivity defects. A model polymer network is realized based on a 4-arm poly(ethylene glycol) (pEG) where each arm is capped with terpyridine moieties that are capable of forming metallo-supramolecular complexes with zinc ions. In this model network, we insert 8-arm pEG macromolecules that are functionalized with terpyridine at different ratios to create connectivity defects. With an increasing amount of 8-arm pEG, the polymer network forms more loops, as quantified by double quantum-NMR. This doped network shows an enhanced self-diffusivity of the building block molecules within the network, as examined by fluorescence recovery after photobleaching, and a higher softness, as investigated by oscillatory shear rheology. With these findings, we show that it is possible to tune the diffusivity and softness of hydrogels with defects in a rational fashion.

In Situ RheoNMR Correlation of Polymer Segmental Mobility with Mechanical Properties during Hydrogel Synthesis

Understanding polymer gelation over multiple length-scales is crucial to develop advanced materials. An experimental setup is developed that combines rheological measurements with simultaneous time-domain 1 H NMR relaxometry (TD-NMR) techniques, which are used to study molecular motion (<10 nm) in soft matter. This so-called low-field RheoNMR setup is used to study the impact of varying degrees of crosslinking (DC) on the gelation kinetics of acrylic acid (AAc) and N,N'-methylene bisacrylamide (MBA) free radical crosslinking copolymerization. A stretched exponential function describes the T2 relaxation curves throughout the gelation process. The stretching exponent β decreases from 0.90 to 0.67 as a function of increasing DC, suggesting an increase in network heterogeneity with a broad T2 distribution at higher DC. The inverse correlation of the elastic modulus G' with T2 relaxation times reveals a pronounced molecular rigidity for higher DC at early gelation times, indicating the formation of inelastic, rigid domains such as crosslinking clusters. The authors further correlate G' with the polymer concentration during gelation using a T1 filter for solvent suppression. A characteristic scaling exponent of 2.3 is found, which is in agreement with theoretical predictions of G' based on the confining tube model in semi-dilute entangled polymer solutions.

Injectable thiol-ene hydrogel of galactoglucomannan and cellulose nanocrystals in delivery of therapeutic inorganic ions with embedded bioactive glass nanoparticles

We propose an injectable nanocomposite hydrogel that is photo-curable via light-induced thiol-ene addition between methacrylate modified O-acetyl-galactoglucomannan (GGMMA) and thiolated cellulose nanocrystal (CNC-SH). Compared to free-radical chain polymerization, the orthogonal step-growth of thiol-ene addition allows a less heterogeneous hydrogel network and more rapid crosslinking kinetics. CNC-SH reinforced the GGMMA hydrogel as both a nanofiller and a crosslinker to GGMMA resulting in an interpenetrating network via thiol-ene addition. Importantly, the mechanical stiffness of the GGMMA/CNC-SH hydrogel is mainly determined by the stoichiometric ratio between the thiol groups on CNC-SH and the methacrylate groups in GGMMA. Meanwhile, the bioactive glass nanoparticle (BaGNP)-laden hydrogels of GGMMA/CNC-SH showed a sustained release of therapeutic ions in simulated body fluid in vitro, which extended the bioactive function of hydrogel matrix. Furthermore, the suitability of the GGMMA/CNC-SH formulation as biomaterial resin to fabricate digitally designed hydrogel constructs via digital light processing (DLP) lithography printing was evaluated.

Finding the sweet spot: a library of hydrogels with tunable degradation for tissue model development

In vitro models are valuable tools for applications, including understanding cellular mechanisms and drug screening. Hydrogel biomaterials are very useful for in vitro models to better mimic the in vivo...

Catalyst Control of Nanoscale Characteristic Length of the Glass Transition in Organic Strong Glass-Formers

We report the nanoscale characteristic length (ξ_α) in organic strong glass-formers at glass transition. Catalyst-dependent ξ_α of the epoxy-based covalent adaptable networks (CANs) is observed, that is, the more efficient the catalyst for the cross-linking reactions, the larger the ξ_α is, and upon thermal heating, the more broad the glass transition will be. The observed structural properties at glass transition can be correlated with the topology freezing transition where the fluctuations of network topology aroused from the bond exchange reactions are frozen. This study proposes the catalyst-dependent structural properties in CANs and may fill the structural gap between the glass transition and topology freezing transition of organic strong glass-formers.

Covalently Crosslinked Hydrogels via Step-Growth Reactions: Crosslinking Chemistries, Polymers, and Clinical Impact

Hydrogels are an important class of biomaterials with the unique property of high-water content in a crosslinked polymer network. In particular, chemically crosslinked hydrogels have made a great clinical impact in past years because of their desirable mechanical properties and tunability of structural and chemical properties. Various polymers and step-growth crosslinking chemistries are harnessed for fabricating such covalently crosslinked hydrogels for translational research. However, selecting appropriate crosslinking chemistries and polymers for the intended clinical application is time-consuming and challenging. It requires the integration of polymer chemistry knowledge with thoughtful crosslinking reaction design. This task becomes even more challenging when other factors such as the biological mechanisms of the pathology, practical administration routes, and regulatory requirements add additional constraints. In this review, key features of crosslinking chemistries and polymers commonly used for preparing translatable hydrogels are outlined and their performance in biological systems is summarized. The examples of effective polymer/crosslinking chemistry combinations that have yielded clinically approved hydrogel products are specifically highlighted. These hydrogel design parameters in the context of the regulatory process and clinical translation barriers, providing a guideline for the rational selection of polymer/crosslinking chemistry combinations to construct hydrogels with high translational potential are further considered.

Experimental Verification of the Balance between Elastic Pressure and Ionic Osmotic Pressure of Highly Swollen Charged Gels

The equilibrium swelling degree of a highly swollen charged gel has been thought to be determined by the balance between its elastic pressure and ionic osmotic pressure. However, the full experimental verification of this balance has not previously been conducted. In this study, we verified the balance between the elastic pressure and ionic osmotic pressure of charged gels using purely experimental methods. We used tetra-PEG gels created using the molecular stent method (St-tetra-PEG gels) as the highly swollen charged gels to precisely and separately control their network structure and charge density. The elastic pressure of the gels was measured through the indentation test, whereas the ionic osmotic pressure was determined by electric potential measurement without any strong assumptions or fittings. We confirmed that the two experimentally determined pressures of the St-tetra-PEG gels were well balanced at their swelling equilibrium, suggesting the validity of the aforementioned relationship. Furthermore, from single-strand level analysis, we investigated the structural requirements of the highly swollen charged gels in which the elasticity and ionic osmosis are balanced at their swelling equilibrium.

Can oppositely charged polyelectrolyte stars form a gel? A simulational study

We present a Langevin molecular dynamics study of an equimolar mixture of monodispersed oppositely charged di-block four-armed polyelectrolyte stars. We used an implicit solvent coarse-grained representation of the polyelectrolyte stars, and varied the length of the terminal charged blocks that reside on each arm. By varying the polymer concentration, we computed PV diagrams and determined the free-swelling equilibrium concentration with respect to a pure water reservoir as a function of the charged block length. We investigated various structural properties of the resulting equilibrium structures, like the number of ionic bonds, dangling arms, isolated stars, and cluster sizes. The ionic bonds featured a broad distribution of the number of arms involved and also displayed a distribution of net charges peaked around the neutral ionic bond. Our main result is that for charged block length equal to 4 and 5 ionized beads the resulting macro-aggregate spans the box and forms a network phase. Furthermore, we investigated the restructuring dynamics of ionic bonds; the results suggested both short bond lifetimes and a high frequency of ballistic association/dissociation events. Bonds result strong enough to yield a stable gel phase, but they are still weak enough to allow network restructuring under thermal fluctuations.

Is the Microgel Collapse a Two-Step Process? Exploiting Cononsolvency to Probe the Collapse Dynamics of Poly-N-isopropylacrylamide (pNIPAM)

Many applications of responsive microgels rely on the fast adaptation of the polymer network. However, the underlying dynamics of the de-/swelling process of the gels have not been fully understood. In the present work, we focus on the collapse kinetics of poly-N-isopropylacrylamide (pNIPAM) microgels due to cononsolvency. Cononsolvency means that either of the pure solvents, e.g., pure water or pure methanol, act as a so-called good solvent, leading to a swollen state of the polymer network. However, in mixtures of water and methanol, the previously swollen network undergoes a drastic volume loss. To further elucidate the cononsolvency transition, pNIPAM microgels with diameters between 20 and 110 μm were synthesized by microfluidics. To follow the dynamics, pure water was suddenly exchanged with an unfavorable mixture of 20 mol% methanol (solvent-jump) within a microfluidic channel. The dynamic response of the microgels was investigated by optical and fluorescence microscopy and Raman microspectroscopy. The experimental data provide unique and detailed insight into the size-dependent kinetics of the volume phase transition due to cononsolvency. The change in the microgel's diameter over time points to a two-step process of the microgel collapse with a biexponential behavior. Furthermore, the dependence between the two time constants from this biexponential behavior and the microgel's diameter in the collapsed state deviates from the square-power law proposed by Tanaka and Fillmore [ J. Chem. Phys. 1979, 70, 1214-1218]. The deviation is discussed considering the adhesion-induced deformation of the gels and the physical processes underlying the collapse.

Role of soft-gel substrates on bouncing-merging transition in drop impact on a liquid film

Liquid droplets impacting on liquid films is common in many industrial and natural processes. It is crucial to understand the impact of droplets on a liquid film resting on soft deformable substrates in some of the applications including 3D printing of engineering structures, prosthetic implants and tissue engineering. By recognizing the practical relevance of soft-substrates, we present an experimental investigation to assess the role of deformable substrates on bouncing-to-merging transition in droplet impact on the liquid film. First, we prepared polyacrylamide (PAAm) soft-gel substrates with various "softness" (i.e., Young's modulus) by modulating the concentration of a crosslinker, N,N-methylene-bis-acrylamide (BIS). We found that the Young's modulus of PAAm initially increases with the concentration of crosslinker, and subsequently becomes almost constant due to inhomogeneity of crosslinking. Next, through the experiments of droplet impact on the liquid film resting on soft substrates with different Young's moduli, we observe that the early merging and corresponding bouncing-to-merging transitional boundaries remain unaffected by the "softness" since such merging occurs further away from the substrate. However, the late merging, which appears during the retraction process of the deformed droplet, occurs relatively close to the substrate, and hence is found to be significantly affected by its "softness". A scaling analysis is presented to quantify the role of change in Young's modulus of the substrate on late merging, which is supported by the experimental data.

Effect of heterogeneous and homogeneous polymerisation on the structure of pNIPAm nanogels

The choice of the polymerisation temperature and initiator in the synthesis of poly(N-isopropylacrylamide)-based nanogels can significantly influence their structure, morphology and thermoresponsive properties.

Digitally Fabricated and Naturally Augmented In Vitro Tissues

Human in vitro tissues are extracorporeal 3D cultures of human cells embedded in biomaterials, commonly hydrogels, which recapitulate the heterogeneous, multiscale, and architectural environment of the human body. Contemporary strategies used in 3D tissue and organ engineering integrate the use of automated digital manufacturing methods, such as 3D printing, bioprinting, and biofabrication. Human tissues and organs, and their intra- and interphysiological interplay, are particularly intricate. For this reason, attentiveness is rising to intersect materials science, medicine, and biology with arts and informatics. This report presents advances in computational modeling of bioink polymerization and its compatibility with bioprinting, the use of digital design and fabrication in the development of fluidic culture devices, and the employment of generative algorithms for modeling the natural and biological augmentation of in vitro tissues. As a future direction, the use of serially linked in vitro tissues as human body-mimicking systems and their application in drug pharmacokinetics and metabolism, disease modeling, and diagnostics are discussed.

Defects and defect engineering in Soft Matter

Soft matter covers a wide range of materials based on linear or branched polymers, gels and rubbers, amphiphilic (macro)molecules, colloids, and self-assembled structures. These materials have applications in various industries, all highly important for our daily life, and they control all biological functions; therefore, controlling and tailoring their properties is crucial. One way to approach this target is defect engineering, which aims to control defects in the material's structure, and/or to purposely add defects into it to trigger specific functions. While this approach has been a striking success story in crystalline inorganic hard matter, both for mechanical and electronic properties, and has also been applied to organic hard materials, defect engineering is rarely used in soft matter design. In this review, we present a survey on investigations on defects and/or defect engineering in nine classes of soft matter composed of liquid crystals, colloids, linear polymers with moderate degree of branching, hyperbranched polymers and dendrimers, conjugated polymers, polymeric networks, self-assembled amphiphiles and proteins, block copolymers and supramolecular polymers. This overview proposes a promising role of this approach for tuning the properties of soft matter.

The hierarchical bulk molecular structure of poly(acrylamide) hydrogels: beyond the fishing net

The polymeric structure of hydrogels is commonly presented in the literature as resembling a fishing net. However, this simple view cannot fully capture all the unique properties of these materials. Crucial for a detailed description of the bulk structure in free-radical polymerized vinylic hydrogels is a thorough understanding of the cross-linker distribution. This work focuses on the precise role of the tetra-functional cross-linker in the hydrogel system: acrylamide-N,N'-methylenebis(acrylamide). Clusters of crosslinker smaller than 4 nm and their agglomerates, as well as polymer domains with sizes from the 100 nm to the μm-range, have been identified by means of both X-ray and visible-light scattering. Placed in the context of the extensive literature on this system, these observations demonstrate the heterogeneous organisation of the polymer within the hydrogel network structure, and can be accounted for by the different polymerization behavior of the monomer and crosslinker. Together with polymer-network chain-length approximations based on swelling experiments and structural observations with scanning electron microscopy, these results indicate a hierarchical structure of the polymer network surrounding pockets of water.

Photopolymerizable Biomaterials and Light-Based 3D Printing Strategies for Biomedical Applications

Since the advent of additive manufacturing, known commonly as 3D printing, this technology has revolutionized the biofabrication landscape and driven numerous pivotal advancements in tissue engineering and regenerative medicine. Many 3D printing methods were developed in short course after Charles Hull first introduced the power of stereolithography to the world. However, materials development was not met with the same enthusiasm and remained the bottleneck in the field for some time. Only in the past decade has there been deliberate development to expand the materials toolbox for 3D printing applications to meet the true potential of 3D printing technologies. Herein, we review the development of biomaterials suited for light-based 3D printing modalities with an emphasis on bioprinting applications. We discuss the chemical mechanisms that govern photopolymerization and highlight the application of natural, synthetic, and composite biomaterials as 3D printed hydrogels. Because the quality of a 3D printed construct is highly dependent on both the material properties and processing technique, we included a final section on the theoretical and practical aspects behind light-based 3D printing as well as ways to employ that knowledge to troubleshoot and standardize the optimization of printing parameters.