Chemistry matters: A side-by-side comparison of two chemically distinct methacryloylated dECM bioresins for vat photopolymerization.

Decellularized extracellular matrix (dECM) is an excellent natural source for 3D bioprinting materials due to its inherent cell compatibility. In vat photopolymerization, the use of dECM-based bioresins is just emerging, and extensive research is needed to fully exploit their potential. In this study, two distinct methacryloyl-functionalized, photocrosslinkable dECM-based bioresins were prepared from digested porcine liver dECM through functionalization with glycidyl methacrylate (GMA) or conventional methacrylic anhydride (MA) under mild conditions for systematic comparison. Although the chemical modifications did not significantly affect the structural integrity of the dECM proteins, mammalian cells encapsulated in the respective hydrogels performed differently in long-term culture. In either case, photocrosslinking during 3D (bio)printing resulted in transparent, highly swollen, and soft hydrogels with good shape fidelity, excellent biomimetic properties and tunable mechanical properties (~ 0.2-2.5 kPa). Interestingly, at a similar degree of functionalization (DOF ~ 81.5-83.5 %), the dECM-GMA resin showed faster photocrosslinking kinetics in photorheology resulting in lower final stiffness and faster enzymatic biodegradation compared to the dECM-MA gels, yet comparable network homogeneity as assessed via Brillouin imaging. While human hepatic HepaRG cells exhibited comparable cell viability directly after 3D bioprinting within both materials, cell proliferation and spreading were clearly enhanced in the softer dECM-GMA hydrogels at a comparable degree of crosslinking. These differences were attributed to the additional hydrophilicity introduced to dECM via methacryloylation through GMA compared to MA. Due to its excellent printability and cytocompatibility, the functional porcine liver dECM-GMA biomaterial enables the advanced biofabrication of soft 3D tissue analogs using vat photopolymerization-based bioprinting.

Amphiphilic Block Copolymers PEG-b-PMTCs: Synthesis, Self-Assembly, Degradation Properties and Biocompatibility.

As a hydrophilic cyclic ketene acetal (CKA), 2-methylene-1,3,6-trioxocane (MTC) has recently attracted a lot of attention owing to its ability to promote a quicker (bio)degradation as compared to other heavily studied CKAs. Here, we prepared amphiphilic block copolymers based on poly-MTC with varying chain lengths by radical ring opening polymerization. Self-assemblies of these amphiphiles were performed in PBS buffer to generate nanoparticles with sizes from 40 to 105 nm, which were verified by dynamic light scattering, electron microscopy, and static light scattering (Zimm plots). Subsequently, fluorescence spectroscopy was applied to study the enzymatic degradation of Nile red-loaded nanoparticles. By performing a point-by-point comparison of fluorescence intensity decline patterns between nanoparticles, we demonstrated that lipase from Pseudomonas cepacia was very efficient in degrading the nanoparticles. Hydrolysis degradations under basic conditions were also carried out, and a complete degradation was achieved after 4 h. Additionally, cytotoxicity assays were carried out on HEK293 cells, and the results affirmed cell viabilities over 90% when incubated with up to 1 mg/mL nanoparticles for 24 h. These biodegradable and biocompatible nanoparticles hence hold great potential for future applications such as drug release.

Self-organization of metallo-supramolecular polymer networks by free formation of pyridine-phenanthroline heteroleptic complexes.

Nature employs spontaneous self-organization of supramolecular bonds to create complex matter capable of adaptation and self-healing. Accordingly, the self-sorting of unlike ligands towards a cooperative heteroleptic complex or narcistic homoleptic association in a mixed ligand system is frequently employed to form interchangeable stimuli-responsive complex geometries with a wide range of applications. This notion is however just rarely employed in the organization of polymer networks. In this paper, we report the free-formation of heteroleptic complexes between tetra-am poly(ethylene glycol) (tetraPEG) precursors functionalized either with pyridine (tetraPy) or phenanthroline (tetraEPhen). Among a wide range of studied metal ions, tetraPy could form a network only in combination with Pd2+, presumably with a square-planar geometry, highlighting the importance of complex strength and stability in forming gels with monodentate ligands. Also, mixed networks with tetraEPhen form only in combination with Pd2+ and Fe2+, with strengths surpassing those of individual components and stabilities incomparable to those of parent networks, indicative of heteroleptic complexation. Extensive rheological, UV-vis, and DFT simulation studies revealed the coexistence of different coordination geometries, with an octahedral arrangement prevailing in the presence of Fe2+ and a square-planar geometry in the presence of Pd2+. Therefore, this study offers new opportunities for the development of stimuli-responsive topology-switching polymer networks.

Metal-ligand complexation and clustering in mussel-inspired side-chain functionalized supramolecular hydrogels.

Byssus threads of mussels have high resistance against abrasion in wave-swept habitats because of their outer cuticle, which is rich in amino acid dopa complexes with Fe3+ ions. This stems from the transient nature of metal-ligand complexes that creates extra relaxation mechanisms. Inspired by this concept, in this work, supramolecular hydrogels based on poly(acrylic acid) functionalized with nitrocatechol groups are synthesized. Polymer chains are physically crosslinked via nitrocatechol-Fe3+ complexes. The hydrogels have different polymer volume fractions as well as different nitrocatechol : Fe3+ molar ratios. The strength of the supramolecular crosslinks strongly depends on the pH of the medium. The dynamics of these hydrogels are studied by stress relaxation experiments followed by calculation of the relaxation time spectrum. Generally, samples have three relaxation modes, including dissociation of distinct metal-ligand complexes, reptation of sticky polymer chains, and disengagement of network segments from supramolecular aggregates and clusters. Such clusters hinder the terminal relaxation and potentially increase the stability of supramolecular hydrogels.

Molecular behavior of fluid gels - the crucial role of edges and particle surface in macroscopic properties.

Fluid gels exhibit unique properties during oral processing and thus are well known in gastronomy as well as for use in dysphagia patients. Agarose fluid gels, which are produced by gelation under shear, in particular, show elastic solid-like behavior at rest but a fluid-like behavior once critical stress is exceeded. In a previous study this special behavior is addressed to the "hairy" structure of the microgel particles - dangling gel parts and chains on the particle surface - which plays a crucial role in the rheological, mechanical and tribological properties of the gels. In this paper, atomic force microscopy (AFM) was used to investigate the underlying microscopic structures and develop a consistent physical model, which relates the irregular particle structures and their heterogonous shape to the experimental observation of the previous studies. One crucial point is the inner structure of the gel particles, which show a dense area in the center, whereas towards the periphery the network and thus the elastic properties change. Agarose gels by forming helices and meshes, which defines the basic length scale for their elastic response in bulk. These properties in turn depend on the concentration and preparation conditions. The present study is meant to address the still prevalent lack of understanding regarding a direct structure-property relationship of these novel fluid gels. Controlling the properties of such fluid gels may play a crucial role in the texture modification of foods and beverages for dysphagia.

Hierarchy of relaxation times in supramolecular polymer model networks.

Supramolecular polymer gels are an evolving class of soft materials with a vast number of properties that can be tuned to desired applications. Despite continuous advances concerning polymer synthesis, sustainability or adaptability, a consistent understanding of the interplay between structure, dynamics, and diffusion processes within transient networks is lacking. In this study, the hierarchy of several relaxation processes is investigated, starting from a microscopic perspective of a single sticker dissociation event up to the center-of-mass diffusion of a star-shaped polymer building block on different length scales, as well as the resulting macroscopic mechanical response to applied external stress. In addition to that, a second focus is placed on the gel micro-structure that is analyzed by light scattering. Conversion of the dynamic light scattering (DLS) inverse length scale into real space allows for a combination of relaxation times with those obtained by forced Rayleigh scattering (FRS). For these investigations, a model-type metallo-supramolecular network consisting of narrowly dispersed tetra-arm poly(ethylene glycol)-terpyridine macromolecules that are interconnected via complexation with zinc ions is chosen. Assembling the obtained activation energies reveals that all complex dissociation-governed relaxation processes exhibit similar activation energies.

Multiparametric Material Functionality of Microtissue-Based In Vitro Models as Alternatives to Animal Testing.

With the definition of the 3R principle by Russel and Burch in 1959, the search for an adequate substitute for animal testing has become one of the most important tasks and challenges of this time, not only from an ethical, but also from a scientific, economic, and legal point of view. Microtissue-based in vitro model systems offer a valuable approach to address this issue by accounting for the complexity of natural tissues in a simplified manner. To increase the functionality of these model systems and thus make their use as a substitute for animal testing more likely in the future, the fundamentals need to be continuously improved. Corresponding requirements exist in the development of multifunctional, hydrogel-based materials, whose properties are considered in this review under the aspects of processability, adaptivity, biocompatibility, and stability/degradability.

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.

Bridging Rigidity and Flexibility: Modulation of Supramolecular Hydrogels by Metal Complexation.

The combination of complementary, noncovalent interactions is a key principle for the design of multistimuli responsive hydrogels. In this work, an amphiphilic peptide, supramacromolecular hydrogelator which combines metal-ligand coordination induced gelation and thermoresponsive toughening is reported. Following a modular approach, the incorporation of the triphenylalanine sequence FFF into a structural (C3 EG ) and a terpyridine-functionalized (C3 Tpy ) C3 -symmetric monomer enables their statistical copolymerization into self-assembled, 1D nanorods in water, as investigated by circular dichroism (CD) spectroscopy and transmission electron microscopy (TEM). In the presence of a terpyridine functionalized telechelic polyethylene glycol (PEG) cross-linker, complex formation upon addition of different transition metal ions (Fe2+ , Zn2+ , Ni2+ ) induces the formation of soft, reversible hydrogels at a solid weight content of 1 wt% as observed by linear shear rheology. The viscoelastic behavior of Fe2+ and Zn2+ cross-linked hydrogels are basically identical, while the most kinetically inert Ni2+ coordinative bond leads to significantly weaker hydrogels, suggesting that the most dynamic rather than the most thermodynamically stable interaction supports the formation of robust and responsive hydrogel materials.

Cell Adhesion on UV-Crosslinked Polyurethane Gels with Adjustable Mechanical Strength and Thermoresponsiveness.

Temperature-responsive polyurethane (PU) hydrogels represent a versatile material platform for modern tissue engineering and biomedical applications. However, besides intrinsic advantages such as high mechanical strength and a hydrolysable backbone composition, plain PU materials are generally lacking bio-adhesive properties. To overcome this shortcoming, the authors focus on the synthesis of thermoresponsive PU hydrogels with variable mechanical and cell adhesive properties obtained from linear precursor PUs based on poly(ethylene glycol)s (pEG) with different molar masses, isophorone diisocyanate, and a dimerizable dimethylmaleimide (DMMI)-diol. The cloud point temperatures of the dilute, aqueous PU solutions depend linearly on the amphiphilic balance. Rheological gelation experiments under UV-irradiation reveal the dependence of the gelation time on photosensitizer concentration and light intensity, while the finally obtained gel strength is determined by the polymer concentration and spacing of the crosslinks. The swelling ratios of these soft hydrogels show significant changes between 5 and 40 °C whereby the extent of this switch increases with the hydrophobicity of the precursor. Moreover, it is shown that the incorporation of a low amount of catechol groups into the networks through the DMMI dimerization reaction leads to strongly improved cell adhesive properties without significantly weakening the gels.

Rational Design of Thermoresponsive Microgel Templates with Polydopamine Surface Coating for Microtissue Applications.

Functional microgels provide a versatile basis for synthetic in vitro platforms as alternatives to animal experiments. The tuning of the physical, chemical, and biological properties of synthetic microgels can be achieved by blending suitable polymers and formulating them such to reflect the heterogenous and complex nature of biological tissues. Based on this premise, this paper introduces the development of volume-switchable core-shell microgels as 3D templates to enable cell growth for microtissue applications, using a systematic approach to tune the microgel properties based on a deep conceptual and practical understanding. Microscopic microgel design, such as the tailoring of the microgel size and spherical shape, is achieved by droplet-based microfluidics, while on a nanoscopic scale, a thermoresponsive polymer basis, poly(N-isopropylacrylamide) (PNIPAAm), is used to provide the microgel volume switchability. Since PNIPAAm has only limited cell-growth promoting properties, the cell adhesion on the microgel is further improved by surface modification with polydopamine, which only slightly affects the microgel properties, thereby simplifying the system. To further tune the microgel thermoresponsiveness, different amounts of N-hydroxyethylacrylamide are incorporated into the PNIPAAm network. In a final step, cell growth on the microgel surface is investigated, both at a single microgel platform and in spheroidal cell structures.

Physics of agarose fluid gels: Rheological properties and microstructure.

Agarose, a strongly gelling polysaccharide, is a common ingredient used to optimize the viscoelastic properties of a multitude of food products. Through aggregation of double helices via hydrogen bonds while cooling under quiescent conditions it forms firm and brittle gels. However, this behavior can be altered by manipulating the processing conditions viz shear. For example, gelation under shear leads to microgel particles with large surface area, which in turn leads to completely different rheological properties and texture. Such fluid gels are shown to play an important role in texture modification of foods and beverages for dysphagia patients. In this study, different concentration of agarose fluid gel (0.5 % wt, 1 % wt and 2 % wt) were considered. Rheological measurements of the microgel particles showed an increase of storage and loss modulus with increasing concentration. However, 1 % wt fluid gel exhibited the lowest viscosity in the low shear range and the shortest LVE range. Furthermore, the effect on the microstructure and size of gel particles were also investigated by using light microscopy and particle size analysis. It was observed that as the concentration of agarose increased the particle size and unordered chains present at the particle surface decreases. Based on our results, we propose specific models suggesting the impact of the particle size, the concentration and the "hairy" projections on the rheological and tribological properties that could help in understanding the differences in characteristics of fluid gels.

An Inverse Thermogelling Bioink Based on an ABA-Type Poly(2-oxazoline) Amphiphile.

Hydrogels are key components in several biomedical research areas such as drug delivery, tissue engineering, and biofabrication. Here, a novel ABA-type triblock copolymer comprising poly(2-methyl-2-oxazoline) as the hydrophilic A blocks and poly(2-phenethyl-2-oxazoline) as the aromatic and hydrophobic B block is introduced. Above the critical micelle concentration, the polymer self-assembles into small spherical polymer micelles with a hydrodynamic radius of approx 8-8.5 nm. Interestingly, this specific combination of hydrophilic and hydrophobic aromatic moieties leads to rapid thermoresponsive inverse gelation at polymer concentrations above a critical gelation concentration (20 wt %) into a macroporous hydrogel of densely packed micelles. This hydrogel exhibited pronounced viscoelastic solid-like properties, as well as extensive shear-thinning, rapid structure recovery, and good strain resistance properties. Excellent 3D-printability of the hydrogel at lower temperature opens a wide range of different applications, for example, in the field of biofabrication. In preliminary bioprinting experiments using NIH 3T3 cells, excellent cell viabilities of more than 95% were achieved. The particularly interesting feature of this novel material is that it can be used as a printing support in hybrid bioink systems and sacrificial bioink due to rapid dissolution at physiological conditions.

Salt partitioning in ionized, thermo-responsive hydrogels: perspective to water desalination.

Charged hydrogels are capable of swelling in aqueous salt solutions, whereby part of the salt ions is repelled due to the presence of fixed charged groups inside the hydrogel. This effect creates a concentration gradient between the absorbed solution and the surrounding fluid known as salt partitioning, offering a potential for these materials to be employed to desalinate saltwater. If the charged hydrogels are thermo-sensitive as well, then the purer, absorbed solution can be recovered by shrinking the hydrogels upon temperature change. To tailor that potential in water-purification and desalination applications, the main parameters influencing the salt partitioning, the deswelling of the hydrogels, and the recovery of water must be understood. In this paper, we analyze these factors based on equations derived from the Donnan theory. In addition, hydrogels composed of N-isopropyl acrylamide and acrylic acid are synthesized, and their salt rejection efficiency in a model desalination experiment is studied. A comparison of the experimental and the theoretical results demonstrates that the charge density of the hydrogels at their equilibrium swelling and the degree of water recovery are two parameters controlling the salt rejection efficiency. These parameters are individually controlled by the content of the ionic groups and the degree of cross-linking of the gel polymer network. In addition, the prediction of the theory and the experimental results demonstrate that the salt rejection efficiency can be significantly improved if a second water recovery step is performed by a secondary increase in the temperature in the deswelling process.

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.

"Dumb" pH-Independent and Biocompatible Hydrogels Formed by Copolymers of Long-Chain Alkyl Glycidyl Ethers and Ethylene Oxide.

The formation and rheological properties of hydrogels based on amphiphilic ABA triblock polyether copolymers are described, relying solely on the hydrophobic interaction of long-chain alkyl glycidyl ether (AlkGE)- based A-blocks that are combined with a hydrophilic poly(ethylene glycol) (PEG) midblock. Via anionic ring-opening copolymerization (AROP), ethylene oxide (EO) and long-chain alkyl glycidyl ethers (AlkGEs) were copolymerized, using deprotonated poly(ethylene glycol) (PEG) macroinitiators (Mn of 10, 20 kg mol-1). The polymerization afforded amphiphilic ABA triblock copolymers with molar masses in the range of 21-32 kg mol-1 and dispersities (D) of D = 1.07-1.17. Kinetic studies revealed random copolymerization of EO and AlkGE, indicating random spacing of the hydrophobic AlkGE units by polar EO units. Following this approach, the hydrophobicity of the apolar blocks of amphiphilic ABA triblock polyethers can be tailored. Detailed rheological measurements confirmed the successful formation of hydrogels at different pH values as a consequence of nonpolar interactions and alkyl chain crystallization. Hydrogel formation was also observed at different ionic strengths (i.e., varied salt concentration), based on the hydrophobic aggregates. This behavior is in contrast to other often-used supramolecular cross-linking strategies, such as Coulomb interactions, complexation, or hydrogen bonding. Micro-differential scanning calorimetry (µ-DSC) measurements of the hydrogels revealed crystalline hydrophobic domains with melting temperatures in the physiological temperature range. In 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazoliumbromide (MTT) assays, diblock copolymers possessing structural analogy to the triblock copolymers were studied to assess the general cytotoxicity of amphiphilic polyethers bearing long alkyl chains at the polyether backbone, using splenic immune cells. At intermediate polymer concentrations, no cytotoxic effects were observed. This indicates that long-chain alkyl glycidyl ethers are promising for the introduction of highly hydrophobic as well as crystalline motifs at the polyether backbone in hydrogels for biomedical purposes.

Efficiency range of the Belousov-Zhabotinsky reaction to induce the self-organization of transient bonds in metallo-supramolecular polymeric systems.

The periodic change of the oxidation state of the metal catalyst in the oscillating Belousov-Zhabotinsky (BZ) reaction has been reported to establish a periodic organization of metallo-supramolecular bonds in polymeric systems, which results in autonomous viscosity oscillations. To appraise the possible extent of quantitative control on the viscosity oscillation features, we assess how the kinetics of the BZ reaction affects the periodic self-organization of the metal-ligand coordination, and vice versa. Our model system includes mono-, bis-, and tetra-functional polyethyleneglycol (PEG) precursors end grafted with terpyridine ligands that are complexed with ruthenium ions, which oscillate between Ru2+ and Ru3+ oxidation states in the BZ reaction medium. The control parameters are divided into microscopic factors, which are responsible for the local reaction rate, and mesoscopic factors, which are responsible for the spatial distribution of the concentration patterns. The reactant concentrations are found to nonlinearly control the amplitude and periods of reduction and oxidation phases, independent of the precursor functionalization degree. An increased medium viscosity, and therewith cease of mixing, accelerates the reaction rate by localization of the reaction phases, even though the diffusion of reaction intermediates causes a periodic chemical wave with distinct harmonics. Time-course viscosity measurements of the tetra-arm precursors in the BZ medium demonstrate an initial overshoot followed by minor oscillations around a plateau that is significantly lower than the viscosity of an equivalent fully associated network. Apparently, the slow association kinetics of Ru2+-bis(terpyridine) limits the frequency and the extent of self-organization, and this way, avoids full establishment of the expected viscosity oscillation.

Bone Scaffolds Based on Degradable Vaterite/PEG-Composite Microgels.

Vaterite, a metastable modification of calcium carbonate, embedded in a flexible microgel packaging with adjustable mechanical properties, functionality, and biocompatibility, provides a powerful scaffolding for bone tissue regeneration, as it is easily convertible to bone-like hydroxyapatite (HA). In this study, the synthesis and physical analysis of a packaging material to encapsulate vaterite particles and osteoblast cells into monodisperse, sub-millimeter-sized microgels, is described whereby a systematic approach is used to tailor the microgel properties. The size and shape of the microgels is controlled via droplet-based microfluidics. Key requirements for the polymer system, such as absence of cytotoxicity as well as biocompatibility and biodegradability, are accomplished with functionalized poly(ethylene glycol) (PEG), which reacts in a cytocompatible thiol-ene Michael addition. On a mesoscopic level, the microgel stiffness and gelation times are adjusted to obtain high cellular viabilities. The co-encapsulation of living cells provides i) an in vitro platform for the study of cellular metabolic processes which can be applied to bone formation and ii) an in vitro foundation for novel tissue-regenerative therapies. Finally, the degradability of the microgels at physiological conditions caused by hydrolysis-sensitive ester groups in the polymer network is examined.

Inverse Thermogelation of Aqueous Triblock Copolymer Solutions into Macroporous Shear-Thinning 3D Printable Inks.

Amphiphilic block copolymers that undergo (reversible) physical gelation in aqueous media are of great interest in different areas including drug delivery, tissue engineering, regenerative medicine, and biofabrication. We investigated a small library of ABA-type triblock copolymers comprising poly(2-methyl-2-oxazoline) as the hydrophilic shell A and different aromatic poly(2-oxazoline)s and poly(2-oxazine)s cores B in an aqueous solution at different concentrations and temperatures. Interestingly, aqueous solutions of poly(2-methyl-2-oxazoline)-block-poly(2-phenyl-2-oxazine)-block-poly(2-methyl-2-oxazoline) (PMeOx-b-PPheOzi-b-PMeOx) undergo inverse thermogelation below a critical temperature by forming a reversible nanoscale wormlike network. The viscoelastic properties of the resulting gel can be conveniently tailored by the concentration and the polymer composition. Storage moduli of up to 110 kPa could be obtained while the material retains shear-thinning and rapid self-healing properties. We demonstrate three-dimensional (3D) printing of excellently defined and shape-persistent 24-layered scaffolds at different aqueous concentrations to highlight its application potential, e.g., in the research area of biofabrication. A macroporous microstructure, which is stable throughout the printing process, could be confirmed via cryo-scanning electron microscopy (SEM) analysis. The absence of cytotoxicity even at very high concentrations opens a wide range of different applications for this first-in-class material in the field of biomaterials.

Thermodynamic control over energy dissipation modes in dual-network hydrogels based on metal-ligand coordination.

Modern polymeric hydrogels use reversible bonds to mimic biological functionalities. However, true biological materials benefit from several supramolecular elements and deliver multiple functions at the same time. To approach similar creation and control of multiple different functional elements in a synthetic soft material, we develop a model dual-network hydrogel in which multiple energy dissipating modes are formed by metal-ligand coordination and regulated by their association thermodynamics. This idea is realized by using linear and tetra-arm poly(ethylene glycol) (PEG) precursors with complementary reactive end groups. The former also carries terpyridine ligands on both ends, which form metallo-supramolecular bonds upon addition of metal ions. Multiple relaxation modes are provided by a combination of different metal ions. The timescale and amplitude of energy dissipating elements are characterized by oscillatory shear deformation. These studies suggest that the composition of metal ions controls the contribution of the corresponding relaxation modes in a linear fashion. A molecular-level confirmation is provided by following the UV-vis absorbance of the linear precursor in combination with mixtures of metal ions, accompanied by a theoretical study on the kinetics of the reversible association process. These results show that the linearity of the aforementioned dependence holds for such systems in which the utilized combination of metal ions and ligands exclusively form stable bis-complexes. By contrast, in many other cases, especially when the ions may compete to form mono-, bis-, or tris-complexes with the ligand, deviation from linearity is expected.