Activation Energies Control the Macroscopic Properties of Physically Cross-Linked Materials

Supramolecular Polymers for Drug Delivery

Supramolecular polymers are constructed through highly reversible and directionally specific non-covalent interactions between monomer units. This unique feature enables supramolecular polymers to undergo controlled structural reconfiguration and functional transformation in response to external stimuli, imparting them with high environmental responsiveness and self-healing properties. In particular, supramolecular polymers exhibit several specific advantages compared to conventional polymers, such as inherent degradability, the ease of preparation and the incorporation of functional units, and smart responsiveness to various biological stimuli. These characters make supramolecular polymers promising candidates for intelligent drug delivery systems in complex biological environments. In this review, we comprehensively summarize the latest developments and representative achievements of supramolecular polymers in drug delivery fields, focusing primarily on the design and synthesis, the properties and functionalities, and the practical applications of supramolecular polymers in small molecule drug delivery, gene therapy, and protein delivery. Finally, we highlight future research directions, focusing on multifunctionality, adaptability, and personalized therapy. We focus on recent studies that address key challenges in the field, providing rational polymer design, important properties, functionality, and understanding delivery strategies. These developments are expected to advance supramolecular polymers as new platforms of intelligent drug delivery systems, offering innovative solutions for the treatment of complex diseases.

Donor-Acceptor Viologens with Through-Space Conjugation for Enhanced Visible-Light-Driven Photocatalysis

A series of novel donor-acceptor (D-A) type viologens with through-space conjugation (TSC) are synthesized. The synergistic effect of D-A conjugation and TSC endow these compounds with a narrower energy gap, excellent redox properties, strong absorption in the visible range (up to 450 nm), and high efficiency of intramolecular charge transfer (ICT). These compounds also exhibit a long-lived charge separation state and stable radical formation. Considering their impressive photophysical and redox properties, TSC viologens with D-A conjugation are explored for visible-light-driven photocatalysis. As a catalyst in oxidative coupling reactions under visible light, compound 9 can achieve a yield of 94%, attributed to its strong ICT effect. Additionally, hybridized into graphitic carbon nitride (g-C₃N₄), compound 9 enhances hydrogen production performance under visible light, achieving an H2 generation rate of 4102 µmol h-1g-1. This study highlights the potential applications of TSC viologens in photocatalytic systems.

The role of macrocycles in supramolecular assembly with polymers

Recently, supramolecular self-assembly has attracted the attention of researchers worldwide because it enables the creation of nanostructures with unique properties without additional costs. Spontaneous organization of molecules allows the design and development of new nanostructures that can interact with drugs and living cells and generate a response. Therefore, supramolecular structures have enormous potential and can be in demand in various fields of healthcare and ecology. One of the widely used building blocks of such supramolecular assemblies is polymers. This review examines the joint aggregation behavior of various macrocycles (cyclodextrins, calixarenes, cucurbiturils, porphyrins, and pillararenes) with polymers, the functional properties of these supramolecular systems and their potential applications.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

High ion barrier hydrogel with excellent toughness achieved by directional structures

Owing to their nontoxicity, environmental friendliness, and high biocompatibility, physically cross-linked hydrogels have become popular research materials; however, their high water content and high free volume, along with the weak bonding interactions inherent to ordinary physically cross-linked hydrogels, limit their application in fields such as flexible devices, packaging materials, and substance transport regulation. Here, a structural barrier approach based on directional freezing-assisted salting out was proposed, and the directional structure significantly enhanced the barrier performance of the hydrogel. When the direction of substance diffusion was perpendicular to the pore channel structure of the directional freezing-PVA hydrogel (DFPVA), the Cl- transmission rate was 57.2% for the uniform freezing-PVA hydrogel (UFPVA). By adjusting the concentration of the salting-out solution and the salting-out time, the crystallinity and crystal domain size of the hydrogel could be further changed, optimizing and regulating the barrier performance of the hydrogel, with the best Cl- unit permeability being 36.02 mg mm per cm2 per day. Additionally, DFPVA had excellent mechanical properties (stress of 6.47 ± 1.04 MPa, strain of 625.85 ± 61.58%, toughness of 25.77 ± 3.72 MPa). Due to the barrier and mechanical properties of the direct structure, DFPVA is suitable as a drug carrier for slow drug release in vitro.

Harnessing Macromolecular Chemistry to Design Hydrogel Micro- and Macro-Environments

Cell encapsulation within three-dimensional hydrogels is a promising approach to mimic tissues. However, true biomimicry of the intricate microenvironment, biophysical and biochemical gradients, and the macroscale hierarchical spatial organizations of native tissues is an unmet challenge within tissue engineering. This review provides an overview of the macromolecular chemistries that have been applied toward the design of cell-friendly hydrogels, as well as their application toward controlling biophysical and biochemical bulk and gradient properties of the microenvironment. Furthermore, biofabrication technologies provide the opportunity to simultaneously replicate macroscale features of native tissues. Biofabrication strategies are reviewed in detail with a particular focus on the compatibility of these strategies with the current macromolecular toolkit described for hydrogel design and the challenges associated with their clinical translation. This review identifies that the convergence of the ever-expanding macromolecular toolkit and technological advancements within the field of biofabrication, along with an improved biological understanding, represents a promising strategy toward the successful tissue regeneration.

Ultrastable, Superrobust, and Recyclable Supramolecular Polymer Networks

Supramolecular polymer networks (SPNs), crosslinked by noncovalent bonds, have emerged as reorganizable and recyclable polymeric materials with unique functionality. However, poor stability is an imperative challenge faced by SPNs, because SPNs are susceptible to heat, water, and/or solvents due to the dynamic and reversible nature of noncovalent bonds. Herein, the design of a noncovalent cooperative network (NCoN) to simultaneously stabilize and reinforce SPNs is reported, resulting in an ultrastable, superrobust, and recyclable SPN. The NCoN is constructed by multiplying the H-bonding sites and tuning the conformation/geometry of the H-bonding segment to optimize the multivalence cooperativity of H-bonds. The rationally designed H-bonding segment with high conformational compliance favors the formation of tightly packed H-bond arrays comprising higher-density and stronger H-bonds. Consequently, the H-bonded crosslinks in the NCoN display a covalent crosslinking effect but retain on-demand dynamics and reversibility. The resultant ultrastable SPN not only displays remarkable resistance to heat up to 120 °C, water soaking, and a broad spectrum of solvents, but also possesses a superhigh true stress at break (1.1 GPa) and an ultrahigh toughness (406 MJ m-3 ). Despite the covalent-network-like stability, the SPN is recyclable through activating its reversibility in a high-polarity solvent heated to a threshold temperature.

Rational Design of DNA Hydrogels Based on Molecular Dynamics of Polymers

In recent years, DNA has emerged as a fascinating building material to engineer hydrogel due to its excellent programmability, which has gained considerable attention in biomedical applications. Understanding the structure-property relationship and underlying molecular determinants of DNA hydrogel is essential to precisely tailor its macroscopic properties at molecular level. In this review, the rational design principles of DNA molecular networks based on molecular dynamics of polymers on the temporal scale, which can be engineered via the backbone rigidity and crosslinking kinetics, are highlighted. By elucidating the underlying molecular mechanisms and theories, it is aimed to provide a comprehensive overview of how the tunable DNA backbone rigidity and the crosslinking kinetics lead to desirable macroscopic properties of DNA hydrogels, including mechanical properties, diffusive permeability, swelling behaviors, and dynamic features. Furthermore, it is also discussed how the tunable macroscopic properties make DNA hydrogels promising candidates for biomedical applications, such as cell culture, tissue engineering, bio-sensing, and drug delivery.

Tailoring Dynamic Hydrogels by Controlling Associative Exchange Rates

Dithioalkylidenes are a newly-developed class of conjugate acceptors that undergo thiol exchange via an associative mechanism, enabling decoupling of key material properties for sustainability, biomedical, and sensing applications. Here, we show that the exchange rate is highly sensitive to the structure of the acceptor and tunable over four orders of magnitude in aqueous environments. Cyclic acceptors exchange rapidly, from 0.95 to 15.6 M-1s-1, while acyclic acceptors exchange between 3.77x10-3 and 2.17x10-2 M-1s-1. Computational, spectroscopic, and structural data suggest that cyclic acceptors are more reactive than their acyclic counterparts because of resonance stabilization of the tetrahedral exchange intermediate. We parametrize molecular reactivity with respect to computed descriptors of the electrophilic site and leverage this insight to design a compound with intermediate characteristics. Lastly, we incorporate this dynamic bond into hydrogels and demonstrate that the characteristic stress relaxation time (τ) is directly proportional to molecular kex.

Reinforcing supramolecular hyaluronan hydrogels via kinetically interlocking multiple-units strategy

Supramolecular hydrogels exhibit promising potential in biological and clinical fields due to their special dynamic properties. However, most existing supramolecular hydrogels suffer from poor mechanical strength, which severely limits their applications. Here in this study, the Kinetically Interlocking Multiple-Units (KIMU) strategy was applied to the hyaluronan networks by introducing different supramolecular interaction motifs in an organized and alternative manner. Our strategy successfully elevated the energy barrier of crosslinker dissociation to 103.0 kJ mol^-1 and increased the storage modulus of hydrogels by 78 % with the intrinsic dynamic properties preserved. It can be expected that this method would bring a convenient and effective route to fabricate novel supramolecular materials with excellent mechanical properties.

Molecular Tuning of a Benzene-1,3,5-Tricarboxamide Supramolecular Fibrous Hydrogel Enables Control over Viscoelasticity and Creates Tunable ECM-Mimetic Hydrogels and Bioinks

Traditional synthetic covalent hydrogels lack the native tissue dynamics and hierarchical fibrous structure found in the extracellular matrix (ECM). These dynamics and fibrous nanostructures are imperative in obtaining the correct cell/material interactions. Consequently, the challenge to engineer functional dynamics in a fibrous hydrogel and recapitulate native ECM properties remains a bottle-neck to biomimetic hydrogel environments. Here, the molecular tuning of a supramolecular benzene-1,3,5-tricarboxamide (BTA) hydrogelator via simple modulation of hydrophobic substituents is reported. This tuning results in fibrous hydrogels with accessible viscoelasticity over 5 orders of magnitude, while maintaining a constant equilibrium storage modulus. BTA hydrogelators are created with systematic variations in the number of hydrophobic carbon atoms, and this is observed to control the viscoelasticity and stress-relaxation timescales in a logarithmic fashion. Some of these BTA hydrogels are shear-thinning, self-healing, extrudable, and injectable, and can be 3D printed into multiple layers. These hydrogels show high cell viability for chondrocytes and human mesenchymal stem cells, establishing their use in tissue engineering applications. This simple molecular tuning by changing hydrophobicity (with just a few carbon atoms) provides precise control over the viscoelasticity and 3D printability in fibrillar hydrogels and can be ported onto other 1D self-assembling structures. The molecular control and design of hydrogel network dynamics can push the field of supramolecular chemistry toward the design of new ECM-mimicking hydrogelators for numerous cell-culture and tissue-engineering applications and give access toward highly biomimetic bioinks for bioprinting.

Structure-Reactivity-Property Relationships in Covalent Adaptable Networks

Polymer networks built out of dynamic covalent bonds offer the potential to translate the control and tunability of chemical reactions to macroscopic physical properties. Under conditions at which these reactions occur, the topology of covalent adaptable networks (CANs) can rearrange, meaning that they can flow, self-heal, be remolded, and respond to stimuli. Materials with these properties are necessary to fields ranging from sustainability to tissue engineering; thus the conditions and time scale of network rearrangement must be compatible with the intended use. The mechanical properties of CANs are based on the thermodynamics and kinetics of their constituent bonds. Therefore, strategies are needed that connect the molecular and macroscopic worlds. In this Perspective, we analyze structure-reactivity-property relationships for several classes of CANs, illustrating both general design principles and the predictive potential of linear free energy relationships (LFERs) applied to CANs. We discuss opportunities in the field to develop quantitative structure-reactivity-property relationships and open challenges.

Anion Binding by Fluorescent Ureido-Hexahomotrioxacalix[3]arene Receptors: An NMR, Absorption and Emission Spectroscopic Study

Fluorescent receptors (4a-4c) based on (thio)ureido-functionalized hexahomotrioxacalix[3]arenes were synthesised and obtained in the partial cone conformation in solution. Naphthyl or pyrenyl fluorogenic units were introduced at the lower rim of the calixarene skeleton via a butyl spacer. The binding of biologically and environmentally relevant anions was studied with NMR, UV-vis absorption, and fluorescence titrations. Fluorescence of the pyrenyl receptor 4c displays both monomer and excimer fluorescence. The thermodynamics of complexation was determined in acetonitrile and was entropy-driven. Computational studies were also performed to bring further insight into the binding process. The data showed that association constants increase with the anion basicity, and AcO-, BzO- and F- were the best bound anions for all receptors. Pyrenylurea 4c is a slightly better receptor than naphthylurea 4a, and both are more efficient than naphthyl thiourea 4b. In addition, ureas 4a and 4c were also tested as ditopic receptors in the recognition of alkylammonium salts.

Star-Polymer-DNA Gels Showing Highly Predictable and Tunable Mechanical Responses

Dynamically crosslinked gels are appealing materials for applications that require time-dependent mechanical responses. DNA duplexes are ideal crosslinkers for building such gels because of their excellent sequence addressability and flexible tunability in bond energy. However, the mechanical responses of most DNA gels are complicated and unpredictable. Here, a DNA gel with a highly homogeneous gel network and well predictable mechanical behaviors is demonstrated by using a pair of star-polymer-DNA precursors with presimulated DNA sequences showing the two-state transition. The melting curve analysis of the DNA gels reveals the good correspondence between the thermodynamic potentials of the DNA crosslinkers and the presimulated values by DNA calculators. Stress-relaxation tests and dissociation kinetics measurements show that the macroscopic relaxation time of the DNA gels is approximately equal to the lifetime of the DNA crosslinkers over 4 orders of magnitude from 0.1-2000 s. Furthermore, a series of durability tests find the DNA gels are hysteresis-less and self-healable after the applications of repeated temperature and mechanical stimuli. These results demonstrate the great potential of star-polymer-DNA precursors for building gels with predictable and tunable viscoelastic properties, suitable for applications such as stress-response extracellular matrices, injectable solids, and soft robotics.

Internal and External Catalysis in Boronic Ester Networks

In dynamic materials, the reversible condensation between boronic acids and diols provides adaptability, self-healing ability, and responsiveness to small molecules and pH. The thermodynamics and kinetics of bond exchange determine the mechanical properties of dynamic polymer networks. Here, we investigate the effects of diol structure and salt additives on the rate of boronic acid-diol bond exchange, binding affinity, and the mechanical properties of the corresponding polymer networks. We find that proximal amides used to conjugate diols to polymers and buffering anions induce significant rate acceleration, consistent with an internal and external catalysis, respectively. This rate acceleration is reflected in the stress relaxation of the gels. These findings contribute to the fundamental understanding of the boronic ester dynamic bond and offer molecular strategies to tune the macromolecular properties of dynamic materials.

ortho-Terphenylene Viologens with Through-Space Conjugation for Enhanced Photocatalytic Oxidative Coupling and Hydrogen Evolution

A series of novel ortho-terphenylene viologen derivatives (o-TPV²⁺) with through-space conjugation (TSC) via the combination of ortho-terphenylene skeletons with viologen structure is reported. Their optoelectronic properties can be adjusted by N-arylation or N-alkylation reactions. Compared with other viologen derivatives, o-TPV²⁺ not only exhibits strong photoluminescence but also retards the charge recombination process and stabilizes the diradical state without forming a quinoid structure due to the special TSC effect. Based on their special redox characteristics, o-TPV²⁺ was applied to the photocatalytic oxidative coupling of benzylamine with 96% yield. In addition, pTA-o-TPV²⁺ (tethered with p-toluic acid)-modified g-C₃N₄ was used for visible-light-driven hydrogen production for the first time, exceeding 15 times the rate over unmodified g-C₃N₄.

Peptide Amphiphile Hydrogels Based on Homoternary Cucurbit[8]uril Host-Guest Complexes

Supramolecular hydrogels based on peptide amphiphiles (PAs) are promising materials for tissue engineering and model extracellular matrixes for biological studies. While PA hydrogels are conventionally formed via electrostatic screening, new hydrogelation mechanisms might help to improve the design and functionality of these materials. Here, we present a host-guest-mediated PA hydrogelation method that relies on the formation of a host-guest homoternary complex with cucurbit[8]uril (CB[8]) and aromatic amino-acid-bearing PA nanofibers. As a result of the host-guest cross-linking between PA nanofibers, hierarchical morphologies and increased stiffness were found when host-guest-mediated PA hydrogels were compared to their ion-based equivalents. Additionally, both families of hydrogels exhibited similar biocompatibilities. These results demonstrate that CB[8]-mediated hydrogelation can be used as an alternative cross-linking method to upgrade the design of PA materials and extend their biomedical applications.

Predictably Engineering the Viscoelastic Behavior of Dynamic Hydrogels via Correlation with Molecular Parameters

Rational design of dynamic hydrogels with desirable viscoelastic behaviors relies on an in-depth understanding of the principles correlating molecular parameters and macroscopic properties. To quantitatively elucidate such principles, a series of dynamic covalent hydrogels crosslinked via hydrazone bonds is designed. The exchange rate of the hydrazone bond is tuned by varying the concentration of an organic catalyst, while maintaining the crosslinking density unchanged. This strategy of independently tuning exchange dynamics of crosslinks and crosslinking density allows unambiguous analysis of the viscoelastic response of the dynamic hydrogels as a function of their network parameters. It is found that the terminal relaxation time of the dynamic hydrogels is primarily determined by two factors: the exchange rate of crosslinks and the number of effective crosslinks per polymer chain, and is independent of the network architecture. Furthermore, a universal correlation is identified between the terminal relaxation time determined from stress relaxation and the exchange rate determined via reaction kinetics, which can be generalized to any viscoelastic hydrogel network, in principle. This quantitative correlation facilitates the development of dynamic hydrogels with a variable desired viscoelastic response based on molecular design.

Supramolecular Host-Guest Hydrogels for Corneal Regeneration

Over 6.2 million people worldwide suffer from moderate to severe vision loss due to corneal disease. While transplantation with allogenic donor tissue is sight-restoring for many patients with corneal blindness, this treatment modality is limited by long waiting lists and high rejection rates, particularly in patients with severe tissue damage and ocular surface pathologies. Hydrogel biomaterials represent a promising alternative to donor tissue for scalable, nonimmunogenic corneal reconstruction. However, implanted hydrogel materials require invasive surgeries and do not precisely conform to tissue defects, increasing the risk of patient discomfort, infection, and visual distortions. Moreover, most hydrogel crosslinking chemistries for the in situ formation of hydrogels exhibit off-target effects such as cross-reactivity with biological structures and/or result in extractable solutes that can have an impact on wound-healing and inflammation. To address the need for cytocompatible, minimally invasive, injectable tissue substitutes, host-guest interactions have emerged as an important crosslinking strategy. This review provides an overview of host-guest hydrogels as injectable therapeutics and highlights the potential application of host-guest interactions in the design of corneal stromal tissue substitutes.

Control Viscoelasticity of Polymer Networks with Crosslinks of Superposed Fast and Slow Dynamics

Depending on the dynamics of the crosslinks, polymer networks can have distinct bulk mechanical behaviors, from viscous liquids to tough solids. Here, by means of designing a crosslink with variable molecular dynamics, we show the control of viscoelasticity of polymer networks in a broad range quantitatively. The hexanoate-isoquinoline@cucurbit[7]uril (HIQ@CB[7]) crosslink exhibits in a combination of protonated and deprotonated states of similar association affinity but distinct molecular dynamics. The molecular property of this crosslink is contributed by linear combination of the parameters at the two states, which is precisely tuned by pH. Using this crosslink, we achieve the quantitative control of viscoelasticity of quasi-ideal networks in 5 orders of magnitude, and we show the reversible control of mechanical response, such as stiffness, strength and extensibility, of tough random polymer networks. This strategy offers a way to tailor the mechanical properties of polymer networks at the molecular level and paves the way for engineering "smart" responsive materials.

Engineering the Dynamics of Cell Adhesion Cues in Supramolecular Hydrogels for Facile Control over Cell Encapsulation and Behavior

The extracellular matrix (ECM) forms through hierarchical assembly of small and larger polymeric molecules into a transient, hydrogel-like fibrous network that provides mechanical support and biochemical cues to cells. Synthetic, fibrous supramolecular networks formed via non-covalent assembly of various molecules are therefore potential candidates as synthetic mimics of the natural ECM, provided that functionalization with biochemical cues is effective. Here, combinations of slow and fast exchanging molecules that self-assemble into supramolecular fibers are employed to form transient hydrogel networks with tunable dynamic behavior. Obtained results prove that modulating the ratio between these molecules dictates the extent of dynamic behavior of the hydrogels at both the molecular and the network level, which is proposed to enable effective incorporation of cell-adhesive functionalities in these materials. Excitingly, the dynamic nature of the supramolecular components in this system can be conveniently employed to formulate multicomponent supramolecular hydrogels for easy culturing and encapsulation of single cells, spheroids, and organoids. Importantly, these findings highlight the significance of molecular design and exchange dynamics for the application of supramolecular hydrogels as synthetic ECM mimics.

Chain-End Effects on Supramolecular Poly(ethylene glycol) Polymers

In this work we present a fundamental analysis based on small-angle scattering, linear rheology and differential scanning calorimetry (DSC) experiments of the role of different hydrogen bonding (H-bonding) types on the structure and dynamics of chain-end modified poly(ethylene glycol) (PEG) in bulk. As such bifunctional PEG with a molar mass below the entanglement mass Me is symmetrically end-functionalized with three different hydrogen bonding (H-bonding) groups: thymine-1-acetic acid (thy), diamino-triazine (dat) and 2-ureido-4[1H]-pyrimidinone (upy). A linear block copolymer structure and a Newtonian-like dynamics is observed for PEG-thy/dat while results for PEG-upy structure and dynamics reveal a sphere and a network-like behavior, respectively. These observations are concomitant with an increase of the Flory-Huggins interaction parameter from PEG-thy/dat to PEG-upy that is used to quantify the difference between the H-bonding types. The upy association into spherical clusters is established by the Percus-Yevick approximation that models the inter-particle structure factor for PEG-upy. Moreover, the viscosity study reveals for PEG-upy a shear thickening behavior interpreted in terms of the free path model and related to the time for PEG-upy to dissociate from the upy clusters, seen as virtual crosslinks of the formed network. Moreover, a second relaxation time of different nature is also obtained from the complex shear modulus measurements of PEG-upy by the inverse of the angular frequency where G' and G'' crosses from the network-like to glass-like transition relaxation time, which is related to the segmental friction of PEG-upy polymeric network strands. In fact, not only do PEG-thy/dat and PEG-upy have different viscoelastic properties, but the relaxation times found for PEG-upy are much slower than the ones for PEG-thy/dat. However, the activation energy related to the association dynamics is very similar for both PEG-thy/dat and PEG-upy. Concerning the segmental dynamics, the glass transition temperature obtained from both rheological and calorimetric analysis is similar and increases for PEG-upy while for PEG-thy/dat is almost independent of association behavior. Our results show how supramolecular PEG properties vary by modifying the H-bonding association type and changing the molecular Flory-Huggins interaction parameter, which can be further explored for possible applications.

Designing Hydrogels for 3D Cell Culture Using Dynamic Covalent Crosslinking

Designing simple biomaterials to replicate the biochemical and mechanical properties of tissues is an ongoing challenge in tissue engineering. For several decades, new biomaterials have been engineered using cytocompatible chemical reactions and spontaneous ligations via click chemistries to generate scaffolds and water swollen polymer networks, known as hydrogels, with tunable properties. However, most of these materials are static in nature, providing only macroscopic tunability of the scaffold mechanics, and do not reflect the dynamic environment of natural extracellular microenvironment. For more complex applications such as organoids or co-culture systems, there remain opportunities to investigate cells that locally remodel and change the physicochemical properties within the matrices. In this review, advanced biomaterials where dynamic covalent chemistry is used to produce stable 3D cell culture models and high-resolution constructs for both in vitro and in vivo applications, are discussed. The implications of dynamic covalent chemistry on viscoelastic properties of in vitro models are summarized, case studies in 3D cell culture are critically analyzed, and opportunities to further improve the performance of biomaterials for 3D tissue engineering are discussed.

Physical networks from entropy-driven non-covalent interactions

Physical networks typically employ enthalpy-dominated crosslinking interactions that become more dynamic at elevated temperatures, leading to network softening. Moreover, standard mathematical frameworks such as time-temperature superposition assume network softening and faster dynamics at elevated temperatures. Yet, deriving a mathematical framework connecting the crosslinking thermodynamics to the temperature-dependent viscoelasticity of physical networks suggests the possibility for entropy-driven crosslinking interactions to provide alternative temperature dependencies. This framework illustrates that temperature negligibly affects crosslink density in reported systems, but drastically influences crosslink dynamics. While the dissociation rate of enthalpy-driven crosslinks is accelerated at elevated temperatures, the dissociation rate of entropy-driven crosslinks is negligibly affected or even slowed under these conditions. Here we report an entropy-driven physical network based on polymer-nanoparticle interactions that exhibits mechanical properties that are invariant with temperature. These studies provide a foundation for designing and characterizing entropy-driven physical crosslinking motifs and demonstrate how these physical networks access thermal properties that are not observed in current physical networks.

Reversible covalent locking of a supramolecular hydrogel via UV-controlled anthracene dimerization

The formation of supramolecular hydrogels is demonstrated based on ternary complexes between anthracene side-chain functionalized polymers and macrocyclic hosts. Photo-induced reversible dimerization enables switching between supramolecular and covalent hydrogels.

Role of Functionalized Pillararene Architectures in Supramolecular Catalysis

The many useful features possessed by pillararenes (PAs; e.g. rigid, capacious, and hydrophobic cavities, as well as exposed functional groups) have led to a tremendous increase in their popularity since their first discovery in 2008. In this Minireview, we emphasize the use of functionalized PAs and their assembled supramolecular materials in the field of catalysis. We aim to provide a fundamental understanding and mechanism of the role PAs play in catalytic process. The topics are subdivided into catalysis promoted by the PA rim/cavity, PA-based nanomaterials, and PA-based polymeric materials. To the best of our knowledge, this is the first overview on PA-based catalysis. This Minireview not only summarizes the fabrications and applications of PAs in catalysis but also anticipates future research efforts in applying supramolecular hosts in catalysis.

Supramolecular Hydrogels via Light-Responsive Homoternary Cross-Links

Host-guest physical cross-linking has been used to prepare supramolecular hydrogels for various biomedical applications. More recent efforts to endow these materials with stimuli-responsivity offers an opportunity to precisely tune their function for a target use. In the context of light-responsive materials, azobenzenes are one prevailing motif. Here, an asymmetric azobenzene was explored for its ability to form homoternary complexes with the cucurbit[8]uril macrocycle, exhibiting an affinity (K_eq) of 6.21 × 10¹⁰ M^-2 for sequential binding, though having negative cooperativity. Copolymers were first prepared from different and tunable ratios of NIPAM and DMAEA, and DMAEA groups were then postsynthetically modified with this asymmetric azobenzene. Upon macrocycle addition, these polymers formed supramolecular hydrogels; relaxation dynamics increased with temperature due to temperature-dependent affinity reduction for the ternary complex. Application of UV light disrupted the supramolecular motif through azobenzene photoisomerization, prompting a gel-to-sol transition in the hydrogel. Excitingly, within several minutes at room temperature, thermal relaxation of azobenzene to its trans state afforded rapid hydrogel recovery. By revealing this supramolecular motif and employing facile means for its attachment onto pre-synthesized polymers, the approach described here may further enable stimuli-directed control of supramolecular hydrogels for a number of applications.

Influence of Network Topology on the Viscoelastic Properties of Dynamically Crosslinked Hydrogels

Biological materials combine stress relaxation and self-healing with non-linear stress-strain responses. These characteristic features are a direct result of hierarchical self-assembly, which often results in fiber-like architectures. Even though structural knowledge is rapidly increasing, it has remained a challenge to establish relationships between microscopic and macroscopic structure and function. Here, we focus on understanding how network topology determines the viscoelastic properties, i.e., stress relaxation, of biomimetic hydrogels. We have dynamically crosslinked two different synthetic polymers with one and the same crosslink. The first polymer, a polyisocyanopeptide (PIC), self-assembles into semi-flexible, fiber-like bundles, and thus displays stress-stiffening, similar to many biopolymer networks. The second polymer, 4-arm poly(ethylene glycol) (starPEG), serves as a reference network with well-characterized structural and viscoelastic properties. Using one and the same coiled coil crosslink allows us to decouple the effects of crosslink kinetics and network topology on the stress relaxation behavior of the resulting hydrogel networks. We show that the fiber-containing PIC network displays a relaxation time approximately two orders of magnitude slower than the starPEG network. This reveals that crosslink kinetics is not the only determinant for stress relaxation. Instead, we propose that the different network topologies determine the ability of elastically active network chains to relax stress. In the starPEG network, each elastically active chain contains exactly one crosslink. In the absence of entanglements, crosslink dissociation thus relaxes the entire chain. In contrast, each polymer is crosslinked to the fiber bundle in multiple positions in the PIC hydrogel. The dissociation of a single crosslink is thus not sufficient for chain relaxation. This suggests that tuning the number of crosslinks per elastically active chain in combination with crosslink kinetics is a powerful design principle for tuning stress relaxation in polymeric materials. The presence of a higher number of crosslinks per elastically active chain thus yields materials with a slow macroscopic relaxation time but fast dynamics at the microscopic level. Using this principle for the design of synthetic cell culture matrices will yield materials with excellent long-term stability combined with the ability to locally reorganize, thus facilitating cell motility, spreading, and growth.

Injectable Cucurbit[8]uril-Based Supramolecular Gelatin Hydrogels for Cell Encapsulation

Recent efforts to develop hydrogel biomaterials have focused on better recapitulating the dynamic properties of the native extracellular matrix. In hydrogel biomaterials, binding thermodynamics and cross-link kinetics directly affect numerous bulk dynamic properties such as strength, stress relaxation, and material clearance. However, despite the broad range of bulk dynamic properties observed in biological tissues, present strategies to incorporate dynamic linkages in cell-encapsulating hydrogels rely on a relatively small number of dynamic covalent chemical reactions and host-guest interactions. To expand this toolkit, we report the preparation of supramolecular gelatin hydrogels with cucurbit[8]uril (CB[8])-based cross-links that form on demand via thiol-ene reactions between preassembled CB[8]·FGGC peptide ternary complexes and grafted norbornenes. Human fibroblast cells encapsulated within these optically transparent, shear thinning, injectable hydrogels remained highly viable and exhibited a well-spread morphology in culture. These CB[8]-based gelatin hydrogels are anticipated to be useful in applications ranging from bioprinting to cell and drug delivery.

Molecular engineering of metal coordination interactions for strong, tough, and fast-recovery hydrogels

Many load-bearing tissues, such as muscles and cartilages, show high elasticity, toughness, and fast recovery. However, combining these mechanical properties in the same synthetic biomaterials is fundamentally challenging. Here, we show that strong, tough, and fast-recovery hydrogels can be engineered using cross-linkers involving cooperative dynamic interactions. We designed a histidine-rich decapeptide containing two tandem zinc binding motifs. Because of allosteric structural change-induced cooperative binding, this decapeptide had a higher thermodynamic stability, stronger binding strength, and faster binding rate than single binding motifs or isolated ligands. The engineered hybrid network hydrogels containing the peptide-zinc complex exhibit a break stress of ~3.0 MPa, toughness of ~4.0 MJ m^-3, and fast recovery in seconds. We expect that they can function effectively as scaffolds for load-bearing tissue engineering and as building blocks for soft robotics. Our results provide a general route to tune the mechanical and dynamic properties of hydrogels at the molecular level.

Cross-linker control of vitrimer flow

The rate of stress relaxation in a vitrimer can be modulated by changing solely the structure of the cross-linker electrophile.

A Highly Stretchable, Tough, Fast Self-Healing Hydrogel Based on Peptide⁻Metal Ion Coordination

Metal coordination bonds are widely used as the dynamic cross-linkers to construct self-healing hydrogels. However, it remains challenging to independently improve the toughness of metal coordinated hydrogels without affecting the stretchability and self-healing properties, as all these features are directly correlated with the dynamic properties of the same metal coordination bonds. In this work, using histidine-Zn2+ binding as an example, we show that the coordination number (the number of binding sites in each cross-linking ligand) is an important parameter for the mechanical strength of the hydrogels. By increasing the coordination number of the binding site, the mechanical strength of the hydrogels can be greatly improved without sacrificing the stretchability and self-healing properties. By adjusting the peptide and Zn2+ concentrations, the hydrogels can achieve a set of demanding mechanical features, including the Young's modulus of 7-123 kPa, fracture strain of 434-781%, toughness of 630-1350 kJ m-3, and self-healing time of ~1 h. We anticipate the engineered hydrogels can find broad applications in a variety of biomedical fields. Moreover, the concept of improving the mechanical strength of metal coordinated hydrogels by tuning the coordination number may inspire the design of other dynamically cross-linked hydrogels with further improved mechanical performance.

Stress Dissipation in Cucurbit[8]uril Ternary Complex Small Molecule Adhesives

The ability to control supramolecular and macroscopic self-assembly and disassembly holds great potential for responsive, reversible adhesives that can efficiently broker stresses accumulated between two surfaces. Here, cucurbit[8]uril is used to directly adhere two functionalized mica substrates creating surface-surface interactions that are held together through photoreversible CB[8] heteroternary complexes. Comparison of single-molecule, bulk, and macroscopic adhesion behavior give insight into cooperativity and stress dissipation in dynamic adhesive systems.

Cucurbit[8]uril-Based Polymers and Polymer Materials

Cucurbit[8]uril (CB[8]) is unique and notable in the cucurbit[n]uril family, since it has a relatively large cavity and thus is able to simultaneously accommodate two guest molecules. Typically, an electron-deficient first guest and an electron-rich second guest can be bound by CB[8] to form a stable 1:1:1 heteroternary supramolecular complex. Additionally, two homo guests can also be strongly dimerized inside the cavity of CB[8] to form a 2:1 homoternary supramolecular complex. During the past decade, by combining polymer science and CB[8] host-guest chemistry, a variety of systems have been established to construct supramolecular polymers with polymer chains typically at the nanoscale/sub-microscale, and CB[8]-based micro/nanostructured polymer materials in the form of polymer networks and hydrogels, microcapsules, micelles, vesicles, and colloidal particles, normally in solution and occasionally on surfaces. This Review summarizes the noncovalent interactions and strategies used for the preparation of CB[8]-based polymers and polymer materials with a focus on the representative and latest developments, followed by a brief discussion of their characterization, properties, and applications.

Preparation of cross-linked supramolecular polymers based on benzo-21-crown-7/secondary ammonium salt host-guest interactions

We found that TC7 not only self-assembles into one-dimensional supramolecular aggregates in chloroform, but also forms cross-linked supramolecular polymers via host-guest complexation between benzo-21-crown-7 and secondary ammonium salts. Compared with one-dimensional linear supramolecular polymers, soft and long viscous fibers were pulled out from a concentrated solution of cross-linked supramolecular polymers as a result of higher viscosity and lower diffusion coefficients.

Dynamic Hydrogels from Host-Guest Supramolecular Interactions

Hydrogel biomaterials are pervasive in biomedical use. Applications of these soft materials range from contact lenses to drug depots to scaffolds for transplanted cells. A subset of hydrogels is prepared from physical cross-linking mediated by host-guest interactions. Host macrocycles, the most recognizable supramolecular motif, facilitate complex formation with an array of guests by inclusion in their portal. Commonly, an appended macrocycle forms a complex with appended guests on another polymer chain. The formation of poly(pseudo)rotaxanes is also demonstrated, wherein macrocycles are threaded by a polymer chain to give rise to physical cross-linking by secondary non-covalent interactions or polymer jamming. Host-guest supramolecular hydrogels lend themselves to a variety of applications resulting from their dynamic properties that arise from non-covalent supramolecular interactions, as well as engineered responsiveness to external stimuli. These are thus an exciting new class of materials.

Supramolecular polymeric biomaterials

Polymeric chains crosslinked through supramolecular interactions-directional and reversible non-covalent interactions-compose an emerging class of modular and tunable biomaterials. The choice of chemical moiety utilized in the crosslink affords different thermodynamic and kinetic parameters of association, which in turn illustrate the connectivity and dynamics of the system. These parameters, coupled with the choice of polymeric architecture, can then be engineered to control environmental responsiveness, viscoelasticity, and cargo diffusion profiles, yielding advanced biomaterials which demonstrate rapid shear-thinning, self-healing, and extended release. In this review we examine the relationship between supramolecular crosslink chemistry and biomedically relevant macroscopic properties. We then describe how these properties are currently leveraged in the development of materials for drug delivery, immunology, regenerative medicine, and 3D-bioprinting (253 references).

Reversibly tuning hydrogel stiffness through photocontrolled dynamic covalent crosslinks

Controlling the physical properties of soft materials with external stimuli enables researchers to mimic and study dynamic systems. Of particular interest are hydrogels, polymer networks swollen by water with broad applicability to biomedicine. To control hydrogel mechanics with light, researchers have relied on a limited number of photochemical reactions. Here we introduce an approach to reversibly tune hydrogel mechanics with light by manipulating the stability of dynamic covalent crosslinks at the molecular level. The equilibrium between a boronic acid and diol to form a boronic ester can be altered by the configuration of an adjacent azobenzene photoswitch. By irradiating branched polymers bearing azobenzene-boronic acid and diol end groups with two different wavelengths of light, we can stiffen or soften the resulting hydrogel. Alternating irradiation induces reversible mechanical changes. Rheological characterization reveals that the hydrogels are viscoelastic, exhibiting stress relaxation on the order of seconds, and the stiffness is tuned independently of the crossover frequency. We have also demonstrated that this approach can be extended to use visible light for both softening and stiffening. These photocontrolled dynamic covalent crosslinks provide a versatile platform for tunable dynamic materials.

Injectable network biomaterials via molecular or colloidal self-assembly

Self-assembly is a powerful tool to create functional materials. A specific application for which self-assembled materials are ideally suited is in creating injectable biomaterials. Contrasting with traditional biomaterials that are implanted through surgical means, injecting biomaterials through the skin offers numerous advantages, expanding the scope and impact for biomaterials in medicine. In particular, self-assembled biomaterials prepared from molecular or colloidal interactions have been frequently explored. The strategies to create these materials are varied, taking advantage of engineered oligopeptides, proteins, and nanoparticles as well as affinity-mediated crosslinking of synthetic precursors. Self-assembled materials typically facilitate injectability through two different mechanisms: i) in situ self-assembly, whereby materials would be administered in a monomeric or oligomeric form and self-assemble in response to some physiologic stimulus, or ii) self-assembled materials that, by virtue of their dynamic, non-covalent interactions, shear-thin to facilitate flow within a syringe and subsequently self-heal into its reassembled material form at the injection site. Indeed, many classes of materials are capable of being injected using a combination of these two mechanisms. Particular utility has been noted for self-assembled biomaterials in the context of tissue engineering, regenerative medicine, drug delivery, and immunoengineering. Given the controlled and multifunctional nature of many self-assembled materials demonstrated to date, we project a future where injectable self-assembled biomaterials afford improved practice in advancing healthcare.