Using Visible Light to Tune Boronic Acid–Ester Equilibria

Precise assembly/disassembly of homo-type and hetero-type macrocycles with photoresponsive and non-photoresponsive dynamic covalent bonds

Dynamic covalent macrocycles offer the advantage of tunable ring-opening/ring-closure and structural transformation, but their control with precision remains a daunting task due to the labile nature of reversible bonds. Herein we demonstrate the precise formation/scission of covalent macrocycles with varied sizes by contrasting the reactivity, stability, and degradability of light-active and light-inactive dynamic covalent bonds. The incorporation of photoswitchable and non-photoresponsive aldehyde sites into one single dialdehyde component afforded the creation of [1 + 1] type macrocycles with primary diamines of suitable lengths. The manipulation of light and acid/base stimuli allowed on-demand breaking/remaking of macrocycles, achieving the interconversion between macrocyclic and linear skeletons. Moreover, a combination of the dialdehyde, primary diamines, and secondary diamines enabled the construction of hetero-type [2 + 1 + 1'] macrocycles via enhanced discrimination and hierarchical assembly. Light-induced kinetic locking/unlocking of dynamic bonds further afforded macrocycle-to-macrocycle conversion when needed. Through leveraging controllable covalent connection/disconnection, switchable formation/disintegration of mechanically interlocked catenanes was further accomplished. The results described showcase the potential of photoinduced dynamic covalent chemistry for preparing complex architectures and should set the stage for molecular recognition, dynamic assemblies, synthetic motors, and responsive materials.

Photoswitchable Topological Regulation of Covalent Macrocycles, Molecular Recognition, and Interlocked Structures

Macrocycles represent one important class of functional molecules, and dynamic macrocycles with the potential of cleavability, adaptability, and topological conversion are challenging. Herein we report photoswitchable allosteric and topological control of dynamic covalent macrocycles and further the use in guest binding and mechanically interlocked molecules. The manipulation of competing ring-chain equilibria and bond formation/scission within reaction systems enabled light-induced structural regulation over dithioacetal and thioacetal dynamic bonds, accordingly realizing bidirectional switching between crown ether-like covalent macrocycles and their linear counterparts. The on-demand photoswitchable topological transformation of macrocycles further allowed guest recognition/release exhibiting controllable binding affinity and selectivity. To showcase the capability light-triggered assembly/disassembly of diverse mechanically interlocked structures, such as rotaxanes and catenanes, was achieved. The realization of photoswitchable topological conversion of covalent macrocycles, which has been rarely reported before, demonstrates the potential of light-triggered reactivity control and structural reconfiguration for enhanced complexity and sophisticated function. The strategies and results should be appealing to endeavors in molecular recognition, dynamic assemblies, molecular machines, and intelligent materials.

Design and advances in antioxidant hydrogels for ROS-induced oxidative disease

Reactive oxygen species (ROS) play a crucial role in human physiological processes, but oxidative stress caused by excessive ROS may lead to a variety of acute and chronic diseases. Despite the development of various strategies and biomaterials, an efficiently and broadly applied method for treatment of ROS-induced oxidative disease remains a bottleneck. Aiming to improve the local oxidative stress environment, numerous bioactive hydrogels with antioxidant properties have emerged and are proven to quickly and continuously eliminate excessive ROS. To deeply understand the design principles and applications of antioxidant hydrogels is highly beneficial for designing antioxidant hydrogels for treatment of oxidative disease. This review provides a detailed summary of recent advances in design and applications of antioxidant hydrogels for various ROS-induced oxidative diseases. In this review, the kinds of antioxidant components in antioxidant hydrogels are outlined in detail. Additionally, the crosslinking methods and the biomedical applications of antioxidant hydrogels are widely summarized and discussed, especially focusing on their usage in different types of diseases and the attention given to the treatment of diseases such as skin wounds, myocardial infarction, and osteoarthritis. Finally, the future development direction of antioxidant hydrogel is further proposed. STATEMENT OF SIGNIFICANCE: Oxidative stress is a pivotal biochemical process that plays a critical role in cellular homeostasis. Excessive cellular oxidative stress triggers an inflammatory response, which is implicated in a spectrum of associated diseases. Given the critical need for managing oxidative stress, antioxidant therapies have become a vital focus in medical research. Hydrogels have garnered substantial interest among biomaterial scientists due to their hydrophilic nature and biocompatibility. The review delves into the realm of antioxidant hydrogels, encompassing the classification of antioxidant components, the synthesis and fabrication of hydrogels, and a comprehensive overview of the biological applications and challenges of these antioxidant hydrogels. Aiming to provide new perspectives for researchers in developing cutting-edge therapeutic approaches that leverage antioxidant hydrogels.

Boronate Ester Hydrogels for Biomedical Applications: Challenges and Opportunities

Boronate ester (BE) hydrogels are increasingly used for biomedical applications. The dynamic nature of these molecular networks enables bond rearrangement, which is associated with viscoelasticity, injectability, printability, and self-healing, among other properties. BEs are also sensitive to pH, redox reactions, and the presence of sugars, which is useful for the design of stimuli-responsive materials. Together, BE hydrogels are interesting scaffolds for use in drug delivery, 3D cell culture, and biofabrication. However, designing stable BE hydrogels at physiological pH (≈7.4) remains a challenge, which is hindering their development and biomedical application. In this context, advanced chemical insights into BE chemistry are being used to design new molecular solutions for material fabrication. This review article summarizes the state of the art in BE hydrogel design for biomedical applications with a focus on the materials chemistry of this class of materials. First, we discuss updated knowledge in BE chemistry including details on the molecular mechanisms associated with BE formation and breakage. Then, we discuss BE hydrogel formation at physiological pH, with an overview of the main systems reported to date along with new perspectives. A last section covers several prominent biomedical applications of BE hydrogels, including drug delivery, 3D cell culture, and bioprinting, with critical insights on the design relevance, limitations and potential.

Internal Catalysis in Dynamic Hydrogels with Associative Thioester Cross-Links

Thioesters are an essential functional group in biosynthetic pathways, which has motivated their development as reactive handles in probes and peptide assembly. Thioester exchange is typically accelerated by catalysts or elevated pH. Here, we report the use of bifunctional aromatic thioesters as dynamic covalent cross-links in hydrogels, demonstrating that at physiologic pH in aqueous conditions, transthioesterification facilitates stress relaxation on the time scale of hundreds of seconds. We show that intramolecular hydrogen bonding is responsible for accelerated exchange, evident in both molecular kinetics and macromolecular stress relaxation. Drawing from concepts in the vitrimer literature, this system exemplifies how dynamic cross-links that exchange through an associative mechanism enable tunable stress relaxation without altering stiffness.

Multiple control of azoquinoline based molecular photoswitches

Multi-addressable molecular switches with high sophistication are creating intensive interest, but are challenging to control. Herein, we incorporated ring-chain dynamic covalent sites into azoquinoline scaffolds for the construction of multi-responsive and multi-state switching systems. The manipulation of ring-chain equilibrium by acid/base and dynamic covalent reactions with primary/secondary amines allowed the regulation of E/Z photoisomerization. Moreover, the carboxyl and quinoline motifs provided recognition handles for the chelation of metal ions and turning off photoswitching, with otherwise inaccessible Z-isomer complexes obtained via the change of stimulation sequence. Particularly, the distinct metal binding behaviors of primary amine and secondary amine products offered a facile way for modulating E/Z switching and dynamic covalent reactivity. As a result, multiple control of azoarene photoswitches was accomplished, including light, pH, metal ions, and amine nucleophiles, with interplay between diverse stimuli further enabling addressable multi-state switching within reaction networks. The underlying structural and mechanistic insights were elucidated, paving the way for the creation of complex switching systems, molecular assemblies, and intelligent materials.

Activation of Anthraquinone's Electrophilicity by Light for a Dynamic C-O Bond

Coupling of photoswitching with dynamic covalent chemistry enables control of the formation and cleavage of covalent bonds by light irradiation. peri-Aryloxyanthraquinones feature an exclusive ability to switch electrophilicity by interconversion between para- and ana-quinone isomers, which was used for the first time for the implementation of a dynamic C-O bond. Photogenerated ana-isomers undergo a concerted oxa-Michael addition of phenols to give hitherto unknown 4-hydroxy-10,10-diaryloxyanthracen-9-ones. These species were found to be in equilibrium with the corresponding ana-quinones, thus forming a dynamic covalent system of a new type. Withdrawal of the colored ana-quinones from the equilibria by visible light irradiation resulted in two para-quinones with "locked" aryloxy groups.

Structural Determinants of Stimuli-Responsiveness in Amphiphilic Macromolecular Nano-assemblies

Stimuli-responsive nano-assemblies from amphiphilic macromolecules could undergo controlled structural transformations and generate diverse macroscopic phenomenon under stimuli. Due to the controllable responsiveness, they have been applied for broad material and biomedical applications, such as biologics delivery, sensing, imaging, and catalysis. Understanding the mechanisms of the assembly-disassembly processes and structural determinants behind the responsive properties is fundamentally important for designing the next generation of nano-assemblies with programmable responsiveness. In this review, we focus on structural determinants of assemblies from amphiphilic macromolecules and their macromolecular level alterations under stimuli, such as the disruption of hydrophilic-lipophilic balance (HLB), depolymerization, decrosslinking, and changes of molecular packing in assemblies, which eventually lead to a series of macroscopic phenomenon for practical purposes. Applications of stimuli-responsive nano-assemblies in delivery, sensing and imaging were also summarized based on their structural features. We expect this review could provide readers an overview of the structural considerations in the design and applications of nanoassemblies and incentivize more explorations in stimuli-responsive soft matters.

Highly Recyclable and Tough Elastic Vitrimers from a Defined Polydimethylsiloxane Network

Despite intensive research on sustainable elastomers, achieving elastic vitrimers with significantly improved mechanical properties and recyclability remains a scientific challenge. Herein, inspired by the classical elasticity theory, we present a design principle for ultra-tough and highly recyclable elastic vitrimers with a defined network constructed by chemically crosslinking the pre-synthesized disulfide-containing polydimethylsiloxane (PDMS) chains with tetra-arm polyethylene glycol (PEG). The defined network is achieved by the reduced dangling short chains and the relatively uniform molecular weight of network strands. Such elastic vitrimers with the defined network, i.e., PDMS-disulfide-D, exhibit significantly improved mechanical performance than random analogous, previously reported PDMS vitrimers, and even commercial silicone-based thermosets. Moreover, unlike the vitrimers with random network that show obvious loss in mechanical properties after recycling, those with the defined network enable excellent thermal recyclability. The PDMS-disulfide-D also deliver comparable electrochemical signals if utilized as substrates for electromyography sensors after the recycling. The multiple relaxation processes are revealed via a unique physical approach. Multiple techniques are also applied to unravel the microscopic mechanism of the excellent mechanical performance and recyclability of such defined network.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

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.

Photoswitchable dynamic conjugate addition-elimination reactions as a tool for light-mediated click and clip chemistry

Phototriggered click and clip reactions can endow chemical processes with high spatiotemporal resolution and sustainability, but are challenging with a limited scope. Herein we report photoswitchable reversible covalent conjugate addition-elimination reactions toward light-addressed modular covalent connection and disconnection. By coupling between photochromic dithienylethene switch and Michael acceptors, the reactivity of Michael reactions was tuned through closed-ring and open-ring forms of dithienylethene, allowing switching on and off dynamic exchange of a wide scope of thiol and amine nucleophiles. The breaking of antiaromaticity in transition states and enol intermediates of addition-elimination reactions provides the driving force for photoinduced change in kinetic barriers. To showcase the versatile application, light-mediated modification of solid surfaces, regulation of amphiphilic assemblies, and creation/degradation of covalent polymers on demand were achieved. The manipulation of dynamic click/clip reactions with light should set the stage for future endeavors, including responsive assemblies, biological delivery, and intelligent materials.

Vat Photopolymerization Additive Manufacturing of Tough, Fully Recyclable Thermosets

To advance the capabilities of additive manufacturing, novel resin formulations are needed that produce high-fidelity parts with desired mechanical properties that are also amenable to recycling. In this work, a thiol-ene-based system incorporating semicrystallinity and dynamic thioester bonds within polymer networks is presented. It is shown that these materials have ultimate toughness values >16 MJ cm^-3, comparable to high-performance literature precedents. Significantly, the treatment of these networks with excess thiols facilitates thiol-thioester exchange that degrades polymerized networks into functional oligomers. These oligomers are shown to be amenable to repolymerization into constructs with varying thermomechanical properties, including elastomeric networks that recover their shape fully from >100% strain. Using a commercial stereolithographic printer, these resin formulations are printed into functional objects including both stiff (E ∼ 10-100 MPa) and soft (E ∼ 1-10 MPa) lattice structures. Finally, it is shown that the incorporation of both dynamic chemistry and crystallinity further enables advancement in the properties and characteristics of printed parts, including attributes such as self-healing and shape-memory.

An alkaline phosphatase-induced immunosensor for SARS-CoV-2 N protein and cardiac troponin I based on the in situ fluorogenic self-assembly between N-heterocyclic boronic acids and alizarin red S

Alkaline phosphatase (ALP)-induced in situ fluorescent immunosensor is less investigated and reported. Herein, a high-performance ALP-labeled in situ fluorescent immunoassay platform was constructed. The developed platform was based on a fluorogenic self-assembly reaction between pyridineboronic acid (PyB(OH)₂) and alizarin red S (ARS). We first used density functional theory (DFT) to theoretically calculate the changes of Gibbs free energy of the used chemicals before and after the combination and simulated the electrostatic potential on its' surfaces. The free ARS and PyB(OH)₂ exist alone, neither emits no fluorescence. However, the ARS/PyB(OH)₂ complex emits strong fluorescence, which could be effectively quenched by PPi based on the stronger affinity between PPi and PyB(OH)₂ than that of ARS and PyB(OH)₂. PyB(OH)₂ coordinated with ARS again in the presence of ALP due to the ALP-catalyzed hydrolysis of PPi, and correspondingly, the fluorescence was restored. We chose cTnI and SARS-CoV-2 N protein as the model antigen to construct ALP-induced immunosensor, which exhibited a wide dynamic range of 0-175 ng/mL for cTnI and SARS-CoV-2 N protein with a low limit of detection (LOD) of 0.03 ng/mL and 0.17 ng/mL, respectively. Moreover, the proposed immunosensor was used to evaluate cTnI and SARS-CoV-2 N protein level in serum with satisfactory results. Consequently, the method laid the foundation for developing novel fluorescence-based ALP-labeled ELISA technologies in the early diagnosis of diseases.

Spatiotemporal Control of Biology: Synthetic Photochemistry Toolbox with Far-Red and Near-Infrared Light

The complex network of naturally occurring biological pathways motivates the development of new synthetic molecules to perturb and/or detect these processes for fundamental research and clinical applications. In this context, photochemical tools have emerged as an approach to control the activity of drug or probe molecules at high temporal and spatial resolutions. Traditional photochemical tools, particularly photolabile protecting groups (photocages) and photoswitches, rely on high-energy UV light that is only applicable to cells or transparent model animals. More recently, such designs have evolved into the visible and near-infrared regions with deeper tissue penetration, enabling photocontrol to study biology in tissue and model animal contexts. This Review highlights recent developments in synthetic far-red and near-infrared photocages and photoswitches and their current and potential applications at the interface of chemistry and biology.

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.

11B NMR Spectroscopy: Structural Analysis of the Acidity and Reactivity of Phenyl Boronic Acid-Diol Condensations

Phenyl boronic acids are valuable for medical diagnostics and biochemistry studies due to their ability to readily bind with carbohydrates in water. Incorporated in carbohydrates are 1,2-diols, which react with boronic acids through a reversible covalent condensation pathway. A wide variety of boronic acids have been employed for diol binding with differing substitution of the phenyl ring, with the goals of simplifying their synthesis and altering their thermodynamics of complexation. One method for monitoring their pK_a's and binding is ¹¹B NMR spectroscopy. Herein, we report a comprehensive study employing ¹¹B NMR spectroscopy to determine the pK_a of the most commonly used phenyl boronic acids and their binding with catechol or d,l-hydrobenzoin as prototypical diols. The chemical shift of the boronic acid transforming into the boronate ester was monitored at pHs ranging from 2 to 10. With each boronic acid, the results confirm (1) the necessity to use pHs above their pK_a's to induce complexation, (2) that the pK_a's change in the presence of diols, and (3) that ¹¹B NMR spectroscopy is a particularly convenient tool for monitoring these interconnected acidity and binding phenomena.

Dynamic combinatorial libraries of a dimercapto-pillar[5]arene

We report the synthesis and dynamic covalent chemistry (DCvC) of an A1/A2-dimercapto-functionalized pillar[5]arene (Di-SH-P5). The introduction of thiol moieties into this macrocyclic host makes it an effective building block for making a dynamic combinatorial library (DCL), giving rise to a set of cyclic trimers with intriguing host-guest properties as the dominant constituents.

Acylhydrazone-based supramolecular assemblies undergoing a converse sol-to-gel transition on trans → cis photoisomerization

Photoisomeric supramolecular assemblies have drawn enormous attention in recent years. Although it is a general rule that photoisomerization from a less to a more distorted isomer causes the destruction of assemblies, this photoisomerization process inducing a converse transition from irregular aggregates to regular assemblies is still a great challenge. Here, we report a converse sol-to-gel transition derived from the planar to nonplanar photoisomer conversion, which is in sharp contrast to the conventional light-induced gel collapse. A well-designed acylhydrazone-linked monomer is exploited as a photoisomer to realize the above-mentioned phase transition. In the monomer, imine is responsible for trans-cis interconversion and amide generates intermolecular hydrogen bonds enabling the photoisomerization-driven self-assembly. The counterintuitive feature of the sol-to-gel transition is ascribed to the partial trans → cis photoisomerization of acylhydrazone causing changes in stacking mode of monomers. Furthermore, the reversible phase transition is applied in the valves formed in situ in microfluidic devices, providing fascinating potential for miniature materials.

Remote-Controlled Exchange Rates by Photoswitchable Internal Catalysis of Dynamic Covalent Bonds

The transesterification rate of boronate esters with diols is tunable over 14 orders of magnitude. Rate acceleration is achieved by internal base catalysis, which lowers the barrier for proton transfer. Here we report a photoswitchable internal catalyst that tunes the rate of boronic ester/diol exchange over 4 orders of magnitude. We employed an acylhydrazone molecular photoswitch, which forms a thermally stable but photoreversible intramolecular H-bond, to gate the activity of the internal base catalyst in 8-quinoline boronic ester. The photoswitch is bidirectional and can be cycled repeatedly. The intramolecular H-bond is found to be essential to the design of this photoswitchable internal catalyst, as protonating the quinoline with external sources of acid has little effect on the exchange rate.

Wavelength Orthogonal Photodynamic Networks

The ability of light to remotely control the properties of soft matter materials in a dynamic fashion has fascinated material scientists and photochemists for decades. However, only recently has our ability to map photochemical reactivity in a finely wavelength resolved fashion allowed for different colors of light to independently control the material properties of polymer networks with high precision, driven by monochromatic irradiation enabling orthogonal reaction control. The current concept article highlights the progress in visible light‐induced photochemistry and explores how it has enabled the design of polymer networks with dynamically adjustable properties. We will explore current applications ranging from dynamic hydrogel design to the light‐driven adaptation of 3D printed structures on the macro‐ and micro‐scale. While the alternation of mechanical properties via remote control is largely reality for soft matter materials, we herein propose the next frontiers for adaptive properties, including remote switching between conductive and non‐conductive properties, hydrophobic and hydrophilic surfaces, fluorescent or non‐fluorescent, and cell adhesive vs. cell repellent properties.

Molecular Solar Thermal Systems towards Phase Change and Visible Light Photon Energy Storage

Molecular solar thermal (MOST) systems have attracted tremendous attention for solar energy conversion and storage, which can generate high-energy metastable isomers upon capturing photon energy, and release the stored energy as heat on demand during back conversion. However, the pristine molecular photoswitches are limited by low storage energy density and UV light photon energy storage. Recently, numerous pioneering works have been focused on the development of MOST systems towards phase change (PC) and visible light photon energy storage to increase their properties. On the one hand, the strategy of simultaneously capturing isomerization enthalpy and PC energy between solid and liquid can not only offer high latent heat, but also promote the development of sustainable energy systems. On the other hand, the efficient photon energy storage in the visible light range opens a tremendously fascinating avenue to fabricate MOST systems powered under natural sunlight. Here, the recent advances of MOST systems towards PC and visible light photon energy storage are systematically summarized, the most promising advantages and current challenges are analyzed, and emerging strategies and future research directions are proposed.

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.

Healable, Recyclable, and Programmable Shape Memory Organogels Based on Highly Malleable Catalyst-Free Carboxylate Linkages

The development of healable and recyclable organogels possessing responsive abilities is mainly hindered by the unavailability of many dynamic covalent linkages that undergo exchange reaction below the boiling temperature of organic swelling medium. Furthermore, the exchange is desired to be effective under catalyst-free conditions to circumvent the issue of catalyst leaching during the swelling process. Especially, imparting swift reversibility to thermostable carboxylate linkages is challenging. In this approach, we have utilized the β-keto anchimeric assistance as the tool to induce swift reversibility into the conventional carboxylate linkage under mild temperature (∼70-90 °C) and catalyst-free conditions. Using this β-keto carboxylate linkage as an associative bond exchange mean, strong (tensile strength = 0.3 MPa) and stretchable (ultimate elongation ≈ 600%) covalent adaptable organogels (CAOs) with anisotropic swelling, remoldable, self-healing, and shape memory ability are derived from commercially available precursors. The shape memory ability of these samples shows dependency on the shape fixing time and can be programmed, targeting further applications. Soft actuators may be fabricated from the CAOs using temperature and solvent as the activating tools. This research demonstrates that the conventional carboxylate linkages can be made labile under mild conditions for further applications.

Connecting the Dynamics and Reactivity of Arylboronic Acids to Emergent and Stimuli-Responsive Material Properties

Over the past two decades, arylboronic acid-functionalized biomaterials have been used in a variety of sensing and stimuli-responsive scaffolds. Their diverse applications result from the diverse reactivity of arylboronic acids,...

Light-Induced Formation/Scission of C-N, C-O, and C-S Bonds Enables Switchable Stability/Degradability in Covalent Systems

The manipulation of covalent bonds could be directed toward degradable, recyclable, and sustainable materials. However, there is an intrinsic conflict between properties of stability and degradability. Here we report light-controlled formation/scission of three types of covalent bonds (C-N, C-O, and C-S) through photoswitching between equilibrium and nonequilibrium states of dynamic covalent systems, achieving dual benefits of photoaddressable stability and cleavability. The photocyclization of dithienylethene fused aldehyde ring-chain tautomers turns on the reactivity, incorporating/releasing amines, alcohols, and thiols reversibly with high efficiency, respectively. Upon photocycloreversion the system is shifted to kinetically locked out-of-equilibrium form, enabling remarkable robustness of covalent assemblies. Reaction coupling allows remote and directional control of a diverse range of equilibria and further broadens the scope. Through locking and unlocking covalent linkages with light when needed, the utility is demonstrated with capture/release of bioactive molecules, modification of surfaces, and creation of polymers exhibiting tailored stability and degradability/recyclability. The versatile toolbox for photoswitchable dynamic covalent reactions to toggle matters on and off should be appealing to many endeavors.

Reversible dynamic mechanics of hydrogels for regulation of cellular behavior

The mechanical properties of the native extracellular matrix play a key role in regulating cell behavior during developmental, healing and homeostatic processes. Since these properties change over time, it may be valuable to have the capacity to dynamically vary the mechanical properties of engineered hydrogels used in tissue engineering strategies to better mimic the dynamic mechanical behavior of native extracellular matrix. However, in situ repeatedly reversible dynamic tuning of hydrogel mechanics is still limited. In this study, we have engineered a hydrogel system with reversible dynamic mechanics using a dual-crosslinkable alginate hydrogel. The effect of reversible mechanical signals on encapsulated stem cells in dynamically tunable hydrogels has been demonstrated. In situ stiffening of hydrogels decreases cell spreading and proliferation, and subsequent softening of hydrogels gives way to an increase in cell spreading and proliferation. The hydrogel stiffening and softening, and resulting cellular responses are repeatedly reversible. This hydrogel system provides a promising platform for investigating the effect of repeatedly reversible changes in extracellular matrix mechanics on cell behaviors. STATEMENT OF SIGNIFICANCE: Since the mechanical properties of native extracellular matrix (ECM) change over time during development, healing and homeostatic processes, it may be valuable to have the capacity to dynamically control the mechanics of biomaterials used in tissue engineering and regenerative medicine applications to better mimic this behavior. Unlike previously reported biomaterials whose mechanical properties can be changed by the user only a limited number of times, this system provides the capacity to induce unlimited alterations to the mechanical properties of an engineered ECM for 3D cell culture. This study presents a strategy for on-demand dynamic and reversible control of materials' mechanics by single and dual-crosslinking mechanisms using oxidized and methacrylated alginates. By demonstrating direct changes in encapsulated human mesenchymal stem cell morphology, proliferation and chondrogenic differentiation in response to multiple different dynamic changes in hydrogel mechanics, we have established a repeatedly reversible 3D cellular mechanosensing system. This system provides a powerful platform tool with a wide range of stiffness tunability to investigate the role of dynamic mechanics on cellular mechanosensing and behavioral responses.

Protodeboronation of (Hetero)Arylboronic Esters: Direct versus Prehydrolytic Pathways and Self-/Auto-Catalysis

The kinetics and mechanism of the base-catalyzed hydrolysis (ArB(OR)₂ → ArB(OH)₂) and protodeboronation (ArB(OR)₂ → ArH) of a series of boronic esters, encompassing eight different polyols and 10 polyfluoroaryl and heteroaryl moieties, have been investigated by in situ and stopped-flow NMR spectroscopy (¹⁹F, ¹H, and ¹¹B), pH-rate dependence, isotope entrainment, ²H KIEs, and KS-DFT computations. The study reveals the phenomenological stability of boronic esters under basic aqueous-organic conditions to be highly nuanced. In contrast to common assumption, esterification does not necessarily impart greater stability compared to the corresponding boronic acid. Moreover, hydrolysis of the ester to the boronic acid can be a dominant component of the overall protodeboronation process, augmented by self-, auto-, and oxidative (phenolic) catalysis when the pH is close to the pK_a of the boronic acid/ester.

Structurally Dynamic Hydrogels for Biomedical Applications: Pursuing a Fine Balance between Macroscopic Stability and Microscopic Dynamics

Owing to their unique chemical and physical properties, hydrogels are attracting increasing attention in both basic and translational biomedical studies. Although the classical hydrogels with static networks have been widely reported for decades, a growing number of recent studies have shown that structurally dynamic hydrogels can better mimic the dynamics and functions of natural extracellular matrix (ECM) in soft tissues. These synthetic materials with defined compositions can recapitulate key chemical and biophysical properties of living tissues, providing an important means to understanding the mechanisms by which cells sense and remodel their surrounding microenvironments. This review begins with the overall expectation and design principles of dynamic hydrogels. We then highlight recent progress in the fabrication strategies of dynamic hydrogels including both degradation-dependent and degradation-independent approaches, followed by their unique properties and use in biomedical applications such as regenerative medicine, drug delivery, and 3D culture. Finally, challenges and emerging trends in the development and application of dynamic hydrogels are discussed.

Photo-responsive host-guest complexation directs dynamic covalent condensation of phenyl boronic acid and d-fructose

Inspired by the way templates have been used to drive dynamic combinatorial libraries by molecular recognition, we exploited the photo-responsive host-guest interaction of an azo-based photoswitch with permethylated cyclodextrin to reversibly manipulate the dynamic covalent interaction of a phenyl boronic acid and d-fructose by irradiation with light.

Taking phototherapeutics from concept to clinical launch

More than four decades have passed since the first example of a light-activated (caged) compound was described. In the intervening years, a large number of light-responsive derivatives have been reported, several of which have found utility under a variety of in vitro conditions using cells and tissues. Light-triggered bioactivity furnishes spatial and temporal control, and offers the possibility of precision dosing and orthogonal communication with different biomolecules. These inherent attributes of light have been advocated as advantageous for the delivery and/or activation of drugs at diseased sites for a variety of indications. However, the tissue penetrance of light is profoundly wavelength-dependent. Only recently have phototherapeutics that are photoresponsive in the optical window of tissue (600-900 nm) been described. This Review highlights these recent discoveries, along with their limitations and clinical opportunities. In addition, we describe preliminary in vivo studies of prospective phototherapeutics, with an emphasis on the path that remains to be navigated in order to translate light-activated drugs into clinically useful therapeutics. Finally, the unique attributes of phototherapeutics is highlighted by discussing several potential disease applications.