Reversibly Softening and Stiffening Organogels Using a Wavelength-Controlled Disulfide-Diselenide Exchange

Dynamic Exchange in 3D Cell Culture Hydrogels Based on Crosslinking of Cyclic Thiosulfinates

Dynamic polymer materials are highly valued substrates for 3D cell culture due to their viscoelasticity, a time-dependent mechanical property that can be tuned to resemble the energy dissipation of native tissues. Herein, we report the coupling of a cyclic thiosulfinate, mono-S-oxo-4-methyl asparagusic acid, to a 4-arm PEG-OH to prepare a disulfide-based dynamic covalent hydrogel with the addition of 4-arm PEG-thiol. Ring opening of the cyclic thiosulfinate by nucleophilic substitution results in the rapid formation of a network showing a viscoelastic fluid-like behaviour and relaxation rates modulated by thiol content through thiol-disulfide exchange, whereas its viscoelastic behaviour upon application as a small molecule linear crosslinker is solid-like. Further introduction of 4-arm PEG-vinylsulfone in the network yields a hydrogel with weeks-long cell culture stability, permitting 3D culture of cell types that lack robust proliferation, such as human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs). These cells display native behaviours such as cell elongation and spontaneous beating as a function of the hydrogel's mechanical properties. We demonstrate that the mode of dynamic cyclic thiosulfinate crosslinker presentation within the network can result in different stress relaxation profiles, opening the door to model tissues with disparate mechanics in 3D cell culture.

Strain-Stiffening Hydrogels with Dynamic, Secondary Cross-Linking

Hydrogels are water-swollen, typically soft networks useful as biomaterials and in other fields of biotechnology. Hydrogel networks capable of sensing and responding to external perturbations, such as light, temperature, pH, or force, are useful across a wide range of applications requiring on-demand cross-linking or dynamic changes. Thus far, although mechanophores have been described as strain-sensitive reactive groups, embedding this type of force-responsiveness into hydrogels is unproven. Here, we synthesized multifunctional polymers that combine a hydrophilic zwitterion with permanently cross-linking alkenes, and dynamically cross-linking disulfides. From these polymers, we created hydrogels that contain irreversible and strong thiol-ene cross-links and reversible disulfide cross-links, and they stiffened in response to strain, increasing hundreds of kPa in modulus under compression. We examined variations in polymer composition and used a constitutive model to determine how to balance the number of thiol-ene vs disulfide cross-links to create maximally force-responsive networks. These strain-stiffening hydrogels represent potential biomaterials that benefit from the mechanoresponsive behavior needed for emerging applications in areas such as tissue engineering.

Hydrolytically Degradable PEG-Based Inverse Electron Demand Diels-Alder Click Hydrogels

Hydrogels cross-linked by inverse electron demand Diels–Alder (iEDDA) click chemistry are increasingly used in biomedical applications. With a few exceptions in naturally derived and chemically modified macromers, iEDDA click hydrogels exhibit long-term hydrolytic stability, and no synthetic iEDDA click hydrogels can undergo accelerated and tunable hydrolytic degradation. We have previously reported a novel method for synthesizing norbornene (NB)-functionalized multiarm poly(ethylene glycol) (PEG), where carbic anhydride (CA) was used to replace 5-norbornene-2-carboxylic acid. The new PEGNB_CA-based thiol-norbornene hydrogels exhibited unexpected fast yet highly tunable hydrolytic degradation. In this contribution, we leveraged the new PEGNB_CA macromer for forming iEDDA click hydrogels with [methyl]tetrazine ([m]Tz)-modified macromers, leading to the first group of synthetic iEDDA click hydrogels with highly tunable hydrolytic degradation kinetics. We further exploited Tz and mTz dual conjugation to achieve tunable hydrolytic degradation with an in vitro degradation time ranging from 2 weeks to 3 months. Finally, we demonstrated the excellent in vitro cytocompatibility and in vivo biocompatibility of the new injectable PEGNB_CA-based iEDDA click cross-linked hydrogels.

Sulfur in Dynamic Covalent Chemistry

Sulfur has been important in dynamic covalent chemistry (DCC) since the beginning of the field. Mainly as part of disulfides and thioesters, dynamic sulfur-based bonds (DSBs) have a leading role in several remarkable reactions. Part of this success is due to the almost ideal properties of DSBs for the preparation of dynamic covalent systems, including high reactivity and good reversibility under mild aqueous conditions, the possibility of exploiting supramolecular interactions, access to isolable structures, and easy experimental control to turn the reaction on/off. DCC is currently witnessing an increase in the importance of DSBs. The chemical flexibility offered by DSBs opens the door to multiple applications. This Review presents an overview of all the DSBs used in DCC, their applications, and remarks on the interesting properties that they confer on dynamic chemical systems, especially those containing several DSBs.

Anisotropic Silk-Inspired Nerve Conduit with Peptides Improved the Microenvironment for Long-Distance Peripheral Nerve Regeneration

A lack of effective bioactivity to create a desirable microenvironment for peripheral nerve regeneration has been challenging in successful treatment of long-distance injuries using nerve guidance conduits (NGCs) clinically. Herein, we developed a silk-inspired phototriggered gelation system combining dual therapeutic cues of anisotropic topography and adhesive ligands for improving peripheral nerve regeneration. Importantly, enhanced cell recruitment and myelination of Schwann cells were successfully achieved by the Arg-Gly-Asp (RGD)-peptide-immobilized hydrogel scaffolds to promote axon growth. Moreover, as the orientated growth of Schwann cells and rapid axon growth were facilitated by aligned grooved micropatterns, this multifunctional bioactive system provides remarkable nerve regeneration with function recovery for long-distance nerve injury. Therefore, this bioengineered silk-inspired nerve guidance conduit delivers a platform for desirable peripheral nerve repair.

Thiol- and Disulfide-Based Stimulus-Responsive Soft Materials and Self-Assembling Systems

Properties and applications of synthetic thiol- and disulfide-based materials, principally polymers, are reviewed. Emphasis is placed on soft and self-assembling materials in which interconversion of the thiol and disulfide groups initiates stimulus-responses and/or self-healing for biomedical and non-biomedical applications.