Reversible maleimide-thiol adducts yield glutathione-sensitive poly(ethylene glycol)-heparin hydrogels

Hyaluronic Acid/Chondroitin Sulfate-Based Dynamic Thiol-Aldehyde Addition Hydrogel: An Injectable, Self-Healing, On-Demand Dissolution Wound Dressing

Frequent removal and reapplication of wound dressings can cause mechanical disruption to the healing process and significant physical discomfort for patients. In response to this challenge, a dynamic covalent hydrogel has been developed to advance wound care strategies. This system comprises aldehyde functionalized chondroitin sulfate (CS-CHO) and thiolated hyaluronic acid (HA-SH), with the distinct ability to form in situ via thiol-aldehyde addition and dissolve on-demand via the thiol-hemithioacetal exchange reaction. Although rarely reported, the dynamic covalent reaction of thiol-aldehyde addition holds great promise for the preparation of dynamic hydrogels due to its rapid reaction kinetics and easy reversible dissociation. The thiol-aldehyde addition chemistry provides the hydrogel system with highly desirable characteristics of rapid gelation (within seconds), self-healing, and on-demand dissolution (within 30 min). The mechanical and dissolution properties of the hydrogel can be easily tuned by utilizing CS-CHO materials of different aldehyde functional group contents. The chemical structure, rheology, self-healing, swelling profile, degradation rate, and cell biocompatibility of the hydrogels are characterized. The hydrogel possesses excellent biocompatibility and proves to be significant in promoting cell proliferation in vitro when compared to a commercial hydrogel (HyStem® Cell Culture Scaffold Kit). This study introduces the simple fabrication of a new dynamic hydrogel system that can serve as an ideal platform for biomedical applications, particularly in wound care treatments as an on-demand dissolvable wound dressing.

Reconnection of cysteine in reduced hair with alkylene dimaleates via thiol-Michael click chemistry

OBJECTIVES

Conventional hair permanent waving (PW) and permanent straightening processes typically involve two steps: reduction, for breaking -S-S- bond in cystine into cysteine and oxidation for -S-S- bond reconnection. However, it is known that the hair incurs damage during the oxidation step. In this work, we proposed a novel strategy to reconnect reduced disulfide bonds in hair via the thiol-Michael click reaction, by using a symmetric Michael reagent.

METHODS

Virgin black Chinese hair was reduced using 8% wt thioglycolic acid and employed as model hair containing a high content of broken disulfide bonds. The reduced hair was treated with 1,4-n-butylene dimaleate. Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) were used to verify the chemical changes occurred in untreated and treated hair fibre. Single-fibre mechanical properties and thermal properties of the hair were evaluated using tensile testing and differential scanning calorimetry (DSC), respectively.

RESULTS

The 1,4-n-butylene dimaleate could reconnect free thiol groups generated by disulfide bond reduction via thiol-Michael click reaction and significantly improve the mechanical strength of hair compared to that of the reduced hair. Secondary conformational resolution analysis of FT-IR results revealed that the content of α-helix structure could be restored after treatment with 1,4-n-butylene dimaleate. The intermolecular forces established by the newly generated C-S bonds compensate the broken disulfide bonds and enhance the fracture strength of the hair compared to that of reduced hair. Michael reagents of similar structure also showed similar performance in restoring the mechanical properties of reduced hair.

CONCLUSIONS

Our data suggest that 1,4-n-butylene dimaleate can restore the mechanical properties of reduced hair by reconnecting reduced disulfide bonds and restoring the secondary conformation of hair keratin.

Therapeutic delivery systems for rheumatoid arthritis based on hydrogel carriers

Rheumatoid arthritis (RA) is an autoimmune disease suffered by millions of people worldwide. It can significantly affect the patient's quality of life by damaging not only the joints but also organs such as the lungs and the heart. RA is normally treated using nonsteroidal anti-inflammatory drugs (NSAIDs), glucocorticoids, disease-modifying antirheumatic drugs (DMARDs), and biologics. These active agents often cause side effects and offer low efficacy due to their lack of specificity and limited retention time. In an attempt to improve RA treatments, hydrogel-based systems have been proposed as drug delivery carriers. Due to their exceptional adaptability and biocompatibility, hydrogels have the potential of enhancing the delivery of RA therapy through different administration routes in an efficient and effective manner. In this review, we explore the application of hydrogel systems as potential carriers in RA treatment. Additionally, we discuss recent work in the field and highlight the required hydrogel properties, depending on the administration route. The outstanding potential of hydrogel systems as carriers for RA was demonstrated; however, there is extensive research yet to be done to improve available treatments for RA.

Recent advances in spirocyclization of maleimides via transition-metal catalyzed C-H activation

In recent years, the maleimide scaffold has received a great deal of attention in C-H activation. Several types of products can be constructed using maleimides as a coupling partner. Alkylation, alkenylation, annulation, dehydrogenative annulation and spirocyclization are various reactions shown by maleimides in C-H activation. Thus, the maleimide scaffold has been extensively studied in the last few years in C-H activation owing to its unique reactivity. Among the diverse class of reactions of maleimides, spirocyclization is a less explored reaction. The spirocycles, in particular the spirosuccinimides are interesting candidates in drug discovery and materials chemistry. Therefore the method of spirocyclization of maleimides via C-H activation becomes an important strategy for the synthesis of a diverse array of spirosuccinimides. This review summarizes the reports available in this field from 2015-2023 and also highlights the scopes and prospects of this method.

Metal-Free Click-Chemistry: A Powerful Tool for Fabricating Hydrogels for Biomedical Applications

Increasing interest in the utilization of hydrogels in various areas of biomedical sciences ranging from biosensing and drug delivery to tissue engineering has necessitated the synthesis of these materials using efficient and benign chemical transformations. In this regard, the advent of "click" chemistry revolutionized the design of hydrogels and a range of efficient reactions was utilized to obtain hydrogels with increased control over their physicochemical properties. The ability to apply the "click" chemistry paradigm to both synthetic and natural polymers as hydrogel precursors further expanded the utility of this chemistry in network formation. In particular, the ability to integrate clickable handles at predetermined locations in polymeric components enables the formation of well-defined networks. Although, in the early years of "click" chemistry, the copper-catalyzed azide-alkyne cycloaddition was widely employed, recent years have focused on the use of metal-free "click" transformations, since residual metal impurities may interfere with or compromise the biological function of such materials. Furthermore, many of the non-metal-catalyzed "click" transformations enable the fabrication of injectable hydrogels, as well as the fabrication of microstructured gels using spatial and temporal control. This review article summarizes the recent advances in the fabrication of hydrogels using various metal-free "click" reactions and highlights the applications of thus obtained materials. One could envision that the use of these versatile metal-free "click" reactions would continue to revolutionize the design of functional hydrogels geared to address unmet needs in biomedical sciences.

Metal-Organic Framework-Based Nanomedicines for Ferroptotic Cancer Therapy

As an iron-dependent, non-apoptosis, regulated cell death (RCD) modality, ferroptosis has gained growing attention for cancer therapy. With the development of nanomaterials in the biomedical field, ferroptotic cancer nanomedicine is extensively investigated. Amongst various nanomaterials, metal-organic frameworks (MOFs) are hybridized porous materials consisting of metal ions or clusters bridged by organic linkers. The superior properties of MOFs, such as high porosity and cargo loading, ease of surface modification, and good biocompatibility, make them appealing in inducing or sensitizing ferroptotic cell death. There are remarkable achievements in the field of MOF-based ferroptosis cancer therapy. However, this topic is not reviewed. This review will introduce the fundamentals of MOF and ferroptosis machinery, summarize the recent progress of MOF-based ferroptotic anticancer drug delivery, discuss the benefits and problems of MOFs as vehicles and sensitizers for cancer ferroptosis, and provide the perspective on future research direction on this promising field.

Recent advances in the design and immobilization of heparin for biomedical application: A review

Heparin, a member of the glycosaminoglycan family, is renowned as the most negatively charged biomolecule discovered within the realm of human biology. This polysaccharide serves a vital role as a regulator for various proteins, cells, and tissues within the human body, positioning itself as a pivotal macromolecule of significance. The domain of biology has witnessed substantial interest in the intricate design of heparin and its derivatives, particularly focusing on heparin-based polymers and hydrogels. This intrigue spans a wide spectrum of applications, encompassing diverse areas such as protein adsorption, anticoagulant properties, controlled drug release, development of implants, stent innovation, enhancement of blood compatibility, acceleration of wound healing, and pioneering strides in tissue engineering. This comprehensive overview delves into a multitude of developed heparin conjugates, employing various methods, and explores their functions in both the biomedicine and electronics fields. The efficacy of materials derived from heparin is also thoroughly investigated, encompassing considerations such as thrombogenicity, drug release kinetics, affinity for growth factors (GFs), biocompatibility, and electrochemical analyses. We firmly believe that by redirecting focus towards research and advancements in heparin-related polymers/hydrogels, this study will ignite further research and accelerate potential breakthroughs in this promising and evolving field of discovery.

Hydrogels with Reversible Crosslinks for Improved Localised Stem Cell Retention: A Review

Successful stem cell applications could have a significant impact on the medical field, where many lives are at stake. However, the translation of stem cells to the clinic could be improved by overcoming challenges in stem cell transplantation and in vivo retention at the site of tissue damage. This review aims to showcase the most recent insights into developing hydrogels that can deliver, retain, and accommodate stem cells for tissue repair. Hydrogels can be used for tissue engineering, as their flexibility and water content makes them excellent substitutes for the native extracellular matrix. Moreover, the mechanical properties of hydrogels are highly tuneable, and recognition moieties to control cell behaviour and fate can quickly be introduced. This review covers the parameters necessary for the physicochemical design of adaptable hydrogels, the variety of (bio)materials that can be used in such hydrogels, their application in stem cell delivery and some recently developed chemistries for reversible crosslinking. Implementing physical and dynamic covalent chemistry has resulted in adaptable hydrogels that can mimic the dynamic nature of the extracellular matrix.

Supramolecular hybrid hydrogels as rapidly on-demand dissoluble, self-healing, and biocompatible burn dressings

Despite decades of efforts, state-of-the-art synthetic burn dressings to treat partial-thickness burns are still far from ideal. Current dressings adhere to the wound and necessitate debridement. This work describes the first "supramolecular hybrid hydrogel (SHH)" burn dressing that is biocompatible, self-healable, and on-demand dissoluble for easy and trauma-free removal, prepared by a simple, fast, and scalable method. These SHHs leverage the interactions of a custom-designed cationic copolymer via host-guest chemistry with cucurbit[7]uril and electrostatic interactions with clay nanosheets coated with an anionic polymer to achieve enhanced mechanical properties and fast on-demand dissolution. The SHHs show high mechanical strength (>50 kPa), self-heal rapidly in ∼1 min, and dissolve quickly (4-6 min) using an amantadine hydrochloride (AH) solution that breaks the supramolecular interactions in the SHHs. Neither the SHHs nor the AH solution has any adverse effects on human dermal fibroblasts or epidermal keratinocytes in vitro. The SHHs also do not elicit any significant cytokine response in vitro. Furthermore, in vivo murine experiments show no immune or inflammatory cell infiltration in the subcutaneous tissue and no change in circulatory cytokines compared to sham controls. Thus, these SHHs present excellent burn dressing candidates to reduce the time of pain and time associated with dressing changes.

From Free-Radical to Radical-Free: A Paradigm Shift in Light-Mediated Biofabrication

In recent years, the development of novel photocrosslinking strategies and photoactivatable materials has stimulated widespread use of light-mediated biofabrication techniques. However, despite great progress toward more efficient and biocompatible photochemical strategies, current photoresins still rely on photoinitiators (PIs) producing radical-initiating species to trigger the so-called free-radical crosslinking/polymerization. In the context of bioprinting, where cells are encapsulated in the bioink, the presence of radicals raises concerns of potential cytotoxicity. In this work, a universal, radical-free (RF) photocrosslinking strategy to be used for light-based technologies is presented. Leveraging RF uncaging mechanisms and Michael addition, cell-laden constructs are photocrosslinked by means of one- and two-photon excitation with high biocompatibility. A hydrophilic coumarin-based group is used to cage a universal RF photocrosslinker based on 4-arm-PEG-thiol (PEG4SH). Upon light exposure, thiols are uncaged and react with an alkene counterpart to form a hydrogel. RF photocrosslinker is shown to be highly stable, enabling potential for off-the-shelf products. While PI-based systems cause a strong upregulation of reactive oxygen species (ROS)-associated genes, ROS are not detected in RF photoresins. Finally, optimized RF photoresin is successfully exploited for high resolution two-photon stereolithography (2P-SL) using remarkably low polymer concentration (<1.5%), paving the way for a shift toward radical-free light-based bioprinting.

The progress of research on the application of redox nanomaterials in disease therapy

Redox imbalance can trigger cell dysfunction and damage and plays a vital role in the origin and progression of many diseases. Maintaining the balance between oxidants and antioxidants in vivo is a complicated and arduous task, leading to ongoing research into the construction of redox nanomaterials. Nanodrug platforms with redox characteristics can not only reduce the adverse effects of oxidative stress on tissues by removing excess oxidants from the body but also have multienzyme-like activity, which can play a cytotoxic role in tumor tissues through the catalytic oxidation of their substrates to produce harmful reactive oxygen species such as hydroxyl radicals. In this review, various redox nanomaterials currently used in disease therapy are discussed, emphasizing the treatment methods and their applications in tumors and other human tissues. Finally, the limitations of the current clinical application of redox nanomaterials are considered.

Breaking the niche: multidimensional nanotherapeutics for tumor microenvironment modulation

Most of the current antitumor therapeutics were developed targeting the cancer cells only. Unfortunately, in the majority of tumors, this single-dimensional therapy is found to be ineffective. Advanced research has shown that cancer is a multicellular disorder. The tumor microenvironment (TME), which is made by a complex network of the bulk tumor cells and other supporting cells, plays a crucial role in tumor progression. Understanding the importance of the TME in tumor growth, different treatment modalities have been developed targeting these supporting cells. Recent clinical results suggest that simultaneously targeting multiple components of the tumor ecosystem with drug combinations can be highly effective. This type of "multidimensional" therapy has a high potential for cancer treatment. However, tumor-specific delivery of such multi-drug combinations remains a challenge. Nanomedicine could be utilized for the tumor-targeted delivery of such multidimensional therapeutics. In this review, we first give a brief overview of the major components of TME. We then highlight the latest developments in nanoparticle-based combination therapies, where one drug targets cancer cells and other drug targets tumor-supporting components in the TME for a synergistic effect. We include the latest preclinical and clinical studies and discuss innovative nanoparticle-mediated targeting strategies.

Thia-Michael Reaction: The Route to Promising Covalent Adaptable Networks

While the Michael addition has been employed for more than 130 years for the synthesis of a vast diversity of compounds, the reversibility of this reaction when heteronucleophiles are involved has been generally less considered. First applied to medicinal chemistry, the reversible character of the hetero-Michael reactions has recently been explored for the synthesis of Covalent Adaptable Networks (CANs), in particular the thia-Michael reaction and more recently the aza-Michael reaction. In these cross-linked networks, exchange reactions take place between two Michael adducts by successive dissociation and association steps. In order to understand and precisely control the exchange in these CANs, it is necessary to get an insight into the critical parameters influencing the Michael addition and the dissociation rates of Michael adducts by reconsidering previous studies on these matters. This review presents the progress in the understanding of the thia-Michael reaction over the years as well as the latest developments and plausible future directions to prepare CANs based on this reaction. The potential of aza-Michael reaction for CANs application is highlighted in a specific section with comparison with thia-Michael-based CANs.

Redox-Responsive Drug Delivery Systems: A Chemical Perspective

With the widespread global impact of cancer on humans and the extensive side effects associated with current cancer treatments, a novel, effective, and safe treatment is needed. Redox-responsive drug delivery systems (DDSs) have emerged as a potential cancer treatment with minimal side effects and enhanced site-specific targeted delivery. This paper explores the physiological and biochemical nature of tumors that allow for redox-responsive drug delivery systems and reviews recent advances in the chemical composition and design of such systems. The five main redox-responsive chemical entities that are the focus of this paper are disulfide bonds, diselenide bonds, succinimide-thioether linkages, tetrasulfide bonds, and platin conjugates. Moreover, as disulfide bonds are the most commonly used entities, the review explored disulfide-containing liposomes, polymeric micelles, and nanogels. While various systems have been devised, further research is needed to advance redox-responsive drug delivery systems for cancer treatment clinical applications.

CCL21 and beta-cell antigen releasing hydrogels as tolerance-inducing therapy in Type I diabetes

Type-I Diabetes (T1D) is caused by defective immunotolerance mechanisms enabling autoreactive T cells to escape regulation in lymphoid organs and destroy insulin-producing β-cells in the pancreas, leading to insulin dependence. Strategies to promote β-cell tolerance could arrest T1D. We previously showed that secretion of secondary lymphoid chemokine CCL21 by CCL21 transgenic β-cells induced tolerance and protected non-obese diabetic (NOD) mice from T1D. T1D protection was associated with formation of lymph node-like stromal networks containing tolerogenic fibroblastic reticular cells (FRCs). Here, we developed a polyethylene glycol (PEG) hydrogel platform with hydrolytically degradable PEG-diester dithiol crosslinkers to provide controlled and sustained delivery of CCL21 and β-cell antigens for at least 28 days in vitro and recapitulate properties associated with the tolerogenic environment of CCL21 transgenic β-cells in our previous studies. CCL21 and MHC-II restricted antigens were tethered to gels via simple click-chemistry while MHC-I restricted antigens were loaded in PEG-based polymeric nanovesicles and incorporated in the gel networks. CCL21 and antigen release kinetics depended on the PEG gel tethering strategy and the linkers. Importantly, in vitro functionality, chemotaxis, and activation of antigen-specific T cells were preserved. Implantation of CCL21 and β-cell antigen gels under the kidney capsule of pre-diabetic NOD mice led to enrichment of adoptively transferred antigen-specific T cells, formation of gp38 + FRC-like stromal cell networks, and increased regulation of specific T cells with reduced accumulation within pancreatic islets. Thus, our platform for sustained release of β-cell antigens and CCL21 immunomodulatory molecule could enable the development of antigen-specific tolerance therapies for T1D.

Fast-Forming Dissolvable Redox-Responsive Hydrogels: Exploiting the Orthogonality of Thiol-Maleimide and Thiol-Disulfide Exchange Chemistry

Fast-forming yet easily dissolvable hydrogels (HGs) have potential applications in wound healing, burn incidences, and delivery of therapeutic agents. Herein, a combination of a thiol–maleimide conjugation and thiol–disulfide exchange reaction is employed to fabricate fast-forming HGs which rapidly dissolve upon exposure to dithiothreitol (DTT), a nontoxic thiol-containing hydrophilic molecule. In particular, maleimide disulfide-terminated telechelic linear poly(ethylene glycol) (PEG) polymer and PEG-based tetrathiol macromonomers are employed as gel precursors, which upon mixing yield HGs within a minute. The selectivity of the thiol–maleimide conjugation in the presence of a disulfide linkage was established through ¹H NMR spectroscopy and Ellman’s test. Rapid degradation of HGs in the presence of thiol-containing solution was evident from the reduction in storage modulus. HGs encapsulated with fluorescent dye-labeled dextran polymers and bovine serum albumin were fabricated, and their cargo release was investigated under passive and active conditions upon exposure to DTT. One can envision that the rapid gelation and fast on-demand dissolution under relatively benign conditions would make these polymeric materials attractive for a range of biomedical applications.

Development of a nanocapsule-loaded hydrogel for drug delivery for intraperitoneal administration

Intraperitoneal (IP) drug delivery of chemotherapeutic agents, administered through hyperthermal intraperitoneal chemotherapy (HIPEC) and pressurized intraperitoneal aerosolized chemotherapy (PIPAC), is effective for the treatment of peritoneal malignancies. However, these therapeutic interventions are cumbersome in terms of surgical practice and are often associated with the formation of peritoneal adhesions, due to the catheters inserted into the peritoneal cavity during these procedures. Hence, there is a need for the development of drug delivery systems that can be administered into the peritoneal cavity. In this study, we have developed a nanocapsule (NCs)-loaded hydrogel for drug delivery in the peritoneal cavity. The hydrogel has been developed using poly(ethylene glycol) (PEG) and thiol-maleimide chemistry. NCs-loaded hydrogels were characterized by rheology and their resistance to dilution and drug release were determined in vitro. Using IVIS® to measure individual organ and recovered gel fluorescence intensity, an in vivo imaging study was performed and demonstrated that NCs incorporated in the PEG gel were retained in the IP cavity for 24 h after IP administration. NCs-loaded PEG gels could find potential applications as biodegradable, drug delivery systems that could be implanted in the IP cavity, for example at a the tumour resection site to prevent recurrence of microscopic tumours.

A KPV-binding double-network hydrogel restores gut mucosal barrier in an inflamed colon

Ulcerative colitis (UC) usually occurs in the superficial mucosa of the colorectum. Here, a double-network hydrogel (PMSP) was constructed from maleimided γ-polyglutamic acid and thiolated γ-polyglutamic acid through crosslinking of thiol-maleimide and self-oxidized thiols. PMSP with a negative charge specifically adhered to the inflamed mucosa with positively charged proteins rather than to the healthy mucosa. PMSP exhibited good mechanical strength with storage modulus (G') of 17.6 Pa and a linear viscoelastic region (LVR) of 107.2% strain. Moreover, PMSP showed a stronger bio-adhesive force toward the inflamed tissue-mimicking substrate than toward its healthy counterpart. In vivo imaging confirmed that PMSP specifically adhered to the inflamed colonic mucosa of rats with TNBS-induced UC. KPV (Lys-Pro-Val) as a model drug was easily captured by PMSP through electrostatic interactions, thus retaining its bioactivity for a longer time under high temperature conditions. Moreover, the alleviating effect of KPV on rats with TNBS-induced colitis was significantly improved by PMSP after intracolonic administration. The epithelial barrier of the colon also effectively recovered following PMSP-KPV treatment. PMSP-KPV also modulated the gut flora, markedly augmenting the abundance of beneficial microorganisms in gut homeostasis. The mechanism by which PMSP-KPV induces a therapeutic effect may be associated with the inhibition of oxidative stress. Conclusively, the PMSP hydrogel seems to be a promising rectal delivery system for the therapy of UC. STATEMENT OF SIGNIFICANCE: Ulcerative colitis (UC) is a chronic and relapsing disease of the gastrointestinal tract. A key therapeutic approach to treat UC is to repair the mucosal barriers. Here, a double-network hydrogel (PMSP) was constructed from maleimided and thiolated γ-polyglutamic acid through crosslinking of thiol-maleimide and self-oxidized thiols. The negatively charged PMSP specifically adhered to the inflamed colon rather than its healthy counterpart and was retained for a longer time. KPV as a model drug was easily captured by PMSP, which provided better stability to KPV when exposed to high temperature of 50 °C. The epithelial mucosal barrier of the colon was effectively recovered by the rectal administration of PMSP-KPV to rats with TNBS-induced UC. Moreover, PMSP-KPV modulated the gut flora of colitic rats, markedly augmenting the abundance of beneficial microorganisms. Conclusively, PMSP seems to be a promising rectal delivery system for UC therapy.

Carrier-free nanodrugs with efficient drug delivery and release for cancer therapy: From intrinsic physicochemical properties to external modification

The considerable development of carrier-free nanodrugs has been achieved due to their high drug-loading capability, simple preparation method, and offering "all-in-one" functional platform features. However, the native defects of carrier-free nanodrugs limit their delivery and release behavior throughout the in vivo journey, which significantly compromise the therapeutic efficacy and hinder their further development in cancer treatment. In this review, we summarized and discussed the recent strategies to enhance drug delivery and release of carrier-free nanodrugs for improved cancer therapy, including optimizing the intrinsic physicochemical properties and external modification. Finally, the corresponding challenges that carrier-free nanodrugs faced are discussed and the future perspectives for its application are presented. We hope this review will provide constructive information for the rational design of more effective carrier-free nanodrugs to advance therapeutic treatment.

Click Chemistry Hydrogels for Extrusion Bioprinting: Progress, Challenges, and Opportunities

The emergence of 3D bioprinting has allowed a variety of hydrogel-based "bioinks" to be printed in the presence of cells to create precisely defined cell-loaded 3D scaffolds in a single step for advancing tissue engineering and/or regenerative medicine. While existing bioinks based primarily on ionic cross-linking, photo-cross-linking, or thermogelation have significantly advanced the field, they offer technical limitations in terms of the mechanics, degradation rates, and the cell viabilities achievable with the printed scaffolds, particularly in terms of aiming to match the wide range of mechanics and cellular microenvironments. Click chemistry offers an appealing solution to this challenge given that proper selection of the chemistry can enable precise tuning of both the gelation rate and the degradation rate, both key to successful tissue regeneration; simultaneously, the often bio-orthogonal nature of click chemistry is beneficial to maintain high cell viabilities within the scaffolds. However, to date, relatively few examples of 3D-printed click chemistry hydrogels have been reported, mostly due to the technical challenges of controlling mixing during the printing process to generate high-fidelity prints without clogging the printer. This review aims to showcase existing cross-linking modalities, characterize the advantages and disadvantages of different click chemistries reported, highlight current examples of click chemistry hydrogel bioinks, and discuss the design of mixing strategies to enable effective 3D extrusion bioprinting of click hydrogels.

Stimuli-responsive destructible polymeric hydrogels based on irreversible covalent bond dissociation

Covalently crosslinked stimuli-destructible hydrogels with the ability of irreversible bond dissociation have attracted great attentions due to their biodegradability, stability against hydrolysis, and controlled solubility upon insertion of desired triggers.

Redox-Responsive Hydrogels with Decoupled Initial Stiffness and Degradation

Disulfide-cross-linked hydrogels have been widely used for biological applications because of their degradability in response to redox stimuli. However, degradability often depends on polymer concentration, which also influences the hydrogel mechanical properties such as the initial stiffness. Here, we describe a one-pot cross-linking approach utilizing both a thiol-ene reaction through a Michael pathway with divinyl sulfone (DVS) to form non-reducible thioether bonds and thiol oxidation promoted by ferric ethylenediaminetetraacetic acid (Fe-EDTA) to form reducible disulfide bonds. The ratio between these two bonds was modulated by varying the DVS concentration used, and the initial shear or elastic modulus and degradation rate of the hydrogels were decoupled. These gels had tunable release rates of encapsulated dextran when exposed to 10 μM glutathione. Fibroblast encapsulation results suggested good cytocompatibility of the cross-linking reactions. This work shows the potential of combining DVS and Fe-EDTA to create thiol-cross-linked hydrogels as redox-responsive drug delivery vehicles and tissue engineering scaffolds with variable degradability.

RGDSP-Decorated Hyaluronate Hydrogels Facilitate Rapid 3D Expansion of Amylase-Expressing Salivary Gland Progenitor Cells

In vitro engineering of salivary glands relies on the availability of synthetic matrices presenting essential cell-instructive signals to guide tissue growth. Here, we describe a biomimetic, hyaluronic acid (HA)-based hydrogel platform containing covalently immobilized bioactive peptides derived from perlecan domain IV (TWSKV), laminin-111 (YIGSR, IKVAV), and fibronectin (RGDSP). The HA network was established by the thiol/acrylate reaction, and bioactive peptides were conjugated to the network with high efficiency without significantly altering the mechanical property of the matrix. When encapsulated as single cells in peptide-modified HA hydrogels, human salivary gland stem/progenitor cells (hS/PCs) spontaneously organized into multicellular spheroids with close cell-cell contacts. Conjugation of RGDSP and TWSKV signals in HA gels significantly accelerated cell proliferation, with the largest spheroids observed in RGDSP-tagged gels. Peptide conjugation did not significantly alter the expression of acinar (AMY1), ductal (TFCP2L1), and progenitor (KRT14) markers at the mRNA level. Characterization of three-dimensional (3D) cultures by immunocytochemistry showed positive staining for keratin-5 (K5), keratin-14 (K14), integrin-β1, and α-amylase under all culture conditions, confirming the maintenance of the secretory progenitor cell population. Two-dimensional (2D) adhesion studies revealed that integrin-β1 played a key role in facilitating cell-matrix interaction in gels with RGDSP, IKVAV, and TWSKV signals. Overall, conjugation of the RGDSP peptide to HA gels improved cell viability, accelerated the formation of epithelial spheroids, and promoted the expansion of the progenitor cell population in 3D. This work represents an essential first step toward the development of an engineered salivary gland.

Regulating the Homogeneity of Thiol-Maleimide Michael-Type Addition-Based Hydrogels Using Amino Biomolecules

Poly(ethylene glycol) (PEG)-based synthetic hydrogels based on Michael-type addition reaction have been widely used for cell culture and tissue engineering. However, recent studies showed that these types of hydrogels were not homogenous as expected since micro domains generated due to the fast reaction kinetics. Here, we demonstrated a new kind of method to prepare homogenous poly(ethylene glycol) hydrogels based on Michael-type addition using the side chain amine-contained short peptides. By introducing such a kind of short peptides, the homogeneity of crosslinking and mechanical property of the hydrogels has been also significantly enhanced. The compressive mechanical and recovery properties of the homogeneous hydrogels prepared in the presence of side chain amine-contained short peptides were more reliable than those of inhomogeneous hydrogels while the excellent biocompatibility remained unchanged. Furthermore, the reaction rate and gelation kinetics of maleimide- and thiol-terminated PEG were proved to be significantly slowed down in the presence of the side chain amine-contained short peptides, thus leading to the improved homogeneity of the hydrogels. We anticipate that this new method can be widely applied to hydrogel preparation and modification based on Michael-type addition gelation.

Long-term stability, high strength, and 3D printable alginate hydrogel for cartilage tissue engineering application

Cartilage damage is one of the main causes of disability, and 3D bioprinting technology can produce complex structures that are particularly suitable for constructing a customized and irregular tissue engineering scaffold for cartilage repair. Alginate is an attractive biomaterial for bioinks because of its good biological safety profile and fast ionic gelation. However, ionically crosslinked alginate hydrogels are recognized as lacking enough mechanical property and long-term stability due to ion exchange. Here, we developed a double crosslinked alginate (DC-Alg) hydrogel for 3D bioprinting, and human umbilical cord mesenchymal stem cells (huMSCs) could differentiate into chondrocytes on its printed 3D scaffold after 4 weeks' culture. We performed sequential modification of alginate with L-cysteine and 5-norbornene-2-methylamine, and the DC-Alg hydrogels were obtained in the presence of CaCl₂and ultraviolet light with stronger mechanical properties than those of the single ionic crosslinked alginate hydrogels, which was similar to natural cartilage. They also had better stability and could be maintained in DMEM medium for over 1 month, as well good viability for huMSCs. Moreover, the DC-Alg as 3D printing inks demonstrated a better printing accuracy (∼200 µm). After 4 weeks culture of huMSCs in the 3D printed DC-Alg scaffolds, the expressions of chondrogenic genes such asaggrecan (agg), collagen II (col II), and SRY-box transcription factor9(sox-9) were obviously observed, indicating the differentiation of huMSCs into cartilage. Immumohistochemical staining analysis further exhibited cartilage tissue developed well in the 3D printed scaffolds. Our study is the first demonstration of DC-Alg in 3D printing for MSC differentiation into cartilage, which shows a potential application in cartilage defect repair.

Novel Multifunctional Silver Nanocomposite Serves as a Resistance-Reversal Agent to Synergistically Combat Carbapenem-Resistant Acinetobacter baumannii

In the face of the abundant production of various types of carbapenemases, the antibacterial efficiency of imipenem, seen as "the last line of defense", is weakening. Following, the incidence of carbapenem-resistant Acinetobacter baumannii (CRAB), which can generate antibiotic-resistant biofilms, is increasing. Based on the superior antimicrobial activity of silver nanoparticles against multifarious bacterial strains compared with common antibiotics, we constructed the IPM@AgNPs-PEG-NOTA nanocomposite (silver nanoparticles were coated with SH-PEG-NOTA as well as loaded by imipenem) whose core was a silver nanoparticle to address the current challenge, and IPM@AgNPs-PEG-NOTA was able to function as a novel smart pH-sensitive nanodrug system. Synergistic bactericidal effects of silver nanoparticles and imipenem as well as drug-resistance reversal via protection of the β-ring of carbapenem due to AgNPs-PEG-NOTA were observed; thus, this nanocomposite confers multiple advantages for efficient antibacterial activity. Additionally, IPM@AgNPs-PEG-NOTA not only offers immune regulation and accelerates tissue repair to improve therapeutic efficacy in vivo but also can prevent the interaction of pathogens and hosts. Compared with free imipenem or silver nanoparticles, this platform significantly enhanced antibacterial efficiency while increasing reactive oxygen species (ROS) production and membrane damage, as well as affecting cell wall formation and metabolic pathways. According to the results of crystal violet staining, LIVE/DEAD backlight bacterial viability staining, and real-time quantitative polymerase chain reaction (RT-qPCR), this silver nanocomposite downregulated the levels of ompA expression to prevent formation of biofilms. In summary, this research demonstrated that the IPM@AgNPs-PEG-NOTA nanocomposite is a promising antibacterial agent of security, pH sensitivity, and high efficiency in reversing resistance and synergistically combatting carbapenem-resistant A. baumannii. In the future, various embellishments and selected loads for silver nanoparticles will be the focus of research in the domains of medicine and nanotechnology.

Controlling the Morphology of Dynamic Thia-Michael Networks to Target Pressure-Sensitive and Hot Melt Adhesives

A series of multistage (pressure-sensitive/hot melt) adhesives utilizing dynamic thia-Michael bonding motifs are reported. The benzalcyanoacetate Michael acceptors used in this work undergo bond exchange under ambient conditions without external catalysis, facilitating pressure-sensitive adhesion. A key feature of this system is the dynamic reaction-induced phase separation that lends reinforcement to the otherwise weakly bonded materials, enabling weak, repeatable pressure-sensitive adhesion under ambient conditions and strong adhesion when processed as a hot melt adhesive. By using different pairs of benzalcyanoacetate cross-linking units, the phase separation characteristics of the adhesives can be directly manipulated, allowing for a tailored adhesive response.

Thiol-Rich Multifunctional Macromolecular Crosslinker for Gelatin-Norbornene-Based Bioprinting

Extrusion-based bioprinting is an emerging and most frequently used technique for the fabrication of cell-laden constructs. A suitable hydrogel-based bioink for cell encapsulation and protection is critical for printability, structural stability, and post-printing cell viability. The thiol-ene chemistry-based gelatin-norbornene (GelNB) hydrogels have drawn much attention as a promising substitution of gelatin methacryloyl (GelMA), owing to the fast and controllable step-growth polymerization mechanism, as well as a significant reduction in reactive oxygen species (ROS) accumulation. Herein, thiolated heparin (HepSH) was synthesized and used as a macromolecular crosslinker for GelNB-based bioprinting, so that GelNB gelation became less sensitive to the thiol/ene ratio. The mechanical stability and moduli of GelNB/HepSH hydrogels were easily manipulated by the concentration and/or degree of thiol substitution. The GelNB/HepSH hydrogel allowed little intracellular ROS for encapsulated cells but provided vascular endothelial growth factor binding affinity for potential facilitation of neovascularization. Finally, the GelNB/HepSH bioink enabled a convenient printing process for both complex-structured bioscaffolds and cell-laden constructs, and resulted in good printability and high post-crosslinking cell viability. The crosslinker HepSH may serve as a multifunctional macromolecule that enables GelNB-based bioprinting in broad applications in regenerative medicine.

Multi-stimuli-responsive, liposome-crosslinked poly(ethylene glycol) hydrogels for drug delivery

The development of hybrid hydrogels has been of great interest over recent decades, especially in the field of biomaterials. Such hydrogels provide various opportunities in tissue engineering, drug delivery, and regenerative medicine due to their ability to mimic cellular environments, sequester and release therapeutic agents, and respond to stimuli. Herein we report the synthesis and characterization of an injectable poly(ethylene glycol) hydrogel crosslinked via thiol-maleimide reactions and containing both chemically crosslinked temperature-sensitive liposomes (TSLs) and matrix metalloproteinase-sensitive peptide crosslinks. Rheological studies demonstrate that the hydrogel is mechanically stable and can be synthesized to achieve a range of physically applicable moduli. Experiments characterizing the in situ drug delivery and degradation of these materials indicate that the TSL gel responds to both thermal and enzymatic stimuli in a local environment. Doxorubicin, a widely used anticancer drug, was loaded in the TSLs with a high encapsulation efficiency and the subsequent release was temperature dependent. Finally, TSLs did not compromise viability and proliferation of human and murine fibroblasts, supporting the use of these hydrogel-linked liposomes as a thermo-responsive drug carrier for controlled release.

Cyclic Thiosulfinates as a Novel Class of Disulfide Cleavable Cross-Linkers for Rapid Hydrogel Synthesis

We recently reported that cyclic thiosulfinates are cysteine selective cross-linkers that avoid the "dead-end" modifications that contribute to other cross-linkers' toxicity. In this study, we generalize the chemistry of cyclic thiosulfinates to that of thiol selective cross-linking and apply them to the synthesis of hydrogels. Thiol-functionalized four-arm poly(ethylene glycol) and hyaluronic acid monomers were cross-linked with 1,2-dithiane-1-oxide to form disulfide cross-linked hydrogels within seconds. The synthesized hydrogel could be reduced with physiological concentrations of glutathione, which modulated hydrogel mechanical properties and degradation kinetics. Bovine serum albumin protein was successfully encapsulated in hydrogel, and diffusion-mediated release was demonstrated in vitro. Hep G2 cells grew in the presence of preformed hydrogel and during hydrogel synthesis, demonstrating acceptable cytotoxicity. We encapsulated cells within a hydrogel and demonstrated cell growth and recovery up to 10 days, with and without cell adhesion peptides. In summary, we report cyclic thiosulfinates as a novel class of cross-linkers for the facile synthesis of biodegradable hydrogels.

Advanced materials for drug delivery across mucosal barriers

Mucus is a viscoelastic gel that traps pathogens and other foreign particles to limit their penetration into the underlying epithelium. Dosage forms containing particle-based drug delivery systems are trapped in mucosal layers and will be removed by mucus turnover. Mucoadhesion avoids premature wash-off and prolongs the residence time of drugs on mucus. Moreover, mucus penetration is essential for molecules to access the underlying epithelial tissues. Various strategies have been investigated to achieve mucoadhesion and mucus penetration of drug carriers. Innovations in materials used for the construction of drug-carrier systems allowed the development of different mucoadhesion and mucus penetration delivery systems. Over the last decade, advances in the field of materials chemistry, with a focus on biocompatibility, have led to the expansion of the pool of materials available for drug delivery applications. The choice of materials in mucosal delivery is generally dependent on the intended therapeutic target and nature of the mucosa at the site of absorption. This review presents an up-to-date account of materials including synthesis, physical and chemical modifications of mucoadhesive materials, nanocarriers, viral mimics used for the construction of mucosal drug delivery systems.

Progress, challenges and future directions of heterocycles as building blocks in iterative methodologies towards sequence-defined oligomers and polymers

Heterocyclic building blocks for iterative methodologies leading to sequence-defined oligomers and polymers are reviewed. Solid- as well as solution-phase methods, challenges surrounding these systems and potential future directions are presented.

On-Demand Dissoluble Diselenide-Containing Hydrogel

On-demand dissolution of hydrogels is being increasingly studied for their potential use in burn wound dressing applications. Herein, a dynamic diselenide-containing hydrogel is developed through a very simple one-pot and two-step process starting from the selenol functionalization of a partially hydrolyzed poly(2-ethyl-2-oxazoline) with γ-butyroselenolactone. The hydrogel spontaneously cross-links via an in situ oxidation of the selenol functionalities in air. The gelation process and the final viscoelastic properties of the gel are characterized by rheological experiments. The mechanical properties of those new diselenide-containing hydrogels are easily tuned by varying the concentration of γ-butyroselenolactone. The materials also show good skin adhesion and UV light responsiveness. A unique feature of the hydrogel is its capability to be fully and rapidly dissolved on-demand, via oxidation or reduction of the diselenide cross-links, making them particularly attractive for burn wound dressing applications.

Tumor microenvironment responsive drug delivery systems

Conventional tumor-targeted drug delivery systems (DDSs) face challenges, such as unsatisfied systemic circulation, low targeting efficiency, poor tumoral penetration, and uncontrolled drug release. Recently, tumor cellular molecules-triggered DDSs have aroused great interests in addressing such dilemmas. With the introduction of several additional functionalities, the properties of these smart DDSs including size, surface charge and ligand exposure can response to different tumor microenvironments for a more efficient tumor targeting, and eventually achieve desired drug release for an optimized therapeutic efficiency. This review highlights the recent research progresses on smart tumor environment responsive drug delivery systems for targeted drug delivery. Dynamic targeting strategies and functional moieties sensitive to a variety of tumor cellular stimuli, including pH, glutathione, adenosine-triphosphate, reactive oxygen species, enzyme and inflammatory factors are summarized. Special emphasis of this review is placed on their responsive mechanisms, drug loading models, drawbacks and merits. Several typical multi-stimuli responsive DDSs are listed. And the main challenges and potential future development are discussed.

Pyrocinchonimides Conjugate to Amine Groups on Proteins via Imide Transfer

Advances in bioconjugation, the ability to link biomolecules to each other, small molecules, surfaces, and more, can spur the development of advanced materials and therapeutics. We have discovered that pyrocinchonimide, the dimethylated analogue of maleimide, undergoes a surprising transformation with biomolecules. The reaction targets amines and involves an imide transfer, which has not been previously reported for bioconjugation purposes. Despite their similarity to maleimides, pyrocinchonimides do not react with free thiols. Though both lysine residues and the N-termini of proteins can receive the transferred imide, the reaction also exhibits a marked preference for certain amines that cannot solely be ascribed to solvent accessibility. This property is peculiar among amine-targeting reactions and can reduce combinatorial diversity when many available reactive amines are available, such as in the formation of antibody-drug conjugates. Unlike amides, the modification undergoes very slow reversion under high pH conditions. The reaction offers a thermodynamically controlled route to single or multiple modifications of proteins for a wide range of applications.

A programmable chemical switch based on triggerable Michael acceptors

Developing an engineerable chemical reaction that is triggerable for simultaneous chemical bond formation and cleavage by external cues offers tunability and orthogonality which is highly desired in many biological and materials applications. Here, we present a chemical switch that concurrently captures these features in response to chemically and biologically abundant and important cues, viz., thiols and amines. This thiol/amine-triggerable chemical switch is based on a Triggerable Michael Acceptor (TMAc) which bears good leaving groups at its β-position. The acceptor undergoes a "trigger-to-release" process where thiol/amine addition triggers cascaded release of leaving groups and generates a less activated acceptor. The newly generated TMAc can be further reversed to liberate the original thiol/amine by a second nucleophile trigger through a "trigger-to-reverse" process. Within the small molecular volume of the switch, we have shown five locations that can be engineered to achieve tunable "trigger-to-release" kinetics and tailored reversibility. The potential of the engineerable bonding/debonding capability of the chemical switch is demonstrated by applications in cysteine-selective and reversible protein modification, universal self-immolative linkers, and orthogonally addressable hydrogels.

Use of pyridazinediones as extracellular cleavable linkers through reversible cysteine conjugation

Herein we report a retro-Michael deconjugation pathway of thiol-pyridazinedione linked protein bioconjugates to provide a novel cleavable linker technology. We demonstrate that the novel pyridazinedione linker does not suffer from off-target modification with blood thiols (e.g., glutathione, human serum albumin (HSA)), which is in sharp contrast to an analogous maleimide linker.

Smart nanomedicine agents for cancer, triggered by pH, glutathione, H2O2, or H2S

Effective tumor diagnosis and therapy have always been a significant but challenging issue. Although nanomedicine has shown great potential for improving the outcomes of tumor diagnosis and therapy, the nonspecial targeted distribution of nanomedicine agents in the whole body causes a low diagnosis signal-to-noise ratio and a potential risk of systemic toxicity. Recently, the development of smart nanomedicine agents with diagnosis and therapy functions that can only be activated by the tumor microenvironment (TME) is regarded as an effective strategy to improve the theranostic sensitivity and selectivity, as well as reduce the potential side effects during treatment. This article will introduce and summarize the latest achievements in the design and fabrication of TME-responsive smart nanomedicine agents, and highlight their prospects for enhancing tumor diagnosis and therapy.

Hydrogels for sustained delivery of biologics to the back of the eye

Hydrogels are water-laden polymer networks that have been used for myriad biological applications. By controlling the chemistry through which a hydrogel is constructed, a wide range of chemical and physical properties can be accessed, making them an attractive class of biomaterials. In this review, we cover the application of hydrogels for sustained delivery of biologics to the back of the eye. In adapting hydrogels to this purpose, success is dependent on careful consideration of material properties, route of administration, means of injection, and control of drug efflux, all of which are addressed. We also provide a perspective on clinical and chemistry, manufacturing and controls (CMC) considerations that are integral to the development of an ocular hydrogel delivery system.

[2]Rotaxane End-Capping Synthesis by Click Michael-Type Addition to the Vinyl Sulfonyl Group

We report the application of the click Michael-type addition reaction to vinyl sulfone or vinyl sulfonate groups in the synthesis of rotaxanes through the threading-and-capping method. This methodology has proven to be efficient and versatile as it allowed the preparation of rotaxanes using template approaches based on different noncovalent interactions (i.e., donor-acceptor π-π interactions or hydrogen bonding) in yields of generally 60-80 % and up to 91 % aided by the mild conditions required (room temperature or 0 °C and a mild base such as Et₃ N or 4-(N,N-dimethylamino)pyridine (DMAP)). Furthermore, the use of vinyl sulfonate moieties, which are suitable motifs for coupling-and-decoupling (CAD) chemistry, implies another advantage because it allows the controlled chemical disassembly of the rotaxanes into their components through nucleophilic substitution of the sulfonates resulting from the capping step with a thiol under mild conditions (Cs₂ CO₃ and room temperature).