Branched polyrotaxane hydrogels consisting of alpha-cyclodextrin and low-molecular-weight four-arm polyethylene glycol and the utility of their thixotropic property for controlled drug release

A quadruple-stimuli responsive supramolecular hydrogel constructed from a poly(acrylic acid) derivative and β-cyclodextrin dimer

The fabrication of stimulus-responsive supramolecular hydrogels as smart materials has attracted much attention in recent years. However, the multi-stimuli responsiveness often requires complicated chemical synthesis and rational molecular design. Herein, a quadruple-stimuli responsive supramolecular hydrogel was designed through the host-guest interaction between a β-CD dimer and a methoxy-azobenzene (mAzo) and ferrocene (Fc) grafted poly(acrylic acid) derivative, as well as through the electrostatic interaction of negatively charged carboxyl side groups. Owing to the dynamic properties of the host-guest and electrostatic interactions, reversible sol-gel transition can be triggered by various stimuli, including temperature, light irradiations, pH changes and chemical redox reagents. As a result, the release of rhodamine B loaded in the hydrogel can be accelerated by green light irradiation, oxidizing agents and low pH, demonstrating potential applications in biomedical materials.

Photo-Controlled Reversible Uptake and Release of a Modified Sulfamethoxazole Antibiotic Drug from a Pillar[5]arene Cross-Linked Gelatin Hydrogel

Pillararene cross-linked gelatin hydrogels were designed and synthesized to control the uptake and release of antibiotics using light. A suite of characterization techniques ranging from spectroscopy (FT-IR, 1H and 13C NMR, and MAS NMR), X-ray crystallographic analysis, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) was employed to investigate the physicochemical properties of hydrogels. The azobenzene-modified sulfamethoxazole (Azo-SMX) antibiotic was noncovalently incorporated into the hydrogel via supramolecular host-guest interactions to afford the A-hydrogel. While in its ground state, the Azo-SMX guest has a trans configuration structure and forms a thermodynamically stable inclusion complex with the pillar[5]arene motif in the hydrogel matrix. When the A-hydrogel was exposed to 365 nm UV light, Azo-SMX underwent a photoisomerization reaction. This changed the structure of Azo-SMX from trans to cis, and the material was released into the environment. The Azo-SMX released from the hydrogel was effective against both Gram-positive and Gram-negative bacteria. Importantly, the A-hydrogel exhibited a striking difference in antibacterial activity when applied to bacterial colonies in the presence and absence of UV light, highlighting the switchable antibacterial activity of A-hydrogel aided by light. In addition, all hydrogels containing pillar[5]arenes have demonstrated biocompatibility and effectiveness as scaffolds for biological and medical purposes.

An injectable hydrogel based on hyaluronic acid prepared by Schiff base for long-term controlled drug release

Drug-loaded injectable hydrogels have been studied widely in biomedical technology while the stable long-term controlled drug release and cytotoxicity are challenges. In this work, an injectable hydrogel with good swelling resistance was in situ synthetized using aminated hyaluronic acid (NHA) and aldehyde β-cyclodextrin (ACD) via Schiff-base reaction. The composition, morphology and mechanical property were characterized with FTIR, ¹³C NMR, SEM and rheology test, respectively. Voriconazole (VCZ) and Endophthalmitis was selected as a model drug and disease, respectively. The drug release, cytotoxicity and antifungal properties were detected in vitro. The results showed a long-term (> 60 days) drug release was realized, the NHA/ACD2/VCZ presented a zero-order release in the later stage. The cytotoxicity of NHA/ACD was detected by live/dead staining assay and Cell Counting Kit-8 (CCK-8). The survival rate of adult retina pigment epithelial cell line-19 (ARPE-19) was over 100 % after 3 d, it indicated a good cytocompatibility. The antifungal experiment presented samples had antifungal property. Biocompatibility in vivo proved NHA/ACD2 had no adverse effects on ocular tissues. Consequently, the injectable hydrogel based on hyaluronic acid prepared by Schiff base reaction provides a new option for long-term controlled drug release in the course of disease treatment from a material perspective.

The Role of Cyclodextrin in the Construction of Nanoplatforms: From Structure, Function and Application Perspectives

Cyclodextrins (CyDs) in nano drug delivery systems have received much attention in pursuit of good compatibility, negligible toxicity, and improved pharmacokinetics of drugs. Their unique internal cavity has widened the application of CyDs in drug delivery based on its advantages. Besides this, the polyhydroxy structure has further extended the functions of CyDs by inter- and intramolecular interactions and chemical modification. Furthermore, the versatile functions of the complex contribute to alteration of the physicochemical characteristics of the drugs, significant therapeutic promise, a stimulus-responsive switch, a self-assembly capability, and fiber formation. This review attempts to list recent interesting strategies regarding CyDs and discusses their roles in nanoplatforms, and may act as a guideline for developing novel nanoplatforms. Future perspectives on the construction of CyD-based nanoplatforms are also discussed at the end of this review, which may provide possible direction for the construction of more rational and cost-effective delivery vehicles.

Injectable Thermoresponsive Hydrogels for Cancer Therapy: Challenges and Prospects

The enervating side effects of chemotherapeutic drugs have necessitated the use of targeted drug delivery in cancer therapy. To that end, thermoresponsive hydrogels have been employed to improve the accumulation and maintenance of drug release at the tumour site. Despite their efficiency, very few thermoresponsive hydrogel-based drugs have undergone clinical trials, and even fewer have received FDA approval for cancer treatment. This review discusses the challenges of designing thermoresponsive hydrogels for cancer treatment and offers suggestions for these challenges as available in the literature. Furthermore, the argument for drug accumulation is challenged by the revelation of structural and functional barriers in tumours that may not support targeted drug release from hydrogels. Other highlights involve the demanding preparation process of thermoresponsive hydrogels, which often involves poor drug loading and difficulties in controlling the lower critical solution temperature and gelation kinetics. Additionally, the shortcomings in the administration process of thermosensitive hydrogels are examined, and special insight into the injectable thermosensitive hydrogels that reached clinical trials for cancer treatment is provided.

Advanced supramolecular design for direct ink writing of soft materials

The exciting advancements in 3D-printing of soft materials are changing the landscape of materials development and fabrication. Among various 3D-printers that are designed for soft materials fabrication, the direct ink writing (DIW) system is particularly attractive for chemists and materials scientists due to the mild fabrication conditions, compatibility with a wide range of organic and inorganic materials, and the ease of multi-materials 3D-printing. Inks for DIW need to possess suitable viscoelastic properties to allow for smooth extrusion and be self-supportive after printing, but molecularly facilitating 3D printability to functional materials remains nontrivial. While supramolecular binding motifs have been increasingly used for 3D-printing, these inks are largely optimized empirically for DIW. Hence, this review aims to establish a clear connection between the molecular understanding of the supramolecularly bound motifs and their viscoelastic properties at bulk. Herein, extrudable (but not self-supportive) and 3D-printable (self-supportive) polymeric materials that utilize noncovalent interactions, including hydrogen bonding, host-guest inclusion, metal-ligand coordination, micro-crystallization, and van der Waals interaction, have been discussed in detail. In particular, the rheological distinctions between extrudable and 3D-printable inks have been discussed from a supramolecular design perspective. Examples shown in this review also highlight the exciting macroscale functions amplified from the molecular design. Challenges associated with the hierarchical control and characterization of supramolecularly designed DIW inks are also outlined. The perspective of utilizing supramolecular binding motifs in soft materials DIW printing has been discussed. This review serves to connect researchers across disciplines to develop innovative solutions that connect top-down 3D-printing and bottom-up supramolecular design to accelerate the development of 3D-print soft materials for a sustainable future.

Advances in innovative delivery systems for antiglaucoma drugs

PURPOSE OF REVIEW

Currently, glaucoma treatment drugs are facing problems such as low bioavailability, poor patient compliance, discontinuous administration affecting the efficacy of intraocular pressure (IOP) lowering and chronic damage to the eye caused by side effects of drugs. In order to solve these problems and to better meet clinical needs, various new dosage forms have been developed and applied in the clinical setting.

RECENT FINDINGS

A number of nano formulations and extended-release gels are in successive animal trials, some tear plugs, implants and contact lenses are in clinical trials, and it is believed that more new carrier materials and formulations to improve the bioavailability of drugs are being developed.

SUMMARY

Novel delivery systems for antiglaucoma drugs offer patients more and better therapeutic options, and ongoing or completed studies are providing clear directions for subsequent research to improve clinical applications.

Glaucoma Treatment and Hydrogel: Current Insights and State of the Art

Aqueous gels formulated using hydrophilic polymers (hydrogels) and those based on stimuli-responsive polymers (in situ gelling or gel-forming systems) attract increasing interest in the treatment of several eye diseases. Their chemical structure enables them to incorporate various ophthalmic medications, achieving their optimal therapeutic doses and providing more clinically relevant time courses (weeks or months as opposed to hours and days), which will inevitably reduce dose frequency, thereby improving patient compliance and clinical outcomes. Due to its chronic course, the treatment of glaucoma may benefit from applying gel technologies as drug-delivering systems and as antifibrotic treatment during and after surgery. Therefore, our purpose is to review current applications of ophthalmic gelling systems with particular emphasis on glaucoma.

Recent Updates on Supramolecular-Based Drug Delivery - Macrocycles and Supramolecular Gels

Supramolecules-based drug delivery has attracted significant recent research attention as it could enhance drug solubility, retention time, targeting, and stimuli responsiveness. Among the different supramolecules and assemblies, the macrocycles and the supramolecular hydrogels are the two important categories investigated to a greater extent. Here, we provide the most recent advancements in these categories. Under macrocycles, reports on drug delivery by cyclodextrins, cucurbiturils, calixarenes/pillararenes, crown ethers and porphyrins are detailed. The second category discusses the supramolecular hydrogels of macrocycles/polymers and low molecular weight gelators. The updated information provided could be helpful to advance R & D in this vital area.

Bioinspired tunable hydrogels: An update on methods of preparation, classification, and biomedical and therapeutic applications

Hydrogels exhibit water-insoluble three-dimensional polymeric networks capable of absorbing large amounts of biological fluids. Both natural and synthetic polymers are used for the preparation of hydrogel networks. Such polymeric networks are fabricated through chemical or physical mechanisms of crosslinking. Chemical crosslinking is accomplished mainly through covalent bonding, while physical crosslinking involves self-healing secondary forces like H-bonding, host-guest interactions, and antigen-antibody interactions. The building blocks of the hydrogels play an important role in determining the mechanical, biological, and physicochemical properties. Hydrogels are used in a variety of biomedical applications like diagnostics (biodetection and bioimaging), delivery of therapeutics (drugs, immunotherapeutics, and vaccines), wound dressing and skin materials, cardiac complications, contact lenses, tissue engineering, and cell culture because of the inherent characteristics like enhanced water uptake and structural similarity with the extracellular matrix (ECM). This review highlights the recent trends and advances in the roles of hydrogels in biomedical and therapeutic applications. We also discuss the classification and methods of hydrogels preparation. A brief outlook on the future directions of hydrogels is also presented.

Advances in Injectable In Situ-Forming Hydrogels for Intratumoral Treatment

Chemotherapy has been linked to a variety of severe side effects, and the bioavailability of current chemotherapeutic agents is generally low, which decreases their effectiveness. Therefore, there is an ongoing effort to develop drug delivery systems to increase the bioavailability of these agents and minimize their side effects. Among these, intratumoral injections using in situ-forming hydrogels can improve drugs' bioavailability and minimize drugs' accumulation in non-target organs or tissues. This review describes different types of injectable in situ-forming hydrogels and their intratumoral injection for cancer treatment, after which we discuss the antitumor effects of intratumoral injection of drug-loaded hydrogels. This review concludes with perspectives on the future applicability of, and challenges for, the adoption of this drug delivery technology.

Tough and Resilient Hydrogels Enabled by a Multifunctional Initiating and Cross-Linking Agent

Many high-strength hydrogels have been developed in recent years; however, few of them are both tough and resilient, and their intrinsic paradoxical nature makes designing a gel with both high toughness and high resilience a great challenge. To address this problem, we introduced both N,N,N,N-pentamethyldiethylenetriamine (PA) and N,N-methylenebisacrylamide (MBA) into polyacrylamide hydrogel networks to construct an entangled network that contains chemically cross-linked chains and branched chains simultaneously. The entanglements of branched chains can act as a physical cross-linking point to uniformly disperse stress on molecular chains, and chemical cross-linking ensures the stability of the hydrogel network. The increase in the number and length of branched chains is able to achieve an enhancement in strength while the slip of the entangled polymer chains can effectively achieve energy dissipation and can improve the toughness of the gel. Moreover, the resultant hydrogels exhibit an excellent resilience (>98%). Therefore, high toughness and resilience are achieved simultaneously. In addition, we also investigated the initiation mechanism of PA. This strategy creates a new way for the preparation of next-generation high toughness and high resilience hydrogel-based materials, which have promising applications in wearable, flexible strain/pressure sensors.

Toward a Greener World-Cyclodextrin Derivatization by Mechanochemistry

Cyclodextrin (CD) derivatives are a challenge, mainly due to solubility problems. In many cases, the synthesis of CD derivatives requires high-boiling solvents, whereas the product isolation from the aqueous methods often requires energy-intensive processes. Complex formation faces similar challenges in that it involves interacting materials with conflicting properties. However, many authors also refer to the formation of non-covalent bonds, such as the formation of inclusion complexes or metal–organic networks, as reactions or synthesis, which makes it difficult to classify the technical papers. In many cases, the solubility of both the starting material and the product in the same solvent differs significantly. The sweetest point of mechanochemistry is the reduced demand or complete elimination of solvents from the synthesis. The lack of solvents can make syntheses more economical and greener. The limited molecular movements in solid-state allow the preparation of CD derivatives, which are difficult to produce under solvent reaction conditions. A mechanochemical reaction generally has a higher reagent utilization rate. When the reaction yields a good guest co-product, solvent-free conditions can be slower than in solution conditions. Regioselective syntheses of per-6-amino and alkylthio-CD derivatives or insoluble cyclodextrin polymers and nanosponges are good examples of what a greener technology can offer through solvent-free reaction conditions. In the case of thiolated CD derivatives, the absence of solvents results in significant suppression of the thiol group oxidation, too. The insoluble polymer synthesis is also more efficient when using the same molar ratio of the reagents as the solution reaction. Solid reactants not only reduce the chance of hydrolysis of multifunctional reactants or side reactions, but the spatial proximity of macrocycles also reduces the length of the spacing formed by the crosslinker. The structure of insoluble polymers of the mechanochemical reactions generally is more compact, with fewer and shorter hydrophilic arms than the products of the solution reactions.

PEG-α-CD/AM/liposome @amoxicillin double network hydrogel wound dressing-Multiple barriers for long-term drug release

Wound infection and poor wound healing are the major challenges of wound treatment. Antibiotic drug treatment is the effective way to inhibit wound infection. It is necessary to achieve sustained release of antibiotics to get a longer treatment for wound infection. The double network hydrogels based on liposome, polyethylene glycol (PEG), α- cyclodextrin (α-CD) and acrylamide (AM) were developed, in which liposome acts as amoxicillin repository. Because the drug would release from the multiple barriers including two cavities of liposome and α-CD, as well as polyethylene glycol -α- cyclodextrin/acrylamide (PEG-CD/AM) double network, the PEG-α-CD/AM/liposome @amoxicillin double network hydrogels could achieve sustained drug release. The drug release assay showed that the dressing could release amoxicillin continuously until 12 days, than that of 8th day for single-network hydrogel releasing. The antibacterial ratio of the hydrogel could reach above 80%. What's more, the hydrogels present adjustable mechanical strength by changing the ratio of the components. The swelling ratio proved that the hydrogel had potential ability to absorb wound exudates. The cytotoxicity test of the hydrogels demonstrated excellent biocompatibility. These results indicated that this study can provide a new thought for antibacterial wound dressing and has a broad application prospect.

"Double-punch" strategy for delivery of viral immunotherapy with prolonged tumor retention and enhanced transfection efficacy

Viral immunotherapy has shown clinical efficacy in treating cancers (e.g., melanoma). Given that viral immunotherapy commonly uses intratumoral injection, prolonging the duration of therapeutic virus at the tumor site can further enhance the antitumor efficacy and reduce potential off-target effects. In this work, we describe a "double-punch" strategy by combining dendrimer platform and injectable hydrogel encapsulation for delivery of an adenovirus encoding Flagrp170 (Adv-Flagrp170), which has been shown to effectively mount a cytotoxic T lymphocyte response through enhanced tumor immunogenicity and optimized antigen cross-presentation. We first complexed PAMAM generation 4 (G4) with Adv (G4/Adv) to strengthen its transfection efficiency and then loaded G4/Adv into a biocompatible and injectable supramolecular hydrogel (SH) made of α-cyclodextrin and 4-arm polyethylene glycol via host-guest interaction. When tested in a murine melanoma model, the G4/Adv complex was shown to have improved retention at the tumor site. The presence of SH facilitated the targeted gene expression in tumor-infiltrating leukocytes, including antigen-presenting dendritic cells. Delivery of Adv-Flagrp170 by both G4 coating and SH encapsulation significantly enhanced its therapeutic efficacy in controlling mouse melanoma (8-fold reduction in tumor volume), which is associated with increased immune activation in the tumor microenvironment as well as decreased adenovirus-reactive antibodies. Taken together, this new formulation may be used to improve the treatment outcome of adenovirus-based cancer immunotherapy.

Drug-loaded chitosan film prepared via facile solution casting and air-drying of plain water-based chitosan solution for ocular drug delivery

Chitosan is a nature-based polymer with low toxicity, excellent biocompatibility and biodegradability. However, the intractable solubility of chitosan in water and most conventional solvents hampers its biomedical applications. Following the dissolution method for dissolving chitosan in plain water developed by us, chitosan was dissolved in ionic liquid followed by overnight freezing at -20 °C and subsequent solvent exchange with plain water at room temperature. In this study, we fabricated a drug-carrying chitosan film via solution casting and air-drying by using the plain water-based chitosan solution. Specifically, brimonidine tartrate (BT), an antiglaucoma drug, was dissolved in the plain-water based solution and used to prepare BT-loaded chitosan film, i.e., chitosan-BT film. The resulting film is transparent, structurally stable, and mucoadhesive. Micro-sized antiglaucoma BT drug crystals form and are well dispersed in the chitosan film. The chitosan-BT film enables BT to have a high corneal permeability with fast drug release kinetics for potential ocular drug delivery.

A tumor-to-lymph procedure navigated versatile gel system for combinatorial therapy against tumor recurrence and metastasis

Application of cancer vaccines is limited due to their systemic immunotoxicity and inability to satisfy all the steps, including loading of tumor antigens, draining of antigens to lymph nodes (LNs), internalization of antigens by dendritic cells (DCs), DC maturation, and cross-presentation of antigens for T cell activation. Here, we present a combinatorial therapy, based on a α-cyclodextrin (CD)-based gel system, DOX/ICG/CpG-P-ss-M/CD, fabricated by encapsulating doxorubicin (DOX) and the photothermal agent indocyanine green (ICG). Upon irradiation, the gel system exhibited heat-responsive release of DOX and vaccine-like nanoparticles, CpG-P-ss-M, along with chemotherapy- and phototherapy-generated abundant tumor-specific antigen storage in situ. The released CpG-P-ss-M acted as a carrier adsorbed and delivered antigens to LNs, promoting the uptake of antigens by DCs and DC maturation. Notably, combined with PD-L1 blocking, the therapy effectively inhibited primary tumor growth and induced tumor-specific immune response against tumor recurrence and metastasis.

Biocompatible Polymers Combined with Cyclodextrins: Fascinating Materials for Drug Delivery Applications

Cyclodextrins (CD) are a group of cyclic oligosaccharides with a cavity/specific structure that enables to form inclusion complexes (IC) with a variety of molecules through non-covalent host-guest interactions. By an elegant combination of CD with biocompatible, synthetic and natural polymers, different types of universal drug delivery systems with dynamic/reversible properties have been generated. This review presents the design of nano- and micro-carriers, hydrogels, and fibres based on the polymer/CD supramolecular systems highlighting their possible biomedical applications. Application of the most prominent hydrophobic aliphatic polyesters that exhibit biodegradability, represented by polylactide and polycaprolactone, is described first. Subsequently, particular attention is focused on materials obtained from hydrophilic polyethylene oxide. Moreover, examples are also presented for grafting of CD on polysaccharides. In summary, we show the application of host-guest interactions in multi-component functional biomaterials for controlled drug delivery.

α-Cyclodextrin-Based Polypseudorotaxane Hydrogels

Supramolecular hydrogels that are based on inclusion complexes between α-cyclodextrin and (co)polymers have gained significant attention over the last decade. They are formed via dynamic noncovalent bonds, such as host-guest interactions and hydrogen bonds, between various building blocks. In contrast to typical chemical crosslinking (covalent linkages), supramolecular crosslinking is a type of physical interaction that is characterized by great flexibility and it can be used with ease to create a variety of "smart" hydrogels. Supramolecular hydrogels based on the self-assembly of polypseudorotaxanes formed by a polymer chain "guest" and α-cyclodextrin "host" are promising materials for a wide range of applications. α-cyclodextrin-based polypseudorotaxane hydrogels are an attractive platform for engineering novel functional materials due to their excellent biocompatibility, thixotropic nature, and reversible and stimuli-responsiveness properties. The aim of this review is to provide an overview of the current progress in the chemistry and methods of designing and creating α-cyclodextrin-based supramolecular polypseudorotaxane hydrogels. In the described systems, the guests are (co)polymer chains with various architectures or polymeric nanoparticles. The potential applications of such supramolecular hydrogels are also described.

Specific Modification with TPGS and Drug Loading of Cyclodextrin Polyrotaxanes and the Enhanced Antitumor Activity Study in Vitro and in Vivo

A kind of specific cyclodextrin polyrotaxanes (PRs) drug delivery system was developed for an effective drug delivery and enhancing antitumor effect. In this work, we prepared the PR by using α-CD derivatives and dicarboxyl-PEG (M_n = 4200) self-assembling and end-capping with β-CD derivatives. Then, we chose d-a-Tocopheryl polyethylene glycol 1000 succinate (TPGS) with an antitumor effect to modify the PR. The modified PRs have a certain anticancer effect and can assist the anticancer drug to treat cancer. The 10-hydroxycamptothecin (HCPT) was combined to the specific PRs by covalent bonds to prepare drug-loaded specificity PRs (PR-TPGS-HCPT). The enhanced antitumor activities of PR-TPGS-HCPT were studied by in vitro and in vivo experiments, and the experiment results proved that the TPGS could effectively assist the drug to treat cancer and prolong the lifetime of the tumor-bearing mice. Therefore, this research provides a promising drug-loaded material for the cancer treatment and the specific water-soluble PRs will have potential applications in the biomedical field.

Modification of Supramolecular Membranes with 3D Hydrophilic Slide-Rings for the Improvement of Antifouling Properties and Effective Separation

A three-dimensional (3D) strategy for the fabrication of ethylene vinyl alcohol (EVAL) membranes with a dynamic surface was developed based on sliding supramolecular polymer brushes (SSPBs). The SSPBs with a 3D hydrophilic structure were introduced into the alkyne-EVAL membrane matrix via an azide-alkyne click coupling reaction. The self-mobile hydrophilic slide-rings in the SSPB provided a proactive exclusion system. This resulted in reduced direct contact of the membrane surface with multiple pollutants such as bovine serum albumin (BSA) and oil droplets. The EVAL-SSPB membrane demonstrated increased surface hydrophilicity, underwater oleophobicity, and antifouling properties. More importantly, the abundant hydrophilic rings in the membrane matrix result in supramolecular assembly and efficient hydrophilic sliding channels. This resulted in a dramatic increase in the water flux [2000 L/(m² h)] while retaining a 96% rejection of BSA and oil/water emulsions. The results of the study indicate that three effects of the cyclodextrins rings, i.e., the hydrophilic effect, the exclusion effect, and the sliding effect, enabled the improved membrane performance. The demonstrated 3D fabrication strategy is versatile, facile, and scalable, which allows for its application to various other membranes. The fabricated materials possess excellent permeability and separation efficiencies, which make them attractive candidates for use as separation membranes with novel functions.

Hydrogel-based ocular drug delivery systems: Emerging fabrication strategies, applications, and bench-to-bedside manufacturing considerations

The physiological barriers of the eye pose challenges to the delivery of the array of therapeutics for ocular diseases. Hydrogels have been widely explored for medical applications and introduce possible solutions to overcoming the medication challenges of the ocular environment. While the innovations in drug encapsulation and release mechanisms, biocompatibility, and treatment duration have become highly sophisticated, the challenge of widespread application of hydrogel formulations in the clinic is still apparent. This article reviews the latest hydrogel formulations and their associated chemistries for use in ocular therapies, spanning from external anterior to internal posterior regions of the eye in order to evaluate the state of recent research. This article discusses the utility of hydrogels in soft contact lens, wound dressings, intraocular lens, vitreous substitutes, vitreous drug release hydrogels, and cell-based therapies for regeneration. Additional focus is placed on the pre-formulation, formulation, and manufacturing considerations of the hydrogels based on individual components (polymer chains, linkers, and therapeutics), final hydrogel product, and required preparations for clinical/commercial applications, respectively.

Advances in crosslinking strategies of biomedical hydrogels

Biomedical hydrogels as sole repair matrices or combined with pre-seeded cells and bioactive growth factors are extensively applied in tissue engineering and regenerative medicine. Hydrogels normally provide three dimensional structures for cell adhesion and proliferation or the controlled release of the loading of drugs or proteins. Various physiochemical properties of hydrogels endow them with distinct applications. In this review, we present the commonly used crosslinking method for hydrogel synthesis involving physical and chemical crosslinks and summarize their current progress and future perspectives.

New injectable two-step forming hydrogel for delivery of bioactive substances in tissue regeneration

A hydrogel based on chitosan, collagen, hydroxypropyl-γ-cyclodextrin and polyethylene glycol was developed and characterized. The incorporation of nano-hydroxyapatite and pre-encapsulated hydrophobic/hydrophilic model drugs diminished the porosity of hydrogel from 81.62 ± 2.25% to 69.98 ± 3.07%. Interactions between components of hydrogel, demonstrated by FTIR spectroscopy and rheology, generated a network that was able to trap bioactive components and delay the burst delivery. The thixotropic behavior of hydrogel provided adaptability to facilitate its implantation in a minimally invasive way. Release profiles from microspheres included or not in hydrogel revealed a two-phase behavior with a burst- and a controlled-release period. The same release rate for microspheres included or not in the hydrogel in the controlled-release period demonstrated that mass transfer process was controlled by internal diffusion. Effective diffusion coefficients, D eff, that describe internal diffusion inside microspheres, and mass transfer coefficients, h, i.e. the contribution of hydrogel to mass transfer, were determined using 'genetic algorithms', obtaining values between 2.64·10-15 and 6.67·10-15 m2/s for D eff and 8.50·10-10 to 3.04·10-9 m/s for h. The proposed model fits experimental data, obtaining an R 2-value ranged between 95.41 and 98.87%. In vitro culture of mesenchymal stem cells in hydrogel showed no manifestations of intolerance or toxicity, observing an intense proliferation of the cells after 7 days, being most of the scaffold surface occupied by living cells.

Injectable Hydrogels for Cardiac Tissue Engineering

In light of the limited efficacy of current treatments for cardiac regeneration, tissue engineering approaches have been explored for their potential to provide mechanical support to injured cardiac tissues, deliver cardio-protective molecules, and improve cell-based therapeutic techniques. Injectable hydrogels are a particularly appealing system as they hold promise as a minimally invasive therapeutic approach. Moreover, injectable acellular alginate-based hydrogels have been tested clinically in patients with myocardial infarction (MI) and show preservation of the left ventricular (LV) indices and left ventricular ejection fraction (LVEF). This review provides an overview of recent developments that have occurred in the design and engineering of various injectable hydrogel systems for cardiac tissue engineering efforts, including a comparison of natural versus synthetic systems with emphasis on the ideal characteristics for biomimetic cardiac materials.

Thermoadaptive Supramolecular α-Cyclodextrin Crystallization-Based Hydrogels via Double Hydrophilic Block Copolymer Templating

Supramolecular hydrogels play a prominent role in contemporary research of hydrophilic polymers. Especially, hydrogels based on α-cyclodextrin/poly(ethylene glycol) (α-CD/PEG) complexation and crystal formation are studied frequently. Here, the effect of double hydrophilic block copolymers (DHBCs) on α-CD/PEG hydrogel properties is investigated. Therefore, a novel DHBC, namely poly(N-vinylpyrrolidone)-b-poly(oligo ethylene glycol methacrylate) (PVP-b-POEGMA), was synthesized via a combination of reversible deactivation radical polymerization and modular conjugation methods. In the next step, hydrogel formation was studied after α-CD addition. Interestingly, DHBC-based hydrogels showed a significant response to thermal history. Heating of the gels to different temperatures led to different mechanical properties after cooling to ambient temperature, i.e., gels with mechanical properties similar to the initial gels or weak flowing gels were obtained. Thus, the hydrogels showed thermoadaptive behavior, which might be an interesting property for future applications in sensing.