Gelatin hydrogels via thiol-ene chemistry

Functional Enhancement of Guar Gum−Based Hydrogel by Polydopamine and Nanocellulose

The development of green, biomedical hydrogels using natural polymers is of great significance. From this viewpoint, guar gum (GG) has been widely used for hydrogel preparation; however, its mechanical strength and adhesion often cannot satisfy the biomedical application. Therefore, in the present study, gelatin and a cellulose nanocrystal (CNC) were first applied to overcome the defects of guar gum hydrogel. Dopamine was self−polymerized into polydopamine (PDA) on the gelatin chain at alkaline condition, and gelatin−polydopamine (Gel−PDA) further cross−linked with guar gum and CNC via the borate−didiol bond, intramolecular Schiff base reaction, and Michael addition. CNC not only interacted with guar gum using borate chemistry but also acted as a mechanical reinforcer. The obtained Gel−PDA+GG+CNC hydrogel had an excellent self−healing capacity, injectability, and adhesion due to the catechol groups of PDA. Moreover, dopamine introduction caused a significant increase in the anti−oxidant activity. This hydrogel was cyto− and hemo−compatible, which implies a potential usage in the medical field.

3D bioprinted colorectal cancer models based on hyaluronic acid and signalling glycans

In cancer microenvironment, aberrant glycosylation events of ECM proteins and cell surface receptors occur. We developed a protocol to generate 3D bioprinted models of colorectal cancer (CRC) crosslinking hyaluronic acid and gelatin functionalized with three signalling glycans characterized in CRC, 3'-Sialylgalactose, 6'-Sialylgalactose and 2'-Fucosylgalactose. The crosslinking, performed exploiting azide functionalized gelatin and hyaluronic acid and 4arm-PEG-dibenzocyclooctyne, resulted in biocompatible hydrogels that were 3D bioprinted with commercial CRC cells HT-29 and patient derived CRC tumoroids. The glycosylated hydrogels showed good 3D printability, biocompatibility and stability over the time. SEM and synchrotron radiation SAXS/WAXS analysis revealed the influence of glycosylation in the construct morphology, whereas MALDI-MS imaging showed that protein profiles of tumoroid cells vary with glycosylation, indicating that sialylation and fucosylation of ECM proteins induce diverse alterations to the proteome of the tumoroid and surrounding cells.

Design and Synthesis of Hybrid Thermo-Responsive Hydrogels Based on Poly(2-oxazoline) and Gelatin Derivatives

The combination of natural and synthetic polymers to form hybrid hydrogels offers the potential of fabricating new materials that possess a combination of properties resulting from both types of polymer classes. Within this work, two alkene-functionalized poly(2-alkyl/aryl–2-oxazoline) (PAOx) copolymers and one gelatin derivative, thiolated gelatin (gel-SH), are synthesized as precursors for hybrid hydrogels through a photo-induced radical thiol-ene crosslinking process. In-situ photo-rheology revealed an increased mechanical stability for hydrogels that possess an excess amount of PAOx precursor. A final qualitative investigation of the thermo-responsive properties of a P(EtOx₂₇₀–norbornenOx₃₀):gel-SH (2:1) hydrogel film revealed a cloud point temperature (T_cp) in the same range as the T_cp of the P(EtOx₂₇₀–norbornenOx₃₀) polymer precursor, which is around 30 °C. This promising result demonstrates that thermo-responsive hybrid poly(2-oxazoline)-gelatin hydrogels could be prepared with predictable T_cps and that further investigation into this appealing feature might be of interest. Ultimately, this work shows a proof-of-concept of using PAOx as potential hybrid hydrogel precursor in combination with cell-interactive gelatin derivatives to potentially improve the mechanical stability of the final scaffolds and introduce additional features such as thermo-responsiveness for the purpose of drug delivery.

Squarate Cross-Linked Gelatin Hydrogels as Three-Dimensional Scaffolds for Biomedical Applications

Hydrogels are useful platforms as three-dimensional (3D) scaffolds for cell culture, drug-release systems, and regenerative medicine applications. Here, we propose a novel chemical cross-linking approach by the use of 3,4-diethoxy-3-cyclobutene-1,2-dione or diethyl squarate for the preparation of 5 and 10% w/v gelatin-based hydrogels. Hydrogels showed good swelling properties, and the 5% gelatin-based hydrogel proved suitable as a 3D cell culture scaffold for the chondrocyte cell line C28/I2. In addition, diffusion properties of different sized molecules inside the hydrogel were determined.

Chemically Modified Biopolymers for the Formation of Biomedical Hydrogels

Biopolymers are natural polymers sourced from plants and animals, which include a variety of polysaccharides and polypeptides. The inclusion of biopolymers into biomedical hydrogels is of great interest because of their inherent biochemical and biophysical properties, such as cellular adhesion, degradation, and viscoelasticity. The objective of this Review is to provide a detailed overview of the design and development of biopolymer hydrogels for biomedical applications, with an emphasis on biopolymer chemical modifications and cross-linking methods. First, the fundamentals of biopolymers and chemical conjugation methods to introduce cross-linking groups are described. Cross-linking methods to form biopolymer networks are then discussed in detail, including (i) covalent cross-linking (e.g., free radical chain polymerization, click cross-linking, cross-linking due to oxidation of phenolic groups), (ii) dynamic covalent cross-linking (e.g., Schiff base formation, disulfide formation, reversible Diels-Alder reactions), and (iii) physical cross-linking (e.g., guest-host interactions, hydrogen bonding, metal-ligand coordination, grafted biopolymers). Finally, recent advances in the use of chemically modified biopolymer hydrogels for the biofabrication of tissue scaffolds, therapeutic delivery, tissue adhesives and sealants, as well as the formation of interpenetrating network biopolymer hydrogels, are highlighted.

Norbornene-functionalized methylcellulose as a thermo- and photo-responsive bioink

Three-dimensional (3D) bioprinting has emerged as an important tool to fabricate scaffolds with complex structures for tissue engineering and regenerative medicine applications. For extrusion-based 3D bioprinting, the success of printing complex structures relies largely on the properties of bioink. Methylcellulose (MC) has been exploited as a potential bioink for 3D bioprinting due to its temperature-dependent rheological properties. However, MC is highly soluble and has low structural stability at room temperature, making it suboptimal for 3D bioprinting applications. In this study, we report a one-step synthesis protocol for modifying MC with norbornene (MCNB), which serves as a new bioink for 3D bioprinting. MCNB preserves the temperature-dependent reversible sol-gel transition and readily reacts with thiol-bearing linkers through light-mediated step-growth thiol-norbornene photopolymerization. Furthermore, we rendered the otherwise inert MC network bioactive through facile conjugation of integrin-binding ligands (e.g. CRGDS) or via incorporating cell-adhesive and protease-sensitive gelatin-based macromer (e.g. GelNB). The adaptability of the new MCNB-based bioink offers an attractive option for diverse 3D bioprinting applications.

Photoinduced Porcine Gelatin Cross-Linking by Homobi- and Homotrifunctional Tetrazoles

Gelatin is a costless polypeptide material of natural origin, able to form hydrogels that are potentially useful in biomaterial scaffold design for drug delivery, cell cultures, and tissue engineering. However, gelatin hydrogels are unstable at physiological conditions, losing their features only after a few minutes at 37 °C. Accordingly, treatments to address this issue are of great interest. In the present work, we propose for the first time the use of bi- and trifunctional tetrazoles, most of them unknown to date, for photoinduced gelatin cross-linking towards the production of physiologically stable hydrogels. Indeed, after UV-B irradiation, aryl tetrazoles generate a nitrilimine intermediate that is reactive towards different functionalities, some of them constitutively present in the amino acid side chains of gelatin. The efficacy of the treatment strictly depends on the structure of the cross-linking agent used, and substantial improved stability was observed by switching from bifunctional to trifunctional cross-linkers.

Click chemistry-based biopolymeric hydrogels for regenerative medicine

Click chemistry is not a single specific reaction, but describes ways of generating products which emulate examples in nature. Click reactions occur in one pot, are not disturbed by water, generate minimal and inoffensive byproducts, and are characterized by a high thermodynamic driving force, driving the reaction quickly and irreversibly towards a high yield of a single reaction product. As a result, over the past 15 years it has become a very useful bio-orthogonal method for the preparation of chemical cross-linked biopolymer-based hydrogel, in the presence of e.g. growth factors and live cells, or in-vivo. Biopolymers are renewable and non-toxic, providing a myriad of potential backbone toolboxes for hydrogel design. The goal of this review is to summarize recent advances in the development of click chemistry-based biopolymeric hydrogels, and their applications in regenerative medicine. In particular, various click chemistry approaches, including copper-catalyzed azide-alkyne cycloaddition reactions, copper-free click reactions (e.g. the Diels-Alder reactions, the strain-promoted azide-alkyne cycloaddition reactions, the radical mediated thiol-ene reactions, and the oxime-forming reactions), and pseudo-click reactions (e.g. the thiol-Michael addition reactions and the Schiff base reactions) are highlighted in the first section. In addition, numerous biopolymers, including proteins (e.g. collagen, gelatin, silk, and mucin), polysaccharides (e.g. hyaluronic acid, alginate, dextran, and chitosan) and polynucleotides (e.g. deoxyribonucleic acid), are discussed. Finally, we discuss biopolymeric hydrogels, cross-linked by click chemistry, intended for the regeneration of skin, bone, spinal cord, cartilage, and cornea. This article provides new insights for readers in terms of the design of regenerative medicine, and the use of biopolymeric hydrogels based on click chemistry reactions.

Design and Synthesis of Chitosan-Gelatin Hybrid Hydrogels for 3D Printable in vitro Models

The development of 3D printable hydrogels based on the crosslinking between chitosan and gelatin is proposed. Chitosan and gelatin were both functionalized with methyl furan groups. Chemical modification was performed by reductive amination with methyl furfural involving the lysine residues of gelatin and the amino groups of chitosan to generate hydrogels with tailored properties. The methyl furan residues present in both polymers were exploited for efficient crosslinking via Diels-Alder ligation with PEG-Star-maleimide under cell-compatible conditions. The obtained chitosan-gelatin hybrid was employed to formulate hydrogels and 3D printable biopolymers and its processability and biocompatibility were preliminarily investigated.

3D Extracellular Matrix Mimics: Fundamental Concepts and Role of Materials Chemistry to Influence Stem Cell Fate

Synthetic 3D extracellular matrices (ECMs) find application in cell studies, regenerative medicine, and drug discovery. While cells cultured in a monolayer may exhibit unnatural behavior and develop very different phenotypes and genotypes than in vivo, great efforts in materials chemistry have been devoted to reproducing in vitro behavior in in vivo cell microenvironments. This requires fine-tuning the biochemical and structural actors in synthetic ECMs. This review will present the fundamentals of the ECM, cover the chemical and structural features of the scaffolds used to generate ECM mimics, discuss the nature of the signaling biomolecules required and exploited to generate bioresponsive cell microenvironments able to induce a specific cell fate, and highlight the synthetic strategies involved in creating functional 3D ECM mimics.

Gelatin-Based Hydrogels for the Controlled Release of 5,6-Dihydroxyindole-2-Carboxylic Acid, a Melanin-Related Metabolite with Potent Antioxidant Activity

The ability of gelatin-based hydrogels of incorporating and releasing under controlled conditions 5,6-dihydroxyindole-2-carboxylic acid (DHICA), a melanin-related metabolite endowed with marked antioxidant properties was investigated. The methyl ester of DHICA, MeDHICA, was also tested in view of its higher stability, and different solubility profile. Three types of gelatin-based hydrogels were prepared: pristine porcine skin type A gelatin (HGel-A), a pristine gelatin cross-linked by amide coupling of lysines and glutamic/aspartic acids (HGel-B), and a gelatin/chitosan blend (HGel-C). HGel-B and HGel-C differed in the swelling behavior, showed satisfactorily high mechanical strength at physiological temperatures and well-defined morphology. The extent of incorporation into all the gelatins tested using a 10% w/w indole to gelatin ratio was very satisfactory ranging from 60 to 90% for either indoles. The kinetics of indole release under conditions of physiological relevance was evaluated up to 72 h. The highest values were obtained with HGel-B and HGel-C for MeDHICA (90% after 6 h), and an appreciable release was observed for DHICA reaching 30% and 40% at 6 h for HGel-B and HGel-C, respectively. At 72 h, DHICA and MeDHICA were released at around 30% from HGel-A at pH 7.4, with an increase up to 40% at pH 5.5 in the case of DHICA. DHICA incorporated into HGel-B proved fairly stable over 6 h whereas the free compound at the same concentration was almost completely oxidized. The antioxidant power of the indole loaded gelatins was monitored by chemical assays and proved unaltered even after prolonged storage in air, suggesting that the materials could be prepared in advance with respect to their use without alteration of their efficacy.

Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature

Thiolated polymers designated "thiomers" are obtained by covalent attachment of thiol functionalities on the polymeric backbone of polymers. In 1998 these polymers were first described as mucoadhesive and in situ gelling compounds forming disulfide bonds with cysteine-rich substructures of mucus glycoproteins and crosslinking through inter- and intrachain disulfide bond formation. In the following, it was shown that thiomers are able to form disulfides with keratins and membrane-associated proteins exhibiting also cysteine-rich substructures. Furthermore, permeation enhancing, enzyme inhibiting and efflux pump inhibiting properties were demonstrated. Because of these capabilities thiomers are promising tools for drug delivery guaranteeing a strongly prolonged residence time as well as sustained release on mucosal membranes. Apart from that, thiomers are used as drugs per se. In particular, for treatment of dry eye syndrome various thiolated polymers are in development and a first product has already reached the market. Within this review an overview about the thiomer-technology and its potential for different applications is provided discussing especially the outcome of studies in non-rodent animal models and that of numerous clinical trials. Moreover, an overview on product developments is given.

(Photo-)crosslinkable gelatin derivatives for biofabrication applications

Over the recent decades gelatin has proven to be very suitable as an extracellular matrix mimic for biofabrication and tissue engineering applications. However, gelatin is prone to dissolution at typical cell culture conditions and is therefore often chemically modified to introduce (photo-)crosslinkable functionalities. These modifications allow to tune the material properties of gelatin, making it suitable for a wide range of biofabrication techniques both as a bioink and as a biomaterial ink (component). The present review provides a non-exhaustive overview of the different reported gelatin modification strategies to yield crosslinkable materials that can be used to form hydrogels suitable for biofabrication applications. The different crosslinking chemistries are discussed and classified according to their mechanism including chain-growth and step-growth polymerization. The step-growth polymerization mechanisms are further classified based on the specific chemistry including different (photo-)click chemistries and reversible systems. The benefits and drawbacks of each chemistry are also briefly discussed. Furthermore, focus is placed on different biofabrication strategies using either inkjet, deposition or light-based additive manufacturing techniques, and the applications of the obtained 3D constructs. STATEMENT OF SIGNIFICANCE: Gelatin and more specifically gelatin-methacryloyl has emerged to become one of the gold standard materials as an extracellular matrix mimic in the field of biofabrication. However, also other modification strategies have been elaborated to take advantage of a plethora of crosslinking chemistries. Therefore, a review paper focusing on the different modification strategies and processing of gelatin is presented. Particular attention is paid to the underlying chemistry along with the benefits and drawbacks of each type of crosslinking chemistry. The different strategies were classified based on their basic crosslinking mechanism including chain- or step-growth polymerization. Within the step-growth classification, a further distinction is made between click chemistries as well as other strategies. The influence of these modifications on the physical gelation and processing conditions including mechanical properties is presented. Additionally, substantial attention is put to the applied photoinitiators and the different biofabrication technologies including inkjet, deposition or light-based technologies.

Gelatin-Based Hydrogels through Homobifunctional Triazolinediones Targeting Tyrosine Residues

Gelatin is a biopolymer with interesting properties that can be useful for biomaterial design for different applications such as drug delivery systems, or 3D scaffolds for tissue engineering. However, gelatin suffers from poor mechanical stability at physiological temperature, hence methods for improving its properties are highly desirable. In the present work, a new chemical cross-linking strategy based on triazolinedione ene-type chemistry towards stable hydrogel is proposed. Two different homobifunctional 1,2,4-triazoline-3,5(4H)-diones, namely 4,4'-hexane-1,6-diylbis(3H-1,2,4-triazoline-3,5(4H)-dione) 1 and 4,4'-[methylenebis(4,1-phenylene)]bis(3H-1,2,4-triazoline-3,5(4H)-dione) 2 were used as cross-linkers in different ratio to tyrosine residues in gelatin. The reaction was proved effective in all experimented conditions and hydrogels featured with different thermal stability were obtained. In general, the higher the cross-linker/tyrosine ratio, the more thermostable the hydrogel. The swelling properties are strictly dependent upon the chemical nature of the cross-linker.

Gelatin-Based Photocurable Hydrogels for Corneal Wound Repair

In this study, an injectable, photocurable gelatin system, consisting of acrylated gelatin and thiolated gelatin, with tunable mechanical, biodegradation, and biological properties was used as a potential cell-supportive scaffold for the repair of focal corneal wounds. The mechanical property of hydrogels can be readily modified (postcure shear modulus of between 0.3 and 22 kPa) by varying the ratio of acrylate to thiol groups, photointensity, and solid content, and the biodegradation times also varied with the change of solid content. More importantly, the generated hydrogels exhibited excellent cell viability in both cell seeding and cell encapsulation experiments. Furthermore, the hydrogels were found to be biocompatible with rabbit cornea and aided the regeneration of a new tissue under a focal corneal wound (exhibiting epithelial wound coverage in <3d), and ultraviolet irradiation did not have any obvious harmful effect on the cornea and posterior eye segment tissues. Along with their injectability and tunable mechanical properties, the photocurable thiol-acrylate hydrogels showed promise as corneal substitutes or substrates to construct a new corneal tissue.

Recent Developments in Thiolated Polymeric Hydrogels for Tissue Engineering Applications

This review focuses on the recent strategy in the preparation of thiolated polymers and fabrication of their hydrogel matrices. The mechanism involved in the synthesis of thiolated polymers and fabrication of thiolated polymer hydrogels is exemplified with suitable schematic representations reported in the recent literature. The 2-iminothiolane namely "Traut's reagent" has been widely used for effectively thiolating the natural polymers such as collagen and gelatin, which contain free amino group in their backbone. The free carboxylic acid group containing polymers such as hyaluronic acid and heparin have been thiolated by using the bifunctional molecules such as cysteamine and L-cysteine via N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide/N-hydroxysuccinimide (EDC/NHS) coupling reaction. The degree of thiolation in the polymer chain has been widely determined by using Ellman's assay method. The thiolated polymer hydrogels are prepared by disulfide bond formation (or) thiol-ene reaction (or) Michael-type addition reaction. The thiolated polymers such as thiolated gelatin are reacted with polyethylene glycol diacrylate for obtaining interpenetrating polymer network hydrogel scaffolds. Several in vitro cell culture experiments indicate that the developed thiolated polymer hydrogels exhibited biocompatibility and cellular mimicking properties. The developed hydrogel scaffolds efficiently support proliferation and differentiation of various cell types. In the present review article, the thiol-functionalized protein-based biopolymers, carbohydrate-based polymers, and some synthetic polymers have been covered with recently published research articles. In addition, the usage of new thiolated nanomaterials as a crosslinking agent for the preparation of three-dimensional tissue-engineered hydrogels is highlighted.

Bioresponsive Hydrogels: Chemical Strategies and Perspectives in Tissue Engineering

Disease, trauma, and aging account for a significant number of clinical disorders. Regenerative medicine is emerging as a very promising therapeutic option. The design and development of new cell-customised biomaterials able to mimic extracellular matrix (ECM) functionalities represents one of the major strategies to control the cell fate and stimulate tissue regeneration. Recently, hydrogels have received a considerable interest for their use in the modulation and control of cell fate during the regeneration processes. Several synthetic bioresponsive hydrogels are being developed in order to facilitate cell-matrix and cell-cell interactions. In this review, new strategies and future perspectives of such synthetic cell microenvironments will be highlighted.