Structural analysis of dextran-based hydrogels obtained chemoenzymatically

Preparation of new hydrogels by visible light cross-linking of dextran methacrylate and poly(ethylene glycol)-maleic acid copolymer

In this work, a series of new biodegradable and biocompatible hydrogels were synthesized by photopolymerization of dextran-methacrylate (DXM) with poly(ethylene glycol)-maleic acid copolymer (poly(PEG-co-MA, PEGMA)) using (-)-riboflavin as a visible light photoinitiator and L-arginine as a co-photoinitiator. DXM was prepared by acylation of dextran (DX) with methacryloyl chloride (MAC), and PEGMA was synthesized by polycondensation of poly(ethylene glycol) (PEG) and maleic acid (MA). The DXM and PEGMA were characterized by FT-IR and ¹HNMR spectroscopy. Different types of hydrogels from various ratios of DXM and PEGMA were prepared and characterized by SEM. The results showed that the prepared hydrogel by photo-cross-linking of DXM (DPHG0) was transparent and flexible, and its physical shape was excellent, but it was sticky. The stickiness was reduced by increasing the PEGMA contents, and different types of DXM/PEGMA hydrogels (DPHG1-4) with various properties were prepared. For example, DPHG2 (PEGMA content was 0.25 g) was transparent and flexible, its physical shape was excellent, and it was not sticky. The prepared hydrogels showed excellent cytocompatibility, and their tensile and compressive strength were also evaluated. Additionally, the in vitro degradation and swelling ratios of the prepared hydrogels were studied in buffer solution at different pHs.

Antibacterial Hydrogel Sheet Dressings Composed of Poly(vinyl alcohol) and Silver Nanoparticles by Electron Beam Irradiation

Advanced wound dressings that can deliver potent antibacterial action are still much in need, especially for treating wound infections caused by drug-resistant bacteria. In this research, we utilized electron beam (EB) irradiation to develop antibacterial hydrogel sheet dressings from poly(vinyl alcohol) (PVA) and silver nanoparticles (AgNPs) in a two-step processing and evaluated their bactericidal efficacy, as well as the AgNP release. The effect of the irradiation dose on the swelling, gel fraction, network parameters, and mechanical properties of the hydrogels was first determined to establish the optimal doses for the two-step processing. The prototypic hydrogel sheets were then formed in the first EB irradiation and served as a matrix for the AgNP synthesis by the reduction of the silver nitrate precursors during the second EB irradiation. The diffusion assay showed that the minimal inhibition concentration (MIC) of the AgNP-load hydrogels was 0.25 and 0.5 mg/cm2 against Escherichia coli and Staphylococcus aureus, respectively. At these MIC levels, the released AgNPs increased sharply before reaching the maximum, ~950 and 1800 ppb, at 24 h as analyzed by atomic absorption. Therefore, we successfully demonstrated that this two-step processing by EB irradiation provides a convenient platform to fabricate AgNP-loaded hydrogel dressings that can be further developed for wound healing.

Insights on Some Polysaccharide Gel Type Materials and Their Structural Peculiarities

Global resources have to be used in responsible ways to ensure the world's future need for advanced materials. Ecologically friendly functional materials based on biopolymers can be successfully obtained from renewable resources, and the most prominent example is cellulose, the well-known most abundant polysaccharide which is usually isolated from highly available biomass (wood and wooden waste, annual plants, cotton, etc.). Many other polysaccharides originating from various natural resources (plants, insects, algae, bacteria) proved to be valuable and versatile starting biopolymers for a wide array of materials with tunable properties, able to respond to different societal demands. Polysaccharides properties vary depending on various factors (origin, harvesting, storage and transportation, strategy of further modification), but they can be processed into materials with high added value, as in the case of gels. Modern approaches have been employed to prepare (e.g., the use of ionic liquids as "green solvents") and characterize (NMR and FTIR spectroscopy, X ray diffraction spectrometry, DSC, electronic and atomic force microscopy, optical rotation, circular dichroism, rheological investigations, computer modelling and optimization) polysaccharide gels. In the present paper, some of the most widely used polysaccharide gels will be briefly reviewed with emphasis on their structural peculiarities under various conditions.

Chitin-Glucan Complex Hydrogels: Optimization of Gel Formation and Demonstration of Drug Loading and Release Ability

Chitin-glucan complex (CGC) hydrogels were fabricated through a freeze–thaw procedure for biopolymer dissolution in NaOH 5 mol/L, followed by a dialysis step to promote gelation. Compared to a previously reported methodology that included four freeze–thaw cycles, reducing the number of cycles to one had no significant impact on the hydrogels’ formation, as well as reducing the total freezing time from 48 to 18 h. The optimized CGC hydrogels exhibited a high and nearly spontaneous swelling ratio (2528 ± 68%) and a water retention capacity of 55 ± 3%, after 2 h incubation in water, at 37 °C. Upon loading with caffeine as a model drug, an enhancement of the mechanical and rheological properties of the hydrogels was achieved. In particular, the compressive modulus was improved from 23.0 ± 0.89 to 120.0 ± 61.64 kPa and the storage modulus increased from 149.9 ± 9.8 to 315.0 ± 76.7 kPa. Although the release profile of caffeine was similar in PBS and NaCl 0.9% solutions, the release rate was influenced by the solutions’ pH and ionic strength, being faster in the NaCl solution. These results highlight the potential of CGC based hydrogels as promising structures to be used as drug delivery devices in biomedical applications.

A chondroitin sulfate based injectable hydrogel for delivery of stem cells in cartilage regeneration

Chondroitin sulfate (CS), as a popular material for cartilage tissue engineering scaffolds, has been extensively studied and reported for its safety and excellent biocompatibility. However, the rapid degradation of pure CS scaffolds has brought a challenge to regenerate neo-tissue similar to natural articular cartilage effectively. Meanwhile, the poly(ethene glycol) (PEG) -based biopolymer is frequently applied as a structural constituent material because of its remarkable mechanical properties, long-lasting in vivo stability, and hypo-immunity. Here, we report that the combination of CS and hyperbranched multifunctional PEG copolymer (HB-PEG) could synergistically promote cartilage repair. The thiol functionalised CS (CS-SH)/HB-PEG hydrogel scaffolds were fabricated via thiol-ene reaction, which exhibits rapid gelation, excellent mechanical properties and prolonged degradation properties. We found that rat adipose-derived mesenchymal stem cells presented great cell viability and improved chondrogenesis in CS-SH/HB-PEG hydrogels. Moreover, the injectable hydrogel scaffolds reduced stem cell inflammatory response, consistent with the well-documented anti-inflammatory activities of CS.

Ion-imprinted sponge produced by ice template-assisted freeze drying of salecan and graphene oxide nanosheets for highly selective adsorption of mercury (II) ion

As a kind of potential heavy metal absorbent, polysaccharide-based materials are limited by the complicated preparation method and bad selectivity toward targeted ion. Here, a fantastic sponge was produced by combining salecan and graphene oxide (GO) nanosheets via ice template-assisted freeze drying and ion-imprinting technologies. The intense intermolecular interactions between salecan and GO gave the sponge high stability. The swelling, morphology, and mechanical stiffness of the material showed highly dependent on the salecan content. Additionally, the influence of salecan content, pH, initial ion concentration, and contact time on Hg²⁺ adsorption was extensively investigated. Adsorption kinetics and equilibrium isotherms perfectly fitted in the pseudo-second-order and Freundlich models, reflecting the multilayer chemical-adsorption mediated mechanism. Most strikingly, the ion-imprinted sponge exhibited strong selectivity toward Hg²⁺ and outstanding stability with recyclability over usage of five times. These investigations provide the guidance for the construction of promising polysaccharide-based adsorbents for the remediation of Hg²⁺-polluted water.

Unraveling the Effect of PEG Chain Length on the Physical Properties and Toxicant Removal Capacities of Cross-Linked Network Synthesized by Thiol-Norbornene Photoclick Chemistry

With the aim to develop chemical adsorbents that are inherently nontoxic to living beings and the environment, a gel system based on thiol-norbornene photoclick chemistry was developed. Norbornene was strategically functionalized with different poly(ethylene glycol) (PEG) chains to produce PEG-functionalized macromonomers. The influence of incorporating PEG in the cross-linked network was evaluated on the basis of its physical properties and dye-removing efficiency from aqueous solutions. The excellent swelling ability of the gels in organic solvents was found to improve with the PEG chain length. The rheological measurements of the as-synthesized materials also exhibited the presence of elasticity in the network, and a decrease in storage and loss moduli was observed with an increase in PEG molecular weight. The materials possess excellent thermal stability, which enhanced with an increase in PEG chain length, as revealed from thermogravimetric analysis (TGA). Differential scanning calorimetry (DSC) studies revealed the tendency of higher-molecular-weight PEG to form a crystalline phase in the network. Kinetic studies of dye removal from aqueous solutions by the as-prepared cross-linked networks indicate that the dye removal proceeds via pseudo-second-order kinetics. The study of adsorption isotherm of the removal process indicates that the adsorption follows the Langmuir isotherm model. In this present work, we have thoroughly evaluated the influence of PEG chain length on several physical properties and toxic cationic dye removal efficiencies of thiol-norbornene photo-cross-linked networks.

Hybrid collagen alginate hydrogel as a platform for 3D tumor spheroid invasion

Extracellular matrix regulates hallmark features of cancer through biochemical and mechanical signals, although mechanistic understanding of these processes remains limited by lack of models that recreate physiology of tumors. To tissue-engineer models that recapitulate three-dimensional architecture and signaling in tumors, there is a pressing need for new materials permitting flexible control of mechanical and biophysical features. We developed a hybrid hydrogel system composed of collagen and alginate to model tumor environments in breast cancer and other malignancies. Material properties of the hydrogel, including stiffness, microstructure and porosimetry, encompass parameters present in normal organs and tumors. The hydrogel possesses a well-organized, homogenous microstructure with adjustable mechanical stiffness and excellent permeability. Upon embedding multicellular tumor spheroids, we constructed a 3D tumor invasion model showing follow-the-leader migration with fibroblasts leading invasion of cancer cells similar to in vivo. We also demonstrated effects of CXCL12-CXCR4 signaling, a pathway implicated in tumor progression and metastasis, in a dual-tumor spheroid invasion model in 3D hydrogels. These studies establish a new hydrogel platform with material properties that can be tuned to investigate effects of environmental conditions on tumor progression, which will advance future studies of cancer cell invasion and response to therapy.

STATEMENT OF SIGNIFICANCE

Our manuscript describes a novel design of hybrid hydrogel system composed of collagen and alginate modeling 3D tumor environments in breast cancer. The hydrogel possesses a well-organized, homogenous microstructure with adjustable mechanical stiffness. Upon embedding tumor spheroids, we successfully showed a 3D tumor invasion model showing follow-the-leader migration with fibroblasts leading invasion of cancer cells similar to in vivo. To the best of our knowledge, this is the first study showing two spheroids invade simultaneously and forming bridge-like connection and effects of chemical gradients in 3D hydrogel environment. This research provides a new model for tumor-stromal interactions in cancer cell migration and establishes a novel hydrogel system for analyzing physical and biochemical signals regulating cancer progression and response to therapy.

Gellan gum-hydroxyapatite composite spongy-like hydrogels for bone tissue engineering

Osteoinductive biomaterials represent a promising approach to advance bone grafting. Despite promising, the combination of sustained biodegradability, mechanical strength, and biocompatibility in a unique biomaterial that can also support cell performance and bone formation in vivo is demanding. Herein, we developed gellan gum (GG)-hydroxyapatite (HAp) spongy-like hydrogels to mimic the organic (GG) and inorganic (HAp) phases of the bone. HAp was successfully introduced within the GG polymeric networks, as determined by FTIR and XRD, without compromising the thermostability of the biomaterials, as showed by TGA. The developed biomaterials showed sustained degradation, high swelling, pore sizes between 200 and 300 μm, high porosity (>90%) and interconnectivity (<60%) that was inversely proportional to the total polymeric amount and to CaCl₂ crosslinker. CaCl₂ and HAp reinforced the mechanical properties of the biomaterials from a storage modulus of 40 KPa to 70-80 KPa. This study also showed that HAp and CaCl₂ favored the bioactivity and that cells were able to adhere and spread within the biomaterials up to 21 days of culture. Overall, the possibility to tailor spongy-like hydrogels properties by including calcium as a crosslinker and by varying the amount of HAp will further contribute to understand how these features influence bone cells performance in vitro and bone formation in vivo. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 479-490, 2018.

Preparation and Characterization of a Polymer-Based "Molecular Accordion"

A urethane-based polymer material, denoted HDI-1, was obtained from the addition reaction of β-cyclodextrin (β-CD) with 1,6-hexamethylene diisocyanate (HDI) at the 1:1 mole ratio. In aqueous solution and ambient temperature conditions, HDI-1 adopts a compact (coiled) morphology where the cross-linker units become coiled and are partially self-included in the annular hydroxyl (interstitial) region of β-CD. As the temperature is raised or as p-nitrophenol (PNP) was included within the β-CD cavity and the noninclusion sites of the polymer, an extended (uncoiled) morphology was adopted. The equilibrium distribution between the extended and the compact forms of HDI-1 is thermally and chemically switchable, in accordance with the hydration properties and host-guest chemistry of this responsive polymer system. The molecular structure of this water-soluble urethane polymer and its host-guest complexes with PNP were investigated using spectroscopic (Raman, (1)H NMR, induced circular dichroism), dynamic light scattering (DLS), and calorimetric (DSC) methods in aqueous solution at ambient pH, and compared with native β-CD. This study reports on the unique supramolecular properties of a polymer that resembles a thermally and chemically responsive "molecular accordion".

Photocrosslinkable Gelatin Hydrogel for Epidermal Tissue Engineering

Natural hydrogels are promising scaffolds to engineer epidermis. Currently, natural hydrogels used to support epidermal regeneration are mainly collagen- or gelatin-based, which mimic the natural dermal extracellular matrix but often suffer from insufficient and uncontrollable mechanical and degradation properties. In this study, a photocrosslinkable gelatin (i.e., gelatin methacrylamide (GelMA)) with tunable mechanical, degradation, and biological properties is used to engineer the epidermis for skin tissue engineering applications. The results reveal that the mechanical and degradation properties of the developed hydrogels can be readily modified by varying the hydrogel concentration, with elastic and compressive moduli tuned from a few kPa to a few hundred kPa, and the degradation times varied from a few days to several months. Additionally, hydrogels of all concentrations displayed excellent cell viability (>90%) with increasing cell adhesion and proliferation corresponding to increases in hydrogel concentrations. Furthermore, the hydrogels are found to support keratinocyte growth, differentiation, and stratification into a reconstructed multilayered epidermis with adequate barrier functions. The robust and tunable properties of GelMA hydrogels suggest that the keratinocyte laden hydrogels can be used as epidermal substitutes, wound dressings, or substrates to construct various in vitro skin models.

Fabrication of a photo-crosslinked gelatin hydrogel for preventing abdominal adhesion

A photocrosslinkable gelatin-based hydrogel membrane of easy-handling and non-toxic degradation was developed for preventing intra-abdominal adhesion formation.

Transfection in the third dimension

An understanding of parameters that modulate gene transfer in 3-D will assist in the formation of gene delivery systems and scaffolds, which can mediate efficient non-viral delivery for guiding in vivo tissue regeneration and therapy. We have previously demonstrated the cell area and length, integrin expression, and RhoGTPase mediated signalling to be pivotal parameters that guide gene transfer to mouse mesenchymal stem cells (mMSCs) cultured in 2-D and are modulated by ECM proteins. In this study, we were interested in determining if cationic polymer mediated gene transfer to cells seeded in 3-D would occur through different mechanisms as compared to those seeded in 2-D. In particular, we examined the endocytosis pathways used to internalize polyplexes, and the role of cytoskeletal dynamics and RhoGTPases in non-viral gene transfer for cells seeded in 2-D and 3-D. Inhibition of clathrin- and caveolae-mediated endocytosis resulted in a more drastic decrease in overall transgene expression for cells seeded in 3-D than for those in 2-D. In addition, polyplex internalization was only significantly decreased in 3-D when clathrin-mediated endocytosis was inhibited, while caveolae-mediated endocytosis inhibition for cells seeded in 2-D resulted in the strongest polyplex internalization inhibition. Actin and microtubule polymerization affected 2-D and 3-D transfection differently. Microtubule depolymerization enhanced transgene expression in 2-D, but inhibited transgene expression in 3-D. Lastly, inhibition of RhoGTPases also affected 2-D and 3-D transfection differently. The inhibition of ROCK effectors resulted in a decrease of transgene expression and internalization for cells seeded in 3-D, but not in 2-D, and the inhibition of the effector PAK1 resulted in an increase of transgene expression for both 2-D and 3-D. Overall, our study suggests that the process of gene transfer occurs through different mechanisms for cells seeded in 2-D compared to those seeded in 3-D.

25th anniversary article: Designer hydrogels for cell cultures: a materials selection guide

Cell culturing, whether for tissue engineering or cell biology studies, always involves placing cells in a non-natural environment and no material currently exist that can mimic the entire complexity of natural tissues and variety of cell-matrix interactions that is found in vivo. Here, we review the vast range of hydrogels, composed of natural or synthetic polymers that provide a route to tailored microenvironments.

Thermoresponsive poly(N-vinylcaprolactam) cryogels: synthesis and its biophysical evaluation for tissue engineering applications

The thermoresponsive poly(N-vinylcaprolactam) (PVCl) based cryogel network were synthesized and characterized with respect to physical and biological properties. The PVCl cryogel crosslinked with polyethylene glycol-diacrylate (PEGda) was synthesized in 1% dimethyl sulfoxide containing aqueous medium at -12°C for 12-14 h. The cryogel synthesized in this manner were highly spongy in nature and can absorb water in its porous network. These polymeric cryogel networks have good physical morphology as confirmed by scanning electron microscopy. The estimated porosity of these cryogels was 90% as demonstrated by various methods based on absorption of water and cyclohexane. The median pore diameter and surface area was 30 μm and 2.0253 m(2)/g, respectively as confirmed by analysis on mercury porosimeter. These materials can interact with biological system without any cytotoxic effects. Change in temperature influenced the adsorption of fetal bovine serum (FBS) on PVCl scaffold which showed maximum protein adsorption at 37°C, as compared to that at 25°C. Furthermore, the fibroblast cell adhesion studies showed the potential of these PVCl based cryogels as tissue engineering scaffolds.

In Vivo Biocompatibility and Biodegradability of Dextrin-based Hydrogels

The in vivo biocompatibility of dextrin hydrogels obtained by polymerization of dextrin-hydroxyethylmethacrylate (dextrin-HEMA) and dextrin-vinyl acrylate (dextrin-VA) are reported in this work. The histological analysis of subcutaneous implants of these hydrogels, featuring inflammatory and reabsorption events, were carried out over a 16-week period in mice. The dextrin-HEMA hydrogel was quickly and completely degraded and reabsorbed, whereas the dextrin-VA degradation occurred slowly and a thin fibrous capsule surrounded the nondegradable hydrogel. The dextrin-HEMA was degraded after 16 weeks with only mild inflammation and a few detectable foamy macrophages around the implant. These events were followed by complete resorption and no sign of capsule formation or fibrosis associated to the implants. The results indicate that the dextrin hydrogels are biocompatible because no toxicity on the tissues surrounding the implants was found. It may be speculated that a controlled degradation rate of the hydrogels may be obtained by grafting dextrin to HEMA and VA in different proportions.

Synthesis of macroporous thermosensitive hydrogels: a novel method of controlling pore size

Macroporous thermosensitive poly(N-isopropylacrylamide) hydrogels were synthesized in the presence of dodecyl dimethyl benzyl ammonium bromide (DDBAB). Poly(vinyl alcohol) (PVA) was involved to control the pore size of the hydrogels. The morphology of the resulting hydrogels was studied by both an optical microscope and a scanning electron microscope. Moreover, the pore size and its distribution were examined by mercury intrusion porosimetry. The results indicated that size of the pores decreased with the increase of the amount of PVA added. The mechanism was explained after dynamic light scattering measurement of the size of hydrophobic initiator DDBAPS aggregates that were formed in situ in PVA aqueous solutions of various concentrations as the product of the reaction between DDBAB and the water soluble initiator ammonium persulfate. Swelling ratio and deswelling/reswelling kinetics of the hydrogels were also measured to investigate the response properties of the hydrogels. It would be a promising method of pore size control for synthesizing hydrogels of other vinyl monomers that could be initiated by persulfates.

Proliferation and differentiation of human osteoblasts within 3D printed poly-lactic-co-glycolic acid scaffolds

Bone repair and regeneration can be enhanced through implantation of biocompatible and biodegradable scaffolds, which serve primarily as osteoconductive moieties. In this study, the mechanical properties and microenviroment of 3D printed poly-lactic-co-glycolic acid (PLGA) scaffolds are examined. Additionally, the proliferation and differentiation of human fetal osteoblasts are evaluated after 3 weeks of in vitro culture on the scaffolds. The results showed that the PLGA scaffolds examined had mechanical properties similar to that of trabecular bone, but was still much weaker compared to cortical bone. In addition to general porosity, the PLGA scaffolds also had micropores within macropore walls. Cultured human osteoblasts could proliferate upon seeding on the PLGA scaffolds. Alkaline phosphatase activity and osteonectin expression of the osteoblasts cultured on the PLGA scaffolds remained stable over three weeks, whilst expression of collagen type I and osteopontin decreased. The alkaline phosphatase activity of osteoblasts cultured on PLGA scaffolds is comparable with that from two commercially-available scaffolds - OPLA and collagen scaffolds (Becton-Dickinson (BD) Inc., Franklin Lakes, NJ, USA). Hence, the results suggested that the PLGA scaffolds examined are conducive for promoting osteogenesis.

Novel glycidyl methacrylated dextran (Dex-GMA)/gelatin hydrogel scaffolds containing microspheres loaded with bone morphogenetic proteins: Formulation and characteristics

Novel thermomechanical hydrogel scaffolds containing our previously prepared microspheres loaded with bone morphogenetic proteins (BMP) were successfully generated by radical crosslinking and low dose gamma-irradiation from combination of two kind of biomaterials: glycidyl methacrylated dextran (Dex-GMA) and gelatin. The structure of those resulting smart hybrid hydrogels was evaluated by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) analyses, and as a function of the degree of Dex-GMA's substitution (DS), the proportion between Dex-GMA and gelatin, and the initial polyethyleneglycol (PEG) concentration used in the preparation of the hydrogels. The swelling and degradation properties and the temperature-sensitive drug release manner were determined by dynamic evaluation methods in vitro, and the gel content was also calculated. MIP analysis showed that by systematically altering the preparation parameters, the overall networks were clearly macroporous with pore sizes ranging from 5.6+/-4.2 to 37.7+/-13.7 microm. As expected, the pore size decreased as DS and initial PEG concentration increased, whereas the opposite was found for the gel content. Moreover, the porosity values ranged from 73.7+/-12.4% up to 89.6+/-6.3%. The SEM results also showed the inter-connective pores as well as microspheres encased into their porous structure of those hydrogels. The swelling and degradation properties of the resultant hydrogels varied according to the DS of Dex-GMA and initial PEG concentration, while the proportion between Dex-GMA and gelatin had no significant influence on those characterizations. By changing the composition ratio of the two precursors, the phase transition temperature (lower critical solution temperature, LSCT) of the hydrogel scaffolds could also be adjusted to be or near the body temperature, so BMP release from microsphere-hydrogel compounds could be accordingly controlled and the release period could be varied from 18 to more than 28 days. These results demonstrated that a novel temperature-sensitive and biodegradable Dex-GMA/gelatin scaffold containing microspheres loaded with BMP could be successfully developed from both dextran- and gelatin-based biomaterials, which could promisingly satisfy the need, desire, and expectation of both self-regulated drug delivery and tissue-engineering applications.