Characterization and Optimization of Injectable In Situ Crosslinked Chitosan-Genipin Hydrogels

In recent years, there has been an increased interest in injectable, in situ crosslinking hydrogels due to their minimally invasive application and ability to conform to their environment. Current in situ crosslinking chitosan hydrogels are either mechanically robust with poor biocompatibility and limited biodegradation due to toxic crosslinking agents or the hydrogels are mechanically weak and undergo biodegradation too rapidly due to insufficient crosslinking. Herein, the authors developed and characterized a thermally-driven, injectable chitosan-genipin hydrogel capable of in situ crosslinking at 37 °C that is mechanically robust, biodegradable, and maintain high biocompatibility. The natural crosslinker genipin is utilized as a thermally-driven, non-toxic crosslinking agent. The chitosan-genipin hydrogel's crosslinking kinetics, injectability, viscoelasticity, swelling and pH response, and biocompatibility against human keratinocyte cells are characterized. The developed chitosan-genipin hydrogels are successfully crosslinked at 37 °C, demonstrating temperature sensitivity. The hydrogels maintained a high percentage of swelling over several weeks before degrading in biologically relevant environments, demonstrating mechanical stability while remaining biodegradable. Long-term cell viability studies demonstrated that chitosan-genipin hydrogels have excellent biocompatibility over 7 days, including during the hydrogel crosslinking phase. Overall, these findings support the development of an injectable, in situ crosslinking chitosan-genipin hydrogel for minimally invasive biomedical applications.     

Preparation of Homogeneous Hydrogels by Controlling the Crosslinker Reactivity and Availability

Network microstructures of polyacrylamide (PAAm) hydrogels were investigated by static light scattering measurements. The gels were prepared by free‐radical crosslinking polymerization of acrylamide (AAm). To suppress the degree of gel inhomogeneity, the crosslinker reactivity during gelation was controlled by decreasing its availability in the reaction system. Our first approach was the addition of the crosslinker N,N′‐methylenebis(acrylamide) (BAAm) in one or three portions during the course of the gelation reactions. As a second approach, a slightly water soluble crosslinker, namely ethylene glycol dimethacrylate (EGDM) was used as a crosslinker in AAm polymerization. Due to the low water solubility of EGDM, EGDM phase in the gelation system act as a reservoir of crosslinker so that the crosslinker can be supplied continuously to the aqueous reaction zone during the course of gelation. It was found that the delayed crosslinker addition technique further increases the degree of inhomogeneity of PAAm hydrogels. The results were explained with increasing viscosity of the reaction solution at the time of the crosslinker addition so that the crosslinking reactions are limited to local regions in the reaction system. The second approach, namely use of the slightly water soluble crosslinker EGDM significantly increases the degree of structural homogeneity of PAAm hydrogels.     

A Nonhydrolyzable-Water Soluble Crosslinker: Tetrallylpiperazinium Dichloride and its Copolymers with Acrylic Acid and Acrylamide

Abstract

A versatile crosslinking agent, tetraallyl piperazinium dichloride (TAP) has been prepared and demonstrated to be useful in copolymerization with water soluble monomers such as acrylic acid and acrylamide. Although the crosslinker itself does not readily homopolymerize in water, it forms transparent rigid hydrogels within a reasonable period of time when copolymerized with acrylic acid. Being nonhydrolysable, the crosslinker presented is superior to other common water soluble crosslinking agents such as methylene or ethylene bis(acrylamides). Since this crosslinker has a low toxicity, it may be used for bioapplications such as gel electrophoresis etc.     

Flexible,stretchable and conductive PVA/PEDOT:PSS composite hydrogels prepared by SIPN strategy

Stretchable conductive hydrogels have received significant attention due to their possibility of being utilized in wearable electronics and healthcare devices. In this work, a semi-interpenetrating polymer network (SIPN) strategy was employed to fabricate a set of flexible, stretchable and conductive composite hydrogels composed of polyvinyl alcohol (PVA) in the presence of glutaraldehyde as the crosslinker, HCl as the catalyst and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) as the conductive medium. The results from FTIR, Raman, SEM and TGA indicate that a chemical crosslinking network and interactions of PVA and PEDOT:PSS exist in the SIPN hydrogels. The swelling ratio of hydrogels decreased with increasing content of PEDOT:PSS. Due to the chemical crosslinking network and interactions of PVA and PEDOT:PSS, PVA networks semi-interpenetrated with PEDOT:PSS exhibited excellent tensile and compression properties. The tensile strength and elongation at breakage of the composite hydrogels with 0.14 wt% PEDOT:PSS were 70 KPa and 239%, respectively. The compression stress of the composite hydrogels with 0.14 wt% PEDOT:PSS at a strain of 50% was about 216 KPa. The electrical conductivity of the hydrogels increased with increasing PEDOT:PSS content. The flexible, stretchable and conductive properties endow the composite hydrogel sensor with a superior gauge factor of up to 4.4 (strain: 100%). Coupling the strain sensing capability to the flexibility, good mechanical properties and high electrical conductivity, we consider that the designed PVA/PEDOT:PSS composite hydrogels have promising applications in wearable devices, such as flexible electronic skin and sensitive strain sensors.     

快速响应温敏水凝胶研究进展

温敏水凝胶是一类具有广泛应用前景的高分子材料,但是由于传统方法合成的水凝胶响应速率较慢因而限制了其应用,因此近年来围绕提高传统水凝胶的响应速率做了大量研究工作。本文从几个方面综述了近年来快速响应的温敏水凝胶的研究进展,并对有关现象进行了解释和说明。     

Enhancement in the stability of alginate gels prepared with mixed solution of divalent ions using a diffusion through dialysis tube (DTDT) approach

In this work, calcium alginate hydrogels have been prepared by a new technique, called ‘diffusion through dialysis tube’ (DTDT) which involved transfer of crosslinking ions into alginate solution via a dialysis tube with a molecular weight cutoff of 8 k Da. The various hydrogels, prepared with varying degree of crosslinking, were characterized by FTIR, XRD and TG analysis. We also investigated their dynamic water absorbency in phosphate buffer saline (PBS) of pH 7.4 and the data were interpreted in terms of various kinetic models. The equilibrium water uptake data was used to determine network parameters of hydrogels. Finally, the stability and water absorbency of these hydrogels were enhanced to a desired level by using crosslinker solutions, composed of Ba⁺⁺/Ca ⁺⁺and Zn⁺⁺ / Ca⁺⁺ ions.     

Copolymerization and Properties of Multicomponent Crosslinked Hydrogels

A series of multicomponent hydrogels were prepared by the copolymerization of hydrophobic silicon-containing monomer 3-bis(trimethylsilyloxy) methylsilylpropyl glycerol methacrylate(Si MA) with the solvent-responsive monomers 2-hydroxyethyl methacrylate(HEMA) and N-vinyl pyrrolidone(NVP) and thermosensitive monomer N,N-dimethyl acrylamide(DMA). 2-Hydroxy-2-methyl phenyl acetone(D-1173) was chosen as UV initiator and five different dienes/triene monomers were selected as crosslinking agent in order to select the best crosslinker. The ethanol extraction experiments as well as the FTIR, DSC and TG results showed that the copolymerization was effective. The optical, permeability, and mechanical analysis results demonstrated that the obtained hydrogels were highly transparent with good oxygen permeability and mechanical properties. And the impact of crosslinker on the mechanical properties of the hydrogels was also discussed in detail. The basic results demonstrated that the obtained hydrogels had good stimuli-responsive effects to both p H value and solvent.     

Synthesis and characterization of new hydrogels through copolymerization of N-acryloyl-tris-(hydroxymethyl)aminomethane and different crosslinking agents

In this study, new hydrogels in rod shape were prepared from N-acryloyl-tris-(hydroxymethyl)aminomethane (NAT) using three different crosslinking agents: poly(2-methyl-2-oxazoline) bismacromonomer (BM), ethylene glycol dimethacrylate (EGDMA) and N,N′-methylenebisacrylamide (BIS). Dimethylformamide (DMF) was used as solvent and 2,2′-azobisisobutyronitrile (AIBN) as initiator. Polymeric matrices with different properties were obtained by free radical polymerization by changing the crosslinker (BM, EGDMA or BIS) or the concentration of BM. The hydrogel structures were characterized by high resolution magic angle spinning (HRMAS) NMR technique. Swelling experiments and rheological studies were used to test the water absorption capacity and viscoelastic properties of the hydrogels, respectively. For a given NAT/crosslinking agent molar ratio, the hydrogel synthesized with BM displays higher water absorptive capacity and larger range of linear viscoelasticity than those synthesized with BIS or EGDMA. The relatively larger hydrophilic character of the former and the lower crosslinking density generated by the longer molecules of BM might be the cause of this behavior. The results also reveal that water diffuses into the network following a non-Fickian mechanism. This is concluded from the value of the diffusion exponent n, which is higher than 0.50. The elastic modulus and the equilibrium water content (EWC) measurements suggest that these materials may have potential application as biomaterials.     

Symmetrical biodegradable crosslinkers for use in polymeric devices

There is a need for biodegradable hydrogels that deteriorate at defined rates under physiological conditions for use in engineered tissue constructs and drug delivery. These hydrogels should contain components that are readily synthesized, biocompatible and easily incorporated into hydrogel networks. This need was addressed through a judiciously designed series of crosslinkers composed of symmetrical oligo-glycolate and oligo-lactate esters terminated with vinylic moieties (1). These materials were incorporated into poly(HPMA) networks via free-radical polymerization. This work describes the preparation of symmetrical, lactate and glycolate ester based crosslinking agents and their incorporation into a hydrogel network composed of 2-hydroxypropyl methacrylamide (HPMA). By varying the number of lactic and glycolic acid residues (n = 0, 1, 2) within the crosslinker, the rate of hydrolytic degradation of the gel can be controlled.     

A galactose polyacrylate-based hydrogel scaffold for the detection of cholera toxin and staphylococcal enterotoxin B in a sandwich immunoassay format

A galactoside-based polyacrylate hydrogel was used as a scaffold to immobilize antibodies for the development of a sandwich immunoassay to detect cholera toxin (CT) and staphylococcal enterotoxin B (SEB). The hydrogel possesses large pores and simulates a solution-like environment allowing easy penetration of large biomolecules. Highly crosslinked hydrogels containing pendant amine or carboxyl functionalities were polymerized through a free-radical polymerization process. Covalent crosslinking of the antibodies on hydrogel films was accomplished using a homobifunctional crosslinker or carbodiimide chemistry. Utilizing the two different crosslinking methodologies, our results demonstrated the effectiveness of repetitive additions of crosslinker reactant into a single location on the gel surface. This approach in fact increased the amount of immobilized antibody. Patterned arrays of the immobilized antibodies for sandwich immunoassay development were achieved using a PDMS template containing micro-channels. This template provided a suitable means for applying reagents in multiple cycles. Fluorescence and three-dimensional (3D) imaging by confocal microscopy and laser scanning confocal microscopy of Cy3-labeled anti-CT and/or Cy3-anti-SEB tracer molecules provided qualitative and quantitative measurements on the efficiency of protein immobilization, detection sensitivity and signal-to-noise ratios. As a result of using the galactose polyacrylate-base hydrogel as a platform for immunoassay development, we have successfully been able to achieve low limits of detection for SEB and cholera toxins (1.0 ng mL(-1)). Repetitive additions (>3 cycles) of the crosslinker and antibody have also shown a dramatic increase in the immobilization of antibody resulting in improved immunoassay sensitivity. Fluorescence signal-to-noise ratios using the hydrogel-based immunoassays have been observed as high a 40:1.     

Designing Visible Light‐Cured Thiol‐Acrylate Hydrogels for Studying the HIPPO Pathway Activation in Hepatocellular Carcinoma Cells

Various polymerization mechanisms have been developed to prepare peptide‐immobilized poly(ethylene glycol) (PEG) hydrogels, a class of biomaterials suitable for studying cell biology in vitro. Here, a visible light mediated thiol‐acrylate photopolymerization scheme is reported to synthesize dually degradable PEG‐peptide hydrogels with controllable crosslinking and degradability. The influence of immobilized monothiol pendant peptide is systematically evaluated on the crosslinking of these hydrogels. Further, methods are proposed to modulate hydrogel crosslinking, including adjusting concentration of comonomer or altering the design of multifunctional peptide crosslinker. Due to the formation of thioether ester bonds, these hydrogels are hydrolytically degradable. If the dithiol peptide linkers used are susceptible to protease cleavage, these thiol‐acrylate hydrogels can be designed to undergo partial proteolysis. The differences between linear and multiarm PEG‐acrylate (i.e., PEGDA vs PEG4A) are also evaluated. Finally, the use of the mixed‐mode thiol‐acrylate PEG4A‐peptide hydrogels is explored for in situ encapsulation of hepatocellular carcinoma cells (Huh7). The effects of matrix stiffness and integrin binding motif (e.g., RGDS) on Huh7 cell growth and HIPPO pathway activation are studied using PEG4A‐peptide hydrogels. This visible light poly­merized thiol‐acrylate hydrogel system represents an alternative to existing light‐cured hydrogel platforms and shall be useful in many biomedical applications.     

Cytocompatible,Photoreversible, and Self‐Healing Hydrogels for Regulating Bone Marrow Stromal Cell Differentiation

Photo‐crosslinking and self‐healing have received considerable attention for the design of intelligent materials. A novel photostimulated, self‐healing, and cytocompatible hydrogel system is reported. A coumarin methacrylate crosslinker is synthesized to modify the polyacrylamide‐based hydrogels. With the [2+2] cyclo‐addition of coumarin moieties, the hydrogels exhibit excellent self‐healing capacity when they are exposed to light with wavelengths at 280 and 365 nm, respectively. To enhance cell compatibility, a poly (amidoamine) crosslinker is also synthesized. Variations in light exposure times and irradiation wavelengths are found to alter the self‐healing property of the hydrogels. The hydrogels are shown to induce a regular cellular pattern. The hydrogels are used to regulate bone marrow stromal cells differentiation. The relative mRNA expressions are recorded to monitor the osteogenic differentiation of the cells.

     

Preparation and Characterization of Attractive Poly(amino acid)Hydrogels Based on 2-Ureido-4[1H]-pyrimidinone

Self-healing hydrogels with the shear-thinning property are novel injectable materials and are superior to traditional injectable hydrogels.The self-healing hydrogels based on 2-ureido-4[1 H]-pyrimidinone(UPy)have recently received extensive attention due to their dynamic reversibility of UPy dimerization.However,generally,UPy-based self-healing hydrogels exhibit poor stability,cannot degrade in vivo and can hardly be excreted from the body,which considerably limit their bio-application.Here,using poly(l-glutamic acid)(PLGA)as biodegradable matrix,branchingα-hydroxy-ω-amino poly(ethylene oxide)(HAPEO)as bridging molecule to introduce UPy,and ethyl acrylate polyethylene glycol(MAPEG)to introduce double bond,the hydrogel precursors(PMHU)are prepared.A library of the self-healing hydrogels has been achieved with well self-healable and shear-thinning properties.With the increase of MAPEG grafting ratio,the storage modulus of the self-healing hydrogels decreases.The self-healing hydrogels are stable in solution only for 6 h,hard to meet the requirements of tissue regeneration.Consequently,ultraviolet(UV)photo-crosslinking is involved to obtain the dual crosslinking hydrogels with enhanced mechanical properties and stability.When MAPEG grafting ratio is 35.5%,the dual crosslinking hydrogels can maintain the shape in phosphate-buffered saline solution(PBS)for at least 8 days.Loading with adipose-derived stem cell spheroids,the self-healing hydrogels are injected and self-heal to a whole,and then they are crosslinked in situ via UV-irradiation,obtaining the dual crosslinking hydrogels/cell spheroids complex with cell viability of 86.7%±6.0%,which demonstrates excellent injectability,subcutaneous gelatinization,and biocompatibility of hydrogels as cell carriers.The novel PMHU hydrogels crosslinked by quadruple hydrogen bonding and then dual photo-crosslinking of double bond are expected to be applied for minimal invasive surgery or therapies in tissue engineering.     

二乙烯基砜交联的可德胶化学水凝胶的制备与表征

以二乙烯基砜(DVS)作为交联剂通过亲电加成反应制备了可德胶化学水凝胶。采用傅里叶变换红外光谱(FT-IR)、扫描电子显微镜(SEM)和质构分析手段表征了所得凝胶的结构与性能,并对其形貌、溶胀率以及水凝胶的力学性质与制备条件的关系进行了研究。结果表明:所得凝胶网络结构较为致密均匀,溶胀率依赖于交联度。可德胶化学水凝胶的强度和韧性可以通过DVS的用量、可德胶的浓度和碱溶液的浓度进行调控。     

Characterization and Evaluation of Acrylic Acid Co-2-acrylamido-2-methylpropane-1-sulfonic Acid Hydrogels for Uranium Recovery

Crosslinked 2-acrylamido-2-methyl-1-propane sulfonic acid -co- acrylic acid (AMPS/AA) hydrogels were synthesized by radical polymerization in the presence of N,N′-methylenebisacrylamide (MBA) as the crosslinking agent using potassium persulfate (KPS) as initiator. Hydrogels with different compositions and crosslinker concentrations were prepared. The structures of hydrogels were characterized by FTIR analysis. Thermal stabilities of the hydrogels were investigated using TGA and DSC analysis. Swelling kinetics and the equilibrium water content (EWC) of the hydrogels were studied. The swelling behavior of these hydrogels was investigated at different pHs at room temperature. Also the swelling behavior of these hydrogels was investigated at different ionic strength. The ability of the prepared hydrogels to bind uranium(VI) was tested under noncompetitive conditions by batch equilibrium procedure. Experimental work using uranyl nitrate hexahydrate bought from the local market was carried out in the safeguards destructive analysis laboratory (KMP-I) in the National Center for Nuclear Safety and Radiation Control.     

Synthesis and functionalization of dendron‐polymer conjugate based hydrogels via sequential thiol‐ene “click” reactions

Fabrication and functionalization of hydrogels from well‐defined dendron‐polymer‐dendron conjugates is accomplished using sequential radical thiol‐ene “click” reactions. The dendron‐polymer conjugates were synthesized using an azide‐alkyne “click” reaction of alkene‐containing polyester dendrons bearing an alkyne group at their focal point with linear poly(ethylene glycol)‐bisazides. Thiol‐ene “click” reaction was used for crosslinking these alkene functionalized dendron‐polymer conjugates using a tetrathiol‐based crosslinker to provide clear and transparent hydrogels. Hydrogels with residual alkene groups at crosslinking sites were obtained by tuning the alkene‐thiol stoichiometry. The residual alkene groups allow efficient postfunctionalization of these hydrogel matrices with thiol‐containing molecules via a subsequent radical thiol‐ene reaction. The photochemical nature of radical thiol‐ene reaction was exploited to fabricate micropatterned hydrogels. Tunability of functionalization of these hydrogels, by varying dendron generation and polymer chain length was demonstrated by conjugation of a thiol‐containing fluorescent dye. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 926–934     

Synthesis of thermosensitive guar‐based hydrogels with tunable physico‐chemical properties by click chemistry

Thermosensitive guar‐based hydrogels are obtained in water solutions by copper‐catalyzed 1,3‐dipolar cycloaddition between alkyne‐functionalized guars and α,ω‐diazido‐poly[(ethylene glycol)‐co‐(propylene glycol)]. Characterization by TGA, HR‐MAS ¹H NMR, and rheology have shown that hydrogels with tunable physico‐chemical properties, such as crosslinking density, viscoelasticity, swelling ratio, and so forth, could be obtained by varying the guar molar mass, the degree of alkyne functionality, the guar/crosslinker weight ratio, and the reaction temperature. Based on swelling measurements, it has been shown that the thermal sensitiveness of guar‐based hydrogels is fast, reversible, and intimately related to the weight fraction of the thermosensitive crosslinker in the network. Finally, the monitoring of doxorubicin hydrochloride release has demonstrated the potential of these hydrogels as temperature‐dependent drug release devices. The robust, efficient, and orthogonal approach described herein represents a general approach towards the development of well‐controlled guar‐based hydrogels using α,ω‐diazido crosslinkers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2733–2742, 2010     

Thermoresponsive super water absorbent hydrogels prepared by frontal polymerization of N‐isopropyl acrylamide and 3‐sulfopropyl acrylate potassium salt

Super water absorbent polymer hydrogels were synthesized by frontal polymerization. These materials were obtained by copolymerizing N‐isopropyl acrylamide (NIPAAm) and 3‐sulfopropyl acrylate potassium salt (SPAK) in the presence of N,N′‐methylene‐bis‐acrylamide as a crosslinker. It was found that their swelling behavior in water can be easily tuned by using either the appropriate monomer ratio or the amount of the crosslinker used. Namely, the swelling ratio was found to range from about 1000% for the NIPAAm homopolymer in the presence of 5.0 mol % of crosslinker, up to 35,000% for the sample containing 87.5 mol % of SPAK and 1.0 mol % of crosslinker. The affinity toward water was also confirmed by contact angle analysis. Moreover, the obtained hydrogels exhibit a thermoresponsive behavior, with a lower critical solution temperature of about 28–30 °C. This value is close to that of poly(NIPAAm) but with a swelling capability that dramatically increases as the amount of SPAK increases. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

丙烯酸乙酯/烯丙基缩水甘油醚共聚物的热可逆共价交联

研究了双环戊二烯基二羧酸及其铵盐对丙烯酸乙酯 /烯丙基缩水甘油醚共聚物的交联反应 ,对比了二者的交联速度 .用DSC评价了它们的热可逆转化行为 .制得了热可逆部分达 79 7%的共价交联环氧型丙烯酸酯橡胶     

Synthesis and characterization of acrylamide/acrylic acid hydrogels crosslinked using a novel diacrylate of glycerol to produce multistructured materials

In this work we propose a new crosslinking agent and the method to use it for the synthesis of acrylate based hydrogels. The use of this diacrylate of glycerol, synthesized in our laboratory, allows the generation of materials with well defined micro‐structures in the dry state, unique meso‐ and macro‐structures during swelling, and enhanced mechanical properties and swelling capacity in water. These properties depend on the crosslinking agent concentration, as well as synthesis thermal history. Poly(acrylamide‐co‐acrylic acid) hydrogels are commonly crosslinked with N, N′‐methylenebisacrylamide or N‐isopropylacrylamide. Here we obtain and use a new crosslinking agent, obtained from the reaction between glycerol and acrylic acid to produce a Diacrylate of glycerol (DAG). Two synthesis methods at equivalent molar ratio of acrylamide/acrylic acid (AM/AA) were analyzed. The mechanical properties, the swelling capacity, and the morphology at microscale of these hydrogels showed a well defined transition at a critical concentration of crosslinking agent. DAG induces the generation of hydrogels with hierarchichal structure. The micro‐structure surface morphology was investigated by scanning electron microscopy, the meso‐structure by polarized light microscopy and the macro‐structure by CCD imaging. The hydrogels with hierarchical structures showed improved mechanical properties when compared with structureless hydrogels. Control of the microstructure allows the generation of materials for different applications, i.e. templates or smart materials that interact with electromagnetic radiation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2667–2679, 2008