Cross-linking Of Polymer Chains Research Articles

Study of Several Alginate-Based Hydrogels for In Vitro 3D Cell Cultures.

Hydrogel, a special system of polymer solutions, can be obtained through the physical/chemical/enzymic crosslinking of polymer chains in a water-based dispersion medium. Different compositions and crosslinking methods endow hydrogel with diverse physicochemical properties. Those hydrogels with suitable physicochemical properties hold manifold functions in biomedical fields, such as cell transplantation, tissue engineering, organ manufacturing, drug releasing and pathological model analysis. In this study, several alginate-based composite hydrogels, including gelatin/alginate (G-A), gelatin/alginate/agarose (G-A-A), fibrinogen/alginate (F-A), fibrinogen/alginate/agarose (F-A-A) and control alginate (A) and alginate/agarose (A-A), were constructed. We researched the advantages and disadvantages of these hydrogels in terms of their microscopic structure (cell living space), water holding capacity, swelling rate, swelling–erosion ratio, mechanical properties and biocompatibility. Briefly, alginate-based hydrogels can be used for three-dimensional (3D) cell culture alone. However, when mixed with other natural polymers in different proportions, a relatively stable network with a good cytocompatibility, mechanical strength and water holding capacity can be formed. The physical and chemical properties of the hydrogels can be adjusted by changing the composition, proportion and cross-linking methods of the polymers. Conclusively, the G-A-A and F-A-A hydrogels are the best hydrogels for the in vitro 3D cell cultures and pathological model construction.

Analyses of structure and thermal properties of synthesized crosslinked poly(1-vinyl-2-pyrrolidone-co-vinyl acetate) hydrogels

This paper describes the process for the synthesis of chemically crosslinked copolymer hydrogels based on monomer 1-vinyl-2-pyrrolidone and comonomer vinyl acetate, using the varied content of ethylene glycol dimethacrylate as a crosslinker, by the free radical polymerization method with thermal initiation. The content of unreacted reactants after poly(1-vinyl-2-pyrrolidone-co-vinyl-acetate), p(VP-VA), hydrogels synthesis was examined using high-pressure liquid chromatography (HPLC) method. Structure characterization of the obtained p(VP-VA) hydrogels was performed using the Fourier transform infrared spectroscopy (FTIR). In this study, the influences of crosslinker content and temperature on the swelling behaviour of p(VP-VA) were studied. Quantities of unreacted comonomers and crosslinker, calculated in relation to the initial amount present in the reaction mixture, confirmed their successful conversion into p(VP-VA) hydrogels. These unreacted values of 1-vinyl-2-pyrrolidone (in range of 0.605-1.609%), vinyl acetate (in range of 2.486-4.798%), and ethylene glycol dimethacrylate (in range of 0.889-3.240%) were within acceptable limits, and they were removed from the final products. FTIR spectra were verified that the copolymerization process was performed, and chemically crosslinking of polymer chains occurred by breaking double bonds from the reactants. Obtained crosslinked copolymers could be classified in the class of negative-thermosensitive hydrogels because they can swell and pass through a phase transition when heated from the swollen state at 25ºC to contracted state at 80ºC.

Synthetic Matrices for Intestinal Organoid Culture: Implications for Better Performance.

Organoids are three-dimensional (3D) self-renewing and self-organizing clusters of cells that imitate an organ’s structure and function, making them an important tool in various fields ranging from regenerative medicine to drug discovery. Organoids can be developed ex vivo by isolating adult stem cells from an organ-specific tissue (e.g., intestine, brain, and lung) and allowing the stem cells to grow and differentiate in an appropriate growth media with some structural support elements. A 3D extracellular matrix (ECM) hydrogel, a network of highly hydrophilic cross-linked polymer chains, provides essential support and cues for ex vivo organoid growth. Commercially available hydrogel matrices (for example, Matrigel and collagen) are primarily derived from animal tissues. Notably, these animal-derived hydrogel matrices are not suitable for controlled modifications and pose risks of immunogen and pathogen transfer, thus diminishing their clinical application. These limitations of animal-derived hydrogel matrices can, however, be overcome using synthetic hydrogel matrices based on polymers such as polyethylene glycol, nanocellulose, alginate, hyaluronic acid, and polylactic-co-glycolic acid. This review highlights some of the current approaches and advantages of developing synthetic ECM-mimic hydrogels, focusing primarily on intestinal organoid culture.

Radiation synthesis of poly(acrylic acid) nanogels for drug delivery applications – post-synthesis product colloidal stability

Abstract Synthesis of polymer nanogels (NGs) for biomedical applications is considered to be a very promising application in radiation engineering. Under high-dose pulse irradiation of dilute aqueous polymer solution, reactive species generated by water radiolysis can create multiple radicals on each macromolecule and consequently induce intramolecular cross-linking of polymer chains, resulting in NG formation. The obtained products are free from harmful monomers, initiators, and cross-linking agents, which makes them potentially applicable for drug delivery applications. One of the biggest challenges in handling and use of nanoparticles, however, is the colloidal stability, when aqueous suspensions are stored for prolonged periods. Therefore, development of the best protocols for the particular nanocarrier storage is key. To address this need, we have performed the prospective study in which we systematically assessed the influence of various processing and storage scenarios feasible in our lab, on the colloidal stability of the radiation-synthesized poly(acrylic acid) (PAA) NG particles in suspension. This allowed us to choose the optimal way of handling the product after its synthesis. We confirmed that none of the strategies we used and tested are substantially detrimental to our product. Filtration with 0.2-μm filters was proven sufficient for sample purification and prolonged storage in aqueous suspension did not exert a negative effect on the colloidal stability of particles suspension. We have also demonstrated that lyoprotectant-free lyophilization was suitable for our polymer nanoparticles. This is an important fact for further application of particles as nanocarriers for biologically active compounds such as targeting ligands or therapeutic moieties.

Tailoring Lifetimes and Properties of Carbodiimide-Fueled Covalently Cross-linked Polymer Networks

The mechanical properties of nonequilibrium polymer hydrogels obtained from the transient cross-linking of polymer chains by a chemical fuel were investigated. Aqueous polymers featuring pendant carboxylic acids were treated with a carbodiimide to give anhydride-cross-linked gels. The anhydrides spontaneously hydrolyze back to the polymer solution, and the cycle can be repeated multiple times. Oscillatory rheology was employed to study the effects of temperature, fuel concentration, chain length, and polymer composition on the storage and loss moduli of the polymeric materials as well as the time taken for the polymers to undergo decross-linking. Regardless of the temperature used, at constant carbodiimide concentration, degelation times are more sensitive to experimental temperature than are the peak storage moduli. Decross-linking times decrease with increasing temperature. As carbodiimide concentration decreases, there is a decrease in moduli and decross-linking times. Within the scope of materials studied, the polymer structure was found to have a relatively small impact on the transient properties of gel networks compared to the fuel concentration and temperature. These findings facilitate the design of tunable on-demand networks and gels.

Structure of Polytetrafluoroethylene Modified by the Combined Action of γ-Radiation and High Temperatures.

By means of X-ray computed microtomography (XCMT), the existence of a developed porous structure with an average pore diameter of ~3.5 μm and pore content of ~1.1 vol.% has been revealed in unirradiated polytetrafluoroethylene (PTFE). It has been found that the combined action of gamma radiation (absorbed dose per PTFE of ~170 kGy) and high temperatures (327–350 °C) leads to the disappearance of the porous structure and the formation of several large pores with sizes from 30 to 50 μm in the bulk of thermal-radiation modified PTFE (TRM-PTFE). It has been established by X-ray diffraction (XRD) analysis that the thermal-radiation modification of PTFE leads to an increase in the interplanar spacings, the degree of crystallinity and the volume of the unit cell, as well as to a decrease in the size of crystals and the X-ray density of the crystalline phase in comparison with the initial polymer. It is assumed that the previously-established effect of improving the deformation-strength and tribological properties of the TRM-PTFE can be due not only to the radiation cross-linking of polymer chains but also to the disappearance of the pore system and to the ordering of the crystalline phase of PTFE.

CO2 laser fabrication of hydrogel-based open-channel microfluidic devices.

This study proposed a rapid and low-cost fabrication method for open-channel hydrogel-based microfluidic devices using CO2 laser ablation. The agarose hydrogel substrate was prepared with agarose gelation in DI water upon microwave heating, then a commercial CO2 laser system was used for the direct laser ablation of microchannels on the surface of agarose hydrogel substrate, the hydrophilic nature of the microchannels fabricated on hydrogel substrate enables the self-driven of the liquid inside the microchannels with capillary force. The profiles of the laser ablated microchannels on agarose hydrogel substrate with various laser power and scan speed were studied in detail. Due to the loss of water when exposed to the atmosphere, significant deformation of the fabricated microchannels was observed, and the profile change was recorded for 48h for comparison. An easy-to-access storage method of hydrogel-based microfluidic device in DI water was also proposed in this study. Unlike compact silicon or polymer-based microfluidic devices, the hydrogel is formed by cross-linked polymer chains filled with water, for a better understanding of the diffusion of small molecules into the bulk hydrogel material during fluid propagation inside the microchannel, the Nile red fluorescent was added into the liquid and the diffusion across the hydrogel-based microchannels with time was measured and discussed in this study. Several open-channel agarose hydrogel-based microfluidic devices were fabricated in this study for the demonstration of the proposed fabrication method. The CO2 laser ablation approach for agarose hydrogel-based microfluidic devices has the advantages of rapid processing time, low-cost, highly biocompatible, and self-driven without pumps and could have wide application potentials in biological and medical fields.

Polymeric materials reinforced by noncovalent aggregates of polymer chains

AbstractMechanical performances are among the most fundamental properties that dictate the applicability and durability of polymeric materials. Reinforcement of polymeric materials is eternally pursued to broaden the applications of polymers with light‐weight, low‐cost and easy‐processing advantages. Noncovalent aggregates of biomacromolecules have been found to play a significant role in the mechanical properties of many natural materials, such as the spider silk. Increasing numbers of reports have demonstrated that the in situ formed noncovalent aggregates of polymer chains in polymeric systems are highly effective for enhancing the mechanical properties of artificial polymeric materials, in terms of strength, stiffness, toughness, and/or elasticity. The in situ formed noncovalent aggregates act as additional crosslinking domains and well‐dispersed “hard” nanofillers in the polymer networks, significantly strengthening, stiffening and/or toughening the polymeric materials. Moreover, the noncovalent crosslinking of polymer chains favors the development of healable and recyclable polymeric materials, thanks to the reversible and dynamic properties of noncovalent bonds. This review provides an overview of the recent advances on the enhancement of the mechanical properties of different polymeric materials by the in situ formed noncovalent aggregates of polymer chains. It is expected to arouse inspirations for the development of novel polymeric materials with extraordinary mechanical performances and functionalities.

UV- and Thermally-Active Bifunctional Gelators Create Surface-Anchored Polymer Networks.

A versatile one-step synthesis of surface-attached polymer networks using small bifunctional gelators (SBG), namely 4-azidosulfonylphenethyltrimethoxysilane (4-ASPTMS) and 6-azidosulfonylhexyltriethoxysilane (6-ASHTES) is reported. A thin layer (≈200nm) of a mixture comprising ≈90% precursor polymer and 10% of 4-ASPTMS or 10% 6-ASHTES on a silicon wafer is deposited. Upon UV irradiation (≈l-254nm) or annealing (>100°C) layers, sulfonyl azides (SAz) release nitrogen by forming singlet and triplet nitrenes that concurrently react with any C─H bond in the vicinity resulting in sulfonamide crosslinks. Condensation among tri-alkoxy groups (i.e., methoxy or ethoxy) in bulk connects the SBG units, which completes the crosslinking. Concurrently, when such functionalities react with hydroxyl groups at the surface, which enable the covalent attachment of the crosslinked polymer chains. A systematic investigation on reaction mechanism and gel formation using spectroscopic ellipsometry (SE) and Fourier-transform infrared spectroscopy in the attenuated total reflection mode (FTIR-ATR) is performed. Analogous thermally initiated gelation for both 4-ASPTMS and 6-ASHTES is found. The 6-ASHTES is UV inactive at ≈l-254nm, while the 4-ASPTMS is active and forms gels. The difference is attributed to the aromatic nature of 4-ASPTMS that absorb UV light at ≈l-254nm due to π-π* transition.

Nano- and microgels: a review for educators

Abstract Nano- and microgels are promising soft polymer materials for different application fields: stabilizers, sensors, catalysts, selective sorbents, drug delivery carriers etc. They are composed of cross-linked polymer chains swollen with a solvent. The building blocks, synthesis approaches and architecture of nano- and microgels are reviewed. The mechanisms of responsiveness to various stimuli are described, examples of applications are provided. Micro- and nanogels are good objects for learning projects and the ideas for learning projects with microgels are described.

Modeling swelling behavior of hydrogels in aqueous organic solvents

Hydrogel, a three-dimensional elastic structure of cross-linked polymer chains, swells or deswells when immersed in aqueous organic solutions. The swelling of hydrogels depends on temperature, solvent composition, and hydrogel structure as the thermodynamic driving forces for hydrogel swelling are due to solvent – hydrogel interactions and expansion of elastic network. In this work, polymer Nonrandom Two-Liquid activity coefficient model is used to account for the solvent – hydrogel interactions and a semi-empirical expression for rubber elasticity is used to describe the expansion of elastic network. The model successfully correlates data for both swelling and equilibrium phase compositions for n-isopropyl acrylamide hydrogels in water – ethanol, water – acetone, water – n-butanol, and water – methyl isobutyl ketone mixed solvents.

Wideband reflection wavelength tuning by bending of cholesteric liquid crystal elastomer films

Cholesteric liquid crystal elastomers (CLCEs), which exhibit selective reflection derived from a helical molecular structure, are receiving a great deal of attention because they deform largely due to the cross-linked polymer chains. Reflection wavelength of a CLCE film can be tuned by mechanical stretching that induces a change in the helical pitch. However, stretch-induced reflection wavelength tuning has some issues such as a large load required and a limited tuning range. In this paper, reflection wavelength of a CLCE film is tuned facilely and widely by bending. Outward and inward bendings cause blue and red shifts, respectively. Bending–buckling load required for the reflection tuning is much lower than stretching one, which is proved experimentally and theoretically. By considering the bending behavior of materials, we can impose large strain on a CLCE film and tune reflection wavelength over 300 nm, which is almost the whole region of visible light. This wideband reflection wavelength tuning by low-load bending leads to expanding applications of CLCEs.

Thermomechanical and gas transport behavior of silica-doped polymer blend nanocomposites

Blending of polymers enhances the transport rate of gases due to cross-linking of polymer chains. Nanofillers dispersed into a single polymer matrix also tend to modify the transport behavior. Polymer blend nanocomposites show changes in their thermomechanical and gas transport properties. Blending plays a vital role in improving gas permeation. Silica nanoparticles are used as filler with a polycarbonate and polysulfone polymer matrix. Membranes were prepared using the solution-casting method, and uniform dispersion of silica nanoparticles was obtained by sonication. A gas permeability test for hydrogen (H2) and carbon dioxide (CO2) was carried at 30 pounds per square inch (207 kPa) constant upstream pressure at room temperature using the constant pressure/variable volume method. Oxygen permeability was determined from the oxygen transmission rate using the differential pressure method. Characterization of thermomechanical properties was carried out using differential scanning calorimetry and dynamic mechanical analysis.

Fast, low-energy additive manufacturing of isotropic parts via reactive extrusion

Abstract Despite the well-documented advantages of additive manufacturing (AM) compared to conventional manufacturing processes, most AM technologies are subject to some significant limitations, including slow processing times, substantial energy needs, and anisotropic part properties, which become even more important as part size increases. To address these limitations, a reactive extrusion AM (REAM) process is introduced, in which successive layers of a thermoset resin are deposited and cured rapidly in situ with no external energy input aside from that required to pump and position the resin. Mechanical properties of tensile specimens fabricated in multiple orientations indicate that the tensile modulus and ultimate strength are isotropic, but elongation at break and toughness depend on orientation. The isotropic properties are attributed to chemical crosslinking of polymer chains that occurs between layers of printed parts. Moreover, the tensile modulus and strength are influenced by the local thermal environment in the build, which varies within the build envelope because of the exothermic nature of the polymerization reaction and affects the degree of curing within the specimen. The degree of curing was measured via differential scanning calorimetry (DSC) and was commensurate with the tensile strength of the tested coupons. When coupled with a large nozzle and high material flow rates, the REAM system can create nearly isotropic structural parts substantially more quickly than other AM systems, while using appreciably less energy at the point of manufacture.

Stimuli-responsive microgels with cationic moieties: characterization and interaction with E. coli cells.

Stimuli-responsive microgel copolymer networks with ionizable functional groups have important applications for encapsulation of drugs, peptides, enzymes, proteins, or cells. Rational design of such networks can be based on characterization of stimuli-induced volume phase transition and spatial distribution of neutral and charged monomer units in crosslinked polymer chains. In this work we successfully synthesized poly(N-vinylcaprolactam-co-1-vinyl-3-methylimidazolium) (poly(VCL-VIM+)) microgels carrying permanent positive charges and demonstrate that 1H high-resolution NMR spectroscopy in combination with transverse (T2) magnetization relaxometry allows investigating separately the behavior of each functional group in the microgel network. The information about comonomer transition temperatures, width of transition, and change in transition entropy were reported and correlated with the concentration of charged functional groups and resulting electrophoretic mobility. A two-state approach was used to describe the temperature-induced volume phase transition separately for neutral and charged polymer segments. The core-corona architecture specific to each functional group was detected revealing that the charged methylated vinylimidazolium groups (VIM+) are concentrated mainly in the corona of the microgel. These biocompatible PVCL-based microgels functionalized with permanent positive charges are shown to serve as an antibacterial system against Gram-negative E. coli strains, due to the positive charge of the incorporated VIM+ comonomer in the polymer network.

Hydrogel: A potential therapeutic material for bone tissue engineering

Current surgical treatments and material applications are not ideal for the treatment of orthopedic clinical injuries, such as large bone defects, cartilage defects, and vascular tendon adhesions that occur after repair. With the continuous development of tissue engineering technology, hydrogels have become important medical biomaterials. Hydrogels are three-dimensional hydrophilic network structures composed of cross-linked polymer chains. They are a new kind of polymeric material for the treatment of orthopedic diseases. Hydrogels have good biocompatibility, biodegradability, drug-carrying capacity, and controllable drug release ability and are less toxic than nanoparticle carriers. They have been widely used in wound repair, guided tissue regeneration, bacteriostasis, hemostasis, postoperative adhesion prevention, drug delivery, and 3D printing. These characteristics can be used to develop a variety of treatments for different diseases. This paper focuses on the innovative progress of hydrogels in promoting and improving bone, cartilage, tendon, and soft tissue regeneration in orthopedic clinical applications. Current and prospective applications of hydrogels in the field of orthopedics are discussed herein.

Alignment of magnetic particles in hydrogel matrix: A novel anisotropic magnetic hydrogels for soft robotics

Magnetic hydrogels are composed of magnetic particles and hydrogel matrix. In recent years, the magnetic hydrogels have been developed rapidly because they have shown promising applications in drug release and artificial muscle. In this paper, we proposed a study to develop novel anisotropic magnetic hydrogels and investigate their mechanical and sensing properties for possible applications in soft robotics. In preparing the anisotropic magnetic hydrogels, the polyacrylamide (PAAm) hydrogel is chosen as a model hydrogel because of its popular application in soft electronics and ionic conductors. A method of free radicals copolymerization is employed to fill (polyacrylic acid/acrylamide) polymers in preparing anisotropic hydrogels under the magnetic field. Unlike most of the previous studies which incorporated magnetic nanoparticles into hydrogels, we mixed the micro-size carbonyl iron particles (CIPs) with the hydrogel and cured them under a magnetic field to form anisotropic structures within its crosslinking polymer chains. The particles and formed particle chains will not only improve the mechanical properties of the hydrogels but also provide sensing function as the electrical resistance changed from mechanical deformation referred to piezoresistivity. We experimentally evaluated the magnetorheological and the piezoresistive behaviors of the magnetic hydrogels, and demonstrated their potential use in soft robots as flexible touch sensors and variable-stiffness devices.

Hydrogel: Promising alternative for water and nutrient management

Nowadays water management is considered as one of the major challenges for all countries. It is estimated that by 2030, global water demand is probable to be 50% higher than today and this will lead us towards water scarcity, at the same time agricultural sector used over 70 percent of freshwater in most regions of the world. Among all the cultivated areas in India 60% lands are under dryland condition and 30% area has less rainfall. So, cultivation under shortage of water is a major problem in India. Under such condition, application of hydrogel can be used as an alternative which can reduce the problem of water scarcity. Hydrogels are the network of crosslinked polymer chains which can swell to absorb huge volume of water. Hydrogel releases water and nutrient to the plants when soils around root zone of plants starts to dry up. Application of hydrogel polymer creates a water reservoir near the root zone of plants, improve the capacity of available water to plant, enhances plant growth and increases yield and hence reduces cost of cultivation. Hence, hydrogel plays an important role in modern agriculture.

Chitosan Oligosaccharide Based Hydrogel: an Insight into the Mechanical, Drug Delivery, and Antimicrobial Studies

Hydrogel is a macromolecular polymer gel constructed of a network of cross-linked polymer chains. Chitosan oligosaccharide (CO) is a natural polymer that has an efficient biodegradability and is used to prepare hydrogels for efficient drug delivery. CO is combined with Carboxy Methyl Cellulose (CMC) to produce a hydrogel in different compositions for the best suitable option where the drug delivery and biocompatibility are most efficient. The chemical hydrogels of the composition CO and CMC were prepared by adding the cross-linker mixture of glutaraldehyde, alcohol, and distilled water. After the hydrogels were prepared, it is subjected to a series of tests, i.e., microscopy, hemocompatibility, mechanical strength, impedance measurement, drug release, antimicrobial activity. Ccross-linking is achieved by adding a mixture of glutaraldehyde, alcohol, and distilled water. Thus, the best composition of the mixture of the two compounds – Chitosan oligosaccharide and the Carboxy Methyl Cellulose is determined and used for efficient drug delivery.

Selective Doping of Positive and Negative Spatial Defects into Polymer Gels by Tuning the Pregel Packing Conditions of Star Polymers

Gels are giant single molecules that consist of a very large number (∼Avogadro’s number) of cross-linked nanometer-size polymer chains. Unlike most low-molecular-weight compounds, the extensively c...