Influence of Nano-Bioactive Glass (NBG) Content on Properties of Gelatin-Hyaluronic Acid/NBG Composite Scaffolds

The development of three-dimensional (3-D) scaffolds with highly open porous structure is one of the most important issues in tissue engineering. A novel nanocomposite scaffold of gelatin (Gel), hyaluronic acid (HA), and nano-bioactive glass (NBG) was prepared by blending NBG with a Gel and HA solution followed by lyophilization. The effects of NBG content on the properties of the Gel-HA/NBG composite scaffolds, including the morphologies, porosity, compressive strength, swelling behavior, cell viability and alkaline phosphatase (ALP) activity, were investigated. Porous composite scaffolds with interconnected pores were obtained and the pores became cylindrical with increasing NBG content. The porosity percent and swelling ability decreased with increasing NBG content; however, the compressive strength, cell viability and ALP activity were enhanced. All the results showed the addition of NBG particles can improve the physicochemical and biological properties and the Gel-HA/NBG composite scaffolds exhibited good potential for tissue engineering applications.     

Preparation and Properties of Polyvinyl Alcohol/Collagen Hydrogel

Hydrogel scaffolds based on poly(vinyl alcohol) (PVA) collagen films were prepared by a chemical cross-linking method. The effects of the contents of PVA and cross-linker on compressive strength and swelling ratio were studied, and the effect of the pH value of the immersion medium on the swelling ratio was also investigated. The results showed that the introduction of PVA improved the compressive strength of PVA/collagen hydrogel, and the swelling ratio of the hydrogel scaffold increased with increasing PVA content in the blends. With increasing cross-linker content, the swelling ratio decreased; however, the compressive strength increased. The swelling ratio of PVA/collagen scaffold increased when pH was decreased. In conclusion, swelling ratio and compressive strength in PVA/collagen blends can be controlled by variation of their contents, cross-linking agent content, and pH value.     

Preparation and Characterization of Chemically Crosslinked Polyvinyl Alcohol/Carboxylated Nanocrystalline Cellulose Nanocomposite Hydrogel Films with High Mechanical Strength

Chemically crosslinked polyvinyl alcohol (PVA)/carboxylated nanocry-stalline cellulose (PVA/CNCC) nanocomposite hydrogel films were fabricated by film-casting of PVA/CNCC mixture solutions and subsequent thermal-curing of the PVA with the CNCC. Gel fractions of the hydrogel films were measured to confirm the occurrence of crosslinking. Morphologies of the hydrogel films were characterized by polarized light microscopy and scanning electron microscopy (SEM). Thermal properties, swelling behavior and mechanical properties of the hydrogel films were investigated to evaluate the influence of CNCC content (10～30% of PVA mass). Equilibrium water content of the hydrogel films was in the range of 40～49%. At swelling equilibrium, the hydrogel films could be stretched to 3～3.4 times their original length, and their tensile strength was in the range of 7.9～11.6 MPa. The results show that the PVA/CNCC nanocomposite hydrogel films were both extensible and highly tough.     

Synthesis and Properties of Polyacrylamide-Based Conducting Gels with Enhanced Mechanical Strength

A nanocomposite conducting hydrogel, polyacrylamide/MWNT/clay (abbreviated as PAM/MWNT/clay), prepared through in situ free radical aqueous polymerization and crosslinked by both clay, as a functional physical crosslinker, and N,N′-methylenebisacrylamide (MBA) as a chemical crosslinker, is reported. The morphology of the gels was characterized by scanning electron microscopy (SEM). The mechanical properties and electrical conductivity were also studied. The results show that the prepared hydrogels had the expected chemical components, with a highly porous structure, and the gels also showed high mechanical strength. The mechanical strength and electrical conductivity value increased with increasing content of multi-walled nanotube (MWNT), and decreased with increasing content of water.     

Rheological Properties and Microstructures of Hydroxyethyl Cellulose/Poly(Acrylic Acid) Blend Hydrogels

A simple in-situ method was introduced to prepare hydroxyethyl cellulose/poly(acrylic acid) (HEC/PAA) blend hydrogels by forming an interpenetrating network (IPN). Storage modulus (G′) and loss modulus (G″) were improved dramatically compared to HEC. To prove that hydrogen bonds and chemical crosslinking played major roles in improving the hydrogel strength and toughening, and to optimize the components of HEC/PAA blend hydrogels, a series of blend hydrogels with different ratios of HEC to PAA were designed and the corresponding Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), rheological tests, and swelling properties were compared. Crosslinked HEC/PAA blend hydrogel (mol/mol = 1:1) showed the best properties appropriate for opening up biomedical applications of the hydrogel.     

Swelling and Gelation Time Behavior of Sulfonated Polyacrylamide/Chromium Triacetate Hydrogels

A hydrogel was prepared by crosslinking of aqueous solutions of sulfonated polyacrylamide/chromium triacetate for use in water shut-off operations in oil reservoirs. The effects of pH, salinity, retarder and temperature, as well as co-polymer and crosslinker concentration, on the gelation time were investigated. The results indicated that as temperature increased, gelation occurred more rapidly. The activation energy was measured as about 86 kJ/mol. The effects of initial pH and retarder on the gelation time were also examined. The results showed that addition of retarder and increasing of pH increased and decreased the gelation time, respectively. The increase of co-polymer concentration in solution increased the gel swelling. However, the increase of crosslinker concentration decreased the gel swelling. In the presence of electrolytes, the gel swelling decreased by about 80%. Finally, some usable practical recommendations are offered for the gelling systems in reservoirs.     

Influence of Mass Ratio of Polyols on Properties of Polycaprolactone-Polyethylene Glycol/Methylene Diphenyl Diisocyanate/Diethylene Glycol Hydrogels

Polyurethane (PU) hydrogels with good hydrophilicity and biocompatibility have been applied as biomedical materials. A series of polyurethane prepolymers based on methylene diphenyl diisocyanate (MDI), polycaprolactone (PCL) and polyethylene glycol (PEG), using diethylene glycol (DEG) as the chain-extender, were synthesized; then the polyurethane hydrogels were obtained from the prepolymers using benzoyl peroxide (BPO) as a cross-linker by free radical polymerization. The influences of the ratio of polyols (PCL and PEG) on the contact angle, swelling ratio and morphology of the polyurethane hydrogel were investigated. The loading capacity and release behavior of chloramphenicol from the PCL-PEG/MDI/DEG hydrogels with different compositions were also studied. The contact angle and swelling degree results showed that the PCL-PEG/MDI/DEG hydrogel with PCL/PEG mass ratio of 3:1 had higher hydrophilicity than that with PCL/PEG mass ratios of 1:1 and 1:3. All PCL-PEG/MDI/DEG hydrogels showed three dimensional porous structures; however, the pore size increased with increasing PEG content. The chloramphenicol release kinetics from PCL-PEG/MDI/DEG hydrogels indicated Fickian diffusion, and the drug release rate increased with increasing PEG content in the PU hydrogels.     

Hydroxyethyl Cellulose-Based Hydrogels with Various Pore Sizes Prepared by Freeze-Drying

Porous hydrogels were prepared from hydroxyethyl cellulose (HEC) using citric acid (CA) as a cross-linker with the pores formed by a freeze-drying technique prior to the cross-linking reaction. The pore size increased with the increase of the mass ratio of water to HEC during the preparation of the hydrogels. The porous structure of the hydrogels was in favor of the protein adsorption. The results of thermogravimetry demonstrated that the thermal stability of HEC was improved by cross-linking with CA. The data of percent weight remaining in buffer solution with different pH indicated that the hydrogels were stable in both weak acid and base media. The data of the swelling ratio demonstrated a fast swelling property of the hydrogel.     

Synthetic–Natural Bionanocomposite Hydrogels on the Basis of Polyvinyl Alcohol and Egg White

A series of bionanocomposite hydrogels composed of polyvinyl alcohol, a synthetic polymer, and egg white, a natural protein containing material, were prepared by the freezing- thawing cyclic method. Na-montmorillonite nanoclay (Na-MMT), as a crosslinker and reinforcing agent, with 0, 5, 10, and 15 wt% loadings (based on the dried mass of the bionanocomposite hydrogel) was incorporated in the polyvinyl alcohol and egg white hydrogel matrix. The microstructural characteristics of the prepared bionanocomposite hydrogels were characterized by the X-ray diffractometry, transmission electron microscopy, Fourier transform infrared spectroscopy, scanning electron microscopy and gel fraction measurements. Thermal and mechanical properties of the samples were also studied using differential scanning calorimetry and dynamic mechanical-thermal analysis. The swelling and drying kinetics and mechanisms of the bionanocomposite hydrogels were also studied. The bionanocomposite hydrogels had an exfoliated Na-MMT morphology with an appropriate dispersion of nanoclay layers in the hydrogel matrix. The results showed that the Na-MMT platelets acted as crosslinkers and created a hydrogel network with a smaller pore size in the bio-nanocomposite hydrogels, compared with the clay-free hydrogel. The obtained thermal and mechanical properties confirmed the reinforcing effect of the nanoclay in the bionanocomposite hydrogels. The swelling and drying rates of the bionanocomposite hydrogels exhibited an inverse dependency on the nanoclay loading. In general, it was concluded that the prepared bionanocomposite hydrogels could be used as appropriate biomaterials in biomedical applications, especially in drug delivery, tissue engineering and wound care.     

Preparation and Properties of Polyurethane Hydrogels Based on Hexamethylene Diisocyanate/Polycaprolactone-Polyethylene Glycol

Hydrogels are considered an optimum material for controlled release drug systems and tissue engineering scaffolds since they are tri-dimensional networks. In this work hexamethylene diisocyanate (HMDI), polycaprolactone (PCL) and polyethylene glycol (PEG) were used to prepare polyurethane prepolymers using diethylene glycol (DEG) as a chain-extender. Then the prepolymer was used to fabricate the HMDI/PCL-PEG/DEG polyurethane hydrogels by free radical polymerization using benzoyl peroxide (BPO) as a cross-linking agent. The influences of the ratio of polyol on the contact angle, swelling ratio, morphology and cytotoxicity in-vitro of the HMDI/PCL-PEG/DEG polyurethane hydrogel were investigated. The biological behavior of the polyurethane hydrogels was analyzed by studying the cell behavior using the standard biological MTT (3–4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide) test. The Fourier transform infrared (FTIR) spectra results showed that the polyurethane hydrogels were successfully synthesized. The change of the molar the ratio of the polyhydric alcohols (PEG and PCL) played important roles in the swelling degree, the contact angle and the pore size. The HMDI/PCL-PEG/DEG polyurethane hydrogel (PCL/PEG = 1:3) was hydrophilic with many more large pores while the polyurethane hydrogel with PCL/PEG = 3:1 had a dense structure. The fibroblastic cell proliferation improved with decreasing relative PEG content; however, there were insignificant differences (P > 0.05) on all days of observation of the samples with various PEG contents compared with the negative control group. The MTT assays revealed that the cells were able to grow and proliferate quite quickly in the extracts of the HMDI/PCL-PEG/DEG polyurethane hydrogels as well as the extract of the negative control.     

Effect of κ-carrageenan on volume phase transition for polyacrylamide (PAAm) hydrogel using the fluorescence technique

Steady-state fluorescence (SSF) technique was employed for studying swelling of polyacrylamide (PAAm) gels with various content of κ-carrageenan (κC). Disc shaped composite hydrogels were prepared by free-radical crosslinking copolymerization of acrylamide (AAm) with various amounts κC. N,N′-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine was monitored during in situ swelling processes of composite gels. It was observed that fluorescence intensity values decreased as swelling is proceeded. Li–Tanaka equation was used to determine the swelling time constants, τ and cooperative diffusion coefficients, D from intensity variations during the swelling processes. It was shown that swelling time constants, τ decreased and diffusion coefficients, D increased as the κC content in the composites are increased.     

Swelling and Drying Mechanisms of Freeze-Thawed Polyvinyl Alcohol/Egg White/Montmorillonite Bionanocomposite Hydrogels

Abstract

Polyvinyl alcohol and egg white bionanocomposite hydrogels loaded with montmorillonite clay were fabricated by a freezing-thawing technique. The bionanocomposite hydrogels showed an exfoliated morphology and they had a more interconnected and dense network as compared with the clay-free sample. The montmorillonite layers acted as multifunctional crosslinkers and the bionanocomposite hydrogels had nanoscale, slit-shaped pores. The swelling ratios of the bionanocomposite hydrogels were increased either by decreasing the content of incorporated montmorillonite or by increasing the pH of the swelling medium. It was found that the bionanocomposite hydrogels having a higher content of montmorillonite exhibited a slightly slower drying process with a longer drying duration. Using the Ritger-Peppas model, it was shown that the swelling and drying mechanisms for all bionanocomposite hydrogels were non-Fickian diffusion. According to the Peppas-Sahlin model, it was found that the absorption of the swelling agent molecules during the swelling process and also the removal of water molecules during the drying process in the early stages of the processes occurred mostly due to their diffusion. At higher swelling or drying times, the contribution of the relaxation (for swelling) and shrinkage (for drying) of the polyvinyl alcohol polymeric chains and egg white protein chains was increased.     

Influence of Addition of Hyaluronic Acid (HA) on the Properties of Collagen/HA Composite Scaffolds

In order to develop a scaffolding material for tissue regeneration, porous matrices containing varying composites of collagen and hyaluronic acid (HA) (from 1:0 to 0:1) were fabricated using a freeze-drying method. The effect of the composition on the morphology, hydrophilicity, swelling behavior, mechanical properties, and in vitro cytotoxicity was investigated. The results showed that all the scaffolds had an interconnected pore structure with sufficient pore size for use as a support for the growth of fibroblasts. The addition of HA improved the swelling property, but reduced the compressive strength. The contact angle decreased with increasing HA content. In in vitro cytotoxicity tests using fibroblastic cells, the collagen/HA scaffolds showed no toxicity. All these results suggest that collagen/HA composite scaffolds are a potential candidate for tissue engineering scaffolds.     

Synthesis and Properties of Tannic Acid-Based Hydrogels

Hydrogels were synthesized by using tannic acid (TA) as a chemical cross-linker for the copolymer of allylglycidyl ether and acrylamide. The swelling ratio of the hydrogels increased with increasing amount of TA and decreased with increasing amounts of allylglycidyl ether. The hydrogels exhibited pH sensitivity; the swelling ratio increased with pH. Adsorption experiments of the hydrogel for Cu (II) ions suggested that the hydrogels can be used as an adsorbent for removal of Cu (II) heavy metal ions from dilute aqueous solutions.     

Polyvinyl Alcohol/Na-Montmorillonite Nanocomposite Hydrogels Prepared by Freezing–Thawing Method: Structural,Mechanical, Thermal,and Swelling Properties

Physically crosslinked nanocomposite hydrogels based on polyvinyl alcohol (PVA) containing Na-montmorillonite were prepared by the cyclic freezing–thawing method. The primarily exfoliated morphology of prepared nanocomposite hydrogels was confirmed by X-ray diffractometry (XRD) and transmission electron microscopy (TEM) as complementary techniques. It is shown that some interactions developed between the hydroxyl groups of PVA chains and Na-montmorillonite silicate layers in the nanocomposite hydrogels. Differential scanning calorimetry (DSC) results indicated some shifting in the glass transition temperature of the PVA hydrogel in the presence of the nanoclay. Swelling measurements showed that the swelling ratios of the nanocomposite hydrogels were increased either by decreasing the Na-montmorillonite content or by increasing the swelling medium temperature. Dynamic mechanical–thermal properties results showed higher storage modulus for nanocomposite hydrogels in temperature ranges both below and above 0°C. It was also found that the hardness of the nanocomposite hydrogels increased by increasing the nanoclay loading level.     

Synthesis of Novel Double Network Hydrogels via Atom Transfer Radical Polymerization

Novel double network (DN) hydrogels were prepared via atom transfer radical polymerization (ATRP) of the first network using two different initiators, and followed by ultraviolet (UV) photo-initiated polymerization for the second network. The first networks of DN hydrogels were prepared by using two different initiators; one is a one-side initiator, and the other is a double-side initiator. The mechanical properties of various DN gels, such as water content and compressive strength, were studied. The network structure of the DN hydrogels was also characterized by dynamic light scattering. Thus, we demonstrated that the ATRP method is useful to control the network structure of the first network for DN gels. Extensive inhomogeneous structure of the first network was obtained by ATRP method and increased both the swelling degree and softness of DN gels.     

pH-Sensitive Poly(Vinyl Alcohol)/Sodium Carboxymethylcellulose Hydrogel Beads for Drug Delivery

A series of pH-sensitive hydrogel beads were prepared composed of poly(vinyl alcohol) (PVA) and sodium carboxymethylcellulose (CMC) by using Fe³⁺ crosslinking and freeze-thawing (FT) cycle techniques. The mixed solution of CMC and PVA was firstly crosslinked with Fe³⁺ to form beads and then subjected to freezing-thawing cycles for further crosslinking. The formation of hydrogel was confirmed by Fourier transform infrared spectroscopy (FTIR). The gelling rate in ferric solution and the swelling and pH-senstive properties of the hydrogel beads were investigated. The encapsulation efficiency and in-vitro release properties of beads were also evaluated using Bovine serum albumin as model drug. The pH sensitivity and the release rate increased with increasing CMC content. These results suggest that the PVA/CMC hgdrogel beads should be useful as pH-sensitive drug delivery systems for bioactive agents.     

Preparation and Properties of Polyurethane Hydrogels Based on Methylene Diphenyl Diisocyanate/Polycaprolactone-Polyethylene Glycol

Polyurethane (PU) hydrogel is an important biomedical material for drug controlled release systems, wound dressings and medical bandages. Three series of polyurethane prepolymers based on methylene diphenyl diisocyanate (MDI), polycaprolactone (PCL) and polyethylene glycol (PEG), using diethylene glycol (DEG), N-methyldiethanolamine (MDEA) or dimethylolpropionic acid (DMPA), as the chain-extender, were prepared. Then the polyurethane hydrogels were obtained from the prepolymers, using benzoyl peroxide (BPO) as a cross-linking agent, by free radical polymerization. The influences of the types of chain-extenders and polyols on the contact angle, swelling ratio and morphology of the polyurethane hydrogels were investigated. The effect of the variety of the chain-extenders in the PU hydrogel on the drug release behavior was also studied. The FT-IR results showed that the PU hydrogels were successfully synthesized. The introduction of PEG improved the hydrophilicity of the PU hydrogels. The MDI/PCL-PEG/DEG hydrogel was hydrophobic, and there were small micropores on its surface; while the MDI/PCL-PEG/DMPA and MDI/PCL-PEG/MDEA hydrogels had high hydrophilicity and a micropouous structure on their surface due to the existence of carboxyl and tertiary amino functional groups. The change of chain-extenders had no significant effect on the cumulative drug release of chloramphenicol from the PU hydrogels. However, the introduction of PEG increased the drug release rate. The chloramphenicol release kinetics from the MDI/PCL-PEG hydrogels indicated non-Fickian diffusion.     

Tuning of Lower Critical Solution Temperature (LCST) of Poly(N-Isopropylacrylamide-co-Acrylic acid) Hydrogels

Temperature-responsive hydrogel with a lower critical solution temperature (LCST) close to human body temperature was prepared. Crosslinked N-isopropylacrylamide (NIPAAm) and acrylic acid (AAc) copolymer networks were synthesized at various monomer ratios in the presence of ammonium persulfate (APS), N,N′-methylenebisacrylamide (NMBA) and N,N,N′,N′-tetramethylethylenediamine (TEMED) via a redox polymerization method. The resulting hydrogels possessed thermo- and pH-responsive characteristics. They were characterized in terms of swelling ratio, volume change, water uptake and diffusivity, water vapor uptake and diffusivity, and phase transition temperature. The water liquid and vapor diffusion coefficients for all the synthesized hydrogels were higher than the literature data, implying higher rates for drug release. The LCST of the hydrogel increased with higher AAc content in the copolymer. The gel containing 1.8% AAc exhibited an LCST similar to human body temperature, demonstrating a potential use in drug controlled release and biomedical applications.     

Effects of Filler Content on Mechanical Properties of Macroporous Composites

The mechanical properties of hydroxyapatite related macroporous biocomposites (MPBs) are influenced by a number of factors, such as the pore size, the filler content and the properties of the matrix and the inclusion. Failure often occurs when the strength of the implant cannot bear the applied mechanical load. In this study, the effects of filler content on the mechanical properties of MPBs have been investigated. A finite element (FE) unit cell model of a macroporous hydroxyapatite–polyetheretherketone (HA–PEEK) biocomposite structure with uniform and interconnected pores has been constructed. In the FE model, the HA particles were assumed to have random distribution, and particle volume fraction would be varied in the PEEK matrix. The material behaviours of both HA and PEEK have been implemented in the ABAQUS finite element code. HA was modelled to exhibit elastic behaviour and undergo plastic softening to a residual strength when a critical stress was reached, while the PEEK matrix would follow elastic–plastic behaviour. The macroscopic compressive stress–strain relations of the macroporous biocomposite structures have been predicted. Increasing particle volume fraction could lead to an increase in the compressive elastic modulus of the structures but a reduction in the compressive strength. The von Mises stress distribution and the effect of stress concentration in the structures with different filler content are also discussed. The proposed model could provide macro-structural and microscopic information of the macroporous biocomposite structure to the designers in order to facilitate the fabrication of this kind of structure with optimum mechanical properties.