Design, synthesis and properties of polyurethane hydrogels for tissue engineering

Tailored Biocompatible Polyurethane-Poly(ethylene glycol) Hydrogels as a Versatile Nonfouling Biomaterial

Polyurethane-based hydrogels are relatively inexpensive and mechanically robust biomaterials with ideal properties for various applications, including drug delivery, prosthetics, implant coatings, soft robotics, and tissue engineering. In this report, a simple method is presented for synthesizing and casting biocompatible polyurethane-poly(ethylene glycol) (PU-PEG) hydrogels with tunable mechanical properties, nonfouling characteristics, and sustained tolerability as an implantable material or coating. The hydrogels are synthesized via a simple one-pot method using commercially available precursors and low toxicity solvents and reagents, yielding a consistent and biocompatible gel platform primed for long-term biomaterial applications. The mechanical and physical properties of the gels are easily controlled by varying the curing concentration, producing networks with complex shear moduli of 0.82-190 kPa, similar to a range of human soft tissues. When evaluated against a mechanically matched poly(dimethylsiloxane) (PDMS) formulation, the PU-PEG hydrogels demonstrated favorable nonfouling characteristics, including comparable adsorption of plasma proteins (albumin and fibrinogen) and significantly reduced cellular adhesion. Moreover, preliminary murine implant studies reveal a mild foreign body response after 41 days. Due to the tunable mechanical properties, excellent biocompatibility, and sustained in vivo tolerability of these hydrogels, it is proposed that this method offers a simplified platform for fabricating soft PU-based biomaterials for a variety of applications.

Effect of PEG Molecular Weight on the Polyurethane-Based Quasi-Solid-State Electrolyte for Dye-Sensitized Solar Cells

Nanosilica was surface modified with polyaniline and incorporated into polyurethane to form a polymer matrix capable of entrapping a liquid electrolyte and functioning as quasi-solid-state electrolyte in the dye-sensitized solar cells. The effect on the S−PANi distribution, surface morphology, thermal stability, gel content, and structural change after varying the PEG molecular weight of the polyurethane matrix was analyzed. Quasi-solid-state electrolytes were prepared by immersing the polyurethane matrix into a liquid electrolyte and the polymer matrix absorbency, conductivity, and ion diffusion were investigated. The formulated quasi-solid-state electrolytes were applied in dye-sensitized solar cells and their charge recombination, photovoltaic performance, and lifespan were measured. The quasi-solid-state electrolyte with a PEG molecular weight of 2000 gmol−1 (PU−PEG 2000) demonstrated the highest light-to-energy conversion efficiency, namely, 3.41%, with an open-circuit voltage of 720 mV, a short-circuit current of 4.52 mA cm−2, and a fill factor of 0.63.

Biomedical Applications of Polyurethane Hydrogels, Polyurethane Aerogels and Polyurethane-Graphene Nanocomposite Materials

BACKGROUND

Increasing new emerging ill-healths have posed therapeutic challenges in modern medicine. Hence polyurethane hydrogels that comprise polyol, copolymer and extender could be prepared from diverse chemical compounds with adjuvants such as ascorbic acid, sorbitol among others. Their mechano-physicochemical properties are functions of their biological activities. Therefore there is need to assess their therapeutic potentials.

METHODS

literature were searched on synthesis and medical uses of polyurethane - hydrogels, polyurethane - aerogels and polyurethane - graphene nanocomposite materials, with a view to identifying their sources, synthesis, mechanical and physiochemical properties, biomedical applications, chirality, and the relevance of Lipinski's rule of five in the synthesis of oral polyurethane nanocomposite materials.

RESULTS

The prepared hydrogels and aerogels could be used as polymer carriers for intradermal, cutaneous and intranasal drugs. They can be fabricated and used as prosthetics. In addition the strength modulus (tensile stress-tensile strain ratio), biodegradability, biocompatibility and non-toxic effects of the polyurethane hydrogels and aerogels are the highly desirable properties. However, body and environmental temperatures may contribute to their instability, hence there is need to improve on the synthesis of aerogels and hydrogels of polyurethane that can last for many years. Alcoholism, diabetes, pyrogenic diseases, mechanical and physical forces, and physiological variability may also reduce the life span of polyurethane aerogels and hydrogels.

CONCLUSION

Synthesis of polyurethane hydrogel-aerogel complex that can be used in complex, rare biomedical cases is of paramount importance. These hydrogels and aerogels may be hydrophobic, hydrophilic, aerophobic-aerophilic or amphiphilic and sometimes lipophilic depending on structural components and the intended biomedical uses. Polyurethane graphene nanocomposite materials are used in the treatment of a myriad of diseases including cancer and bacterial infection.

Soft Polymer-Based Technique for Cellular Force Sensing

Soft polymers have emerged as a vital type of material adopted in biomedical engineering to perform various biomechanical characterisations such as sensing cellular forces. Distinct advantages of these materials used in cellular force sensing include maintaining normal functions of cells, resembling in vivo mechanical characteristics, and adapting to the customised functionality demanded in individual applications. A wide range of techniques has been developed with various designs and fabrication processes for the desired soft polymeric structures, as well as measurement methodologies in sensing cellular forces. This review highlights the merits and demerits of these soft polymer-based techniques for measuring cellular contraction force with emphasis on their quantitativeness and cell-friendliness. Moreover, how the viscoelastic properties of soft polymers influence the force measurement is addressed. More importantly, the future trends and advancements of soft polymer-based techniques, such as new designs and fabrication processes for cellular force sensing, are also addressed in this review.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

A Robust, Tough and Multifunctional Polyurethane/Tannic Acid Hydrogel Fabricated by Physical-Chemical Dual Crosslinking

Commonly synthetic polyethylene glycol polyurethane (PEG-PU) hydrogels possess poor mechanical properties, such as robustness and toughness, which limits their load-bearing application. Hence, it remains a challenge to prepare PEG-PU hydrogels with excellent mechanical properties. Herein, a novel double-crosslinked (DC) PEG-PU hydrogel was fabricated by combining chemical with physical crosslinking, where trimethylolpropane (TMP) was used as the first chemical crosslinker and polyphenol compound tannic acid (TA) was introduced into the single crosslinked PU network by simple immersion process. The second physical crosslinking was formed by numerous hydrogen bonds between urethane groups of PU and phenol hydroxyl groups in TA, which can endow PEG-PU hydrogel with good mechanical properties, self-recovery and a self-healing capability. The research results indicated that as little as a 30 mg·mL-1 TA solution enhanced the tensile strength and fracture energy of PEG-PU hydrogel from 0.27 to 2.2 MPa, 2.0 to 9.6 KJ·m-2, respectively. Moreover, the DC PEG-PU hydrogel possessed good adhesiveness to diverse substrates because of TA abundant catechol groups. This work shows a simple and versatile method to prepare a multifunctional DC single network PEG-PU hydrogel with excellent mechanical properties, and is expected to facilitate developments in the biomedical field.

Water-based non-isocyanate polyurethane-ureas (NIPUUs)

This review aims at discussing the achievements and the remaining challenges in the development of water-soluble NIPUUs, NIPUUs-based hydrogels and water-borne NIPUU dispersions.

Structural and Biomechanical Characterization of a Scaffold-Free Skin Equivalent Model via Biophysical Methods

AIMS

Among in vitro skin models, the scaffold-free skin equivalent (SFSE), without exogenous material, is interesting for pharmacotoxicological studies. Our aim was to adapt in vivo biophysical methods to study the structure, thickness, and extracellular matrix of our in vitro model without any chemical fixation needed as for histology.

METHODS

We evaluated 3 batches of SFSE and characterized them by histology, transmission electron microscopy (TEM), and immunofluorescence. In parallel, we investigated 3 biophysical methods classically used for in vivo evaluation, optical coherence tomography (OCT), and laser scanning microscopy (LSM) imaging devices as well as the cutometer suction to study the biomechanical properties.

RESULTS

OCT allowed the evaluation of SFSE total thickness and its different compartments. LSM has a greater resolution enabling an evaluation at the cell scale and the orientation of collagen fibers. The viscoelasticity measurement by cutometry was possible on our thin skin model and might be linked with mature collagen bundles visible in TEM and LSM and with elastic fibers seen in immunofluorescence.

CONCLUSION

Our data demonstrated the simplicity and sensitivity of these different in vivo biophysical devices on our thin skin model. These noninvasive tools allow to study the morphology and the biomechanics of in vitro models.

Osseointegrated membranes based on electro-spun TiO2/hydroxyapatite/polyurethane for oral maxillofacial surgery

Membranes which have an osseointegration abilty are often selected as biomaterials in oral and maxillofacial surgery. Although these membranes are often the best option for certain uses, it is a challenge to create functionally attractive membranes. In this research, electro-spun titanium oxide (TiO2)/hydroxyapatite (HA)/polyurethane (PU) membranes were fabricated with different ratios of HA and TiO2: 100: 0, 70:30, 50:50, 30:70 and 0:100 w/w. The morphologies of the different mixtures were assessed with a Scanning Electron Microscope (SEM) and Field Emission Microscope (FESEM). Element analysis was performed with EDX. The physical properties of the water contact angles and mechanical strength were tested and the membranes cultured with osteoblasts to evaluate their biological functions, cell adhesion, viability, proliferation, alkaline phosphatase (ALP) activity, and calcium content. The results showed that the membranes with TiO₂ and HA had smaller fibers than those without TiO₂ and HA. The TiO2- and HA-including compounds showed the formation of particle aggregation on the surface of the fibers. They also had higher water contact angles, mechanical strength, and stiffness than those without TiO₂ and HA, and they had better cell adhesion, viability, proliferation, ALP activity and calcium content. The membrane with a 50:50 TiO₂:HA ratio had more unique biological functions than the others. Finally, our research demonstrated that osseointegrated membranes with 50:50 TiO2:HA are promising for oral and maxillofacial surgery.

Biodegradable, anti-adhesive and tough polyurethane hydrogels crosslinked by triol crosslinkers

The mechanical and biodegradable properties of hydrogels are two essential properties for practical biomaterial applications. In this work, a series of biodegradable polyurethane (PU) hydrogels were successfully synthesized using two kinds of triol crosslinkers with different chain structures. One crosslinker is normal glycerol (GC) with short chain length, and the other is biodegradable poly (ε-caprolactone)-triol (CAPA) with long chain length. All PU hydrogels showed considerable water uptake around ~60%, excellent strength (above 3 MPa), advisable modulus (0.9~1.7 MPa), high elasticity (above 700%), as well as good biodegradability and biocompatibility. Hydrogen bonds served as reversible sacrificial bonds in the PU hydrogels endow them good toughness with partial hysteresis during deformation. The biodegradable long chain crosslinker CAPA can certainly accelerate the degradation of PU hydrogels compared with the GC crosslinked hydrogels. The degradation of these hydrogels was a process of continuous erosion from the surface to interior, which contributes to the high remain of mechanical properties after 30 days-degradation. Besides, the hydrogels also show excellent antifouling ability of protein and anti-adhesion of cells. Therefore, these hydrogels suggest great potential used as biological anti-adhesive membranes or catheters.

Polyurethanes Crosslinked with Poly(vinyl alcohol) as a Slowly-Degradable and Hydrophilic Materials of Potential Use in Regenerative Medicine

Novel, slowly-degradable and hydrophilic materials with proper mechanical properties and surface characteristics are in great demand within the biomedical field. In this paper, the design, synthesis, and characterization of polyurethanes (PUR) crosslinked with poly(vinyl alcohol) (PVA) as a new proposition for regenerative medicine is described. PVA-crosslinked PURs were synthesized by a two-step polymerization performed in a solvent (dimethylsulfoxide, DMSO). The raw materials used for the synthesis of PVA-crosslinked PURs were poly(ε-caprolactone) (PCL), 1,6-hexamethylene diisocyanate (HDI), and PVA as a crosslinking agent. The obtained materials were studied towards their physicochemical, mechanical, and biological performance. The tests revealed contact angle of the materials surface between 38-47° and tensile strength in the range of 41-52 MPa. Mechanical characteristics of the obtained PURs was close to the characteristics of native human bone such as the cortical bone (TSb = 51-151 MPa) or the cancellous bone (TSb = 10-20 MPa). The obtained PVA-crosslinked PURs did not show significant progress of degradation after 3 months of incubation in a phosphate-buffered saline (PBS). Accordingly, the obtained materials may behave similar to slowly-degradable materials, which can provide long-term physical support in, for example, tissue regeneration, as well as providing a uniform calcium deposition on the material surface, which may influence, for example, bone restoration. A performed short-term hemocompatibility study showed that obtained PVA-crosslinked PURs do not significantly influence blood components, and a cytotoxicity test performed with the use of MG 63 cell line revealed the great cytocompatibility of the obtained materials. According to the performed studies, such PVA-crosslinked PURs may be a suitable proposition for the field of tissue engineering in regenerative medicine.

Advanced Microengineered Lung Models for Translational Drug Discovery

Lung diseases impose a significant socioeconomic burden and are a leading cause of morbidity and mortality worldwide. Moreover, respiratory medicine, unlike several other therapeutic areas, faces a disappointingly low number of new approved therapies. This is partly due to lack of reliable in vitro or in vivo models that can reproduce organ-level complexity and pathophysiological responses of human lung. Here, we examine new opportunities in application of recently emerged organ-on-chip technology to model human lung alveolus and small airway in preclinical drug development and biomarker discovery. We also discuss challenges that need to be addressed in coming years to further enhance the physiological and clinical relevance of these microsystems, enable their increased accessibility, and support their leap into personalized medicine.

Mechanical viability of a thermoplastic elastomer hydrogel as a soft tissue replacement material

Hydrogels are a class of synthetic biomaterials composed of a polymer network that swells with water and as such they have both an elastic and viscous component making them ideal for soft tissue applications. This study characterizes the compressive, tensile, and shear properties of a thermoplastic elastomer (TPE) hydrogel and compares the results to published literature values for soft tissues such as articular cartilage, the knee meniscus, and intervertebral disc components. The results show the TPE hydrogel material is viscoelastic, strain rate dependent, has similar surface and bulk properties, displays minimal damping under dynamic load, and has tension-compression asymmetry. When compared to other soft tissues it has a comparable equilibrium compressive modulus of approximately 0.5MPa and shear modulus of 0.2MPa. With a tensile modulus of only 0.2MPa though, the TPE hydrogel is inferior in tension to most collagen based soft tissues. Additional steps may be necessary to reinforce the hydrogel system and increase tensile modulus depending on the desired soft tissue application. It can be concluded that this material could be a viable option for soft tissue replacements.

The Alginate Layer for Improving Doxorubicin Release and Radiolabeling Stability of Chitosan Hydrogels

PURPOSE

Chitosan hydrogels (CSH) formed through ionic interaction with an anionic molecule are suitable as a drug carrier and a tissue engineering scaffold. However, the initial burst release of drugs from the CSH due to rapid swelling after immersing in a biofluid limits their wide application as a drug delivery carrier. In this study, alginate layering on the surface of the doxorubicin (Dox)-loaded and I-131-labeled CSH (DI-CSH) was performed. The effect of the alginate layering on drug release behavior and radiolabeling stability was investigated.

METHODS

Chitosan was chemically modified using a chelator for I-131 labeling. After labeling of I-131 and mixing of Dox, the chitosan solution was dropped into tripolyphosphate (TPP) solution using an electrospinning system to prepare spherical microhydrogels. The DI-CSH were immersed into alginate solution for 30 min to form the crosslinking layer on their surface. The formation of alginate layer on the DI-CSH was confirmed by Fourier transform infrared spectroscopy (FT-IR) and zeta potential analysis. In order to investigate the effect of alginate layer, studies of in vitro Dox release from the hydrogels were performed in phosphate buffered in saline (PBS, pH 7.4) at 37 °C for 12 days. The radiolabeling stability of the hydrogels was evaluated using ITLC under different experimental condition (human serum, normal saline, and PBS) at 37 °C for 12 days.

RESULTS

Formatting the alginate-crosslinked layer on the CSH surface did not change the spherical morphology and the mean diameter (150 ± 10 μm). FT-IR spectra and zeta potential values indicate that alginate layer was formed successfully on the surface of the DI-CSH. In in vitro Dox release studies, the total percentage of the released Dox from the DI-CSH for 12 days were 60.9 ± 0.8, 67.3 ± 1.4, and 71.8 ± 2.5 % for 0.25, 0.50, and 1.00 mg Dox used to load into the hydrogels, respectively. On the other hand, after formatting alginate layer, the percentage of the released Dox for 12 days was decreased to 47.6 ± 1.4, 51.1 ± 1.4, and 57.5 ± 1.6 % for 0.25, 0.50, and 1.00 mg Dox used, respectively. The radiolabeling stability of DI-CSH in human serum was improved by alginate layer.

CONCLUSIONS

The formation of alginate layer on the surface of the DI-CSH is useful for improving the drug release behavior and radiolabeling stability.

The effect of the preparation process on the swelling behavior of silk fibroin-polyurethane composite hydrogels using a full factorial experimental design

Polyurethane prepolymer (PUP) was synthesized by polyethylene glycol (PEG) and polypropylene glycol (PPG) as the soft segments, isophorone diisocyanate (IPDI) as the hard segment and dimethylol propionic acid (DMPA) and diethylene glycol (DEG) as chain extenders. Silk fibroin (SF)-PU composite hydrogels were prepared by SF and PUP through chemical crosslinking and physical crosslinking interactions. A full factorial experimental design with four factors and four levels was applied to optimize the craft of preparing SF-PU composite hydrogels. The molecular weight of PEG, IPDI/(PEG+PPG) (molar ratio), PEG/(PEG+PPG) (molar ratio) and SF/(SF+PU) (mass ratio) were the factors. The swelling behavior of hydrogels was tested in deionized water at 30°C. The results showed that the equilibrium swelling ratio (ESR) was the largest by tuning the molecular weight of PEG to 4000, IPDI/(PEG+PPG)(molar ratio) to 3, PEG/(PEG+PPG) (molar ratio) to 40% and SF/(SF+PU) (mass ratio) to 2%. Fickian diffusion played a dominant role in the initial stage of swelling. For the whole process, the results fitted well into the Schott second-order kinetic equation.

Photocrosslinkable kappa-carrageenan hydrogels for tissue engineering applications

Kappa carrageenan (κ-CA) is a natural-origin polymer that closely mimics the glycosaminoglycan structure, one of the most important constituents of native tissues extracellular matrix. Previously, it has been shown that κ-CA can crosslink via ionic interactions rendering strong, but brittle hydrogels. In this study, we introduce photocrosslinkable methacrylate moieties on the κ-CA backbone to create physically and chemically crosslinked hydrogels highlighting their use in the context of tissue engineering. By varying the degree of methacrylation, the effect on hydrogel crosslinking was investigated in terms of hydration degree, dissolution profiles, morphological, mechanical, and rheological properties. Furthermore, the viability of fibroblast cells cultured inside the photocrosslinked hydrogels was investigated. The combination of chemical and physical crosslinking procedures enables the formation of hydrogels with highly versatile physical and chemical properties, while maintaining the viability of encapsulated cells. To our best knowledge, this is the first study reporting the synthesis of photocrosslinkable κ-CA with controllable compressive moduli, swelling ratios and pore size distributions. Moreover, by micromolding approaches, spatially controlled geometries and cell distribution patterns could be obtained, thus enabling the development of cell-material platforms that can be applied and tailored to a broad range of tissue engineering strategies.

Biodegradable IPN hydrogel beads of pectin and grafted alginate for controlled delivery of diclofenac sodium

A novel diclofenac sodium (DS) loaded interpenetrating polymer network (IPN) beads of pectin and hydrolyzed polyacrylamide-graft-sodium alginate (PAAm-g-SA) was developed through ionotropic gelation and covalent cross-linking. The graft copolymer was synthesized by free radical polymerization under the nitrogen atmosphere followed by alkaline hydrolysis. The grafting, alkaline hydrolysis, and characterization of beads were confirmed by Fourier transforms infrared spectroscopy. The crystalline structure of drug after encapsulation into IPN beads were evaluated by differential scanning colorimetry and X-ray diffraction analyses. DS encapsulation was up to 96.45 %. The effect of hydrolyzed graft copolymer/pectin ratios and glutaraldehyde concentration on drug release in acidic and phosphate buffer solutions were investigated. The release of drug was significantly increased with increase of pH. The release of drug depends on the extent of cross-linking. The results indicated that IPN beads of hydrolyzed PAAm-g-SA and pectin could be used for sustained release of DS.

Synthesis and rheology of thermoreversible polyurethane hydrogels

Polyurethanes based on poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, succinic or citric acid, and ethyl ester L-lysine diisocyanate were synthesized and characterized by FT-IR and GPC. Wetting properties were evaluated by contact angle determinations. The critical micelle concentration was determined from surface tension measurements. Their abillity to undergo self-aggregation and temperature induced gelation was investigated through dynamic oscillatory measurements. The gelation point, sol-gel transitions, and gel structure are influenced by polymer structure, composition, molecular weight, concentration, and heating rate.

Review: Synthetic Polymer Hydrogels for Biomedical Applications

Synthetic polymer hydrogels constitute a group of biomaterials, used in numerous biomedical disciplines, and are still developing for new promising applications. The aim of this study is to review information about well known and the newest hydrogels, show the importance of water uptake and cross-linking type and classify them in accordance with their chemical structure.

The tensile properties of poly(ethylene oxide)-based segmented block copolymers in the dry and wet state

The tensile properties of poly(ethylene oxide)-based segmented block copolymers were studied in the dry and wet state. The hard segments were made up of monodisperse crystallisable tetra-amide segments (T6T6T) comprising terephthalate (T) and hexamethylenediamine (6) groups. The length of the segments making up the soft phase was varied from 600 to 10,000 g/mol. The water absorption of the PEO-based copolymers was found to increase exponentially with the PEO concentration. The modulus and yield strength decreased with water absorption and this effect seemed to be mainly the result of a lowering hard segment content caused by their swelling with water. Upon wetting, the copolymers demonstrated an increased yield strain. Furthermore, the ultimate properties were sensitive to the hard segment content, the molecular weight of the copolymer and whether or not strain-hardening could take place. Upon wetting, the fracture stresses decreased whereas the fracture strains increased. The true fracture stresses of the wet samples were as high as those of the dry samples for reasonable amounts of absorbed water.

Structure and Drug Release in a Crosslinked Poly(Ethylene Oxide) Hydrogel

Hydrogels are a continuously expanding class of pharmaceutical polymers designed for sustained or controlled drug release. The structure and intermolecular interactions in such systems define their macroscopic properties. The aim of this study was to investigate the mechanism of swelling, drug impregnation, and drug release from poly(ethylene oxide) (PEO) gel crosslinked by urethane bonds. A combination of SAXS/WAXS/SANS techniques enabled us to determine the phase transition between lamellar and extended gel network, and to apply different descriptions of crystallinity, based on lamellar and crystal lattice structures. It is shown that even low (1-7% w/w) loading of model drugs acetaminophen and caffeine, produced significant disorder in the polymer matrix. This effect was particularly pronounced for acetaminophen due to its specific ability to form complexes with PEO. The drug-release profiles were analyzed using a general cubic equation, proposed for this work, which allowed us to determine the gel hydration velocity. The results indicate that the release profiles correlate inversely with the polymer crystallinity.

Effect of hydrophobic modification on rheological and swelling features during chemical gelation of aqueous polysaccharides

Rheological characteristics during chemical gelation with the cross-linker ethylene glycol diglycidyl ether (EGDE) of semidilute aqueous solutions of hydroxyethylcellulose (HEC) and of two hydrophobically modified analogues (HM-1-HEC and HM-2-HEC) are reported. In addition, rheological features of gelling samples (dextran and its hydrophobically modified analogue (HM-dextran)) of a different structure have been examined. Some swelling experiments on these gels in the postgel region are also reported. The gelation time of the hydroxyethylcellulose systems decreased with increasing cross-linker concentration, and incorporation of hydrophobic units of HEC resulted in a slower gelation. The time of gelation for the dextran system was only slightly affected by the incorporation of hydrophobic groups (HM-dextran). At the gel point, a power law frequency dependence of the dynamic storage modulus (G' proportional to omegan') and loss modulus (G'' proportional to omegan'') was observed for all gelling systems with n' = n'' = n. The attachment of hydrophobic moieties on the dextran chains had virtually no impact on the value of n (n = 0.77), and the percolation model describes the incipient dextran gels. By increasing the number of hydrophobic groups of the HEC polymer, the value of n for the corresponding incipient gel drops significantly, and the value of the gel strength parameter increases strongly. Incorporation of hydrophobic units in the HEC chains promotes the formation of stronger incipient gels because of the contribution from the hydrophobic association effect. The frequency dependence of the complex viscosity reveals that all the investigated gels become more solidlike in the postgel domain. Far into the postgel region, the hydrophobicity of HEC plays a minor role for the strength of the gel network, whereas the values of the complex viscosity are significantly higher for HM-dextran than for the corresponding dextran gel. The swelling experiments on HEC, HM-1-HEC, and HM-2-HEC systems disclose that the degree of swelling of the postgels in water is quite different, depending on the relative distance from the gel point at which the cross-linker reaction is quenched. At a given distance from the gel point, the swelling of the HEC gel is less pronounced than for the corresponding hydrophobically modified samples. At this stage, the swelling of the HM-dextran gel is stronger than for the dextran gel.

A flexible approach to the preparation of polymer scaffolds for tissue engineering

Porous polyester thermoset xerogels have been produced via sol-gel chemistry as a first step in the development of sol-gel derived tissue engineering scaffolds templated by replica molding and/or salt leaching. The pore structure of these untemplated thermosets is tunable and can be altered independent of or in tandem with alterations in composition. Cytocompatibility studies on these xerogels imply the effects of both pore size and materials chemistry, with fully aliphatic polyesters with large pore structures allowing the growth of mammalian cells. To the best of our knowledge, this represents the first report examining the preparation and potential of sol-gel derived porous polymer xerogels as tissue engineering scaffolds.

Morphology and gelation of thermosensitive xyloglucan hydrogels

Galactose modified xyloglucan is a thermally reversible hydrogel that is increasingly used in the biomedical field due to the ease of altering the gelation time and temperature by modifying the galactose removal ratio. However there is little information concerning the morphology and rheological properties of the hydrogel under physiological conditions. Differential scanning microcalorimetry (DSmicroC) showed the thermal gelation process to occur over a broad temperature range (5-50 degrees C). The rheological properties of the hydrogels were investigated as a function of concentration, temperature and ionic strength. The final elastic moduli of the hydrogels increased with increases in concentration. Isothermal rheology suggests that the gelation occurred in two distinct stages, which was influenced by the solution media. Scanning electron microscopy (SEM) was used to characterize the morphology of the xyloglucan which were thermally gelled at 37 degrees C. The resultant morphology was strongly dependent on the concentration of the hydrogel. Strong hydrogels were only obtained at 3 wt.% at 37 degrees C, and the morphology characterized by an open 3-dimensional network, comprised of thin membranes. It is proposed that the first stage of the isothermal gelation is the formation and growth of the thin membranes, followed by the formation of a three dimensional network.

Three-dimensional Nanohydroxyapatite/Chitosan Scaffolds as Potential Tissue Engineered Periodontal Tissue

The development of suitable three-dimensional scaffold for the maintenance of cellular viability and differentiation is critical for applications in periodontal tissue engineering. In this work, different ratios of porous nanohydroxyapatite/chitosan (HA/chitosan) scaffolds are prepared through a freeze-drying process. These scaffolds are evaluated in vitro by the analysis of microscopic structure, porosity, and cytocompatibility. The expression of type I collagen and alkaline phosphatase (ALP) activity are detected with real-time polymerase chain reaction (RT-PCR). Human periodontal ligament cells (HPLCs) transfected with enhanced green fluorescence protein (EGFP) are seeded onto the scaffolds, and then these scaffolds are implanted subcutaneously into athymic mice. The results indicated that the porosity and pore diameter of the HA/chitosan scaffolds are lower than those of pure chitosan scaffold. The HA/chitosan scaffold containing 1% HA exhibited better cytocompatibility than the pure chitosan scaffold. The expression of type I collagen and ALP are up-regulated in 1% HA/chitosan scaffold. After implanted in vivo, EGFP-transfected HPLCs not only proliferate but also recruit surrounding tissue to grow in the scaffold. The degradation of the scaffold significantly decreased in the presence of HA. This study demonstrated the potential of HA/ chitosan scaffold as a good substrate candidate in periodontal tissue engineering