Photo-cross-linked PLA-PEO-PLA hydrogels from self-assembled physical networks: mechanical properties and influence of assumed constitutive relationships

Poly(lactide)-block-poly(ethylene oxide)-block-poly(lactide) (PLA-PEO-PLA) triblock copolymers are known to form physical hydrogels in water as a result of the polymer's amphiphilicity. Their mechanical properties, biocompatibility, and biodegradability have made them attractive for use as soft tissue scaffolds. However, the network junction points are not covalently cross-linked, and in a highly aqueous environment these hydrogels adsorb more water, transform from gel to sol, and lose the designed mechanical properties. In this article, a hydrogel was formed by the use of a novel two-step approach. In the first step, the end-functionalized PLA-PEO-PLA triblock was self-assembled into a physical hydrogel through hydrophobic micelle network junctions, and in the second step, this self-assembled physical network structure was locked into place by photo-cross-linking the terminal acrylate groups. In contrast with physical hydrogels, the photo-cross-linked gels remained intact in phosphate-buffered solution at body temperature. The swelling, degradation, and mechanical properties were characterized, and they demonstrated an extended degradation time (approximately 65 days), an exponential decrease in modulus with degradation time, and a tunable shear modulus (1.6-133 kPa). We also discuss the various constitutive relationships (Hookean, neo-Hookean, and Mooney-Rivlin) that can be used to describe the stress-strain behavior of these hydrogels. The chosen model and assumptions used for data fitting influenced the obtained modulus values by as much as a factor of 3.5, which demonstrates the importance of clearly stating one's data fitting parameters so that accurate comparisons can be made within the literature.     

Cross-linking and degradation of step-growth hydrogels formed by thiol-ene photoclick chemistry

Thiol-ene photoclick hydrogels have been used for a variety of tissue engineering and controlled release applications. In this step-growth photopolymerization scheme, four-arm poly(ethylene glycol) norbornene (PEG4NB) was cross-linked with dithiol containing cross-linkers to form chemically cross-linked hydrogels. While the mechanism of thiol-ene gelation was well described in the literature, its network ideality and degradation behaviors are not well-characterized. Here, we compared the network cross-linking of thiol-ene hydrogels to Michael-type addition hydrogels and found thiol-ene hydrogels formed with faster gel points and higher degree of cross-linking. However, thiol-ene hydrogels still contained significant network nonideality, demonstrated by a high dependency of hydrogel swelling on macromer contents. In addition, the presence of ester bonds within the PEG-norbornene macromer rendered thiol-ene hydrogels hydrolytically degradable. Through validating model predictions with experimental results, we found that the hydrolytic degradation of thiol-ene hydrogels was not only governed by ester bond hydrolysis, but also affected by the degree of network cross-linking. In an attempt to manipulate network cross-linking and degradation of thiol-ene hydrogels, we incorporated peptide cross-linkers with different sequences and characterized the hydrolytic degradation of these PEG-peptide hydrogels. In addition, we incorporated a chymotrypsin-sensitive peptide as part of the cross-linkers to tune the mode of gel degradation from bulk degradation to surface erosion.     

Preparation and characterization of imogolite/DNA hybrid hydrogels

Imogolite is one of the clay minerals contained in volcanic ash soils. The novel hybrid hydrogels were prepared from imogolite nanofibers and DNA by utilizing strong interaction between the aluminol groups on imogolite surface and phosphate groups of DNA. The hybrid hydrogels of imogolite and DNA were prepared in various feed ratios, and their physicochemical properties and molecular aggregation states were investigated in both dispersion and gel states. The maximum DNA content in the hybrid gels was shown in equivalent molar ratio of imogolite and DNA. The physical properties of the hybrid gels were changed by varying DNA blend ratios. In the dispersion state, the hybrid gels showed a fibrous structure of imogolite, whereas a continuous network structure was observed in pure imogolite, indicating that the hybrid with DNA enhanced the dispersion of imogolite. In the gel state, DNA and imogolite nanofibers formed a 3D network structure.     

New aspects of the formation of physical hydrogels of chitosan in a hydroalcoholic medium

New aspects concerning the mechanism of formation of chitosan physical hydrogels without any cross-linking agent were studied. The gelation took place during the evaporation of a hydroalcoholic solution of chitosan. We first demonstrated that it was possible to form a physical hydrogel from a hydrochloride form of chitosan. Chromatographic methods showed that during the gel formation, when the initial concentration is over C, the critical concentration of chain entanglement, the water and acid used for the solubilization of the polymer were both eliminated. This particular situation contributed to decrease the dielectric constant of the medium and the apparent charge density of chitosan chains, thus inducing the formation of a three-dimensional network through hydrophobic interactions and hydrogen bonding. In the gelation process, this step was kinetically determining. The speed of evaporation of water and acid were determined and different initial conditions were compared. Thus, we investigated the influence of: the initial polymer concentration, the nature of the counterion and the alcohol, the temperature and the geometry of the reactor. Our results allowed us to confirm the existence of a second critical initial concentration C, from which the evaporation of water became more difficult. We suggested that C corresponded to a reorganization of the solution involving the presence of gel precursors. Then, a mechanism of formation of physical hydrogels of chitosan in a hydroalcoholic medium could be proposed. For the first time, we demonstrated that it was possible to generate physical hydrogels in the presence of various diols, which size of the carbonated chain appeared as a limiting factor for the gelation process. These physical hydrogels of chitosan are currently used in our laboratory for tissue engineering in the treatment of third degree burns with the possibility to adapt their mechanical properties from the choice of both the acid or the alcohol used.     

Superporous IPN hydrogels having enhanced mechanical properties

The objective of this study was to improve the mechanical properties of superporous hydrogels (SPHs), which were used to develop gastric retention devices for long-term oral drug delivery. The main approach used in this study was to form an interpenetrating polymer network by incorporating a second polymer network inside an SPH structure. Polyacrylonitrile was used as the second network inside an SPH. Mechanical properties including compression strength and elasticity were significantly improved, up to 50 times as compared with the control SPHs. The enhanced mechanical properties were a result of the scaffold-like fiber network structures formed inside the cell walls of SPHs. The fast swelling property of SPHs was not affected by the incorporation of the second polymer network because the interconnected pore structures were maintained. Gastric retention devices based on superporous IPN hydrogels (SPIHs) with the improved mechanical properties are expected to withstand compression pressure and mechanical frictions in the stomach better than the control SPHs.     

Cell wall polysaccharides from Gelidium species: physico-chemical studies using MRI techniques

Magnetic resonance imaging (MRI) has already been successively used to investigate polysaccharide matrices. In particular, MRI at microscopic resolution (MR microscopy) is now one of the most powerful techniques for studying the physical properties of natural hydrogels. To contribute to a better understanding of the correlation between chemical and physical properties of agar gels, we report here the measurement of the water magnetic parameters for agar gels extracted from different species of Gelidium: T1 and T2 relaxation times, magnetisation transfer (Ms /M0) and diffusion (D) were measured to evaluate their use for studying the gel characteristics. MR microscopic images were acquired at 7.05 Tesla using various pulse sequences. The results obtained confirmed the possibility to use quantitative MRI for the characterisation of physical parameters correlated with the type of agar chemical structure. In particular, T2 data obtained for gels at different concentrations indicate that this magnetic parameter is very sensitive to the agar concentration and hence particularly useful for the gel strength determination. This revised version was published online in June 2006 with corrections to the Cover Date.     

An approach to modulate degradation and mesenchymal stem cell behavior in poly(ethylene glycol) networks

A simple, sequential approach for creation of hydrolytically degradable poly(ethylene glycol) (PEG) hydrogels has been developed and characterized. The chemistry involves an initial step growth polymerization reaction between PEG-diacrylate and dithiothreitol (DTT) to form acrylate-terminated (-PEG-DTT-)n PEG chains, followed by photocross-linking to form a hydrogel network. Varying the extent of step growth polymerization prior to photocross-linking allowed for control over the equilibrium swelling ratio, degradation, and erosion of PEG hydrogels. Hydrogel degradability had a significant effect on behavior of human mesenchymal stem cells (hMSCs) encapsulated within PEG hydrogels, both in the presence and absence of an RGDSP cell adhesion ligand. In particular, enhanced network degradability resulted in enhanced hMSC viability and spreading during in vitro culture. Comparison of degradable and nondegradable hydrogels with similar physical properties (e.g., equilibrium swelling ratio) demonstrated that hMSC viability and spreading were dependent on network degradability. This study demonstrates that hydrolytically degradable PEG hydrogels can be formed via a sequential step growth polymerization and photocross-linking process and the resulting materials may serve as promising matrices for 3-dimensional stem cell culture and tissue engineering applications.     

Design of biomimetic cell-interactive substrates using hyaluronic acid hydrogels with tunable mechanical properties

Hyaluronic acid (HA) is a natural polysaccharide abundant in biological tissues with excellent potential for constructing synthetic extracellular matrix analogues. In this work, we established a simple and dependable approach to prepare hyaluronic acid-based hydrogels with controlled stiffness and cell recognition properties for use as cell-interactive substrates. This approach relied on a new procedure for the synthesis of methacrylate-modified HA macromers (HA-MA) and, on photorheometry allowing real time monitoring of gelation during photopolymerization. We showed in this way the ability to obtain gels that encompass the range of physiologically relevant elastic moduli while still maintaining the recognition properties of HA by specific cell surface receptors. These hydrogels were prepared from HA macromers having a degree of methacrylation <0.5, which allows to minimize compromising effects on the binding affinity of HA to its cell receptors due to high substitution on the one hand, and to achieve nearly 100% conversion of the methacrylate groups on the other. When the HA hydrogels were immobilized on glass substrates, it was observed that the attachment and the spreading of a variety of mammalian cells rely on CD44 and its coreceptor RHAMM. The attachment and spreading were also shown to be modulated by the elastic properties of the HA matrix. All together, these results highlight the biological potential of these HA hydrogel systems and the needs of controlling their chemical and physical properties for applications in cell culture and tissue engineering.     

Supramolecular hydrogels formed from biodegradable ternary COS-g-PCL-b-MPEG copolymer with α-cyclodextrin and their drug release

On the basis of the synthesis of novel biodegradable amphiphilic MPEG-b-PCL-grafted chitooligosaccharide (COS-g-PCL-b-MPEG) copolymers, supramolecular hydrogels were fabricated rapidly via their inclusion complexation with α-cyclodextrin (α-CD) in aqueous solutions. The graft copolymers were characterized by ¹H NMR spectroscopy, gel permeation chromatography (GPC), and fluorescence measurement, and the supramolecular structure of the resultant hydrogels was confirmed by X-ray diffraction measurements. Rheological studies of as-obtained hydrogels indicate that the physical properties could be modulated by controlling the concentration and the graft content of the graft copolymers as well as the molar feed ratio of the graft to α-CD. The in vitro release kinetics studies of bovine serum albumin (BSA) entrapped in the hydrogels show that the drug release profiles are dependent on the supramolecular hydrogel compositions.     

Nanostructured biodegradable polymer networks using lyotropic liquid crystalline templates

The physical properties, porosity, and physiological behavior of synthetic biodegradable hydrogels have been identified as highly critical design parameters in most tissue engineering materials applications. Nanotechnology may provide the means to manipulate these parameters by accessing control over the network structure of the biomaterial, providing unique property relationships that often result from nanostructured materials. In this study, a lyotropic liquid crystal (LLC) was used as a polymerization template in the formation of a photopolymerizable biodegradable PLA-b-PEG-b-PLA (PEG = poly(ethylene glycol); PLA = poly(lactic acid)) material with nanoscale lamellar morphology. Through ordering of the biodegradable monomer within the liquid crystal assembly, a 2-fold increase in maximum polymerization rate and a 30% increase in double bond conversion were realized over isotropic monomer formulations. The resulting network structure of the templated PLA-b-PEG-b-PLA material has a dramatic affect on the physical properties of the hydrogel including an 80% increase in network swelling and an approximately 230% increase in diffusivity. This increase in permeability and solvent uptake leads to rapid degradation of the lamellar templated samples, further demonstrating the influence of the LLC directed network structure on the porosity and physical properties of the biodegradable material. The ability to control the porosity, physical properties, and behavior of a biodegradable hydrogel simply by imparting LLC network structure, without changing the chemistry or biocompatibility of the polymer, could prove highly advantageous in the design of synthetic biomaterials for potential medical applications.     

Microporous cell‐laden hydrogels for engineered tissue constructs

In this article, we describe an approach to generate microporous cell‐laden hydrogels for fabricating biomimetic tissue engineered constructs. Micropores at different length scales were fabricated in cell‐laden hydrogels by micromolding fluidic channels and leaching sucrose crystals. Microengineered channels were created within cell‐laden hydrogel precursors containing agarose solution mixed with sucrose crystals. The rapid cooling of the agarose solution was used to gel the solution and form micropores in place of the sucrose crystals. The sucrose leaching process generated homogeneously distributed micropores within the gels, while enabling the direct immobilization of cells within the gels. We also characterized the physical, mechanical, and biological properties (i.e., microporosity, diffusivity, and cell viability) of cell‐laden agarose gels as a function of engineered porosity. The microporosity was controlled from 0% to 40% and the diffusivity of molecules in the porous agarose gels increased as compared to controls. Furthermore, the viability of human hepatic carcinoma cells that were cultured in microporous agarose gels corresponded to the diffusion profile generated away from the microchannels. Based on their enhanced diffusive properties, microporous cell‐laden hydrogels containing a microengineered fluidic channel can be a useful tool for generating tissue structures for regenerative medicine and drug discovery applications. Biotechnol. Bioeng. 2010; 106: 138–148. © 2010 Wiley Periodicals, Inc.     

Encapsulating chondrocytes in degrading PEG hydrogels with high modulus: engineering gel structural changes to facilitate cartilaginous tissue production

A major challenge when designing cell scaffolds for chondrocyte delivery in vivo is creating scaffolds with sufficient mechanical properties to restore initial function while simultaneously controlling temporal changes in the gel structure to facilitate tissue formation. To address this design challenge, degradable photocrosslinked hydrogels based on poly(ethylene glycol) were investigated. To alter the gel's initial mechanical properties, hydrogels were fabricated by varying the initial macromer concentration from 10% to 15% to 20%. A twofold increase in macromer concentration resulted in an eightfold increase in the initial compressive modulus from 60 to 500 kPa. Gel degradation was tailored by incorporating fast-degrading crosslinks that enable maximal extracellular matrix (ECM) diffusion with time and a minimal number of nondegrading (or slowly degrading) crosslinks to maintain scaffold integrity and prevent complete gel erosion during tissue formation. Chondrocytes encapsulated in these gels produced cartilaginous tissue rich in glycosaminoglycans and collagen as seen biochemically and histologically. Interestingly, mass loss appeared to more closely match tissue secretion in gels fabricated from a 15% macromer concentration. However, the spatial ECM distribution was grossly similar in all three gels. By tailoring gel degradation and controlling network evolution during degradation, gels with optimal properties can be fabricated to support initially physiologic compressive loads while simultaneously supporting the formation of a neotissue.     

Development of a hybrid dextrin hydrogel encapsulating dextrin nanogel as protein delivery system

Dextrin, a glucose polymer with low molecular weight, was used to develop a fully resorbable hydrogel, without using chemical initiators. Dextrin was first oxidized (oDex) with sodium periodate and then cross-linked with adipic acid dihidrazide, a nontoxic cross-linking molecule. Furthermore, a new bidimensional composite hydrogel, made of oxidized dextrin incorporating dextrin nanogels (oDex-nanogel), was also developed. The oDex hydrogels showed good mechanical properties and biocompatibility, allowing the proliferation of mouse embryo fibroblasts 3T3 cultured on top of the gel. The gelation time may be controlled selecting the concentrations of the polymer and reticulating agent. Both the oDex and oDex-nanogel hydrogels are biodegradable and present a 3-D network with a continuous porous structure. The obtained hybrid hydrogel enables the release of the dextrin nanogel over an extended period of time, paralleling the mass loss curve due to the degradation of the material. The dextrin nanogel allowed the efficient incorporation of interleukin-10 and insulin in the oDex hydrogel, providing a sophisticated system of controlled release. The new hydrogels present promising properties as an injectable carrier of bioactive molecules. Both proteins and poorly water-soluble low-molecular-weight drugs are efficiently encapsulated in the nanogel, which performs as a controlled release system entrapped in the hydrogel matrix.     

Molecular dynamics study of the influence of solvents on the structure and mechanical properties of poly(vinyl alcohol) gels

Molecular dynamics (MD) simulations were employed to study the influence of solvents on the structure and mechanical properties of physically crosslinked poly(vinyl alcohol) (PVA) gels. Firstly, three kinds of PVA precursor gels were made by adding water, dimethyl sulfoxide (DMSO) and a mixture of DMSO and water (4:1 by weight), respectively. The solvents in the precursor gels were then exchanged with water to obtain three kinds of PVA hydrogels. Solvent in the precursor gel with a mixture of DMSO and water was also exchanged with ethanol and DMSO, respectively. It was found that the tensile strength and failure strain of the PVA hydrogel prepared from precursor gel with a mixture of DMSO and water was the highest, and the polymer network was more homogeneous than the other two PVA hydrogels. The polymer network of PVA gel with ethanol or with DMSO was more heterogenous than with water, and the tensile strength and failure strain were much lower. The torsional activity of polymer chains of PVA gel with ethanol was much stronger than PVA gel with water and DMSO.     

Beyond bread and beer: whole cell protein extracts from baker’s yeast as a bulk source for 3D cell culture matrices

Here, we present a novel approach to form hydrogels from yeast whole cell protein. Countless hydrogels are available for sophisticated research, but their fabrication is often difficult to reproduce, with the gels being complicated to handle or simply too expensive. The yeast hydrogels presented here are polymerized using a four-armed, amine reactive crosslinker and show a high chemical and thermal resistance. The free water content was determined by measuring swelling ratios for different protein concentrations, and in a freeze-drying approach, pore sizes of up to 100 μm in the gel could be created without destabilizing the 3D network. Elasticity was proofed to be adjustable with the help of atomic force microscopy by merely changing the amount of used protein. Furthermore, the material was tested for possible cell culture applications; diffusion rates in the network are high enough for sufficient supply of human breast cancer cells and adenocarcinomic human alveolar basal epithelial cells with nutrition, and cells showed high viabilities when tested for compatibility with the material. Furthermore, hydrogels could be functionalized with RGD peptide and the optimal concentration for sufficient cell adhesion was determined to be 150 μM. Given that yeast protein is one of the cheapest and easiest available protein sources and that hydrogels are extremely easy to handle, the developed material has highly promising potential for both sophisticated cell culture techniques as well as for larger scale industrial applications.

     

Study of the near-neutral pH-sensitivity of chitosan/gelatin hydrogels by turbidimetry and microcantilever deflection

The fundamental properties and pH-sensitivity of chitosan/gelating hydrogels were investigated using spectroscopic and microelectro mechanical (MEMS) measurement approaches. Turbidimetric titration revealed that there were electrostatic attractive interactions between tripolyphosphate (TPP), chitosan, and gelatin in the acidic pH range, depending on their degree of ionization. The pH-sensitive swelling behavior of the hydrogels was investigated by monitoring the deflection of hydrogel-coated microcantilevers, which exhibited a sensitive and repeatable response to solution pH. The deflection of the microcantilever increased as the pH decreased, and the response speed of the system exhibited a nearly linear relationship with pH. The effects of the pH and concentration of TPP solution, as well as the ratio of chitosan to gelatin in gel precursor solutions, on the pH sensitivity of the hydrogels were also investigated. It was found that the swelling of the hydrogel is mainly a result of chain relaxation of chitosan-TPP complexes caused by protonation of free amino groups in chitosan, which depends on the crosslinking density set during the formation of the network. An increase in initial crosslink density induced a decrease in swelling and pH sensitivity. It can be concluded from this study that pH-sensitive chitosan gel properties can be tuned by preparatory conditions and inclusion of gelatin. Furthermore, microcantilevers can be used as a platform for gaining increased understanding of environmentally sensitive polymers.     

Rheometric study of the gelation of chitosan in aqueous solution without cross-linking agent

A new process of formation of chitosan physical hydrogels in aqueous solution, without any organic solvent or cross-linking additive, was studied. The three conditions required for the physical gelation were an initial polymer concentration over C*, a critical value of the balance between hydrophilic and hydrophobic interactions, and a physicochemical perturbation responsible for a bidimensional percolating mechanism. The time necessary to reach the gel point was determined by rheometry, and gelations were compared according to different initial conditions. Thus, we investigated the influence of the polymer concentration and the degree of acetylation (DA) of chitosan on gelation. The number of junctions per unit volume at the gel point varied with the initial polymer concentration, i.e., the initial number of chain entanglements per unit volume or the number of gel precursors. The time to reach the gel point decreased with both higher DAs and concentrations. For a chitosan of DA = 36.7%, a second critical initial concentration close to 1.8% (w/w) was observed. Above this concentration, the decrease of the time to reach the gel point was higher and fewer additional junctions had to be formed to induce gelation. To optimize these physical hydrogels, to be used for cartilage regeneration, their final rheological properties were studied as a function of their degree of acetylation and their polymer concentration. Our results allowed us to define the most appropriate gel for the targeted application corresponding to a final concentration of chitosan in the gel of near 1.5% (w/w) and a DA close to 40%.     

Viscoelastic properties and nanoscale structures of composite oligopeptide-polysaccharide hydrogels

Biocompatible and biodegradable peptide hydrogels are drawing increasing attention as prospective materials for human soft tissue repair and replacement. To improve the rather unfavorable mechanical properties of our pure peptide hydrogels, in this work we examined the possibility of creating a double hydrogel network. This network was created by means of the coassembly of mutually attractive, but self-repulsive oligopeptides within an already-existing fibrous network formed by the charged, biocompatible polysaccharides chitosan, alginate, and chondroitin. Using dynamic oscillatory rheology experiments, it was found that the coassembly of the peptides within the existing polysaccharide network resulted in a less stiff material as compared to the pure peptide networks (the elastic modulus G' decreased from 90 to 10 kPa). However, these composite oligopeptide-polysaccharide hydrogels were characterized by a greater resistance to deformation (the yield strain γ grew from 4 to 100%). Small-angle neutron scattering (SANS) was used to study the 2D cross-sectional shapes of the fibers, their dimensional characteristics, and the mesh sizes of the fibrous networks. Differences in material structures found with SANS experiments confirmed rheology data, showing that incorporation of the peptides dramatically changed the morphology of the polysaccharide network. The resulting fibers were structurally very similar to those forming the pure peptide networks, but formed less stiff gels because of their markedly greater mesh sizes. Together, these findings suggest an approach for the development of highly deformation-resistant biomaterials.     

Porphyrin-cross-linked hydrogel for fluorescence-guided monitoring and surgical resection

We demonstrate that porphyrins can be used as efficient cross-linkers to generate a new class of hydrogels with enabling optical properties. Tetracarboxylic acid porphyrins reacted with PEG diamines to form a condensation polyamide in a range of appropriate conditions, with respect to reaction time, diisopropylethylamine initiator concentration, porphyrin-to-PEG ratio, porphyrin concentration, and PEG size. The network structure of the hydrogel maintained a porphyrin spacing that prevented excessive fluorescence self-quenching despite high porphyrin density. The near-infrared properties readily enabled low background, noninvasive fluorescence monitoring of the implanted hydrogel in vivo, as well as its image-guided surgical removal in real time using a low-cost fluorescence camera prototype. Emission could be tuned by incorporating copper metalloporphyrins into the network. The approach of creating hydrogels using cross-linking porphyrin comonomers creates opportunities for new polymer designs with strong optical character.