Non-porous Hydrogels Research Articles

Fabrication of three-dimensional porous cell-laden hydrogel for tissue engineering

For tissue engineering applications, scaffolds should be porous to enable rapid nutrient and oxygen transfer while providing a three-dimensional (3D) microenvironment for the encapsulated cells. This dual characteristic can be achieved by fabrication of porous hydrogels that contain encapsulated cells. In this work, we developed a simple method that allows cell encapsulation and pore generation inside alginate hydrogels simultaneously. Gelatin beads of 150–300 µm diameter were used as a sacrificial porogen for generating pores within cell-laden hydrogels. Gelation of gelatin at low temperature (4 °C) was used to form beads without chemical crosslinking and their subsequent dissolution after cell encapsulation led to generation of pores within cell-laden hydrogels. The pore size and porosity of the scaffolds were controlled by the gelatin bead size and their volume ratio, respectively. Fabricated hydrogels were characterized for their internal microarchitecture, mechanical properties and permeability. Hydrogels exhibited a high degree of porosity with increasing gelatin bead content in contrast to nonporous alginate hydrogel. Furthermore, permeability increased by two to three orders while compressive modulus decreased with increasing porosity of the scaffolds. Application of these scaffolds for tissue engineering was tested by encapsulation of hepatocarcinoma cell line (HepG2). All the scaffolds showed similar cell viability; however, cell proliferation was enhanced under porous conditions. Furthermore, porous alginate hydrogels resulted in formation of larger spheroids and higher albumin secretion compared to nonporous conditions. These data suggest that porous alginate hydrogels may have provided a better environment for cell proliferation and albumin production. This may be due to the enhanced mass transfer of nutrients, oxygen and waste removal, which is potentially beneficial for tissue engineering and regenerative medicine applications.

X-Ray Imaging of Poly(Ethylene Glycol) Hydrogels Without Contrast Agents

Hydrogels have shown promise for a number of tissue engineering applications. However, their high water content results in little or no image contrast when using conventional X-ray imaging techniques. X-ray imaging techniques based on phase-contrast have shown promise for biomedical application due to their ability to provide information about the X-ray refraction properties of samples. Nonporous and porous poly(ethylene glycol) hydrogels were synthesized and imaged using a synchrotron light source employing a silicon analyzer crystal and an X-ray energy of 40-keV. Data were acquired at 21 angular analyzer positions spanning the range of -5 to 5 μrad. Images that depict the projected X-ray absorption, refraction, and ultra-small-angle scatter (USAXS) properties of the hydrogels were reconstructed from the measurement data. The poly(ethylene glycol) hydrogels could be discerned from surrounding water and soft tissue in the refraction image but not the absorption or USAXS images. In addition, the refraction images of the porous hydrogels have a speckle pattern resulting in increased image texture in comparison to nonporous hydrogels. To our knowledge, this is the first study to show that X-ray phase-contrast imaging techniques can identify and provide detail on hydrogel structure without the addition of contrast agents.

Designed biodegradable hydrogel structures prepared by stereolithography using poly(ethylene glycol)/poly(d,l-lactide)-based resins

Designed three-dimensional biodegradable poly(ethylene glycol)/poly(d,l-lactide) hydrogel structures were prepared for the first time by stereolithography at high resolutions. A photo-polymerisable aqueous resin comprising PDLLA-PEG-PDLLA-based macromer, visible light photo-initiator, dye and inhibitor in DMSO/water was used to build the structures. Porous and non-porous hydrogels with well-defined architectures and good mechanical properties were prepared. Porous hydrogel structures with a gyroid pore network architecture showed narrow pore size distributions, excellent pore interconnectivity and good mechanical properties. The structures showed good cell seeding characteristics, and human mesenchymal stem cells adhered and proliferated well on these materials.

Gamma radiation synthesis of rapid swelling superporous polyacrylamide hydrogels

In this report a simple route for gamma radiation induced synthesis of superporous hydrogel (SPH) is described. Conventional SPH synthesis requires foaming and cross-linking reactions to take place simultaneously. However, in radiation synthesis it is difficult to introduce foaming during the cross-linking reactions. In order to overcome this limitation, the foaming and radiation cross-linking reactions were decoupled and carried out in two stages. The polyacrylamide SPH synthesized by this approach has very fast swelling kinetics compared to the non-porous hydrogel.

Initial evaluation of vascular ingrowth into superporous hydrogels

There is a need for new materials and architectures for tissue engineering and regenerative medicine. Based upon our recent results developing novel scaffold architecture, we hypothesized that this new architecture would foster vascularization, a particular need for tissue engineering. We report on the potential of superporous hydrogel (SPH) scaffolds for in vivo cellular infiltration and vascularization. Poly(ethylene glycol) diacrylate (PEGDA) SPH scaffolds were implanted in the dorsum of severe combined immunodeficient (SCID) mice and harvested after 4 weeks of in vivo implantation. The SPHs were visibly red and vascularized, as apparent when compared to the non-porous hydrogel controls, which were macroscopically avascular. Host cell infiltration was observed throughout the SPHs. Blood cells and vascular structures, confirmed through staining for CD34 and smooth muscle alpha-actin, were observed throughout the scaffolds. This novel soft material may be utilized for cell transplantation, tissue engineering and in combination with cell therapies. The neovasularization and limited fibrotic response suggest that the architecture may be conducive to cell survival and rapid vessel development.

A New Cationic Poly[1-vinyl-3-ethyl imidazolium iodide], P(VEII) Hydrogel for the Effective Removal of Chromium (VI) from Aqueous Solution

Cationic porous and non-porous hydrogels have been prepared via free radical solution polymerization using newly synthesized 1-vinyl-3-ethyl imidazolium iodide (VEII) monomer and N,N′-methylenebisacrylamide as crosslinker. The hydrogel has been characterized by FT-IR, mercury porosimetery and TGA analysis. Their surface morphology was observed using scanning electron microscope (SEM). The swelling kinetics of porous and non-porous P(VEII) hydrogels in solutions of chromium(VI) was determined using a gravimetric method. A batch system was applied to study the adsorption of chromium(VI) from aqueous solutions by cationic hydrogels. The effect of treatment time, pH of the medium, amount of adsorbent dose and initial feed concentration of metal ion on adsorption of chromium(VI) from their solution were also investigated. Adsorption of chromium(VI) increases with the increase in treatment time, adsorbent dose and initial feed concentration and decreases with the increase in pH of the medium. Desorption of chromium(VI) was carried out using various eluents and almost 100% desorption was observed, so the P[VEII] hydrogel can be used repeatedly for the adsorption of chromium(VI).

Unconfined compression properties of a porous poly(vinyl alcohol)–chitosan-based hydrogel after hydration

A poly(vinyl alcohol) (PVA) hydrogel composite scaffold containing N, O-carboxymethylated chitosan (NOCC) was tested to assess its potential as a scaffold for cartilage tissue engineering in a weight-bearing environment. The mechanical properties under unconfined compression for different hydration periods were investigated. The effect of supplementing PVA with NOCC (20 wt.% PVA:5 vol.% NOCC) produced a porosity of 43.3% and this was compared against a non-porous PVA hydrogel (20 g PVA: 100 ml of water, control). Under non-hydrated conditions, the porous PVA–NOCC hydrogel behaved in a similar way to the control non-porous PVA hydrogel, with similar non-linear stress–strain response under unconfined compression (0–30% strain). After 7 days’ hydration, the porous hydrogel demonstrated a reduced stiffness (0.002 kPa, at 25% strain), resulting in a more linear stiffness relationship over a range of 0–30% strain. Poisson’s ratio for the hydrated non-porous and porous hydrogels ranged between 0.73 and 1.18, and 0.76 and 1.33, respectively, suggesting a greater fluid flow when loaded. The stress relaxation function for the porous hydrogel was affected by the hydration period (from 0 to 600 s); however the percentage stress relaxation regained by about 95%, after 1200 s for all hydration periods assessed. No significant differences were found between the different hydration periods between the porous hydrogels and control. The calculated aggregate modulus, H A, for the porous hydrogel reduced drastically from 10.99 kPa in its non-hydrated state to about 0.001 kPa after 7 days’ hydration, with the calculated shear modulus reducing from 30.92 to 0.14 kPa, respectively. The porous PVA–NOCC hydrogel conformed to a biphasic, viscoelastic model, which has the desired properties required for any scaffold in cartilage tissue engineering.

Macroporous hydrogel beads of high toughness and superfast responsivity

Macroporous hydrogel beads based on the monomers acrylamide and 2-acrylamido-2-methylpropane sulfonic acid sodium salt were prepared by dropwise addition of the monomer solution into the paraffin oil as the continuous phase at subzero temperatures. The beads prepared between −15 and −20 °C have irregular large pores of 1–10 μm in diameter, typical for macroporous networks created by the cryogelation technique, while nonporous hydrogels were obtained at room temperature. Swelling measurements show that the low temperature beads swell within seconds to attain their equilibrium states in water. The beads formed at subzero temperatures were very tough and can be compressed up to 94% strain without any crack development while those formed at room temperature were fragile and broke at a strain of about 40%. The results indicate that the tough hydrogel beads formed at subzero temperatures can be used in separation processes in which the separated compounds can easily be recovered by compression of the beads under a piston.

Formation of macroporous poly(acrylamide) hydrogels in DMSO/water mixture: Transition from cryogelation to phase separation copolymerization

Poly(acrylamide) hydrogels were prepared by free-radical crosslinking copolymerization of acrylamide and N, N′-methylenebis(acrylamide) at −18 oC in aqueous DMSO solutions of various composition. The hydrogels formed in the solvent mixture with less than 25% DMSO by volume have irregular large pores of about 10 1 μm in diameter, typical for macroporous networks created by the cryogelation technique. Non-porous hydrogels were obtained in solutions containing 25% DMSO, while at larger DMSO contents, the structure of the hydrogel networks consists of aggregates of microspheres, which looks as cauliflowers, typical for a macroporous network formed by reaction-induced phase separation mechanism. Swelling measurements show that fast responsive PAAm hydrogels can be obtained as the DMSO content in the mixed solvent is decreased or increased starting from 25 v/v%. The results were interpreted as the transition from cryogelation to the phase separation copolymerization due to the marked freezing point depression of the solvent mixture as well as due to the action of the mixed solvent as a poor solvating diluent at −18 oC. It was also shown that the initial temperature of the cryogelation reactions in water strongly affects the hydrogel properties. Hydrogels formed at an initial temperature T ini of 0 oC were very tough and can be compressed up to about 100% strain without any crack development while those formed at T ini = 21 oC were fragile.

Fabrication of macroporous hydrogel membranes using photolithography for enzyme immobilization

AbstractBACKGROUND: The main problem when immobilizing enzyme inside hydrogel is the activity loss of the trapped enzymes due to the limitation on diffusion. In this study, macroporous poly(ethylene glycol) (PEG) hydrogel membrane was fabricated using photolithography to enhance the activity of immobilized enzyme.RESULT: Photopolymerization through the designed photomask produced crosslinked hydrogel with tunnel‐like macropores which were completely opened from top to bottom of the hydrogel membranes. Arrays of pores with 50 µm and 100 µm diameters were successfully created in the hydrogel membranes, and the pore density could be controlled by changing the photomask design. An activity study demonstrated that there was an enhancement of more than 50% in glucose oxidase activity in the macroporous membrane in comparison to the nonporous membrane. The activity of immobilized enzyme could be further enhanced by increasing the pore density of the hydrogel membranes or by using a higher molecular weight PEG.CONCLUSION: Increased enzymatic activity was observed in macroporous hydrogel membranes compared with nonporous hydrogel membranes due to the higher surface to volume ratio provided by the macroporous structure. Compared with other methods of obtaining porous hydrogel, the proposed method is much simpler and can minimize the deactivation of enzyme. Copyright © 2007 Society of Chemical Industry

Superporous hydrogels containing poly(acrylic acid- co-acrylamide)/ O-carboxymethyl chitosan interpenetrating polymer networks

Superporous hydrogels containing poly(acrylic acid- co-acrylamide)/ O-carboxymethyl chitosan interpenetrating polymer networks (SPH-IPNs) were prepared by cross-linking O-carboxymethyl chitosan ( O-CMC) with glutaraldehyde (GA) after superporous hydrogel (SPH) was synthesized. The structures of the SPH-IPNs were characterized with FT-IR, 13C-NMR and DSC. SEM, CLSM and light images revealed that the SPH-IPNs possessed both the IPN network and large numbers of pores and the cross-linked O-CMC molecules were located on the peripheries of these pores. The swelling behavior of SPH-IPNs was dependent on the O-CMC content, GA amount and cross-linking time. Due to the cross-linked O-CMC network, in vitro muco -adhesive force and mechanical properties, including compression and tensile modulus, of the SPH-IPN were greatly improved when compared with the CSPH. An enhanced loading capacity for insulin could be obtained by the SPH-IPNs as compared to non-porous hydrogel and CSPH, and more than 90% of the insulin was released within 1 h. With the improved mechanical properties, in vitro muco-adhesive force and loading capacities, the SPH-IPN may be used as a potential muco-adhesive system for peroral delivery of peptide and protein drugs.

Macroporous hydrogels based on 2-hydroxyethyl methacrylate

Crosslinked macroporous hydrogels based on 2-hydroxyethyl methacrylate (HEMA)-[2-(methacryloyloxy)ethyl]trimethylammonium chloride (MOETACl) copolymer, HEMA-MOETACl-methacrylic acid (MA) terpolymer, and on a polyelectrolyte complex of HEMA-MA copolymer with poly(MOETACl) were prepared. All the hydrogels were prepared in the presence of fractionated sodium chloride particles. The hydrogels were characterized by the number of pores and the total volume of all pores in unit volume, the average volume and the average diameter of single pore. Morphology of the hydrogels was investigated by confocal and scanning electron microscopy. The hydrogels based on polyelectrolyte complexes were also characterized by chemical composition. Homogeneous (non-porous) hydrogels with the same composition as macroporous hydrogels were prepared and characterized by their biocompatibility.

Ultrarapid Molecular Release from Poly(N‐isopropylacrylamide) Hydrogels Perforated Using Silica Nanoparticle Networks

AbstractSummary: Porous poly(N‐isopropylacrylamide) (PNIPAAm) hydrogels were successfully prepared in the presence of nanosized silica particles with network morphology and subsequent silica extraction by HF treatment. The nanosized pores induced very rapid deswelling of the hydrogels, accompanied by volume phase transitions from swelling to deswelling states at approximately 30 °C. Deswelling, which was analyzed by a quick change in temperature of the aqueous media from 20 to 40 °C, was facilitated by increased silica particle content. The deswelling rate constant of the hydrogel prepared using silica nanoparticles with a network morphology (NT gel) was about 1 900‐fold greater than that of nonporous hydrogels and 13‐fold larger than that of porous hydrogels perforated using spherical silica nanoparticles (SP gel). The releasing rate of the low‐molecular weight chromophore from the NT gel was higher than that from the SP gel. In the chromophore‐labeled macromolecules, the network morphology was effective for increasing the releasing rate based on the transfer inside the hydrogel, as well as for increasing the amount released based on the surface pore morphology. magnified image

Porous poly(2-hydroxyethyl acrylate) hydrogels prepared by radical polymerisation with methanol as diluent

Dynamic-mechanical and water sorption properties of porous and non-porous hydrogels have been studied as a function of their porosity and crosslinking density. Porous hydrogels with different crosslinking densities were prepared by co-polymerisation of 2-hydroxyethyl acrylate and ethyleneglycol dimethacrylate in solution in methanol. Pores were formed due to the segregation of the solvent from the polymer network during the polymerisation process. The porosity of the samples was observed by scanning electron microscopy. The pores collapse during the drying process after polymerisation but they reopen when the xerogel is immersed in liquid water. Bulk polymer networks with varying crosslinking densities were also synthesised and used as a reference in the analysis of the porous hydrogels. Water sorption from the gas phase and from the liquid phase was studied by means of equilibrium sorption isotherms and immersion experiments, respectively. Dynamic-mechanical spectroscopy conducted on the xerogels shows that the elastic modulus in the rubber-like state highly depends on the amount of solvent used in the polymerisation what is attributed to the presence of discontinuity surfaces in the xerogel although the pores are closed.

Preparation of porous hydrolyzable polyrotaxane hydrogels and their erosion behavior

—We have prepared porous polyrotaxane hydrogels by using the salt leaching technique. Porous hydrogels were found to have a uniform and highly porous structure. The size of pores in each hydrogel was directly proportional to the size of the sodium chloride particle used. Structural uniformity of the hydrogels is useful not only for uniform cell distribution, but also for wellcontrolled material properties. Uniform pore size and distribution may ensure the diffusion of nutrients throughout of the gel and the removal of metabolic wastes from the system. The results of an erosion study in phosphate-buffered saline showed that the erosion time of porous polyrotaxane hydrogels was controlled by the poly(ethylene glycol) (PEG) content in the hydrogels. The erosion time of the porous polyrotaxane hydrogel was observed to be almost the same with the non-porous polyrotaxane hydrogel with the same PEG content. From the erosion study, the erosion time of the polyrotaxane hydrogel may be independent of its morphology. Easy control of the erosion time in the polyrotaxane hydrogels is useful in the preparation of scaffolds for tissue engineering.

Rapid temperature/pH response of porous alginate- g-poly( N-isopropylacrylamide) hydrogels

To improve the swelling and deswelling rate, comb-type macroporous hydrogels were prepared. The temperature-sensitive poly( N-isopropylacrylamide) (PNIPAAm) was grafted on the surface or bulk of the pH-responsive alginate. The larger surface area in pores and free chain mobility by comb-type graft copolymer reached its equilibrium swollen state within a minute. The swelling ratio of porous hydrogel in equilibrium state was over fifteen times greater than that of nonporous hydrogels. The increase of surface area by pores caused water molecules to transfer easily in and out of the matrix, resulting in a rapid deswelling. The degree of change in swelling ratio during deswelling might be affected by the phase transition behavior of PNIPAAm attached on only the surface of the pore rather than PNIPAAm grafted into mainchain of alginate. Surface-grafted alginate/PNIPAAm gels had a suitable mechanical strength without collapsing during the repeatable shrinkage and expansion due to swelling and deswelling, whereas bulk-grafted gels easily collapsed.

Thermoresponsive properties of porous poly(N‐isopropylacrylamide) hydrogels prepared in the presence of nanosized silica particles and subsequent acid treatment

AbstractPorous poly(N‐isopropylacrylamide) hydrogels were prepared by the free‐radical polymerization of its monomer and a suitable crosslinker in the presence of spherical silica particles of different sizes (74 and 1600 nm) and by the subsequent acid extraction of silica. The yields were 81–83%, and the yields were not affected by the silica content. Scanning electron microscopy observations revealed the porous structure of the hydrogels. Porous and nonporous hydrogels showed volume phase transitions from swelling states to deswelling states at approximately 30 °C, as analyzed by the ratio of the diameter of cylinder‐shaped hydrogels to that of the glass tube used for the hydrogel preparation at the corresponding temperature. Deswelling, which was analyzed by rapid changes in the temperature of the aqueous media from 20 to 40 °C, was facilitated by decreased silica particle size and increased silica content. The deswelling rate constant of the hydrogel prepared with 74‐nm silica at 10 v/v % (silica/solvent) was more than 1500 times greater than that of conventional hydrogels. Swelling was similarly analyzed through changes in the temperature from 40 to 20 °C and was independent of the pore structure. The deswelling–swelling cycle was repeatable with reasonable reproducibility. Moreover, the mechanical strength of the porous hydrogels was significantly maintained compared with that of conventional nonporous hydrogels. This method produced thermoresponsive hydrogels of suitable mechanical strength and remarkable deswelling properties. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4228–4235, 2002

Temperature-dependent property development in hydrogels derived from hydroxypropylcellulose

Hydrogels constitute an important class of responsive materials that are employed in numerous biomedical and personal-care applications, most notably of which are controlled drug delivery, separations and superabsorbency. Since aqueous hydroxypropylcellulose (HPC) solutions exhibit lower critical solution behavior, hydrogels produced from this cellulose ether are temperature-responsive, swelling at low temperatures and contracting at high temperatures. If HPC hydrogels are synthesized at temperatures in the single-phase regime, they remain nonporous, whereas those crosslinked in the biphasic regime become microporous. In this work, we employ the modified temperature-induced phase separation (TIPS) protocol to generate nonporous and microporous HPC hydrogels crosslinked at different temperatures. The dynamic mechanical properties and swelling capacities of these hydrogels are reported as functions of crosslinking temperature.

PH-sensitive freeze-dried chitosan–polyvinyl pyrrolidone hydrogels as controlled release system for antibiotic delivery

The aim of this study was to develop a pH-sensitive chitosan/polyvinyl pyrrolidone (PVP) based controlled drug release system for antibiotic delivery. The hydrogels were synthesised by crosslinking chitosan and PVP blend with glutaraldehyde to form a semi-interpenetrating polymer network (semi-IPN). The semi-IPN formation was confirmed by Fourier transform infrared spectroscopic (FTIR) analysis. Semi-IPNs, viz, air-dried and freeze-dried, were compared for their surface morphology, wettability, swelling properties and pH-dependent swelling. Air- and freeze-dried membranes were also incorporated with amoxicillin and antibiotic release was studied. Porous freeze-dried hydrogels (pore diameter, 39.20±2.66 μm) exhibited superior pH-dependent swelling properties over non-porous air-dried hydrogels. A high octane contact angle (144.20±0.580) of hydrogel was indicative of its hydrophilic nature. Increased swelling of hydrogels, under acidic conditions, was due to the protonation of a primary amino group on chitosan, as confirmed by FTIR analysis. Freeze-dried membranes released around 73% of the amoxicillin (33% by air-dried) in 3 h at pH 1.0 and, thus, had superior drug-release properties to air-dried hydrogels. Freeze-dried membranes could serve as potent candidates for antibiotic delivery in an acidic environment.

Effect of reaction parameters on the dispersion polymerization of 1-vinyl-2-pyrrolidone

Nonporous hydrogel microspheres 0.1–1.3 μm in diameter were prepared by the dispersion copolymerization of 1-vinyl-2-pyrrolidone and ethylene dimethacrylate as a crosslinking agent. The crosslinking was evidenced by solid state 13C NMR and elemental analysis. The effect of various parameters including selection of solvent (cyclohexane, butyl acetate), initiator (4,4′-azobis(4-cyanopentanoic acid), 2,2′-azobisisobutyronitrile, dibenzoyl peroxide) and stabilizer on the properties of resulting microspheres has been studied. Dynamic light scattering and photographic examination were used for determination of the diameter and polydispersity of microspheres. Increasing concentration of steric stabilizer in the initial polymerization mixture decreased the particle size. The particle size depended on the molecular weight of polystyrene-block-hydrogenated polyisoprene stabilizer, but not on the number of PS and polybutadiene blocks in the styrene–butadiene block copolymer stabilizers. Dibenzoyl peroxide used as an initiator resulted in agglomeration of particles. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 653–663, 2000