Dextran permeation through poly(N-isopropylacrylamide) hydrogels

On-Chip Fabrication of Colloidal Suprastructures by Assembly and Supramolecular Interlinking of Microgels

In this report, a versatile method is demonstrated to create colloidal suprastructures by assembly and supramolecular interlinking of microgels using droplet-based microfluidics. The behavior of the microgels is systematically investigated to evaluate the influence of their concentration on their distribution between the continuous, the droplet phase, and the interface. At low concentrations, microgels are mainly localized at the water-oil interface whereas an excess of microgels results, following the complete coverage of the water-oil interface, in their distribution in the continuous phase. To stabilize the colloidal suprastructure, on-chip gelation is introduced by adding natural polyphenol tannic acid (TA) in the water phase. TA forms interparticle linking between the poly(N-vinylcaprolactam) (PVCL) microgels by supramolecular interactions. The combination of supramolecular interlinking with the variation of the microgel concentration in microfluidic droplets enables on-chip fabrication of defined colloidal suprastructures with morphologies ranging from colloidosomes to colloidal supraballs. The obtained supracolloidal structures exhibit a pH-responsive behavior with a disintegration at alkaline conditions within a scale of seconds. The destabilization process results from the deprotonation of phenolic groups and destruction of hydrogen bonds with PVCL chains at higher pH.

Biomedical Application, Patent Repository, Clinical Trial and Regulatory Updates on Hydrogel: An Extensive Review

Hydrogels are known for their leading role in biomaterial systems involving pharmaceuticals that fascinate material scientists to work on the wide variety of biomedical applications. The physical and mechanical properties of hydrogels, along with their biodegradability and biocompatibility characteristics, have made them an attractive and flexible tool with various applications such as imaging, diagnosis and treatment. The water-cherishing nature of hydrogels and their capacity to swell-contingent upon a few ecological signals or the simple presence of water-is alluring for drug conveyance applications. Currently, there are several problems relating to drug delivery, to which hydrogel may provide a possible solution. Hence, it is pertinent to collate updates on hydrogels pertaining to biomedical applications. The primary objective of this review article is to garner information regarding classification, properties, methods of preparations, and of the polymers used with particular emphasis on injectable hydrogels. This review also covers the regulatory and other commerce specific information. Further, it enlists several patents and clinical trials of hydrogels with related indications and offers a consolidated resource for all facets associated with the biomedical hydrogels.

Mechanical confinement via a PEG/Collagen interpenetrating network inhibits behavior characteristic of malignant cells in the triple negative breast cancer cell line MDA.MB.231

To decouple the effects of collagen fiber density and network mechanics on cancer cell behavior, we describe a highly tunable in vitro 3D interpenetrating network (IPN) consisting of a primary fibrillar collagen network reinforced by a secondary visible light-mediated thiol-ene poly(ethylene glycol) (PEG) network. This PEG/Collagen IPN platform is cytocompatible, inherently bioactive via native cellular adhesion sites, and mechanically tunable over several orders of magnitude-mimicking both healthy and cancerous breast tissue. Furthermore, we use the PEG/Collagen IPN platform to investigate the effect of mechanical confinement on cancer cell behavior as it is hypothesized that cells within tumors that have yet to invade into the surrounding tissue experience mechanical confinement. We find that mechanical confinement via the IPN impairs behavior characteristic of malignant cells (i.e., viability, proliferation, and cellular motility) in the triple negative breast cancer cell line MDA.MB.231, and is more effective than removal of soluble growth signals. The PEG/Collagen IPN platform is a useful tool for studying mechanotransductive signaling pathways and motivates further investigation into the role of mechanical confinement in cancer progression.

STATEMENT OF SIGNIFICANCE

In this study, we have developed, optimized, and applied a novel 3D in vitro cell culture platform composed of an interpenetrating network (IPN) that is both mechanically tunable and inherently bioactive. The IPN consists of a primary fibrillar collagen type-1 network reinforced by a secondary thiol-ene poly(ethylene glycol) (PEG) network. The IPNs are formed via a novel strategy in which cell-laden collagen gels are formed first, and soluble PEG monomers are added later and crosslinked via visible light. This approach ensures that the collagen gels contain a fibrillar architecture similar to the collagen architecture present in vivo. We applied our IPN platform to study the effect of mechanical confinement on cancer cell behavior and found that it inhibits malignant-like behavior.

Hydrogel Actuators and Sensors for Biomedical Soft Robots: Brief Overview with Impending Challenges

There are numerous developments taking place in the field of biorobotics, and one such recent breakthrough is the implementation of soft robots-a pathway to mimic nature's organic parts for research purposes and in minimally invasive surgeries as a result of their shape-morphing and adaptable features. Hydrogels (biocompatible, biodegradable materials that are used in designing soft robots and sensor integration), have come into demand because of their beneficial properties, such as high water content, flexibility, and multi-faceted advantages particularly in targeted drug delivery, surgery and biorobotics. We illustrate in this review article the different types of biomedical sensors and actuators for which a hydrogel acts as an active primary material, and we elucidate their limitations and the future scope of this material in the nexus of similar biomedical avenues.

Swelling-induced structural changes and microparticle uptake of gelatin gels probed by NMR and CLSM

Gelatin gels are increasingly involved in many industrial applications due to several advantages including cost efficiency and biocompatibility. Generally, their production requires the use of aqueous solvents, which cause significant swelling, due to the ability of solvent molecules to penetrate through the gel microstructure and increase its volume. Since swelling mechanisms and their effect on the gel structure are not fully understood, further investigations are required. In this work, we combine macroscopic measurements of the swelling ratio (SR) with Nuclear Magnetic Resonance (NMR) and Confocal Laser Scanning Microscopy (CLSM) to investigate changes in the gelatin structure as a function of both polymer concentration and swelling time. SR values increase as a function of time until a maximum is reached and then show a slight drop for all the gelatin concentrations after 24 h swelling time, probably due to a network relaxation process. NMR allows determination of mass transport and molecular dynamics of water inside the gelatin pores, while CLSM is used to visualize the penetration of tracers (polystyrene microbeads) with a diameter much larger than the gel pores. Structural parameters, such as average pore size and tortuosity, are estimated. In particular, the pore size decreases for higher polymer concentration and increases during swelling, until reaching a maximum, and then dropping at longer times. The penetration of tracers provides evidence of the heterogeneity of the gel structure and shows that single microcarriers can be loaded in gelatin gels upon swelling.

Novel magneto-responsive membrane for remote control switchable molecular sieving

Magneto-responsive separation membrane: reversible change of molecule sieving through pore-confined polymeric hydrogel network by remote control of immobilized “nano heaters” with alternating magnetic field.

Immunoisolating semi-permeable membranes for cell encapsulation: focus on hydrogels

Cell-based medicine has recently emerged as a promising cure for patients suffering from various diseases and disorders that cannot be cured/treated using technologies currently available. Encapsulation within semi-permeable membranes offers transplanted cell protection from the surrounding host environment to achieve successful therapeutic function following in vivo implantation. Apart from the immunoisolation requirements, the encapsulating material must allow for cell survival and differentiation while maintaining its physico-mechanical properties throughout the required implantation period. Here we review the progress made in the development of cell encapsulation technologies from the mass transport side, highlighting the essential requirements of materials comprising immunoisolating membranes. The review will focus on hydrogels, the most common polymers used in cell encapsulation, and discuss the advantages of these materials and the challenges faced in the modification of their immunoisolating and permeability characteristics in order to optimize their function.

Synthetic sandwich culture of 3D hepatocyte monolayer

The sandwich culture of hepatocytes, between double layers of extra-cellular matrix (ECM), is a well-established in vitro model for re-establishing hepatic polarity and maintaining differentiated functions. Applications of the ECM-based sandwich culture are limited by the mass transfer barriers induced by the top gelled ECM layer, complex molecular composition of ECM with batch-to-batch variation and uncontrollable coating of the ECM double layers. We have addressed these limitations of the ECM-based sandwich culture by developing an 'ECM-free' synthetic sandwich culture, which is constructed by sandwiching a 3D hepatocyte monolayer between a glycine-arginine-glycine-aspatic acid-serine (GRGDS)-modified polyethylene terephthalate (PET) track-etched membrane (top support) and a galactosylated PET film (bottom substratum). The bioactive top support and bottom substratum in the synthetic sandwich culture substituted for the functionalities of the ECM in the ECM-based sandwich culture with further improvement in mass transfer and optimal material properties. The 3D hepatocyte monolayer in the synthetic sandwich culture exhibited a similar process of hepatic polarity formation, better cell-cell interaction and improved differentiated functions over 14-day culture compared to the hepatocytes in collagen sandwich culture. The novel 3D hepatocyte monolayer sandwich culture using bioactive synthetic materials may readily replace the ECM-based sandwich culture for liver tissue engineering applications, such as drug metabolism/toxicity testing and hepatocyte-based bioreactors.

Improved hepatocyte excretory function by immediate presentation of polarity cues

Liver tissue constructs with excretory function are crucial to developing realistic hepatocyte models for engineering effective bioartificial liver-assisted devices and for modeling the in vivo tissue. Current hepatocyte in vitro models suffer from limited or inefficient hepatocyte repolarization, which results in poor removal of xenobiotics and other waste products from the cells. We hypothesized that the temporal and spatial presentation of the cell matrix and cell-cell contacts as polarity cues would be important to define the axis of polarization to improve the excretory function of hepatocytes. The spatial presentation of polarity cues can be best achieved with sandwich configuration. We improve the temporal presentation of polarity cues by introducing the collagen overlay immediately in synchrony with cell-cell contacts instead of after 24 h in conventional sandwich culture. We demonstrate that the immediate presentation of the collagen matrix overlay enhances the formation of apicobasolateral domains, tight junctions, and the recovery of the functional activity of 2 canalicular transporters, the multidrug resistance-associated protein (Mrp2) and P-glycoprotein (P-gp) at 48 h of culture, and enhances the albumin secretion, urea production, and 7-ethoxyresorufin-O-deethylation cytochrome P450 activities of hepatocytes over 14 days of culture as compared to the 24-h overlay controls. The improvement in the excretory function of hepatocytes for the removal of waste products deleterious to cells may improve the functional maintenance and the in vivo fidelity of tissue-engineered liver constructs.

Analysis using a linear viscoelastic model of the in vitro osmotic kinetics of polydisperse synthetic colloids

This study clarifies the contribution to overall osmotic kinetics of colloid osmotic pressure (Pi) and the interaction of synthetic colloids with the membrane. Solutions (6%) of dextran with weight average molecular weight (MW(w)) 68 800 (DEX 70), dextran with MW(w) 40 000 (DEX 40), hydroxyethyl starch with MW(w) 70 000 (HES 70), gelatin with MW(w) 60 000 and albumin were tested. An osmotic flow cell fitted with membranes of molecular weight cutoff size 30 000 or 50 000 was used to measure time-dependent changes in Pi for each of these solutions. A linear viscoelastic model was fitted to the curve describing changes to Pi as a function of time. Values of total effective Pi for DEX 40 and DEX 70 were larger than those for HES 70, gelatin, and albumin. As an index of solute-solvent exchange rate at the membrane surface, these values were in the order DEX 40 > DEX 70, HES 70 > gelatin, albumin. The findings suggest that DEX 40 may be preferable for the temporary restoration of plasma volume because of a heightened initial osmotic force. In contrast, the osmotic force exerted by gelatin is slower to increase but is likely to be longer lasting in vivo as a result of the inhibition of gelatin from penetrating the capillary membrane due to its interaction with negatively charged groups in the endothelial glycocalyx.

Native fibrillar collagen membranes of micron-scale and submicron thicknesses for cell support and perfusion

Fibrillar type I collagen is nontoxic, biocompatible, and possesses considerable strength and stability. In a study of scaffolds for use in laminated tissue substitutes, we examined the properties of membranes made from air-dried hydrogels of collagen fibrils that were polymerized from native, monomeric collagen. Planar collagen membranes (CMs) of 0.1-5.3 microm dry thickness were made by variation of the collagen concentration and/or the volume of the hydrogel. The planar CMs, which were comprised of a dense feltwork of long collagen fibrils 70-100 nm in diameter, showed considerable resistance to rupture and retained their membranous character after 6 weeks in tissue culture medium at 37 degrees C. CMs that were relatively thick when dry exhibited a greater proportional increase in rehydrated thickness and a greater diffusivity (when rehydrated) to 4.3 kDa dextran than did CMs that were relatively thin when dry. Hollow, tubular CMs of several configurations were prepared by embedment of solid, removable forms into collagen hydrogels prior to drying. By use of special fixtures, a planar CM that incorporated multiple, parallel tubes was fabricated. In summary, hydrogels of fibrillar collagen can be transformed into membranous structures suitable for tissue engineering applications.

Protein Encapsulation via Porous CaCO3Microparticles Templating

Porous microparticles of calcium carbonate with an average diameter of 4.75 microm were prepared and used for protein encapsulation in polymer-filled microcapsules by means of electrostatic layer-by-layer assembly (ELbL). Loading of macromolecules in porous CaCO3 particles is affected by their molecular weight due to diffusion-limited permeation inside the particles and also by the affinity to the carbonate surface. Adsorption of various proteins and dextran was examined as a function of pH and was found to be dependent both on the charge of the microparticles and macromolecules. The electrostatic effect was shown to govern this interaction. This paper discusses the factors which can influence the adsorption capacity of proteins. A new way of protein encapsulation in polyelectrolyte microcapsules is proposed exploiting the porous, biocompatible, and decomposable microparticles from CaCO3. It consists of protein adsorption in the pores of the microparticles followed by ELbL of oppositely charged polyelectrolytes and further core dissolution. This resulted in formation of polyelectrolyte-filled capsules with protein incorporated in interpenetrating polyelectrolyte network. The properties of CaCO3 microparticles and capsules prepared were characterized by scanning electron microscopy, microelectrophoresis, and confocal laser scanning microscopy. Lactalbumin was encapsulated by means of the proposed technique yielding a content of 0.6 pg protein per microcapsule. Horseradish peroxidase saves 37% of activity after encapsulation. However, the thermostability of the enzyme was improved by encapsulation. The results demonstrate that porous CaCO3 microparticles can be applied as microtemplates for encapsulation of proteins into polyelectrolyte capsules at neutral pH as an optimal medium for a variety of bioactive material, which can also be encapsulated by the proposed method. Microcapsules filled with encapsulated material may find applications in the field of biotechnology, biochemistry, and medicine.

Effect of drug physicochemical properties on swelling/deswelling kinetics and pulsatile drug release from thermoresponsive poly(N-isopropylacrylamide) hydrogels

The effect of drug physicochemical properties on swelling/deswelling kinetics and pulsatile drug release from a thermoresponsive hydrogel was examined. Hydrogels were loaded with drug and thermally triggered swelling/deswelling and release experiments were performed. Two series of drugs of contrasting hydrophilicity and varying physicochemical properties were examined. Benzoic acid (BA), its methyl and propyl esters, and diltiazem base were used as model hydrophobic drugs. Sodium benzoate (NaB), diltiazem HCl (DHCl), vitamin B12 (VB12) and various dextrans (MW 4300, 10,200, 42,000, 68,800) were used as model hydrophilic agents of increasing size. The hydrogel swelling rate was slowed by the presence of the hydrophobic drugs and this decreased rate was solubility dependant for the benzoates. The hydrophilic series increased the rate of swelling compared to the unloaded system. In all cases, the magnitude and rate of hydrogel contraction were proportional to the extent of swelling prior to temperature switch. Drug release was by diffusion below the lower critical solution temperature (LCST), while a solubility-dependent drug pulse release on temperature switch was observed for the hydrophobic series. Effectiveness of thermal control of hydrophobic drug release increased with increasing solubility. The hydrophilic series produced a molecular size-dependent drug pulse on temperature switch above the LCST. Pulsatile on-off drug release was shown with DHCl, VB12 and the various dextrans. Drug solubility, size and chemical nature were shown to be of particular importance in the control of hydrogel swelling and drug release from thermosensitive hydrogels.

Hydrogels for biomedical applications

This article reviews the composition and synthesis of hydrogels, the character of their absorbed water, and permeation of solutes within their swollen matrices. The most important properties of hydrogels relevant to their biomedical applications are also identified, especially for use of hydrogels as drug and cell carriers, and as tissue engineering matrices.

New antibody purification procedure using a thermally responsive poly(N-isopropylacrylamide)-dextran derivative conjugate

Through their specificity and affinity, antibodies are useful tools in research and medicine. In this study, we investigated a new type of chromatographic method using a thermosensitive polymer for the purification of antibodies against a dextran derivative (DD), as a model. The thermally reversible soluble-insoluble poly(N-isopropylacrylamide)-dextran derivative conjugate, named poly(NIPAAm)-DD, has been synthesized by conjugating amino-terminated poly(N-isopropylacrylamide) to a DD via ethyl-3-(3-dimethylaminopropyl)-carbodiimide. On one hand, this report describes the two steps of poly(NIPAAm)-DD conjugation and characterization. On the other hand, the poly(NIPAAm)-DD conjugate was used as a tool to purify polyclonal antibodies in serum samples from rabbits subcutaneously immunized with the derivatized dextran. Antibodies were purified and quantified by immunoenzymatic assays. Our results indicate that antibodies recognized both DD and poly(NIPAAm)-DD. In contrast, they did not bind to native poly(NIPAAm) or poly(NIPAAm) conjugated with another anionic dextran. We conclude that the conjugation of a polysaccharide to poly(NIPAAm) leads to an original and efficient chromatographic method to purify antibodies. Moreover, this novel method of purification is rapid, sensitive, inexpensive and could be used to purify various types of antibodies.

Insulin release from islets of Langerhans entrapped in a poly(N-isopropylacrylamide-co-acrylic acid) polymer gel

A copolymer of N-isopropylacrylamide (98 mol% in feed) and acrylic acid, poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAAm-co-AAc)), was prepared by free radical polymerization for development of a thermally reversible polymer to entrap islets of Langerhans for a refillable biohybrid artificial pancreas. A 5 wt% solution of the polymer in Hanks' balanced salt solution forms a gel at 37 degrees C that exhibits no syneresis. Diffusion of fluorescein isothiocyanate (FITC) dextrans having molecular weights of 4400 and 70000 were used to evaluate mass transport in the gel at 37 degrees C. Insulin secretion from islets in the polymer gel was also investigated in both static and dynamic systems. The polymer gel exhibited excellent diffusion of FITC dextran 4400 and FITC dextran 70000 with diffusion ratios, D/D0 (ratio of diffusion in the gel to diffusion in water), of 0.20+/-0.04 and 0.35+/-0.17, respectively. Human islets entrapped in the polymer gel showed prolonged insulin secretion in response to basal (5.5 mM) glucose concentration compared to free human islets. Rat islets showed prolonged insulin secretion in response to high (16.5 mM) glucose concentrations compared to free rat islets. Rat islets in the polymer gel maintained insulin secretion in response to the higher glucose concentration for over 26 days. Rat islets entrapped by the polymer also released higher quantities of insulin more rapidly in response to changes in concentrations of glucose and other stimulants than rat islets entrapped in an alginate control. These results suggest that this material would provide adequate diffusion for rapid insulin release in an application as a synthetic extracellular matrix for a biohybrid artificial pancreas.

Factors determining hydrogel permeability

Developing hydrogel membranes and coatings of appropriate permeability characteristics is key to the success of a number bioartificial organ technologies. Key principles relevant to the design and application of hydrogels for such applications were reviewed. The first key point is that permeability is a function of both transport and thermodynamic properties, the diffusion coefficient and partition coefficient, respectively, and that these parameters can be evaluated separately. Although the aspect of partitioning often emphasized is size exclusion, this review points out that many other relevant interactions come into play, especially hydrophobic and electrostatic interactions, and that these phenomena can dominate size exclusion. Similarly, while the diffusion coefficient also is strongly dependent upon size, other interactions can also cause diffusivity to deviate from theories which consider only solute size and gel swelling. For example, the heterogeneity of hydrogel networks can result in permeabilities that fail to decline as much as might be anticipated if networks were uniform.