Structures and interactions in collapsed hydrogels of thermoresponsive interpenetrating polymer networks

External Stimuli-Responsive Characteristics of Poly(N,N′-diethylacrylamide) Hydrogels: Effect of Double Network Structure

Swelling experiments and NMR spectroscopy were combined to study effect of various stimuli on the behavior of hydrogels with a single- and double-network (DN) structure composed of poly(N,N′-diethylacrylamide) and polyacrylamide (PAAm). The sensitivity to stimuli in the DN hydrogel was found to be significantly affected by the introduction of the second component and the formation of the double network. The interpenetrating structure in the DN hydrogel causes the units of the component, which is insensitive to the given stimulus in the form of the single network (SN) hydrogel, to be partially formed as globular structures in DN hydrogel. Due to the hydrophilic PAAm groups, temperature- and salt-induced changes in the deswelling of the DN hydrogel are less intensive and gradual compared to those of the SN hydrogel. The swelling ratio of the DN hydrogel shows a significant decrease in the dependence on the acetone content in acetone–water mixtures. A certain portion of the solvent molecules bound in the globular structures was established from the measurements of the ¹H NMR spin–spin relaxation times T₂ for the studied DN hydrogel. The time-dependent deswelling and reswelling kinetics showed a two-step profile, corresponding to the solvent molecules being released and absorbed during two processes with different characteristic times.

Sodium alginate/N-(Vinylcaprolactam) based supramolecular self-assembled subcutaneously administered in situ formed gels depot of 5-fluorouracil: Rheological analysis, in vitro cytotoxic potential, in vivo bioavailability and safety evaluation

In current study, novel in situ formed injectable self-assembled thermoreversible depot gels based on N-(Vinylcaprolactam) were synthesized with a carbohydrate polymer i.e. sodium alginate in aqueous solution using cold method. The prepared gels preparations were intended to be utilized as 5-FU delivery depot after injectable administration through subcutaneous route. The structural characterization of self-assembled gels samples were studied through FTIR. The thermal properties of newly formed gels complexes were investigated by DSC and TGA. While the morphology of gels were assessed through SEM. The tunable gelation temperatures and phase transition of optimized formulations were confirmed by tube inverting, rheology determination and optical transmittance test. Thermo and pH response was evaluated at different temperatures and in various acidic and basic buffer solutions. In vitro release experiments were conducted using Franz diffusion system to monitor the controlled delivery fashion of gels matrices. Results concluded that depot gels exhibit controlled delivery fashion with maximum release at pH 7.4 and 37 ± 2 °C. The biocompatible nature of blank gels samples was assessed by MTT assay against Vero cell lines and was found safe. While killing ability of 5-FU encapsulated gels was evaluated against HeLa (19 ± 0.22 μg/ml; 23 ± 0.55 μg/ml) and MCF-7 (21 ± 0.06 μg/ml and 22 ± 0.34 μg/ml) cancer cell lines and were found effective to kill cancer cells. Histopathological study showed that gels depot is safe with no harmful effects on major organs. The in vivo bioavailability in rabbits showed controlled release (Cmax, 4465.78 ± 1.99 ng/ml) in comparison to free drug (Cmax, 4883.73 ± 3.32 ng/ml) administration after subcutaneous injection.

Poly (N-Vinylcaprolactam-Grafted-Sodium Alginate) Based Injectable pH/Thermo Responsive In Situ Forming Depot Hydrogels for Prolonged Controlled Anticancer Drug Delivery; In Vitro, In Vivo Characterization and Toxicity Evaluation

This study was aimed to develop novel in situ forming gels based on N-vinylcaprolactam, sodium alginate, and N,N-methylenebisacrylamide. The in situ Poly (NVRCL-g-NaAlg) gels were developed using the cold and free radical polymerization method. The structure formation, thermal stability, and porous nature of gels was confirmed by FTIR, NMR, DSC, TGA, and SEM. The tunable gelation temperature was evaluated by tube titling and rheological analysis. Optical transmittance showed that all formulations demonstrated phase transition around 33 °C. The swelling and release profile showed that gels offered maximum swelling and controlled 5-FU release at 25 °C and pH (7.4), owing to a relaxed state. Porosity and mesh size showed an effect on swelling and drug release. The in vitro degradation profile demonstrated a controlled degradation rate. An MTT assay confirmed that formulations are safe tested against Vero cells. In vitro cytotoxicity showed that 5-FU loaded gels have controlled cytotoxic potential against HeLa and MCF-7 cells (IC50 = 39.91 µg/mL and 46.82 µg/mL) compared to free 5-FU (IC50 = 50.52 µg/mL and 53.58 µg/mL). Histopathological study demonstrated no harmful effects of gels on major organs. The in vivo bioavailability in rabbits showed a controlled release in gel form (Cmax, 1433.59 ± 45.09 ng/mL) compared to a free drug (Cmax, 2263.31 ± 13.36 ng/mL) after the subcutaneous injection.

Poly(N,N'-Diethylacrylamide)-Based Thermoresponsive Hydrogels with Double Network Structure

Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N,N'-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) or poly(N,N`-dimethylacrylamide) (PDMAAm) was studied by a combination of swelling measurements, differential scanning calorimetry (DSC) and 1H NMR and UV-Vis spectroscopies. Presence of the second hydrophilic network in DN hydrogels influenced their thermal sensitivity significantly. DN hydrogels show less intensive changes in deswelling, smaller enthalpy, and entropy changes connected with phase transition and broader temperature interval of the transition than the single network (SN) hydrogels. Above the transition, the DN hydrogels contain significantly more permanently bound water in comparison with SN hydrogels due to interaction of water with the hydrophilic component. Unlike swelling and DSC experiments, a rather abrupt transition was revealed from temperature-dependent NMR spectra. Release study showed that model methylene blue molecules are released from SN and DN hydrogels within different time scale. New thermodynamical model of deswelling behaviour based on the approach of the van't Hoff analysis was developed. The model allows to determine thermodynamic parameters connected with temperature-induced volume transition, such as the standard change of enthalpy and entropy and critical temperatures and characterize the structurally different states of water.

The effect of saccharides on equilibrium swelling of thermo-responsive gels

Mechanical and optical properties of thermo-responsive (TR) gels change drastically at their volume phase transition temperature. As the critical temperature is strongly affected by the presence of small amounts of additives in aqueous solutions, TR gels can be employed as sensors for detection and recognition of multiple analytes (from specific ions to hazardous biochemicals to pathogenic proteins) and actuators for biomedical applications. A simplified mean-field model is developed for equilibrium swelling of TR gels in aqueous solutions of additives. Its advantage is that the model involves a relatively small (compared with the conventional approaches) number of material constants and accounts for changes in the thermo-mechanical response at transition from the swollen to collapsed state. The ability of the model to describe experimental swelling diagrams and to predict the influence of additives on the equilibrium degree of swelling and the volume phase transition temperature of TR gels is confirmed by comparison of observations on poly(N-isopropylacrylamide) gel in aqueous solutions of saccharides (glucose, sucrose and galactose) with results of numerical analysis.

Atomistic Molecular Dynamics Simulations of the Lower Critical Solution Temperature Transition of Poly( N-vinylcaprolactam) in Aqueous Solutions

Poly( N-vinylcaprolactam) (PVCL) is a thermo-responsive polymer, which exhibits a lower critical solution temperature (LCST) in an aqueous solution. The LCST of this hydrophilic-to-hydrophobic transition is found to be strongly dependent on the salt-type and salt-concentration as well as on the molecular weight and concentration of the polymer. Here, atomistic molecular dynamics simulations have been successfully conducted for the first time to investigate the LCST transition of a 100 degree of polymerization PVCL chain in water, 1 M NaCl, 3.5 M NaCl, and 0.5 M CaCl₂ solutions. Our results show that steric hindrance resulting from the bulky 7-member ring on the PVCL chain plays a critical role in the conformational transition. Moreover, the degrees of hydration and dehydration below or above the transition temperature are highly dependent on the specific solution condition and temperature. Water molecules are found to be trapped inside the collapsed polymer chains leading to the varying degrees of hydration and dehydration of the polymer chain in different solutions. Calculated water diffusion coefficients for both trapped and free water molecules agree very well with experimental measurements.

Interpenetrating Polymer Network Microgels in Water: Effect of Composition on the Structural Properties and Electrosteric Interactions

Microgels of cross-linked interpenetrating polymer networks (IPNs) are very versatile systems combining the properties of colloids and polymers. Herein we study IPN microgels composed of poly(N-isopropylacrylamide) and poly(acrylic acid) to understand how weight composition and reactant concentrations affect their structural, conformational and electrosteric properties in water. The results show that it is possible to drive the formation of microgels with the desired properties by adjusting IPN composition and preparation method during the synthesis. During synthesis, the polymerization of acrylic acid triggers the merging among IPNs via covalent linking, giving rise to microgels with larger mass and size, the effect being larger for higher concentration of the reactants. In addition, a close relation between the microgel internal conformation and the colloidal stability is observed, due to the presence of screened groups inside the microgel.

The role of unique spatial structure in the volume phase transition behavior of poly(N-isopropylacrylamide)-based interpenetrating polymer network microgels including a thermosensitive poly(ionic liquid)

By comparing with the linear homopolymer mixture, the influence of spatial structure on the phase behavior of thermosensitive interpenetrating polymer network (IPN) microgels was clarified.

Mechanical properties of PNIPAM based hydrogels: A review

Materials which adjust their properties in response to environmental factors such as temperature, pH and ionic strength are rapidly evolving and known as smart materials. Hydrogels formed by smart polymers have various applications. Among the smart polymers, thermoresponsive polymer poly(N-isopropylacrylamide)(PNIPAM) is very important because of its well defined structure and property specially its temperature response is closed to human body and can be finetuned as well. Mechanical properties are critical for the performance of stimuli responsive hydrogels in diverse applications. However, native PNIPAM hydrogels are very fragile and hardly useful for any practical purpose. Intense researches have been done in recent decade to enhance the mechanical features of PNIPAM hydrogel. In this review, several strategies including interpenetrating polymer network (IPN), double network (DN), nanocomposite (NC) and slide ring (SR) hydrogels are discussed in the context of PNIPAM hydrogel.

Stimuli Responsive Poly(Vinyl Caprolactam) Gels for Biomedical Applications

Poly(vinyl caprolactam) (PNVCL) is one of the most important thermoresponsive polymers because it is similar to poly(N-isopropyl acrylamide). PNVCL precipitates from aqueous solutions in a physiological temperature range (32⁻34 °C). The use of PNVCL instead of PNIPAM is considered advantageous because of the assumed lower toxicity of PNVCL. PNVCL copolymer gels are sensitive to external stimuli, such as temperature and pH; which gives them a wide range of biomedical applications and consequently attracts considerable scientific interest. This review focuses on the recent studies on PNVCL-based stimuli responsive three dimensional hydrogels (macro, micro, and nano) for biomedical applications. This review also covers the future outlooks of PNVCL-based gels for biomedical applications, particularly in the drug delivery field.