Phase Transitions in Swollen Networks. 3. Swelling Behavior of Radiation Cross-Linked Poly(vinyl methyl ether) in Water†

Chitosan-Based Biocompatible Copolymers for Thermoresponsive Drug Delivery Systems: On the Development of a Standardization System

Chitosan is a natural polysaccharide that is considered to be biocompatible, biodegradable and non-toxic. The polymer has been used in drug delivery applications for its positive charge, which allows for adhesion with and recognition of biological tissues via non-covalent interactions. In recent times, chitosan has been used for the preparation of graft copolymers with thermoresponsive polymers such as poly-N-vinylcaprolactam (PNVCL) and poly-N-isopropylamide (PNIPAM), allowing the combination of the biodegradability of the natural polymer with the ability to respond to changes in temperature. Due to the growing interest in the utilization of thermoresponsive polymers in the biological context, it is necessary to increase the knowledge of the key principles of thermoresponsivity in order to obtain comparable results between different studies or applications. In the present review, we provide an overview of the basic principles of thermoresponsivity, as well as a description of the main polysaccharides and thermoresponsive materials, with a special focus on chitosan and poly-N-Vinyl caprolactam (PNVCL) and their biomedical applications.

Thermoresponsive Chitosan-Grafted-Poly(N-vinylcaprolactam) Microgels via Ionotropic Gelation for Oncological Applications

Microgels can be considered soft, porous and deformable particles with an internal gel structure swollen by a solvent and an average size between 100 and 1000 nm. Due to their biocompatibility, colloidal stability, their unique dynamicity and the permeability of their architecture, they are emerging as important candidates for drug delivery systems, sensing and biocatalysis. In clinical applications, the research on responsive microgels is aimed at the development of "smart" delivery systems that undergo a critical change in conformation and size in reaction to a change in environmental conditions (temperature, magnetic fields, pH, concentration gradient). Recent achievements in biodegradable polymer fabrication have resulted in new appealing strategies, including the combination of synthetic and natural-origin polymers with inorganic nanoparticles, as well as the possibility of controlling drug release remotely. In this review, we provide a literature review on the use of dual and multi-responsive chitosan-grafted-poly-(N-vinylcaprolactam) (CP) microgels in drug delivery and oncological applications.

Phase Equilibrium and Interdiffusion in Poly(Vinyl Methyl Ether)-Water System

The phase state diagram of the poly(vinyl methyl ether)-water system in a wide concentration range was obtained by the optical interferometry method. It was shown that this system was characterized by a complicated phase equilibrium with two lower critical solution temperatures, one of which was located in the concentrated region at 21 °C, and the other one in the region of a dilute solution at 31 °C. In the framework of the Flory-Huggins theory, pair interaction parameters were calculated for different parts of the binodal curves, and an attempt was made to reverse simulate the diagram in different conditions. It was suggested that the unusual character of the diagram was associated with the formation of a complicated complex between PVME and water in the middle region of the compositions. Concentration profiles for different temperatures were constructed. For the first time for this system, the numerical values of the diffusion coefficients of poly(vinyl methyl ether) (PVME) into water and water in PVME were obtained. Concentration and temperature dependences of diffusion coefficients were constructed and analyzed. The kinetics of water sorption in PVME was plotted, the clustering integral was calculated, and the approximate number of molecules in a water cluster was estimated. It was shown that in the dilute solution region upon passing through the binodal curve, the interphase disappeared immediately, and the remaining fluctuation of the concentration decreased in size with time. The kinetics of this process was estimated from the change in the size of such a particle.

Volume Phase Transition in Gels: Its Discovery and Development

 The history of volume phase transition of responsive gels from its theoretical prediction to experimental discovery was described and the major role of mixing Gibbs energy function in theoretical models was stressed. For detailed analysis and fine tuning of the volume phase transition, the generalized Flory-Huggins model with concentration and temperature dependent interaction function coupled with Maxwell construction as a tool is very suitable. Application of expansive stresses can uncover the potential of various swelling gels for volume phase transition. Experimentally, the abrupt, equilibrium-controlled phase transition is often hard to achieve due to passage of gel through states of mechanical instability and slow relaxation processes in macroscopic objects.

Synthesis and Applications of Hydrogels in Cancer Therapy

Hydrogels are water-insoluble, hydrophilic, cross-linked, three-dimensional networks of polymer chains having the ability to swell and absorb water but do not dissolve in it, that comprise the major difference between gels and hydrogels. The mechanical strength, physical integrity and solubility are offered by the crosslinks. The different applications of hydrogels can be derived based on the methods of their synthesis, response to different stimuli, and their different kinds. Hydrogels are highly biocompatible and have properties similar to human tissues that make it suitable to be used in various biomedical applications, including drug delivery and tissue engineering. The role of hydrogels in cancer therapy is highly emerging in recent years. In the present review, we highlighted different methods of synthesis of hydrogels and their classification based on different parameters. Distinctive applications of hydrogels in the treatment of cancer are also discussed.

How to Force Polymer Gels to Show Volume Phase Transitions

Relatively few polymer gels are known to show volume phase transition where the gels undergo an abrupt change in the degree of swelling by passing through a three-phase equilibrium. Characteristic for such transition is the existence of van der Waals (vdW) loop on the dependence of solvent chemical potential versus polymer concentration. For the χ-induced transition, the existence of vdW loop is determined by the concentration dependence of the interaction function. It is shown that expansive mechanical strains can assist in development of the vdW loop. Systems characterized by continuous change of the degree of swelling transform upon such strain into ones where the degree of swelling changes much and abruptly. Also, expansive modes of strain can make the transition wider and more robust in gels where transition is already observed under free swelling condition. The possibility to induce the volume phase transition by external stresses can be utilized for finding other stimuli sensitive gels, strengthening of gel response, and in modeling of properties of gel constructs by Finite Element Method.

Unusual Phase Transition Behavior of Poly(N-isopropylacrylamide)-co-Poly(tetrabutylphosphonium styrenesulfonate) in Water: Mild and Linear Changes in the Poly(N-isopropylacrylamide) Part

In this paper, one LCST-type thermoresponsive poly(ionic liquid) (PIL), poly(tetrabutylphosphonium styrenesulfonate) (P[P4,4,4,4][SS]), was introduced to poly(N-isopropylacrylamide) (PNIPAM) by two different ways, mixing and copolymerization. Interestingly, they show distinct thermoresponsive phase transition behaviors, evidenced by temperature-variable (1)H nuclear magnetic resonance and Fourier transform infrared in combination with the perturbation correlation moving window (PCMW) technique. The PNIPAM/P[P4,4,4,4][SS] mixture exhibits a sharp and drastic phase transition, similar to that of pure PNIPAM. In the statistical copolymer, PNIPAM-co-P[P4,4,4,4][SS], the thermosensitivity of P[P4,4,4,4][SS] is largely suppressed, resulting in a linear, mild, and incomplete phase transition, which has never been reported before. This abnormal phenomenon is shown to arise from the outstanding hydration ability of P[P4,4,4,4][SS]. Our findings should be conducive to improving our understanding of the interaction between LCST-type polymers with distinct structures and provide a new perspective for preparing thermoresponsive materials with linear phase transition behavior.

Smart micro/nanoparticles in stimulus-responsive drug/gene delivery systems

New achievements in the realm of nanoscience and innovative techniques of nanomedicine have moved micro/nanoparticles (MNPs) to the point of becoming actually useful for practical applications in the near future. Various differences between the extracellular and intracellular environments of cancerous and normal cells and the particular characteristics of tumors such as physicochemical properties, neovasculature, elasticity, surface electrical charge, and pH have motivated the design and fabrication of inventive "smart" MNPs for stimulus-responsive controlled drug release. These novel MNPs can be tailored to be responsive to pH variations, redox potential, enzymatic activation, thermal gradients, magnetic fields, light, and ultrasound (US), or can even be responsive to dual or multi-combinations of different stimuli. This unparalleled capability has increased their importance as site-specific controlled drug delivery systems (DDSs) and has encouraged their rapid development in recent years. An in-depth understanding of the underlying mechanisms of these DDS approaches is expected to further contribute to this groundbreaking field of nanomedicine. Smart nanocarriers in the form of MNPs that can be triggered by internal or external stimulus are summarized and discussed in the present review, including pH-sensitive peptides and polymers, redox-responsive micelles and nanogels, thermo- or magnetic-responsive nanoparticles (NPs), mechanical- or electrical-responsive MNPs, light or ultrasound-sensitive particles, and multi-responsive MNPs including dual stimuli-sensitive nanosheets of graphene. This review highlights the recent advances of smart MNPs categorized according to their activation stimulus (physical, chemical, or biological) and looks forward to future pharmaceutical applications.

Volume phase transition of electron beam cross-linked thermo-responsive PVME nanogels in the presence and absence of nanoparticles: with a view toward rheology and interactions

We investigate the effect of nanoparticles and radiation dose on interactions in the PVME-based nanogel system and its phase behavior (swelling/deswelling behavior and phase separation mechanism) by rheological and FTIR measurements.

Modified biopolymer-dextrin based crosslinked hydrogels: application in controlled drug delivery

This review describes hydrogels and their classifications along with the synthesis and properties of biopolymer-dextrin based crosslinked hydrogels towards potential application in controlled drug delivery.

Phase diagrams of smart copolymers poly(N-isopropylacrylamide) and poly(sodium acrylate)

The phase behavior of linear poly(N-isopropylacrylamide) (PNIPA), linear copolymer poly(N-isopropylacrylamide) and poly(sodium acrylate) (PNIPA-SA), and chemically cross-linked PNIPA in water has been determined by temperature modulated differential scanning calorimetry (TM-DSC). Experiments related to linear polymers (PNIPA and PNIPA-SA) indicated nontypical demixing/mixing behavior with a lower critical solution temperature (LCST), which do not correspond to the three classical types of limiting critical behavior. Some similarities and differences are observed in comparison to our literature data using standard TM-DSC for PNIPA/water. Furthermore no influence of composition cross-linked PNIPA/water system on demixing/mixing temperature was observed.

LCST transition of PNIPAM-b-PVCL in water: cooperative aggregation of two distinct thermally responsive segments

The self-aggregation behavior of poly(N-isopropylacrylamide)-b-poly(N-vinylcaprolactam) (PNIPAM-b-PVCL) during the thermal-induced phase transition in water was explored by a combination of calorimetric, turbidity, dynamic light scattering (DLS) and FTIR measurements. Only one transition can be observed via all detecting methods, revealing the cooperative aggregation of the two distinct temperature-sensitive segments. What is more, the combination of strong hydrophobic interactions among the entire polymer chains and hydrogen bonds of C=O…H-N within PNIPAM segments results in the sharp variations at the LCST and gradual variations above the LCST during the phase transition of PNIPAM-b-PVCL aqueous solution upon heating. Additional analysis by perturbation correlation moving window (PCMW) and two-dimensional correlation spectroscopy (2Dcos) indicates that the hydrophobic C-H groups have an earlier response than the relatively hydrophilic C=O groups during the LCST transition.

Infrared spectroscopic insight into hydration behavior of poly(N-vinylcaprolactam) in water

IR spectroscopy in combination with two-dimensional correlation spectroscopy (2DCOS) and the perturbation correlation moving window (PCMW) technique is employed to illustrate the dynamic hydration behavior of poly(N-vinylcaprolactam) (PVCL) in water, which exhibits a typical type I continuous lower critical solution temperature (LCST) behavior. PCMW easily determined the transition temperature to be ca. 43.5 °C during heating and ca. 42.5 °C during cooling and the transition temperature range to be 39.5-45 °C. On the other hand, 2DCOS was used to discern the sequence order of different species in PVCL and concluded that hydrogen bonding transformation predominates at the first stage below LCST while hydrophobic interaction predominates at the second stage above LCST. In combination with molecular dynamics simulation results, we find that there exists a distribution gradient of water molecules in PVCL mesoglobules ranging from a hydrophobic core to a hydrophilic surface. Due to the absence of self-associated hydrogen bonds and topological constraints, PVCL mesoglobules would form a "sponge-like" structure which can further continuously expel water molecules upon increasing temperature, while poly(N-isopropylacrylamide) (PNIPAM) with self-associated hydrogen bonds forms mesoglobules with a "cotton-ball-like" structure without an apparent distribution gradient of water molecules and does not change much upon increasing temperature.

Phase separation of poly(vinyl methyl ether) aqueous solution: a near-infrared spectroscopic study

The thermosensitive phase separation of poly(vinyl methyl ether) (PVME) aqueous solutions has been investigated using near-infrared spectroscopy in combination with two-dimensional correlation analysis, and a two-step phase separation mechanism during gradual heating has been established. Two-dimensional near-infrared (2D NIR) analysis results indicate that during this two-step process the dehydration of CH 2 groups occurs earlier than that of CH 3 groups. This result suggests that it is the change of the hydrophobic hydrocarbon chain conformation induced by heating that indirectly leads to the dehydration of the hydrophilic ether oxygen side groups.

Phase Transformations in Aqueous Low Molar Mass Poly(vinyl methyl ether) Solutions: Theoretical Prediction and Experimental Validation of the Peculiar Solvent Melting Line, Bimodal LCST, and (Adjacent) UCST Miscibility Gaps

Supported by theoretical predictions based on the Wertheim Lattice Thermodynamic Perturbation Theory, modulated temperature differential scanning calorimetry (MTDSC) was used to further the knowledge of the phase behavior of aqueous poly(vinyl methyl ether) (PVME) solutions. Using a narrowly dispersed low molar mass PVME, we determined the following phase boundaries: (i) a bimodal lower critical solution temperature (LCST) miscibility gap at physiological temperature (around 37 degrees C), (ii) an upper critical solution temperature (UCST) two-phase area at sub-zero temperatures and high polymer concentration, and (iii) the melting line of the solvent across the entire concentration range, showing a peculiar stepwise decrease with composition. The location of the glass transition region and its influence on the crystallization/melting behavior of the solvent is discussed.

FT-IR and 2D-IR Spectroscopic Studies on the Effect of Ions on the Phase Separation Behavior of PVME Aqueous Solution

Thermosensitive phase transition behavior of poly(vinyl methyl ether) (PVME) in an aqueous solution and the effect of inorganic ions on the coil-globule transition have been investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) accessory. ATR-IR spectra of PVME aqueous solution indicate that in water-PVME-inorganic salts system, the phase separation temperature of PVME aqueous solution decreased with the increase of ion concentration and the increase of anion electronegativity. Meanwhile, two-dimensional infrared (2D-IR) measurements have been made to clarify the microcosmic conformational changes of PVME during the coil-globule transition. Results show that the conformation changes of main chains occur earlier than those of ether groups during heating. Furthermore, the 2D correlation spectroscopy of PVME aqueous solution during heating and the increase of concentration of potassium chloride have been studied. The features of 2D-IR spectra during heating did not change compared to the features of PVME aqueous solution during the increase of concentration of potassium chloride. This result implies that, although the addition of inorganic ions shifts the phase separation temperature, it does not alter the internal mechanism of the coil-globule transition of PVME.