Salt-Induced Thermoresponsivity of Cross-Linked Polymethoxyethylaminophosphazene Hydrogels: Energetics of the Volume Phase Transition

Thermosensitive P(AAc-co-NIPAm) hydrogels display enhanced toughness and self-healing via ion-ligand interactions

Hydrogels containing thermosensitive polymers such as poly(N-isopropylacrylamide) (P(NIPAm)) may contract during heating and show great promise in fields ranging from soft robotics to thermosensitive biosensors. However, these gels often exhibit low stiffness, tensile strength, and mechanical toughness, limiting their applicability. Through copolymerization of P(NIPAm) with poly(Acrylic acid) (P(AAc)) and introduction of ferric ions (Fe³⁺ ) that coordinate with functional groups along the P(AAc) chains, we here introduce a thermoresponsive hydrogel with significantly enhanced mechanical extensibility, strength, and toughness. Using both experimentation and constitutive modeling, we find that increasing the ratio of m(AAc):m(NIPAm) in the prepolymer decreases strength and toughness but improves extensibility. In contrast, increasing Fe³⁺ concentration generally improves strength and toughness with little decrease in extensibility. Due to reversible coordination of the Fe³⁺ bonds, these gels display excellent recovery of mechanical strength during cyclic loading and self-healing ability. While thermosensitive contraction imbued by the underlying P(NIPAm) is reduced slightly with increased Fe³⁺ concentration, the temperature transition range is widened and shifted upwards towards that of human body temperature (between 30 and 40°C), perhaps rendering these gels suitable as in vivo biosensors. Finally, these gels display excellent adsorptive properties with a variety of materials, rendering them possible candidates in adhesive applications. This article is protected by copyright. All rights reserved.

Dual stimuli-responsive polyphosphazene-based molecular gates for controlled drug delivery in lung cancer cells

A switchable silane derived stimuli-responsive bottle-brush polyphosphazene (PPz) was prepared and attached to the surface of mesoporous silica nanoparticles (MSNs). The hybrid polymer with PEG-like Jeffamine® M-2005 side-arms undergo conformational changes in response to both pH and temperature due to its amphiphilic substituents and protonatable main-chain, hence were investigated as a gatekeeper. Safranin O as control fluorophore or the anticancer drug camptothecin (CPT) were encapsulated in the PPz-coated MSNs. At temperatures below the lower critical solution temperature (LCST), the swollen conformation of PPz efficiently blocked the cargo within the pores. However, above the LCST, the PPz collapsed, allowing release of the payload. Additionally, protonation of the polymer backbone at lower pH values was observed to enhance opening of the pores from the surface of the MSNs and therefore the release of the dye. In vitro studies demonstrated the ability of these nanoparticles loaded with the drug camptothecin to be endocytosed in both models of tumor (A549) and healthy epithelial (BEAS-2B) lung cells. Their accumulation and the release of the chemotherapeutic drug, co-localized within lysosomes, was faster and higher for tumor than for healthy cells, further, the biocompatibility of PPz-gated nanosystem without drug was demonstrated. Tailored dual responsive polyphosphazenes thus represent novel and promising candidates in the construction of future gated mesoporous silica nanocarriers designs for lung cancer-directed treatment.

Binding Energetics of Charged Amphiphilic Ligands to Thermoresponsive Biodegradable Poly(methoxyethylaminophosphazene) Hydrogels

Changes in the affinity of the swollen and collapsed forms of a thermoresponsive polymer gel for targeted ligands can be directly estimated using a thermodynamic approach based on high-sensitivity differential scanning calorimetry (HS-DSC). For macromolecular ligands (proteins) bound to the gel, this method provides information on changes in their conformational stability, which is of crucial importance for the biological or pharmaceutical activity of the protein. We used HS-DSC for the study of interactions of two widely administrated drugs-gemfibrozil and ibuprofen-and two globular proteins-α-lactalbumin and BSA-with hydrogels of the cross-linked poly(methoxyethylaminophosphazene). The gel collapse resulted in a substantial increase in the gel affinity for the drugs. We obtained quantitative estimations of the affinity of the collapsed gels depending on the gel structure, pH, concentration of NaCl, and phosphate buffer (an inductor of the thermoresponsivity). The gels retained a high affinity for the drugs in the near-physiological conditions (ionic composition and pH). The binding curves of globular proteins to the gels in the swollen and collapsed states were obtained. The different proteins demonstrated the preferential binding to the swollen or collapsed state of the gels, presumably depending on the protein surface hydrophobicity. The proteins bound to the gel subchains retain their native tertiary structure and, therefore, maintain their functionality when immobilized in the polyphosphazene hydrogels.

Conformation-Dependent Affinity of Thermoresponsive Biodegradable Hydrogels for Multifunctional Ligands: A Differential Scanning Calorimetry Approach

We investigated energetics of binding of multifunctional pyranine ligands to hydrogels of the cross-linked poly(methoxyethylaminophosphazene) (PMOEAP) from data on the thermotropic volume phase transition of the gels by means of high-sensitivity differential scanning calorimetry. Dependences of the transition temperature, enthalpy, and width on the concentration of pyranines were obtained, and the excess transition free energy as a function of the pyranine concentration was calculated. We found that the affinity of the gels for the pyranine ligands increased very significantly upon the gel collapse. The intrinsic binding constants and free energies of binding of the ligands to the gels in the collapsed state were estimated from the DSC data. They revealed a significant increase in the hydrogel affinity for pyranines proportional to the number of anionic groups in the ligand structure. The affinity of the PMOEAP hydrogels for the multifunctional ligands was not affected by an increase in the cross-linking density of the gels and only slightly reduced by physiological salt concentrations.