Critical Micellar Temperature Research Articles

Tuning Pluronic F127 phase transitions by adding physiological amounts of salts: A rheology, SAXS, and NMR investigation

Specific ion (Hofmeister) effects in colloid and biological systems represented a scientific challenge for more than 100 years. Recently, possible applications, based on their rationalization, are emerging. Here, Cl-, SO42-, SCN- anions and Na+, K+, Mg2+ cations are added at physiological concentration (∼0.15 mol/kg) to Pluronic F127 20 wt% aqueous solutions to suitably tune phase transitions for a smart drug delivery platform. Rheological measurements, along with SAXS and NMR self-diffusion experiments, are used to carefully characterize the prepared F127/salt-based formulations. The critical micellar temperature (cmt), the hard-gel formation temperature (Thg), liquid crystal structures, and self-diffusion coefficients are determined. The cmt and Thg values of F127/salt formulations are lower than that of F127 20 wt% sample, following an anionic Hofmeister series: SO42-< Cl-< SCN-. All added salts significantly increase storage modulus and complex viscosity with maximum values occurring at T around 40 °C. SAXS data confirm that added salts preserve cubic liquid crystal phases. NMR self-diffusion analysis highlights that the intermolecular interactions and mobility of F127 unimers/aggregates are ion specific at 16 °C but not at 40 °C. These findings suggest that F127/salt-based formulations may constitute a versatile thermosensitive platform for drug delivery able to assure sustained release in topical or surgery administrations, in the range of temperatures 30–45 °C.

Co‐assembly behavior of oppositely charged thermoresponsive elastin‐like polypeptide block copolymers

AbstractThe formation of complex coacervate core micelles (C3Ms) was observed by mixing two oppositely charged elastin‐like polypeptides (ELP)‐based diblock copolymers, each composed of a neutral and a charged moiety. The physical properties of C3Ms as a function of pH, temperature, and salt concentration were analyzed by UV–visible spectrometry, dynamic light scattering (DLS), 1H NMR, and small angle X‐ray scattering (SAXS). In distilled water (or low salt conditions), below the transition temperature (Tt), ELP mixtures induced the C3Ms with a complex coacervate core and a neutral corona; meanwhile, above the Tt, they aggregated by the hydrophobic interaction among the neutral coronas. However, as the salt concentration increased, the induced C3Ms gradually dissociated; while above the critical micellar temperature (CMT) and critical salt concentration (CSC), inverted hydrophobic core micelles surrounded by the charged blocks were created. Based on these results, we demonstrate that the co‐assembly behavior of two oppositely charged diblock ELPs is harnessed by the salt concentration and temperature.

A Novel Study on the Self-Assembly Behavior of Poly(lactic-co-glycolic acid) Polymer Probed by Curcumin Fluorescence.

Understanding the self-assembly behavior of block copolymers is of great importance due to their usefulness in a wide range of applications. In this work, the physical properties of poly(lactic-co-glycolic acid) (PLGA polymer) are studied for the first time in solution using the fluorescence technique and curcumin as a molecular probe. First, curcumin at a concentration of 2 μM was added to different concentrations of PLGA, and the fluorescence of curcumin was tracked. It was found that the critical micellar concentration (CMC) was equal to 0.31 g/L and the critical micellar temperature (CMT) was obtained to be 25 °C. Furthermore, an insight on the effect of NaCl salt on the CMC value of PLGA is assessed through curcumin probing. A decrease in the CMC has been observed with the increase in the concentration of NaCl, which could be due to the salting out effect. Moreover, in order to understand the aggregation behavior of PLGA in different solutions, CMC experiments were investigated using chloroform as a solvent. Results showed that the solvent does not affect the CMC value of the polymer; however, it only affects the shape of the obtained micelle forming a reversed micelle. Finally, fluorescence quenching of curcumin with hydrophobic cetyl-pyridinium bromide (CPB) and hydrophilic KI quenchers was established, where it was proved that curcumin is located near the hydrophobic pocket of the Stern layer of the PLGA micelle.

The effect of erythrosine-B on the structuration of poloxamer 407 and cellulose derivative blends: In silico modelling supporting experimental studies

Erythrosine is a dye approved for medical use that has shown promising photodynamic activity, allowing for the inactivation of microorganisms and activity against malignant cells. Despite the great photodynamic potential, erythrosine exhibits hydrophilicity, negatively impacting its action in biological membranes. Therefore, the incorporation of erythrosine in micellar polymeric systems, such as poloxamers, may overcome this limitation. Moreover, using bioadhesive and thermoresponsive polymers to combine in situ gelation and bioadhesion may enhance retention of this topically applied drug. In this work, mucoadhesive and thermoresponsive micellar systems were prepared containing erythrosine in two states: the native form (ERI) and the disodium salt (ERIs). The systems were evaluated based on the effect of ERI/ERIs on the micellar structure of the binary polymer mixtures. Optimised combinations of poloxamer 407 (polox407) and mucoadhesive sodium carboxymethylcellulose (NaCMC) or hydroxypropyl methylcellulose (HPMC) were used as micellar systems for ERI or ERIs delivery. The systems were studied with respect to theoretical interactions, qualitative composition, morphology, and micellar properties. In silico modelling indicated a higher interaction of the drug with poly(ethylene oxide) (PEO) than poly(propylene oxide) (PPO) fragments of polox407. Systems containing NaCMC displayed a repulsive effect in the presence of erythrosine, due to the polymer's charge density. Both systems could convert the photosensitizer in its monomeric form, ensuring photodynamic activity. In these mixtures, crystallinity, critical micellar temperature and enthalpy of polox407 micellisation were reduced, and micellar size, evaluated by transmission electron microscopy (TEM), showed low impact of ERI/ERIs in HPMC preparations. Aiming toward photodynamic applications, the findings showed how ERI or ERIs can affect the micellar formation of gels composed of 17.5% (w/w) polox407 and 3% (w/w) HPMC or 1% (w/w) NaCMC, important for understating their behaviour and future utilisation as erythrosine delivery systems.

Custom-design of intrinsically antimicrobial polyurethane hydrogels as multifunctional injectable delivery systems for mini-invasive wound treatment

• Infections represent the major obstacle hindering the healing of chronic wounds. • The development of polymers intrinsically antimicrobial reduces the need of antibiotics. • The chemical versatility of polyurethanes results in polymers with ad-hoc properties. • System thermosensitivity allows shape-adaptability and rapid post-injection gelation. • The micellar hydrogel network opens the way to versatile delivery carriers. Effective management of hard-to-close skin wounds is a challenging issue due to several co-morbidities in affected patients. Particularly, infections represent a major obstacle in wound healing. The design of efficient wound treatments thus represents an urgent need. Injectable drug delivery hydrogels with intrinsic antimicrobial and antifungal properties were herein designed for perspective application in the mini-invasive treatment of hard-to-close wounds. First, an amphiphilic polyurethane was synthesized from Poloxamer® 407 macrodiol and N-Boc diethanolamine chain extender (DHP407, M ¯ w =33 kDa). Chain-extension reaction step was optimized to maximize the formation of -NH groups along the polymer chains (4.5 × 10 20 ±1.8 × 10 19 –NH groups/g polymer ), after Boc-caging group removal (D-DHP407). DHP407 and D-DHP407 water-based solutions were thermosensitive with slightly different Critical Micellar Concentration (17.5 μg/mL vs . 19.7 μg/mL) and cluster hydrodynamic diameter (235.6 ± 19.9 nm vs . 260.1 ± 20.5 nm), and similar Critical Micellar Temperature (22.5 °C vs . 23.1 °C). A polyurethane solution concentration (15% w/V) was selected by tube-inverting test and rheological analysis showing injectability, as evidenced by sol-to-gel transition at 27.7 ± 0.6 °C for DHP407 and 29.7 ± 0.6 °C for D-DHP407, within few minutes, at similar gelation kinetics. DHP407 and D-DHP407 hydrogels showed controlled release of Bovine Serum Albumin (BSA) model protein (1 mg/mL), with no burst phenomena. BSA released from DHP407 and D-DHP407 hydrogels at 24 h was 33.7 ± 5.0% and 24.6 ± 1.2%, respectively. D-DHP407 hydrogel was biocompatible and able to support NIH-3T3 cell proliferation. Furthermore, D-DHP407 hydrogel showed intrinsic antifungal and antibacterial activity against C. albicans and Gram-positive S. aureus and Gram-negative E. coli bacteria, injectability and capability to retain shape post-injection, making it promising for future use in the management of hard-to-close skin wounds.

Poloxamine/D-α-Tocopheryl polyethylene glycol succinate (TPGS) mixed micelles and gels: Morphology, loading capacity and skin drug permeability

The combination of polymeric surfactants with different features into mixed micelles give access to properties that may be superior to the single-component micelles. In this work, we investigated synergistic effects in mixtures of D-α-Tocopheryl polyethylene glycol succinate (TPGS) with poloxamines (also known as Tetronic), pH-responsive and thermogelling polyethylene oxide (PEO)-polypropylene oxide (PPO) 4-arm block copolymers. We examined the morphology of the self-assembled micelles of TPGS with Tetronic 1107 (T1107) and 908 (T908) in the presence of naproxen (NA), used as a model drug, and assessed the capacity of the single and mixed micelles to trap the guest, using a combination of small-angle neutron scattering (SANS) and NMR spectroscopy (1D, 2D-NOESY and diffusion NMR), over a range of compositions and temperatures, in the dilute regime and gel state. NA did not interact with T1107 or T908 in their unimer form, but it was incorporated into the hydrophobic core of the micelles above the critical micellar temperature (CMT). In contrast, TPGS dissolved NA at any temperature, mainly in the tocopherol core, with some partitioning in the PEG-shell. The micellar structure was not altered by the presence of NA, except for an expansion of the core size, a result of the preferential accumulation of NA in that compartment. The solubility of the drug in single component micelles increased markedly with temperature, while mixed micelles produced an intermediate enhancement of the solubility between that of TPGS and the poloxamines, which increased at higher TPGS/poloxamine ratios. Micellar hydrogels formed by the packing of the polymeric mixed micelles in a BCC macrolattice, whose structure was not altered by the presence of the drug (at least at 0.2 wt%). The applicability of the drug-loaded gels for topical formulations was explored by transdermal diffusion testing using a synthetic model of skin, showing that the diffusion of NA across the membrane was enhanced by incorporating small amounts of TPGS to the hydrogel, especially with the more hydrophilic T908.

Using Poloxamer® 407 as Building Block of Amphiphilic Poly(ether urethane)s: Effect of its Molecular Weight Distribution on Thermo-Sensitive Hydrogel Performances in the Perspective of Their Biomedical Application

Due to its hydroxyl terminal groups, Poloxamer® 407 (P407), a commercially available poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer can be used as macrodiol for the synthesis of high molecular weight amphiphilic poly(ether urethane)s (PEUs). This work was aimed at studying the effect of P407 purification by removing PEO-PPO diblock copolymer by-products on the chemical properties of PEU polymer and the physical properties of PEU hydrogels. Removal of PEO-PPO diblock copolymers (P407_P) was found to preserve the thermo-responsiveness of resulting hydrogels, although slightly lower gelation onset temperature (Tonset) was found for P407_P (15.3°C) vs. P407 (16.7°C) hydrogels (25% w/V) as assessed through temperature ramp test. P407 and P407_P were then reacted with 1,6-diisocyanatohexane and 1,4-cyclohexanedimethanol to synthesize two different PEUs, coded as CHP407 and CHP407_P, respectively. Lower Number Average Molecular Weight (Mn¯) and higher polydispersity Index (D) was measured for CHP407 (Mn¯: 34 kDa, D: 1.6) respect to CHP407_P (Mn¯: 40 kDa, D: 1.4) as a consequence of macrodiol purification. CHP407_P hydrogels formed bigger micelles (43.9 ± 4.1 nm vs. 28.7 ± 4 nm) while showed similar critical micellar temperatures (22.1°C vs. 21.6°C) respect to CHP407 formulations. Sol-to-gel transition of CHP407 and CHP407_P hydrogels was similar while CHP407_P gelation time at 37°C was longer as assessed by tube inverting test. The rheological analysis showed slightly lower Tonset for CHP407_P hydrogels (15% w/V), probably due to larger micelle size, promoting micellar assembly. However, CHP407_P hydrogels showed a significantly lower critical strain than CHP407 hydrogels, as assessed by strain sweep test, suggesting their higher brittleness due to a lower density of intermicellar bridge chains. Nano-scale hydrogel characterization by Low-Field Nuclear Magnetic Resonance spectroscopy supported previous findings, showing lower spin-spin relaxation time (i.e., 1,259 ms) for CHP407_P than for CHP407 hydrogels (i.e., 1,560 ms) at 37°C, which suggested the formation of a more tightly packed network for CHP407_P than CHP407 hydrogel. Finally, lower swelling capability and resistance against dissolution were measured for CHP407_P hydrogels. Overall, the here‐reported results suggested that the heterogeneous structure in the CHP407 hydrogel network caused by the presence of diblock copolymer-based macrodiols improved PEU hydrogel properties in light of their applicability in the biomedical field.

Thermosensitive Micellar Hydrogels as Vehicles to Deliver Drugs With Different Wettability.

The design of adaptable drug delivery systems able to encapsulate and release drugs with different wettability has been attracting widespread interest. Additionally, many attempts have been made to tune hydrophobic/hydrophilic drug release kinetics over time, avoiding the so-called burst release. In this scenario, hydrogels resulting from the assembly of micellar structures showing a hydrophobic core and a hydrophilic shell could represent a promising alternative to design versatile drug vehicles. In this regard, this work aimed at designing new thermosensitive micellar hydrogels starting from a custom-made amphiphilic poly(ether urethane) (PEU). Specifically, a commercial triblock copolymer (Poloxamer® 407), selected to ensure the temperature-driven chain arrangement into micelles, was reacted with 1,6-diisocyanatohexane and 1,4-cyclohexanedimethanol. The successful PEU synthesis was proved by size-exclusion chromatography (w 50000 Da) and infrared spectroscopy. Subsequently, the wettability-driven drug arrangement within the micelle network as well as the influence of drug-loading on the resultant formulation thermosensitivity was investigated by selecting ibuprofen (IBU) and ibuprofen sodium salt (IBUSS) as hydrophobic and hydrophilic drugs, respectively. Specifically, growing drug amounts were loaded into PEU solutions, and the average hydrodynamic micelle diameters and the critical micellar temperatures (CMT) were measured. Systems containing IBU at the highest tested concentration (i.e., 20 mg/mL) showed a significantly higher micelle average diameter (58.2 ± 4.7 nm) and a remarkably lower CMT (8.9°C) with respect to both the control (40.1 ± 1.4 nm and 21.6°C) and IBUSS-loaded formulations (37.3 ± 2.1 nm and 22.4°C). Then, the influence of drug encapsulation on the temperature at which micelles begin to aggregate was rheologically assessed, showing that IBU-loading induced a decrease in this parameter (14.6, 8.7, and 13.7°C for virgin, IBU-loaded, and IBUSS-loaded hydrogel, respectively). Finally, IBU and IBUSS releasing mechanism was analysed using the Korsmayer–Peppas model (n value of 0.63 ± 0.007 and 0.89 ± 0.003 for IBU- and IBUSS-loaded gels, respectively). Thanks to their micellar organisation, the here-developed hydrogel platform allowed the encapsulation of a high number of molecules with different wettability. Additionally, these systems exhibited tunable payload-releasing time without burst release and open the way toward the engineering of smart systems for the sustained co-delivery of multiple drugs in a target tissue/organ.

Molecular interactions between Pluronic F127 and saponin in aqueous solution

Nanotechnology is important for the development of new materials capable to improve the development of novel drug delivery systems. Such systems may result in more efficient therapeutic effects with reduced toxicity and extended time in the organism and increase in bioavailability, biocompatibility, and solubility of drugs. Here it was investigated the interaction between the triblock copolymer Pluronic F127 (F127) and a natural bioactive amphiphilic saponin (SAP), extracted from Glycyrrhiza glabra roots, aiming to form micelle-like F127-SAP complexes in a pH 7.4 HEPES buffered aqueous solution, with potential nanobiotechnological applications. Differential scanning calorimetry (DSC), dynamic light scattering (DLS), and dynamic electrophoretic light scattering (DESL) were the techniques used in this investigation, at varied SAP concentrations below and above CMC at fixed 500 μM F127. The SAP-F127 molecular interactions were verified by changes in the critical micellar temperature (CMT) of F127 obtained from the DSC thermograms, while DLS and DELS data showed, respectively, the formation larger structures of the complexes relative to the single Pluronic micelles and zeta potential (ζ) values ranging from ca − 6.5 to − 9 mV, on varying the SAP concentration. Combined, these results indicate the formation of micelle-like complexes, with structure dependent on the SAP concentration, as highly promising agents in nanobiotechnology applications.

Polyurethane-based thiomers: A new multifunctional copolymer platform for biomedical applications

During the past few decades, thiomers have attracted interest as responsive-polymers in the design of smart hydrogels, due to thiol reactivity to physical stimuli (i.e., pH, oxygen, UV/Vis irradiation). However, thiomer potentialities have been limited by their low thiolation degree and lack of multi-responsiveness to physical cues. In this work, a new class of thiomers with tailored thiolation degree and sensitivity to multiple physical stimuli was developed. To this aim, a polyurethane (33,000 Da) was synthesised from Poloxamer® 407, 1,6-diisocyanatohexane and N-Boc diethanolamine. Secondary amino groups were exposed through Boc-removal (yield 80%, assessed by 1H NMR) giving up to 4.5 × 1020 ± 1.8 × 1019 –NH units/gpolymer (Orange II Sodium Salt colorimetric assay) without polymer chemical degradation. Polymer thermo-responsiveness was proved by the increase in micelle hydrodynamic diameter upon system heating up to 45 °C and through estimated critical micellar temperatures. Thioglycolic acid grafting to exposed amines via carbodiimide chemistry was optimised to give up to 1.73 × 1019 thiols/gpolymer (Ellman's method), while minimising disulphide bond formation. Finally, the best storage conditions against oxidation were investigated by quantifying free thiols at different time points.

Chain conformation: A key parameter driving clustering or dispersion in polyelectrolyte – Colloid systems

The work presents the characterization of Pluronic F127 micellar solutions in presence of hyaluronic acid in semi-dilute regime. The effects of the nature and salt concentration are investigated by differential scanning calorimetry and small angle neutron scattering. Hyaluronic acid reduces the critical micellar temperature to the same extend as an increase of the ionic strength. Within the investigated HA concentration range, the size and shape of the micelles are not modified by the addition of HA but their dispersion state depends on the salt concentration. By increasing the ionic strength we observe the formation of small micellar clusters which organize into a face-centered cubic liquid crystalline phase at high salt concentration. This behavior is reinforced by increasing the HA concentration or molecular weight. The nature of the salt plays also a role and divalent cations such as Ca2+ promote the clustering of micelles and their crystallization. The origin of the aggregative behavior is the change of the HA chain conformation –from stretched to coil- by addition of salt which in turn induces an excluded volume around the micelles and exerts a depletion interaction.

Salt and bile salt accelerate self-assembly behavior of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) probed by curcumin fluorescence

Understanding self-assembly aspects of block copolymers is of great importance due to their utility in a wide range of applications whether in chemistry, pharmacy, or medicine. In this work, properties of poly(ethylene oxide) -block- poly(propylene oxide) -block- poly(ethylene oxide) (F108) are studied in solution using fluorescence technique and curcumin as the molecular probe. Fluorescence of curcumin has been tracked in solutions of different concentrations of F108. The CMC (critical micellar concentration) and CMT (critical micellar temperature) have been found to be 23.2 μM and 35 °C respectively. First time curcumin fluorescence-based method has been proven to be useful to estimate CMT. Furthermore, fluorescence quenching technique using hydrophobic cetyl pyridinium bromide and hydrophilic KI quenchers has established the position of curcumin is located near the hydrophobic pocket of Stern-layer of F108 micelle. New insight on effect of ionic strength and bile salt on the CMC and CMT values of F108 is evaluated through curcumin probing. CMC has decreased with the increase in the concentration of the three salts except for NaC. The effect has been arranged in decreasing order as follows: NaDC > NaCl > NaC. On the other hand, the effect of the three salts on the CMT of F108 has been found to be less remarkable, with a 1-fold decrease for NaCl and NaDC and almost no change for NaC.

Surface excess at water/air interface and micellization in solutions of an amphiphilic triblock copolymer

PEO-PPO-PEO triblock copolymers (usually referred as EPE) have been employed in different fields, with a remarkable emphasis in biomedical-related applications. They are amphiphilic macromolecules, in which PEO segments are soluble in the aqueous phase, while PPO blocks have hydrophobic character. In this work we carried out experiments with surface tensiometry of aqueous solutions of an EPE copolymer, well-known as Kolliphor P188, as a function of temperature and concentration. From the resultant data, thermodynamic relationships were related to adsorption at non-saturated liquid/air interface (low copolymer concentrations). Surface excess curves were calculated from Kohlrausch-William-Watts (KWW) functions fitted to experimental data and critical micellar concentration, was determined. The relationship of and temperature was similar to the observed for the adsorption of EPE copolymer at non-saturated liquid/air interface: there was a drastic change in enthalpy and entropy of adsorption/micellization at which was correlated to the critical micellar temperature, for this copolymer.

Structural Characterization of Pluronic Micelles Swollen with Perfume Molecules.

The insertion in nonionic polymer micelles (Pluronics F127) of seven essential oils and some of the pure compounds that compose them was investigated by complementary differential scanning calorimetry, small-angle X-ray, and neutron scattering (SAXS and SANS). The study revealed various insertion and swelling behaviors for the different oil molecules, an evidence of different interaction mechanisms involved between oils and Pluronic monomers. Thermodynamically, the addition of oil increased the micellization enthalpy due to an enhanced release of water molecules, leading subsequently to a decrease of the critical micellar temperature (CMT). Structurally, with oil, SANS revealed the presence of large aggregates at lower temperature than the CMT for which their size is maximal. Above the CMT, the size decreased and the equilibrium was reached a few degrees after the temperature corresponding to the maximum of the endothermic peak. At 37 °C, the detailed combined SANS and SAXS analysis demonstrated a partial phase separation between the oil and the poly(propylene oxide) core. The hydrophilic stabilizing poly(ethylene oxide) shell remains unchanged.

Study of the Protein Bovine Serum Albumin (BSA) and the Triblock Copolymer P103 Interface By Differential Capacitance

Triblock copolymers are considered as non-ionic surfactants due to their self-assembling nature. Triblock copolymers can form structures similar to micelles and larger structures at a certain concentration and temperature. The triblock copolymers used here are biodegradable, water-soluble and are formed by chains of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), which can be arranged as A-B-A or B-A-B; where A are chains of PEO and B chains of PPO. And in this case they are referred by the commercial name Pluronics. The physicochemical properties of these surfactants (Critical micellar temperature (CMT), critical micellar concentration (CMC), density, molecular weight, etc.) depend on their molecular weight, the number of PPO and PEO chains present in the triblock copolymer, on the solvent (polar or no polar) and on the presence of salts in the solution. Applications of triblock copolymers are diverse; for instance, in the field of biotechnology they have been used for the controlled release of drugs, proteins, DNA. However, even when these compounds are already used and applied in several topics, there is a lack in the knowledge concerning to the effect of the interactions of them with other compounds, such as biopolymers. The main interest is to understand the effects of these interactions in the characteristic shape or size of aggregates formed by triblock copolymers. In order to obtain information about this topic, an initial approach is to study the interactions of some triblock copolymer and some well-characterized protein, as bovine serum albumin (BSA). BSA protein is cheap, easy to isolate and purify. In this way, BSA is an excellent option for the interactions study that exist between proteins and triblock copolymers. The objective of this work is to contribute to the knowledge of the interactions of triblock copolymers and proteins through the use of electrochemical techniques to obtain information about the amount of species adsorbed on the electrodes, as well as the size of the interface formed by the interaction of both, protein and triblock copolymers, once they are adsorbed on a metal surface. This information is important in the design of the sensors and is also necessary for the application of drug delibery and other fields of bioengineering and biomedicine. Thus, this investigation is focused on the interaction between BSA and the termosensible triblock copolymer Pluronic® P103 (PEO17-PPO60-PEO17). The interactions are studied by the adsorption of both compounds onto a gold disk electrode immersed in a phosphate buffer solution. Some electrochemical techniques, such as cyclic voltammetry and Differential-capacitance vs. Polarization potential are presented and analyzed in a wide range of temperatures and different P103 and BSA concentration. The thickness of the interface and the effect on the polarization potential are reported.

Development of Pluronic® nanocarriers comprising Pheophorbide, Zn-Pheophorbide, Lapachol and β-lapachone combined drugs: Photophysical and spectroscopic studies

Therapy efficiency is ruled by a single agent, which can be enhanced using an adjuvant, increasing cell inhibition in a synergetic aspect. Taking this into account, the photosensitizers Pheophorbide and Zinc-Pheophorbide, and their physical chemical and photodynamic properties, were evaluated with Lapachol and β-lapachone drugs. They were incorporated into triblock copolymers P123 and F127 as biocompatible nanocarriers. Spectrophotometric studies showed photosensitizer monomerization in the combined formulation, a fundamental aspect for optimal photodynamic results. The apparent pKa studies of Lapachol incorporated in copolymeric micelles indicated small variations when compared to the data in a homogeneous medium (water/ethanol). This demonstrated the composition of 76% and 56% for deprotonated forms of Lapachol at the physiological pH, after administration of the formulated systems in F127 and P123, respectively. This preferential location is in the poly (ethylene oxide) interfacial bulk of the Pluronic®. The characterization of the photophysical properties of the combined systems showed an expressed decrease of fluorescence quantum yield of the PS in the β-lapachone based systems. This was attributed to greater suppression by collisions with water molecules due to variations in the accommodation region of the photosensitizer after simultaneous encapsulation. The combined system showed a decrease of singlet oxygen quantum yield due to high local micellar microviscosity and low molecular oxygen concentration. However, a five-fold increase in singlet oxygen lifetime was noted in systems within the nanostructured microenvironment, which enhances the range of photosensitizer action. The combined system showed marked thermal stability when subjected to refrigeration, which was below the critical micellar temperature for nanocarriers. This was true after subsequent reheating as well. In addition, the formulations were stored for prolonged periods in the lyophilized state that demonstrated the maintenance of the monomeric state of the photosensitizers. This is a desirable effect for photodynamic applications. The majority of the combined systems incorporated into the nanostructured microenvironment showed temporal stability for more than 96 h in an aqueous solution.

Characterization of chlorophyll derivatives in micelles of polymeric surfactants aiming photodynamic applications

The spectrophotometric properties of chlorophylls' derivatives (Chls) formulated in the Pluronics® F-127 and P-123 were evaluated and the results have shown that the Chls were efficiently solubilized in these drug delivery systems as monomers. The relative location of the Chls in the Pluronics® was estimated from the Stokes shift and micropolarity of the micellar environment. Chls with phytyl chain were located in the micellar core, where the micropolarity is similar to ethanol, while phorbides' derivatives (without phytyl chain) were located in the outer shell of the micelle, i.e., more polar environment. In addition, the thermal stability of the micellar formulations was evaluated through electronic absorption, fluorescence emission and resonance light scattering with lowering the temperature. The Chls promote the stability of the micelles at temperatures below the Critical Micellar Temperature (CMT) of these surfactants. For F-127 formulations, the water molecules drive through inside the nano-structure at temperatures below the CMT, which increased the polarity of this microenvironment and directly affected the spectrophotometric properties of the Chls with phytyl chain. The properties of the micellar microenvironment of P-123, with more hydrophobic core due to the small PEO/PPO fraction, were less affected by lowering the temperature than for F-127. These results enable us to better understand the Chls behavior in micellar copolymers and allowed us to design new drug delivery system that maintains the photosensitizer's properties for photodynamic applications.

Interaction between Albumin and Pluronic F127 Block Copolymer Revealed by Global and Local Physicochemical Profiling.

The interaction of human serum albumin (HSA) with amphiphilic block copolymer Pluronic F127 has been investigated by several physical methods. Interest in studying this system stems from a broad range of bioactivities involving both macromolecules. Serum albumins constitute a significant class of proteins in the circulatory system, acting as carriers for a wide spectrum of compounds or assemblies. Pluronic block copolymers have revealed their capacity to ferry a variety of biologically active compounds. Circular dichroism, rheological measurements, and differential scanning microcalorimetry (μDSC) were employed to get insight into the interaction betweeen the two macromolecules. The results reveal that Pluronic F127 induces conformational changes to albumin if it is organized in a micellar form, while albumin influences the self-assembly of Pluronic F127 into micelles or gels. F127 micelles, however, induce smaller conformational changes compared to ionic surfactants. The μDSC thermograms obtained for HSA and/or F127 show that HSA shifts the critical micellar temperature (cmt) to lower values, while concurrently the HSA denaturation behavior is influenced by F127, depending on its concentration. Rheological measurements on solutions of F127 17% have shown that a sol-to-gel transition occurs at higher temperatures in the presence of HSA and the resulting gel is weaker. The global profile on HSA/F127 systems was complemented by local information provided by EPR measurements. A series of X-band EPR experiments was performed with spin probes 4-(N,N'-dimethyl-N-hexadecyl)ammonium-2,2',6,6'-tetramethylpiperidine-1-oxyl iodide (CAT16) and 5-doxyl stearic acid (5-DSA). These spin probes bind to albumin sites and are sensitive to phase transformations in Pluronic block copolymer solutions. For a given F127 concentration, the spin probe binds only to HSA below cmt and migrates to the F127 micelles above cmt. The collective data suggest soft interactions between the macromolecules, with the emerging results projecting potential applications linked to reaching optimal conditions for certain drug formulations.

Structural Evolution of a Stimulus-Responsive Diblock Polypeptide Micelle by Temperature Tunable Compaction of its Core

With a perfectly defined primary structure, both in terms of monomer sequence and chain length, recombinant polypeptides obtained by protein engineering techniques allow the investigation of structure property relationships at a level of detail that is difficult to achieve with traditional synthetic polymers because of the precision with which their sequence can be defined. In the present work, we have studied the behavior and temperature-triggered self-assembly of a series of diblock recombinant elastin-like polypeptides (ELPs) with the goal of elucidating the mechanism of their self-assembly into micelles. Aqueous solutions of diblock ELPs were studied below and above their critical micellar temperature (CMT) by multiangle light scattering and small-angle neutron scattering techniques. Below the CMT, the radius of gyration of soluble ELP chains follows a power law as a function of molecular weight with an exponent value close to 0.5 that is characteristic of Gaussian coil conformations. As the temperature reaches the CMT, attractive interactions between the more hydrophobic block of diblock ELP chains leads to the self-assembly of monodisperse spherical micelles at thermodynamic equilibrium. Above the CMT, micelles expel water molecules from their core whose densification is evidenced by the monotonic increase in the light and neutron scattering intensities as a function of temperature. The behaviors of these different diblock ELPs in solution and as self-assembled nanoparticles above the CMT following universal experimental scaling laws make them analogous to synthetic amphiphilic diblock copolymers (star-like vs crew-cut micelle models). These studies also shed light on the important role of water in the thermal behavior of these thermally responsive self-assembling diblock polypeptides and suggest a new design parameter thermally triggered desolvation and densification of the core of micelles that can be fine-tuned at the sequence level to control the density of self-assembled polymer nanoparticles.

Determination of the critical micellar temperature of F127 aqueous solutions at the presence of sodium bromide by cyclic voltammetry

The properties of Pluronic F127 aqueous solutions with the variation of concentration and temperature were studied by cyclic voltammetry at the presence of sodium bromide. By this method, the relationship between the peak currents of electroactive probe, 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO), and the viscosity of F127 aqueous solutions within some temperature ranges was discussed, and the diffusion coefficient (D) of TEMPO in F127 aqueous solutions at various concentrations under different temperatures was determined. The critical micellar temperature (CMT) of F127 aqueous solution at various concentrations can be obtained from the curve of the peak current i p against the square root of reciprocal viscosity (1/η)1/2 and from the curve of logarithm diffusion coefficient (logD) of TEMPO as a function of reciprocal temperature (1/T). All the results are in good agreement with the results from 1H NMR measurements. Besides, the thermodynamic parameters such as diffusion activation energy (E D) and hydrodynamic radius (R H) can be also obtained from the above two curves. A new method was introduced to determine the CMT of copolymer systems, which is simple, quick, and accurate.