Structural Changes in PEO–PPO–PEO Gels Induced by Methylparaben and Dexamethasone Observed Using Time-Resolved SAXS

Effect of Acetaminophen on Poloxamer 407 Micelles and Hydrogels: The Relationship between Structural and Physical Properties

Poloxamer hydrogel possesses thermosensitive sol-gel transition characteristics and is widely used as a drug-controlled-release carrier for topical or injectable formulations. In this study, the effect of loading of a drug, acetaminophen (ACE), on the physical and structural properties of poloxamer 407 (P407) micelles and hydrogels was investigated. Differential scanning calorimetry measurements revealed that ACE reduced the critical micelle temperature and enthalpy of micellization of P407 solutions. The P407 micellization was promoted by ACE incorporation. Rheometry showed that ACE increased the sol-gel transition temperature and reduced the gel strength of P407. In situ small-angle X-ray scattering (SAXS) using synchrotron radiation revealed that ACE altered the structure of P407 micelles and their packing in the P407 gels. As ACE concentration increased, the P407 micelle packing changed from a face-centered cubic phase to a body-centered cubic phase. Furthermore, ACE disordered the micelle packing structure and induced the formation of an amorphous phase. Structural analysis of the P407 micelle packing indicated that ACE reduced the aggregation number (Nagg) of P407 micelles in the gels. The SAXS study for diluted P407 solutions revealed that ACE reduced the P407 micelle size and its uniformity. The structural changes in P407 micelles by ACE loading (e.g., the reduction of Nagg, size, and size uniformity) would alter the micelle packing structure. It was found that these structural changes of micelle packing, especially the formation of an amorphous phase, could destabilize the P407 gel. As a result, the physical properties of P407 gels, such as gelation temperature and gel strength, were changed. This relationship between the structure and physical property of drug-loaded P407 gels was well-explained by correlating the micelle and gel structures. The mechanistic understanding of the change in the physical properties of P407 gels by drug loading is essential for the effective development of poloxamer gel formulations.

Impact of small molecule and reverse poloxamer addition on the micellization and gelation mechanisms of poloxamer hydrogels

Poloxamer 407 (P407) is widely used for targeted drug-delivery because it exhibits thermoresponsive gelation behavior near body temperature, stemming from a disorder-to-order transition. Hydrophobic small molecules can be encapsulated within P407; however, these additives often negatively impact the rheological properties and lower the gelation temperatures of the hydrogels, limiting their clinical utility. Here we investigate the impact of adding two BAB reverse poloxamers (RPs), 25R4 and 31R1, on the thermal transitions, rheological properties, and assembled structures of P407 both with and without incorporated small molecules. By employing a combination of differential scanning calorimetry (DSC), rheology, and small-angle x-ray scattering (SAXS), we determine distinct mechanisms for RP incorporation. While 25R4 addition promotes inter-micelle bridge formation, the highly hydrophobic 31R1 co-micellizes with P407. Small molecule addition lowers thermal transition temperatures and increases the micelle size, while RP addition mitigates the decreases in modulus traditionally associated with small molecule incorporation. This fundamental understanding yields new strategies for tuning the mechanical and structural properties of the hydrogels, enabling design of drug-loaded formulations with ideal thermal transitions for a range of clinical applications.

A meta-analysis of bone cement mediated antibiotic release: Overkill, but a viable approach to eradicate osteomyelitis and other infections tied to open procedures

A number of clinical studies have highlighted the success of antibiotics formulated at concentrations between 0 and 6% w/w into bone cements to address localized infections. Separately, some commercial manufacturers have produced gentamycin-infused bone cement mixtures as a countermeasure to infection. The anecdotal evidence suggests that antibiotic infused cements can help eradicate or delay the onset of infections. Quantifying the functionality of that response is more challenging. We have surveyed the literature to identify studies in which controlled drug release or mechanical behavioral assessments have been conducted on drug-infused cements. The focus here is on vancomycin (VAN) in part due to its higher potency relative to gentamycin and its more common usage for staph infections. Takeaways from the limited pool of research studies indicate that large fractions (>99%) of the infused vancomycin remain sequestered in the cement and aren't bioavailable after solidification. Antibiotic fluence ranged from 1 to 283 μg/cm²hr. The initial strength of the various antibiotic loaded samples as produced were 52-96 MPa. Simulated exposures in a fluid environment by submersion reduced the antibiotic loaded strengths between 3 and 29%. Some strength measurements were noted below the ASTM F451 standard for acrylic bone cement although drug releasing spacers likely have different requirements. The glassy behavior of the cured cement led to both vancomycin and gentamicin having low permeability and a burst response. Smaller drug molecules and more gel-like immobilization matrices with lower glass transition temperatures offer higher potential for larger and more comprehensive drug bioavailability.

Dynamic Behavior of the Structural Phase Transition of Hydrogel Formation Induced by Temperature Ramp and Addition of Ibuprofen

Understanding the dynamic behavior of hydrogel formation induced by a temperature ramp is essential for the design of gel-based injectable formulation as drug-delivery vehicles. In this study, the dynamic behavior of the hydrogel formation of Pluronic F108 aqueous solutions within different heating rates was explored in both macroscopic and microscopic views. It was discovered that when the heating rate is increased, the gelation temperature window (hard gel region) shrinks and the mechanical strength of the hydrogel decreases. A given system at different heating rates would lead to different crystalline structural evolutions. The time-resolved small-angle X-ray scattering (SAXS) experiments at a heating rate of 10 °C/min disclose that the crystalline structure of micelle packing in the hydrogel exhibits a series of transitions: hexagonal close-packed (HCP) to face-centered cubic (FCC) and body-centered cubic (BCC) structures coexisting and then to the BCC structure along with the increasing temperature. For the system at equilibrium, the BCC structure exclusively dominates the system. Furthermore, the addition of a hydrophobic model drug (ibuprofen) to the F108 aqueous solution promotes hard gel formation at even lower temperatures and concentrations of F108. The SAXS results for the system with ibuprofen at a heating rate of 10 °C/min demonstrate a mixture of FCC and BCC structures coexisting over the whole gelation window compared to the BCC structure that exclusively dominates the system at equilibrium. The addition of ibuprofen would alter the structural evolution to change the delivery path of the encapsulated drug, which is significantly related to the performance of drug release.

Near-Infrared Optogenetic Genome Engineering Based on Photon-Upconversion Hydrogels

Photon upconversion (UC) from near-infrared (NIR) light to visible light has enabled optogenetic manipulations in deep tissues. However, materials for NIR optogenetics have been limited to inorganic UC nanoparticles. Herein, NIR-light-triggered optogenetics using biocompatible, organic TTA-UC hydrogels is reported. To achieve triplet sensitization even in highly viscous hydrogel matrices, a NIR-absorbing complex is covalently linked with energy-pooling acceptor chromophores, which significantly elongates the donor triplet lifetime. The donor and acceptor are solubilized in hydrogels formed from biocompatible Pluronic F127 micelles, and heat treatment endows the excited triplets in the hydrogel with remarkable oxygen tolerance. Combined with photoactivatable Cre recombinase technology, NIR-light stimulation successfully performs genome engineering resulting in the formation of dendritic-spine-like structures of hippocampal neurons.

The effect of cisplatin on the nanoscale structure of aqueous PEO-PPO-PEO micelles of varying hydrophilicity observed using SAXS

Aqueous solutions of polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) copolymers form micelles and cubic lattices as their temperature is raised. The presence of added solutes within the dispersions can also affect the kinetics of structure formation. Here, we investigate the structures formed in the amphiphiles P104, P105, and F108 solutions at 31% mass per v both neat and co-formulated with the drug cisplatin (0.02% to 0.1% mass per v) using small-angle X-ray scattering. P104 formed BCC colloidal crystals while P105 and F108 formed FCC structures. Cisplatin had a minor influence of the formation and stability of the crystals during these thermal excursions. The largest interaction between the amphiphiles and cisplatin was P104 where there was a 2% reduction in the BCC lattice parameter of P104 as cisplatin loading rose to 0.1% at 28 °C. The F108 unit cell swelled ∼2% upon cisplatin loading of 0.1%. A progressive evolution and breakdown of these structures was noted as the temperature rose from 10 °C to 35 °C. For the different amphiphiles, crystal thermal expansion coefficients of ∼1 × 10-2 °C-1 were determined in neat and loaded amphiphiles with cisplatin and all the crystals swelled with increasing temperature.

Rheological Study and Molecular Dynamics Simulation of Biopolymer Blend Thermogels of Tunable Strength

The temperature-induced gelation of chitosan/glycerophosphate (Chs/GP) systems through physical interactions has shown great potential for various biomedical applications. In the present work, hydroxyethyl cellulose (HEC) was added to the thermosensitive Chs/GP solution to improve the mechanical strength and gel properties of the incipient Chs/HEC/GP gel in comparison with the Chs/GP hydrogel at body temperature. The physical features of the macromolecular complexes formed by the synergistic interaction between chitosan and hydroxyethyl cellulose in the presence of β-glycerophosphate disodium salt solution have been studied essentially from a rheological point of view. The temperature and time sweep rheological characterizations of the thermogelling systems revealed that the sol-gel transition temperature of the Chs/HEC/GP blends is equal to 37 °C at neutral pH; with increasing HEC content in the solutions, more compact networks with considerably improved gel strength are formed without influencing the gelation time. The formed hydrogel matrix has enough mechanical integrity and adequate strength for using it as injectable in situ forming matrices for biomedical applications. The classical Winter-Chambon (W-C) and Fredrickson-Larson (F-L) theories were applied to determine the gel point. In view of the obtained results, it is shown that the F-L theory can be employed as a robust and less tedious method than the W-C approach to precisely determine the gel point in these systems. At the end, molecular simulation studies were conducted by using ab initio quantum mechanics (QM) calculations carried out on Chs and HEC models, and molecular dynamics (MD) simulations of solvated Chs/HEC blend systems showed the binding behavior of Chs/HEC polymers. Analyses of interaction energy, radial distribution function, and hydrogen bonding from simulation studies strongly supported the experimental results; they all disclosed that hydrogen-bond formation between Chs moieties with regard to HEC chains plays an important role for the stabilization of the complexes.

Thermoresponsive physical hydrogels of poly(lactic acid)/poly(ethylene glycol) stereoblock copolymers tuned by stereostructure and hydrophobic block sequence

CBABC-type poly(lactic acid) (PLA)/poly(ethylene glycol) (PEG) pentablock copolymers composed of a central PEG block (A) and enantiomeric poly(l-lactic acid) (PLLA, B), poly(d-lactic acid) (PDLA, C) blocks were synthesized. Such pentablock copolymers form physical hydrogels at high concentrations in an aqueous solution, which stem from the aggregation and physical bridging of copolymer micelles. These gels are thermoresponsive and turn into sols upon heating. Physical gelation, gel-to-sol transition, crystalline state, microstructure, rheological behavior, biodegradation, and drug release behavior of PLA/PEG pentablock copolymers and their gels were investigated; they were also compared with PLA-PEG-PLA triblock copolymers containing the isotactic PLLA or atactic poly(d,l-lactide) (PDLLA) endblocks and PLLA-PEG-PLLA/PDLA-PEG-PDLA enantiomeric mixtures. PLA hydrophobic domains in pentablock copolymer gels changed from a homocrystalline to stereocomplexed structure as the PLLA/PDLA block length ratio approached 1/1. The gel of symmetric pentablock copolymer exhibited a wider gelation region, higher gel-to-sol transition temperature, higher hydrophobic domain crystallinity, larger intermicellar distance, higher storage modulus, and slower degradation and drug release rate compared to those of the asymmetric PLA/PEG pentablock copolymers or triblock copolymers. SAXS results indicated that the PLLA/PDLA blocks stereocomplexation in pentablock copolymers facilitated the intermicellar aggregation and bridging. Cylindrical ordered structures were observed in all the gels formed from the PLA/PEG pentablock and triblock copolymers. The stereocomplexation degree and intermicellar distance of the pentablock copolymer gels increased with heating.

Solubilization of parabens in aqueous Pluronic solutions: investigating the micellar growth and interaction as a function of paraben composition

The influence of methyl paraben (MP) and butyl paraben (BP) on the aggregation characteristics of Pluronics in an aqueous medium has been investigated by DLS, SANS, viscometry, and fluorescence measurement techniques. Parabens are extensively used as preservatives in cosmetic, pharmaceutical, and food products. In this paper, we show that their influence on the restructuring and growth of Pluronics micelles vary quite significantly with their aqueous solubility and with the composition of Pluronics. In the case of P105 and P104, MP reduces the sphere-to-rod transition temperature down to room temperature, but BP with significantly less aqueous solubility than MP suppresses such micellar transition and leads to the formation of micellar clusters due to the onset of intermicellar attractive interaction. In the case of more hydrophobic Pluronic P103, on the other hand, both MP and BP are able to induce rapid room temperature sphere-to-rod micellar growth, which is not observed in the presence of water structure making salts like NaCl and Na(3)PO(4). These observations have been attributed to modulation of growth and restructuring processes of the Pluronic micelles arising due to different locations of parabens within the micellar corona as determined by their aqueous solubility and the hydrophobicity of the Pluronics.