"Bitter-Sweet" Polymeric Micelles Formed by Block Copolymers from Glucosamine and Cholic Acid.

Natural compounds glucosamine and cholic acid have been used to make acrylic monomers which are subsequently used to prepare amphiphilic block copolymers by reversible addition-fragmentation chain transfer (RAFT) polymerization. Despite the striking difference in polarity and solubility, three diblock copolymers consisting of glucosamine and cholic acid pendants with different hydrophilic and hydrophobic chain lengths have been synthesized without the use of protecting groups. They are shown to self-assemble into polymeric micelles with a "bitter" bile acid core and "sweet" sugar shell in aqueous solutions, as evidenced by dynamic light scattering and transmission electron microscopy. The critical micelle concentration varies with the hydrophobic/hydrophilic ratio, ranging from 0.62 to 1.31 mg/L. Longer chains of polymers induced the formation of larger micelles in range of 50-70 nm. These micelles can solubilize hydrophobic compounds such as Nile Red in aqueous solutions. Their loading capacity mainly depends upon the hydrophobic/hydrophilic ratio of the polymers, and may be also related to the length of the hydrophilic block. These polymeric micelles allowed for a 10-fold increase in the aqueous solubility of paclitaxel and showed no cytotoxicity below the concentration of 500 mg/L. Such properties make these polymeric micelles interesting reservoirs for hydrophobic molecules and drugs for biomedical applications.

Two-Dimensional Magnesium Phosphate Nanosheets Form Highly Thixotropic Gels That Up-Regulate Bone Formation.

Hydrogels composed of two-dimensional (2D) nanomaterials have become an important alternative to replace traditional inorganic scaffolds for tissue engineering. Here, we describe a novel nanocrystalline material with 2D morphology that was synthesized by tuning the crystallization of the sodium-magnesium-phosphate system. We discovered that the sodium ion can regulate the precipitation of magnesium phosphate by interacting with the crystal's surface causing a preferential crystal growth that results in 2D morphology. The 2D nanomaterial gave rise to a physical hydrogel that presented extreme thixotropy, injectability, biocompatibility, bioresorption, and long-term stability. The nanocrystalline material was characterized in vitro and in vivo and we discovered that it presented unique biological properties. Magnesium phosphate nanosheets accelerated bone healing and osseointegration by enhancing collagen formation, osteoblasts differentiation, and osteoclasts proliferation through up-regulation of COL1A1, RunX2, ALP, OCN, and OPN. In summary, the 2D magnesium phosphate nanosheets could bring a paradigm shift in the field of minimally invasive orthopedic and craniofacial interventions because it is the only material available that can be injected through high gauge needles into bone defects in order to accelerate bone healing and osseointegration.

Recent Developments and Optimization of Lipase-Catalyzed Lactone Formation and Ring-Opening Polymerization.

To obtain materials useful for the biomedical field, toxic catalysts should be removed from the synthetic route of polymerization reactions and of their precursors. Lipase-catalyzed ring-opening polymerization and the synthesis of cyclic precursors can be performed with the same catalyst under different conditions. Here, we highlight the use of lipases as catalysts and optimization of their performance for both ring-closing and ring-opening polymerization, via varying parameters such as ring size, concentration, substrate molar ratio, temperature, and solvent. While the conditions for ring-closing reactions and ring-opening polymerizations of small molecules, such as ε-caprolactone, have been extensively explored using Candida antarctica lipase B (CALB), the optimization of macrocyclization, especially for more bulky substrates is surveyed here. Finally, recent methods and polymer architectures are summarized with an emphasis on new procedures for more sustainable chemistry, such as the use of ionic liquids as solvents and recycling of polyesters by enzymatic pathways.

Invertible vesicles and micelles formed by dually-responsive diblock random copolymers in aqueous solutions.

Dually responsive diblock random copolymers poly(nPA0.8-co-DEAEMA0.2)-block-poly(nPA0.8-co-EA0.2) were made from N-n-propylacrylamide (nPA), 2-(diethylamino)ethyl methacrylate (DEAEMA) and N-ethylacrylamide (EA) via reversible addition-fragmentation chain transfer (RAFT) polymerization. Copolymers of different block length ratios, poly(nPA28-co-DEAEMA7)-block-poly(nPA29-co-EA7) (P1) and poly(nPA28-co-DEAEMA7)-block-poly(nPA70-co-EA18) (P2), self-assemble into vesicles and micelles, responding to external stimuli in aqueous solutions, and both show "schizophrenic" inversion behavior when the pH and temperature are varied. The relative lengths of the two blocks are shown to affect the self-assembly of amphiphilic diblock copolymers. P1 has a similar length for both blocks and forms spherical vesicles with the first block poly(nPA29-co-EA7) as the membrane inner layer at pH 7 and 37 °C (above the cloud point of the more hydrophobic block, CP1), while spherical micelle-like aggregates are obtained at pH 10 and 25 °C (above CP1) with the second block poly(nPA28-co-DEAEMA7) as the core. In comparison, P2 has a different block length ratio (1 : 3, thus a much longer second block) and forms spherical micelles above CP1 at both pH 7 (the second block as the core) and pH 10 (the first block as the core). Further aggregation was observed by heating the polymer solution above the cloud point of the more hydrophilic block (CP2). The variation of the length and chemical composition of the blocks allows the tuning of the responsiveness of the block copolymers toward both pH and temperature and determines the formation of either micelles or vesicles during the aggregation.

PEGylated bile acids for use in drug delivery systems: enhanced solubility and bioavailability of itraconazole.

Itraconazole is a drug of choice for the treatment of severe fungal infections and parasitic diseases, but its use is limited by its low water solubility and varying bioavailability. New self-emulsifying drug delivery systems (SEDDS) based on PEGylated bile acids (BA-PEGs) were designed and prepared, where the number and length of PEG arms were varied to optimize the loading of itraconazole in the final drug formulation. The use of both BA-PEGs and oleic acid improved the solubilization and absorption of the drug, which was in a glassy state in the SEDDS prepared with the melting method. High loading efficiencies of itraconazole (up to 20%) and stable liquid formulations were obtained at neutral pH, and full dispersion of itraconazole was reached in 2 h in simulated intestinal fluid (pH 6.8). Aqueous emulsions consisting of spherical micelles with mean hydrodynamic diameters (Dh) of ca. 75-220 nm, as verified by transmission electron microscopy and dynamic light scattering, are expected to improve the intestinal absorption of the drug. The new SEDDS showed good cytocompatibility by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays of BA-PEGs with Caco-2 and RAW 264.2 cells, and a low degree of hemolysis of human erythrocytes. The SEDDS based on PEGylated bile acids provide a controlled release system with significant improvement of the bioavailability of itraconazole in rats, as demonstrated by the pharmacokinetic studies.

Switchable vesicles formed by diblock random copolymers with tunable pH- and thermo-responsiveness.

The thermo-responsiveness of polymers in aqueous media can be tuned by the choice of comonomers used in the synthesis of block copolymers made of random sequences of the same comonomers but of different molar ratios. The same synthetic approach may be applied to other stimuli and we have made diblock random copolymers with both pH- and thermo-responsiveness and studied the formation of vesicles whose membrane core and coronas may be inverted in aqueous media. Sequential reversible addition-fragmentation chain transfer (RAFT) polymerization was used to prepare well-defined block copolymers in the form of AnBm-b-ApCq, where A, B, and C are N-n-propylacrylamide (nPA), 2-(diethylamino)ethyl methacrylate (DEAEMA), and N-ethylacrylamide (EA), respectively. This polymer shows interesting "schizophrenic" behavior in aqueous solutions. Both blocks are thermo-responsive, and one block is pH-responsive in which the tertiary amine group of DEAEMA may be protonated at a lower pH. A molecularly dissolved polymer is obtained at neutral pH and ambient temperature. At pH 7 and 37 °C, the polymer self-assembles into vesicles with the poly(nPA0.8-co-EA0.2) block as the membrane core (mean hydrodynamic diameter of the vesicles Dh = 148 nm). In an alkaline medium (pH 10) at 25 °C, the membrane core and the coronas of the vesicles are inverted with poly(nPA0.8-co-DEAEMA0.2) block forming the core (Dh = 60 nm). In addition, two-step phase transitions are observed in both alkaline and neutral solutions corresponding to the cloud points of the individual blocks. Here, the random nature of the blocks allows fine-tuning the thermo-responsiveness based solely on lower critical solution temperatures and its combination with pH-sensitivity provides vesicles with switchable membrane core and corona in aqueous solution.

Aggregation behavior of pegylated bile acid derivatives.

Bile acids are amphiphilic endogenous steroids that act as anionic surfactants in the digestive tract and aggregate in aqueous solutions. Nonionic surfactants were synthesized by grafting poly(ethylene glycol) chains of various lengths (pegylation) to three bile acids (lithocholic, deoxycholic, and cholic acid) using anionic polymerization. The aggregation properties of the derivatives were studied with viscosity measurements and light scattering as well as with steady-state and time-resolved fluorescence techniques, and the aggregates were visualized by transmission electron microscopy to elucidate the effect of pegylation on the aggregation process. The fluorescence results showed a good correlation with the capacity of the bile acid derivatives to solubilize a hydrophobic drug molecule. The solubilization of ibuprofen depends on the length and the number of grafted PEG chains, and the solubilization efficiency increases with fewer PEG chains on the bile acid. The results indicate their potential for use in the design of new bile acid-based drug-delivery systems.

Thermosensitivity of bile acid-based oligo(ethylene glycol) stars in aqueous solutions.

Amphiphilic star-shaped oligo(ethylene glycol)s with a hydrophobic bile acid core and varying number of hydrophilic arms have been made. Their thermal behavior in aqueous solutions depends on the number rather than the length of the arms. The two-armed lithocholate derivative showed the strongest tendency for association and exhibited the lowest cloud point (79 °C) of the oligomers made, as well as another phase separation at a lower temperature (31 °C). The "double thermosensitivity" arising both from the salt-dependent LCST of the oligo(ethylene glycol) segments and the temperature-responsive self-assembly of amphiphilic bile acid derivative provides an interesting path in the design of bile acid-based smart materials.

Origin of the fast negative ERG component from isolated aspartate-treated mouse retina.

The leading edge of the a-wave of the ERG is generally believed to accurately reflect the changes in the circulating current through the cGMP-gated channels in the outer segment plasma membrane of rods and cones. The aspartate-isolated mammalian electroretinogram (ERG) to a rod-saturating flash contains a fast "nose"-like wave temporally overlapping with the a-wave. We characterize the nature of this nose, investigate the membrane current mechanisms involved in the nose mechanism, and propose a model that can explain the generation of the nose component in the rod inner segment. On the basis of pharmacological treatments and perfusate ion composition alterations we rule out the possible role of most of the known rod membrane current mechanisms that might participate in the generation of the ERG nose component and we propose that the nose is generated by the interplay of voltage-dependent K(x) and h channels together with the Na(+)/K(+) ATPase. Our results strengthen the view that the kinetics of the leading edge of the ERG photoresponses should correspond to that of the outer segment light-sensitive current.

Validation and application of a nondestructive and contactless method for rheological evaluation of biomaterials.

Hydrogels are extensively used for tissue engineering, cell therapy or controlled release of bioactive factors. Nondestructive techniques that can follow their viscoelastic properties during polymerization, remodeling, and degradation are needed, since these properties are determinant for their in vivo efficiency. In this work, we proposed the viscoelastic testing of bilayered materials (VeTBiM) as a new method for nondestructive and contact-less mechanical characterization of soft materials. The VeTBiM method measures the dynamic displacement response of a material, to a low amplitude vibration in order to characterize its viscoelastic properties. We validated VeTBiM by comparing data obtained on various agar and chitosan hydrogels with data from rotational rheometry, and compression tests. We then investigated its potential to follow the mechanical properties of chitosan hydrogels during gelation and in the presence of papain and lysozyme that induce fast or slow enzymatic degradation. Due to this nondestructive and contactless approach, samples can be removed from the instrument and stored in different conditions between measurements. VeTBiM is well adapted to follow biomaterials alone or with cells, over long periods of time. This new method will help in the fine tuning of the mechanical properties of biomaterials used for cell therapy and tissue engineering. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2565-2573, 2017.

Self-Healing Supramolecular Hydrogels Based on Reversible Physical Interactions.

Dynamic and reversible polymer networks capable of self-healing, i.e., restoring their mechanical properties after deformation and failure, are gaining increasing research interest, as there is a continuous need towards extending the lifetime and improving the safety and performance of materials particularly in biomedical applications. Hydrogels are versatile materials that may allow self-healing through a variety of covalent and non-covalent bonding strategies. The structural recovery of physical gels has long been a topic of interest in soft materials physics and various supramolecular interactions can induce this kind of recovery. This review highlights the non-covalent strategies of building self-repairing hydrogels and the characterization of their mechanical properties. Potential applications and future prospects of these materials are also discussed.

Supramolecular hydrogelation with bile acid derivatives: structures, properties and applications.

Hydrogelation of small molecules in aqueous solutions results from a balance between solubilization and precipitation (or crystallization). The hydrophobic moieties of amphiphiles tend to aggregate and the hydrophilic units may stabilize the aggregates in aqueous solutions. Morphologies vary according to the chemical structure of the amphiphiles. The formation of nanofibers or worm-like micelles is a prerequisite for hydrogels. Molecular hydrogels often show better degradability and functional diversity than polymeric hydrogels and may be useful in biomedical applications. Bile acids have attracted increasing attention for designing various biomaterials, including molecular hydrogels. They are naturally occurring amphiphilic compounds that exist in our body and help with the dissolution and digestion of fat by the formation of micelles. This review highlights the recent progress in the field of molecular hydrogelators based on bile acids, including bile salts, anionic, cationic and neutral bile acid derivatives, two-component hydrogelation systems, and polymeric supramolecular hydrogels, along with their potential applications.

Genipin-crosslinked catechol-chitosan mucoadhesive hydrogels for buccal drug delivery.

Drug administration via buccal mucosa is an attractive drug delivery strategy due to good patient compliance, prolonged localized drug effect, and avoidance of gastrointestinal drug metabolism and first-pass elimination. Buccal drug delivery systems need to maintain an intimate contact with the mucosa lining in the wet conditions of the oral cavity for long enough to allow drug release and absorption. For decades, mucoadhesive polymers such as chitosan (CS) and its derivatives have been explored to achieve this. In this study, inspired by the excellent wet adhesion of marine mussel adhesive protein, we developed a buccal drug delivery system using a novel catechol-functionalized CS (Cat-CS) hydrogel. We covalently bonded catechol functional groups to the backbone of CS, and crosslinked the polymer with a non-toxic crosslinker genipin (GP). We achieved two degrees of catechol conjugation (9% and 19%), forming Cat9-CS/GP and Cat19-CS/GP hydrogels, respectively. We confirmed covalent bond formation during the catechol functionalization and GP crosslinking during the gel formation. The gelation time and the mechanical properties of Cat-CS hydrogels are similar to those of CS only hydrogels. Catechol groups significantly enhanced mucoadhesion in vitro (7 out of the 10 Cat19-CS hydrogels were still in contact with porcine mucosal membrane after 6 h, whereas all of the CS hydrogels lost contact after 1.5 h). The new hydrogel systems sustained the release of lidocaine for about 3 h. In-vivo, we compared buccal patches made of Cat19-CS/GP and CS/GP adhered to rabbit buccal mucosa. We were able to detect lidocaine in the rabbit's serum at concentration about 1 ng/ml only from the Cat19-CS patch, most likely due to the intimate contact provided by mucoadhesive Cat19-CS/GP systems. No inflammation was observed on the buccal tissue in contact with any of the patches tested. These results show that the proposed catechol-modified CS hydrogel is a promising mucoadhesive and biocompatible hydrogel system for buccal drug delivery.

Self-assembly of bile acid-PEG conjugates in aqueous solutions.

Thermoresponsive characteristics of oligo(ethylene glycol) derivatives of lithocholic acid (LCA) depend on the hydrophilic/hydrophobic balance of the compounds. Below a threshold temperature (∼30 °C), one of the derivatives, LCA(EG(4))(2), self-assembles in water into hollow nanotubes that form thixotropic gels at high concentrations. Two concentration regimes were observed in the plateau moduli (G(0) ∼ G'). At high concentrations, the mobility of the nanotubes is restricted at the ordered microdomains acting as cross-links. In semidilute aqueous solutions, the linkers between the aggregated domains in the fragile gel rupture easily, and this effect depends strongly on dilution. Organic hydrophobic additives perturb the self-assembling process, leading to changes in the two transition temperatures and poorer mechanical properties of the gel.