Imaging the In Vivo Degradation of Tissue Engineering Implants by Use of Supramolecular Radiopaque Biomaterials.

For in situ tissue engineering (TE) applications it is important that implant degradation proceeds in concord with neo-tissue formation to avoid graft failure. It will therefore be valuable to have an imaging contrast agent (CA) available that can report on the degrading implant. For this purpose, a biodegradable radiopaque biomaterial is presented, modularly composed of a bisurea chain-extended polycaprolactone (PCL2000-U4U) elastomer and a novel iodinated bisurea-modified CA additive (I-U4U). Supramolecular hydrogen bonding interactions between the components ensure their intimate mixing. Porous implant TE-grafts are prepared by simply electrospinning a solution containing PCL2000-U4U and I-U4U. Rats receive an aortic interposition graft, either composed of only PCL2000-U4U (control) or of PCL2000-U4U and I-U4U (test). The grafts are explanted for analysis at three time points over a 1-month period. Computed tomography imaging of the test group implants prior to explantation shows a decrease in iodide volume and density over time. Explant analysis also indicates scaffold degradation. (Immuno)histochemistry shows comparable cellular contents and a similar neo-tissue formation process for test and control group, demonstrating that the CA does not have apparent adverse effects. A supramolecular approach to create solid radiopaque biomaterials can therefore be used to noninvasively monitor the biodegradation of synthetic implants.

Cooperative and reciprocal chiral structure formation of an alanine-based peptide confined at the surface of cationic surfactant membranes.

The confinement of anionic oligoalanine peptides at the surface of cationic membranes can cooperatively reinforce peptide/peptide interactions and induce secondary-structure formation, and, reciprocally, induce chirality expression of the membrane at the mesoscopic level, thus leading to the formation of three-dimensional chiral fibrillar networks. Such a strong binding effect of peptides with cationic membranes and the resulting cooperative assembly behaviors are observed with two different types of cationic surfactant, namely, two-head two-tail gemini and one-head two-tail surfactants. The ensemble of assembly properties, such as critical micellar concentration (cmc), Krafft temperature (T(k) ), molecular area at the air/water interface, molecular organization (as studied by FTIR attenuated total reflectance (ATR) measurements and small-angle X-ray scattering), and morphology of the aggregates (as observed by optical and electron microscopy studies), are reported. The results clearly demonstrate that the molecular organization and mesoscopic supramolecular structures are controlled by a subtle balance between the peptide/peptide interactions, ionic interactions between the membranes and peptides, and the interactions the between surfactant molecules, which are governed by hydrophobicity and steric interactions. Investigation into such cooperative organization can shed light on the mechanism of supramolecular chirality expression in membrane systems and allow understanding of the structure of peptides in interactions with lipid bilayers.

Monodisperse hydrogel microspheres by forced droplet formation in aqueous two-phase systems.

This paper presents a method to form micron-sized droplets in an aqueous two-phase system (ATPS) and to subsequently polymerize the droplets to produce hydrogel beads. Owing to the low interfacial tension in ATPS, droplets do not easily form spontaneously. We enforce the formation of drops by perturbing an otherwise stable jet that forms at the junction where the two aqueous streams meet. This is done by actuating a piezo-electric bending disc integrated in our device. The influence of forcing amplitude and frequency on jet breakup is described and related to the size of monodisperse droplets with a diameter in the range between 30 and 60 µm. Rapid on-chip polymerization of derivatized dextran inside the droplets created monodisperse hydrogel particles. This work shows how droplet-based microfluidics can be used in all-aqueous, surfactant-free, organic-solvent-free biocompatible two-phase environment.

Introduction of curvature in amphipathic oligothiophenes for defined aggregate formation.

In this study the possibility to control the size and shape of self-assembled structures through the local curvature of their molecular building blocks has been investigated. To this end a series of amphipathic conjugated oligothiophenes with a well-defined curvature of their backbone has been designed and synthesized. The molecular (local) curvature of these oligothiophenes resulted from a preference for cis instead of trans conformations at specific positions along the oligothiophene backbone, which can be controlled by the sequence of hydrophilic and hydrophobic groups, while their ratio was kept constant. The self-assembly of ter-, sexi-, and dodecathiophenes appeared to be a low-cooperative process, involving the formation of premicellar aggregates at sub-millimolar concentrations, which at concentrations in the millimolar regime transformed into micelles and cylindrical micelles. The aggregates display fine structures with dimensions reminiscent of the thiophene molecules. The structure-morphology relationship of the ter- and sexithiophenes could be described by conventional packing theory. However, with the dodecathiophene, the backbone curvature governed the formation of cylindrical aggregates with a well-defined diameter. These results demonstrate that it is possible to control the aggregation morphology of simple amphipathic oligothiophenes by implementation of an additional structural motif namely, the curvature.

Size control and compartmentalization in self-assembled nano-structures of a multisegment amphiphile.

A "multisegment amphiphile" has been synthesized by covalently connecting two well known building blocks, a gelator and a micelle forming surfactant. Self-assembly results in the formation of compartmentalized nano-object displaying properties inherited from both parents.

Molecular and supramolecular chirality in gemini-tartrate amphiphiles studied by electronic and vibrational circular dichroisms.

This contribution presents an application of electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) to study the molecular and supramolecular chirality in assemblies of gemini-tartrate amphiphiles. Nonchiral dicationic n-2-n amphiphiles (n = 14-20) can self-organize into right- or left-handed structures upon interacting with chiral tartrate counterions. Micellar solutions can also be obtained for shorter alkyl chains (n = 12). First, the conformation of tartrate counterions has been investigated in various environments (micellar solutions and chiral ribbons). ECD and VCD spectra recorded in micellar solutions are independent from the solvent and from the nature of the cations (sodium, cetyl-trimethylammonium, or dimeric surfactant 12-2-12) used and are representative of the anticonformation of the tartrate dianions. On the other hand, drastic changes in the ECD and VCD spectra have been observed in multilayered chiral assemblies of 16-2-16 tartrate. These strong spectral modifications are associated with the chiral arrangement of the tartrate molecules at the surface of the bilayers. Moreover, chirality transfer from counterions to achiral amphiphiles has been clearly evidenced by VCD since circular dichroism has been observed on vibrations related to alkyl chains and gemini headgroups. Finally, ECD and VCD experiments were performed varying the enantiomeric excess of the tartrate. The ECD and VCD intensities do not vary linearly with the enantiomeric excess of the anion and different behaviors have been observed from the two spectroscopic methods: ECD intensities are correlated to the pitch of the ribbons, whereas the VCD intensities are correlated to the dimension of the chiral ribbons.

Self-assembled interpenetrating networks by orthogonal self assembly of surfactants and hydrogelators.

Interpenetrating networks (IPN) consist of two or more networks of different components which are entangled on a molecular scale and cannot be separated without breaking at least one of the networks. They are of great technological interest because they allow the blending of two or more otherwise incompatible properties or functions, and furthermore synergistic effects might arise from the simultaneous operation of the two networks. So far, the preparation of interpenetrating network gels by self-assembly approaches was doomed to fail because the conventional polymers and surfactant building blocks either phase separate or form mixed assemblies, respectively. Here we report on self-assembled interpenetrating networks obtained by the orthogonal self-assembly of small molecular hydrogelators and surfactants. Preliminary studies on the self-assembly behaviour and viscoelastic properties of these systems revealed that these self-assembled IPN have a number of intriguing properties. For instance, the presence of two coexisting networks offers new possibilities for compartmentalization, and will allow one to adjust the viscoelastic properties between 'soft' and 'hard' gels. The non-covalent character of such IPN makes their formation fully reversible, which can be exploited for dual responsive systems. Most interestingly, self-assembled IPN can also act as a very primitive, yet unique, model for biological interpenetrating networks like the extracellular matrix and the cytoskeleton, and thereby contribute to our understanding of these very complex systems.

Individualized silica nanohelices and nanotubes: tuning inorganic nanostructures using lipidic self-assemblies.

Diverse chiral nanometric ribbons and tubules formed by self-assembly of organic amphiphilic molecules could be transcribed to inorganic nanostructures using a novel sol-gel transcription protocol with tetraethoxysilane (TEOS) in the absence of catalyst or cosolvent. By controlling parameters such as temperature or the concentration of the different reactants, we could finely tune the morphology of the inorganic nanostructures formed from organic templates. This fine-tuning has also been achieved upon controlling the kinetics of both organic assembly formation and inorganic polycondensation. The results presented herein show that the dynamic and versatile nature of the organic gels considerably enhances the tunability of inorganic materials with rich polymorphisms.

Control of nano-micrometric twist and helical ribbon formation with gemini-oligoalanine via interpeptidic beta-sheet structure formation.

Confinement of anionic oligo-alanine peptides at the surfaces of cationic membrane by ionic interaction can induce their secondary structure formation; such organized peptides reciprocally transfer their chirality to membranes with non-chiral amphiphiles and their supramolecular chiral structures can be tuned both by peptides and amphiphiles structures.

Counterion, temperature, and time modulation of nanometric chiral ribbons from gemini-tartrate amphiphiles.

Amphiphile supramolecular assemblies result from the cooperative effects of multiple weak interactions between a large number of subcomponents. As a result, prediction of and control over the morphologies of such assemblies remains difficult to achieve. Here, we described the fine-tuning of the shape, size, and morphology transitions of twisted and helical membranes formed by non-chiral dicationic n-2-n gemini amphiphiles complexed with chiral tartrate anions. We have reported that such systems express the chirality of the tartrate components at a supramolecular level and that the mechanism of the chiral induction by counterions involves specific anion cation recognition and the induction of conformationally labile chirality in the cations. Here, we demonstrate that the morphologies and dimensions of twisted and helical ribbons, as well as tubules, can be controlled and that interconversion between these structures can be induced upon modifying temperature, upon introducing small amounts of additives, or slightly modifying molecular structure. Specifically, electron microscopy, IR spectroscopy, and small-angle X-ray scattering show that (i) varying the hydrophobic chain length or adding gemini having bromide counterions (1%) or the opposite enantiomer (10%) leads to an increase of the diameter of membrane tubules from 33 to 48.5 nm; (ii) further addition (1.5%) of gemini bromide or a slight increase in temperature induces a transition from tubules to twisted ribbons; (iii) the twist pitch of the ribbons can be continuously tuned by varying enantiomeric excess; and (iv) it was also observed that the morphologies of these ribbons much evolve with time. Such unprecedented observations over easy tuning of the chiral supramolecular structures are clearly related to the original feature that the induction of chirality is solely due the counterions, which are much more mobile than the amphiphiles.

Asp-Gly based peptides confined at the surface of cationic gemini surfactant aggregates.

Cationic gemini surfactants complexed with anionic oligoglycine-aspartate (called gemini peptides hereafter) were synthesized, and their aggregation behaviors were studied. The effects of the hydrophobic chain length (C10-C22) and the length of the oligoglycine (0-4) were investigated, and it was clearly shown by critical micellar concentration, Krafft temperature, and isothermal surface pressure measurements that the hydrophobic effect and interpeptidic interaction influence the aggregation behavior in a cooperative manner. Below their Krafft temperatures, some of them formed both hydro- and organogels with three-dimensional networks and the Fourier transform infrared measurements show the presence of interpeptidic hydrogen bonds.

Mixing behavior of fluorinated and hydrogenated gemini surfactants at the air-water interface.

Mixing behavior of hydrogenated and fluorinated cationic gemini surfactants was studied at the air-water interface by Brewster angle microscopy and pi-A isotherm curves. In the bulk, these two molecules did not mix and showed phase separation. At the air-water interface, if a monolayer was formed by separate deposition of the two solutions, they formed separate domains, and the compression occurred in two steps: first the domains with hydrogenated gemini surfactant were compressed until they showed collapse; then the domains with fluorinated gemini surfactant were compressed. If the two solutions were mixed before the deposition, they remained mixed upon compression; on the other hand, separate domains under separate deposition were shown to mix if the subphase was heated.

Chirality Effects in Self-assembled Fibrillar Networks.

Chirality seems to be intimately associated with the growth and stability of self-assembled fibrillar networks and with the most common macroscopic property of these networks, which is the thermoreversible gelation of the solvent. The presence and the relative configurations of stereogenic centers in the structure of a small molecule gelator are generally (but not always) observed to be critical to its ability to form gels. Symmetry considerations of chiral molecular packing provide thermodynamic and kinetic arguments that may explain why chirality favors fiber growth. Additionally, molecular chirality is sometimes expressed at a scale of nanometers or micrometers and gives rise to twisted or coiled fiber structures that are readily observable by microscopic techniques. These chiral fiber morphologies have already found some applications as templates for helical protein crystallization or for the growth of chiral inorganic replicas. The chiroptical properties of assembled chiral molecules, e.g., circular dichroism, allow monitoring of aggregation and may sometimes give insights into molecular packing. But determining chiral molecular arrangements in the fibers remains a challenge and requires the use of multiple techniques.