Functional π-Gelators and Their Applications

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping ...

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

Recent advances in the stabilization of emulsions and foams by particles of nanoscale and microscopic dimensions are described. Ongoing research in this highly active field is providing insight into (i) the molecular factors controlling particle wettability and adsorption, (ii) the structural and mechanical properties of particle-laden liquid interfaces, and (ii) the stabilization mechanisms of particle-coated droplets and bubbles. There is much potential for exploiting the emerging knowledge in new food product applications. The preparation of cheap and effective colloidal particles based on food-grade ingredients, especially proteins, is the key technological challenge.

Food emulsions and foams: Stabilization by particles

1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Colloidal Suspension Rheology

Aromatic peptide amphiphiles are gaining popularity as building blocks for the bottom-up fabrication of nanomaterials, including gels. These materials combine the simplicity of small molecules with the versatility of peptides, with a range of applications proposed in biomedicine, nanotechnology, food science, cosmetics, etc. Despite their simplicity, a wide range of self-assembly behaviours have been described. Due to varying conditions and protocols used, care should be taken when attempting to directly compare results from the literature. In this review, we rationalise the structural features which govern the self-assembly of aromatic peptide amphiphiles by focusing on four segments, (i) the N-terminal aromatic component, (ii) linker segment, (iii) peptide sequence, and (iv) C-terminus. It is clear that the molecular structure of these components significantly influences the self-assembly process and resultant supramolecular architectures. A number of modes of assembly have been proposed, including parallel, antiparallel, and interlocked antiparallel stacking conformations. In addition, the co-assembly arrangements of aromatic peptide amphiphiles are reviewed. Overall, this review elucidates the structural trends and design rules that underpin the field of aromatic peptide amphiphile assembly, paving the way to a more rational design of nanomaterials based on aromatic peptide amphiphiles.

Design of nanostructures based on aromatic peptide amphiphiles

Biocompatible hydrogels have a wide variety of potential applications in biotechnology and medicine, such as the controlled delivery and release of cells, cosmetics and drugs, and as supports for cell growth and tissue engineering. Rational peptide design and engineering are emerging as promising new routes to such functional biomaterials. Here, we present the first examples of rationally designed and fully characterized self-assembling hydrogels based on standard linear peptides with purely alpha-helical structures, which we call hydrogelating self-assembling fibres (hSAFs). These form spanning networks of alpha-helical fibrils that interact to give self-supporting physical hydrogels of >99% water content. The peptide sequences can be engineered to alter the underlying mechanism of gelation and, consequently, the hydrogel properties. Interestingly, for example, those with hydrogen-bonded networks of fibrils melt on heating, whereas those formed through hydrophobic fibril-fibril interactions strengthen when warmed. The hSAFs are dual-peptide systems that gel only on mixing, which gives tight control over assembly. These properties raise possibilities for using the hSAFs as substrates in cell culture. We have tested this in comparison with the widely used Matrigel substrate, and demonstrate that, like Matrigel, hSAFs support both growth and differentiation of rat adrenal pheochromocytoma cells for sustained periods in culture.

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Rational design and application of responsive α-helical peptide hydrogels

We demonstrate a generic new approach to produce homogeneous and reproducible hydrogels from low molecular weight hydrogelators using the controlled hydrolysis of glucono-δ-lactone (GdL). GdL slowly hydrolyses in water to give gluconic acid, which controllably lowers the pH. This hydrolysis is slower than the rate of dissolution; hence uniform pH change throughout the sample is possible. This results in homogeneous hydrogels that are unaffected by their shear or mixing history. A further advantage of this method is that it allows the gelation process to be monitored, giving further insight into the mechanism by which gelation occurs.

A new method for maintaining homogeneity during liquid–hydrogel transitions using low molecular weight hydrogelators

Using a range of complementary experiments, a detailed investigation into the behaviour of air-in-water foams stabilised by a mixture of silica nanoparticles and pure cationic surfactant has been made. At high pH where particles are significantly negatively charged and surfactant is positively charged, no foam is possible with particles alone whereas surfactant-stabilised foams break down completely within one day at all concentrations. In particle–surfactant mixtures, a synergism occurs with respect to foam formation and stability due to the adsorption of surfactant molecules onto particle surfaces. The foamability of mixed dispersions is substantially reduced compared with surfactant solutions alone. However, the foam stability passes through a maximum with respect to surfactant concentration and these foams are remarkably stable. Based on our findings from dispersion stability measurements, particle ζ potentials, the adsorption isotherm of surfactant on particles and relevant contact angles of water in air on silica surfaces, we conclude that foams are most stable when particles are strongly flocculated corresponding to them possessing a low charge, being maximally hydrophobic and containing an adsorbed monolayer of surfactant. Cryo-scanning electron microscopy (cryo-SEM) analysis of the same foams leads us to propose that foam stabilisation changes from being surfactant dominated at low surfactant concentration to being particle dominated at intermediate concentrations and reverting to surfactant dominated at higher concentrations.

Origin of stabilisation of aqueous foams in nanoparticle–surfactant mixtures

Hydrogels can be prepared using the commercially available Fmoc-phenylalanine or Fmoc-tyrosine as the gelator. Gelation is triggered by careful adjustment of the pH of the solution using glucono-delta-lactone (GdL). Model dyes have been entrapped in the hydrogels, and the release of the dyes from the hydrogels has been monitored. The release ratios indicate that the systems are under Fickian diffusion control. A range of dyes with different radii of gyration diffuse from the Fmoc-phenylalanine hydrogels with similar diffusion coefficients, implying that the network is not specifically retaining even relatively large (5 nm) dyes. On the other hand, the larger dyes are restricted in their diffusion from Fmoc-tyrosine hydrogels. These results correlate with the rheological measurements for the hydrogels, where those formed from Fmoc-tyrosine were shown to have significantly higher storage moduli than those formed from Fmoc-phenylalanine. In addition, the frequency-dependent behavior of the hydrogels demonstrates that Fmoc-tyrosine shows the classic response of a strong gel with a storage modulus that is nearly independent of frequency. However, for Fmoc-phenylalanine, the frequency dependence of moduli is very strong and very similar to that displayed by a transient network, where the interconnections between junction zones in the network are highly flexible and able to withstand large deformations.

Controlled release from modified amino acid hydrogels governed by molecular size or network dynamics.

The flow behavior of particle stabilized oil-in-water emulsions with different dispersed volume fractions was analyzed in steady shear on a rotational rheometer employing a coaxial cylinder geometry. The dispersed phase of the emulsion was a mixture of equal volumes of a polar oil, isopropyl myristate, and a nonpolar oil, dodecane. The continuous phase was an aqueous suspension of hydrophilic colloidal silica particles of 8nm diameter with the pH adjusted to pH2 in order to stabilize the emulsion [Binks and Whitby, Colloids Surf., A 253, 105–115 (1995)]. Droplet diameters were of the order of a few micrometers, and droplet surfaces apparently show dense particle coverage. We show that the markedly different interfacial structure in particle stabilized emulsions when compared to surfactant stabilized emulsions is reflected in the rheological behavior. To illustrate these differences, the rheological behavior of a comparable surfactant stabilized emulsion with the particles in the aqueous phase replaced by ...

Mark Kirkland

Papers

Rational design and application of responsive α-helical peptide hydrogels

A new method for maintaining homogeneity during liquid–hydrogel transitions using low molecular weight hydrogelators

Origin of stabilisation of aqueous foams in nanoparticle–surfactant mixtures

Controlled release from modified amino acid hydrogels governed by molecular size or network dynamics.

Shear thickening of an emulsion stabilized with hydrophilic silica particles