Peter C. Griffiths

Tuneable mechanical properties in low molecular weight gels

The mechanical properties of gels are critical to the final targeted applications. Depending on the application, different properties may be required. Here, we show that the mechanical strength and ability to recover of gels formed using a low molecular weight gelator can be controlled by two independent factors (i) the volume fraction of co-solvent (in this case DMSO) in the system and (ii) the temperature cycle used. These differences correlate with the large scale structure of the network that is formed from the self-assembled fibres. This opens up the potential to prepare gels with very different properties at the same final conditions, allowing the effect of microstructure to be probed.

Nanoparticles decorated with proteolytic enzymes, a promising strategy to overcome the mucus barrier.

The intestinal mucus gel layer represents a stumbling block for drug adsorption. This study is aimed to formulate a nanoparticulate system able to overcome this barrier by cleaving locally the glycoprotein substructures of the mucus. Mucolytic enzymes such as papain (PAP) and bromelain (BRO) were covalently conjugated to poly(acrylic acid) (PAA). Nanoparticles (NPs) were then formulated via ionic gelation method and characterized by particle size, zeta potential, enzyme content and enzymatic activity. The NPs permeation quantified by rotating tube studies was correlated with changes in the mucus gel layer structure determined by pulsed-gradient-spin-echo NMR (PGSE-NMR), small-angle neutron scattering (SANS) and spin-echo SANS (SESANS). PAP and BRO functionalized NPs had an average size in the range of 250 and 285 nm and a zeta potential that ranged between -6 and -5 mV. The enzyme content was 242 μg enzyme/mg for PAP modified NPs and 253 μg enzyme/mg for BRO modified NPs. The maintained enzymatic activity was 43% for PAP decorated NPs and 76% for BRO decorated NPs. The rotating tube technique revealed a better performance of BRO decorated NPs compared to PAA decorated NPs, with a 4.8-fold higher concentration of NPs in the inner slice of mucus. Addition of 0.5 wt% of enzyme functionalized NPs to 5 wt% intestinal mucin led to c.a. 2-fold increase in the mobility of the mucin as measured by PGSE-NMR indicative of a significant break-up of the structure of the mucin. SANS and SESANS measurements further revealed a change in structure of the intestinal mucus induced by the incorporation of the functionalized NPs mostly occurring at a length scale longer than 0.5 μm. Accordingly, BRO decorated NPs show higher potential than PAP functionalized NPs as mucus permeating drug delivery systems.

Architecture in the microcosm: biocolloids, self-assembly and pattern formation

Complex microscopic structure is a common feature in biology; the mineral shells of single–celled aquatic plants and animals such as diatoms, coccolithophores, radiolaria, the organic coatings of pollen grains and the surfaces of many seeds are all familiar examples. To the human eye, viewing this exquisite complexity, the method of construction is often far from obvious. Operating on the microscopic scale, at the size range called the colloidal dimension by synthetic chemists, is a gamut of interactions between the various components, which in many cases can lead to the formation of complex structure as an entropically favourable process. The importance of these ‘colloidal interactions’ is becoming increasingly apparent to biologists seeking the link between the genetic basis of structure and its ultimate expression. It is an emerging theme that through the evolutionary history of life, self–assembly of structure from colloidal building blocks has become integral to the process of organismal development. Colloidal interactions, however, are themselves complex. Chemists therefore tend to restrict the number and diversity of components within any system being studied in order to minimize this complexity. The interactions of spherical polystyrene particles in an aqueous or organic fluid, for example, have been well documented. The introduction of a third component into such a system clearly increases the diversity of interaction (and concomitantly, the difficulty of interpretation). Yet such a system is unrealistically simple to the biologist! The investigation of the behaviour of mixed colloidal systems is essential in the formulation of concepts regarding microscopic structural development in order to further both our understanding of biological construction, and to give rise to new developments in microscopic materials technology. Here we assess the developments in the understanding of colloidal systems in microscopic biological construction and demonstrate how these have given rise to new concepts regarding the relationships and evolution of the gene and organismal structure. We show how development of these new concepts may give rise to new materials with properties that have been tried and tested by organisms over millions of years of evolution and which, by their very nature, are more compatible with humans and their environment. We suggest how self–assembling microstructure might be used in the development of new surface coatings and drug delivery mechanisms.

Nanoparticle diffusion within intestinal mucus: Three-dimensional response analysis dissecting the impact of particle surface charge, size and heterogeneity across polyelectrolyte, pegylated and viral particles.

Multiple particle tracking (MPT) methodology was used to dissect the impact of nanoparticle surface charge and size upon particle diffusion through freshly harvested porcine jejunum mucus. The mucus was characterised rheologically and by atomic force microscopy. To vary nanoparticle surface charge we used a series of self-assembly polyelectrolyte particles composed of varying ratios of the negatively charged polyacrylic acid polymer and the positively charged chitosan polymer. This series included a neutral or near-neutral particle to correspond to highly charged but near-neutral viral particles that appear to effectively permeate mucus. In order to negate the confounding issue of self-aggregation of such neutral synthetic particles a sonication step effectively reduced particle size (to less than 340 nm) for a sufficient period to conduct the tracking experiments. Across the polyelectrolyte particles a broad and meaningful relationship was observed between particle diffusion in mucus (×1000 difference between slowest and fastest particle types), particle size (104-373 nm) and particle surface charge (-29 mV to +19.5 mV), where the beneficial characteristic promoting diffusion was a neutral or near-neutral charge. The diffusion of the neutral polyelectrolyte particle (0.02887 cm S(-1)×10(-9)) compared favourably with that of a highly diffusive PEGylated-PLGA particle (0.03182 cm(2) S(-1)×10(-9)), despite the size of the latter (54 nm diameter) accommodating a reduced steric hindrance with the mucin network. Heterogeneity of particle diffusion within a given particle type revealed the most diffusive 10% sub-population for the neutral polyelectrolyte formulation (5.809 cm(2) S(-1)×10(-9)) to be faster than that of the most diffusive 10% sub-populations obtained either for the PEGylated-PLGA particle (4.061 cm(2) S(-1)×10(-9)) or for a capsid adenovirus particle (1.922 cm(2) S(-1)×10(-9)). While this study has used a simple self-assembly polyelectrolyte system it has substantiated the pursuance of other polymer synthesis approaches (such as living free-radical polymerisation) to deliver stable, size-controlled nanoparticles possessing a uniform high density charge distribution and yielding a net neutral surface potential. Such particles will provide an additional strategy to that of PEGylated systems where the interactions of mucosally delivered nanoparticles with the mucus barrier are to be minimised.

Methods to determine the interactions of micro- and nanoparticles with mucus

The present review provides an overview of methods and techniques for studying interactions of micro- and nanoparticulate drug delivery system with mucus. Nanocarriers trapped by mucus are featuring a change in particle size and zeta potential that can be utilized to predict their mucus permeation behavior. Furthermore, interactions between nanoparticulate drug delivery systems and mucus layer modify the viscoelasticity of mucus which can be detected via rheological studies and quartz crystal microbalance with dissipation monitoring (QCM-D) analysis. Having a closer look at molecular interactions between drug carrier and mucus small-angle neutron scattering (SANS) is an appropriate analysis technique. Moreover, different methods to determine particle diffusion in mucus such as the newly established Transwell diffusion system, rotating silicone tube technique, multiple-particle tracking (MPT) and diffusion NMR are summarized within this review. The explanations and discussed pros and cons of collated methods and techniques should provide a good starting point for all those looking forward to move in this interesting field.

Aggregate Properties of Sodium Deoxycholate and Dimyristoylphosphatidylcholine Mixed Micelles

Mixed micelles of the phospholipid dimyristoylphosphatidylcholine (DMPC) and bile salts of sodium deoxycholate (NaDC) were investigated by a combination of techniques, including time-resolved fluorescence quenching (TRFQ), electron spin resonance (ESR), viscometry, pulsed-gradient spin-echo NMR (PGSE-NMR), and surface tensiometry. Aggregation numbers, and bimolecular collision rate constants of guest molecules confined in the micelles (by TRFQ), interfacial hydration index and microviscosity, (by ESR), axial ratio (from solution viscosity), micelle self-diffusion coefficient (by PGSE-NMR), and the critical micelle concentrations (from surface tension) were determined for various molar compositions defined by the ratio R identical with [NaDC]/[DMPC] and concentrations ([NaDC]+[DMPC]). The data interpretation showed the micelles to be polydisperse rods. Aggregate properties depend on the ratio, R and reveal behavior unlike that in micelles of surfactants with aliphatic nonpolar chains. With increase in concentration from [NaDC] = 0.010 M to [NaDC] = 0.200 M, the hydration index and the aggregation number exhibit non-monotonic variations. Formulation of a polar shell model for cylindrical micelles yielded a set of nonlinear equations for the structural features of the micelle. The solutions give the microstructural description of the mixed micelle that includes the length, diameter, number of water molecules in the hydration shell, and the monomer organization in the micelle.

A multi-technique approach for probing the evolution of structural properties during crystallization of organic materials from solution

We are engaged in a multidisciplinary study of fundamental aspects of the crystallization of organic molecular materials from solution, focusing on polymorphic systems under the recognition that such systems represent an ideal opportunity for obtaining a systematic understanding of competing pathways in crystallization processes. The range of techniques employed in this work are sensitive to structural properties on different length scales and are thus appropriate for mapping the changes that occur at different stages of the crystallization process, starting from the early aggregation events in solution (probed by solution-state NMR and molecular dynamics simulations, including studies of diffusion properties), leading to the growth of molecular aggregates (probed by small-angle neutron scattering), then the emergence of solid microcrystals dispersed in the crystallization solution (probed by small-angle neutron scattering and solid-state NMR) and finally the formation of the bulk solid crystalline phase (probed by powder X-ray diffraction). This paper reports preliminary results on the application of this multi-technique approach to study the crystallization of glycine (which has three known polymorphic forms under ambient conditions) from aqueous solution.

Locus-specific microemulsion catalysts for sulfur mustard (HD) chemical warfare agent decontamination.

The rates of catalytic oxidative decontamination of the chemical warfare agent (CWA) sulfur mustard (HD, bis(2-chlororethyl) sulfide) and a range (chloroethyl) sulfide simulants of variable lipophilicity have been examined using a hydrogen peroxide-based microemulsion system. SANS (small-angle neutron scattering), SAXS (small-angle X-ray scattering), PGSE-NMR (pulsed-gradient spin-echo NMR), fluorescence quenching, and electrospray mass spectroscopy (ESI-MS) were implemented to examine the distribution of HD, its simulants, and their oxidation/hydrolysis products in a model oil-in-water microemulsion. These measurements not only present a means of interpreting decontamination rates but also a rationale for the design of oxidation catalysts for these toxic materials. Here we show that by localizing manganese-Schiff base catalysts at the oil droplet-water interface or within the droplet core, a range of (chloroethyl) sulfides, including HD, spanning some 7 orders of octanol-water partition coefficient (K(ow)), may be oxidized with equal efficacy using dilute (5 wt. % of aqueous phase) hydrogen peroxide as a noncorrosive, environmentally benign oxidant (e.g., t(1/2) (HD) approximately 18 s, (2-chloroethyl phenyl sulfide, C(6)H(5)SCH(2)CH(2)Cl) approximately 15 s, (thiodiglycol, S(CH(2)CH(2)OH)(2)) approximately 19 s {20 degrees C}). Our observations demonstrate that by programming catalyst lipophilicity to colocalize catalyst and substrate, the inherent compartmentalization of the microemulsion can be exploited to achieve enhanced rates of reaction or to exert control over product selectivity. A combination of SANS, ESI-MS and fluorescence quenching measurements indicate that the enhanced catalytic activity is due to the locus of the catalyst and not a result of partial hydrolysis of the substrate.

Aqueous solutions of transition metal containing micelles

Incorporation of d- or f-block metals into ligand systems that renders a metal complex surface-active or drives its partitioning into surfactant phases enables the localisation of chemical functionality at interfaces. This article discusses a number of fundamental aspects of these interesting materials and examines potential applications.

Evaluation of the physical and biological properties of hyaluronan and hyaluronan fragments

Hyaluronan (HA) has been extensively used for various medical applications, including osteoarthritis, tissue augmentation and ocular surgery. More recently, it has been investigated for use in polymer therapeutics as a carrier for drugs and biologically active proteins, thanks to its biodegradability, biocompatibility and inherent biological properties. Such biological functions are strongly dependent on HAs chain length, yet the molecular weight of HAs used in polymer conjugates varies widely and is inconsistent with its intended application. Therefore, this study aimed to determine the ideal chain length of HA to be used in polymer conjugates for enhanced tissue repair. HA fragments (M(w) 45,000-900,000g/mol) were prepared by acid hydrolysis of rooster comb HA and their physicochemical and biological properties were characterized. Such HA fragments had a highly extended, almost rod-like solution conformation and demonstrated chain length- and concentration-dependent viscosity, while exposure to HAase caused a rapid reduction in HA viscosity, which was most significant for the native HA. Initial HA hydrolysis rate by HAase varied strongly with HA chain length and was dependent on the formation of a stable enzyme-substrate complex. When normal human dermal fibroblasts were exposed to the different HA fragments for 72h, only native (900,000g/mol) HA reduced proliferation at 1000μg/mL. Conversely, only the smallest HA fragment (70,000g/mol) reduced the proliferation of chronic wound fibroblasts, at 1000μg/mL. The 70,000g/mol HA fragment also promoted the greatest cell attachment. These observations demonstrate that low molecular weight (70,000-120,000g/mol) HA fragments would be best suited for the delivery of proteins and peptides with applications in chronic wound healing and paves the way for the rationalized development of novel HA conjugates.