Self-assembly of small peptide amphiphiles, the structures formed and their applications. (A foods and home and personal care perspective).

In this opinion piece, some specific challenges in the field of peptide self-assembly and gel formation are discussed. One major hurdle to finding functional small peptides is that there are a huge number of compounds to explore, which increases exponentially with the peptide size. This in itself creates a barrier to the discovery and application of materials, both through the difficulty of finding the peptides, and because protecting inventions also becomes more difficult. Recent work has shown that computer simulations may provide us a route to explore such a huge compound space; this is discussed along with the prospect for future developments. At the microscopic scale, many fibril-forming peptides form gels, apparently through a process of lateral association of primary self-assembled filaments, which leads to a relatively coarse-grained structure of rigid interconnects. However, recent data obtained on Fmoc-tyrosine gels appear to indicate that the gel microstructure is both more flexible and finer grained than previously believed. As such, it is clear that there is a considerable amount that is still not understood regarding this class of gel.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Hierarchical self-assembly of chiral fibres from achiral particles.

We investigate, by molecular dynamics simulation, the behaviour of discotic particles in a solvent of Lennard-Jones spheres. When chromonic disc-sphere interactions are imposed on these systems, three regimes of self-assembly are observed. At moderate temperatures, numerous short threads of discs develop, but these threads remain isolated from one another. Quenching to low temperatures, alternatively, causes all of the discs to floc into a single extended aggregate which typically comprises several distinct sections and contains numerous packing defects. For a narrow temperature range between these regimes, however, defect-free chiral fibres are found to freely self-assemble. The spontaneous chirality of these fibres results from frustration between the hexagonal packing and interdigitation of neighbouring threads, the pitch being set by the particle shape. This demonstration of aggregate-wide chirality emerging owing to packing alone is pertinent to many biological and synthetic hierarchically self-assembling systems.

Interfacial tension in aqueous biopolymer-surfactant mixtures.

The current study offers a first insight into the interfacial properties of pullulan-sodium dodecyl sulphate (SDS) aqueous two-phase systems (ATPS) in the presence of sodium chloride (NaCl). The effect of composition on the interfacial tension (sigma) in these ATPS was investigated over a wide range of pullulan, SDS and NaCl concentrations. An increase in the interfacial tension was observed with increasing pullulan and SDS concentrations and a small increase was also observed as the NaCl concentration was increased. In both cases the interfacial tension increases were closely related to the phase behaviour of these systems; as a consequence of increasing the pullulan, SDS and/or NaCl concentrations, the system moves further away from the critical point. In all systems interfacial tensions (of the order of muN/m) were comparable with those reported for polymer-polymer ATPS. Interfacial tensions sigma can be well correlated with the difference in pullulan and SDS concentrations between the phases (DeltaC pul and DeltaC SDS) and also the tie-line length (TLL); all yield straight lines on a log-log scale.

Interfacial tension in phase-separated gelatin/dextran aqueous mixtures.

The effect of solute concentrations on interfacial tension was investigated in phase-separated mixtures of dextran and gelatin over a range of concentrations that covered different tie-lines and different positions on one tie-line. The investigations were carried out using equilibrated gelatin-rich and dextran-rich phases in a computer-controlled Couette device at 40 degrees C (above the gelation point of gelatin) and interfacial tensions were measured using the retracting drop method. The results show that the interfacial tension can be related to the length of the tie-line or to the difference in the concentration of dextran (or gelatin) in the separated phases. Interfacial tension increases as either of these parameters increases. For concentrations lying on any single tie-line, the interfacial tension is constant and independent of the concentration of biopolymers. Also, the addition of small amounts of low molecular weight dextran to a dextran-rich phase does not significantly affect the interfacial tension between the gelatine-rich and dextran-rich phases. Experimental results were also compared with theoretical predictions of the interfacial tension using a Flory-Huggins based analysis of the measured tie-line data. Reasonable agreement was found between predicted and measured values, indicating that this approach captures the basic physics of the system.

Dynamic experimentation on the confocal laser scanning microscope: application to soft-solid, composite food materials.

Confocal laser scanning microscopy (CLSM) is used to follow the dynamic structural evolution of several phase-separated mixed biopolymer gel composites. Two protein/polysaccharide mixed gel systems were examined: gelatin/maltodextrin and gelatin/agarose. These materials exhibit 'emulsion-like' structures, with included spherical particles of one phase (i.e. polymer A) within a continuous matrix of the second (i.e. polymer B). Compositional control of these materials allows the phase order to be inverted (i.e. polymer B included and polymer A continuous), giving four basic variants for the present composites. Tension and compression mechanical tests were conducted dynamically on the CLSM, with crack/microstructure interactions investigated using a notched compact tension geometry. Gelatin/maltodextrin composites exhibit a 'pseudo-yielding' stress/strain response in both tension and compression, when the gelatin-rich phase is continuous, which was attributed to debonding of the particle/matrix interface. This behaviour is significantly less apparent for both the gelatin/agarose composites, and the maltodextrin continuous gelatin/maltodextrin composites, with these materials responding in a nominally linear elastic manner. Values of the interfacial fracture energy for selected compositions of the two biopolymer systems were determined by 90 degrees peel testing, where a gelatin layer was peeled from either a maltodextrin or agarose substrate. For biopolymer layers 'cast' together, a value of 0.2 +/- 0.2 J m-2 was obtained for the fracture energy of a gelatin/maltodextrin interface, while a significantly higher value of 6.5 +/- 0.2 J m-2 was determined for a gelatin/agarose interface. The interfacial fracture energy of the two mixed systems was also determined following an indirect elastomer composite debonding model. An interfacial fracture energy of approximately 0.25 J m-2 was determined using this approach for the gelatin continuous gelatin/maltodextrin composite, which compares favourably with the value calculated directly by peel testing (i.e. approximately 0.2 J m-2). A somewhat higher value was estimated for the gelatin continuous gelatin/agarose system (1.0-2.0 J m-2), using this model, although there are severe limitations to this approach for this mixed gel system. In the present case, it is believed that the differing mechanical response of the two mixed biopolymer systems, when the gelatin phase is continuous, arises from the order of magnitude difference in interfacial fracture energy. It is postulated that polymer interdiffusion may occur across the interface for the gelatin/agarose system, to a significantly greater extent than for interfaces between gelatin and maltodextrin, resulting in a higher interfacial fracture energy.

The relative effectiveness of a hidden versus a visible speed camera programme.

Overtly operated mobile speed cameras have been used in New Zealand since late 1993. Their operation has been confined to specific sites (called 'speed camera areas') which are mainly road sections with a record of speed-related crashes. A trial of hidden speed cameras began in mid-1997 in 100 km/h speed limit areas in one of New Zealand's four Police regions. This trial was still in progress at the time of writing and the current paper reports the results of an evaluation of the first year of the trial. During that period, the hidden cameras and related publicity were found (compared with the generally highly visible speed camera enforcement in the rest of New Zealand) to be associated with net falls in speeds, crashes and casualties both in speed camera areas and on 100 km/h speed limit roads generally. There were initial changes in public attitudes in response to the programme that later largely reverted to pre-trial levels. Compared with the localised effect of visible cameras on speeds and crashes mainly in speed camera areas, the hidden cameras had a more general effect on all roads. As further crash, speed and attitude data become available, the longer-term effects of the hidden camera programme will be evaluated.

Dynamic experimentation on the confocal laser scanning microscope: application to soft-solid, composite food materials.

Confocal laser scanning microscopy (CLSM) is used to follow the dynamic structural evolution of several phase-separated mixed biopolymer gel composites. Two protein/polysaccharide mixed gel systems were examined: gelatin/maltodextrin and gelatin/agarose. These materials exhibit 'emulsion-like' structures, with included spherical particles of one phase (i.e. polymer A) within a continuous matrix of the second (i.e. polymer B). Compositional control of these materials allows the phase order to be inverted (i.e. polymer B included and polymer A continuous), giving four basic variants for the present composites. Tension and compression mechanical tests were conducted dynamically on the CLSM, with crack/microstructure interactions investigated using a notched compact tension geometry. Gelatin/maltodextrin composites exhibit a 'pseudo-yielding' stress/strain response in both tension and compression, when the gelatin-rich phase is continuous, which was attributed to debonding of the particle/matrix interface. This behaviour is significantly less apparent for both the gelatin/agarose composites, and the maltodextrin continuous gelatin/maltodextrin composites, with these materials responding in a nominally linear elastic manner. Values of the interfacial fracture energy for selected compositions of the two biopolymer systems were determined by 90 degrees peel testing, where a gelatin layer was peeled from either a maltodextrin or agarose substrate. For biopolymer layers 'cast' together, a value of 0.2 +/- 0.2 J m-2 was obtained for the fracture energy of a gelatin/maltodextrin interface, while a significantly higher value of 6.5 +/- 0.2 J m-2 was determined for a gelatin/agarose interface. The interfacial fracture energy of the two mixed systems was also determined following an indirect elastomer composite debonding model. An interfacial fracture energy of approximately 0.25 J m-2 was determined using this approach for the gelatin continuous gelatin/maltodextrin composite, which compares favourably with the value calculated directly by peel testing (i.e. approximately 0.2 J m-2). A somewhat higher value was estimated for the gelatin continuous gelatin/agarose system (1.0-2.0 J m-2), using this model, although there are severe limitations to this approach for this mixed gel system. In the present case, it is believed that the differing mechanical response of the two mixed biopolymer systems, when the gelatin phase is continuous, arises from the order of magnitude difference in interfacial fracture energy. It is postulated that polymer interdiffusion may occur across the interface for the gelatin/agarose system, to a significantly greater extent than for interfaces between gelatin and maltodextrin, resulting in a higher interfacial fracture energy.

Cycle helmet effectiveness in New Zealand.

Since late 1989, the cycle helmet wearing rate in New Zealand has risen from around 20% for adults and teenagers, and 40% for younger children, to more than 90% in all age groups. Cycle helmet wearing became mandatory under New Zealand law in January 1994. This paper considers the effect of cycle helmet wearing on hospitalised head injuries between 1990 and 1996, using cyclist limb injuries as a measure of exposure to the risk of cycling trauma. Non-motor vehicle crashes were treated separately from those involving a motor vehicle. Non-motor vehicle crashes were further subdivided by age group. Cyclist head injuries decreased with increasing helmet wearing rates for non-motor vehicle crashes in all age groups, and for motor vehicle crashes. For an increase of 5 percentage points in the helmet wearing rate, the corresponding decreases in head injuries in non-motor vehicle accidents were estimated to be 10.2, 5.3 and 3.2% for children of primary school age (5-12 years), secondary school age (13-18 years), and adults respectively. The corresponding decrease in hospitalisations for motor vehicle crashes was 3.6%. All results were significant at the 95% level. The relatively large increase in helmet wearing associated with the passing of a compulsory helmet wearing law in 1994 reduced head injuries by between 24 and 32% in non-motor vehicle crashes, and by 20% in motor vehicle crashes. No increase or decrease in the severity of head injuries for which cyclists were hospitalised over this period could be detected. This may have been due to the small and highly variable number of 'high severity' injuries.

Rapid MRI and velocimetry of cylindrical Couette flow.

A narrow-gap, temperature-controlled Couette flow rheometer has been developed to study fluid velocities within the annular gap between two concentric cylinders by nuclear magnetic resonance (NMR) imaging and velocimetry. Alternative pulsed-field-gradient-based nuclear magnetic resonance imaging strategies which may be used for measurement of velocity within the Couette flow device have been evaluated. These include two-dimensional (2-D) imaging techniques with acquisition times of several minutes and a one-dimensional (1-D) projection method which exploits the symmetry of the device to reduce overall measurement time to less than 1 min. Velocity measurements made using each technique are presented for a Newtonian fluid undergoing Couette flow at shear rates of approximately 20 and 60 s(-1).

The Grand Rapids dip revisited.

This analysis addresses an issue that has concerned road safety authorities for some 28 years: the celebrated "Grand Rapids Dip." This, most readers will recognise, is the below-baseline excursion, which occurs in the blood alcohol concentration (BAC) interval of .01%-.04%, of the relative risk curve for accident risk versus blood alcohol, derived from the 1964 Grand Rapids Study data. The present analysis has its starting place in the explanation advanced by Allsop, who noted that the case/control comparisons were biased due to the disproportionate representation of demographic subgroups in different blood alcohol concentration class intervals. Indeed, when relative-risk curves are derived separately for subgroups of differing drinking habits, the resulting separate risk curves all show monotonic increases at all blood alcohol concentration ranges. Such separate relative risk curves are unpopular, and most of the road safety community pays them little heed. Thus, the original concept of the "dip" remains with us. For this reason, we have derived, using a simple but realistic statistical model, a single relative-hazard curve from the Grand Rapids data, one that is free from the distortion introduced by unequal representation of different demographic subgroups in different blood alcohol concentration class intervals. This curve indicates that accident risk increases with increased blood alcohol concentration regardless of self-reported drinking frequency. However more frequent drinkers have less risk at all blood alcohol concentration levels, including zero, than less frequent drinkers at the times and places sampled.

Pedestrian exposure and the risk of child pedestrian injury.

The authors used data from the New Zealand Household Travel Survey to examine the extent to which children's pedestrian exposure varies with age, sex and household income. Pedestrian injury morbidity data were combined with pedestrian exposure data to examine age-specific pedestrian injury risk. The annual number of road crossings for girls was greater than that for boys. Pedestrian exposure increased with increasing age. Children aged 5-9 years in the lowest household income bracket crossed approximately 50% more roads than those in the middle and upper income brackets. However, for children aged 10-14 years there was little variation with household income. Sex differences in pedestrian injury rates cannot be explained by differences in exposure although increased exposure may partly explain the increased injury rates for children in lower socio-economic groups. Prevention strategies might aim to reduce pedestrian exposure for alternatively to reduce pedestrian injury risk per unit of exposure by making safer urban living environments.

Intersection control by stop and give way signs--the conclusions of polus.

Polus (1985) after studying accidents and gap acceptance data concluded that replacing give way (yield) signs with stop signs tended to increase vehicle accidents and reduce pedestrian accidents. This paper expresses the authors' reservations about these conclusions, makes suggestions for further study and quotes the results of a study of a sample of New Zealand urban intersections where give way signs have been replaced with stop signs. The results though not statistically significant estimate a favourable effect on accident rates and indicate that any important advantage from give way signs is unlikely.

The safety implications of some control changes at urban intersections.

Injury accident data for a sample of New Zealand urban intersections were analysed, looking for an increase or decrease which coincided with a change in control. Increases or decreases were also looked for in certain types of accidents and in accident severity. The control changes considered were from give way signs, stop signs and no control to signals and small roundabouts and also from no control to give way and stop signs.

Accidents and injuries in child vehicle occupants in New Zealand.

Records of reported child vehicle occupant fatalities and injuries were examined. Child vehicle occupants accounted for approximately 5 percent of all road injuries and deaths in 1979, with an approximate hospital cost of one million dollars per year. It is thus worth spending considerable sums of money on countermeasures. The problem of parents' perception of risk is illustrated by the observation that annually only one child in 1 000 is injured and one in 50 000 dies as an occupant of a vehicle. International comparisons show New Zealand child occupant accident rates per 100 000 population to be of the same order as those of a selection of other developed countries.