Temporal development and collapse of an Arctic plant-pollinator network

<p>Abstract</p> <p>Background</p> <p>The temporal dynamics and formation of plant-pollinator networks are difficult to study as it requires detailed observations of how the networks change over time. Understanding the temporal dynamics might provide insight into sustainability and robustness of the networks and how they react to environmental changes, such as global warming. Here we study an Arctic plant-pollinator network in two consecutive years using a simple mathematical model and describe the temporal dynamics (daily assembly and disassembly of links) by random mechanisms.</p> <p>Results</p> <p>We develop a mathematical model with parameters governed by the probabilities for entering, leaving and making connections in the network and demonstrate that A. The dynamics is described by very similar parameters in both years despite a strong turnover in the composition of the pollinator community and different climate conditions, B. There is a drastic change in the temporal behaviour a few days before the end of the season in both years. This change leads to the collapse of the network and does not correlate with weather parameters, C. We estimate that the number of available pollinator species is about 80 species of which 75-80% are observed in each year, D. The network does not reach an equilibrium state (as defined by our model) before the collapse set in and the season is over.</p> <p>Conclusion</p> <p>We have shown that the temporal dynamics of an Arctic plant-pollinator network can be described by a simple mathematical model and that the model allows us to draw biologically interesting conclusions. Our model makes it possible to investigate how the network topology changes with changes in parameter values and might provide means to study the effect of climate on plant-pollinator networks.</p

Lubrication by biomacromolecules: mechanisms and biomimetic strategies

Biomacromolecules play a key role in protecting human biointerfaces from friction and wear, and thus enable painless motion. Biomacromolecules give rise to remarkable tribological properties that researchers have been eager to emulate. In this review, we examine how molecules such as mucins, lubricin, hyaluronic acid and other components of biotribological interfaces provide a unique set of rheological and surface properties that leads to low friction and wear. We then highlight how researchers have used some of the features of biotribological contacts to create biomimetic systems. While the brush architecture of the glycosylated molecules present at biotribological interfaces has inspired some promising polymer brush systems, it is the recent advance in the understanding of synergistic interaction between biomacromolecules that is showing the most potential in producing surfaces with a high lubricating ability. Research currently suggests that no single biomacromolecule or artificial polymer successfully reproduces the tribological properties of biological contacts. However, by combining molecules, one can enhance their anchoring and lubricating capacity, thus enabling the design of surfaces for use in biomedical applications requiring low friction and wear

Tribology and its growing use towards the study of food oral processing and sensory perception

Here we provide a comprehensive review of the knowledge base of soft tribology, the study of friction, lubrication and wear on deformable surfaces, with consideration for its application towards oral tribology and food lubrication. Studies on 'soft-tribology' have emerged to provide knowledge and tools to predict oral behaviour and assess the performance of foods and beverages. We have shown that there is a now a comprehensive set of fundamental literature, mainly based on soft contacts in the Mini-traction machine with rolling ball on disk configuration, which provides a baseline for interpreting tribological data from complex food systems. Tribology-sensory relationships do currently exist. However, they are restricted to the specific formulations and tribological configuration utilised, and cannot usually be applied more broadly. With a careful and rigorous formulation/experimental design, we envisage tribological tools to provide insights into the sensory perception of foods in combination with other in vitro techniques such as rheology, particle sizing or characterisation of surface interactions. This can only occur with the use of well characterised tribopairs and equipment; a careful characterisation of simpler model foods before considering complex food products; the incorporation of saliva in tribological studies; the removal of confounding factors from the sensory study and a global approach that considers all regimes of lubrication

Responsive polysaccharide-grafted surfaces for biotribological applications

The elucidation of biolubrication mechanisms and the design of artificial biotribological contacts requires the development of model surfaces that can help to tease out the cues that govern friction in biological systems. Polysaccharides provide an interesting option as a biotribological mimic due to their similarity with the glycosylated molecules present at biointerfaces. Here, pectin was successfully covalently grafted at its reducing end to a polydimethylsiloxane (PDMS) surface via a reductive amination reaction. This method enabled the formation of a wear resistant pectin layer that provided enhanced boundary lubrication compared to adsorbed pectin. Pectins with different degrees of methylesterification and blockiness were exposed to salt solutions of varying ionic strength and displayed responsiveness to solvent conditions. Exposure of the grafted pectin layers to solutions of between 1 and 200 mM NaCl resulted in a decrease in boundary friction and an increase in the hydration and swelling of the pectin layer to varying degrees depending on the charge density of the pectin, showing the potential to tune the conformation and friction of the layer using the pectin architecture and environmental cues. The robust and responsive nature of these new pectin grafted surfaces makes them an effective mimic of biotribological interfaces and provides a powerful tool to study the intricate mechanisms involved in the biolubrication phenomenon

Oral tribology: Bridging the gap between physical measurements and sensory experience

Soft-tribology is emerging as an important field of research for quantifying the physics occurring during oral tribological processes. In a food oral processing context, recent research indicates that tribology measurements are providing insights into several texture-related sensory percepts, but obtaining quantitative empirical relationships between the two is challenging. Choosing a physiologically relevant tribological 'system' is paramount to the successful application of tribology as an indicator for texture perception; the choice of instrument, surfaces and model food system, as well as the role of saliva, should be carefully considered. Tribology and sensory perception are affected by multiple physico-chemical properties, and therefore integrative approaches that combine tribology with other characterisation techniques are necessary for mechanistic understanding on their inter-relationship

Lubrication by biomacromolecules: mechanisms and biomimetic strategies

Biomacromolecules play a key role in protecting human biointerfaces from friction and wear, and thus enable painless motion. Biomacromolecules give rise to remarkable tribological properties that researchers have been eager to emulate. In this review, we examine how molecules such as mucins, lubricin, hyaluronic acid and other components of biotribological interfaces provide a unique set of rheological and surface properties that leads to low friction and wear. We then highlight how researchers have used some of the features of biotribological contacts to create biomimetic systems. While the brush architecture of the glycosylated molecules present at biotribological interfaces has inspired some promising polymer brush systems, it is the recent advance in the understanding of synergistic interaction between biomacromolecules that is showing the most potential in producing surfaces with a high lubricating ability. Research currently suggests that no single biomacromolecule or artificial polymer successfully reproduces the tribological properties of biological contacts. However, by combining molecules, one can enhance their anchoring and lubricating capacity, thus enabling the design of surfaces for use in biomedical applications requiring low friction and wear

Hydrolytically degradable polyrotaxane hydrogels for drug and cell delivery applications

Self-assembled pseudopolyrotaxane (PPR) hydrogels formed from Pluronic polymers and α-cyclodextrin (α-CD) have been shown to display a wide range of tailorable physical and chemical properties that may see them exploited in a multitude of future biomedical applications. Upon the mixing of both components, these self-assembling hydrogels reach a metastable thermodynamic state that is defined by the concentrations of both components in solution and the temperature. However, at present, their potential is severely limited by the very nature by which they form and hence also disassemble. Even if the temperature is kept constant, PPR hydrogels will dissociate and collapse within a few hours when immersed in a liquid (such as cell culture media) that contains a lower concentrations of, or no, Pluronic or α-CD due to differences in chemical potential driving dissolution. In this article, an enzymatically mediated covalent cross-linking function and branched eight-arm poly(ethylene glycol) (PEG) were thus introduced into the PPR hydrogels to improve their robustness to such environmental changes. The eight-arm PEG also acted as an end-capping group to prevent the dethreading of the α-CD molecules. The covalent cross-linking successfully extended the lifetime of the hydrogels when placed in cell culture media from a few hours to up to 1 week, with the ability to control the degradation rate (now initiated by hydrolysis of the introduced ester bonds and not by dissolution) by changing the amount of eight-arm PEG present in the hydrogels. Highly tunable hydrogels were obtained with an elastic modulus between 20 and 410 kPa and a viscous modulus between 150 Pa and 22 kPa by varying the concentrations of α-CD and eight-arm PEG. Sustained release of a model drug from the hydrogels was achieved, and viability of mouse fibroblasts encapsulated in these hydrogels was assessed. These self-assembling, hydrolytically degradable, and highly tunable hydrogels are seen to have potential applications in tissue engineering relying on controlled drug or cell delivery to sites targeted for repair

Improvement of the wet tensile properties of nanostructured hydroxyapatite and chitosan biocomposite films through hydrophobic modification

Hydrophobic modification of chitosan/hydroxyapatite composites and chitosan films was performed using four n-alkyl acids with varying alkyl chain lengths (6 to 16 carbons), and these films were characterized for their hydrophobicity and tensile properties under wet conditions. Grafting was performed by a heterogeneous reaction in a 2.0 mol L-1 DMF solution of the acid at 80 degrees C for either one or five days. Characterisation by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) verified the introduction of amide functionalities and alkyl chains. The degree of substitution (DS) ranged from 2-5% for the composite films (based on XPS N 1s narrow scans) reflecting the heterogeneous nature of the reaction. Water contact angle measurements and the tensile testing under wet conditions showed, in general, that the grafting time and alkyl chain length had little effect on the surface hydrophobicity and the tensile properties of films, respectively. However, these modifications had a pronounced effect on decreasing the water content of the films, suggesting that the grafting had occurred in the bulk of the film for longer grafting times. In contrast, the tensile properties of the grafted composite films were significantly improved relative to non-grafted films and a modulus of 393 +/- 68 MPa and an UTS of 18.7 +/- 1.5 MPa were reached for composite films grafted for one day. This study provides a simple method to improve the wet tensile properties of chitosan/hydroxyapatite composite films making them more suitable for biomedical applications

Gelation Kinetics and Viscoelastic Properties of Pluronic and α‑Cyclodextrin-Based Pseudopolyrotaxane Hydrogels

The results of a systematic investigation into the gelation behavior of α-cyclodextrin (α-CD) and Pluronic (poly­(ethylene oxide)-poly­(propylene oxide)-poly­(ethylene oxide) block copolymers) pseudopolyrotaxane (PPR) hydrogels are reported here in terms of the effects of temperature, α-CD concentration, and Pluronic type (Pluronic F68 and Pluronic F127). It was found that α-CD significantly modifies the gelation behavior of Pluronic solutions and that the PPR hydrogels are highly sensitive to changes in the α-CD concentration. In some cases, the addition of α-CD was found to be detrimental to the gelation process, leading to slower gelation kinetics and weaker gels than with Pluronic alone. However, in other cases, the hydrogels formed in the presence of the α-CDs reached higher moduli and showed faster gelation kinetics than with Pluronic alone and in some instances α-CD allowed the formation of hydrogels from Pluronic solutions that would normally not undergo gelation. Depending on composition and ratio of α-CD/Pluronic, these highly viscoelastic hydrogels displayed elastic shear modulus values ranging from 2 kPa to 7 MPa, gelation times ranging from a few seconds to a few hours and self-healing behaviors post failure. Using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), we probed the resident structure of these systems, and from these insights we have proposed a new molecular mechanism that accounts for the macroscopic properties observed