Natural Fibre (Nano)composites

There is a need to develop composites from sustainable materials due to dwindling oil resources. Landfill requirements through EU legislation will be prohibitive enough for a number of industries (e.g. automotive, packaging, electronics) to consider using natural fibres (cellulose) to reinforce plastics. From a scientific and engineering perspective the properties of plant fibres are very versatile, competing in strength/weight and stiffness/weight ratios with conventional materials like glass. Cellulose also has a very low coefficient of thermal expansion and can be taken from renewable resources. There are many problems however involved in the use of cellulose and my group is addressing some of these. One is the characterisation of the interface between fibres and polymer resins typically used in these materials. We currently use Raman spectroscopy to do this which enables spectroscopic measurements of local stresses within natural fibre composites to be measured. There are of course opportunities to be exploited when using cellulose in composites, such as the ability of it to respond to changes in moisture, actuating shape control. We also work extensively on cellulose nanofibres and nanocomposites (see - left for an Atomic Force Microscope (AFM) image of cellulose whiskers. These are just some of the themes that my group is developing and we welcome new members, particularly from overseas who wish to work on natural resources available in their home countries (jute, kenaf, sisal, cotton).

 

Biomimetic Materials

Biology uses many strategies to enhance the mechanical properties of the materials at its disposal e.g. crack deviation and bridging in shells, fibril orientation in wood. The concept of biomimicry or biomimetic materials is to replicate or copy some of these mechanisms or strategies in engineering systems. Principally the group is interested in mechanical properties, but work also extends to mutlifunctionality where mechanical properties can be combined with colour and shape control. The group has so far worked on measuring the fracture properties of biomimetic crystals of calcium carbonate with polymer particles embedded inside them. When the crystals fracture it is found that polymer bridges the crack zones (see image left), which is a toughening mechanism seen in nature. The group has also extended this work to measure the mechanical properties of calcium carbonate crystals with co-polymer micelles in them. We are also interested in using cellulose nanofibres to make materials that respond to changes in moisture and temperature; particularly in applying Raman spectroscopy to understand the mechanisms underlying the mechanical properties.

Electrospinning

Electrospinning is a technique for producing fibres with nanoscale to micron-scale dimensions. The approach uses a charged polymer solution which deforms under the action of a high voltage; due to repulsion of like charges on the surface of the polymer. This repulsion becomes so significant at high voltages (10-40 kV) that it overcomes the surface tension of the polymer and then a jet extends from the deformed droplet of resin solution towards an earthed (or charged) target. The group, in collboration with the Medical School at Manchester (Prof. Geoff Parker and Dr. Penny Hubbard) have developed a technique for near-field electrospinning uisng a moving target to collect fibres. This allows patterning of the fibres, and with a co-axial syringe set-up, core-shell fibres can also be produced (see image left). We are interested in extending this technique for a range of applications. Further work on electrospinning has made structured fibres containing carbon nanotubes, which are aligned and isolated through the process. This has allowed a detailed study of the densities of states of individual nanotubes to take place. 

 

Mechanical Properties of Natural Materials

Ranging from shells to fingernails the group is especially interested in the mechanical properties of natural materials, particularly their resistance to crack growth. One of the first studies we conducted showed how residual stress inside a razor shell led to crack deviation between the laminations in the structure, enhancing the fracture resistance of the material. To do this we used synchrotron X-ray diffraction to map the local changes in lattice strain within the shell structure. This is a technique that we use on a regular basis to perform studies on natural materials. Another study has shown that the moisture content of fingernails has a profound effect mechanical properties, with an optimum value for the maximising of fracture resistance. This result has implications for cosmetic nail care.

 

Biomaterials

Recent work has focussed on using cellulose as a tissue engineering material. As a material cellulose is already used widely in the biomedical sector; for instance for wound healing, surgical implants and as a dialysis membrane. Cellulose is biocompatible and shows very low levels of cytotoxicity and immunogenicity. Initial work began on looking at the modification of simple paper sheets with short amino acids to modulate the shape of tissue cells. It was shown that the shape of fibroblasts and oestoblasts could be controlled on these surfaces by using certain types of amino acids (mid-range hydrophobic amino acids made the cells spread - not something we were expecting!). In the most recent work from the group we have shown that cellulose nanowhiskers aligned on glass slides can guide the growth and proliferation direction of muscle cells (see image left). This has potential for use in a wide range of applications to grow tissue in particular directions on guided substrates.

 

 
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