Holly Warren

Enhanced gelation properties of purified gellan gum

Gellan gum is a hydrogel-forming polysaccharide when combined with monovalent or divalent cations such as sodium, magnesium, potassium or calcium. Commercially, gellan gums are sold with trace amounts of these cations, which have been proven to affect the gelation and mechanical properties of the resultant hydrogels. A new method based on impedance analysis for determining the gel transition temperature of purified and un-purified gellan gum is presented. The sodium salt form of gellan gum is shown to have lower dissolution and gel transition temperatures.

Effect of heterocyclic capping groups on the self-assembly of a dipeptide hydrogel

The mechanism and design rules associated with the self-assembly of short peptides into hydrogels is currently not well understood. In this work, four diphenylalanine-based peptides have been synthesised, bearing heterocyclic capping groups which have different degrees of hydrogen bonding potential and nitrogen substitution. For these four peptides, zeta potential and electrical impedance spectroscopy measurements were undertaken to monitor gelation, with the impedance data showing different gelation times for each peptide hydrogel. Through a combination of atomic force microscopy and rheological measurmeents, including dynamic strain and frequency sweeps, and thixotropic tests, the relationship between the mechanism of self-assembly in these hydrogels and their macroscopic behaviour can be established. It is observed that the degree of nitrogen substitution affects the self-assembly mechanisms of the hydrogels and as such, that there is an interplay between branching and bundling self-assembly pathways that are responsible for the final properties of each hydrogel.

Conducting hydrogels for edible electrodes

The development of highly swollen, strong, conductive hydrogel materials is necessary for the advancement of edible device research. Using a gellan gum/gelatin ionic-covalent entanglement (ICE) hydrogel, a simple method of producing conductive, edible hydrogels is described. ICE gels containing NaCl or CsCl were developed which exhibited conductivities of 200 ± 20 mS cm-1 and 380 ± 30 mS cm-1, respectively. Furthermore, the potential of food grade products for use as edible electrodes was examined by analysing the electrical properties of alginate-gelatin hydrogels, Vegemite, Marmite, jelly and gold leaf. Lastly, these edible ICE gels were used to demonstrate a capacitive pressure sensor from consumable materials, which displayed a sensitivity of 0.80 ± 0.06 pF kPa-1 for a range of 4-20 kPa. The pressure exerted by the GI tract on its contents is standardly 0.7 kPa to 6.3 kPa. This suggests potential for application in the detection of digestive pressure abnormalities such as intestinal motility disorders.

Sonication-induced effects on carbon nanofibres in composite materials

The preparation and characterization of carbon nanofibre–gellan gum composite materials is presented. Electron microscopy analysis reveals that nanofibres are affected by sonolysis, i.e. fibre length reduces, while filling occurs. Spectroscopic analysis suggests that the nanofibres are modified during the preparation of the dispersions. It is shown that despite these effects, composite materials prepared using a short period of sonolysis (4 min) exhibit robust conductivity, strain at failure and Youngs modulus values of 35 ± 2 S cm−1, 20 ± 1% and 1.3 ± 0.3 MPa, respectively.

Electrical conductivity and impedance behaviour of hydrogels

The impedance and electrical conductivity behavior of gellan gum hydrogels containing the conducting fillers poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and vapour grown carbon nanofibers (VGCNF) is presented. Impedance analysis showed that an equivalent circuit consisting of a Warburg element in series with a resistor could be used to model the gels’ behavior. It is demonstrated that the addition of the conducting fillers PEDOT:PSS and VGCNFs can result in a measurable improvement in the conductivity of hydrogels with high water content and swelling ratios. Incorporation of combinations of these conducting fillers resulted in an improvement of the conductivity of gellan gum-containing hydrogels with water content (swelling ratio) of at least 97.5% (40) from 1.2 ± 0.1 mS/cm to 4 ± 0.6 mS/cm.

Films, Buckypapers and Fibers from Clay, Chitosan and Carbon Nanotubes

The mechanical and electrical characteristics of films, buckypapers and fiber materials from combinations of clay, carbon nanotubes (CNTs) and chitosan are described. The rheological time-dependent characteristics of clay are maintained in clay–carbon nanotube–chitosan composite dispersions. It is demonstrated that the addition of chitosan improves their mechanical characteristics, but decreases electrical conductivity by three-orders of magnitude compared to clay–CNT materials. We show that the electrical response upon exposure to humid atmosphere is influenced by clay-chitosan interactions, i.e., the resistance of clay–CNT materials decreases, whereas that of clay–CNT–chitosan increases.