Doctor of Philosophy
Department of Chemistry
Gelmi, Amy Alexis, Probing nanoscale properties of organic conducting polymer interfaces using atomic force microscopy, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3826
The development of new biomaterials for the field of bionics- the fusion of electronic devices with biological tissue- is an exciting area of research. The ability to control the interaction between the electronic and biological interface in devices, such as the cochlear implant, are crucial for enhancing and improving their biocompatibility and performance.
Organic conducting polymers are widely studied for use as biomaterials to replace the conventional metallic materials currently used as electrodes and coatings in medical devices. These polymers are highly interesting due to fine level of control over material properties and the ability to incorporate biological components into the composition of the polymer itself. Thorough characterisation is needed to fully understand how these biological components influence the properties of the polymer itself and how they influence the interaction with living cells. Within this thesis I have used Atomic Force Microscopy to characterise an organic conducting polymer doped with various biological and non-biological molecules. This technique was applied so that the biomaterial could be studied on the nanoscale and on scales relevant to single cell interactions.
Characterisation of the physical properties of the biomaterial demonstrated that irrespective of whether the dopant was biologically derived, the physical properties tended to group together with films having either a low roughness, low modulus and high strain, or vice versa. When compared to previous work, which investigated these polymers as potential biomaterials for supporting the growth and differentiation of skeletal muscle cells, these two groupings of the parameters correlated with the differing ability of the polymer substrates to support the cells.
Using AFM surface characterisation techniques, namely phase imaging, current sensing and Kelvin force probe scanning, it was deduced that the polymer displays variable dopant distribution depending on the dopant. This dopant distribution created regions of attractive and repulsive interactions across the surface, which is dependent on the redox state and degree of dopant loading of the polymer.
I developed a single protein force spectroscopy technique to measure the interfacial forces and interactions between a cell adhesion protein, fibronectin, and the biomaterial depending on dopant. This technique was able to resolve sub-molecular binding specificity between the dopants and binding domains of fibronectin. The interaction exploits a form of biological ‘charge complementarity’ to enable specificity. This specificity and the adhesion force were demonstrated to be influenced spatially by the distribution of dopant throughout the polymer using single protein force volume spectroscopy.
In addition, the effect of stimulus on the organic conducting polymers – protein interface was investigated. When an electrical stimulus was applied to the biomaterial, the specific interaction was switched to a non-specific, high affinity binding state that was shown to be reversibly controlled using electrochemical processes. Both the specific and non-specific interactions are integral for controlling protein conformation and dynamics – the details of which give new molecular insight into controlling cellular interactions on these polymer surfaces. A different organic polymer was stimulated using an optical signal. The change in the surface charge was demonstrated to influence the level of adhesion of a non-specific interaction between the protein and polymer in a reversible manner.