Nezakati Esmaeelzadeh, T; (2016) Development of a Graphene-POSS-Nanocomposite Conductive Material for Biomedical Application. Doctoral thesis , UCL (University College London).

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Abstract

There is currently an unmet clinical need for biocompatible conductive materials used for biomedical applications, which is crucial for the next generation surgical implants and medical devices development. Metallic alloys are ubiquitous in conductive materials, but in medical applications may not be suitable due to their toxicology. The aim of this work was development of conductive materials using graphene nanoparticles incorporated with polymer. This thesis started with the characterization of pristine monolayer nano-diamond and terminated ND with hydrogen and oxygen. First, the monolayer NDs coated on the cover slip then terminated with hydrogen and finally terminated with oxygen. The AFM indicated the RMS roughness of ND, HND, and OND increased respectively. The contact angle indicated the most hydrophilic on the oxygen terminated ND sample, due to the oxygen termination, its C-O, or C=O polar bonding. In contrast the hydrogen terminated ND showed the least hydrophilicity, due to its hydrogen bonding. I also cultured Schwann cell on all samples and results showed promising metabolic activity and cell proliferation in comparison with TCP. The thesis continued with investigation on the development and subsequent elucidation of a novel biocompatible conductive material applying polyhedral oligomeric silsesquioxane (POSS) nano-cages incorporated into modified poly[caprolactone based urea-urethane] (PCL)/graphene hybrid nanocomposite, POSS-PCL-G. Manufacturing methods, characterizations and biological assays are presented to establish the role of the biocompatible conductive POSS-PCL-G with unique physiochemical surface properties. Multilayer graphene flakes were homogeneously dispersed into POSS-PCL using two methods, non-thermally and thermally controlled with different concentration ranges and then casted. Methods of processing were binary mixing and solution intercalation (non-thermally controlled) at range of 0.1 to 10wt.%. In second method (thermally controlled), the graphene and POSS-PCL were mixed in presence of hydrazine hydrate and the temperature maintained at 100°C at ranges 0.08 to 4wt.% respectively. Chemical properties on both methods indicated the non-covalent bonding, and Raman spectroscopy depicted the higher G peak as the graphene concentration increased. The Raman spectroscopy also indicated less disorder as the graphene concentration increased. The morphology measured by AFM showed the phases are merged at the interfaces on both methods. Furthermore, SEM illustrated that the POSS particle in the POSS-PCL observed at the surface and as the graphene concentration increased the POSS particles covered by graphene. Mechanical properties on the non-thermally controlled indicated at 10wt.% the Young modulus drastically increased. However, on thermally controlled method the Young modulus introduced very small change and almost the same as POSS-PCL. Electrical conductivity was measured with Impedance spectroscopy and the electron mobility and charge concentration conducted by Hall measurement. The Impedance spectroscopy measurement was implemented from 0.1Hz to 1MHz, and the Hall effect voltage measured within the temperature range 10 to 50oC.In the non-thermally controlled method, the electrical conductivity of 1.30x10-4 SCm-1 achieved at 10wt.%. In contrast the thermally controlled showed 4wt.% with electrical conductivity of 9.34x10-5Scm-1, which is approximately the same conductivity as the 10wt.% with previous method. From the non-thermally controlled method the percolation threshold occurred at 5wt.% graphene flakes concentration with conductivity of 2x10-5SCm-1 and thereafter used for biomedical application. Subsequently, the thermally controlled solution intercalation of the graphene and POSS-PCL was conducted to reduce the percolation threshold to 0.08wt% and resulted in conductivity of 1.49x10-7SCm-1. In addition the electron concentration observed at 5.0 and 10.0wt.% on the non-thermally controlled with 2x1015 and 4x1017cm-3 respectively, and the mobility showed 4 and 10cm2/Vs respectively at room temperature. In contrast I could measure the electron concentration at 1.6 and 4.0wt% on thermally controlled method with 9x1014 and 9x1017cm-3 respectively, and the mobility showed 2.5 and 7.5cm2/Vs respectively. There weren't any mobility observed at 2.0wt.% on the non thermally controlled. In addition, on the non-thermally controlled even 3.0 or 4.0% didn't show any electrical conductivity. Material characterization and toxicity study performed on the finished product demonstrated superior and optimum graphene flakes composite into polymer forming conductive polymer. In vitro metabolic activity and proliferation based on Schwann cells cultured on the samples studied. All samples indicated better metabolic activity and proliferation compared to the POSS-PCL. One-way analysis of variance (ANOVA) used to analysed the data. A statistically significant result justifies the null hypothesis rejection, for a probability less than 0.05, 0.01, and 0.005 (significance level). It is therefore envisioned that the conductive graphene-POSS-PCL may be used as a novel biomaterial for medical applications. In my knowledge this is first bio absorbable conductive Nanocomposite material developed for biomedical application.

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