Design of graphene based PVA composites for biomedical devices

This work conducted within Prof. Paradossi’s group at the Department of Chemical Science and Technology in the University of Rome Tor Vergata aimed to develop an easily applicable strategy enabling functionalization of graphene, suspended in an aqueous medium in its pristine form, to biocompatible polymeric surfaces, in particular “poly(vinylalcohol)”platforms, designed for biomedical applications. Nowadays graphene, this new two-dimensional material with fascinating properties, is emerging in many scientific fields. Exploitation of graphene propertieshas been one of the motivations to implement biomedical applications of interest for our laboratory. Ourinvestigations are aboutwhat kind of functionalities can be introduced ina polymeric platform, such as hydrogel sponges for anti-tumor drug delivery or the surface of microbubbles, still inthedevelopment phase as multimodal imaging contrast agents mainly for ultrasoundsand photoacoustics. To this regard, photoacoustic imaging is a high-resolution preclinical fast developing diagnostic tool. Evolution of this imaging methodology can bring to a new clinical tool designed for human investigationandis a major challenge that can be tackled by ad hocengineered contrast agents. To this aiman optimized selection of advanced hybrid platforms is needed. Graphene derivatives, mainly graphene oxide (GO) and reduced graphene oxide (RGO),exhibiting depleted properties with respect to pristine graphene, but chemically more versatile, have beenhighlighted in the recent literature. These forms derive from the chemical modifications of the 2D structure of graphene in very harsh conditions, which introduce kinks and irregularities in the carbonic material. Such modifications make GO and RGO more reactive and more processable than pristine graphene, but jeopardize the electrical, optical and mechanical properties of this material. Despite this fact, we have not found a study thatreports theincorporation of pristine graphene into biomedical devicesstarting from its suspended form in aqueous media. The challenge herein was to preserve the graphene original properties which are important for the applications we address;and find the intermediate key to adequately tether graphene on our studied poly(vinylalcohol) composites. For this, Prof. Paradossi’s group strong background about poly(vinylalcohol) hydrogels and microbubbles was a great advantage.The first chapter of the thesis is a brief general introduction about graphene and poly(vinylalcohol) providing the necessary details of why these materials could be interesting candidates for our research, taking into accountthe main problems concerning the separate materials as well as their assembly and formulatinghypothesis regarding the efficiency and biocompatibilityof the hybrid systems.The second chapter is a proof of concept on the method allowing pristine graphene entrapment into a poly(vinylalcohol) hydrogel matrix with potential in drug release at physiological temperature by using thermosensitive moieties.The third chapter of this thesisis a general introduction to the photoacoustic imaging. It provides the basic theoretical foundation for understanding this method and the physical mechanisms related to photoacoustic generation in biological tissues. First, we describe light propagation mechanisms in tissues, and the deposition of heat via optical absorption. Assimilating the biological tissue to a liquid medium, wethen introduce the fundamental equations describing the photoacoustic issue, and the assumptions used in imaging to improve diagnosis. We also introduce ultrasound imaging and its inherent clinical approach for diagnostic and therapy. Finally, the contrast agents used in both ultrasound and photoacoustic imaging modalities are discussed through two important examples: microbubbles and NIR absorbing agents. The chapter IV details the state of the art in the context of the objectives we pursued during this thesis regarding hybrid contrast agents for photoacoustic imaging based pristine graphene and poly(vinylalcohol) microbubbles. Details on the microdevice fabrication as well as its physico-chemical characterization are provided. Finally, the potential of the graphene poly(vinylalcohol) microbubbles in enhancing the photoacoustic signal is assessed in vitro and in vivo. In the chapter V, we present a study on the influence of diamine intermediates and PEGylation used as links between graphene and the PVA microbubbles on the colloidal behavior, acoustic properties, and cytotoxicity of the overall system.An appendix is presented at the end of the thesis describing a preliminarywork carried recently on the realization of new “phase-change”ultrasound contrast agents with a photo-polymerized surfactant monolayer shell structureencapsulating perfluorocarbon. These systems are in normal conditions droplets and upon ultrasound irradiation,they convert into microbubbles by “acoustic droplet vaporization”. The phase change efficiency is studied and the experimental setup and operating conditionsaredetailed.