Electrically conductive bacterial cellulose for tissue-engineered neural interfaces

Bacterial cellulose (BC) with its high crystallinity, tensile strength, degree of polymerisation, and water holding capacity (98%) becomes increasingly attractive as 3D nanofibrillar material for biomedical applications. Such multi-scale fibrillary BC networks can be potentially functionalised with electrically conductive moieties to facilitate the conductive properties required for various smart biomedical devices, in particular, in the construction of bioelectronic neural interfaces. In this thesis, BC fibres are chemically modified with poly(4-vinylaniline) (PVAN) interlayer for further enhancement of electrical conductivity and cell viability of subsequent polyaniline (PANI) coatings as a bilayer grafted BC nanocomposite. This functional poly(4-vinylaniline)/polyaniline (PVAN/PANI) bilayer can be efficiently anchored onto BC fibrils through successive surface-initiated atom transfer radical polymerisation and in situ chemical oxidative polymerisation. PVAN is found to have promoted the formation of a uniform PANI layer with 1D nanofiber- and nanorod-like supramolecular structures, with an overall augmentation of PANI yield, hence further improved electrical performance. Compositional and microstructural analysis reveals such a PVAN/PANI bilayer with a thickness up to ~2 µm on BC formed through a significant growth of PANI with rough surface morphology due to the insertion of PVAN, which has improved the functional properties of the BC nanocomposites. Successful impregnation of both layers onto BC fibrils was corroborated with systematic microstructural and chemical analysis. The solid-state electrical conductivity of such synthesised BC nanocomposites with PVAN interlayer reaches as high as (4.5±2.8)×10-2 S.cm-1 subject to the amounts of PVAN chemically embraced. Electrochemical examination evinces the switching in the electrochemical behaviour of BC/PVAN/PANI nanocomposites at -0.70/0.74 V (at 100 mV.s-1 scan rate) due to the existence of PANI, where the maximal electrical performance can be achieved at charge transfer resistance of as low as 21 Ω and capacitance of as high as 39 μF. Both electrochemical and mechanical properties can be tailored onto an incomplete BC dehydration, where a mathematical model is herein developed to predict BC water loss accordingly. BC/PVAN/PANI nanocomposites are thermally stable up to 200 ºC. Furthermore, further improvement of the electrical conductivity has been achieved through grafting Carbon Nano Tubes (CNTs) into the BC/PVAN/PANI nanocomposites, where the interactions between PANI and CNTs present new electrochemical characteristics with enhanced capacity. PANI/CNTs coatings with a nanorod-like morphology can promote the efficient ions diffusion and charge transfer, resulting in the increased electrical conductivity up to (1.0±0.3)×10-1 S.cm-1. An escalating amplification of the double charge capacity to ~54 mF of the CNTs grafted BC nanocomposites was also detected through electrochemical analysis. In addition, the thermal stability of CNTs grafted BC/PVAN/PANI nanocomposites are improved, and they become stable up to 234 ºC. Cytocompatibility tests conducted using two neuronal cell linages show non-cytotoxic effects for PC-12 Adh cells and SVZ neural stem cells, confirming cell viability that can be over 80 % and neuronal differentiation capability of the electrically functionalised BC-based nanocomposite membranes, which can induce neurites outgrowth up to 115±24 μm long. These voltage-sensible nanocomposites can hence interact with neural cells, thereby significantly stimulate specialised response. These findings pave the path to the new tissue engineered neural interfaces which embraces electronic functions into the tissue regeneration, to enable full functional neural tissue recovery.