Defence of doctoral thesis: Yagmur Bas – From Wood to Advanced Materials: Multifunctional TEMPO- Oxidized Wood Nanofibril Networks as Wound Dressings and Energy Storage Device Separators

The overall aim of this thesis is to study (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidation of wood particles from a structure-property relationship. Herein, the focus is on the characterization of materials after oxidation, fibrillation, and network formation from various wood species as raw materials, namely softwood- and hardwood particles. This thesis also compares the properties of corresponding nanofibrils, and networks produced through TEMPO-oxidation of raw wood and never dried softwood pulp (NDSP), as well as to commercial TEMPO-oxidized pulp, offering insights into the one-step delignification and oxidation process of wood. It further examines the applications of the nanofibril networks in liquid-absorbed state, in relation to their use as wound dressings and supercapacitor separators, while comparing their performance with commercial materials such as bacterial cellulose (BC) and polyolefin membranes.

This thesis demonstrates that TEMPO-oxidation of wood provides a simplified and direct method to produce cellulose nanofibril (CNF) networks, eliminating separate pulping and bleaching processes prior to TEMPO-oxidation. The method streamlines the process by reducing the treatment and washing steps along with the time required, offering a straightforward route compared to traditional multi-step approaches, while utilizing cost-efficient by-products of wood processing as raw materials, such as sawdust. TEMPO-oxidized wood nanofibril networks are shown to be multifunctional materials with potential use for biomedical and electrochemical applications. They exhibit promising properties for use as advanced wound dressings, including high liquid absorption, wet mechanical integrity, thermal stability, transparency, and biocompatibility with skin cells, in relation to commercially available BC wound dressings. Furthermore, the thickness and fabrication methods, such as suspension casting and vacuum-assisted filtration, were found to significantly influence the interconnection of nanofibril layers and allow tunable design of network properties.  

The thesis also highlights the effect of wood specie on the chemical and mechanical properties of the TEMPO-oxidized wood nanofibril networks. Hardwood particles were found more prone to TEMPO-oxidation, and their nanofibrils exhibited higher carboxylate content than TEMPO-oxidized softwood nanofibrils (TO-SWNFs). However, TO-SWNFs displayed lower cytotoxicity with primary skin cells and their networks displayed better mechanical properties in wet state, making them a more suitable material for wound dressing applications. Furthermore, it has also been studied here that reducing the amount of TEMPO catalyst in the adapted method could be achieved without negative effects on the properties of the resulting hydrogel networks when advanced wound dressing applications are considered. Additionally, other application areas of TEMPO-oxidized wood nanofibril networks as supercapacitor separators were explored, with the addition of kraft lignin (KL) into developed networks, which improved the network uniformity while enhancing the electrochemical performance in coin-cell assemblies. An optimal KL content of 10 wt% provided the best balance of mechanical integrity and capacitance, with higher KL content leading to a decline in both material properties and electrochemical performance. 

In conclusion, this thesis establishes that TEMPO-oxidation of wood particles is a promising approach for production of CNFs that self-assembles to mechanically robust, transparent and cytocompatible hydrogel networks with tunable material properties for different potential applications, namely wound dressings and energy storage device separators as studied herein.  

Link to thesis: https://ltu.diva-portal.org/smash/record.jsf?pid=diva2:1998783