Cellulose nanofibril- and chitin nanofibril-based materials: fibers, scaffolds and tubes

• Materialien auf Basis von Zellulose- und Chitin-Nanofasern: Faser, Gerüststrukturen und Röhren

Torres Rendon, Jose Guillermo; Möller, Martin (Thesis advisor); Pich, Andrij (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

Renewable cellulose nanofibrils (CNFs) and chitin nanofibrils (ChNFs) are very attractive bionanoparticles due to their outstanding properties such as remarkable mechanical properties, flexible surface functionalities, thermostability, barrier properties, large surface area, biocompatibility and for their global availability from renewable resources and food waste. Nowadays, the research concerning materials based on CNFs and ChNFs is still in its infancy. This thesis deals with fundamental and groundbreaking research regarding CNF- and ChNF-based materials (fibers, hollow fibers and scaffolds) in connection with additive manufacturing techniques. The merging of defined nanoscale building blocks with advanced additive manufacturing techniques is of eminent importance for the preparation of multiscale and highly functional materials with de-novo designed architectures.In terms of mechanical properties, one major bottleneck to maximize those of materials based on CNFs and ChNFs (e.g. nanopapers and macroscale fibers) is to find pathways to control their direction of alignment and understand how preferred alignment correlates with macroscale mechanical properties. In this context, I processed CNF and ChNF dispersions into high-performance macroscopic fibers using much more environmentally friendly routes compared to the established processes to produce cellulose- and chitin-based macrofibers. In chapter 2, I show that strain-rate controlled wet-stretching of rehydrated macroscale fibers composed of CNFs and ChNFs induces a high degree of orientation and also that the degree of alignment scales with macroscale mechanical stiffness. I find similar degrees of alignment in both types of nanofibril-based macrofibers, yet substantially different macroscale stiffness, with the CNF-based fibers (ECNF = 33 GPa) outperforming the ChNF-based ones (EChNF = 12 GPa) considerably. These differences can be correlated to the mechanical properties of the underlying cellulose I and α-chitin crystals and the degree of crystallinity of the nanofibrils, which both govern the stiffness of an individual nanofibril. This study likely demonstrates the maximum performance in terms of stiffness of materials prepared by CNFs and ChNFs and reveals a critical difference in the performance of both classes of bionanoparticles. Beyond mechanical properties, I explored the capabilities of CNFs and ChNFs in the field of tissue engineering by fabricating scaffolds with potential applications as temporal supports during the regeneration of bone tissue. This study is shown in chapter 3. The development of hierarchically ordered materials having designed porosities from the macro to nanoscales remains very challenging in material science and engineering. Such materials are essential to a wide range of technologies, in particular for tissue engineering, where producing hydrogel scaffolds with multi-scale topographies potentially allows the instruction of cellular behavior to target the organization of tissues. To address these issues, I developed a simple inverse templating strategy that enables the preparation of nanofibrillar hydrogel scaffolds with defined porosities in the macro- and nanoscale. First, together with the Wessling group (DWI), we designed a lithographic process that furnishes sacrificial gyroid scaffolds, based on a resin that degrades in alkaline media. After infiltration with hydrogels of highly crystalline and stiff CNFs and ChNFs, the templates were simply dissolved in mild alkaline solution, and hydrogel replicas of the templates were obtained. This simple approach represents a platform fabrication method for a range of hydrogel-based materials with de novo designed pore geometries. Subsequent cell studies in collaboration with Dr. John Hardy (presently at Lancaster University, UK) and Laura de Laporte (DWI) confirmed the biocompatibility of the CNF and ChNF-based scaffolds and revealed important differences in terms of cell attachment. Differentiation of human mesenchymal stem cells (HMSCs) into osteogenic outcome could be facilitated using a collagen bone mimetic coating, rendering these scaffolds interesting for bone tissue engineering.In the last study performed in this thesis (shown in chapter 4), I continued using the concept of reverse templating but this time using CNF hollow tubes as sacrificial templates to create macroscale tubular cell constructs. I demonstrate that dispersions of CNFs can be processed into complex shapes, and used as a sacrificial template to prepare freestanding cell constructs. I showcase the approach for the fabrication of hollow fibers using a controlled extrusion through a circular die into a coagulation bath. The dimensions of the hollow fibers are tunable, and the final tubes combine the nanofibrillar porosity of the CNF hydrogel with a sub-millimeter wall thickness and centimeter-scale length provided by the additive manufacturing technique. I also demonstrate that covalent and supramolecular cross-linking of the CNFs can be used to tailor the mechanical properties of the hydrogel tubes within one order of magnitude and in an attractive range for the mechanosensation of cells. The resulting tubes are highly biocompatible and allow for the growth of mouse fibroblasts into confluent cell layers in their inner lumen. A detailed screening of several cellulases enables to degrade the scaffolding, temporary CNF hydrogel tube in a quick and highly cell-friendly way, and allows the isolation of coherent cell tubes. I foresee that the growing capabilities of 3D printing techniques in combination with the attractive features of CNFs - sustainable, globally abundant, biocompatible and enzymatically degradable - will allow to make plant-based biomaterials with hierarchical structures and on-demand degradation useful for instance to engineer complex tissue structures to replace animal models, and for implants.