Lightweight iron oxide nanoparticle (IONP)/TEMPO-oxidized cellulose nanofibril (TOCNF) hybrid foams with an anisotropic structure and a high IONP content were produced using magnetic field-enhanced unidirectional ice-templating. Coating the IONP with tannic acid (TA) improved the processability, the mechanical performance, and the thermal stability of the hybrid foams. Increasing the IONP content (and density) increased the Young's modulus and toughness probed in compression, and hybrid foams with the highest IONP content were relatively flexible and could recover 14% axial compression. Application of a magnetic field in the freezing direction resulted in the formation of IONP chains that decorated the foam walls and the foams displayed a higher magnetization saturation, remanence, and coercivity compared to the ice-templated hybrid foams. The hybrid foam with an IONP content of 87% displayed a saturation magnetization of 83.2 emu g⁻¹, which is 95% of the value for bulk magnetite. Highly magnetic hybrid foams are of potential interest for environmental remediation, energy storage, and electromagnetic interference shielding.

By forming and directionally freezing an aqueous foam containing cellulose nanofibrils, methylcellulose, and tannic acid, we produced a stiff and tough anisotropic solid foam with low radial thermal conductivity. Along the ice-templating direction, the foam was as stiff as nanocellulose–clay composites, despite being primarily methylcellulose by mass. The foam was also stiff perpendicular to the direction of ice growth, while maintaining λ_r < 25 mW m^–1 K^–1 for a relative humidity (RH) up to 65% and <30 mW m^–1 K^–1 at 80% RH. This work introduces the tandem use of two practical techniques, foam formation and directional freezing, to generate a low-density anisotropic material, and this strategy could be applied to other aqueous systems where foam formation is possible. 

The biopolymer lignin is deposited in the cell walls of vascular cells and is essential for long-distance water conduction and structural support in plants. Different vascular cell types contain distinct and conserved lignin chemistries, each with specific aromatic and aliphatic substitutions. Yet, the biological role of this conserved and specific lignin chemistry in each cell type remains unclear. Here, we investigated the roles of this lignin biochemical specificity for cellular functions by producing single cell analyses for three cell morphotypes of tracheary elements, which all allow sap conduction but differ in their morphology. We determined that specific lignin chemistries accumulate in each cell type. Moreover, lignin accumulated dynamically, increasing in quantity and changing in composition, to alter the cell wall biomechanics during cell maturation. For similar aromatic substitutions, residues with alcohol aliphatic functions increased stiffness whereas aldehydes increased flexibility of the cell wall. Modifying this lignin biochemical specificity and the sequence of its formation impaired the cell wall biomechanics of each morphotype and consequently hindered sap conduction and drought recovery. Together, our results demonstrate that each sap-conducting vascular cell type distinctly controls their lignin biochemistry to adjust their biomechanics and hydraulic properties to face developmental and environmental constraints. 

Wood-foam hierarchical composites were produced via the shear-forced infiltration of shear-thinning nanocellulose-based foams or gels into the tracheids of Picea abies. Shear processing viscoelastic and shear-thinning aqueous foams composed of cellulose nanocrystals, methylcellulose, and tannic acid (total solids content: 2 wt %) resulted in foam-filled wood composites containing 15-20 wt % foam, with open foam structures and compression strengths similar to those of unmodified P. abies. An amino-functionalized nanocellulose-containing foam confined in wood reversibly adsorbed CO2, retaining 15% of its theoretical uptake capacity over 50 cycles in the thermogravimetric analyzer, and a citronellol-loaded foam released this mosquito-repellent compound over four days, as evaluated using solid-phase microextraction. Shear-forced infiltration of functional foams into wood is an operationally simple route to hierarchically porous composites based on renewable materials.

The mechanical performance in the wet state needs to be significantly improved and the intrinsic functionalities should be fully utilized to promote the replacement of fossil-based plastics with renewable biobased materials. We demonstrate a leather-inspired approach to produce multifunctional materials with a high wet strength that is based on tannin-induced precipitation of gelatin grafted onto surface-modified cellulose nanofibrils (CNF). The leather-inspired CNF-based films had a wet tensile strength of 33 MPa, a Young's modulus of 310 MPa, and a strain at failure of 22%, making the wet materials stronger than, for example, dry conventional low-density polyethylene and more ductile than paper-based food packaging materials. The tannin-containing films displayed excellent antioxidant and UV-blocking properties, rapidly scavenging more than 90% of added free radicals and absorbing 100% of light in the UV-B/UV-C range. This work illustrates the prospect of combining renewable materials in a leather-inspired approach to form wet strong and multifunctional films with potential application in food packaging.

Nanocellulose-based materials and nanocomposites show extraordinary mechanical properties with high stiffness, strength, and toughness. Although the last decade has witnessed great progress in understanding the mechanical properties of these materials, a crucial challenge is to identify pathways to introduce high wet strength, which is a critical parameter for commercial applications. Because of the waterborne fabrication methods, nanocellulose-based materials are prone to swelling by both adsorption of moist air or liquid water. Unfortunately, there is currently no best practice on how to take the swelling into account when reporting mechanical properties at different relative humidity or when measuring the mechanical properties of fully hydrated materials. This limits and in parts fully prevents comparisons between different studies. We review current approaches and propose a best practice for measuring and reporting mechanical properties of wet nanocellulose-based materials, highlighting the importance of swelling and the correlation between mechanical properties and volume expansion.

Renewable and biodegradable alternatives to fossil-based materials are essential as concerns over depleting finite resources and the pollution of our ecosystems are growing. Abundant, highly anisotropic, and mechanically strong cellulose nanofibrils (CNF) are attractive building blocks for the fabrication of high-performance biobased materials that can compete with their conventional fossil-based counterparts. This thesis presents potential solutions to key challenges in the production and properties of CNF and CNF-based materials, such as low moisture resistance and energy-intense processing, by using the physicochemical properties of tannins. The benchmarking of CNF to improve energy-efficient production was investigated and the ability of plant-derived tannins to precipitate proteins, react with nucleophiles when oxidized, and coordinate to metal ions was exploited to produce multifunctional films and foams that were inspired by Nature or traditional processes.

Wet strong, antioxidant, and UV-blocking CNF-based films were produced by mimicking the traditional process of leather tanning. Oxidized CNF were grafted with gelatin that was precipitated with a water-soluble tannin. The polyphenolic tannin provided the films with good radical scavenging properties and efficient blocking of light in the UV-B/UV-C range. The insoluble gelatin–tannin complexes conferred upon the material wet mechanical properties that were comparable to the dry mechanical performance of fossil-based packaging films. So far, there is no universally accepted approach to account for how the swelling of a hygroscopic CNF-based film influences its mechanical properties in humid or wet conditions. Here, a best practice for determining and reporting wet strength is suggested.

Inspired by the sclerotization of insect cuticle, a scalable route towards moisture-resilient, strong, and thermally insulating CNF-based foams was developed. The CNF were modified with a polyamine, ice-templated, treated with an oxidized tannin, solvent-exchanged to ethanol, and evaporatively dried. The cross-linked structure had a high compressive modulus and a thermal conductivity close to that of air, even at high relative humidities.

A method to produce micron-sized patterns on CNF films based on the traditional Bògòlanfini dyeing technique is presented. The films were pre-impregnated with a tannin and patterned using microcontact printing with a metal-salt-soaked stamp. The line and dot patterns were analyzed and their colors were tuned by changing the metal ion in the printing ink or the pH.

The final part of the thesis describes a novel approach to assess the degree of CNF fibrillation during energy-efficient grinding by analyzing the structure and properties of anisotropic foams. The optimal energy input during fiber disintegration that produced CNF foams with the best mechanical and thermal insulation properties, as well as the highest CNF and foam cell wall orientation, was identified.

Thermally insulating foams and aerogels based on cellulose nanofibrils (CNFs) are promising alternatives to fossil-based thermal insulation materials. We demonstrate a scalable route for moisture-resilient lightweight foams that relies on sclerotization-inspired Michael-type cross-linking of amine-modified CNFs by oxidized tannic acid. The solvent-exchanged, ice-templated, and quinone-tanned cross-linked anisotropic structures were mechanically stable and could withstand evaporative drying with minimal structural change. The low-density (7.7 kg m–3) cross-linked anisotropic foams were moisture-resilient and displayed a compressive modulus of 90 kPa at 98% relative humidity (RH) and thermal conductivity values close to that of air between 20 and 80% RH at room temperature. Sclerotization-inspired cross-linking of biobased foams offers an energy-efficient and scalable route to produce sustainable and moisture-resilient lightweight materials.

Inspired by the Bogolanfini dyeing technique, we report how flexible nanofibrillated cellulose (CNF) films can be functionalized and patterned by surface-bound nanoparticles of hydrolyzable tannins and multivalent metal ions with tunable colors. Molecular dynamics simulations show that gallic acid (GA) and ellagic acid (EA) rapidly adsorb and assemble on the CNF surface, and atomic force microscopy confirms that nanosized GA assemblies cover the surface of the CNF. CNF films were patterned with tannin-metal ion nanoparticles by an in-fibre reaction between the pre-impregnated tannin and the metal ions in the printing ink. Spectroscopic studies show that the Fe-III/II ions interact with GA and form surface-bound, stable GA-Fe-III/II nanoparticles. The functionalization and patterning of CNF films with metal ion-hydrolyzable tannin nanoparticles is a versatile route to functionalize films based on renewable materials and of interest for biomedical and environmental applications.

Cellulose nanofibrils (CNFs) are a unique nanomaterial because of their abundant, renewable, and biocompatible origin. Compared with synthetic nanoparticles, CNFs are commonly produced from cellulose fibers (e.g., wood pulp) by repetitive high-shear mechanical disintegration. Yet, this process is still highly demanding in energy and costly, slowing down the large-scale production and commercialization of CNFs. Reducing the energy consumption during fibers fibrillation without using any chemical or enzymatic pretreatments while sustaining the CNF quality is challenging. Here, we show that the anisotropic properties of the CNF foams are directly connected to the degree of nanofibrillation of the cellulose fibers. CNFs were produced from wood pulps using a grinder at increasing specific energy consumptions. The anisotropic CNF foams were made by directional ice templating. The porous architecture, the compressive behavior of the foams, and the CNF alignment in the foam cell walls were correlated to the degree of fibrillation. A particular value of specific energy consumption was identified with respect to the highest obtained foam properties and CNF alignment. This value indicated that the optimal degree of fibrillation, and thus CNF quality, was achieved for the studied cellulose pulp. Our approach is a straightforward tool to evaluate the CNF quality and a promising method for the benchmarking of different CNF grades.