Accurate evaluation of electrophysiological and morphological characteristics of the skeletal muscles is critical to establish a comprehensive assessment of the human neuromusculoskeletal function in vivo. However, current technological challenges lie in unsynchronized and unparallel operation of separate acquisition systems such as surface electromyography (sEMG) and ultrasonography. Key problem is the lack of ultrasound transparency of current electrophysiological electrodes. In this work, ultrasound (US) transparent electrode based on cellulose nanofibrils (CNF) substrate are proposed to solve the issue. US transparency of the electrodes are evaluated using a standard US phantom. The effects of nanocellulose type and ion-bond introduction on electrode performance is investigated. Simultaneous US image and sEMG signal acquisition of biceps brachii during isometric muscle contraction are studied. Reliable correlation analysis of the US and sEMG signals is realized which is rarely reported in the previous literatures. Recyclability and biodegradability of the current electrode are evaluated. The reported technology opens up new pathways to provide coupled anatomical and electrical information of the skeletal muscles, enables reliable anatomical and electrical information correlation analysis and largely simplify the sensor integration for assessment of the human neuromusculoskeletal function.

Metallic wood combines the unique structural benefits of wood and the properties of metals and is thus promising for applications ranging from heat transfer to electromagnetic shielding to energy conversion. However, achieving metallic wood with full use of wood structural benefits such as anisotropy and multiscale porosity is challenging. A key reason is the limited mass transfer in bulk wood where fibers have closed ends. In this work, programmed removal of cell-wall components (delignification and hemicellulose extraction) was introduced to improve the accessibility of cell walls and mass diffusion in wood. Subsequent low-temperature electroless Cu plating resulted in a uniform continuous Cu coating on the cell wall, and, furthermore, Cu nanoparticles (NPs) insertion into the wood cell wall. A novel Cu NPs-embedded multilayered cell-wall structure was created. The unique structure benefits compressible metal-composite foam, appealing for stress sensors, where the multilayered cell wall contributes to the compressibility and stability. The technology developed for wood metallization here could be transferred to other functionalizations aimed at reaching fine structure in bulk wood.

Converting omnipresent environmental energy through the assistance of spontaneous water evaporation is an emerging technology for sustainable energy systems. Developing bio-based hydrovoltaic materials further pushes the sustainability, where wood is a prospect due to its native hydrophilic and anisotropic structure. However, current wood-based water evaporation-assisted power generators are facing the challenge of low power density. Here, an efficient hydrovoltaic wood power generator is reported based on wood cell wall nanoengineering. A highly porous wood with cellulosic network filling the lumen is fabricated through a green, one-step treatment using sodium hydroxide to maximize the wood surface area, introduce chemical functionality, and enhance the cell wall permeability of water. An open-circuit potential of ≈140 mV in deionized water is realized, over ten times higher than native wood. Further tuning the pH difference between wood and water, due to an ion concentration gradient, a potential up to 1 V and a remarkable power output of 1.35 µW cm−2 is achieved. The findings in this study provide a new strategy for efficient wood power generators.

Hydrovoltaic energy harvesting offers the potential to utilize enormous water energy for sustainable energy systems. Here, we report the utilization and tailoring of an intrinsic anisotropic 3D continuous microchannel structure from native wood for efficient hydrovoltaic energy harvesting by Fe3O4 nanoparticle insertion. Acetone-assisted precursor infiltration ensures the homogenous distribution of Fe ions for gradience-free Fe3O4 nanoparticle formation in wood. The Fe3O4/wood nanocomposites result in an open-circuit voltage of 63 mV and a power density of ∼52 μW/m2 (∼165 times higher than the original wood) under ambient conditions. The output voltage and power density are further increased to 1 V and ∼743 μW/m2 under 3 suns solar irradiation. The enhancement could be attributed to the increase of surface charge, nanoporosity, and photothermal effect from Fe3O4. The device exhibits a stable voltage of ∼1 V for 30 h (3 cycles of 10 h) showing good long-term stability. The methodology offers the potential for hierarchical organic-inorganic nanocomposite design for scalable and efficient ambient energy harvesting.

Owing to the hierarchical structure, easy multi-functionalization and favorable mechanical properties, wood could harvest electricity from mechanical energy through piezoelectric behavior. In this work, a scalable method to synthesize wood/ZnO composite with multilayered ZnO morphologies is reported for efficient mechanical energy conversion. The synthesis includes charged wood template fabrication, precursor infiltration, and ZnO hydrothermal growth, resulting in controlled ZnO morphologies and distributions while maintaining the hierarchical structure of the wood. Stereo-digital image correlation (stereo-DIC) investigated the relationship between deformation and piezoelectric performance, which revealed the homogeneous distribution of multilayered ZnO enhance piezoelectric performance. The output voltage of wood/ZnO was 1.5 V under periodic mechanical compression (8–10 N) for 300 cycles, while the output current was 2.91 nA. The scalable synthesis strategy and piezoelectric performance are significant for the design of advanced wood nanocomposites for sustainable and efficient energy conversion systems.

Polymer shape-memory aerogels (PSMAs) are prospects in various fields of application ranging from aerospace to biomedicine, as advanced thermal insulators, actuators, or sensors. However, the fabrication of PSMAs with good mechanical performance is challenging and is currently dominated by fossil-based polymers. In this work, strong, shape-memory bio-aerogels with high specific surface areas (up to 220 m2/g) and low radial thermal conductivity (0.042 W/mK) were prepared through a one-step treatment of native wood using an ionic liquid mixture of [MTBD]+[MMP]−/DMSO. The aerogel showed similar chemical composition similar to native wood. Nanoscale spatial rearrangement of wood biopolymers in the cell wall and lumen was achieved, resulting in flexible hydrogels, offering design freedom for subsequent aerogels with intricate geometries. Shape-memory function under stimuli of water was reported. The chemical composition and distribution, morphology, and mechanical performance of the aerogel were carefully studied using confocal Raman spectroscopy, AFM, SAXS/WAXS, NMR, digital image correlation, etc. With its simplicity, sustainability, and the broad range of applicability, the methodology developed for nanoscale reassembly of wood is an advancement for the design of biobased shape-memory aerogels.

Functional wood materials often rely on active additives due to the weak piezoelectric response of wood itself. Here, we chemically modify wood to form functionalized, eco-friendly wood veneer for self-powered vibration sensors. Only the piezoelectricity of the cellulose microfibrils is used, where the drastic improvement comes only from molecular and nanoscale wood structure tuning. Sequential wood modifications (delignification, oxidation, and model fluorination) are performed, and effects on vibration sensing abilities are investigated. Wood veneer piezoelectricity is characterized by the piezoresponse force microscopy mode in atomic force microscopy. Delignification, oxidation, and model fluorination of wood-based sensors provide output voltages of 11.4, 23.2, and 60 mV by facilitating cellulose microfibril deformation. The vibration sensing ability correlates with improved piezoelectricity and increased cellulose deformation, most likely by large, local cell wall bending. This shows that nanostructural wood materials design can tailor the functional properties of wood devices with potential in sustainable nanotechnology. 

Highly porous aerogels based on renewable materials that possess structural functionality are appealing for sustainable energy regulation and harvesting. Achieving structural anisotropy provides advantageous directional diffusion and mechanical strength, however, introduces great engineering challenges, such as complex, costly, and time-consuming processing. Direct use of wood, where nanocellulose is favorably orientated, offers the opportunity of forming low-cost, scalable, and eco-friendly aerogels.

This thesis explores a new type of nanostructured wood material design by filling the empty wood space with cellulosic aerogel structures based on its intrinsic biopolymers. The structure control is achieved through selective reassembly of the cell wall nanocomponents by cell wall partial dissolution and regeneration. The resultant structures, named integrated wood aerogels, show a unique combination of high specific surface area and strength due to partial retention of the wood hierarchical structure and formation of mesoporous nanofibrillated networks occupying the lumen. Different chemical systems are investigated, including DMAc/LiCl, ionic liquid (IL), and aqueous NaOH, and the processing-structure-property relationships are investigated. DMAc/LiCl is successfully used as proof of concept for integrated wood aerogel formation, but moisture sensitivity and toxicity of the system hinder further development. The IL [MTBD][MMP] is developed to solve the issues and to improve the structure control in cell wall dissolution and regeneration. An aqueous NaOH system advances the integrated cellulosic wood aerogel preparation further, considering low cost and greener chemistry. Wood composition, lignin in particular, is critical to the processing and final properties of the integrated wood aerogel. The influence of lignin content is investigated based on IL and NaOH systems. The influence of processing (such as chemical system, time and temperature) on the structure and properties (e.g. porosity, specific surface area, mechanical performance, thermal conductivity and charge density) of the aerogels are studied. 

Ascribing to the structure-property profile, the application of the integrated aerogel for efficient thermal insulation is demonstrated. Inspired by the water uptake in plants, high-performing pH-responsive wood power generators are formed based on water evaporation-induced electricity. The integrated aerogel structure greatly increases the solid/liquid interphase while allowing excellent mass diffusion.

The methodologies presented in this thesis for selective nanoscale reassembly of the wood cell wall pave the way for advanced wood nanostructure control. The integrated wood aerogel structure reported here provides a universal material platform for advanced material design, such as a self-sustaining wood power generator. The facile and scalable processing contribute toward sustainable high-performing bioaerogels which can compete with fossil-based materials.

Högporösa aerogeler baserade på förnybara material med strukturell funktionalitet är attraktiva för hållbar energireglering och energiutvinning. Strukturell anisotropi har potential att ge dessa material fördelaktiga diffusiva och mekaniska egenskaper, men framställning av ordnade strukturer kräver mer kostsamma och tidskrävande processer. Direkt användning av trä, där nanocellulosa redan är gynnsamt orienterad ger möjlighet att framställa billiga, skalbara och miljövänliga aerogeler.

I den här avhandlingen utforskas en ny typ av nanostrukturerade trämaterial skapade genom att fylla det tomma utrymmet i trä med aerogelstrukturer baserade på cellväggens egna biopolymerer. Strukturen kan kontrolleras genom att selektivt återuppbygga nanokomponenterna i cellväggen genom partiell upplösning och utfällning. De erhållna strukturerna, här benämnda integrerade träaerogeler, uppvisar en unik kombination av hög ytarea och styrka på grund av att träets hierarkiska struktur delvis bibehålls och att mesoporösa nätverk av nanofibriller bildas i lumen. Olika kemiska system undersöks, bland annat DMAc/LiCl, jonvätska och vattenbaserad NaOH, och förhållandet mellan framställning, struktur och egenskaper undersöks. DMAc/LiCl används med framgång för att visa det integrerade träaerogelkonceptets gångbarhet, men systemets fuktkänslighet och toxicitet hämmar fortsatt utveckling. Jonvätskan [MTBD][MMP] utvecklades för att underlätta processen och för att förbättra strukturkontrollen vid upplösning och utfällning av cellväggens komponenter. Ett vattenbaserat NaOH-system förbättrade framställningen av integrerade träaerogeler ytterligare, speciellt med avseende på lägre kostnad och miljövänligare kemi. Träets sammansättning, särskilt lignin, är avgörande för framställningen och för de integrerade träaerogelernas egenskaper. Lignininnehållets inverkan undersöks utifrån jonvätska- och NaOH-systemen. Framställningsparametrars (t.ex. kemiskt system, tid och temperatur) inverkan på aerogelens struktur och egenskaper (t.ex. porositet, specifik yta, mekanisk prestanda, värmeledningsförmåga och laddningstäthet) studeras.

Med de erhållna strukturegenskaperna kunde de integrerade träaerogelerna användas för effektiv värmeisolering och  med inspiration av den naturliga vattenledningsförmågan i träd designades högpresterande och pH-responsiva kraftgeneratorer. Den integrerade aerogelstrukturen ökade interaktionen mellan trämaterialet och vätskan samtidigt som den möjliggjorde bättre vätsketransport.

Metoderna som presenteras i den här avhandlingen visar en ny strategi för avancerad kontroll av träets nanostruktur genom selektiv återuppbyggnad av träets cellvägg. Den träbaserade aerogelstrukturen som uppvisas här utgör en helt biobaserad materialplattform för avancerad materialdesign. Den enkla och skalbara framställningen från trä bidrar i hög grad till hållbara och högpresterande bioaerogeler som kan konkurrera med fossilbaserade material.

Eco-friendly materials with superior thermal insulation and mechanical properties are desirable for improved energy- and space-efficiency in buildings. Cellulose aerogels with structural anisotropy could fulfill these requirements, but complex processing and high energy demand are challenges for scaling up. Here we propose a scalable, nonadditive, top-down fabrication of strong anisotropic aerogels directly from wood with excellent, near isotropic thermal insulation functions. The aerogel was obtained through cell wall dissolution and controlled precipitation in lumen, using an ionic liquid (IL) mixture comprising DMSO and a guanidinium phosphorus-based IL [MTBD][MMP]. The wood aerogel shows a unique structure with lumen filled with nanofibrils network. In situ formation of a cellulosic nanofibril network in the lumen results in specific surface areas up to 280 m2/g and high yield strengths >1.2 MPa. The highly mesoporous structure (average pore diameter ∼20 nm) of freeze-dried wood aerogels leads to low thermal conductivities in both the radial (0.037 W/mK) and axial (0.057 W/mK) directions, showing great potential as scalable thermal insulators. This synthesis route is energy efficient with high nanostructural controllability. The unique nanostructure and rare combination of strength and thermal properties set the material apart from comparable bottom-up aerogels. This nonadditive synthesis approach is believed to contribute significantly toward large-scale design and structure control of biobased aerogels.

The need for sustainable development creates opportunities for biomass-based materials design toward piezo-electric mechanical energy harvesting. Wood is promising due to its hierarchical, porous structure. Here, piezoelectric nanogenerators (PENGs) were prepared through nanostructure-controlled zinc oxide (ZnO) growth inside the outer wood layers of veneers. Mechanisms for formation of various ZnO nanostructures in wood are analyzed. Controlled morphologies of nanoparticles, nanorods, nanowires, and nanoflakes were realized and characterized by field emission-scanning electron microscopy (FE-SEM) and small angle x-ray scattering (SAXS), allowing tunable piezoelectric output. Nanostructures with higher aspect ratios i.e. nanorods and nanowires resulted in higher voltage during cyclic loading. An optimum voltage of 1.3-1.4 V was obtained with wood/ZnO nanowire or nanorod composites at a force of approximate to 8 N. The current output is in the range of 0.85-11 nA, which could be scaled up to ~130 nA with a larger area device. When mounted in shoe soles, these wood/ZnO PENGs generated 1-4 V from walking/jogging motions. The hydrothermal growth method is scalable, which facilitates practical applications.