Biodegradable and Flexible Wood-Gelatin Composites for Soft Actuating Systems.

Compliant materials are indispensable for many emerging soft robotics applications. Hence, concerns regarding sustainability and end-of-life options for these materials are growing, given that they are predominantly petroleum-based and non-recyclable. Despite efforts to explore alternative bio-derived soft materials like gelatin, they frequently fall short in delivering the mechanical performance required for soft actuating systems. To address this issue, we reinforced a compliant and transparent gelatin-glycerol matrix with structure-retained delignified wood, resulting in a flexible and entirely biobased composite (DW-flex). This DW-flex composite exhibits highly anisotropic mechanical behavior, possessing higher strength and stiffness in the fiber direction and high deformability perpendicular to it. Implementing a distinct anisotropy in otherwise isotropic soft materials unlocks new possibilities for more complex movement patterns. To demonstrate the capability and potential of DW-flex, we built and modeled a fin ray-inspired gripper finger, which deforms based on a twist-bending-coupled motion that is tailorable by adjusting the fiber direction. Moreover, we designed a demonstrator for a proof-of-concept suitable for gripping a soft object with a complex shape, i.e., a strawberry. We show that this composite is entirely biodegradable in soil, enabling more sustainable approaches for soft actuators in robotics applications.

Anisotropic wood-hydrogel composites: Extending mechanical properties of wood towards soft materials' applications.

Delignified wood (DW) offers a versatile platform for the manufacturing of composites, with material properties ranging from stiff to soft and flexible by preserving the preferential fiber directionality of natural wood through a structure-retaining production process. This study presents a facile method for fabricating anisotropic and mechanically tunable DW-hydrogel composites. These composites were produced by infiltrating delignified spruce wood with an aqueous gelatin solution followed by chemical crosslinking. The mechanical properties could be modulated across a broad strength and stiffness range (1.2-18.3 MPa and 170-1455 MPa, respectively) by varying the crosslinking time. The diffusion-led crosslinking further allowed to manufacture mechanically graded structures. The resulting uniaxial, tubular structure of the anisotropic DW-hydrogel composite enabled the alignment of murine fibroblasts in vitro, which could be utilized in future studies on potential applications in tissue engineering.

Natural Wood-Based Catalytic Membrane Microreactors for Continuous Hydrogen Generation.

The development of controlled processes for continuous hydrogen generation from solid-state storage chemicals such as ammonia borane is central to integrating renewable hydrogen into a clean energy mix. However, to date, most reported platforms operate in batch mode, posing a challenge for controllable hydrogen release, catalyst reusability, and large-scale operation. To address these issues, we developed flow-through wood-based catalytic microreactors, characterized by inherent natural oriented microchannels. The prepared structured catalysts utilize silver-promoted palladium nanoparticles supported on metal-organic framework (MOF)-coated wood microreactors as the active phase. Catalytic tests demonstrate their highly controllable hydrogen production in continuous mode, and by adjusting the ammonia borane flow and wood species, we reach stable productivities of up to 10.4 cmH23 min-1 cmcat-3. The modular design of the structured catalysts proves readily scalable. Our versatile approach is applicable for other metals and MOF combinations, thus comprising a sustainable and scalable platform for catalytic dehydrogenations and applications in the energy-water nexus.

Sustainability in wood materials science: an opinion about current material development techniques and the end of lifetime perspectives.

Wood is considered the most important renewable resource for a future sustainable bioeconomy. It is traditionally used in the building sector, where it has gained importance in recent years as a sustainable alternative to steel and concrete. Additionally, it is the basis for the development of novel bio-based functional materials. However, wood's sustainability as a green resource is often diminished by unsustainable processing and modification techniques. They mostly rely on fossil-based precursors and yield inseparable hybrids and composites that cannot be reused or recycled. In this article, we discuss the state of the art of environmental sustainability in wood science and technology. We give an overview of established and upcoming approaches for the sustainable production of wood-based materials. This comprises wood protection and adhesion for the building sector, as well as the production of sustainable wood-based functional materials. Moreover, we elaborate on the end of lifetime perspective of wood products. The concept of wood cascading is presented as a possibility for a more efficient use of the resource to increase its beneficial impact on climate change mitigation. We advocate for a holistic approach in wood science and technology that not only focuses on the material's development and production but also considers recycling and end of lifetime perspectives of the products. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Luminescent and Hydrophobic Wood Films as Optical Lighting Materials.

Most materials used for optical lighting applications need to produce a uniform illumination and require high mechanical and hydrophobic properties. However, they are rarely eco-friendly. Herein, a bio-based, polymer matrix-free, luminescent, and hydrophobic film with excellent mechanical properties for optical lighting purposes is demonstrated. A template is prepared by turning a wood veneer into porous scaffold from which most of the lignin and half of the hemicelluloses are removed. The infiltration of quantum dots (CdSe/ZnS) into the porous template prior to densification resulted in almost uniform luminescence (isotropic light scattering) and could be extended to various quantum dot particles, generating different light colors. In a subsequent step, the luminescent wood film is coated with hexadecyltrimethoxysilane (HDTMS) via chemical vapor deposition. The presence of the quantum dots coupled with the HDTMS coating renders the film hydrophobic (water contact angle ≈ 140°). This top-down process strongly eliminates lumen cavities and preserves the orientation of the original cellulose fibrils to create luminescent and polymer matrix-free films with high modulus and strength in the direction of fibers. The proposed optical lighting material could be attractive for interior designs (e.g., lamps and laminated cover panels), photonics, and laser devices.

Enzyme immobilization inside the porous wood structure: a natural scaffold for continuous-flow biocatalysis.

Enzymes are often immobilized on solid supports to enable their recovery from reaction solutions, facilitate their reuse and hence increase cost-effectiveness in their application. Immobilized enzymes may even be used for flow-through applications in continuous processes. However, the synthesis of traditional immobilization scaffolds and immobilization techniques lack sustainability as they are often based on fuel-based materials and tedious synthesis- and immobilization approaches. Here, we present the natural material wood as a green alternative for enzyme immobilization. Its natural structure provides a mechanically stable porous scaffold with a high inner surface area that allows for directional flow-through of liquids. Enzymes were immobilized by nanoparticle-mediated adsorption, a simple, versatile and completely water-based process. The resulting wood-enzyme hybrids were intensely investigated for the model enzyme laccase. Reaction kinetics, as well as catalytic activities at various pH-values, temperatures, and ionic strengths were determined. The wood-enzyme hybrids could quickly and completely be removed from the reaction solution. Hence, they allow for multifold reusability. We show a series of 25 consecutive reaction cycles with a remaining activity in the last cycle of 90% of the maximal activity. Moreover, the anisotropic porosity of wood enabled the application of the hybrid material as a biocatalytic flow-through reactor. Flow-rate dependent productivity of a single-enzyme reaction was determined. Moreover, we show a two-step reaction cascade in continuous flow by the immobilization of the enzymes glucose oxidase and horseradish peroxidase. Therefore, the natural material wood proved to be a promising material for application in continuous-flow biocatalysis.