Water pollution caused by industrialization and human activities poses a significant global challenge. Current water purification methods often involve multiple, time-consuming processes, separating physical filtration from chemical catalysis. This separation limits efficiency and creates challenges in optimizing both mechanical robustness (necessary for high throughput) and catalytic performance. This research addresses these limitations by developing a structure-function integrated system based on bio-inspired metamaterials. Metamaterials, artificial structures with tailored properties, offer a promising approach. Microlattice metamaterials, with their interconnected units, struts, and pores, control mechanical strength (determined by struts) and fluid transport (influenced by pore size distribution). They have found applications in diverse fields, including bioengineering (mimicking bone properties) and thermal management. However, traditional periodic microlattices often suffer from coupled properties—high strength often means reduced porosity and transport. This limitation hinders the optimization of multiple functions. Biomimicry provides inspiration for overcoming these limitations. Natural structures, such as the Douglas fir, have evolved efficient designs that integrate strength and transport. The Douglas fir’s unique staggered, bimodal pore distribution effectively decouples these properties. This research mimics this design using a microlattice overlap strategy, generating bimodal pores to enhance both strength and transport capacity within a single, integrated system. Additive manufacturing techniques, specifically selective laser melting (SLM), are utilized to fabricate complex metamaterial structures from 316L stainless steel, chosen for its strength and corrosion resistance. Electrochemical deposition adds a cobalt coating, boosting catalytic functionality. This integrated design of structure and function, termed a 'metamaterial catalyst', opens up opportunities for improved water purification systems and other flow-related applications.

Existing literature highlights the limitations of current water purification technologies, emphasizing the need for efficient, high-throughput systems. Studies have explored nanotechnology applications for water security, showcasing the potential of advanced materials. Research in 3D-printed catalysts demonstrates the possibility of integrating catalytic function within the structural framework. However, these studies often focus separately on either the catalytic material or the structural design, not achieving the synergistic improvement possible through integrated design. The use of metamaterials in various applications has also been explored, with a focus on tuning mechanical and transport properties. Bio-inspired designs, mimicking natural structures such as bamboo and lotus roots, have demonstrated superior performance in terms of strength, stiffness, and transport. This research builds on this prior work, using the Douglas fir as a model to design a novel metamaterial structure optimized for both mechanical strength and fluid transport. The selected literature showcases prior research in the areas of water purification, metamaterials, and bio-inspired design, laying the foundation for the innovative approach presented in this study.

This research involves a multi-step methodology, starting with the design and fabrication of the wood-inspired metamaterial catalyst. The design process uses computational modeling and biomimicry, inspired by the structural properties of Douglas fir wood. **Metamaterial Fabrication:** The metamaterials were fabricated using selective laser melting (SLM), a 3D printing technique. 316L stainless steel powder was used as the base material due to its high strength and corrosion resistance. Different strut diameters (0.30, 0.35, and 0.40 mm) and overlap rates (0, 30, 50, and 70%) were tested to optimize the design. The SLM process parameters were carefully controlled to ensure the desired microstructure and mechanical properties. The process parameters include input laser power (320W), scan speed (650mm/s), layer thickness (50µm), and hatch distance (140µm). **Cobalt Coating:** After 3D printing, the stainless steel metamaterials were coated with cobalt using an electrochemical deposition process. This coating enhances the catalytic activity of the metamaterial. The electrochemical deposition bath contained cobalt chloride hexahydrate, boric acid, and other reagents. A three-electrode system (saturated Ag/AgCl reference electrode, graphite counter electrode) was used for the coating process. **Characterization:** Various characterization methods were employed to analyze the fabricated metamaterials. Scanning electron microscopy (SEM) was used to examine the microstructure. Micro-focus computed tomography (micro-CT) was used to quantify the geometric deviations introduced by the SLM process. Mechanical properties were measured through quasi-static compression tests. Computational fluid dynamics (CFD) simulations were used to study fluid flow behavior and permeability. **Water Purification Testing:** The catalytic activity of the metamaterials was tested using a fixed-bed reactor. The degradation of sulfamethoxazole (SMX) in the presence of peroxymonosulfate (PMS) was used as a benchmark reaction. The degradation process was analyzed using high-performance liquid chromatography (HPLC). Total organic carbon (TOC) analysis was used to quantify the mineralization of the pollutants. Free radical quenching experiments and electron paramagnetic resonance (EPR) spectroscopy were used to elucidate the catalytic mechanism. X-ray photoelectron spectroscopy (XPS) was used to study the changes in surface oxidation state of the catalyst before and after water treatment. Electrochemical measurements were also employed to investigate electron transfer during the degradation process. Long-term stability tests were performed to evaluate the catalyst's performance over extended periods.

The study's key findings highlight the superior performance of the wood-inspired metamaterial catalyst compared to traditional microlattices. The 70% overlap design demonstrated significantly enhanced properties: * **Mechanical Robustness:** Three times higher compressive strength compared to traditional microlattices, attributed to the increased relative density and the optimized stress distribution provided by the overlapping microlattices and bimodal pores. Finite element analysis confirmed stress concentration near the nodes, indicating effective stress dispersion. * **Transport Properties:** The bimodal pore structure generated by the overlap strategy significantly impacts fluid flow. While traditional microlattices show high permeability, the wood-inspired design reduces permeability, but this is beneficial for catalysis by providing a longer contact time between the pollutant and catalyst. CFD simulations show slower fluid velocities and higher shear rates, which promotes better pollutant-catalyst interactions. * **Catalytic Efficiency:** Four times higher normalized reaction kinetics compared to traditional microlattices, directly related to the increased surface area and improved mass transport provided by the unique porous structure. The catalyst effectively degrades sulfamethoxazole (SMX) within 15 minutes, achieving a high degree of mineralization (TOC analysis confirmed >80% mineralization within 30 min). The mechanism involves both radical and non-radical pathways, with hydroxyl radicals (OH) playing a dominant role (46.3% contribution according to radical quenching experiments). * **Long-Term Stability:** The catalyst maintained over 90% degradation efficiency for over 96 hours in continuous flow experiments, demonstrating remarkable long-term stability. Deactivation after prolonged use could be reversed by reduction treatment. * **Robustness to Impurities:** The catalyst exhibits robust performance in the presence of various impurities, maintaining high degradation efficiency even with 200 ppm of common anions (sulfate, nitrate, chloride, bicarbonate) and 20 ppm humic acid, demonstrating potential for real-world application. * **Broad Applicability:** The catalyst demonstrated high efficiency in degrading other pollutants such as bisphenol A (BPA) and norfloxacin (NOR).

The findings demonstrate the successful integration of structural and functional design in a water purification system. The wood-inspired metamaterial catalyst addresses the limitations of current methods by effectively decoupling mechanical strength, mass transport, and catalytic efficiency. The significantly improved performance metrics—strength, permeability, and reaction kinetics—validate the effectiveness of the bio-inspired design. The high mineralization rates and long-term stability highlight the practical potential of this technology for large-scale water treatment applications. The mechanism analysis revealed that both radical and non-radical pathways contribute to pollutant degradation. The dominance of hydroxyl radicals suggests that the catalyst effectively activates PMS, generating highly reactive species for efficient pollutant removal. The observed long-term stability, even in the presence of various interfering substances, suggests that the catalyst is robust and suitable for practical application. This research has significant implications for developing advanced water treatment technologies, offering a more efficient and sustainable solution to a pressing global problem. The design principles can be extended to other flow-related applications, such as flow batteries and other chemical reactors.

This study presents a novel wood-inspired metamaterial catalyst for water purification, demonstrating superior performance compared to traditional methods. The integrated design successfully decouples mechanical, transport, and catalytic properties, resulting in a robust and highly efficient system. The high degradation efficiency, long-term stability, and adaptability to various pollutants and conditions highlight the potential for large-scale applications. Future research could explore the use of other materials and 3D printing techniques to further optimize the design and expand the range of applications. Investigating the catalyst's performance with a wider array of pollutants and under various environmental conditions would also enhance its practical relevance.

While the study demonstrates promising results, some limitations should be noted. The experiments were conducted primarily with sulfamethoxazole (SMX), and further investigation is needed to assess the catalyst's performance against a broader range of pollutants. The manufacturing process, using selective laser melting (SLM), may limit the scalability and cost-effectiveness for large-scale production, although the use of alternative 3D printing techniques could potentially mitigate this. Long-term studies over extended periods are needed to fully assess the durability and long-term efficacy of the metamaterial catalyst under real-world conditions. Further investigation into the potential toxicity of any degradation by-products should be conducted.