Freeze casting is a manufacturing technique used to produce highly porous materials with custom-designed hierarchical architectures, well-defined pore orientation, and multifunctional surface structures. These materials are versatile and can be used in various applications, including biomedicine, environmental engineering, and energy technologies. An article in Nature Reviews Methods Primer provides a guide to freeze-casting methods, highlighting current and future applications and emphasizing X-ray tomoscopy for characterization.
The Primer, authored by Prof. Ulrike Wegst and experts in tomoscopy, explores various freeze casting processes and lyophilization before delving into characterization techniques for analyzing complex material architectures. X-ray tomoscopy is specifically highlighted for its ability to analyze crystal growth and structure formation in materials like polymers, ceramics, metals, and composites in real-time and 3D, making it ideal for quantifying anisotropic crystal growth in different materials.
Freeze casting was initially developed over 40 years ago for tissue scaffolds and has since been used in various applications in biomedicine and engineering. The highly porous structure of freeze-cast materials allows for integration with host tissues and support for healing processes. These materials are now used in a wide range of applications, from innovative catalysts to porous electrodes for batteries or fuel cells, using different solvents, solutes, and particles to achieve desired structures, shapes, and functionalities.
The process of freeze casting involves dissolving or suspending a substance in water, placing it in a mold, and applying a defined cooling rate to directionally solidify the sample. Phase separation then occurs, with the ice templating the solute/particle phase. After full solidification, the solid solvent is removed through lyophilization, revealing a highly porous, ice-templated scaffold with cell walls composed of the solute/particle that self-assembled during solidification. The properties of the material, including pore size, geometry, orientation, and surface characteristics, can be tailored for specific applications.
Future experiments on the International Space Station are planned to gain further insights into the freeze casting process under microgravity conditions. The reduced gravitational force in microgravity minimizes effects of sedimentation and convection, allowing for the study of structure formation with fewer disruptions. The experts hope that these experiments will lead to a better understanding of freeze casting processes and the production of defect-free materials with custom-designed properties.
