[Image above] Research on ice-templated ceramics (left) guided the work of Carnegie Mellon scientists on ice-templated artificial blood vessels. The picture on right is from an earlier study in which the Carnegie Mellon scientists developed the freeform 3D ice-templating method they used to create the artificial blood vessels. Credit: (left) White et al., International Journal of Applied Ceramic Technology; (right) Garg et al., Advanced Science (CC BY 4.0)
By Becky Stewart
If, like me, you live in a region underlain by silty clay soil, you may have emerged on a late fall or early spring morning to find your yard disrupted by frost heaving. It’s a bit disconcerting to find your carefully mulched garden beds suddenly looking like Hobbiton, with rows of needle-like ice crystals shoved up into mounds above the ground.
Although these frost-heaving structures are merely a curiosity for many people, for scientists in the 1980s, they were the inspiration for early applications of ice templating.
Ice templating, or freeze casting, is a controllable, scalable, and environmentally friendly process for creating all kinds of porous materials, traditionally rigid ones such as ceramics and metals. The technique involves harnessing the phase transformation of a solvent from liquid to crystalline solid. As the crystals grow, they redistribute particles of the target material within the solution or slurry. Removing the crystals leaves pores in the formed material.
Water is the most used solvent because it is cheap and abundant, and its freezing properties are well understood. But though the solidification behavior of water is well understood, the complex interactions between water as the solvent and the other target materials in the process are less well researched. As such, the complete range of properties that could be achieved in the final material using this process is still unknown.
The early days of ice templating
Early ice templating research focused on ceramic materials. The goal was to create ceramics with a higher degree of porosity but comparable compressive strength to the unaltered material. Some of the applications proposed for ceramics fabricated by this process include scaffolds for antibacterial silver loading in biocompatible materials, in composites with negative thermal expansion and reduced material costs, and in dye-sensitized solar cells. However, commercial applications of ice-templated rigid materials are currently limited to alumina foams or cellulose foams in thermal insulation.
The extensive work on ice templating of ceramics in the past decade has provided great insight into how solvent choice, particle size and concentration of the solute powder, and freezing conditions impact the templating process. This knowledge enabled researchers to begin applying these insights to other materials, as described in the section below.
New applications of ice templating
Advancements in the ceramics field have often supported innovation and discoveries in other fields. For example, modern electronics would not be possible without the early development of ceramic insulators. Space exploration would be much more challenging without high-quality telescope lenses, which are made from glass or glass-ceramics.
Similarly, the use of ice templating to create porous materials has moved beyond rigid ceramic and metal materials to soft matter, such as polymers. It is also enabling the creation of bioinspired materials with hierarchical architectures, as demonstrated in a recent study by scientists at Carnegie Mellon University.
In their study, the Carnegie Mellon scientists used a freeform 3D ice-templating method they previously developed to create artificial blood vessels in engineered tissue. The Carnegie Mellon team drew on historical research on ice-templated ceramics (and other materials) for inspiration to make this advancement into flexible materials.
Ice templating of biomimetic materials holds enormous potential. Fabrication of microporous polymeric materials by this method can provide tissue-growth scaffolds, injectable microcarriers for drug delivery, or silk fibroin nacre for smart materials.
The knowledge gained from early work on ice-templated ceramics is making these new applications possible. Ice templating may remain in the forefront of science in the future, enabling the development of many more materials with tunable porosity and other properties.
Further reading
Deville, S., “The lure of ice-templating: Recent trends and opportunities for porous materials,” Scripta Materialia 2018, 147: 119–124.
Deville, S., Meille, S., and Seuba, J., “A meta-analysis of the mechanical properties of ice-templated ceramics and metals,” Science and Technology of Advanced Materials 2015, 16(4): 043501.
Hautcoeur, D., “Freeze casting,” Encyclopedia of Materials: Technical Ceramics and Glasses 2021, 1: 195–202.
Li, M., Dai, X.G., Gao, W.W., and Bai, H., “Ice-templated fabrication of porous materials with bioinspired architecture and functionality,” Accounts of Materials Research 2022, 3(11): 1173–1185.