Shape-memory is the ability of a material to change and hold its appearance, then revert to its original shape in response to an external stimulus. Materials with this function have many useful applications, including in the automotive, aerospace and robotics industries.
Porous varieties of shape-memory materials have even greater uses, due to their ability to interact with biological molecules not just on their surfaces, but throughout their bulk. This makes them potentially very useful as medical implants, both as scaffolds for tissue regeneration and as drug-delivery devices.
However, although shape-memory effects are well-documented in metal alloys and ceramics, they are poorly understood and rarely studied in crystalline porous materials.
In a paper published in the journal Science Advances, a team led by Susumu Kitagawa of Kyoto University’s Institute for Integrated Cell-Material Sciences delivers a new set of observations in the field.
The team created a flexible metal-organic material (FMOM) by dissolving a variety of compounds, including zinc nitrate hexahydrate, in a colourless organic liquid called dimethylformamide.
Using a form of X-ray diffraction, the researchers were able to observe the structure of the resultant crystals and detected three distinct phases.
The first, called the alpha phase, exhibited a porosity — the volume available for incorporation of new molecules — almost half that of the total crystal volume. The second phase occurred in response to heating the alpha crystals at 130 degrees Celsius for 12 hours, and produced crystals that were denser, less porous and more distorted.
The researchers then added and removed carbon dioxide numerous times and found that the crystals retained their shape – the ‘shape-memory’ gamma phase.
The addition of nitrogen and carbon monoxide at different temperatures also induced the beta-gamma transition in the crystals.
The team found that it was possible to return to the original alpha phase by soaking the crystals in dimethylformamide for five minutes.
Kitagawa and his colleagues used the results to formulate a series of “crystal engineering principles” to assist the design of future porous shape-shifters.