Crystallization driven by microgravity includes a continuous state of free-fall where we remove the lens of gravity. The near absence of the force of gravity experienced on Earth results in changes in the physical environment experienced by matter and by living organisms. For materials like macromolecules (e.g., large chemical compounds including polymers, proteins or other biological molecules) the absence of gravity can benefit chemical reactions and physical processes like crystallization by reducing turbulent flow and/or slowing down the rate of nucleation, aggregation, and crystal formation. These changes can result in the formation of physically larger and more homogeneous crystals that in turn can be visually inspected on Earth to generate higher resolution images of the physical structure of the macromolecule.

The process of crystallization conducted in the microgravity environment of space for the pharmaceutical industry can be divided into 4 categories:

Drug development and discovery

New target identification

New formulations

Biomarker discovery

Currently, the following methods are used for large molecule crystallization for work that has been done on the ISS:

Vapor Diffusion – this is a hanging drop method where the solution with the target molecule is exposed to the precipitation solution as it vaporizes and gets adsorbed into the drop.

Batch – all materials (solution and precipitant solution) are in the same container (usually done in a well plate) and each solution is frozen as a separate layer. Once on the ISS, and when the experiment is ready to be performed, the entire system is defrosted at a specified temperature and the two solutions are left to contact one another. Other hardware keeps the two solutions separate until an astronaut initiates mixing them at the time of experiment initiation.

Capillary Counter Diffusion – a solution is placed on one side of an auger plug in a capillary and the precipitant solution is placed on the other side of the plug in the capillary. The precipitation fluid diffuses through the plug, and a concentration gradient develops in the solution with the target molecules. The best crystals form naturally where the concentration is ideal.

Additional methods used on the ground are:

Dialysis – this technique utilizes diffusion and equilibration of precipitant molecules through a semi-permeable membrane to slowly approach the concentration at which the macromolecule crystallizes. Provided that the precipitant is a small molecule like a salt or an alcohol, it can easily penetrate the dialysis membrane, and the protein is slowly brought into equilibrium with the precipitant solution.

Free Interface Diffusion – in a capillary both the precipitant solution and the protein solution are placed into the capillary and the ends are sealed and the precipitant and protein slowly mix via diffusion. The small diameter of a capillary minimizes convection with the capillary. This is similar to the counter diffusion method currently used on the ISS.