Table of Contents

Sciences

Septech Experiment

Septech experiments were conducted in 1998 onboard MissionSTS-95. Conducted by Dr. Donald Brooks of the University of British Columbia, SepTech aimed at improving the effectiveness of phase partitioning, a technique widely used for the separation of different types of living cells, including cancer cells. The target of the experiment was to measure the distribution of derivative latex particles in phase-separated mixtures of aqueous solutions; the solutions of incompatible polymers were composed in the two-phase region of the phase diagram.

The phase partitioning method for cell separation uses two liquids that do not dissolve in each other. After mixing with a sample of biological cells, the two liquids tend to separate into two "phases" (just as salad dressing made from oil and vinegar will separate with the oil floating on top of the vinegar). The separation of cells into different types is achieved because some cells will remain with one of the phases and others with the second phase. On Earth, when cell mixtures are shaken with the two-phase system and the mixture is allowed to separate into upper and lower phases, it has been found that when the phases are sinking and floating through each other, some of the cells leave the phase which they are attracted to, resulting in reduced separation efficiency at the end of the experiment. It is believed that this loss of efficiency is caused by gravitational forces acting on cell during the sinking and floating of the two phases. Performing this kind of partitioning experiment in space greatly reduces such forces.

The experiment flew in the ADvanced SEParations (ADSEP) apparatus developed by Space Hardware Optimization Technology, Inc. of Indiana. The hardware contained 22 individual experiment chambers. Two-phase systems containing plastic "model" cells were loaded into each chamber. In space the samples were mixed and allowed to separate, and the phases were isolated. The samples were returned to Earth in order to be analyzed.

Conducting of the experiment

Ground-based work has shown that the dependence of the partition coefficient on the contact angle between the particle and the phase interface is not as strong as predicted by a thermodynamic model of the process, probably because of the stochastic effects of buoyancy-driven flows on particle distributions as the phases separate following mechanical mixing. Reduced gravity should eliminate this interference. Localization of the phases in low gravity depends on finding appropriate materials for the two halves of the container that are each wet by one of the phases. Equilibrium is reached with the complete separation of the phases, one wetting one container half and the other wetting the second, with a flat interface in between. 

Results

The simple partitioning experiment performed on STS-95 in the ADSEP apparatus with optimally coated latex particles demonstrated for the first time that the disposition of the demixed phases in reduced gravity could be controlled by controlling surface wetting of the two halves of the chamber walls. Analysis of the experiments compared to a ground control showed:

• in virtually all chambers the phase separated as expected on the basis of the surface wetting difference

• on average the particle distributions observed differed significantly from the ground control.

Interpretation of the particle distributions is confounded by uncertainty regarding which chamber stirred in space, as many chambers did not stir when the unit was examined post-flight. Based on only those chambers that showed post-flight stirring, the microgravity partition into the interface was much higher than on the ground, as thermodynamic equilibrium predicts.