Keeping water where it's wanted

Hydroponic systems are a good choice for growing plants in space, because they allow plants to grow directly in a nutrient solution rather than heavy and bulky soil or compost substrates. These systems also enable safe recycling of the nutrient solution, which is not the case in systems using substrates. Additionally, ensuring the nutrient solution contains the right balance for the plants at different stages of growth requires careful monitoring, which is most easily achieved in a substrate-free system.

A deep-water culture system is a common hydroponic setup in which plants are suspended above a tank of nutrient solution, and their roots grow down into it. On Earth, gravity keeps the solution at the bottom of the tank. However, in microgravity, water can flow in all directions and cling to surfaces – which means that roots can struggle to find moisture while leaves and stems are drowning in it.

Designing a system that keeps the nutrient solution where it's needed, around plant roots, is an interesting challenge. In the Substrateless Root Module for Hydroponic Crop Cultivation in Fractional Gravity (SULEROMO) activity, researchers from NTNU Samfunnsforskning AS – CIRiS in Norway explored whether it’s possible to develop a membrane that plant roots can grow through, but which keeps the solution in the root zone.

"Gravity does this separation job for us on Earth. Once you take it away, water is free to go anywhere, and that's the core problem this membrane is designed to solve," says Achim Gerstenberg, Senior Researcher at CIRiS and SULEROMO project lead.

Finding a material that plant roots can grow through isn't difficult, and it's not hard to find a waterproof material – combining those two characteristics is where it gets tricky. The proof-of-concept membrane the team developed has two layers – a felt top layer with a water-repellent silicone-based coating on the underside. Wetting the felt layer allows seeds to germinate, after which the roots will grow through the felt and the coating, finding the nutrient solution beneath. The felt is flexible enough to allow roots to grow through and expand, while the coating ensures impermeability around the stem to control leakage.

To test the RSM, the team grew two plant species in 3D-printed deep-water culture tanks. Kale and pak choi were chosen as the test plants, as they have different morphological traits. The test conditions used uncut plain felt or an RSM (felt coated using a screen-printing process). Seeds were germinated directly on the felt, in membrane holders that were moved to the deep-water culture tanks on day nine.

Around three-quarters of the seedlings successfully penetrated the membrane and grew roots that reached down into the nutrient solution. After six weeks, all the plants looked healthy, with no visible signs of stress. However, the plants grown on the RSM produced less biomass than plants grown in a standardised deep-water culture system with pre-cut felt to allow for stem expansion. This difference was probably due to a slight delay as the roots crawled across the felt to find a way through and some restriction in stem expansion.

Leak tests performed at intervals showed that while the nutrient solution passed freely through the uncoated felt, the RSM leaked considerably less, at just over one millilitre a minute when mildly pressurised. As the plants grew and their stems moved and thickened, leaks became more frequent. However, some leaks went on to reseal themselves.

These early results show that the RSM concept has the potential to enable low-leakage hydroponic systems in low-gravity environments. There is a trade-off between ensuring an entirely leak-free system and allowing plants to thrive, and the team has identified two possible directions for further development. The first involves refining the materials and production methods to create an RSM that better accommodates a moving, growing plant stem. The second would adapt the growing system itself to reduce plant stem movement, perhaps by mechanically supporting the plant or reducing the pressure used to circulate the nutrient solution.

The team are now working to publish their initial findings, and to secure funding to continue the project.

"Reliable, substrate-free cultivation systems will be an important technology for future exploration missions, where every kilogram of hardware and every hour of crew time is precious. This activity has given us a much clearer picture of where the engineering challenges lie," says Christel Paille, ECLS Engineer and ESA lead for the activity.

The SULEROMO activity originated as an idea submitted through ESA's Open Space Innovation Platform, seeking out promising new concepts for space research, and was funded by the Discovery element of ESA's Basic Activities.