Some friends and I came across this challenge while searching for a final project for our High School Senior year Humanitarian Design class. We have spent the past two months, designing, planning, and re-designing our methodology for growing lettuce in a zero-gravity environment.
Designing for growth in microgravity presents many unique challenges not present when designing for growth on Earth. We took into many factors, such as space-efficiency, ventilation, lighting, and hydration. The most prominent feature of our design is a set of screw-shafts controlled by a Raspberry Pi 4B+, allowing us to overlap the foliage of different plants in order to be more space-efficient. Additionally, we set up our Pi with humidity and temperature sensors, and a 7” touchscreen GUI, so that the internal conditions of the setup could be monitored without disturbing the equilibrium. If you enjoy this guide, please consider voting for our design!

Step 1: Brainstorming Phase

About a month was spent coming up with designs for our project. During this time, we were able to identify several potential problems that our design needed to deal with.

Soil - In microgravity environments, there is nothing holding soil into its container, so dirt particles may float away and cause disturbances.
Lack of gravity - On Earth, many seedlings will grow in the opposite direction to gravity before they have broken the surface of the soil because they don’t have a reference frame of light. In microgravity, germinated seeds don’t have this gravitational reference frame.
Hydration - Plants require water to grow. In microgravity, water tends to ball up due to surface tension, meaning that it is harder to get plants to absorb it.
Nitrogenation - Seedlings don’t only require water to grow; they also require many other nutrients, primarily including nitrates.
Airflow - During photosynthesis, plants pull carbon dioxide out of the air to create glucose. Airflow is necessary because of this since air needs to be displaced for the plants to have access to new carbon.
Space - Nasa imposed a size constraint: our design had to fit within a cube with 50cm side lengths.

Step 2: Preliminary Designs

We drew up two main design iterations changing everything from the shape of the housing to the type of pump used in the irrigation system.
1st design: Our initial design was spherical in shape to maximize the space given. Initially, we planned to embed electronics and irrigation systems inside the walls of the sphere. Plants were placed along the wall with a light source in the middle causing the plants to grow inward. We hoped that this would maximize the growing space each plant had access to.

Problems:

As the plants grew larger, they choked each other’s line of sight to the light source, resulting in the death of smaller plants.
The spherical design was extremely hard to create prototypes of, and we suspected that the reward would not be worth the opportunity cost of creation.

2nd design: The second iteration of our design our first concept to include a screw drive. Because we realized that the plants would tend to choke out each other’s light as they grew, we decided to use a screw drive to lower the plants as they grew to give them more space as they needed it. We created a system of drives that lowered a platform of twelve plants. Because our design needed only 25cm of vertical space, we realized that we could stack the design on top of itself to accommodate twenty-four plants. In addition to improving our space utilization, we also changed our gear pumps to peristaltic pumps in order to increase the efficiency in a zero-gravity environment. Problems:

Some plants grew at different rates, which resulted in continued light choking.

Step 3: Final Design

For our final design, we further iterated our screw-drive design. Instead of utilizing a single screw drive, we created several cups that could be raised and lowered independently of one another. This allowed us to write code via our Raspberry Pi to ensure that the plants wouldn’t interfere with each other. This design optimized our plants’ growth because cups with small plants were able to grow near the light source, and as they grew they were allowed more space via a lowering of their cup.

Step 4: Seed Container

We designed our seed cups to be used with silica gel beads. The gels were hyper-absorbent, and the plants’ root systems were able to draw water from the beads, meaning that there was no loose water in our system. In addition, we inserted nutrients into the pump system which were then absorbed by the beads, meaning that the plants could draw their nutrients directly from them. A net wraps around the top only allowing the plant to get through and not the silica gel beads.

Step 5: Walls

We created our container with carbon fiber, with acrylic windows in our prototype allowing easy visual feedback. Because we want to cut out light pollution in the final design in order to accurately track the plants’ growth given a light source, the acrylics would be removed in a working model.

Step 6: Irrigation

We provided water to our plants via a peristaltic pump positioned on the interior of the growing chamber. We ran tubing from the pump to each cup in the system, as well as to a reservoir. Inside each cup, tubing was perforated in order to dispense the nutrient-rich water which was then instantly absorbed by the silica beads.

Step 7: Lighting

We provided light to our plants with variable-intensity RGB lights. This allows for experiments to be run on the efficiency of different light intensities across different color spectra.

Step 8: Airflow

We provided airflow to our system via four small fans on each side of the box, each moving eight liters of air per minute, resulting in a total airflow of sixty-four liters of air per minute.

Step 9: Future Considerations

We would like to have each cup individually controlled by a single motor, rather than grouped via shaft. We designed our model as such, but due to budget restrictions, we were not able to create a working prototype in this manner.
We would like to improve our germination system. As currently designed, our system requires each seed to be manually inserted into its chamber.
We would like to automate this process in the future. We would like to automate our harvesting system. Similar to the germination process, harvesting plants is currently a completely manual process; we would like to automate this too in the future.
We would like to add a camera system via the Raspberry Pi. Since the Pi has such good native support for cheap, high-resolution cameras at a low framerate, we would like to implement this into our design. It would remove the light-pollution issue from our prototype while still allowing for easy monitoring of the interior.

We hope you enjoyed our design; if you did, please consider voting for us!