Starting with the NASA-made prototype, Tupperware worked with Techshot Inc., an official spaceflight implementation partner of the ISS, to test its hardware designs.
The device needed to be "passive," he said, "meaning that it did not require any additional energy in order to make it work."
"It just uses natural forces," Kusuma added. "One of the things we had to solve was the fluid dynamics in zero gravity."
Tupperware also "leveraged" the expertise of contacts at NASA "to solve the problem," Kusuma said.
"The big challenge from an engineering point of view is how do you get water where you need it to be," he said.
Unlike on Earth, Kusuma said, "where if you pour water into a potted plant, you know everything will follow a natural order."
In space, he said, water "just wants to go everywhere."
Tupperware used "generous curved surfaces" in PONDS instead of sharp inner corners, where water "tends to want to cling," Kusuma said.
It also used its materials "to wick the water from the reservoir to the root cylinder where the plant is grown," he said.
So far, Tupperware has launched PONDS three times to the ISS for testing.
On Tupperware's first attempt, which was launched in April 2018, the PONDS units were "overwatered."
"The next point of focus was to allow less water to flow," Kusuma said. During the second mission, he said, "everything germinated but the seeds were too dry."
NASA launched Tupperware's third mission to the ISS in March 2020.
During this test, Kusuma said, the plants grown in PONDS grew stunted, or smaller than they should have been. NASA said the seeds were "overirradiated," he said, a process NASA applies to organic material to ensure no bacteria or pathogens enter the ISS.
Tupperware's third PONDS test is currently still on the ISS and expected to return to Earth at the end of November, he said.
Astronauts can now grow three types of lettuce with PONDS: Chinese cabbage, mizuna mustard and red Russian kale, Kusuma said. They have also successfully grown zinnia flowers and hope to begin growing cherry tomatoes.
"Those have always been difficult to grow in space … and that's because tomatoes use a lot of water," he added.