If part of the bulky, complex life-support system on the International Space Station breaks down, there’s always a relatively easy (if expensive) way to fix it — send up a spacecraft with spare parts from Earth. Crew members on a long journey to Mars wouldn’t have that option. Water Walls is meant to be a simpler, more reliable life-support system inspired by nature. It replaces complicated active mechanical systems with passive “cells” that transfer fluids by forward osmosis, removing CO2, revitalizing oxygen, recycling urine, processing solid waste and growing algae just in case the astronauts run out of food.