Mechanism of Mold-Induced Degradation of Silicone Rubber Under Microgravity in Space Environment

Inside the sealed, climate-controlled cabins of a space station, a quiet battle is taking place between human technology and the microscopic life that inevitably travels with us. Silicone rubber, a material prized for its flexibility and ability to withstand extreme temperatures, serves as the critical sealant that keeps air in and the vacuum of space out. It is the unsung hero holding together life-support systems and electrical components. However, this material is not immune to the tiny fungi that hitch a ride on astronauts and equipment. In the warm, humid, and nutrient-rich environment of a spacecraft, these fungi can settle on rubber surfaces, feeding on the organic additives left behind during manufacturing. The question for engineers has long been whether the unique conditions of space—specifically the absence of gravity—would change how these organisms attack their host. Would the floating, weightless environment make the rubber more vulnerable, or would it somehow protect the material by disrupting the fungi's ability to grow?

To answer this, researchers from China's space program and universities conducted a direct comparison, sending samples of silicone rubber into orbit aboard the Shenzhou spacecraft while keeping identical samples on the ground. They inoculated both sets of samples with a common mold known as Aspergillus brasiliensis, a fungus often found in soil and decaying vegetation. The space samples spent ninety days in the real microgravity of the China Space Station, while the ground samples grew in a controlled laboratory environment that mimicked the station's temperature and humidity. By bringing the space samples back to Earth and analyzing them alongside their terrestrial twins, the team could observe exactly how the lack of gravity influenced the mold's behavior and the subsequent damage to the rubber.

The results revealed a surprising difference in how the mold behaved in the two environments. On Earth, the mold thrived. The spores, which are the tiny reproductive units of the fungus, settled heavily onto the rubber surface, forming dense clusters. As they grew, they secreted a significant amount of organic acids, including a specific type called butyric acid. These acids acted like a chemical solvent, penetrating the rubber and breaking down its internal structure. The researchers found that the total amount of acid produced by the ground-based mold was ten times higher than that produced by the mold in space. This chemical assault led to visible damage: the rubber surface became rough, pitted, and flaky, losing its smooth finish and, crucially, its ability to resist the flow of electricity. The material's insulation properties dropped from approximately 600 kΩ·cm to about 330 kΩ·cm, a reduction of nearly 50%, a sign that its internal molecular chains had been severely compromised.

In contrast, the mold in space struggled to establish a foothold. Without gravity to pull the spores down and keep them settled on the surface, the liquid containing the spores behaved differently, driven mostly by surface tension. This caused the spores to gather only at the edges of the liquid droplets rather than spreading evenly across the rubber. Consequently, the mold in space remained sparse and largely inactive. It produced very little of the damaging organic acids, and the rubber surface remained largely intact. While there were some minor cracks where the liquid had dried, the material did not suffer the widespread erosion seen on Earth. The researchers confirmed this by examining the rubber under powerful microscopes and using light to detect chemical changes; the space samples showed far fewer signs of the molecular breakdown that characterizes fungal decay.

The study concludes that the microgravity environment significantly alters the degradation process, resulting in less material damage compared to Earth conditions. The absence of gravity prevents the mold from forming the dense, acid-producing colonies that are so destructive on the ground. Instead of a uniform layer of decay, the space environment results in a patchy, weak colonization that leaves the material with a lower degree of degradation. This finding is vital for the future of long-duration spaceflight, as it suggests that silicone rubber seals may experience less biological wear in orbit than on Earth. It provides engineers with a clearer picture of how materials will age in space, allowing them to design better protection strategies and ensure that the critical seals keeping astronauts safe remain reliable for years to come.