The functionality of a balloon for any purpose is dependent upon the lift it can generate and the lifespan of its membrane. To generate lift, a balloon must contain lighter than air gas at a volume that counteracts the weight of its membrane. However, it must be able to withstand the strain of expansion and compression under low pressures and volatile temperatures. The extreme atmospheric conditions and radiation climate present on many extraterrestrial bodies form a challenging setting for balloon exploration.
Biological membranes are a key component of any living organism. However, most naturally occurring membranes are optimized for containing liquids and have little functionality with gas. Our team investigated the biological production of both synthetic and organic membranes resilient enough to contain hydrogen in high atmospheric conditions. To create a flexible membrane, we synthesized latex bacterially, and designed a composite membrane of elastin and collagen based on their dynamic properties in vivo. To strengthen our membranes against high stress, we investigated p-aramid fibers, the main constituent monomer of Kevlar. To increase our membranes’ radiation resistance, we produced bacterial melanin, and designed a novel binding agent to incorporate the pigment directly into our balloons. The results of these investigations are covered in more detail in the various subproject pages, but a quick justification for each membrane material is warranted.
Team:Stanford-Brown/SB16 BioMembrane Overview
BioMembranes team member Elias introduces the BioMembranes component of the project
The challenge of a biological membrane
While there is no shortage of biological membranes, from the phospholipid bilayer at the smallest scale to the keratin-based epidermis of mammals, the structure that has to tolerate the highest pressures is probably the arterial wall. The natural solution to the high pressures and repeated expansion and contraction is a mixture of the proteins elastin and collagen. Elastin, as the name suggests, forms highly elastic fibers that provide membranes with resilience, but it has a low ultimate tensile strength. Collagen picks up the slack in that regard; up to a certain strain, collagen provides low resistance as the fibers orient parallel to the stress and then “un-krimp,” but after that point it sharply limits extension and strengthens the membrane. When the force required to extend a collagen-elastin composite is graphed against the extension of the fiber, the resulting plot takes the general shape of the plot on the right.
The area under the green curve is a measure of energy stored at a certain extension. Compared to a single material with comparable ultimate tensile strength and strain, a combined elastin and collagen membrane stores much less energy when stretched. This energy is what drives fracture propagation, so less energy stored makes the composite more robust to minor damage.
Both elastin and collagen are made up of long, repetitive protein monomers; this makes them difficult to synthesize recombinantly via most standard methods, but opens up new avenues for production. Elastin and collagen, however, lend themselves to vastly different means of production.
