There are many concerns regarding what materials we choose to use for the clams. We have to assume that they will be lost and that animals will try to eat them. Additionally, an eventual goal is to develop clams that are benthic and can perform long-term missions. Instead of staying planktonic, the clams would settle out of the water column onto the seafloor, and either take measurements where they are situated, or periodically ascend to take measurements. Therefore, even in normal operation there is the potential for plastics to leach into the environment.

For the initial prints, I believe that we should focus on using polylactic acid (PLA). It is a common filament for 3D printers and it is naturally derived and biodegradable. As of yet, I do not know it’s seaworthiness, but that should be a focus of testing. If anyone has a fish tank, saltwater and a PLA print that they are willing to sacrifice, I would be grateful for their observations about how well the PLA stands up over time. Future testing could use the sensors we incorporate into the clam to measure leaching, but for right now, sight and touch would be good enough to measure seaworthiness.

In addition to knowing how well it stands up to saltwater, we need to know how well PLA performs with biofouling. Biofouling is a serious concern for water quality instruments. It is especially concerning for 3D prints, which tend to have significant gaps and crevices on their surfaces. This is another area where we will have to perform extensive testing.

A common way for mitigating biofouling is to clean water quality instruments after use. It would be too laborious to require researchers to clean each clam individually, so we should try to design them to be cleaned in the dishwasher. As far as I know, PLA softens in dishwashers to a degree that might not be acceptable. We could try to design the shells, so that they can soften without losing their shape or impairing their function. This would also require testing.

If we tried to use a different plastic that is higher temperature, we might lose the advantages of having a naturally derived and biodegradable plastic. A thin conformal coating of silicone or another high temperature plastic might be a good compromise that could fill the gaps to prevent biofouling, and protect the shells from melting and deforming.

Alternatively, we could borrow a solution from natural clams and use calcium carbonate. I have done some experiments with mixing plastics (PLA and epoxy) with calcium carbonate, and it is a wonderful material. It feels like bone and shell, and it is very strong (here are links to vegan taxidermy I made with the material: antlers, scrimshaw).

PLA and calcium carbonate filament is available, so using that material might be a good area to explore. Using calcium carbonate would not add any unnatural chemicals, and I believe that it might help with the dishwasher issue. Also, it might allow the clams to degrade somewhat gracefully, especially if we could figure out a way to build up the shells primarily in calcium carbonate with just a little PLA binder.