In addition to the previous considerations about shells and batteries, the materials for the seals, bladders and circuits need to be considered too. All of the clams will be put in the water, and face the potential of getting lost. They all need to withstand the environment and not pollute it.
Here are some thoughts on the different materials, and their durability, toxicity, accumulation in the environment, and fabrication. The choices available right now for rubbers and electronics are not ideal, but are adequate for this stage of the project. Further research should be conducted and a complete life-cycle assessment of the clam should be made.
Seals and Bladders
Silicone seems like the best rubber for the seals and bladders right now. It is extremely durable, has low toxicity, and it is easy to fabricate parts with it. On the other hand, because of its low degradation, it has very high accumulation in the environment.
Silicone rubber is a siloxane made of long chains of silicon and oxygen with attached organic groups. It gets its durability from the silicon-oxygen bond. It is resistant to many chemicals, including salt water, hydrogen peroxide, acetic acid and sodium bicarbonate, so it is suitable to be used as part of the buoyancy system in the clam.
High molecular weight siloxanes, like silicone rubber are not toxic, but low molecular weight siloxanes, like silicone fluid used in cosmetics and as defoaming agents, can be toxic. Because the clam will use high molecular weight siloxanes, toxicity to the environment is probably not a concern.
On the other hand, accumulation in the environment is a major concern. As a result of the durability of silicone rubber, it has low degradation. It does not photodegrade or biodegrade, and is considered a 'very persistent chemical.' This is an area of active research, so I would like to find studies that look at it in a marine environment (here is an example study that I found for a lake).
It would be nice to use a rubber that does not accumulate. I experimented with natural rubber latex, but I did not find it suitable. I did not vulcanize the rubber and it deteriorated rapidly in use.
Natural rubber is made of polyisoprene chains. During vulcanization, polyisoprene chains are cross-linked with sulfur, making a more durable rubber. Natural rubber and vulcanized rubber can be degraded by a variety of mechanisms, including chlorine, metals, photodegradation and biodegradation.
Natural rubber in a marine environment has been extensively studied by the Navy, and the aging process is well understood. If a simple DIY vulcanization process could be figured out, it might be possible to use natural rubber for the seals and bladders. It would be beneficial to use natural rubber, but because of the difficulties involved in making it suitable, I will not be investigating it for now, but will use silicone instead.
It is easy to fabricate parts with silicone. I used Oogoo for the seals for the clam, and laid them up in place by hand using tape. I like using Oogoo, because it is made with readily available silicone I caulk from the hardware store, but it contains cornstarch, which may make it unsuitable for long-term use. I did not notice any problems with the seals, but silicone is permeable and the cornstarch could attract microorganisms. I made the bladders by coating a form with rubber (natural rubber latex or Smooth-on Sorta-Clear).
For the new design, I will translate the process, so that molds can be 3D printed. Instead of one seal between the shells, the clams will have two seals (one for each shell). The seals will be made by casting silicone on the lip of the shell, bounded by a 3D printed mold that will look like a mouth guard or dental tray. The bladders will either be cast in two parts and bonded, or they could be cast around a dissolvable filament form. Silicone does not perform well with terpenes, so the dissolvable filament will need to be PVA or Hydrofill and not HIPS. Another...
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