Microbattery to monitor migrating salmon

Scientists at the US Department of Energy’s Pacific Northwest National Laboratory (PNNL) have created a microbattery to power an injectable transmitter, which will monitor the movements of salmon through rivers. Details of the battery have been published in the journal Scientific Reports.

The researchers explained that the Juvenile Salmon Acoustic Telemetry System (JSATS) was developed several years ago by PNNL “to estimate the survival and observe the behaviour of juvenile salmonoids passing through hydropower dams in the Federal Columbia River Power System”. The system currently uses acoustic microtransmitters which are surgically implanted in the fish.

The US Army Corps of Engineers and other agencies use the information from these tags to chart the welfare of endangered fish and to help determine the optimal manner to operate dams. But the Corps sought a smaller transmitter which could be injected, instead of implanted, into the fish.

“The weight and volume of the transmitter needed to be reduced and the transmitter’s dimensions and shape optimised for passage through the shaft of an injection needle in order to decrease the amount of handling required to implant transmitters, the effect of the tags on fish health and the cost of implantation,” the researchers stated.

The batteries currently powering the transmitters are two 1.55 V silver oxide button cells (SR416), which supply the 3 V voltage required. These comprise around 60% of the weight and 20% of the volume of the transmitter. A team led by materials science expert Jie Xiao have now designed their own microbatteries (MB306) to replace these, which weigh just 70 mg (compared to 135 mg) and measure 6 x 3 mm - slightly larger than a long grain of rice.

The microbattery created by Jie Xiao, Daniel Deng and colleagues, amid grains of rice. Image courtesy of PNNL under CC BY-NC-SA 2.0.

Lithium/carbon fluoride batteries were chosen to replace the silver oxide cells, said the researchers, “because of their many advantages, including high power density, high average operating voltage, long shelf life and wide operating temperature”. The service life requirement of a JSATS power source is at least 20 days; this is extended in the MB306-powered transmitter due to an improvement in gravimetric and energy density (with an energy density of 240 Wh/kg, compared to 100 Wh/kg for the SR416 batteries).

The MB306 cell has a ‘jellyroll’ structure, commonly used to make large household cylindrical batteries. Xiao’s team laid down layers of the battery materials one on top of the other in a process known as lamination, then rolled them up together. According to Xiao, “It’s a bit like flattening wads of Play-Doh, one layer at a time, and then rolling them up together, like a jellyroll.”

This allowed the team to increase the area of the electrodes without increasing the battery thickness, size or impedance. High impedance occurs when so many electrons are packed into a small place that they don’t flow easily; low impedance, however, has led to “significantly improved electrochemical performances over a wide temperature range”, the researchers said. This is important, they said, as “the cold-blooded juvenile salmon migrate from freshwater into saltwater”, with the water temperature varying along the way.

In a PNNL field trial, 700 transmitters powered by MB306 were injected into juvenile salmon, which were then released into the Snake River of Washington State and tracked as they migrated through the Federal Columbia Power System. The transmitter’s smaller size means it can be inserted into younger, smaller fish, allowing scientists to track their welfare earlier in the life cycle.

The battery holds enough energy to send out a strong acoustic signal even in noisy environments and over long distances. It can power a 744 microsecond signal sent every three seconds for about three weeks, or about every five seconds for a month. It’s the smallest battery the researchers know of with enough energy capacity to maintain that level of signalling.

“The invention of this battery essentially revolutionises the biotelemetry world and opens up the study of earlier life stages of salmon in ways that have not been possible before,” said M Brad Eppard, a fisheries biologist with the US Army Corps of Engineers.

“For years, the chief limiting factor to creating a smaller transmitter has been the battery size. That hurdle has now been overcome.”

The researchers believe the development of the microbattery, and with it the injectable transmitter, “will result in significant reduction in cost of use, biological benefits for the tagged fish and provide information for the development of fish-friendly hydro systems internationally”.