The human vagus nerve contains around 100,000 individual nerve fibres, which branch out to reach various organs. But the amount of electricity needed to trigger neural activity can vary from fibre to fibre by as much as 50-fold.
Yaakov Levine, a former graduate student of Tracey’s, has worked out that the nerve fibres involved in reducing inflammation have a low activation threshold. They can be turned on with as little as 250-millionths of an amp — one-eighth the amount often used to suppress seizures. And although people treated for seizures require up to several hours of stimulation per day, animal experiments have suggested that a single, brief shock could control inflammation for a long time10. Macrophages hit by acetylcholine are unable to produce TNF-α for up to 24 hours, says Levine, who now works in Manhasset at SetPoint Medical, a company established to commercialize vagus-nerve stimulation as a medical treatment.
By 2011, SetPoint was ready to try the technique in humans, thanks to animal studies and Levine’s optimization efforts. That first trial was overseen by Paul-Peter Tak, a rheumatologist at the University of Amsterdam and at the UK-based pharmaceutical company GlaxoSmithKline. Over the course of several years, 18 people with rheumatoid arthritis have been implanted with stimulators, including Katrin.
For the images of the actual device, check Core77. They also have implantable bioelectronic devices.