The Neutronic Sick Patch is an invention that makes whoever wears it very sick. It appears in the episode Journey to the Center of Carl.
The sick patch first appears when Jimmy shows it to Goddard and even says it's his greatest invention yet (even though he says that a lot). When Jimmy first puts it on, it makes him very sick. Goddard gets worried but Jimmy tells not to, and then takes it off which makes him well again. He uses it to fool his Mom to make her think he's sick so she would not allow him to go.
Later on, Jimmy shows the sick patch to Carl and Sheen. As soon as they see it, they start wanting one their own sick patches, as well as Cindy and Libby, Jimmy agrees but warns them to not tell anyone else. Although, despite Jimmy telling his friends not to, they end up telling the rest of the kids of Retroville Elementary School anyway, causing all the children each wanting a sick patch so they don't have to go to school. Jimmy soon opens a sick patch stand where each kid gets a free sick patch, making them all very sick. This caused all of Ms. Fowl's classroom to get empty and she starts worrying about all her students and misses them (except Sheen of course).
However, while wearing their sick patches, Jimmy and his friends' parents did things they didn't like one bit with them: Jimmy's Dad homeschooling him, Carl's Dad putting him in a giant plastic bubble and calling him a bubble boy, Sheen's Dad sprinkling pepper on his face, Cindy's Mom sticking needles all over her body and Libby's Mom using a wasp to cure her by letting it sting her. As Jimmy couldn't take it anymore, he suggested it was time for him to get well. But when Jimmy tried to take the sick patch off, he noticed the patch got growing skin which suddenly absorbed over it. Jimmy's friends then called him and begged him to cure everyone or he'll face the consequences.
Back in Jimmy's Lab, Jimmy discovers the sick patch dissolved in he and his friends' bloodstreams. The only way to cure everyone was for Jimmy to shrink down to microscopic size and travel inside one of his friends. Jimmy and Sheen use Jimmy's shrink ray to shrink themselves down and travel inside Carl. While inside him, Jimmy manages to extract 1 mitochondria from the germs that have spreaded in their bloodstreams and Jimmy was soon able to cure all the kids and get them ready to go back to school.