It's a microchip. We put something similar into animals. To identify them in case they're lost. Yours looks like a, like ones we use on really expensive pets, show dogs and such. They have a tracer in them in case they're stolen. They can be tracked, wherever they are.

–Dr. Martinez, The Angel Experiment

I think I have a tracer chip implanted in me. I'm not positive, but it showed up on an X-ray, and that's what it looked like.

–Max, The Angel Experiment


In the Angel Experiment, when Max was shot, she stayed at Dr. Valencia Martinez's house until it compeltely healed. Dr. Martinez wanted for Max to get an X-ray so they would have a better idea of Max's injured arm. She took Max to her vet place and told the office people that Max was a friend of Ella's and she's doing a repot on how to be a vet, and Dr. Martinez was giving her a guided tour. Max saw a doctor in a whitecoat and instantly froze but Dr. Martinez pulled her into an exam room and that's where they took her X-ray. They were interrupted from discussing the discovery of Max's chip, and Dr. Martinez 

Max's Microchip

Max's microchip, as shown in the manga

literally shoved Max into the closet and confronted the Erasers. The Erasers asked if she'd treated anyone unusual and she snapped that last week she treated a cow that had bicornuate uterus and demanded if that helped. Finally, the Erasers left and she let Max out of the closet.

Max had wanted Dr. Martinez to take her chip out (so the School couldn't track her down anymore and send Erasers and endanger the Flock and there would be no more Voice in her head and all that), but Dr. Martinez apologized that she couldn't surgically remove it because it seemed to be implanted a long time ago, when her arm was much smaller. Dr. Martinez said if she did tried to take it out, Max could possibly lose the use of her hand. But in Saving the World and Other Extreme Sports, when Max visited her again, she did take it out (when Fang pointed out that she tried to scrape herself to death at the beach in School's Out - Forever) -- and Max did lose the use of her hand but the Voice was still with them and Fang had believed some mushy emotion she didn't even mean through Valium talking. When the Flock was captured yet again from the School, Jeb and Anne Walker appeared and told them they never left the School and everything had been a dream. Max could use her left hand again and there were no scars from the operation and Total went missing from the opposite bed, but Max didn't believed them, and, in the end, they were lying and the Flock escaped from the School yet again with the help of Ari.