There's been other posts on this. I actually looked into it and Old Mogs chip (done by the local RSPCA 14 years ago) wouldn't be recognised, so a bit pointless for me really. It would let the other three in, but not her!
Apparently they are good but the snag is, you need a socket nearby. That link is expensive @ £127 - you can get them for about £90I like the bit "Available in 2 exciting colours" - white or brown