You put it in your pants, duh. Just unzip your fly a bit, then just prop it at an angle against your groin, then you have protection on the go, ready to be used at any moment for those sneaky muggers. Just don't walk into anything.
My question is, if the pen just captures their DNA, then what use is that DNA if its not comparable against anything? I mean rape victims usually have the attackers DNA via semen right? Unless buddy uses a prophylactic. Although having a stabbing device that doubles as a pen is handy.