Wallet Pen
A pen when you need one...
Wallet pens aren't a new idea, but most of them are bulky, and not designed to fit comfortably into a wallet.  This invention is a wallet pen that fits in your wallet like a credit card, yet is still comfortable to hold and to write with naturally.
Original wallet pens are essentially smaller versions of normal pens.  Although this may sound like a good idea, the smaller size cramps the hand, and the roundness of the pen makes the wallet significantly bulkier.
Some current wallet pens such a PicoPad do fit into a credit card slot, however the pen in this case is basically only a cylinder with a small amount of padding which can be very uncomfortable to hold.
A brainstorm page out of my notebook.
Renderings in Illustrator of the concept for the pen.
My first prototype.  It consists of two expired credit cards glued together around the internal part of a pen, then wrapped in duct tape.  Using this, I quickly discovered the size of a credit card is too large to hold in your hand, even with curves can mimic how a person normally holds a pen.  I also realized people hold pens in LOTS of different ways.  

My final design is the same height but shorter in length.  This makes it easier to hold in general, but specifically gives the user more control over how they hold the pen, allowing them to adapt it more to how they naturally hold a pen.
A screen shot of the CAD design of the pen body.  Unfortunately I haven't been able to invest in CAD software yet, so this CAD model was done by an engineering student in college.
3D printed prototype compared to the size of a credit card.
The pen with an actual pen body put in it.  (Also a bit beat up from being used so much!).
The pen as it fits into the credit card slot of a normal wallet.