I'm not sure how practical this is, but it's a nice weird idea.
This is a Syringe Pen. A fully working pen in the shape of a syringe, that comes supplied with red liquid to replicate blood. While the pen itself writes with normal black ink.
Apparently this item is popular with medical professionals. In fact, if you are a medical professional, particularly a nurse, then you may find the site this came from quite useful.
The site is called Nifty Nurse and is apparently run by actual nurses, so they claim to "understand the needs of today's nurses and student nurses".
They can supply you with nursing tools, kits, traditional fob watches, stethoscopes and sphygmomanometers (hmm, I think I shall name my next pet that).
But more importantly, they can supply you with a super-dooper Syringe Pen for £1.49 or the equally silly, but maybe not as cool looking, Bone Pen, also a mere £1.49.
Dog owners may want to avoid this one though, as you may end up having to keep retrieving it from a shallow grave in the garden, (the pen that is - not the dog. Although maybe eventually the dog, haha).