With the idea of hacking a microscope as seen in http://www.bidouille.org/hack/mousecam and use the CCD included in all mouses, we decided to organize this as our first workshop in our first meeting.
However, there were problems with the software, even some friends helped a lot trying to fix it. Eventually, we realize that the most important part in such optical mouse was the optics itself as shown in the next diagram.
A video with the blood sample. It is even possible to distinguish the serum from the blood cells: