Ask HN: Product idea/need

I wonder how hard it would be to design a pen-mouse that relies on the pressure of the pen grip to know when to switch to write mode...Whenever you need to write with a pen there is a discernible difference in the amount of pressure you apply to your grip versus when you are just holding the pen. All the products I have seen just can't seem to pull off the concept of a true pen-mouse. I imagine a device exactly resembling a Bic pen and would work on any smooth surface. How hard is it to convert the technology of a regular optical mouse for this use-case?