Upvote:1
Have you considered getting a pannier rather than using a back pack? I cycle to work with a change of clothes and a laptop as well as all my cycling maintenance gear in my pannier and it's great. You also avoid arriving at your destination with a sweaty back.
Also you are probably better off asking questions like this on the Bicycles Stack Exchange site.
Upvote:2
I think I would start with going to a big and respectable bike shop and ask there. There seem to be quite a selection of backpacks especially designed for bikers, you can google for it to get an idea. Then you can ask specifically what is different in these special bike backpacks, etc.
If you want to carry your laptop in the backpack, I strongly recommend one with a dedicated inner pocket with thick soft sides, so that your laptop is not going to be damaged incidentally.