Bubbler
Bubbler is an artifact for Isaac that gives you 1 Bubble for every 4 cards played.
Clarification
It has a counter that increments when you play an Isaac card. This counter does not reset between combats. Every time it hits 4, it resets and gives you 1 Bubble.
Bubble is gained after the Isaac card resolves, so if an Isaac card that deploys midrow objects sets this to 4, it will not get a Bubble Shield; instead the next midrow object you deploy (from an Isaac card or otherwise) will.
Strategy
Bubble Shields on your Midrow Objects are quite valuable, preserving your drones and other midrow objects on their first hit, so with careful planning and plenty of Isaac cards, you can launch drones that simultaneously block enemy fire and stick around to deal quite a bit of damage to the enemy. It can also be used on Asteroids and Geodes to double their blocking effectiveness, but using it to protect drones (especially Jupiter Drones) is probably the most efficient way to use Bubble Shields. (Think of it this way: the Bubble Shield blocks one shot anyway, but if it's used to protect a drone, it also means the drone stays longer, which means the drone gets to do more shooting; Asteroids don't benefit from living longer).
It is possible to use the fact that it carries over from fight to fight with careful play - if you have a fight under control, you may look to stall to end it with this artifact's counter on 3. That way, you'll gain Bubble the first time you play a Isaac card next combat. (Note that you won't benefit from the bubble until you deploy a midrow object after that; if the first Isaac card you play deploys midrow objects it will not gain the Bubble. It isn't possible to preserve Bubble between combats, so this is the fastest benefit you can get from this artifact.)