Over time, from killing enemies, destroying crates, etc, you will get Somatic gel, which is used to either create health packs, or upgrade your RIG. For a Large Med Pack, you need 240 Somatic Gel. This will not take long to accumulate, but if you want this fast, I found that a good way to get it is to sell your small Med Packs. Over the course of the story, you will get riddled with Small Med Packs, and quite a good number of medium ones. What I did was keep the medium ones, and sell the small ones. Doing this quickly got me the necessary 240 Somatic Gel, and I now have 3 Large Med Packs doing this.
In all, starting from scratch, you would need to sell 12 small med packs. However, since you gain the gel from enemies and drops, you won't need to sell as many. I only needed to sell about 5.