8mm is worth considering if 4mm isn't hitting tissue correctly or at deeper injection sites - if your current approach is working without issues, that's the more relevant data point than the general preference.
Bacteriostatic saline is the main backup - same preservative principle. The DIY route is 0.9% benzyl alcohol in sterile water, with pharmaceutical-grade sourcing as the bottleneck.
Not a doctor, but I've used expired doses with no issues. Pens were stored correctly, liquid was clear, and I didn't notice any change in how well it worked.
I know that protocol. I've made both a serum and the cream like I mentioned before.
I'm using both, but I prefer the cream because it's easier to apply. The serum is too runny for my liking. I think serum is better for microneedling only. The picture shows the cream I made (1% concentration)...
I know the risks of buying from those sources. I got the vendor recommendation from another user in a private group. The vendor provided a COA for the powder and seemed professional. Still, I can't be 100% certain it's not junk. I'll share more after I've used it.
I'm starting with a 1%...
I'm already using GHK-Cu as part of a "GLOW" blend that I do every few months. Since I've dropped around 30 lbs and plan to shed another 40 or so, I'm worried about loose skin. I've heard that topical GHK-Cu might help with that.
I'm not sure if it'll work, but I'm willing to try it out.