While the spell lasts, you can deliver the substance with a mere touch. To do so to an opponent, you must make a successful melee [touch attack]
Some attacks completely disregard armor, including shields and natural armor—the aggressor need only touch a foe for such an attack to take full effect. In these cases, the attacker makes a touch attack roll (either ranged or melee).
Can I deliver the effect from Touch Injection using a weapon (by using the Spell Strike ability)?
I presume no. I can't find a concrete definition of touch spell, but I presume it's supposed to be in here:
You must touch a creature or object to affect it. A touch spell that deals damage can score a critical hit just as a weapon can.
It is only implied that spells with a range of touch are touch spells, and it is only implied, if at all, that spells with a range of touch are the only things that can be considered touch spells.
Can Spell Strike deliver the Touch Injection spell's touch attack?