A method for mixing a medicament with a patch-like, self-contained multi-component substance infusion device (1000) which provides one or more substantially hidden patient needles (222) which can be placed in fluid communication with a fluid reservoir assembly that includes a rigid bladder portion used in conjunction with a non-distensible bladder film, such as a metalized film. The device can he attached to a skin surface via an adhesive contact and a pressurization system provides a pressure to the contents of a fluid reservoir assembly. The mixing takes place in a container (3000) below atmospheric pressure in order to reduce bubble formation. Improvements to dry powdered formulations for reconstitution for preferred use in the device are also disclosed.
