A device for adding solvent to a pneumatic system comprises an inlet structure to receive solvent from a supply and a reservoir to hold solvent from the inlet structure. A preferred use of the system is early, rapid, accurate, diagnosis of stroke, especially whether the stroke is due to blockage of a blood vessel or leakage from the blood vessel. The formulation is bio-compatible, stable and injectable. The strain layer is associated with a critical thickness that is dependent on a component of the strain layer, and the strain layer has a thickness lower than or equal to the critical thickness. The wheel suspension does not have to be compressed on retraction.