To measure formula_69 in the Pa range, “pico-indentation” using an optical tweezers system is suitable. Here, a laser beam is used to trap a translucent bead which is then brought into contact with the soft sample so as to indent it. The trap stiffness (formula_63) depends on the laser power and bead material, and a typical value is ~50 pN/μm. The probe size formula_68 can be a micron or so. Then the optical trap can measure formula_69 (≈formula_63/formula_68)in the Pa range.