"u" is an acceptable replacement for "µ". I've never seen "milimili" (mm) before. You can spell it out, like "91 micronewtons". You can also use exponential

"u" is an acceptable replacement for "µ". I've never seen "milimili" (mm) before. You can spell it out, like "91 micronewtons". You can also use exponential notation like 91e-6 N or 91x10^(-6) N. To be fancy you can copy-and-paste unicode characters and write 91×10⁻⁶ N.