R regex gsub separate letters and numbers

I have a string that's mixed letters and numbers:

"The sample is 22mg"

I'd like to split strings where a number is immediately followed by letter like this:

"The sample is 22 mg"

I've tried this:

gsub('[0-9]+[[aA-zZ]]', '[0-9]+ [[aA-zZ]]', 'This is a test 22mg')

but am not getting the desired results.

Any suggestions?