How do I replace the string exactly using gsub()

I have a corpus: txt = "a patterned layer within a microelectronic pattern." I would like to replace the term "pattern" exactly by "form", I try to write a code:

txt_replaced = gsub("pattern","form",txt)

However, the responsed corpus in txt_replaced is: "a formed layer within a microelectronic form."

As you can see, the term "patterned" is wrongly replaced by "formed" because parts of characteristics in "patterned" matched to "pattern".

I would like to query that if I can replace the string exactly using gsub()? That is, only the term with exactly match should be replaced.

I thirst for a responsed as below: "a patterned layer within a microelectronic form."

Many thanks!