  Membrane computing is a branch of natural computingwhich abstracts fromthe
structure and the functioning of living cells. The computation models obtained
in the field of membrane computing are usually called P systems. P systems have
been used to solve computationally hard problems efficiently on the assumption
that the execution of each rule is completed in exactly one time-unit (a global
clock is assumed for timing and synchronizing the execution of rules). However,
in biological reality, different biological processes take different times to
be completed, which can also be influenced by many environmental factors. In
this work, with this biological reality, we give a time-free solution to
independent set problemusing P systems with active membranes, which solve the
problem independent of the execution time of the involved rules.
