a pair of plastic containers are compressed intermediate a pair of jaws and rf energy is applied as a function of the change in impedance of the melting plastic between the jaws to sterilize with heat the plastic intermediate the pair of jaws and to form a sterile rupturable membrane encircled by a seal to permit fluid communication on demand between the containers upon rupture of the membrane . the use of rf energy to heat bipolar plastic material both sterilizes the plastic material and welds the plastic material at a proper temperature to form the rupturable membrane without the use of any instrumentation indicative of temperature or other control parameters .