ULTRA COLD:  All fluids can be safely frozen in Liquid Nitrogen in AFC's freezing bags.  FEP remains flexible and stretchable at liquid nitrogen temperatures.  FEPs Glass transition temperature is -208 degrees Celsius.  FEP remains flexible at -196. FEP is not only flexible, but elastic, it can stretch while in liquid nitrogen.  The inability to stretch is the principle reason that other plastics break in Liquid Nitrogen service.

THERMAL TRANSFER:  AFC's FEP bags are  0.127 millimeters (0.005 inches) thin for good thermal equilibrium during freezing.  FEP is transparent for easy visual inspection during thawing.

FEAR OF BREAKING:  Over the years, lesser bags have earned the reputation that a percentage of them will break in the Liquid Nitrogen, contaminating the Liquid Nitrogen with pathogens from the frozen cells.

BREAKAGE ON THAWING:  Actually, lesser bags will more frequently break on thawing; causing loss or contamination of the valuable frozen cells.

WHY DO BAGS BREAK:

SLICING:  Sharp ice edges cut the bag while freezing or while being handled.  This is virtually the only mechanism of concern in FEP bags.

MECHANICAL: Ice expands; stretching the bag or its fittings beyond their elastic limit while freezing.  This is not an issue with FEP because FEP remains elastic even in liquid nitrogen.

PNEUMATIC:  Gas expands in the bag while thawing, and a non flexible bag breaks rather than stretch or bend.  FEP just stretches.