NO. 3549 FREEZE-DRY PRESERVATION — HOWER 3 



Sublimation. — If biological tissue is first frozen to give it mechanical 

 rigidity and water is then removed by sublimation, most tissue can be 

 dehydrated without apparent physical change. 



Consider this process in terms of plain frozen water: ice. Each ice 

 crystal is a geometrically sound structure, made up of myriads of 

 water molecules that are retained in their positions by the gravitational 

 field of the surrounding molecules. Within the restricting confines of 

 this lattice, each molecule moves randomly, and there is a possibility 

 that one of the motions of a surface molecule will be great enough to 

 propel it out of the confines of the lattice. The greater the crystal 

 mass, the greater the probability of such escapes. As temperature is 

 increased and molecular motion becomes accelerated, the probability 

 of escape becomes greater; thus, the average number of water molecules 

 that will escape from a given mass at a given temperature can be 

 statistically predicted. Ice within a biological specimen behaves in 

 nearly an identical manner. 



Sublimation begins at the outer surface of a specimen and continues 

 at the boundary between the frozen and the dried tissue. This 

 boundary recedes toward the center of the specimen as di'ying pro- 

 ceeds. As water molecules continue to escape from ice crystals on the 

 boundary, they move about at high velocity, colliding constantly with 

 other molecules and with the structm'e of the dried tissue surrounding 

 them. (As they are buffeted from collision to collision, they are virtu- 

 ally independent of external forces.) There is a heavy concentration 

 of water-vapor molecules at the sublimation boundary, due to the 

 gi'eat number of molecules escaping; consequently, there are more colli- 

 sions, which ricochet molecules along the line of least resistance toward 

 the outer shell of the specimen. The force of the molecules' collisions 

 following their escape from the ice crystals on the sublimation 

 boundary supports their drive through the dried tissue of the speci- 

 men and into the atmosphere beyond its outer shell. For ice to 

 sublime efficiently, the vapor pressure within the specimen chamber 

 must be lower than the vapor pressure of ice within the specimen 

 itself. 



Vapor pressure. — If the temperature of a vacuum chamber con- 

 taining ice is mamtained at —10° C, evacuated with a vacuum pump, 

 and then valved off so that no external ah can enter, moleciiles will 

 begin to escape from the ice crystals within the chamber (fig. 1). 

 Some of these molecules will ricochet about, colliding with one another 

 and with the sides of the chamber, while others will relocate them- 

 selves upon other ice crystals. 



As the concentration of the vapor formed by these free molecules 

 reaches a specific point (which is dependent upon the temperatures of 

 their atmosphere), the rate at which molecules return to the ice will 



