Essential Physics of Low Temperature and Freezing - Freeze-drying of Pharmaceuticals and Biopharmaceuticals - page 44

principles as classical materials science, e.g. silicates, borates, metal
alloys, etc., but applied to water-soluble or water-sensitive materials. 39
Thus, as will be made clear in the following chapters, the avoidance,
rather than the promotion of crystallisation in pharmaceutical proc-
esses, may be a worthwhile objective. It has, for instance, been reported
that insulin in the freeze-dried amorphous solid state is substantially
more stable than in the crystalline form, 40 which led the authors to
doubt whether the ''rule'' that the crystalline phase always provides the
higher stability towards degradation is universally applicable. The fol-
lowing chapters explore the relative merits of the two most important
physical states in freeze-drying technology.
4.9 Freeze-Drying of Complex Biological Materials
Although not strictly germane to the subject matter of this topic,
mention should be made of attempts that describe the application of
freeze-drying to the long-term storage and eventual functional recovery
of a range of biological materials of much higher complexities than
isolated biomacromolecules or formulated solutions that contain these
molecules. Such materials can be of plant or animal origin and are, in
increasing order of complexity: single cells, cell clusters (e.g. embryos),
tissues, organs and even simple organisms. (Excluded are biological
materials, such as dried flowers, where there can be no question of
''recovering functional activity''.) Although there are journals in which
freeze-drying reports appear and there are even patents that protect such
inventions, to the author's knowledge, such claimed ''inventions'' have
not yet been successfully reduced to practice, clinical or otherwise.
However, low-temperature preservation by deep chill and dormancy is
indeed a very effective method to preserve in vivo functioning. The
technical problems are then to find methods of preventing water from
freezing, or at least to control the degree of freezing. The principles and
techniques have been described earlier in this chapter. Results are shown
in Figures 2 (in vivo)and3(in vitro). To extrapolate such results to achieve
long-term laboratory preservation of ''real'' biological material in the
unfrozen state, the same principle, i.e. inhibition of ice nucleation, must be
employed. Freeze-drying cannot therefore be the strategy of choice, but
freeze avoidance is. Success has been achieved with cells and even some
plant tissues, by encapsulating the material in aqueous microdroplets and
emulsifying the aqueous phase in an oil, the molecules of which cannot act
as catalysts for ice nucleation. 16,41 Figure 18 shows two such emulsions of
human erythrocytes (left) and Saccharomyces cerevisiae cells (right). Both
these preparations were stored at 201C for several weeks, after which