Ultimately it is the local relative humidity that determines the
end moisture content of the sample during storage, so the environmental conditions for sample handling after the drying process and
the storage atmosphere must also be optimized. For example, the
equilibrium moisture content of amorphous trehalose at 43% RH is
very close to the amount of water required to form crystalline
trehalose dihydrate [10]. Exposure of trehalose samples to RH
levels above this value can thus result in the undesirable crystallization of glassy samples. To limit exposure to high RH, dry chambers
can be constructed to maintain a low and stable relative humidity
for rapid drying and handling. For example, ~11% RH was used for
all studies mentioned in this chapter. This was achieved by flowing
dried air into a chamber containing the experimental setup and
monitoring the RH with a temperature and RH logger. Maintaining a low relative humidity also facilitates the heat-assisted drying
process.
In addition, the drying substrate can play an important role in
the drying process. Two substrates have been used for the drying
studies described in this chapter, glass coverslips and filter paper.
On the coverslip, the sample maintains a hemispherical droplet
shape, while on the filter paper, the drop spreads out and absorbs
into the filter paper, creating a film on the top surface. The surface
tension of the droplets on coverslips decreases the evaporation rate.
In contrast, the filter paper disrupts the integrity of the droplet,
reducing the surface tension and causing the sample to “wet out,”
thereby increasing the surface area for drying. For big cells like
oocytes, fiber paper substrates are used to facilitate quick dehydration of samples, but for smaller cells like sperm that can become
trapped in the filter paper, recovery from this matrix can be challenging. Although drying on the glass coverslips can be slower than
drying on filter paper and require more energy delivery, the glass
coverslips allow for easier recovery and rehydration after processing. Samples on coverslips can also be monitored for cracking
and/or crystal formation in the droplet matrix during storage, a
task which is much harder on opaque and porous filter papers. The
pros and cons of each substrate thus need to be considered for the
biological sample of interest.
Another important factor that influences the overall drying
time is the initial moisture level. Generally, the initial moisture
contents of samples are inversely proportional to the concentration
of protective agents such as trehalose. The characteristics of
biological samples and the type of substrates used are also factors
in determining the overall drying rate. The gradient in the chemical
potential of water provides the driving force for evaporation, but
the rate at which water exits will depend also on diffusion through
the sample and sample characteristics at and near the liquid-air
interface. Although lower initial moisture level may facilitate a faster
drying time due to a lower water removal requirement, a higher
Drying Technology for Preservation of Biologics
205
end moisture content of the sample during storage, so the environmental conditions for sample handling after the drying process and
the storage atmosphere must also be optimized. For example, the
equilibrium moisture content of amorphous trehalose at 43% RH is
very close to the amount of water required to form crystalline
trehalose dihydrate [10]. Exposure of trehalose samples to RH
levels above this value can thus result in the undesirable crystallization of glassy samples. To limit exposure to high RH, dry chambers
can be constructed to maintain a low and stable relative humidity
for rapid drying and handling. For example, ~11% RH was used for
all studies mentioned in this chapter. This was achieved by flowing
dried air into a chamber containing the experimental setup and
monitoring the RH with a temperature and RH logger. Maintaining a low relative humidity also facilitates the heat-assisted drying
process.
In addition, the drying substrate can play an important role in
the drying process. Two substrates have been used for the drying
studies described in this chapter, glass coverslips and filter paper.
On the coverslip, the sample maintains a hemispherical droplet
shape, while on the filter paper, the drop spreads out and absorbs
into the filter paper, creating a film on the top surface. The surface
tension of the droplets on coverslips decreases the evaporation rate.
In contrast, the filter paper disrupts the integrity of the droplet,
reducing the surface tension and causing the sample to “wet out,”
thereby increasing the surface area for drying. For big cells like
oocytes, fiber paper substrates are used to facilitate quick dehydration of samples, but for smaller cells like sperm that can become
trapped in the filter paper, recovery from this matrix can be challenging. Although drying on the glass coverslips can be slower than
drying on filter paper and require more energy delivery, the glass
coverslips allow for easier recovery and rehydration after processing. Samples on coverslips can also be monitored for cracking
and/or crystal formation in the droplet matrix during storage, a
task which is much harder on opaque and porous filter papers. The
pros and cons of each substrate thus need to be considered for the
biological sample of interest.
Another important factor that influences the overall drying
time is the initial moisture level. Generally, the initial moisture
contents of samples are inversely proportional to the concentration
of protective agents such as trehalose. The characteristics of
biological samples and the type of substrates used are also factors
in determining the overall drying rate. The gradient in the chemical
potential of water provides the driving force for evaporation, but
the rate at which water exits will depend also on diffusion through
the sample and sample characteristics at and near the liquid-air
interface. Although lower initial moisture level may facilitate a faster
drying time due to a lower water removal requirement, a higher
Drying Technology for Preservation of Biologics
205