where W is the mass of water load (g), ΔT is the change in temperature (
C), and t is the microwave time (s) at full microwave power
[22, 23].
For microwave devices with nonadjustable power output, intermittent microwave exposure can be used to avoid overheating the
biomaterials. In the case of intermittent drying, samples can be
heated at full power and then cooled inside the microwave cavity
with the door open. In our study of drying macrophage cells, a 30 s
heating/30 s cooling was found to avoid thermal excursions above
typical physiological temperatures (i.e., maintenance of temperature below 40
C), whereas significant heating was observed for
45 s active heating periods (temperatures above 42
C can induce a
heat shock response in mammalian cells).
In the studies of oocytes and sperm, a customized microwave
device was used, which allowed power levels to be modulated
[24, 25]. After determining the power output at each power level
setting and the associated thermal excursions during drying at each
level, to minimize thermal damage, all biological samples were
processed with 20% microwave power for timed increments ranging from 0 to 50 min, depending on the goal of the experiments.
The temperature at the center of the droplet was recorded to
determine upper limits for power setting and duration of
processing.
1.3 Laser-Assisted
Drying
Laser-assisted drying (LAD) uses illumination by near-infrared laser
light to support dehydration of the sample. Laser radiation is
routinely used in a variety of therapeutic procedures in medicine
ranging from cataract surgery to tattoo removal [26, 27]. Lasers are
the light sources of choice for these procedures as they are monochromatic and collimated and thus can deliver precise amount of
energy to a target. Most therapeutic laser procedures in medicine
are based on the idea of selective photo-thermolysis, which is the
precise targeting of chromophores (e.g., water, melanin, hemoglobin, and even tattoo ink) in tissue using a specific wavelength of
light with the intention of selective absorption of light into a target
tissue without absorption in surrounding tissue [28]. LAD implements the principle of selective absorption for dehydration of samples in preparation for anhydrous storage. LAD selectively heats
water to overcome cooling due to evaporation and speeds dehydration of the samples. The drying rate and sample temperature during
processing can be altered by changing the incident laser power,
processing time, and/or laser wavelength.
LAD can be used for inline processing via delivery of laser
radiation through an optical fiber and has the benefit of being
able to deliver precise amounts of energy to each sample. Precise
control over energy deposition means precise control over EMC
which has implications for sample storage temperature. LAD also
Drying Technology for Preservation of Biologics
207
C), and t is the microwave time (s) at full microwave power
[22, 23].
For microwave devices with nonadjustable power output, intermittent microwave exposure can be used to avoid overheating the
biomaterials. In the case of intermittent drying, samples can be
heated at full power and then cooled inside the microwave cavity
with the door open. In our study of drying macrophage cells, a 30 s
heating/30 s cooling was found to avoid thermal excursions above
typical physiological temperatures (i.e., maintenance of temperature below 40
C), whereas significant heating was observed for
45 s active heating periods (temperatures above 42
C can induce a
heat shock response in mammalian cells).
In the studies of oocytes and sperm, a customized microwave
device was used, which allowed power levels to be modulated
[24, 25]. After determining the power output at each power level
setting and the associated thermal excursions during drying at each
level, to minimize thermal damage, all biological samples were
processed with 20% microwave power for timed increments ranging from 0 to 50 min, depending on the goal of the experiments.
The temperature at the center of the droplet was recorded to
determine upper limits for power setting and duration of
processing.
1.3 Laser-Assisted
Drying
Laser-assisted drying (LAD) uses illumination by near-infrared laser
light to support dehydration of the sample. Laser radiation is
routinely used in a variety of therapeutic procedures in medicine
ranging from cataract surgery to tattoo removal [26, 27]. Lasers are
the light sources of choice for these procedures as they are monochromatic and collimated and thus can deliver precise amount of
energy to a target. Most therapeutic laser procedures in medicine
are based on the idea of selective photo-thermolysis, which is the
precise targeting of chromophores (e.g., water, melanin, hemoglobin, and even tattoo ink) in tissue using a specific wavelength of
light with the intention of selective absorption of light into a target
tissue without absorption in surrounding tissue [28]. LAD implements the principle of selective absorption for dehydration of samples in preparation for anhydrous storage. LAD selectively heats
water to overcome cooling due to evaporation and speeds dehydration of the samples. The drying rate and sample temperature during
processing can be altered by changing the incident laser power,
processing time, and/or laser wavelength.
LAD can be used for inline processing via delivery of laser
radiation through an optical fiber and has the benefit of being
able to deliver precise amounts of energy to each sample. Precise
control over energy deposition means precise control over EMC
which has implications for sample storage temperature. LAD also
Drying Technology for Preservation of Biologics
207