does not require a Faraday cage. LAD system bets for processing a
small number of samples simultaneously, while microwave-assisted
drying is capable of batch processing a large number of samples.
1.4 Optical
and Mechanical
Characterization of
Dehydrated Samples
Characterization of dehydrated samples is essential for monitoring
drying quality (e.g., homogeneity). Although biological functional
investigations such as cell viability, membrane integrity, and DNA
fragmentation are critical for determination of preservation success,
optical and physical technologies can be used as the first screening
to quickly determine the suitability of an amorphous matrix for
biopreservation. Several techniques used in our studies to characterize dried samples are reviewed below.
1.4.1 Polarized Light
Imaging (PLI)
Polarized light imaging is used to measure the homogeneity of
amorphous materials and locate areas of crystallization. When
light passes through a linear polarizer, it only transmits the component of the electric field that is in the direction of the transmission
axis, producing linearly polarized light. When that light passes
through a secondary linear polarizer (analyzer) with a transmission
axis oriented perpendicular to the first polarizer, the light is
completely absorbed. This is referred to as crossed polarizers.
When a crystal is placed between a pair of crossed polarizers, its
optical anisotropy will cause the polarization state of linear light
passing through it to rotate. Depending on the amount of rotation,
a component of the electric field will pass through the analyzer
allowing it to be detected. In contrast, when an optically isotropic
material, such as glass, is placed between crossed polarizers, it will
not undergo any change in polarization state and will not pass
through the analyzer. This technique allows imaging of crystals
that might be present in the preservation matrix, generally an
undesirable outcome.
1.4.2 Scanning White
Light Interferometry (SWLI)
Scanning white light interferometry is used to measure the thickness and surface morphology of dried samples. In a Michelson
interferometric objective, white light is amplitude split into a fixed
length reference beam and a measurement beam whose coherence
plane matches the focal plane of the objective. Vertically scanning
the measurement beam generates fringes as the coherence plane
reflects off the sample surface. The height value for each pixel across
the sample corresponds to the z location of maximum fringe visibility. Variations in thickness across the sample may indicate an
uneven distribution of the trehalose preservation matrix, which
could impact the overall functionality of embedded biologics. Figure 1 shows representative profiles of a LAD processed sample
along two directions [29, 30]. This sample exhibits a dome-shaped
profile with the thickest part of the sample lying slightly off center.
208
Shangping Wang et al.
small number of samples simultaneously, while microwave-assisted
drying is capable of batch processing a large number of samples.
1.4 Optical
and Mechanical
Characterization of
Dehydrated Samples
Characterization of dehydrated samples is essential for monitoring
drying quality (e.g., homogeneity). Although biological functional
investigations such as cell viability, membrane integrity, and DNA
fragmentation are critical for determination of preservation success,
optical and physical technologies can be used as the first screening
to quickly determine the suitability of an amorphous matrix for
biopreservation. Several techniques used in our studies to characterize dried samples are reviewed below.
1.4.1 Polarized Light
Imaging (PLI)
Polarized light imaging is used to measure the homogeneity of
amorphous materials and locate areas of crystallization. When
light passes through a linear polarizer, it only transmits the component of the electric field that is in the direction of the transmission
axis, producing linearly polarized light. When that light passes
through a secondary linear polarizer (analyzer) with a transmission
axis oriented perpendicular to the first polarizer, the light is
completely absorbed. This is referred to as crossed polarizers.
When a crystal is placed between a pair of crossed polarizers, its
optical anisotropy will cause the polarization state of linear light
passing through it to rotate. Depending on the amount of rotation,
a component of the electric field will pass through the analyzer
allowing it to be detected. In contrast, when an optically isotropic
material, such as glass, is placed between crossed polarizers, it will
not undergo any change in polarization state and will not pass
through the analyzer. This technique allows imaging of crystals
that might be present in the preservation matrix, generally an
undesirable outcome.
1.4.2 Scanning White
Light Interferometry (SWLI)
Scanning white light interferometry is used to measure the thickness and surface morphology of dried samples. In a Michelson
interferometric objective, white light is amplitude split into a fixed
length reference beam and a measurement beam whose coherence
plane matches the focal plane of the objective. Vertically scanning
the measurement beam generates fringes as the coherence plane
reflects off the sample surface. The height value for each pixel across
the sample corresponds to the z location of maximum fringe visibility. Variations in thickness across the sample may indicate an
uneven distribution of the trehalose preservation matrix, which
could impact the overall functionality of embedded biologics. Figure 1 shows representative profiles of a LAD processed sample
along two directions [29, 30]. This sample exhibits a dome-shaped
profile with the thickest part of the sample lying slightly off center.
208
Shangping Wang et al.