Standard techniques for the analysis of biological tissues like immunohistochemical staining are typically invasive and
lead to mortification of cells. Non-invasive monitoring is an important element of regenerative medicine because
implants and components of implants should be 100% quality-checked with non-invasive and therefore also marker-free
methods. We report on a new bioreactor for the production of collagen scaffolds seeded with Mesenchymal Stem Cells
(MSCs). It contains a computer controlled mechanical activation and ultrasonic online monitoring and has been
constructed for the in situ determination of ultrasonic and rheological parameters.
During the cultivation period of about two weeks the scaffold is periodically compressed by two movable pistons for
improved differentiation of the MSCs. This periodic compression beneficially ensures the supply with nutrition even
inside the sample. During the physiological stimuli, rheological properties are measured by means of highly sensitive
load cells. In addition measurements of the speed of sound in the sample and in the culture medium, with frequencies up
to 16 MHz, are performed continuously. Therefore piezoceramic transducers are attached to the pistons and emit and
detect ultrasonic waves, travelling through the pistons, the sample and the culture medium. The time-of-flight (TOF) of
the ultrasonic signals is determined in real time with the aid of chirped excitation and correlation procedures with a
resolution of at least 10 ps. The implemented ultrasonic measurement scheme allows beside the speed of sound
measurements the detection of the distance between the pistons with a resolution better than 100 nm.
The developed monitoring delivers information on rigidity, fluid dynamics and velocity of sound in the sample and in the
culture medium. The hermetically sealed bioreactor with its life support system provides a biocompatible environment
for MSCs for long time cultivation.
Microscopic objects including living cells on a planar substrate are investigated in bio-medical applications of scanning
acoustic microscopy. Beside of the observation of lateral structures, the determination of sample properties such as
density, sound velocity, and attenuation is desired, from which elastic properties can be derived. This can be achieved
with the aid of the acoustic phase and magnitude contrast represented in a polar plot. For homogeneous and sufficiently
planar objects the contrast in magnitude and phase is a function of the properties of the substrate and the coupling fluid,
which both can easily be determined, and of the mechanical properties of the sample under observation. For observation
in reflection and variable thickness of the sample the signal will depend on the actual thickness. This signature of the
object can be fitted based on a conventional ray model for the sound propagating in the coupling medium and the
sample. The model includes also the refraction and reflection at all interfaces between transducer, lens material, coupling
fluid, object, and substrate. The method is demonstrated for a chitosan film deposited on a glass substrate. The scheme
presented here is capable to reach a resolution of about and even below 1% for relevant quantities in applications
involving imaging at 1.2 GHz in aqueous coupling fluids.
Coulomb excitation and detection of ultrasonic waves in piezoelectric crystals by spherical electrical probes is discussed
in view of the opening angle of the cone of longitudinal waves coupling to such a probe. The electric field distribution in
the piezoelectric crystal under the probe is modeled by means of finite elements in order to determine the effective size
of the probe normalized to the sphere radius. The dynamic impedance of the probe is estimated, and it is shown that a
probe of a size appropriate to illuminate or detect from the piezoelectric half space has a frequency-independent
impedance of about 3 k&OHgr; under idealizing assumptions. Measurements of the directionality of ultrasound emission and
detection at a frequency of about 100 MHz are presented for three probes with different tip radii, varying from about
30 &mgr;m to 2.5 mm. As expected, larger probes yield a higher directionality. A relatively large forward contribution is
observed even for small spheres.
Ultrasonic monitoring schemes for the detection of the solid-liquid interface during directional solidification have been developed including electronic equipment for the Material Science Laboratory (MSL) of the International Space Station (ISS). Special signal and data processing suitable for automatic monitoring, on board signal averaging, and operation under a limited data transfer condition is discussed. The achievable resolution in the micrometer regime as well as post experimental processing and evaluation for high resolution monitoring are presented and exemplified for typical applications.
Electric surface excitation of ultrasound in the Coulomb field of scanned electrically conductive spherical local probes and similar detection has been employed for imaging of the transport properties of acoustic waves in piezoelectric materials including singlecrystalline wafers. The employed Coulomb scheme leads to a fully predictable and almost ideal point excitation and detection. In combination with two-channel quadrature transient detection it allows high precision spatially and temporally resolved holographic imaging. Via modeling of the excitation and propagation properties, the effective elastic tensor and the piezoelectric properties of the observed materials can be determined with high resolution from a single measurement. The generation and detection scheme as well as the theoretical background are demonstrated and applications are exemplified.
Surface focused acoustic transmission microscopy is employed for projection (tomographic) imaging of bonded
materials including wafers. Short pulse excitation with apodized focusing transducers operated in transmission and two
channel quadrature transient detection are employed for multiple contrast imaging. The achievable contrast schemes are
based on mode selection for longitudinal, transverse, mode converted, and scattered modes. The identification of the
involved modes including conversion schemes is experimentally accessible by time-gating of the recorded signal and by
observation of spatially selected holograms. Perfect bonding, disbonding, and weak bonding can be studied and
characterized by the developed mode selective imaging scheme. The characteristic features of weak bonding phenomena
are demonstrated and characterized.
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