Biomedical Engineering Application Brief
Massachusetts General Hospital
Ulysses J. Balis, M.D.
Center for Engineering in Medicine at the Massachusetts General
Hospital, Shriners Hospitals for Children, Harvard Medical School
and Organogenesis, Inc.
In the rapidly growing field of tissue engineering, researchers are
striving to develop suitable artificial environments that allow
human tissue to be grown outside the human body. In an effort to
refine their bioreactor designs, researchers developing a
bioartificial liver are using DADiSP to help them understand the
oxygen uptake behavior of liver cells.
Cambridge, Boston, and Canton, MA
Of primary interest in the field of tissue engineering is the effective
reutilization of living cells in man-made housings to effectively
reproduce organ function. Devices, known as bioreactors or
bioartificial constructs, are highly dependent upon selection of
effective fabrication geometries, such that all cells within the device
receive adequate nutrients and oxygen.
Oxygen consumption rate is an important design parameter in the
fabrication of the BioArtificial Liver (BAL), now under development at
the Massachusetts General Hospital and Shriners Hospitals for Children
in Boston, MA. Unlike many other types of somatic cells, hepatocytes
have the capability of operating at sustained hypermetabolic levels,
under certain stimulatory conditions. While this is a highly desirable
attribute from a BAL design perspective, as fewer total cells are
needed, it also presents a formidable design challenge, as the
bioreactor must be constructed in such a manner that the cells do not
suffocate themselves from their unusually high oxygen consumption. To
help solve this problem, it is therefore important to have precise data
concerning hepatocyte oxygen consumption characteristics.
Measuring Oxygen Uptake
The measurement of cellular oxygen uptake involves the use of a closed
reaction chamber with an embedded Clark polarographic oxygen electrode.
A typical experiment is conducted by measuring oxygen tension over
time, after the chamber has been isolated from the ambient environment.
Since it is a closed system during the experiment, oxygen tension
decreases with time, as cells respire. Key BAL modeling parameters, Km
and Vmax, which are derived from experimental oxygen tension curve
data, are the goal of this class of experimentation.
Accurate generation of oxygen tension-time curves is not trivial,
however, in that the Clark electrodes exhibit a significant noise
spectrum, which is further compounded by the signal drift resulting
from the mixing within the reactor chamber. As a result,
interpretation of raw 16-bit oxygen tension data proved essentially
intractable in initial experimentation, as the meaningful slope data
was buried in a sea of spectral noise, even after signal enhancement by
online numerical averaging techniques.
Frequency Domain Filtering
To remove the spectral noise, it became evident that frequency-domain
based filtering techniques would be necessary. Additionally as each
experiment generated several megabytes of data, a relatively robust
data management system was necessary.