Biomedical Engineering – From Theory to Applications

Design Principles for Microfluidic Biomedical Diagnostics in Space
= 75 dynes/cm 2, they found that a suspension of isolated platelets with viscosity
=1.1 mPa·s was significantly more likely to become activated (similarly for whole blood at
=83 dynes/cm 2, =3.8 mPa·s). Even a few seconds of exposure can be sufficient to sensitize
platelets (Dayananda et al., 2010). The viscosity of the fluid medium is important because it
is the shear stress, not the shear rate reported in many studies, that governs the behavior.
(In a homogeneous fluid, shear stress is the shear rate multiplied by viscosity.) Clearly, the
microfluidic design must avoid such levels of shear stress. But given the dependence on
shear stress history, the sample should avoid wall proximity to limit the potential for
adhesion. This is a strong argument for rigorously avoiding fluid separations (recirculating
flow cells), which may occur near sharp bends, corners, and steps. It also suggests that
sample dilution can be beneficial by lowering fluid viscosity and reducing protein and cell
concentration.
Fig. 2. Solids found in blood and urine samples: (a) blood cells; (b) erythrocyte, activated
platelet, and leukocyte; (c) urinary erythrocyte cast; (d) calcium oxalate crystals (arrows)
(e) uric acid crystals; (f) triple phosphate crystals with amorphous phosphate.
(a)-(b) reprinted from the National Cancer Institute, Frederick, Md.; (c)-(f) reprinted from
the National Institutes of Health Clinical Center Department of Laboratory Medicine,
Bethesda, Md.
Urine separation presents a different set of challenges. Although the solids content is far
less than blood, the size and other properties of the particulates vary more widely. These
can include cells, casts (which include blood and tissue cells that have passed through the
renal system, retaining the renal tubule’s shape, Fig. 2(c)), proteins, and a wide array of
crystals (Fig. 2(d)-(f)). Size, electromagnetic properties, rigidity, and staining propensities
are also quite variable. Obtaining well-filtered urine should be straightforward with
branching techniques (§3.2.2). Examination of urinary sediment can also yield critical
diagnostic information, but it is somewhat more challenging to accommodate in a
microfluidic context. Typically, a urine sample of 10-15 mL is centrifuged at ~2000-3000
rpm for five minutes to concentrate the solids content. Pure fluid is withdrawn until ~0.2-
0.5 mL remains. A drop of this fluid is then examined under a microscope (Simerville et
al., 2005). To examine urinary sediment in the space environment, an effective
microfluidic concentrator would be required as well as a means of staining and imaging
the concentrated sample.
Saliva is of interest to space biodiagnostics because of its ready availability and marginal
invasiveness to the astronaut, particularly since wound healing proceeds more slowly in
space (Delp, 2008). Saliva’s non-Newtonian viscoelastic behavior is a result of its high mucin
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