Biomedical Engineering Reference
In-Depth Information
ADULT HUMOR IN BIOMEMS
Paul Letourneau may have managed to contribute
the only bit of adult
entertainment to the otherwise sexually-neutral BioMEMS literature. his was back in
1975, when Letourneau was a graduate student at Stanford University, ater he conducted
a study with 8-day-old chick embryo neurons cultured on micropatterned substrates.
Intentionally or not, he decided to label the polyornithine tracks as “porn” (see
Figure
2.25a
), and neither the reviewers nor the journal editor seem to have challenged his choice
of words. Perhaps they were amused by it, as well as by the inevitable conclusion, recorded
forever in history: that chicks prefer porn to palladium. he pun takes such a hilarious
turn if one reads the abbreviation of palladium “Pd” in French slang that it is hard not to
conclude that Paul Letourneau is, in fact, a comical genius.
In 1985, reasoning that metals are foreign to in vivo microenvironments, a team led by Letourneau,
now at the University of Minnesota, uniformly coated a surface with laminin and placed a microfab-
ricated TEM grid atop the surface. Next, they UV-irradiated the laminin to “inactivate” it. Because of
some unknown mechanism, the cells (both the somas and their axons) were repelled by the “inacti-
vated laminin” areas (
Figure 2.25c
). he “laminin inactivation” method by Letourneau—which has
not been conirmed to work for other cell types or even other neuronal types, or other media formu-
lations—is not used nowadays (yet it is still being cited), but it is reproduced in this textbook because
it represents the irst efort to use only natural biomolecules to microengineer cell attachment.
2.6.1 Micropatterns of Physisorption-Repellent Background
Once researchers realized that the right approach to biopatterning was to use biomolecules for the
cell-adhesive part of the pattern, the bulk of the research eforts shited to inding approaches and
materials that would repel cells for the nonadhesive part of the pattern. his challenge is twofold:
(a) there are cell-adhesive proteins in the cell culture medium that will adhere in the background
areas; and (b) cell-dependent or cell-independent processes might cause the degradation of non-
adherent coatings and promote their cell invasion. We shall see that not all approaches are equally
efective at keeping the cells at bay, and many are cell type-dependent. Typically, cell types that
can secrete large amounts of ECM (such as ibroblasts) and cells that can project processes over
small nonadhesive regions (such as neurons sprouting neurites) are more diicult to contain.
2.6.1.1 HEG-Thiol SAM
In an article that has been cited more than 700 times since 1994, a team led by George Whitesides
at Harvard University and Donald Ingber at Children's Hospital (Boston) microstamped patterns
of methyl-terminated alkanethiol SAM on gold and illed the background with alkanethiols ter-
minated in a
hexa(ethylene glycol)
(
HEG
) group (
Figure 2.26
). When such a surface (colloqui-
ally referred to as “
PEG thiol
”) is exposed to ECM protein solution, protein physisorbed onto the
methyl-terminated areas only, and selective cell attachment onto the protein templates could be
demonstrated. SAMs of tri(propylene sulfoxide)-terminated alkanethiols were also shown to resist
protein adsorption and the attachment of endothelial cells for at least 24 hours. Most strikingly,
the HEG-derivatized areas prevent not only cellular attachment but spreading as well. As shown
in
Figure 2.26b
, the spreading and shape of individual cells can be constrained within adhesive
“islands” surrounded by HEG-terminated areas. Because the function and growth of a cell depends
on its degree of attachment and spreading onto the substrate, engineering a large population of
cells with a particular cell shape could be used to elicit, identify, or simply discern certain cellular
responses dictated by cell shape. In 1994, this team observed that a reduction in DNA synthesis and
an increase in apoptosis rates in hepatocyte micropatterned cultures correlated with a reduction of
