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Figure 4: Microfl uidic-based separation of circulating tumor cells
Figure 3: Immunomagnetic separation of circulating tumor cells
is used for such type of cell size based sorting. Specially
designed fi lter are employed to allow blood components
antibodies as they undergo EMT transitions while some to percolate through them. CTCs being bigger in size
other tumor cells belonging to a smaller sub-population will not be able to pass through the membrane and hence
might also be ignored. Some CTCs remain undetected remain over it. They can be then collected from over the
throughout this process. Hence, although this method membrane fi lter and subjected to analysis. [32]
[27]
is being used currently for experimental purposes, there
is yet lot of scope for improvisation in the quantitative as Other techniques
well as qualitative aspects of tumor cell detection. The FDA approved cell detection method has quite some
Microfl uidics method limitations. Hence, a lot of attempts are being made to
invent better techniques which are highly effi cient low on
As antibody-dependent cell sorting is not a completely cost, less labor intensive, and time savers too. Metacell
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reliable source. There are a lot of hurdles in is another cell size-based sorting method which has been
accomplishing higher percent enrichment of cells from introduced lately. [32,33] Microchips and micro slides are
the whole blood. Hence, it is important to take into being designed to exploit various differential properties
consideration other methods which rely on antibody-free of cells. Lu et al. have introduced a device which
[34]
systems. In this method, cell size-based sorting is they refer to as Nan Velcro CTCs Chip. They claim that
accomplished using microfl uidic technology. [28] The this device is much more effi cient and reproducible as
microfl uidic chamber is made up of special materials compared to CellSearch kit. This kit is composed of a
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and is usually spiral or curvilinear. When whole blood patterned silicon nanowire substrate which is overlaid with
is allowed to pass through this micro-chamber, inertial polydimethylsiloxane mixture. While another cell surface
[34]
lift forces and drag forces help in sorting of the cells. marker-based systems is a fl ow cytometry fl uorescence-
These forces rely on differential sizes of cell in the activated cell sorting. Another emerging technique
[35]
sample. In case of CTCs, whole blood or leukocyte is making use of dielectric constants of cells such as
along with CTCs fraction can be used as a feed in the DEPArray system. Ju et al. have described a
[36]
[26]
input. As they pass through the microfl uidic chamber, method where they make use telomerase activity to isolate
the forces will act on the cells and start separating them melanoma cells in peripheral blood. As telomerase activity
based on size. The CTCs incline more towards the is elevated in cancerous cells rather than normal cells,
inner wall (larger size) while other cells such as white they made use of an adenoviral vector human telomerase
blood cells and red blood cells will incline towards the reverse transcriptase to drive the expression of green
outer side of the wall (smaller size). [29] They can be fl uorescent protein which can be used to isolate CTCs
collected in separate fragments at the end of the tube, in this method. An interesting device called VeriFAST
where it bifurcates into collecting chambers as shown in is an integrated system which can isolate cells as well
Figure 4. Recent advances have allowed the procedure as perform down streaming processes including staining
to be carried out with minimal loss of cell types. [30,31] with EpCAM and other antibodies to isolate CTCs.
[37]
Size based separation method Many more such technological advancements have been
reported by scientists all over the world. There are several
As CTCs are usually bigger in size compared to other newer assays are being introduced which are focused on
components, this characteristic is put to use. This method marker free isolation such as chromatography, fi ltration,
can even be used to detect the presence of a single tumor and dielectrophoresis for capturing CTCs from cancer
cell in a quantity of blood as little as 1 mL (shown in patients. Few of them have been mentioned under
[38]
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Figure 5). ISET is one such established method which specifi c cancer categories discussed ahead in this review.
Journal of Cancer Metastasis and Treatment ¦ Volume 1 ¦ Issue 2 ¦ July 15, 2015 ¦ 47