Nanostructure Embedded Microchips for Detection, Isolation,
and Characterization of Circulating Tumor Cells
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Abstract
ConspectusCirculating
tumor cells (CTCs) are cancer cells that break away
from either a primary tumor or a metastatic site and circulate in
the peripheral blood as the cellular origin of metastasis. With their
role as a “tumor liquid biopsy”, CTCs provide convenient
access to all disease sites, including that of the primary tumor and
the site of fatal metastases. It is conceivable that detecting and
analyzing CTCs will provide insightful information in assessing the
disease status without the flaws and limitations encountered in performing
conventional tumor biopsies. However, identifying CTCs in patient
blood samples is technically challenging due to the extremely low
abundance of CTCs among a large number of hematologic cells. To address
this unmet need, there have been significant research endeavors, especially
in the fields of chemistry, materials science, and bioengineering,
devoted to developing CTC detection, isolation, and characterization
technologies.Inspired by the nanoscale interactions observed
in the tissue microenvironment,
our research team at UCLA pioneered a unique concept of “NanoVelcro”
cell-affinity substrates, in which CTC capture agent-coated nanostructured
substrates were utilized to immobilize CTCs with high efficiency.
The working mechanism of NanoVelcro cell-affinity substrates mimics
that of Velcro: when the two fabric strips of a Velcro fastener are
pressed together, tangling between the hairy surfaces on two strips
leads to strong binding. Through continuous evolution, three generations
(gens) of NanoVelcro CTC chips have been established to achieve different
clinical utilities. The first-gen NanoVelcro chip, composed of a silicon
nanowire substrate (SiNS) and an overlaid microfluidic chaotic mixer,
was created for CTC enumeration. Side-by-side analytical validation
studies using clinical blood samples suggested that the sensitivity
of first-gen NanoVelcro chip outperforms that of FDA-approved CellSearch.
In conjunction with the use of the laser microdissection (LMD) technique,
second-gen NanoVelcro chips (i.e., NanoVelcro-LMD), based on polymer
nanosubstrates, were developed for single-CTC isolation. The individually
isolated CTCs can be subjected to single-CTC genotyping (e.g., Sanger
sequencing and next-generation sequencing, NGS) to verify the CTC’s
role as tumor liquid biopsy. Created by grafting of thermoresponsive
polymer brushes onto SiNS, third-gen NanoVelcro chips (i.e., Thermoresponsive
NanoVelcro) have demonstrated the capture and release of CTCs at 37
and 4 °C, respectively. The temperature-dependent conformational
changes of polymer brushes can effectively alter the accessibility
of the capture agent on SiNS, allowing for rapid CTC purification
with desired viability and molecular integrity.This Account
summarizes the continuous evolution of NanoVelcro
CTC assays from the emergence of the original idea all the way to
their applications in cancer research. We envision that NanoVelcro
CTC assays will lead the way for powerful and cost-efficient diagnostic
platforms for researchers to better understand underlying disease
mechanisms and for physicians to monitor real-time disease progression