(funded by the NSF XYZ-on-a-chip program, September, 2000)
The goal of this project is the development
of low-cost sensing systems capable of multiple simultaneous analyses
of biological systems. These sensing systems will be applied to
the electronic detection of cell motion and/ or cell division.
Information obtained electronically will be used directly or in
combination with microscopic techniques. Sensing is based on the
use of large-area microelectronic fabrication techniques, which
will ensure low cost, disposability, redundancy, accuracy, and
high speed data collection. A direct interface with digital electronics
will facilitate rapid analysis and throughput at least an order
of magnitude higher than existing techniques (which will lead
to improved productivity in pharmaceutical and biological research).
We have developed an array for live cell screening (referred to as LCAST, for Live Cell Array Sensing Technology) which will make it possible to perform direct electrical sensing of cell motility and/or division. At present, optical sensing is used in cell proliferation and screening studies; individual cells are placed in wells on a partitioned plastic substrate. In this methodology, cells are exposed to drugs or other reagents and the response of the cell is observed microscopically. Each experimental site is analyzed sequentially, which involves microscope translation, focus, and complex image processing. In our approach thousands of sites will be interrogated electronically for cell activity. Electrical sensing will increase the data rate, facilitate direct computer analysis, and reduce the number of lost cells. This sensor will have applications in combinatorial cell biology where, for example, the effects of cytokines and other molecular factors on cellular motility and/or proliferation rate can be tested.
The work was conducted primarily at two
institutions: Carnegie Mellon University (Pittsburgh, PA) and
Lehigh University (Bethlehem, PA). Carnegie Mellon is the lead
institution for this program. Lehigh University received a
subcontract from Carnegie Mellon. Lehigh worked on developing a TFT
technology for the active-matrix array. Carnegie Mellon performed
biological experiments on medium-sized electrodes; modeled the
effect of cells on the electrode impedance; performed several
biological experiments using medium-sized electrodes fabricated at
Carnegie Mellon; designed a CMOS active matrix chip; developed
techniques for post-processing the CMOS chip; and demonstrated the use
of the active matrix array to address individual cell-sized electrodes.
The figure below shows the sensing concept,
originally proposed by Giaever and Keese. The electrical impedance is
measured between a small sensing electrode and a large
counter-electrode.
The figures below show the electric potential
(colors) and current density (arrows) for a cell on an electrode at
various frequencies. Current flows laterally beneath the celll at low
frequencies. The cell blocks the current flow at intermediate
frequencies. At high frequencies the impedance is largely due to
spreading resistance which is weakly dependent on the cell coverage.
The simulated impedance
magnitude change as a
function of frequency is shown in the figure below for two values of
the cell-electrode gap.
The next figure shows the measured normalized
impedance change as a function of frequency for fibroblasts on an
electrode as a function of time after dosing the cells.
This basic approach has been used to perform a
number of biological experiments as detailed in our publications. Some
of the conference proceedings paper drafts are linked from our publication list.
A particular goal of this project is the fabrication
of an active matrix array. Several figures are presented below.
Active matrix array
schematic diagram.
Layout of the CMOS chip.
Photograph of
gold-coated sensing electrodes.
Chip in package with
passivated bond wires and pads.