research activities in 2007 - CSEM

Nanoporous Membranes for Medical Diagnostics and Drug Discovery
C. Santschi, R. Pugin, V. Spassov, S. Berchtold, A. Hoogerwerf
Understanding cellular signaling controlled by cell surface receptors, ion channels and pumps is a key issue of modern biological research and drug
development. A platform for high throughput measurements, namely, impedance spectroscopy and fluorescence microscopy, has been developed
and is described below. A nanoporous SiO2/SixNy membrane, where native vesicles can be self-assembled, forms the core of the device. These
membranes combined with microfluidic channels are the main components of the modularly built-up unit.
A rapidly growing number of molecular targets discovered by
functional genomics and potential therapeutic compounds
produced by chemical synthesis increases the need to
downscale probe formats in order to accelerate functional
screening, and to reduce reagent consumption [1], and costs.
Recent progress in micro- and nanotechnology allows the
fabrication of suspended nanoporous SiO2/SixNy membranes
appropriate for screening transmembrane receptors and
signaling pathways of mammalian cells. In the frame of this
project financed by CCMX, a versatile platform allowing highthroughput
electrochemical spectroscopy and fluorescence
measurements has been developed. The realization of this
platform is a challenging task merging micro, and
nanotechnology. The modularly built platform consists of three
parts: a micro-fluidic component 1), PCB based Si chip-holder
containing SiO2/SixNy membrane 2), and cover 3) (Figure 1).
3)
2)
1)
Figure 1: Schematic drawing of the modular platform with 1) cover 2)
chip-holder 3) micro-fluidic component.
A system of micro-fluidic channels is sealed between two
glass cover slides. The chip-holder is fabricated using a PCB
board commonly used in industrial electronics. An image of
the chip-holder PCB is displayed in Figure 2. The PCB
contains rectangular apertures, where the individual silicon
chips are placed. Moreover, electric contacts are directly
integrated on both sides of the double layered board. The
cover consists of a glass cover slide and a PDMS sealing
layer.
Figure 2: a) Image of the PCB chip-holder with mounted Si chips.
The size of the board is 4 x 10 cm 2 . b) Closed view showing a blank
cutout where the individual chips can be mounted.
The maximum thickness of the cover is governed by the focal
distance of a fluorescence microscope, which can be as short
54
Glass cover slide
PDMS-Sealing
Chip holder
PDMS-Sealing
Glass cover slide
Fluidic channel
Glass cover slide
Microscope
Fluidic in- and
outlets
Si chip (Figure 3)
PDMS
as 350 μm. The micro-fluidic part of the device is sealed
between two glass cover slides.
SiO2 SiN
Pt
Figure 3: Schematic cross-section a Si chip containing a membrane
The SiO2/SixNy membranes are fabricated from a 4 inch silicon
wafer using standard microfabrication techniques. The wafer
is divided into chips of 5 x 10 mm 2 each containing a
50 x 50 μm 2 freestanding membrane. A schematic crosssection
of the chip is displayed in Figure 3. For size selective
positioning of the vesicles of interest, each membrane
contains nine cavities etched in the SiO2 layer. Furthermore,
each of the cavities contains a single nanopore of 50 nm in
diameter in the center of the underlying SiN membrane as
illustrated in Figure 3. The diameter of the cavities lies in the
range of 1 – 2 μm. For prototyping, cavities and nanopores
have been fabricated using a Focused Ion Beam (FIB)
technique. FIB is a versatile tool which allows surface
structuring by local material removal. The interaction between
a focused Ga + -beam and the substrate results in a physical
removal of material due to a momentum transfer from the
incident ions to the target atoms. Using FIB, structures of
almost any user-defined geometry larger than a few tens of
nanometers can be realized; thus, it is an appropriate tool for
the fabrication of nanopores [2] .
500 nm
Figure 4: Image of the Si-wafer and SEM micrograph of an individual
cavity fabricated using FIB technique
Figure 4 shows an image of a wafer containing several thin
membranes and a SEM micrograph of an individual cavity with
nanopore fabricated with FIB. The next step in this project will
be to perform impedance spectroscopy and fluorescence
microscopy measurements in order to validate the platform
performances for drug discovery applications. This project has
been financed by the Competence Center for Materials
Science and Technology (CCMX) and the CSEM.
[1] S. A. Sundberg, Current Opinion in Biotechnology, 11, (2000)
47-53.
[2] C. Danelon, C. Santschi, J. Brugger, H. Vogel, Langmuir 22,
(2006) 10711-10715.