Microfabricated nanomotion detectors for rapid cells viability tests
The widespread use of antibiotics and antifungal drugs has provoked an increasing number
of multi-resistant bacteria and fungi. The emergence of multiresistance to antibiotics has
been now recognized as a very serious public health issue. In order to target infections with
the appropriate antibiotics as soon as possible, there is an urgent need of rapid diagnostic
tools. Presently, the conventional antimicrobial testing techniques require between 24 h and
one month depending on the replication speed of the microorganism, which leads to the use
of broad-spectrum antibiotics treatments and further favors the development of resistant
pathogens.
This thesis proposes a rapid nanomechanical sensing - based diagnostic technique for studying
the viability of living organisms. This instrument is a nanomechanical sensor borrowed from
the Atomic Force Microscopy-based technology that uses a SU-8 lever as an optical fiber.
This combination is a promising approach for further integration and miniaturization of the
diagnostic apparatus. This thesis shows a first use of the microfabricated optomechanical
sensors for monitoring activity of living cells.
Its working principle is the following. The organism of interest is attached onto a cantilever and
its nanoscale movements induce the oscillations of the cantilever. To detect the oscillations,
a laser light is coupled into the SU-8 structure and travels to the free end of the cantilever.
The displacement of the output light spot induced by the oscillations of the cantilever is
recorded with a camera or a 4-quadrants photodiode. This allows to monitor the viability of
the microorganisms, that were previously attached to the cantilever surface.
The first part of this thesis briefly introduces the basic operating principle of the Atomic Force
Microscope, discusses the importance of the problem of the antimicrobial resistance as one
of the major health threats, and lists available alternative solutions to fight the resistance.
Then the state of the art, working principle, applications of the nanomechanical antimicrobial
sensors, and optomechanical mass sensors are discussed.
The second chapter introduces the methods for calculating the mechanical properties of the
cantilever beams, as well as discusses the fundamental sources of the noise in nanomotion
detectors, that limits their sensitivity. This chapter also includes calculations and finite elements
simulations of the mechanical properties of SU-8 - based cantilevers, as well as
estimations of the electronic and thermal noise of the system.
Third chapter explains in details the proposed sensing devices and elaborates on its design
and different strategies for the microfabrication of SU-8 - based optomechanical sensors.
It also describes the developed experimental setup for optomechanical measurements, the
light-coupling procedure and data acquisition.
The fourth chapter reports the optomechanical measurements conducted by actuating a
nanomechanical motion of the cantilever chips, and describes the preparation of the yeast
cells and cantilevers for the nanomotion experiments. Subsequently, the results of the viability
test are presented, along with a discussion of the mechanical and optical origins of it.
Finally, the thesis ends with a conclusion of this work and outlook of the presented results.
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