Biosensing Detection - Bioinspired Approaches for Human-Centric Technologies - page 100

3.12 Stress-Driven Out-of-Plane Cantilever Technology
In this section, we introduce the design of a stress-driven artificial hair cell (AHC).
The technology is based on exploiting the release of the material stress difference
among the constituent layers of a nitride-based cantilever beam to obtain the
upward bending of its tip and to project it inside a flow stream. The beam layer
characteristics (material properties and geometrical dimensions) are designed in
order to exploit a nitride-based (either aluminum nitride or silicon nitride) cantile-
vers, equipped by a nichrome piezoresistive strain gauge read-out, for realization
AHC mechanoreceptor-like sensors. A parylene waterproofing coating allows the
employment in liquids for applications such as acoustic prosthetics and underwater
robotics.
In the last years, advances in micro-fabrication of microelectromechanical
systems (MEMS) allowed the development of several approaches to artificial hair
cell mechanoreceptors based on different technological principles. In case of a
passive AHC, the bending of a micro-mechanical element (Fan et al. 2002 ; Dijkstra
et al. 2005 ) was the preferential approach. A strain gauge is placed on the most
strained point of a mechanically deformable structure, reading-out the curvature
and behaving as a mechanoreceptor for flow (liquid or air) and tactile sensing. The
most investigated structure is a vertical cilium: an SU8 vertical pillar subjected to a
mechanical pulse transfers its momentum to a planar cantilever (Fan et al. 2002 )or
a capacitive suspended membrane (Dijkstra et al. 2005 ). However the fragility of
this approach can cause fracture upon mechanical overloading. In case of active
AHC, an alternative approach was based on embedding a conductor inside a
polyimide cilium. Thermally controlling the expansion of the heated aluminum
conductor (Suh et al. 1996 ) or suspending the cilium on an ITO (indium tin oxide)
electrode and applying a voltage between the two conductive layers (den Toonder
et al. 2008 ), the system works as bimorph biomimetic cilium microactuator. An
array of these cilia, each one electronically addressable, works like a micromanipu-
lation tool. Finally, a cilium was realized by a polymeric matrix, filled by magnetic
nanoparticles (iron oxide particles in this device). The switching-on of a magnetic
field causes the cilium to be manipulated: an electromagnet below the device array
is used to bend and move the cilia (Evans et al. 2007 ). These biomimetic
AHC-based microactuators are interesting proof of principle devices; however,
they have a few important drawbacks limiting their actual applicability to real
implants. First, their response is not triggered by the input of a sensor; conse-
quently, the switch-on of every single microactuators is driven manually and not by
a natural time-varying signal. Second, their operation principle is not intrinsic to the
device materials (like in a piezoelectric cantilever) but it is based on external
electrostatic or magnetic fields, which might suffer the harsh physiologic liquid
environments and could be detrimental for the thermal control or the external field
action.
Stress-driven artificial hair cell has been recently proposed as a new and alter-
native approach to artificial hair cell design. A stress-driven cantilever, suspended