Biomedical Engineering Reference
In-Depth Information
1.7 Softness Sensing
The softness/hardness of objects is defined as the resistance of its material to deformation
(or indentation) [21, 22]. Hardness sensing is already in use in industry and there are
currently specific procedures to measure the hardness of objects. The softness of objects
is most commonly measured by the Shore (durometer) test. This method measures the
resistance of the object toward indentation and provides an empirical hardness number
that does not have an explicit relation to other properties or fundamental characteristics.
The Shore hardness using the Shore A, D, or OO scale is the preferred method for rubbers
and elastomers. While the Shore OO scale is used to measure the softness of very soft
materials, the Shore A scale is used for soft rubbers, and the Shore D scale is used for
harder ones. The Shore A softness is the relative softness of elastic materials such as rubber
or soft plastics and can be determined with an instrument called a Shore A durometer
[21]. International Rubber Hardness Degrees (IRHD) also introduces a measurement scale
for this purpose. Although several researchers have attempted to measure the softness of
objects in different ways, it is usually Young's modulus that is used to relate the softness
of objects by a nonlinear relationship which represents how much spring force a rubber
component will exert when subjected to deformation.
However, in order to measure the softness of tissues, one must also consider the behav-
ior of the contact object itself. Since soft tissues are nonlinear and are comprised of
viscoelastic materials, they show hysteresis in loading/unloading cycles. Furthermore,
variation in characteristics of the different soft tissues adds even more complexity to the
problem. Characterization of the soft tissues has also been restricted by the fact that the
behavior of the soft tissues differs between
in vivo
and
ex vivo
conditions.
One method is to differentiate between the natural frequency shift of the piezoelectric
material and the contact object which, in the case of wood and silicone gum, for example,
is about 750 Hz [23]. Yamamoto and Kawai [24] used a rotational step motor to create a
screw-like motion in soft tissue and then measured the transient response resulting from
these mechanical torsional steps. At the moment, the viscoelasticity of the epidermis
is evaluated by analyzing the voltage waveform of the step-motor inducting coil. This
waveform is characterized by overshoot, damping ratio and undamped natural frequency.
Hardness evaluation was carried out by Bajcsy [25] who pressed a robotic finger, fitted
with a low spatial resolution tactile sensor, against an object. This loading and subsequent
unloading process was performed in small incremental displacement steps and the sensor
output reading on each occasion was recorded.
Material hardness was ranked according to the slopes of the linear parts of the loading
and unloading sensor outputs. Work along similar lines was reported by Dario
et al
. [26],
using a single element sensor made of a piezoelectric polymer pressed against flat sheets
of rubbery materials of different compliance and backed by a reference load cell.
Hardness ranking was associated with the slope of the straight line obtained in the
sensor output reference cell signal plane under loading. De Rossi
et al
. [27] proposed
the use of charged polymer hydrogels as materials useful in tactile sensing, in particular
for softness perception, because of their ideal compliance matching with human skin.
Softness sensing has been applied as a diagnostic tool, such as in the case of diabetic
neuropathic subjects in which the hardness of foot-sole soft tissue increases in different
foot-sole areas [28]. The use of an active palpation sensor for detecting prostate cancer
and hypertrophy is reported by Tanaka
et al
. [29].
