- Thin-film device may provide a sense of touch for minimally invasive surgery
Gwynne D. Koch
Doctors have reported that minimally invasive
surgery suffers from a lack of the sense of touch, causing frustration and even
errors during these procedures. Incorporating a high-resolution sensor that provides
an analogue to touch into an endoscope would, for example, enable surgeons to superimpose
the representation of a tumor’s texture over its optical image. With such
an instrument, surgeons could determine during a procedure whether all cancerous
tissues had been removed, rather than waiting days for results of a pathological
According to Ravi F. Saraf, a chemical engineer
at the University of Nebraska-Lincoln, proposed touch sensors can be described by
two main characteristics: sensitivity and resolution. Sensitivity is the threshold,
or the lowest level, of stress that the sensor can measure, and resolution is how
finely it can measure the stress distribution.
Engineers have developed a large-area, high-resolution touch sensor
based on a thin-film design. Pressure images of portions of coins obtained with
the touch sensor show remarkable correspondence with their optical images (left
pair). The device can capture images with such detail that the letters “TY”
in “LIBERTY” and the outline of the embossed number 5 are clearly visible
Although touch sensors currently offer
sensitivity comparable to that of the human finger during a caress (an applied load
of ~10 kPa or higher), emulating the finger’s resolution of about 40
μm has been a challenge for researchers. For example, small-area devices, such
as an 8 x 8 array of capacitance sensors, have demonstrated 100-μm spatial
resolution. Large-area (>1 cm2) sensors, however, provide only 1- to 2-mm resolution.
Saraf and his colleague, Vivek Maheshwari,
have developed a 2.5 x 2.5-cm sensor that offers spatial resolution of ~40
μm. Their goal was to take a different approach by making a “continuous
sensor” — similar to the human finger — as opposed to using an
array of devices, to image stress distribution.
The electro-optical device comprises
stacks of alternating layers of 10-nm gold and 3-nm semiconducting cadmium-sulfide
nanoparticles sequentially deposited by dipping in four solutions. The layers are
separated by ~3-nm-thick polymer dielectric barriers and are flanked by two
electrodes — one gold, coated with flexible plastic, and one transparent,
on glass — resulting in a thin-film sensor about 100 nm thick.
To test the device, the scientists
pressed a penny against the flexible electrode. Current increases locally where
pressure is applied, and the semiconducting particles respond to the increased flow.
At biases exceeding 8 V, the applied force enhanced electron tunneling between the
layers, inducing electroluminescence from the cadmium-sulfide nanoparticles that
was detected through the transparent electrode with a CCD camera. The optical intensity
and current were linearly proportional to the applied stress, resulting in a touch
sensor that was not jumpy or hypersensitive.
The sensor overcomes several of the
challenges facing its precursors. Devices based on silicon technology, such as organic
thin-film flex circuits, rely on expensive lithographic or chemical vapor deposition
methods. Also, flex circuit devices are limited in the degree to which they can
conform to a surface because bending produces internal stresses that cause a background
signal. The team’s sensor can be fabricated in ambient conditions and is self-assembled,
costing significantly less, and it could be deposited onto large-area, complex-shaped
The researchers’ next goal is
to map temperature and ultrasound radiation using a similar device architecture.
Science, June 9, 2006, pp. 1501-1504.
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