IEEE Robotics & Automation Magazine - June 2017 - 55

were reported [1]. Surgical resection of the tumors is the preferred treatment for early stages of lung cancer, but open surgery is associated with significant risk and morbidity [2].
RAMIS, however, has several advantages for the patient, such
as reduced trauma, less pain and scarring, faster recovery, and
a lower risk of infection [3]. But it prevents the surgeon from
using the sense of touch for palpating internal organs to locate
subsurface tumors. One alternative is to use a RAMIS ultrasound probe intraoperatively, but the images obtained can be
of poor quality, especially in the lungs, preventing reliable
diagnosis [4].
The use of a tactile sensor array has been explored in the
past to enable virtual tactile feedback for palpation during
minimally invasive surgery (MIS) [5]. A tactile sensor array
(referred to as simply tactile sensor from this point on) is a
cluster of small, discrete contact pressure-sensing elements
that can be used to detect variations in underlying stiffness
when pressed against a surface. Since tumors have a different
stiffness when compared to the surrounding soft tissue, a
tactile sensor can be used to locate them [6].
This article presents the design and preliminary evaluation
of TactUS, a novel robotic palpation instrument designed to
be compatible with the da Vinci Classic surgical robot. The
instrument has been designed with simplicity, robustness,
easy manufacturing, and low cost in mind. It incorporates a
custom, disposable tactile sensor and an ultrasound transducer back-to-back in its end effector to enable the use of multiple modalities to achieve higher tumor localization accuracy
[7]. The design allows the surgeon to quickly and frequently
switch between the two modalities during a surgery to verify
the location of a tumor. This work was awarded the Best
Innovation Prize at the 2015 Surgical Robot Challenge held at
the Hamlyn Symposium on Medical Robotics in June 2015.
Figure 1(a) shows the prototype mounted on a da Vinci Classic system. Figure 1(b) and (c) shows closeup views of the
end effector and wrist of the prototype.
Previously developed RAMIS tactile-sensing instruments,
such as those in [8] and [9], utilize nonstandard robotic platforms. Such devices have demonstrated promising results but
are not able to benefit from the established use of the da Vinci
surgical robot platform. Furthermore, there is no existing
RAMIS instrument that combines both tactile sensing and
ultrasound imaging in a single unit.
TactUS has three serial, cable-driven, decoupled internal
degrees of freedom (DoF) that provide the necessary dexterity
and range of motion to comfortably palpate tissue in a wide
range of orientations. It fits through a 12-mm nominal-size
trocar and is designed to safely withstand up to 20 N applied
laterally at the tip. It has been designed to be biocompatible
and sterilizable. It also allows the disposable tactile sensor to
be replaced between uses in under 5 s.
Tactile Sensor
There are several research groups and commercial organizations that have developed different tactile sensors for MIS,
but they have some major limitations when they need to

be employed on RAMIS instruments with a wrist. The limitations include a lack of sterilizability or disposability, a large
number of wires, and a high cost due to expensive manufacturing processes [10], [11]. To overcome these limitations, we
have developed novel, disposable tactile sensors with low cost,
a simple interface, and ease of construction in mind [12].
Both piezoresistive and capacitive sensing methods were
deemed to be suitable. Hence, two versions of the tactile
sensor, identical in appearance and interface, were developed to assess which sensing method is better suited for different tissue types.
The sensors are 49 × 10 × 2.5 mm in size and have 90 sensing elements with a 30-Hz update rate. The elements cover a
36 × 10-mm sensing area, with a 2 × 2-mm spatial resolution.
The manufacturing cost of the sensors is about US$35 when
produced in batches of 100. The low cost allows the sensors to
be disposable, requiring them to be sterilized only once, during manufacturing. This avoids damage due to repeated sterilization. They have onboard analog-to-digital conversion, and
hence require only four connection terminals: two for 3-V
power and two for digital communication. The small number
of connection terminals allows for the use of quick-disconnect spring contacts to enable easy sensor replacement, even
within the space restrictions of a RAMIS instrument.

(a)

(b)

(c)
Figure 1. A prototype of the instrument: (a) the entire instrument
when mounted on the da Vinci Classic system, (b) a closeup
view of the ultrasound transducer, and (c) a closeup view of the
tactile sensor.

June 2017

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