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Sensors & transducers


Designing and manufacturing haptic sensors for robotic surgery platforms


Surgical robotic companies are pursuing a marketable product that incorporates haptic feedback. In this article, FUTEK explains the challenges that need to be addressed when designing and manufacturing haptic sensors


S


urgery is a centuries-old healing practice whose etymology comes from the Greek word cheirourgia (performed by the


hands). Surgical procedures evolved over the centuries concerning instruments, pain control, infection prevention and, most importantly, surgical techniques, resulting in enhanced medical treatment and ultimately extending human lifespan. Minimally invasive surgery (MIS) is a modern


technique that allows interventionists to perform operations through small incisions (usually 5 – 15mm). Besides its clear advantages, MIS is more difficult to perform compared to traditional procedures. Other inherent drawbacks are (a) limited motion due to straight laparoscopic instruments and fixation enforced by the small incision in the abdominal wall; (b) impaired vision, due the two-dimensional imaging; (c) usage of long instruments amplifies the effects of surgeon’s tremor; (d) poor ergonomics imposed to the surgeon; and (e) loss of haptic feedback, which is distorted by friction forces on the instrument and reactionary forces from the abdominal wall.


GAINS AND LOSSES OF MINIMALLY INVASIVE ROBOTIC SURGERY Minimally invasive robotic surgery (MIRS) offers solutions to either minimise or eliminate many of the pitfalls associated with traditional laparoscopic surgery. Current available MIRS platforms such as the Da Vinci Surgical System, approved by the US Food and Drug Administration in 2000, was a


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historical milestone of surgical treatments. The ability to leverage laparoscopic surgery advantages while augmenting surgeons’ dexterity and visualisation and eliminating the ergonomic discomfort of long surgeries, makes MIRS undoubtedly an essential technology for the patient, surgeons and hospitals. However, despite all improvements brought


by currently commercially available MIRS, haptic feedback is still a major limitation reported by robot-assisted surgeons. Since the interventionist no longer manipulates the instrument directly, the natural haptic feedback is eliminated. Haptics is a conjunction of both kinesthetic (form and shape of muscles, tissues and joints) as well as tactile (cutaneous texture and fine detail) perception, and is a combination of many physical variables such as force, distributed pressure, temperature and vibration. Direct benefits of sensing interaction forces at the surgical end-effector are (a) improved organic tissue characterisation and manipulation, (b) assessment of anatomical structures, (c) reduction of sutures breakage and (d) overall increase on the feeling of assisted robotics surgery. Haptic feedback also plays a fundamental role in


shortening the learning curve for young surgeons in MIRS training. A tertiary benefit of accurate real-time direct force measurement is that the data collected from these sensors can be utilised to produce accurate tissue and organ models for surgical simulators used in MIS training.


TECHNICAL AND ECONOMIC CHALLENGES Adding to the inherent complexity of measuring haptics, engineers and neuroscientists also face important issues that require consideration prior to the sensor design and manufacturing stages. The location of the sensing element, which significantly influences the measurement consistency, presents MIRS designers with a dilemma: should they place the sensor outside the abdomen wall near the actuation mechanism driving the end-effector (also known as Indirect Force Sensing), or inside the patient at the instrument tip, embedded on the end-effector (also known as Direct Force Sensing). The pros and cons of these two approaches are


associated with measurement accuracy, size restrictions and sterilisation and biocompatibility requirements. The table in Figure 1 compares these two force measurement methods. In the MIRS applications where very delicate


instrument-tissue interaction forces need to give precise feedback to the surgeon, measurement accuracy is sine qua non, which makes intra- abdominal direct sensing the ideal option. However, this novel approach not only brings


the design and manufacturing challenges described in Figure 1 but also demands higher reusability. Commercially available MIRS systems that are modular in design allow the laparoscopic instrument to be reutilised approximately 12 to 20 times. Adding the sensing element near to the end-effector invariably increases the cost of the


April 2019 Instrumentation Monthly


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