A3.3 - A Miniaturised Piezoresistive Multi-Component Force Sensor for Minimally Invasive Surgery

Abstract

Conventional tools used in minimally invasive abdominal surgery (MIS) suffer from workspace restrictions and loss of haptic perception of the manipulated tissue.Operations are more complicated, expert surgeons are needed and longer operation time is the consequence.On this, a novel hand-held instrument with a force feedback system for gentle manipulation of soft tissue is being developed [1].Driven by integrated piezoelectric actuators an operation platform with four additional degrees of freedom carries manipulators and positions them inside the surgical area.To assist the surgeon during the operation the relating forces between soft tissue and tool are measured with a force sensor.Typically the force vector consists of three force components F i , each one with an absolute value of |F i | ! 5 N [2].An active control element is feeding back the haptic information to the surgeons' hand.In this paper we report of the progress of a miniaturised multi-component force sensor.It will be integrated in the tool tip of the operating platform and detect at minimum three force components (shear and normal forces).Following the requirements of humans' haptic perception the sensor detects both static and dynamic forces up to 1000 Hz.According to forces arising in typical tasks especially during liver surgery forces up to 5 N can be measured at the tool tip. IntroductionMinimally invasive abdominal surgery (MIS) has become a state-of-the-art technique in surgery over the past decades.Several standard procedures and suitable instruments have been developed [3].The interventions cover a wide range of possible treatments e.g. in thorastic surgery, brain surgery and abdominal surgery.The main advantage of minimally invasive techniques is shorter patient recovery time in comparison to open surgery due to the reduction of tissue trauma and risks of postoperative infection.Though many interventions like cholecystectomy (removal of gall bladder) for example are already standard procedures in laparoscopic surgery.The main disadvantage of conventional laparoscopes is the loss of intracorporeal workspace, because the number of degress of freedom is limited to four.Additionally, the haptic feedback is deminished due to friction in trocars and restoring forces of the abdominal wall.Thus, the surgeon is loosing one of his most important senses -the sense of haptic perception.It disables him to feel tissue hardness, evaluate tissue properties and anatomical structures.Furthermore, it inhibits him to commit appropriate force controlled actions for safe tissue manipulation.Several approaches have been suggested to implement force measurement for haptic feedback in laparoscopic tools [4 ,5].On the one hand conventional laparoscopic tools are equiped with force sensing elements usually strain gauges (for example [6,7]), on the other hand laparoscopic tools have been modified or even redesigned integrating a force sensor [7,8,9,11,12].The itemised examples vary in sensing principle, dimensions and location of the force sensor.ROSEN et al. at the University of Washington [7] integrate sensing elements outside of the abdominal cavity (see Fig. 1(b)).They motorised a laparoscopic grasper with DC motors controlling Bowden cables and evaluated the acting forces integrating conventional strain gauges within the driving mechanism.Because of the extracorporeal location the sensors have not to be sterilisable.Forces up to 60 N have been measured.According to the distance between sensor and interaction point, the friction of every joint and the backlash of the kinematics itself decrease the accuracy.Furthermore the dynamics are limited because the kinematics act like a mechanical low-pass-filter.BERKELMAN et al. [8] developed a microsurgical tip force sensing instrument (see Fig. 1(a)).The piezoresisitve tri-axial force sensor is integrated into the hand-held and has an outer diameter of 12.5 mm and a height of 15 mm.Nominal load of +/-1 N at a resolution of 0.5 mN are detectable.Other research groups measure forces at the distal end of an instrument.Either being sterilisable or a sterile one-way product and thus miniaturised 1 , the requirements on dimensions of these sensing elements and their packaging increase.At the Katholieke Universiteit 1 diameters ! 10 mm are state of the art for minimally invasive surgery tools A3.3