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FORCE UNIAXIAL SENSOR
1.INTRODUCTION
Minimally invasive surgery (MIS) techniques are becoming increasingly popular because they offer lower risks and faster recovery times for the patient. However, surgeons lose the critical sense of touch during MIS because instruments are passed through ports that mask the surgeon’s feel of the internal operating environment. The need for force sensing in beating heart surgery is particularly important as the continuous movement of the heart as well as the pressurization of the heart chambers greatly reduces the sensory feedback a surgeon receives using conventional tools.. Researchers have focused on flexure designs for miniature force sensors due to the well-known characteristics of beam deflection as a function of force.
However, there are several limitations with such designs for beating heart applications. The customary use of electrically driven strain gauge transducers is problematic due to the potential for interference from the electrically active cardiac environment and due to safety concerns from the introduction of electrical currents into the heart. In addition, flexure-based configurations typically require relatively complex geometries to produce significant strains in the axial direction and to minimize the potential for damage due from overloading. The design is focused on the requirements of beating heart surgery and therefore requires excellent robustness and environmental isolation due to the dynamic movement and pressures within the heart. The proposed design avoids metal flexures in favor of an elastomer element, which ensures a waterproof seal, simplifies construction, and enhances robustness. The design also features a hollow core through which surgical instrumentation may be passed. Electrical passivity is maintained through the use of fiber optic sensing. The sensor is characterised in vitro, with subsequent in vivo validation in a beating heart mitral valve annuloplasty procedure.
2. SENSOR DESIGN
A number of considerations guide the design of the force sensor for our system. First, the sensor should be located at the instrument tip to accurately measure contact forces. Second, its use inside the heart dictates that it be small, completely sealed from blood, and electrically passive to avoid disrupting conduction in the heart. Finally, to beuseful in beating heart mitral annuloplasty, the sensor must be compatible with the
deployment of surgical anchors
Optically-based sensing is attractive in this setting because it does not require electrical trasmission to the sensor, has low noise, is readily miniaturized, and permits inexpensive, disposable sensors. The sensing principle relies on measuring small displacements of a reflective plate relative to the ends of optical fiber pairs. Three pairs of optical fibers, with each pair comprised of one transmitting and one receiving fiber, are placed in an equilateral triangle formation at the base of the sensor to ensure that the reflective plate deflection is captured entirely. An elastomer element is placed between the optical fiber ends and the reflective plate to convert force to displacement. The displacement modifies the light intensity measured by the receiving fiber, which is converted to a voltage by a phototransistor circuit.
It is built to encompass a 14 gauge needle for the deployment of surgical anchors. Polysiloxane elastomer provides low modulus and hysteresis. The rigid housing is made of Delrin for good appearance in ultrasound images. The external diameter and length of the force sensor are 5.5 mm and 12 mm, respectively. A thin film of silicone seals the exterior surface of the sensor to shield the internal components from fluid motion. Characterization of the force sensor against a commercial sensor (ATI mini40) indicates that our sensor has a 0.17 N RMS accuracy. This was determined by applying 10 Hz bandlimited loads from 0-5 N and up to 30° incidence angle. Calibration was performed in 37° C water to match in vivo thermal conditions.
2.1.FORCE SENSING IN BEATING HEART ANNULOPLASTY
Mitral valve annuloplasty restores the function of defective mitral valves by reshaping the mitral annulus (the tissue perimeter around the valve) using a stiff ring. This ring can either be sutured or secured with metal anchors. The surgical instruments for anchor deployment are thin rigid tubes that are inserted into the left atrium through a purse-string suture on the exterior heart wall. The end of the instrument is then pressed against the annulus with a force of approximately 1.5 N to deploy an anchor to secure the ring, but less than about 4 N to avoid injuring delicate tissue. This proves to be difficult to accomplish manually due to the movement of the annulus with the beating of the heart. Therefore, the procedure requires an accurate force sensor that is robust to intracardiac conditions.
This application imposes following design constraints:
1) Water proof: A waterproof seal is required in order to keep blood from the pressurized heart chamber from entering the sensor.
2) Electrical passivity: Surgical instrumentation must be electrically passive to avoid disrupting normal electrical activity in the heart. Optical fibers present a promising
solution by eliminating the need for electrical signals in the force sensor. Miniature optical force sensors have been developed recently but they rely on metal flexures for force transduction.
3) Miniaturization and location : The sensor must be located at the instrument tip to avoid the effect of friction and ancillary forces from the incision and purse string suture.
For use within the heart, size must be minimized. A further constraint is that anchors must pass through the force sensor. Due to the largely uniaxial motion of the mitral valve annulus, only a single axis of force sensing is required, which aids miniaturization.
4) Material constraints: Beating heart procedures use 3-D ultrasound for guiding instrumentation inside the beating heart. Metals tend to create artifacts in ultrasound images, so polymers are preferred for clear imaging.
Fig. 1 presents the configuration and operating principle of the sensor. It uses three pairs of fiber optic cables, with each pair comprising a light-transmitting fiber and a light-receiving fiber, coupled to LED and phototransistor circuits, respectively. A white plate 4 mm from the fibers reflects light from the emitting fiber to the receiving fiber, so the intensity of the light returned varies with the distance between the plate and fiber ends. Three pairs of optical fibers are placed in a triangle formation to minimize sensitivity to rotation of the reflective plate from off axis loads or uneven tissue contact. A solid elastomer element between the reflective plate and the optical fibers converts force to displacement, avoiding air gaps in the sensing element.