**2.2.3 Catheter design**

140 Haptics Rendering and Applications

For a recently aborted fetus with diagnosed congenital malformation, continuation of the pregnancy until termination with a Cesarean delivery, change in timing mode, or place of delivery are the only three options available. Fetal surgery may provide a further solution in these cases. Open fetal surgery is state of the art (Harrison, 2003) (Berris and Shoham, 2006) and is already available in over a dozen clinical centers worldwide. Endoscopic fetal surgery, which is commonly referred as Fetoscopy (Sydorak and Albanese, 2003) is however a medical discipline that is still in its early development stages. Fetal surgery has given the possibility to treat some fetal congenital malformation in utero (diaphragmatic hernia,

The R&D of a surgical tool for interventions on the fetus during pregnancy requires highly advanced micro/nanotechnologies from the system integration point of view and a close collaboration with medical doctors to develop it according to their real needs. At present fetal surgery lacks purposely developed micro instrumentations because paediatric surgeons are trying to apply standard minimal invasive instruments to fetal surgery applications. These instruments are far too large for interventions like treatment of pulmonary atresia in such early

During pregnancy the oxygen necessary is not supplied through the fetal lungs but by the placenta. The Foramen Ovale is an opening between the right and the left atrium, that allows blood to pass by the ventricles. After birth this opening is usually closed. Pulmonary Atresia is a malfunction that may appear during pregnancy: it is an incorrectly developed pulmonary valve. Instead of a valve there is just a membrane. Thus no blood supply to the lungs is possible, and this causes the death of newborns when oxygen supplied by the placenta is not supplied anymore. Pulmonary atresia occurs in about one out of every 20,000

Anatomic obstruction to the right or left ventricular outflow tract may cause ventricular dysfunction, can divert fetal blood flow in the uterus and result in cardiac chamber hyperplasia. Thus severe aortic or pulmonary stenosis can result in a hypoplastic left or right ventricle with an inability for the ventricular chambers to support the systemic or pulmonary circulation. Theoretically early relief of the fetal aortic or pulmonary stenosis may prevent such occurrence and might preserve the right or left ventricular function .

Pulmonary atresia can be diagnosed in the 12-14th week of gestation. The surgical intervention should be performed as soon as possible. In the 14th week the fetus size is about 9-14cm and has a weight in the range of 60 - 200g. In this development stage the

In the case of pulmonary atresia an early surgical intervention is the only alternative to abortion and could allow normal development of the pulmonary valve and the right ventricle. A suitable surgical procedure for the treatment of pulmonary atresia is by accessing the right ventricle with a steerable micro catheter, identifying the pulmonary valve/membrane and then perforating it. However one must consider that tissues in this

**2.2 Sensorized catheter** 

**2.2.2 Pulmonary atresia** 

**2.2.1 Introduction to fetoscopy** 

myelomeningocele, lower urinary tract obstruction).

stage and also do not offer capabilities such as tissue classification.

live births. (Daubeney et al., 2005) (Litovsky et al., 2005)

pulmonary membrane has a diameter of approximately 1mm.

The catheter prototype is fabricated from polyurethane with a flexible end of 40 mm length and an outer diameter of 3,5 mm. The diameter of the main inner lumen is 2,5 mm . Four 380 µm thin lumen are integrated in the wall (thickness: 550 µm) and serve as guidance for four Bowden cables. A micromachined tip (Fig 7a and 7b) fabricated from Polyether Ether Ketone (PEEK) is bonded to the flexible end of the catheter and mainly acts as a support for four impedance measurement electrodes and two optical fibers. Furthermore it offers a main channel leading to the inner lumen of the catheter. This inner lumen serves as a supply of washing solution (physiological saline solution).

Fig. 7. a. Catheter tip prototype; b. Catheter tip 3D model

To be able to reach the point of interest, the catheter needs to be equipped with steering capabilities. The multi lumen catheter consists of a very flexible ending and a less flexible part. In the walls of the catheter four lumen are integrated, each one for one steering wire. Such wires are usually referred as Bowden cables. Pulling on these four wires and releasing at the same time the wire which is on the opposite side in the catheter will primarily result in a bending movement of the flexible end part of the catheter. Two microcontroller driven servo drives are used to pull and release the wires. This microcontroller is then connected to a personal computer, which is equipped with a haptic force feedback joystick allowing a precise control of the catheter. A third degree of freedom is realized by either manually or servo supported driving the catheter forward and backwards. The arm on the servo controller, which is actuating the Bowden wire is equipped with a strain gauge, which allows measurement of the actuation forces on the Bowden wire. As the actuation force reflects the force required to bend the catheter tip, it also reflects a force on the catheter tip itself, thus it can be used for haptic force feedback. For the readout of the strain gauges a similar circuit, but just with two AD converters, like described in 2.1, figure 4 was used. Figure 8 shows the first prototype of the catheter controlled by a commercial joystick.

Sensorized Tools for Haptic Force Feedback in Computer Assisted Surgery 143

The recorded impedance spectrum is then mainly defined by the electrical impedance characteristics of the tissue exactly in front of the small sensing electrode. This electrode configuration is especially interesting from the miniaturisation point of view, as the centre electrode could then also be used for radio frequency cutting needed for the perforation of

It must be kept in mind that for tissue classification it is not necessary to record accurate impedance data from the electrical point of view. It is important that the training data sets

Even if a two electrode configuration may be suitable for tissue classification, four electrodes are integrated in the peek tip of this first catheter prototype so that two and four electrodes configurations can be compared to each other. In the test setup a TEGAM 3550 LCR Meter is used to record impedance data. It is connected to a Personal Computer

Another way of characterisation of tissues is optical spectroscopy. Therefore two optical fibers were integrated in the front of the catheters tip. A suitable light source illuminates the tissue with light guided to the point of interest through an optical fiber. Reflected light is then received with a second fiber leading to an optical spectrophotometer. The spectral range must be chosen according to the ranges where the tissues have characteristic

In normal conditions the heart is filled with blood. Haemoglobin is a strong light absorber, where the wavelength dependent light absorption is furthermore dependent on the oxygenation status of haemoglobin. Measuring tissue characteristics in such an environment with a spectrophotometric method is not possible. The solution for this problem was solved with the integration of another lumen in the catheter to provide washing solution. With a small amount of physiological saline solution blood in front of the catheters tip can be washed away. Blood in the measuring zone is thus substituted with the optical clear

Impedance spectrum recording and spectrophotometric spectrum recording requires steady state conditions – the tissue in front of the catheter should not move relatively to the catheter, which is difficult to realize in a beating heart environment. To solve this problem the washing system describes above can be used in a second way: After the blood is washed away with a small amount of physiological washing solution, the washing solution pump will be used to suck in washing solution thus creating suction in front of the catheter which will suck the tissue in front of the catheter to the tip thus establishing a reliable electrical connection to the electrode(s) and steady conditions for the measurements. A differential

Once tissue is classified as "pulmonary valve tissue" the perforation of the membrane is the next step. In this case an electrode is used for RF cutting. This electrode can either be a separate electrode or one of the impedance electrodes on the catheters tip. For this task a tissue fixation as described above is also a great advantage, as it allows a safe and controlled perforation in a quasi-static environment. Figure 9 shows the principle function of the

washing solution, which enables spectrophotometric reflectance measurement.

pressure sensor is integrated to allow precise suction force control.

catheter for electrical and optical tissue characterisation

are recorded with the same electrode configuration to give comparable recordings.

the pulmonary membrane.

through a GPIB link.

reflectance spectra.

For electrical characterisation of tissues in front of the sensors an electrical impedance sensor was integrated. Bio-impedance spectroscopy allows tissue classification and identification by recording and analyzing the electrical impedance at different frequencies. From the electrical point of view cell membranes behave like capacitors. In comparison to low frequency electrical current where the current path is leading mainly through extra cellular fluid, high frequency electrical current is able to penetrate the cells. Thus impedance over frequency and phase over frequency plots (or both combined in "Wessel plots") are characteristics of tissues as they reflect the electrical characteristics of the cells and the tissue composition. Principle Component Analysis can then be used to classify a tissue by a recorded data set. For impedance spectroscopy two or four electrodes configuration are state of the art. Four electrodes impedance measurement allows higher accuracy, as two electrodes are used to drive in the electrical current and the other two, which are normally arranged in between the first two ones, are used for measuring the voltage drop, so called four probe measurements. The electrical contact impedance (usually current dependent) of the two voltage sensing electrodes can when be neglected, as the sensing input of impedance meter is high – so there is almost no current flowing.

Fig. 8. Operation of the steerable catheter tip

In comparison to the four electrodes measurement technique the two electrodes configuration has the disadvantage of sensing the voltage through the same electrodes which are used to drive the electrical current. The recordings therefore show tissue impedance in serial with the electrode impedances. A two electrode configuration has the advantage of needing less space, which is an important aspect in designing a miniaturized catheter. Furthermore the two electrodes configuration can be divided into a small electrode on the tip of the catheter and a large area counter electrode which is attached to the body.

For electrical characterisation of tissues in front of the sensors an electrical impedance sensor was integrated. Bio-impedance spectroscopy allows tissue classification and identification by recording and analyzing the electrical impedance at different frequencies. From the electrical point of view cell membranes behave like capacitors. In comparison to low frequency electrical current where the current path is leading mainly through extra cellular fluid, high frequency electrical current is able to penetrate the cells. Thus impedance over frequency and phase over frequency plots (or both combined in "Wessel plots") are characteristics of tissues as they reflect the electrical characteristics of the cells and the tissue composition. Principle Component Analysis can then be used to classify a tissue by a recorded data set. For impedance spectroscopy two or four electrodes configuration are state of the art. Four electrodes impedance measurement allows higher accuracy, as two electrodes are used to drive in the electrical current and the other two, which are normally arranged in between the first two ones, are used for measuring the voltage drop, so called four probe measurements. The electrical contact impedance (usually current dependent) of the two voltage sensing electrodes can when be neglected, as the sensing input of

In comparison to the four electrodes measurement technique the two electrodes configuration has the disadvantage of sensing the voltage through the same electrodes which are used to drive the electrical current. The recordings therefore show tissue impedance in serial with the electrode impedances. A two electrode configuration has the advantage of needing less space, which is an important aspect in designing a miniaturized catheter. Furthermore the two electrodes configuration can be divided into a small electrode on the tip of the catheter and a large area counter electrode which is attached to the body.

impedance meter is high – so there is almost no current flowing.

Fig. 8. Operation of the steerable catheter tip

The recorded impedance spectrum is then mainly defined by the electrical impedance characteristics of the tissue exactly in front of the small sensing electrode. This electrode configuration is especially interesting from the miniaturisation point of view, as the centre electrode could then also be used for radio frequency cutting needed for the perforation of the pulmonary membrane.

It must be kept in mind that for tissue classification it is not necessary to record accurate impedance data from the electrical point of view. It is important that the training data sets are recorded with the same electrode configuration to give comparable recordings.

Even if a two electrode configuration may be suitable for tissue classification, four electrodes are integrated in the peek tip of this first catheter prototype so that two and four electrodes configurations can be compared to each other. In the test setup a TEGAM 3550 LCR Meter is used to record impedance data. It is connected to a Personal Computer through a GPIB link.

Another way of characterisation of tissues is optical spectroscopy. Therefore two optical fibers were integrated in the front of the catheters tip. A suitable light source illuminates the tissue with light guided to the point of interest through an optical fiber. Reflected light is then received with a second fiber leading to an optical spectrophotometer. The spectral range must be chosen according to the ranges where the tissues have characteristic reflectance spectra.

In normal conditions the heart is filled with blood. Haemoglobin is a strong light absorber, where the wavelength dependent light absorption is furthermore dependent on the oxygenation status of haemoglobin. Measuring tissue characteristics in such an environment with a spectrophotometric method is not possible. The solution for this problem was solved with the integration of another lumen in the catheter to provide washing solution. With a small amount of physiological saline solution blood in front of the catheters tip can be washed away. Blood in the measuring zone is thus substituted with the optical clear washing solution, which enables spectrophotometric reflectance measurement.

Impedance spectrum recording and spectrophotometric spectrum recording requires steady state conditions – the tissue in front of the catheter should not move relatively to the catheter, which is difficult to realize in a beating heart environment. To solve this problem the washing system describes above can be used in a second way: After the blood is washed away with a small amount of physiological washing solution, the washing solution pump will be used to suck in washing solution thus creating suction in front of the catheter which will suck the tissue in front of the catheter to the tip thus establishing a reliable electrical connection to the electrode(s) and steady conditions for the measurements. A differential pressure sensor is integrated to allow precise suction force control.

Once tissue is classified as "pulmonary valve tissue" the perforation of the membrane is the next step. In this case an electrode is used for RF cutting. This electrode can either be a separate electrode or one of the impedance electrodes on the catheters tip. For this task a tissue fixation as described above is also a great advantage, as it allows a safe and controlled perforation in a quasi-static environment. Figure 9 shows the principle function of the catheter for electrical and optical tissue characterisation

Sensorized Tools for Haptic Force Feedback in Computer Assisted Surgery 145

tweezer arms are actively moved with a microservo. The closed loop control does not only allow a precise control of the forces excerted by the haptic tweezer to the fingers, but also the behavior of the device can be programmed. In the easiest form, the "virtual" spring

The main idea for the R&D of our force feedback joystick was to use the same sensing/actuation principle we used for the haptic tweezer for a device with more DOFs. A first 1DOF demonstrator was built consisting of a high torque servo and a 12 cm PEEK handle mounted on the servo actuator. The diameter of the handle was 12mm, but reduced to a square profile of 5x5 mm² in a 15mm long section close to the *axis of rotation of the handle* 

A microcontroller (Atmel ATmega32L) was used to read out the strain gauges and control


Simple scenarios were successfully tested, including the simple spring model (the torque on the handle increases with the angle), zero force actuation (a minimum force is required to move the handle) and mixtures of both. The latter was especially important as it reflects the requirements for the catheter control. When the catheter tip is freely moving without touching, zero actuation force is used. Alternatively, one might consider to program a low

in the high strain area. Here 2 strain gauges were positioned in this area.

the servo. A closed loop control was implemented allowing to

constants of the tweezer can be set.


Fig. 10. Torque on the handle of the actuator


example a spring model)


Fig. 9. Multifunctional catheter tip for impedance and optical spectroscopy with integrated suction fixature
