Sensor Embedded Soft Pneumatic Actuator for an Endonasal Instrument Merve Acer, Chansu Suh, Amir Firouzeh, Philippe Pasche, Christos Ikonomidis, Charles Baur and Jamie Paik, Member, IEEE
Abstract—The skull based endonasal surgical procedures are often performed by introducing MIS (minimally invasive surgery) tools through nasal cavities to avoid open skull surgery. However, to reach legions through densely packed neurons, narrow and curvy cavities, the procedure requires specific MIS tools with unique set of geometrical, mechanical and functional requirements. Here, we present anon going project on the development of a novel endonasal surgical instrument. This project encompasses the full spectrum of engineering design processes starting from the definition of the required design parameters directly from the surgeons. We suggest actuator and sensor options for the proposed instrument that will be flexible with controllable impedance: the soft pneumatic actuator (SPA) embedded with customizable low profile sensors are presented here. As these are novel components for any medical instruments, we illustrate the design tool for the components as well as the final instrument control consoles. The proposed actuator and sensor units are unique for the instrument but are highly customizable for diverse soft robotic applications. Index Terms—Soft actuator, Surgical tool, Piezoelectric sensor, Flexible sensor, I.
INTRODUCTION
Endonasal surgeries have been transformed into minimally invasive surgery (MIS) with the development of endoscopic instruments. Firstly these tools are used for paranasal sinus surgeries, throat surgeries and now they are used in brain tumor surgeries with the development of new endoscopic tools. These tools are operating on the skull front base by entering from nose and sinus cavities and resecting the tumors located in the median and paramedian skull base which are difficult to reach by external approach without damaging neighboring healthy tissues [1-2].
*This research is supported by National Centers for Competence in Research (NCCR), in Robotics M. Acer is with the Mechanical Engineering Department of Istanbul Technical University, Istanbul 34437, Turkey (corresponding author e-mail:
[email protected]) and currently she is a visiting scholar in Reconfigurable Robotics Laboratory (RRL), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland. C. Suh (
[email protected]), A. Firouzeh (
[email protected]) and J. Paik (
[email protected]) are with the Reconfigurable Robotics Laboratory (RRL), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland. P. Pasche (
[email protected]) and C. Ikonomidis (
[email protected]) are with Otolaryngology, Head and Neck Surgery department of the Centre Hospitalier Universitaire Vaudois (CHUV), Lausanne 1011 in Switzerland. C. Baur (
[email protected]) is with INSTANT Lab, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland.
There are a lot of MIS tools in the market which can be used in endonasal surgeries. However, the neurologists still require tools with a high maneuverability with a minimum radius of curvature and instrument diameter: the complex skull shape with dense population of neurons impose high physical tool path limitations. Furthermore, there are still parts on the skull base that can not be reached easily by using the available tools. The main difficulty of the surgical procedure with the current tools is the manipulation of straight and rigid instruments in narrow space. Available tools have a fixed tip which limits the manipulation and make it difficult to reach lateral, superior or inferior lesions without removing unnecessary healthy tissues. Besides, the robustness of these tools can be increased by adding feedback control rather than only having manual actuation. If the tip for force and the angle position of the joints are measured and limited according to the patient’s surgery type the operations will be more reliable and the surgeons will be more comfortable while doing the surgery. Many flexible MIS tools use wire driven systems for transferring motion to the tool tip [3-6]. The tools have also been designed as snake like robots which are composed of multiple modules [7-9]. Although these are all improving the flexibility of the tools the tool tip motion is limited by the actuation methodology. If there is a small enough and local actuation method that can be used to articulate the tool tip it will improve the maneuverability of the tool while using in an endonasal surgery. Therefore, we define a new design concept for developing endonasal tools using soft pneumatic actuators (SPAs). These actuators are highly customizable so we have developed a soft actuator design tool to be able to modify our structures and optimize our design. An endonasal surgical simulator has also been started to be developed in order to test our structures. We have also developed new sensors that can be embedded in soft actuators which will give feedback to surgeons and improve their operations. II. DESIGN PARAMETER DEFINITION Endonasal MIS procedures impose quite specific design requirements for the tool. Most due to the surgical environment and nature of diverse tissue types. In the literature, there are presented discussions about the gaps of endonasal surgical tools [10-11] however there is no information about the parameters that are set by the surgeons. Therefore a parameterization study for developing a novel instrument for the need of endonasal endoscopic skull base surgeries has been done with the Otolaryngology (ORL) surgeons from the Centre Hospitalier Universitaire Vaudois (CHUV) in Switzerland.
Fig. 1 shows schematics of a tool with definable parameters. In order to quantify the suggested parameters, including our coauthors, five additional ORL and neurosurgeons were involved in defining these values (shown in Table 1). A test setup has been developed using prototyped tool tips with varied lengths and angles for selecting the proper parameters for the tool tip. The authors have selected 21 mm length for the tool tip and preferred the tip to be locked at 0º, 30º and 60º during the surgery. The forces have also been estimated by using a piece of pork meat placed on the scale (see Table 2). The authors have operated on the pork meet by using the example shaft and tool tips which is placed inside a hole having a diameter of 8 mm to resemble the nasal opening. The normal forces were measured while the surgeons were mimicking their surgical procedure on the meat.
a. The structure of the endoscopic tool.
b. Roll motion (DoF #1)
c. Pitch motion (DoF #2) Figure 1a. The surveyed design parameters for the proposed endonasal instrument, b. The controllable 1st joint that provides roll motion for the tool tip, c. the controllable 2nd joint that provides pitch motion for the tool tip.
The ideal design parameters for the instrument are presented in Table 1. The most important parameters that should be satisfied are the tip length having a controllable roll motion (A) and the tips range of motion angle (E) which is varied as ± 60°. The next important parameters were set as the diameter of the tool and the place of 2nd controllable joint (B) which provides the tool pitch. Then the latching angles during the surgery (E') was selected to be the next priority. Finally the handle distance (C) and the 3rd manual DoF (D) are the least important parameters for the endoscopic instrument. TABLE I.
DESIGN PARAMETERS OF THE TOOL ITERATED AND CONFIRMED BY THE ORL SURGEONS
Definition The length of the tool tip The length between two controllable joints The distance for the placement of 2nd controllable joint The distance for the placement of 3rdmanual joint The tip angular motion Latching angular positions of the tip Tool diameter
Parameters A B
Ideal values 20 mm 20 mm
C
200 mm
D
30 mm
E E' d
± 60° 3 sets (0°, 30°, 60°) 4 mm
TABLE II.
FUNCTIONAL PARAMETERS OF THE PROPOSED INSTRUMENT
Description Normal Forces Joint stiffness Tip torque Sensitivity Speed
Normal Forces while operating on the tissue Stiffness of the tool body excluding the tool tip Torque at the tip of the instrument Surface sensor reading Tool tip travel speed
Minimum or values 0-1.4 N
maximum
> 7 Nm/rad (when stiffened) < 0.2 Nm/rad (when soft) > 0.7 Nm < 100 µm > 2 rad/s
III. TOOL DEVELOPMENT We have set our goals according to the discussions with ORL surgeons. There are various tool tips for endonasal surgery procedure so we have decided to start our design process with using monopolar cautery blade for the tool tip which uses current in order to resect the tumors. Then the endonasal tool will be developed in order to use other tool tips such as forceps, shavers and drills. We have proposed to use soft pneumatic actuators (SPA) for providing actuation and flexibility for the tool tip. But the achievable tool diameter is 6 mm because of the limitation of tubes inserted in the SPA which has a diameter of 1.5 mm. If we want to have an SPA providing multi DoF we have to insert multiple tubes which make the diameter larger. However we can control the stiffness of the SPA according to the procedure of the surgery. The tool can be flexible when it is trying to reach the tumor from nasal cavity to the skull base and it can be stiff while doing the procedure such as cutting the tumors. We can also propose to provide distributed sensors along the tool for measuring the position and pressure at the tool tip which will improve the endonasal surgeries. Low profile sensors can be used for this purpose by embedding in the SPAs. The soft endonasal tool tip in Fig. 1has been designed in order to provide multi DoF and measurement for bending positions and pressure on the tool tip during the operation on the skull base. The diameter of the tool tip is 6 mm. Fig. 2a shows the soft endonasal tool tip composed of 3 air chambers for actuation. Flexible curvature sensors are placed on each bending curvature surfaces to measure the angular position of the tool tip and a sensitive skin composed of distributed piezoelectric (PZT) materials is placed at the tip in order to measure the pressure while operating. Fig. 2b shows how the actuator bends in three axes when the air chambers are inflated. The tool tip position pi(i=1,2,3) depends on the bending angles θi (i= 1,2,3) which is measured with flexible curvature sensors.
TABLE III. AVERAGE SPA FORCE AND TORQUE MEASUREMENTS PZT Pressure sensor
p3
θ3
p1
p2
Flexible Curvature sensors θ1
θ2
y
5 mm x
a b Figure 2a.The endonasal tool tip actuated by SPA having three air chambers. Flexible curvature sensors are embedded under the SPA skin to measure the angular positions and distributed PZT sensor embedded in the tip to measure pressures, b. An illustration of endonasal tool tip motion for every inflation of air chambers.
A. Customizable Multi DoF Actuator The MIS tools for endonasal surgeries in the market are cable driven systems using DC motors which are limiting the flexibility of the tools and preventing the direct control of the joints. Another option is using Shape Memory Alloys (SMAs)but these tools are also cable driven systems and the motion is transferred by using SMAs as cables which don’t provide direct actuation fully. Our solution is to use SPAs which provide direct actuation and flexibility having varied stiffness [12-13]. The soft endonasal tool tip having a diameter of 6 mm has been fabricated and tested. A proper mold was designed that locates the 3 air chambers which enable the bending in three different curvature axes. Exofex 00-30® silicone rubber (Smooth-On Inc. elastic modulus (E) =69 kPa) in the liquid form was poured into the mold and cured. The tubes were inserted in the air chambers and glued. Fig. 3 shows the 3 DoF SPA for the tool tip and an example for bending in one axis. The amount of pressure applied for bending 90º is 25kPa and for 135º is 30kPa. Table 3 shows the average output forces and torques of the tested SPA. For bendable SPAs the force and torque values changes with the thickness and height of the air chambers and the material properties [12].
x
p
O
25 mm
at 135º* 30 kPa 1N
Tout
230 Nmm
370 Nmm
B. Embeddable Flexible Sensors The measurements for the MIS tools are mostly based on visual feedback using fiber optics transport the internal sensor images to a camera producing a 2D-display or small cameras entering from the natural openings of the human body. They do not provide any tactile feedback to the surgeon so researchers focused on using flexure sensors for force measurements [14-17] and stiffness measurements [1819]. Embeddable sensors are needed to provide position measurement directly and pressure measurement for sensing while operating. We have developed our low profile sensors for measuring position of the SPAs and pressure on the endonasal tool tip by using layer by layer fabrication technique. The flexible curvature sensors were designed to measure the bending angles of the tool and distributed PZT sensor was designed to measure the pressures on the tip of tool. 1) Flexible Curvature Sensor The flexible curvature sensor was developed using carbon ink layer. The electrical resistance of the sensor changes under strain which is caused by the micro cracks in the carbon layer [20]. We have also scored the polyimide substrate with the laser micromachining system in order to increase the resistance change and to regulate this effect [21]. Fig. 4 shows the fabricated flexible curvature sensors and their fabrication layers.
Score marks for stress concentration
a Kapton Layer
Score
Mask
Carbon ink
Outline Polyurethane
O
135º
Kapton
O
p 25 mm
b Figure 4a.The flexible curvature sensors and closer look at the parallel score marks that are the sensing parts on the sensor, b. The fabrication process and the three layers of the developed sensor.
x
x
y
at 90º* 25 kPa 0.5 N
Carbon ink
p 25 mm
Parameters Pin Fout
*. Values are estimated from our previous results presented in [12].
Air chambers
z
Description Applied pressure Average output force Average torque
y
y
b c a Figure 3 a. Soft endonasal tip actuator made of Ecoflex 00-30, b.The tip position “p” is rotated 90º according to the point “O” when SPA is actuated, c. The tip position “p” is rotated 135º according to the point “O” when SPA is actuated.
The sensor readings that have been taken from 100 repetitions are presented in Fig. 5. The readings are normalized to their maximum voltage. These sensors provide robust and repeatable measurements.
PZT elements ( 2 mm x 2 mm)
Normalized reading
Flexible circuit
i)
Bending angle º Figure 5 Curvature sensor reading. The maximum error is 2º for the range of motion 25-140º. Data taken from 100 repetitions.
2) Distributed PZT Sensor PZT material provides dynamic voltage changes when they are under strain and they are extremely sensitive. The relationship between the mechanical deformation on the material and the amplitude of the output signal is directly proportional. The deformation on the material causes a change on the surface charge density which results a voltage difference between the top and bottom surface of the material. PZT sensors can not be used for static measurements however they are extremely sensitive to the strain changes (around 3.5V/nm). They can be used to measure the changes in pressure, force, acceleration and strain. The idea for the pressure sensor on the endonasal tool tip is to cover with a thin layer composed of distributed PZT material. The sensitive layer can also be adapted to other kinds of tips that is used in the endonasal surgeries such as forceps, drillers, shavers etc. We have developed a distributed PZT sensor in order to test the feasibility for the endonasal tool application for pressure measurements. The PZT used for the sensor is PSI5H4E (Piezo Systems Inc.) having 127 µm of thickness. Fig. 6a shows the schematic for the tested sensor having five 2x2 mm PZTs distributed on 12x12 mm area. The fabrication process of the sensor is presented in Fig. 6b. Firstly a mold for the proper placement of the PZTs has been built then Ecoflex 00-10® silicone rubber in liquid form poured on the mold using the spin coating process and a top silicone layer having marks for the PZTs was obtained. A conductive cloth was cut for the bonding of electrodes for each PZT. The electrodes were bonded at the top and the bottom properly by using cold soldering epoxy. Then the bonded PZTs are placed on the silicone layer that is half cured. Finally the bottom silicone layer is poured on the sensor using the spin coater process. The fabricated distributed piezoelectric sensor is presented in Fig. 6c. This sensor can cover the endonasal tools and provide high sensitivity pressure measurement while operating. The sensor can detect the transient forces which is applied under 0.1 sec. Table 4 shows the sensitivity of the material when the strain is parallel to polarization and perpendicular to polarization.
A B C D E
ii)
15 mm
iii) Figure 6 i) Top view of distributed PZT sensor with electrode paths for each PZT. The dimensions are highly customizable but the presented model has 2x2 mm PZT elements, ii) The five layers of the PZT sensor—(A): top silicone layer with marks for the placement of PZTs, (B): top conductive layer for top electrode of the PZTs, (C): distributed PZT layer, (D): bottom conductive layer for bottom electrode of the PZTs and (E): bottom silicone layer, iii) Fabricated distributed PZT sensor under a silicone skin. TABLE IV. THE PZT MATERIAL PROPERTIES FOR PRESSURE SENSING Description Piezoelectric strain coefficient in longitudinal direction (parallel to polarization) Piezoelectric strain coefficient in transverse direction (perpendicular to polarization) Amount of pressure at 1 V for longitudinal strain of the PZT Amount of pressure at 1 V for transverse strain of the PZT
Parameters d33
Values* 0.65 nm/V
d31
-0.32 nm/V
p33
1.2 MPa
p31
-1.95 MPa
*.Values are converted from Piezo Systems Inc.
IV. USER INTERFACES User interfaces for endonasal tool development are composed of design tool for the SPA, the control of the tool using the sensor feedback and a surgical simulator for trajectory planning for endonasal surgeries. A. Actuator Design Tool The SPAs static and dynamic behaviors have been investigated in the Reconfigurable Robotics Lab in [12-13]. According to the experimental results a design tool for
designing SPAs has been developed. The software is coded in C++ and third party libraries such as OpenGL (graphical display) and Qt (user interface) are used for user interface that allows easy reconfiguration of the parameters. Fig. 7 shows the user interface for designing a surgical tool using SPAs. The bendable surgical tool composed of serially connected soft pneumatic actuator units. User can control either the pressure of the air chamber of the soft pneumatic actuator units or the position of the tip of the surgical tool. This simulator will help engineers to decide the design parameters for the surgical tool. d Figure 8 a. The scanned skull and the endonasal tool structure, b. The tool is manipulated by the used to enter inside the skull from the nose cavity, c. A collision is detected while the tool is manipulated inside the skull. d.The surgical simulator connected to the phantom surgery
V. CONCLUSION Parameter selection GUI for SPAs
The tool structure made of SPAs connected in series
Figure 7 Design tool for developing SPA structures for surgical tool. The user can control the dimensions, the air chambers of the SPAs. The behavior of the tool can be simulated by applying the pressures in the SPAs.
B. Tool Control A closed loop control will be applied to the endonasal tool using low profile sensors. The fabricated endonasal tool structures will be connected to the user interface which will be followed up from what is shown in Fig. 8. The flexible curvature sensors will provide the bending position measurement to control the tip position and the PZT sensor will give the pressure measurements during the operation. The software will give us the opportunity to select the proper control methodology and tune our parameters. C. Tool Path Trajectory Planner The simulator will be developed be used as a surgical simulator for trajectory planning of the tool before the surgery and training purposes. Fig. 8 shows the initial development of the simulator that consists of a surgical tool and a real patient's CT image of the skull with the tumor legion marked. We use the simulator for testing our SPA tip endonasal tools trajectories. The tool can be controlled by using our user interface that currently is a 3 DoF joystick with 2 additional switches. Currently it measures only the collisions, we envision to add feedback system to the handheld interface.
Endonasal surgeries require MIS tools that are not only compact with a high DoF, but with a flexibility with a sensing capability throughout its body while having a tight radius curvature. In this work, we have defined the engineering parameters that translated and quantified the "need" from the neurologists. We also have proposed actuation and sensor solutions to the defined parameters. The proposed SPAs are highly customizable for diverse applications but also is able to embed sensors that are not unique to the develop done within this scope. We also report on two different types of low profile sensors (PZT-based and carbon ink-based) that are easy to embed within the top layer of the instrument regardless of its stiffness. The presented flexible curvature sensors (carbonink based) have been fabricated for measuring the angular positions of the SPA to provide motion feedback of the tool tip. The distributed pressure sensors (PZT) can cover the tool tip and provide high sensitive measurements. Finally the user interfaces for designing SPAs for the tool and controlling the tool has been introduced. The initial development of the surgical simulator which can detect the collisions while operating with the tool in the skull has been presented. The integration of the sensors and the SPAs will be our future work for developing the endonasal tool. The simulator will be developed to process the sensors’ feedback that will enable to control the tool. ACKNOWLEDGMENT This work was supported by the Swiss National Centre for Competence in Research (NCCR) Robotics fund. REFERENCES [1]
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