Sensing Principle and Sensor Design and Fabrication 

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Chapter 3 Sensing Principle and Sensor Design and Fabrication

Sensory Modalities

In clinical oesophageal bolus transit evaluation, several methods are combined to give a more complete picture of the swallowing functions. The most common investigated parameters are the radial pressure, food clearance, and visual outline of the oesophagus [5]. For the oesophageal swallowing robot, embedded sensing elements serves as built-in measurement probes that provide information pertaining to the state of the actuator and the interaction between the conduit wall and food bolus. Three types of sensors were proposed (Figure 3.1):
Pressure sensors to measure the force imparted by the wall of the oesophagus on food bolus in the direction normal to the surface.
Shear stress sensors to measure the force parallel to the surface during the propulsion.
Strain sensors to measure the deformation of the conduit wall during the bolus transit simulation, and thus eliminate the need for an external imaging technique to visualise the process.
These sensors were arranged in an array and were embedded inside the surface of the robot’s conduit.

Sensing Pri nciples

There ar e many techniques that can be used to measure pressure, strain, and shear stress. Among the most common transduction methods are capacitive, piezoresistive, piezoelectric, and optical The capacitiv e method w as chosen for this research because a typic al parallel plate capacitor constructi on can be used for all the three sensing modalities an d hence greatly simplifies the design and the fabrication pro cess. This type of construction consists of a dielectric layer sandwiched between two conductive plates (Figure 3.2 a). The structure a nd the material of the top and the bottom plates make it easy for grid interconnections between sensors to be designed and fabricated together with the se nsors. A capacitance is g iven by the formula.
plates changes the separation distance, whereas applyin g a force parallel to on e of the pl ate or both plates in opposing directions changes th e overlappi ng area (Figure 3.2 b).

St retchable Interconn ects

A sensor array con sists of two main elements, which are the sensors and interconnect s. An intercon nect is an electrically conductive element con necting one sensor to o ther sensors and components in the circuit. This element has traditionally been a barrier to achie ving a stretchable electronics circuit a it is most commonly made of rigid material s. Nowadays, there are three main str ategies to realise a deformable cond uctive mate rial. One is by incorporating condu ctive fillers in the form of particles, nanowires, a nd flakes into an elasto mer matrix, therefore making the resulting stretchable composite electrically conductive. The second strategy is by synthesis ing new types of elastomers, gels, o r liquids th at are intrins ically cond uctive. The other strategy is by patter ning or stru cturing a rigid conductive material in such a way that will alter the stres s distributio n in the structure, allowing it to wit hstand a certain amount of strain without failure.
For this research, a horseshoe in-plane mea ndering technique, which is one of the rigid material patterning strategies, was used [68], [90]. This techniq ue involves patterning a metal thin foil into a horseshoe shape that is co planar (in-plane) with the surface o f the foil. It is advanta geous in the sense that th e patterned structure inherits the high and sta ble conductivity of the rigid counterparts, which is desirable for an interconnect. The horseshoe h as been found to be the most optimal shape that c an yield high stretchability and fatig ue performance [90]. In addition, metal thin foil materials can be easily obtained at a low cost and a tra ditional printed circuit board manufacturing technique can be employed for the patterning process. These make it among the most cost effective t chniques.
Figure 3.3 illustrate s the horseshoe meandering pattern with three geometrical parameters that have imp acts on the mechanical and electri cal performa nces of a m eandered in terconnect; trace width w, radius of curvature r, and meander turning angle ϴ. Ap art from these, the material thicknes and the intrinsic prop erties such a s electrical conductivity and Young’s modulus will also contribute to fi nal attribute of this type of interconnnect. Table 3.1 summarises the optimal design criteria for each of the parameters.

De sign Requ irements and Constraints

The sensors and the interconnects designs to ok into consideration t he functional requirement for the oeso phageal bol us transit evaluation and physical c onstrains imposed by the structure of the robotic oesophagus.

Pressure and Shear Stress

Pressures generated during the oesophageal transport vary, depen ding on th e properties of a swallow ed food bolus. The pressure range induced by a peristaltic w ave in a hea lthy swallo w has been recorded to be less than 10 0 mmHg [9 1], [92]. For an effecti ve clearance of food bolus, a minimum wave pressure of 30 mmHg in th e distal oesophagus and 12 mmHg in the proximal oesophagus is required [93]. A pressure of less than 10 mmHg is associated with a failed peristalsis. However, for some abnormal motility functions such as hypertensive peristalsis or nutcracker oesophagus, it can reach well beyond 200 mmHg [94]. To be able to simulate a wide range of healthy and impaired oesophageal bolus transit behaviours, pressure sensors with a minimum and maximum detectable pressure of less than 10 mmHg (1.3 kPa) and more than 200 mmHg (26.7 kPa) respectively were designed. In terms of a pressure response, a rise rate of more than 400 mmHg.s−1 (53 kPa.s−1) is recommended for an accurate swallowing process recording A traditional water perfused catheter has a response range of 300 – 400 mmHg.s−1 while a solid state catheter system can have up to ~4000 mmHg.s−1 [96]. For the spatial resolution, a measurement point interval of 4 cm or less is recommended [97].
Currently there is no clinical instrument that can measure shear stress on the surface of the oesophageal wall. The estimates of the stress values during peristaltic transport come from computational modelling studies. One study using a finite element simulation of oesophageal transport found that the values vary from 0 to ~1.3kPa throughout the length of the oesophagus [98], while another more generic simulation of peristaltic transport suggest a range of 0 – 2.5 kPa under various combinations of simulation parameters [99]. The latter is proposed to be the basis for the shear stress sensor measurement range.

Strain

A peristaltic wave inside the robotic oesophageal conduit causes strains in radial and longitudinal directions. The magnitudes vary with the shape, amplitude, and length of the wave travelling in longitudinal direction down the oesophagus. The characteristics of the peristaltic wave can be determined by measuring the strain induced by the wall deformation in this direction. The strain sensors therefore need to be able to measure the whole range of axial strain values at the surface of the wall during the simulation. Similarly, all interconnects that are connecting the sensor array need to withstand the same amount of deformations without failure. Figure 3.4 shows cross-sectional views of the oesophageal swallowing robot, which reveal the longitudinal and radial arrangements of the air chambers. A peristaltic wave can be generated by sequentially pressurizing the chambers (Figure 3.5). The red curves represent the outlines of the deformed surface while the green line and curve represent the undeformed state. Under the maximum radial strain, the length of the deformed surface can be approximated to be twice the radius of the conduit. So the maximum radial strain is given by.
For the o esophageal swallowing robot, the sinusoidal function has b een used to model the shape of the peristaltic wa e. The mean of the maximum longitudinal strains can be o btained fro m the arc length of the sin e wave with the wave amplitude equal to the radius of the conduit and the wavefro t length set to values w ithin the biologically i nspired range of 40 – 60 mm [8]. Figure 3.6 shows the plot of the mean lo ngitudinal strain values.
In terms of the spatial resolution, Nyquist theory sugge sts a sampling interval of at least twice the wave length to e nsure that th e wave being sampled can be cor rectly repro duced [100]. The peristaltic wave of t he oesophageal swallowing robot was configureed to have a sinusoidal shape in the axial direction of the conduit, while the deformation in the ra dial directio n was assum ed to be axisy mmetric. Therefore, only the axial strain was measured by the strain sensors. Takin g the smallest wavefront length of 40 mm as a reference, t he smallest peristaltic wavelength to be produced by the swallowing rob ot will be 80 mm. This suggests a sp acing betw een the sensors to be at most half of the value, which is 40 m m. Table 3.2 summarises the target specificatio ns for the strain , pressure, a nd shear stress sensors.

Design Const raints

During the full closu re of the robotic oesop hageal conduit, the non -occluded ar ea at the peak of the deformed surface will be th eoretically reduced to zero, assum ing a perfectly symmetrical occlusio n (Figure 3.5 a). Based on this con strain, the radial dime nsion of the sensing ele ment should be as small as possible. Howeve r, current available fabrication techniques using photolithography, wet etching, a nd laser engraving, pose a lower limit to the dimension o f the structure that can be patterned. In this research, 0.1 mm was used as the lower limit for dimension of the electrode and interconnect patterns to guarantee a clean edge and uniform trace width. For a capacitive circuit design, the spacing between adjacent conductive elements is an important factor. The sensor array tight configuration involves routing many parallel interconnects close to each other. A design guideline suggests a separation distance of twice the conductor width [101]. This is to reduce unwanted cross-talk between sensors and interconnects that could interfere with the measurement outputs.

Multimodal Sensor Design

Before proceeding with detailed descriptions of the sensor design, it is important to clarify several terminologies related to the structure of the sensor that are used throughout this and subsequent chapters. This chapter presents the design and fabrication of one unit of the stretchable multimodal sensor, which is referred to as a sensor unit or sensing unit in this thesis. This sensor unit is also termed a sensor element when discussing a sensor array in Chapter 5 and This is due to the fact that the sensor unit forms a building block that makes up the element of the sensor array. Each sensor unit consists of three sensor capacitors, which are also called sensor components. The terms sensor capacitor and sensor component are used interchangeably throughout this thesis. Similarly, the sensing capacitor, or simply a capacitor refer to the same thing, which is the sensor capacitor. Each sensor component or capacitor was designed to measure one specific parameter.
Figure 3.7 shows the novel design of the capacitive flexible and stretchable multimodal sensor unit consisting of pressure, shear stress, and strain sensing capacitors. Using a thin copper-polyimide laminate with the copper and polyimide thickness of 9 µm and 15 µm as the base material, the sensor structure consists of one common bottom layer and a top layer, which contains three separate conductor lines. The bottom layer serves as a common bottom plate for the pressure, shear, and strain capacitors, while the three conductors on the top layers form three separate top plates of the capacitors. As depicted in Figure 3.7, the pressure and strain capacitor plates were design to be parallel to the sensor plane while the shear capacitor plates were bent to be perpendicular. The trace width was design to be equal to or wider than 0.1 mm, which is the lower limit of our fabrication process, to ensure good trace uniformity. At sections where some levels of stretchability are required, an in-plane horse-shoe meandering technique was utilised. This technique allows an otherwise a rigid copper trace to sustain a higher level of strain without failure. This is achieved by distributing the strain over a longer trace in different directions.
As discussed in Section 3.3, three important variables that define the geometry of a meandered traces are the trace width, curvature radius and turning angle (Figure 3.3). While the trace width was limited by the fabrication process capability, the curvature radius and turning angle were limited by spatial constrains and strain characteristics and requirement of the target application, which in this case, the oesophageal swallowing robot. In general, a larger radius combined with a larger turning angle, and smaller trace width allow for the structure to accommodate higher strain, which is translated into higher fatigue performance. In the case of the oesophageal swallowing robot, the strain sensor was used to measure conduit surface deformation, which produces two-dimensional strain on its surface. The top and bottom sensor structures and traces on the other hand were designed to be stretchable only along the axial and radial directions respectively. In order for them not to be affected by strains on any other irrelevant directions, the overall width of the stretchable structures must be kept as narrow as possible. In addition, the top layer structure has three separate conductor traces running in parallel. The spacing between the adjacent traces must be kept at least twice the trace width to reduce signal cross-talk. Based on the aforementioned constrains and 3% strain requirement, the top layer stretchable structures were designed to have up to a maximum of 1.25 mm curvature radius and 45° turning angle. The bottom layer structure on the other hand was designed with a maximum curvature radius and turning angle of 1.8 mm and 30° respectively to accommodate a higher strain of 27% (Section 3.4.2).
All meandered traces are backed by a layer of polyimide on one side. In this sensor design, the polyimide layer has several functions as follows:
Hold all the separate conductor structures in the top layer together.
Help strengthen meandered traces by better distributing strain along the trace length [72].
Increase stiffness of the meandered traces, which helps them restore their original length after strain is removed.
Alter stiffness of certain parts of sensor capacitor plates.
Electrically insulate the top layer from the bottom layer and prevent parallel adjacent traces from touching each other.
Acts as a dielectric layer for strain sensor capacitor.
For the meandered traces, a wider polyimide structure support can better protect the copper traces through a better stress distribution, but increases the overall stiffness of the traces. Furthermore, wider traces occupy larger space, which is undesirable. In regard to this, a polyimide trace width that satisfies the aforementioned requirements and constrains was chosen for this design. Some sections of the sensor structures are not stretchable and the dimensions of these sections were kept to a minimum whenever possible so as not to reduce the overall stretchability of the sensor.

Contents
Abstract 
Acknowledgements 
Contents 
List of Figures 
List of Tables 
1 Introduction 
1.1 Oesophageal Bolus Transit Evaluation
1.2 Oesophageal Swallowing Robot
1.3 Motivation
1.4 Aims and Objectives
1.5 Scope
1.6 Research Contributions .
1.7 Thesis Synopsis
2 Literature Review 
2.1 Introduction
2.2 Methods of Food Rheological Testing
2.3 Food and Physiology of Swallowing
2.4 Food Bolus Transit Evaluation
2.5 Swallowing Robot
2.6 Stretchable Circuit Technology
2.7 Chapter Summary
3 Sensing Principle and Sensor Design and Fabrication 
3.1 Sensory Modalities
3.2 Sensing Principles
3.3 Stretchable Interconnects
3.4 Design Requirements and Constraints .
3.5 Multimodal Sensor Design
3.6 Feasibility Studies .
3.7 Sensor Fabrication
3.8 Chapter Summary
4 Test and Characterisation 
4.1 Characterisation Steps
4.2 Characterisation Tool
4.3 Fatigue Test
4.4 Sensor Characterisation
4.5 Chapter Summary
5 Sensor Array Fabrication, Calibration, and Installation 
5.1 Array Design
5.2 Fabrication
5.3 Interface Board Design
5.4 Calibration
5.5 Sensor Installation
5.6 Chapter Summary
6 Biomimetic Oesophageal Bolus Transit Evaluation 
6.1 Food Bolus Preparation
6.2 Setup
6.3 Sensor Measurement
6.4 Results
6.5 Chapter Summary
7 Conclusions and Future Works 
7.1 Research Outcomes
7.2 Research Contributions to the State of the Art
7.3 Proposed Future Works
List of Publications
References
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Deformable Array of Strain and Tactile Sensors for a Biomimetic Oesophageal Swallowing Robot

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