Technische Universität Hamburg-Harburg

Investigation of Materials for the Coating of a Flexible Endoscope

Project Report

by

Trisha Sadhu

April 2013

Supervisors: Dipl.-Ing. Kristina Kaiser

Prof. Dr.-Ing. Jörg Müller

TUHH

ii

ABSTRACT

The various applications of endoscopy procedures require passage of an external object, i.e. an endoscope through narrow channels. To minimize the discomfort on the passage of an endoscope through the nasal passage and via the Eustachian tube for the examination of the middle ear, the endoscope coating should be of a bio-compatible material which is supple towards the human tissue but strong enough to undergo less wear and no tear during usage.

This project work deals with the selection of an endoscope coating which fulfills the above criteria and generates a minimum of friction during movement through the Eustachian tube. All steps in producing the endoscope layer, the experiments for transferring the layer on a needle representing the endoscope, and the experimental setup for measurement of the friction are shown.

iii

STATEMENT

I, Trisha Sadhu, solemnly declare that I have written this project report independently, and that I have not made use of any aid other than those acknowledged in this project. Neither this project report, nor any other similar work, has been previously submitted to any examination board.

Trisha Sadhu Hamburg, April, 2013.

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ACKNOWLEDGEMENTS

I would like to thank Professor Jörg Müller for giving me the opportunity to develop this work at the Institute of Microsystems Technology.

Also, the invaluable advice and constant support of my supervisor, Kristina Kaiser, is gratefully acknowledged.

I would also like to thank all the other members from the department of Microsystems Technology of the Technical University of Hamburg-Harburg for their assistance during the progression of this project work.

Last, but not the least, I thank my family and friends.

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Contents

ABSTRACT .................................................................................................................................................ii

STATEMENT ............................................................................................................................................. iii

ACKNOWLEDGEMENTS ........................................................................................................................... iv

LIST OF FIGURES ..................................................................................................................................... vii

LIST OF TABLES ........................................................................................................................................ ix

1.0. INTRODUCTION ................................................................................................................................ 6

1.1. MOTIVATION ................................................................................................................................ 6

2.0. MATERIALS ....................................................................................................................................... 7

2.1. MUCUS ......................................................................................................................................... 7

2.2. SILICONE ....................................................................................................................................... 9

2.3. PRECURSORS ................................................................................................................................ 6

3.0. FRICTION MINIMIZATION ................................................................................................................. 6

3.1. STATIC FRICTION ........................................................................................................................... 6

3.2. FRICTIONAL FORCE CALCULATION ............................................................................................... 7

3.3. STRUCTURES OF SILICONE FOR MINIMIZING FRICTION ............................................................. 11

3.4. SILICONE STRUCTURES ............................................................................................................... 13

4.0. PROCESSING ................................................................................................................................... 14

4.1. SPIN COATING PARAMETERS FOR A PHOTORESIST THICKNESS OF 20 µm ................................ 14

4.2. STRUCTURING OF THE PYREX WAFER ........................................................................................ 15

4.3. CASTING OF SILICONE STAMPS .................................................................................................. 16

5.0. PREPARATION OF SILICONE STAMPS ............................................................................................. 20

5.1. SETUP FOR PREPARING SILICONE LAYERS .................................................................................. 20

6.0. SETUP FOR THE FRICTION MEASUREMENT .................................................................................... 23

6.1. FRICTION MEASUREMENT CURVE .............................................................................................. 24

6.2. PRESSURE SENSOR...................................................................................................................... 26

6.3. LabVIEW PROGRAM TO MOVE THE STAGE ................................................................................ 27

6.3.1. WORKING OF THE LABVIEW PROGRAM .............................................................................. 27

6.3.2. CALCULATION OF LINEAR DISPLACEMENT PER ENCODER COUNT ..................................... 35

7.0. MAKING OF THE SILICONE ENDOSCOPE COATINGS ....................................................................... 37

7.1. METHOD 1 .................................................................................................................................. 37

7.2. METHOD 2 .................................................................................................................................. 38

7.3. METHOD 3 .................................................................................................................................. 39

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7.4. METHOD 4 .................................................................................................................................. 40

7.5. METHOD 5 .................................................................................................................................. 41

8.0. CONCLUSION AND OUTLOOK ......................................................................................................... 42

9.0. REFERENCES ................................................................................................................................... 43

10.0. APPENDIX ..................................................................................................................................... 44

10.1. MASKS ...................................................................................................................................... 44

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LIST OF FIGURES

Figure 1 The diagram shows the cilia in the nasal passage with the mucus carrying the load and the periciliary fluid lubricating the cilia [2]......................................................................... 8

Figure 2 Structural formula of silicone ...................................................................................... 9

Figure 3 Structural formula DDMS ........................................................................................... 6

Figure 4 Structural Formula of FOTS ........................................................................................ 6

Figure 5 Structural diagram of the monolayer on silicon containing substrates ........................ 7

Figure 6 Friction forces between solid surfaces in contact [9]................................................... 6

Figure 7 Schematic of the meniscus formation from a liquid condensate between two flat surfaces ..................................................................................................................................... 10

Figure 8 Diagram of contact angle hysteresis with liquid droplet on a surface tilted at an angle [8] ............................................................................................................................................. 11

Figure 9 Surface with hemispherically topped cylindrical asperities [8] ................................. 12

Figure 10 Side view of the substrate showing a structured well .............................................. 14

Figure 11 Cross section of the silicone stamp showing the curvature length (in red), the height (in yellow) and the length (in green) of a 20 µm spacing well structure ................................. 16

Figure 12 Cross section of the silicone stamp showing the area (in yellow), the height (in red) and the length (in green) of a 20 µm spacing well structure .................................................... 17

Figure 13 Side view of the 20 µm well structures and the top view showing the dimensions of the basin .................................................................................................................................... 18

Figure 14 Side view of setup for gluing silicone layers around the needle ............................. 20

Figure 15 Top view of setup for gluing silicone layers around the needle .............................. 21

Figure 16 Top view of anti stiction layer coated wafer with silicone in the wells. bubbles are formed due to insufficient vacuum while drying of the silicone for twenty four hours .......... 21

Figure 17 Top view of metal needle of diameter of 0.9 mm .................................................... 22

Figure 18 Side view of the final setup to test silicone stamps on needle ................................. 23

Figure 19 View of the needle screwed on the force transducer ............................................... 24

Figure 20 Expected friction measurement curve ...................................................................... 25

Figure 21 Diagram showing the parts of the pressure sensor................................................... 26

Figure 22 Frontpanel of the LabVIEW program showing the mg17motor ActiveX control ... 27

Figure 23 Frontpanel of the LabVIEW program showing the folder paths generated by the program where the recorded values are stored ......................................................................... 28

Figure 24 Frontpanel of the labview program showing the activex control dataqsdk for the data acquisition device, the stop button for the program, the numerical values of the reverse position and forward position of the movable stage................................................................. 28

Figure 25 Frontpanel of the LabVIEW program showing the DQChart used to display the movement of the sensor ............................................................................................................ 28

Figure 26 Blockdiagram showing the openning of ActiveX control ....................................... 29

Figure 27 Blockdiagram showing the unique filename generator code ................................... 30

Figure 28 Blockdiagram for panel 1 showing the initialisation section ................................... 31

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Figure 29 Blockdiagram showing the starting of operation of the data acquisition device ..... 32

Figure 30 Blockdiagram showing the initial movement for the movable stage ....................... 32

Figure 31 Blockdiagram showing the DAQ section ................................................................ 33

Figure 32 Blockdiagram showing the false path in DAQ section ............................................ 33

Figure 33 Blockdiagram showing the forward path ................................................................. 34

Figure 34 Blockdiagram showing the false path ...................................................................... 34

Figure 35 Sub-diagram showing the stopping of the DAQ ...................................................... 34

Figure 36 Sub-diagram showing the closing of the activex control of the DAQ section ........ 35

Figure 37 Blockdiagram of LabVIEW program to display the values on XY graph ............... 36

Figure 38 Silicone coating on the wafer ................................................................................... 37

Figure 39 The incomplete transfer of the silicone stamp on to the surface of the needle ........ 38

Figure 40 Side view of the needle of diameter 0. 53 µm with the silicone globule at the tip .. 38

Figure 41 Top view showing the tip of the needle of diameter 0.53 µm showing the unequal spacing of the capillaries and also the excess silicone covering the capillaries ....................... 39

Figure 42 Side view of the above needle with silicone covering the capillaries and the glue leaving spots of uneven thickness ............................................................................................ 39

Figure 43 Top view of the needle showing the capillaries which having changed position and are bundled together on the needle ........................................................................................... 40

Figure 44 Side view of the needle with diameter 0.53 µm with capillaries coated with a thin film of silicone ......................................................................................................................... 41

Figure 45 Top view of the same needle showing the tip of the needle coated with a thick layer of silicone covering the capillaries completely ........................................................................ 41

Figure 46 Mask for the making of the structured wells on the silicon-pyrex wafer ................ 44

Figure 47 Mask for the second photolithography process ....................................................... 45

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LIST OF TABLES

Table 1 Composition of a healthy mucus [2] ............................................................................. 7

Table 2 Properties of sylgard 184 elastomer by Dow Corning [6] ............................................ 9

Table 3 Dimensions of the silicone structures ......................................................................... 13

Table 4 Spin coating time for the photoresist AZ 9260 vs thickness of the photoresist on wafer .................................................................................................................................................. 14

Table 5 Final parameters used in the wafer processing ........................................................... 15

Table 6 Values of silicone to be dribbled in the structured wells ............................................ 17

Table 7 Values used for calculation of amount of silicone to be dribbled in the wells ........... 19

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1.0. INTRODUCTION Endoscopy is a type of medical examination in which an instrument called endoscope is passed into an area of the body. With a suitable endoscopic device, endoscopy of the middle ear and the Eustachian tube could be a non-invasive diagnostic method. The nasal passage is composed of the respiratory epithelium which has the purpose to keep the passage moist and to protect the nasal passage. Also, the mucus in the nasal passage is present to protect it.

Moving the endoscope (external object) through the nasal passage and Eustachian tube requires a material which will not cause damage to the tissue. This project work has been done to recognize the suitable materials and their texture for the coating of the endoscope in order to be able to pass it through the Eustachian tube with minimum wear or discomfort to the person.

1.1. MOTIVATION The present day silica endoscopes do not allow the intensive examination of the middle ear without operation of the ear drum. The method of passage of an endoscope via the nasal passage and through the Eustachian tube into the middle ear would allow for the examination of the middle ear without operation. With the available flexible endoscopes, this procedure is not yet possible because of severe danger of tissue injury due to the rigid silica material. This is why a novel, highly flexible endoscope made of silicone is topic of research at the Institute of Microsystems Technology at the Technical University of Hamburg- Harburg.

The purpose of this project is the selection of the coating for the silicone endoscope which ensures a minimum of friction. The materials selected for the friction test are Sylgard-184 by Dow Corning, pure or coated with a monolayer of halogen-silanes or a sub-µm thick layer of Teflon. A metal needle representing the silicone endoscope is coated with a combination of structured or unstructured layers of the tested materials. For the experiment, the passage of the needle through a cylindrical structure made of plastic of a length of 10 mm and a diameter of 4 mm filled with human nose tissue, gives the resistance, which is measured in terms of friction. The cylindrical tube represents the Eustachian tube for the experiment. The friction is measured by an ultra-low force transducer which converts measured force into voltage. The voltage values are recorded by the computer at each instant of time. The plotting on the graph of the voltage recorded vs. the movement of the needle gives the friction characteristic of the silicone layer used on the needle for the particular experiment. The graph with the lowest peak gives the optimal silicone layer. The frictional values are measured as the needle enters and passes the tube and back passing and leaving the tube.

The project report first describes the materials used on the metal needle. A description of the process required to make structured silicone stamps from these materials is given. It is followed by a description of the setup to be used for testing the material and finding the one with the least friction with emphasis on the pressure sensor used for the measurement and the LabVIEW program used to move the stage holding the metal needle. Finally, the results of the silicone layer generation on the metal needle are presented, followed by the conclusion.

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2.0. MATERIALS The Eustachian tube is covered with mucus. In order to understand the behavior of the liquid in the Eustachian tube on passage of the silicone endoscope, the first section of this chapter deals with the properties of mucus. Then, in the second, the silicone that is to be tested as coating material for the endoscope and its properties are presented. The third subchapter deals with the precursors used for deposition of the anti-stiction monolayer on the silicone. The fourth subchapter deals with the choice of silicone structures to minimize friction.

2.1. MUCUS The mucus is the first line of defense against any respiratory tract infection. It has a function of trapping airborne particles entering with air inhaled through the nose. The chemical composition of healthy mucus is listed in table 1

TABLE 1 COMPOSITION OF A HEALTHY MUCUS [2]

Water 95% Proteins and glycoprotein 2-3% Lipids 1% Minerals 1% DNA 0.02%

The human mucus consists of two layers, a superficial gel layer with a depth of 2 - 5 µm and an aqueous layer called periciliary layer with a depth of 7 - 10 µm [2]. The flow of dry air develops a more viscous periciliary layer. It is produced by a normal individual at a resting rate of 0.5 - 1.0 ml/cm2 over a 24 hour period [2]. The periciliary fluid lubricates the beating cilia while the mucus carries the particles in the nasal passage.

Newtonian fluids have a single coefficient of viscosity for a specific temperature. However, the nasal mucus is a non-Newtonian fluid, so a constant coefficient of viscosity cannot be defined.

There are different ways to measure the viscoelastic properties of mucus. One method is using a magnetic rheometer as proposed by King [4]. It consists of a small steel ball with a diameter between 50 and 150 µm which is placed on a small sample of mucus on a glass container. Since, the mucus produced in a healthy human being is of very small quantity, and causing an increase in the production of mucus would result in a change in its consistency, the experiment is carried out using a micro-sample of mucus. The steel ball oscillates between two electromagnets placed on either side. A beam of light is passed through the sample causing the oscillations of the steel ball through the sample to be picked up by a photoelectric cell on the opposite side. The speed and frequency of the steel ball is projected on an oscilloscope and the values are used for computing the viscoelastic properties.

Majima et - al. [1] have determined the storage modulus G’ and the dynamic viscosity η’ in a specimen of the nasal mucus using an oscillating sphere magnetic rheometer at frequencies of 0.5 Hz and 20 Hz at a constant temperature of 25 °C. The dynamic elasticity and viscosity of the specimen is calculated according to the formulae, using the determined values of the

8

amplitude of displacement (X0, cm) and the phase lag of the sphere (δ, rad) with respect to the oscillatory driving force (F0, dyne).

Storage modulus G’

G′ =F0

6πrX0cos δ +

29

ρs r2 ω2 (2.1.1)

Loss modulus G’’

G′′ =F0

6πrX0sin δ

(2.1.2)

Dynamic Viscosity η’

η’ =G′′

ω

(2.1.3)

Where

r is the radius of the iron sphere in cm

ρs is the density of the iron sphere in g/cm3

ω is the angular frequency in rad/s

G′is the measure of the elastic behavior of the specimen

η′ is indicative of viscous behavior of the specimen

It was demonstrated by experiment that a maximum value of mucociliary transport was obtained with 𝐺’ = 2 𝑃𝑎 and 𝜂’ = 0, 2 𝑃𝑎𝑠 at a frequency of 1 Hz at 25 °C.

For a patient with sinusitis, the η' and G' values of mucus determined at 1 Hz are 1.6 ± 1.5 Pas and 31.8 ± 31.0 Pa, respectively, and these values were higher than normal values [3].The mucus flow rate depends on the ciliary beat rate in the nasal passage. The ciliary beat rate has two components – the effective stroke and the recovery stroke. During the effective stroke the cilium is perpendicular to the cell surface such that its tip is in contact with the mucus.

FIGURE 1 THE DIAGRAM SHOWS THE CILIA IN THE NASAL PASSAGE WITH THE MUCUS CARRYING THE LOAD AND THE PERICILIARY FLUID LUBRICATING THE CILIA [2].

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One method used, subjects the respiratory mucus to cyclic deformation, and then observing the deformation response. The cilia are beaten with a fast effective stroke and a slow recovery stroke. The ciliary beat frequency of the nasal mucosa is 11.1 ± 2.5 Hz in healthy adults [5].

The viscoelastic properties of nasal mucus mean that both viscosity and elasticity must be taken into account. As the cilia move, they deform the mucous whose elastic properties will try to regain original shape. This can vary. If it does not have enough elasticity, it will flow like a sheet or if it has too much elasticity, the mucus will not flow. It is proposed the rate of mucociliary transport is directly proportional to the elastic recoil and inversely proportional to mucus viscosity [2].

2.2. SILICONE The silicone that is to be tested as suitable material for the endoscope coating is the silicone elastomer Sylgard 184 by Dow Corning.

Sylgard 184 is a transparent encapsulant and is widely used in biological and microfluidic applications. It has a rapid and versatile curing process which is temperature dependent and it is highly transparent which allows for easy inspection of components.

Further properties of Sylgard 184 are summarized in the table below.

TABLE 2 PROPERTIES OF SYLGARD 184 ELASTOMER BY DOW CORNING [6]

Property Value Viscosity 3.5 Pa-sec Specific Gravity of cured base 1.04 Heat cure time @100°C 35 minutes Tensile Strength 7.1 MPa Tear Strength 2 N/cm

In the friction measurements, the silicone layer is either used purely or covered with a monolayer of Teflon produced by gas phase deposition of the precursors described in the next section. The structural formula of silicone is given below in figure 2.

S

n

R

R

FIGURE 2 STRUCTURAL FORMULA OF SILICONE

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2.3. PRECURSORS In the following, the precursors used for the gas phase deposition of a monolayer of halogen-silanes on silicon containing materials are presented. The deposition of the monomolecular Teflon ensures an anti-stiction layer on the substrate.

I. Dichlorodimethylsilane (DDMS)

The DDMS precursor has the chemical formula(CH3)2SiCl2. The structural formula is given in Figure 3.

FIGURE 3 STRUCTURAL FORMULA DDMS

II. 1H, 1H, 2H, 2H-perflurooctyltrichlorosilane (FOTS)

The FOTS precursor has the chemical formulaCF3 (CF2)5 (CH2)2 SiCl3. The structural formula is given below in figure 4.

FIGURE 4 STRUCTURAL FORMULA OF FOTS

The vapor phase self-assembling monolayer (SAM) deposition is used to generate organized and dense molecular layers of short-chain and long-chain molecules. The SAMs are chemically bonded via covalent bonds to the substrate [7]. The precursors of DDMS and FOTS are used to deposit carbon-fluorine containing layers on silicon, silicone and glass substrates.

Cl

CH3

Cl

Si

H3C

F3C

(CH2)2

(CF2)5

Si

Cl Cl Cl

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The SAM deposition apparatus consists of a turbo-molecular pump which is used to create vacuum in the chamber where the wafer stack is placed. The vacuum in the chamber is sensed using IONIVAC combi transmitters using Oerlikon. Once the chamber is evacuated, the precursor and water vapors enter the chamber through the delivery line. The reaction takes place which is observed by the reduction of pressure. The pressure reaches a stable value and the reaction is complete. Then, the chamber is evacuated and flooded with nitrogen to end the reaction. The vacuum is broken and the chamber is washed with nitrogen to remove the hydrochloric acid vapors that are generated as a chemical byproduct of the reaction. The chemical reactions are -

n(−Si(OH)3) → [RSi − O − SiR]n +32

nH2O

(2.3.1)

RSiCl3 + 3H2O → [−RSi(OH)3] + 3HCl (2.3.2)

The resulting monolayer consists of a mixture of short and long chain silanes bound to the silicon atoms of the substrates as shown in figure 5.

FIGURE 5 STRUCTURAL DIAGRAM OF THE MONOLAYER ON SILICON CONTAINING SUBSTRATES

F3C

(CH2)2

(CF2)

Si

O O

Si

O

Si Si

Si

H3CH

CH3C

O

Si

O

Si

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3.0. FRICTION MINIMIZATION The material used to encapsulate the metal needle has to have minimum friction. The encapsulated metal needle is surrounded by nasal mucus on all sides in the cylinder. In the first section static friction is described. The second chapter deals with the frictional force calculation that is used for the experiment. The third chapter deals with the explanation for the structures developed. The structures are then described in the last chapter.

3.1. STATIC FRICTION

FIGURE 6 FRICTION FORCES BETWEEN SOLID SURFACES IN CONTACT [9]

𝑅 = 𝑛𝑜𝑟𝑚𝑎𝑙 𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒 𝑓𝑜𝑟𝑐𝑒

𝑅𝑇 = 𝑡𝑎𝑛𝑔𝑒𝑛𝑡𝑖𝑎𝑙 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒 𝑓𝑜𝑟𝑐𝑒

𝐺 = 𝑚𝑎𝑠𝑠 × 𝑓𝑜𝑟𝑐𝑒 𝑜𝑓 𝑔𝑟𝑎𝑣𝑖𝑡𝑦

𝑆 = 𝑠ℎ𝑖𝑓𝑡𝑖𝑛𝑔 𝑓𝑜𝑟𝑐𝑒

The static friction is explained by the Coulomb’s law of friction using figure 6 [9].

A body is at rest on a horizontal plane under the action of the normal force G and tangential force S. It is assumed that 𝐺 ≠ 0

If the value 𝑆 = 0, the body is at rest and the force G is reacted by the normal reaction force R of the plane, that is 𝑅 = + 𝐺. If the shifting force is progressively increased, the state of rest is maintained due to the existing force of resistance to motion RT from the plane and this force balances the force S i.e. 𝑅𝑇 = +𝐺

The body stays in a state of rest until the force S and RT reach the critical value.

𝑅𝑇 = 𝐹𝑖 = 𝑆𝑚𝑎𝑥 = µ+𝑅 (3.1.1) This value of 𝐹𝑖 is referred to as the maximum force of the static friction or simply the static friction.

µ + = 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑠𝑡𝑎𝑡𝑖𝑐 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛.

When the value of S reaches its maximum, it is the critical point of equilibrium. As long as the tangential force 𝑅𝑇 = µ+𝑅 , the equilibrium will be maintained.

R

RT S

G

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When a very small force greater than Smax is applied, the body will begin to move. When the body moves the sliding friction coefficient will change depending on the velocity of sliding and other factors.

µ𝑠 ≤ µ+

The law of dry friction assumes the coefficient of sliding friction equal to µ+ to be constant throughout the sliding. The force of sliding friction being equal to the value of static friction is the force of dry friction.

𝑅𝑇 = �< µ+𝑅 , 𝑣 = 0= µ+𝑅 , 𝑣 ≠ 0�

where

𝑣 = 𝑠𝑙𝑖𝑑𝑖𝑛𝑔 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦

The drawbacks of the Coulomb’s model of friction are given below [9].

The Coulomb’s model mathematically follows from the assumption that the surfaces are in atomically close contact only in a small fraction of their overall area, and that this contact area is proportional to the normal reactive force and that frictional force is equal to the tangential component of the reactive force independent of the contact area. This is not always the case and real area of contact needs to be taken into account.

When surfaces become conjoined, Coulomb friction becomes a very poor approximation. E.g. adhesive tape resists sliding even when there is no normal force. In this case, the frictional force is strongly dependent on the area of contact.

Also, it considers nanoscale and microscale friction as unrelated.

3.2. FRICTIONAL FORCE CALCULATION The calculation of the frictional force is done for the case of dynamic viscosity [10].

𝜎 = 𝜇 ∗ 𝑒 (3.2.1) where

𝜎 𝑖𝑠 𝑡ℎ𝑒 𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 𝑖𝑛 𝑃𝑎 𝜇 𝑖𝑠 𝑡ℎ𝑒 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡

The 𝑠𝑡𝑟𝑎𝑖𝑛 𝑟𝑎𝑡𝑒 𝑒 is expressed as

𝑒 = 1𝑥∗𝑑𝑥𝑑𝑡

where

𝑥 𝑖𝑠 𝑡ℎ𝑒 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 𝑡 𝑖𝑠 𝑡ℎ𝑒 𝑡𝑖𝑚𝑒 𝑖𝑛 𝑠𝑒𝑐𝑜𝑛𝑑𝑠

𝑑𝑥𝑑𝑡

𝑖𝑠 𝑡ℎ𝑒 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 𝑣

(3.2.2)

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𝑒 =

𝑣𝑥

(3.2.3)

𝑣 =𝑑𝑥𝑑𝑡

(3.2.4)

The 𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 𝜎 is given below [11].

𝜎 =𝐹𝑣𝐴

where 𝐹𝑣 𝑖𝑠 𝑡ℎ𝑒 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑡ℎ𝑒 𝑡𝑤𝑜 𝑠𝑢𝑟𝑓𝑎𝑐𝑒𝑠 𝑖𝑛 𝑐𝑜𝑛𝑡𝑎𝑐𝑡. It is also called the dynamic fluid friction.

𝐴 𝑖𝑠 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑖𝑛 𝑐𝑜𝑛𝑡𝑎𝑐𝑡

(3.2.5)

𝐹𝑣 = 𝜇 ∗ 𝐴 ∗ 𝑣𝑥

(3.2.6)

The viscosity constant for mucus is calculated using the formula 2.1.3. The velocity gradient in a fluid is the velocity with which the metal needle moves along the nose tissue in the cylinder. To ensure minimum friction the surface of the metal needle should have low adhesion and stiction [8].

For any liquid wetting liquid or a liquid forming small contact angles on surfaces, there is formation of capillary condensate in the contact area. The formation of menisci is due to the proximity of the two surfaces and the affinity of the two surfaces for the condensing liquid. The meniscus force is present due to surface tension and a rate dependent viscous force. This force is greater for smoother surfaces or surfaces with smaller gaps. The liquid mediated forces are of two types – meniscus force due to surface tension and the force due to viscous effects. These forces are higher when the gaps in the surface are small and the surface is smooth [12].

The contact angle depends on many factors such as roughness of the surface and the cleanliness of the surface. Water repellant surfaces are preferred as wetting of the surface leads to formation of menisci at the point of contact during sliding friction and this causes adhesion. This leads to an increased wet friction force compared to dry friction force. Wetting is defined by the contact angle between the solid and liquid surfaces [8].

If the liquid wets the surface (hydrophilic surface) the static contact angle lies between 0° ≤ 𝜃 ≤ 90°

In case the liquid does not wet the surface (hydrophobic surface) the static contact angle lies between 90° ≤ 𝜃 ≤ 180° . To reduce friction due to wetting the surface roughness should be increased for 𝜃 > 90°. Also for wetting liquids the contact angle decreases with increasing roughness while for non-wetting liquids it increases.

9

The contact angle also depends on the surface tension of the liquid in contact with the solid surface. As the molecules and atoms on the surface have a different energy level, it is called surface free energy or surface tension 𝜰. It is equal to the work that is required to create a unit area of the surface at constant volume and temperature [8]. It is a surface property and its unit is N/m.

The Dupre’ equation gives the relation for a solid in contact with a liquid.

𝑊𝑆𝐿 = 𝛶𝑆𝐴 + 𝛶𝐿𝐴 − 𝛶𝑆𝐿 (3.2.7)

where

𝑊𝑆𝐿 = 𝑤𝑜𝑟𝑘 𝑜𝑓 𝑐𝑜ℎ𝑒𝑠𝑖𝑜𝑛 𝑝𝑒𝑟 𝑢𝑛𝑖𝑡 𝑎𝑟𝑒𝑎 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑡ℎ𝑒 𝑡𝑤𝑜 𝑠𝑢𝑟𝑓𝑎𝑐𝑒𝑠

𝛶𝑆𝐴 = 𝑠𝑢𝑓𝑎𝑐𝑒 𝑡𝑒𝑛𝑠𝑖𝑜𝑛 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑠𝑜𝑙𝑖𝑑 𝑎𝑛𝑑 𝑎𝑖𝑟

𝛶𝑆𝐿 = 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑡𝑒𝑛𝑠𝑖𝑜𝑛 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑠𝑜𝑙𝑖𝑑 𝑎𝑛𝑑 𝑙𝑖𝑞𝑢𝑖𝑑

𝛶𝐿𝐴 = 𝑠𝑢𝑓𝑎𝑐𝑒 𝑡𝑒𝑛𝑠𝑖𝑜𝑛 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑙𝑖𝑞𝑢𝑖𝑑 𝑎𝑛𝑑 𝑎𝑖𝑟

On placing a liquid droplet on the solid smooth surface, the contact angle formed is 𝜃0. The total energy is given by 𝐸𝑡𝑜𝑡

𝐸𝑡𝑜𝑡 = 𝛶𝐿𝐴(𝐴𝐿𝐴 + 𝐴𝑆𝐿) − 𝑊𝑆𝐿𝐴𝑆𝐿 (3.2.8)

where

𝐴𝐿𝐴 = 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 𝑤𝑖𝑡ℎ 𝑎𝑖𝑟

𝐴𝑆𝐿 = 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑜𝑙𝑖𝑑 𝑤𝑖𝑡ℎ 𝑙𝑖𝑞𝑢𝑖𝑑

At equilibrium, the 𝑑𝐸𝑡𝑜𝑡 = 0

𝛶𝐿𝐴(𝐴𝐿𝐴 + 𝐴𝑆𝐿) − 𝑊𝑆𝐿𝐴𝑆𝐿 = 0 (3.2.9)

Thus for a droplet of constant volume

𝑑𝐴𝐿𝐴𝑑𝐴𝑆𝐿

= cos 𝜃0 (3.2.10)

Thus, the Young’s equation of contact angle is given below.

𝑐𝑜𝑠 𝜃0 =(𝛶𝑆𝐴 − 𝛶𝑆𝐿)

𝛶𝐿𝐴

(3.2.11)

The above equation gives the static contact angle for different surface tensions.

10

FIGURE 7 SCHEMATIC OF THE MENISCUS FORMATION FROM A LIQUID CONDENSATE BETWEEN TWO FLAT SURFACES

The meniscus formed between the cylinder with the nasal mucus and the surface of the metal needle is equivalent to two flat surfaces separated by a thin film of liquid. The menisci for the hydrophobic surface which has contact angle greater than 90° is convex in shape as shown in figure 7. The meniscus force 𝐹𝑚 is dependent on the roughness parameters, the type of the liquid, and its film thickness [12].

𝐹𝑚 =𝐴𝑚 ∗ 𝛾𝑙 ∗ (cos𝜃1 + cos𝜃2)

ℎ

(3.2.12)

where

𝛾𝑙 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 𝑓𝑖𝑙𝑚

𝜃1 𝑎𝑛𝑑 𝜃2 𝑎𝑟𝑒 𝑡ℎ𝑒 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑛𝑔𝑙𝑒𝑠 𝑤𝑖𝑡ℎ 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 𝑓𝑖𝑙𝑚

ℎ 𝑖𝑠 𝑡ℎ𝑒 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 𝑓𝑖𝑙𝑚

𝐴𝑚 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑒𝑛𝑖𝑠𝑐𝑢𝑠 𝑎𝑟𝑒𝑎

The surface tension of water at 35°C is 70.41 N/m [18]. The contact angle dependence on the roughness factor for a surface is given by equation 3.3.1. In case of a cylinder, the surface area of the outer cylinder 𝐴𝑚 = 2𝜋 ∗ 𝑥 ∗ 𝑙 .

where

𝑙 𝑖𝑠 𝑡ℎ𝑒 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑒𝑛𝑖𝑠𝑢𝑠 𝑎𝑙𝑜𝑛𝑔 𝑡ℎ𝑒 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟

𝑥 𝑖𝑠 𝑡ℎ𝑒 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑒𝑛𝑖𝑠𝑐𝑢𝑠

The net frictional force 𝐹 is given as the sum of the intrinsic frictional force 𝐹𝑖 and the stiction force 𝐹𝑠 which is a combination of meniscus force 𝐹𝑚 and the viscous force 𝐹𝑣.

𝐹 = 𝐹𝑖 + 𝐹𝑠 (3.2.13)

𝐹 = (µ𝑟 ∗ 𝑊) + 𝐹𝑚 + 𝐹𝑣 (3.2.14)

where

µ𝑟 𝑖𝑠 𝑡ℎ𝑒 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛

𝑊 𝑖𝑠 𝑡ℎ𝑒 𝑛𝑜𝑟𝑚𝑎𝑙 𝑙𝑜𝑎𝑑

µ𝑟𝑊 𝑑𝑒𝑝𝑒𝑛𝑑𝑠 𝑜𝑛 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑝𝑟𝑜𝑝𝑒𝑟𝑡𝑖𝑒𝑠 𝑎𝑛𝑑 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑡𝑜𝑝𝑜𝑔𝑟𝑎𝑝ℎ𝑖𝑒𝑠

liquid

h 𝜭1

𝜭2 x

11

3.3. STRUCTURES OF SILICONE FOR MINIMIZING FRICTION The structure of the silicone stamps plays an important role in minimizing the friction. The forces of adhesion and stiction need to be reduced in order to minimize the friction.

The flat surface of the substrate is covered with a hydrophobic film. Then the surface is made superhydrophobic by introducing roughness so that the contact angles are greater than 150°. The surface of the silicone stamps is made superhydrophobic to make it water repellent. For a droplet moving along a solid surface, the contact angle hysteresis is defined as the difference between the contact angle at the front of the droplet and the contact angle at the back of the droplet. This difference exists due to the roughness of the solid surface. If the contact angle hysteresis for the surface is low, the surface has a low water roll-off angle. This is called the Lotus effect. It is a self-cleaning effect which causes the water droplets to take the contaminants along with it as it rolls off the surface [8]. The self-cleaning property allows the hydrophobic surface to pass through the nasal tissue with minimum change in frictional values.

FIGURE 8 DIAGRAM OF CONTACT ANGLE HYSTERESIS WITH LIQUID DROPLET ON A SURFACE TILTED AT AN ANGLE [8]

Bhushan et al. [8] understood the lotus effect by the study of the lotus leaf surface. The surface of lotus leaves is covered with water repellant hydrocarbons. Also, the leaf surfaces have submicron asperities which make it hydrophobic. The hydrophobicity of the silicone surface is further increased by increasing the surface roughness.

The surface roughness is described by the roughness factor. The roughness factor for the heterogeneous surface is calculated using the Wenzel equation.

cos 𝜃 = 𝑅𝑓 ∗ cos𝜃0 (3.3.1) where

𝜃 𝑖𝑠 𝑡ℎ𝑒 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑑𝑟𝑜𝑝𝑙𝑒𝑡 𝑜𝑛 𝑎 𝑟𝑜𝑢𝑔ℎ 𝑠𝑜𝑙𝑖𝑑 𝑠𝑢𝑟𝑓𝑎𝑐𝑒

𝜃0 𝑖𝑠 𝑡ℎ𝑒 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑑𝑟𝑜𝑝𝑙𝑒𝑡 𝑜𝑛 𝑎 𝑠𝑚𝑜𝑜𝑡ℎ 𝑠𝑢𝑟𝑓𝑎𝑐𝑒

𝑅𝑓 𝑖𝑠 𝑡ℎ𝑒 𝑟𝑜𝑢𝑔ℎ𝑛𝑒𝑠𝑠 𝑓𝑎𝑐𝑡𝑜𝑟

Θrec

Θadv

α

12

The roughness factor is also given by equation 3.3.2.

𝑅𝑓 = 𝐴𝑆𝐿𝐴𝐹

(3.3.2)

where

𝐴𝑆𝐿 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑜𝑙𝑖𝑑 − 𝑙𝑖𝑞𝑢𝑖𝑑 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑤𝑎𝑡𝑒𝑟 𝑑𝑟𝑜𝑝𝑙𝑒𝑡

𝐴𝐹 𝑖𝑠 𝑡ℎ𝑒 𝑝𝑟𝑜𝑗𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑜𝑙𝑖𝑑 − 𝑙𝑖𝑞𝑢𝑖𝑑 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑟𝑒𝑎 𝑜𝑛 𝑡ℎ𝑒 ℎ𝑜𝑟𝑖𝑧𝑜𝑛𝑡𝑎𝑙 𝑝𝑙𝑎𝑛𝑒

The roughness factor for a surface having a profile with equally spaced hemispherically topped cylindrical asperities is calculated to have a high contact angle [8].

FIGURE 9 SURFACE WITH HEMISPHERICALLY TOPPED CYLINDRICAL ASPERITIES [8]

The roughness along with the wetting liquid results in a decrease in the contact angle. For creating a superhydrophobic surface with roughness, a hydrophobic film is used [8].

13

3.4. SILICONE STRUCTURES The needles taken are of a diameter of 0.90 mm, 0.67 mm and 0.53 mm. The metal needle of 0.90 mm is to be used for the experiment while the rest is used for testing. The thickness of the silicone structure 1 is calculated in chapter 4.3 using the microscopy measurement values obtained for silicone structure 2. The thickness of silicone structure 1 is calculated such that the surface area is same as that of silicone structure 2.

TABLE 3 DIMENSIONS OF THE SILICONE STRUCTURES

Silicone structure 1

𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝑛𝑒𝑒𝑑𝑙𝑒 = 0.9 mm 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑖𝑙𝑖𝑐𝑜𝑛𝑒 𝑖𝑠 𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑

Silicone structure 2

𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝑛𝑒𝑒𝑑𝑙𝑒 = 0.9 mm

𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑖𝑙𝑖𝑐𝑜𝑛𝑒 = 20 µm

14

4.0. PROCESSING This chapter describes the microsystem technology steps applied in the production of the structures on the substrate. In order to produce the different (structured and unstructured) coatings for the metal needle – as test layers for friction measurement experiments - you need a preform, which is the mold form for the silicone coating. The 20 µm thick resist layer at the borders of the basins ensures that each silicone stamp has a base thickness of 20 µm. This also makes it easier to remove the resulting silicone stamp from the substrate. The cavities in the structured wells are etched to achieve silicone stamps with half-cylindrical structures as shown in figure 10.

FIGURE 10 SIDE VIEW OF THE SUBSTRATE SHOWING A STRUCTURED WELL

The first section deals with the experiment to achieve a photoresist thickness of 20 µm. Then, the second deals with the manufacturing process. The third section describes the calculations for making the silicone stamps.

4.1. SPIN COATING PARAMETERS FOR A PHOTORESIST THICKNESS OF 20 µm The experimental procedure to achieve the optimum parameters required for a photoresist thickness of 20 µm is described below.

A pyrex wafer is spin coated with Ti Primer. The spin coater revolves with a velocity of 1000rpm. This is followed by vacuum heating 120 °C for 2 minutes. Then further spin coating is done with AZ 9260 by MicroChemicals. This is followed by vacuum heating 120 °C for 5 minutes. The wafer is exposed at a light density of 365 nm for 2 minutes and developed with 1% NaOH for 85 seconds.

TABLE 4 SPIN COATING TIME FOR THE PHOTORESIST AZ 9260 VS THICKNESS OF THE PHOTORESIST ON WAFER

Spinning Time (seconds)

Thickness (µm)

8 18 6 21 5 20

20 µm (AZ 9260)

15

TABLE 5 FINAL PARAMETERS USED IN THE WAFER PROCESSING

Final Parameters Wafer Pyrex Spin coating with Ti Primer 1000rpm for 20 s Baking of Ti Primer 120°C for 2min Spin coating with AZ 9260 1000rpm for 5s Postbake 110°C for 5 min 30s Exposure 2min Development (1% NaOH) 1 min 25s

The final parameters given in Table 5 are used in the wafer processing to achieve a photoresist thickness of 20 µm.

4.2. STRUCTURING OF THE PYREX WAFER The substrate is divided into four quadrants containing basins with different sized well structures. The different spacing widths are 20 µm, 50 µm, 100 µm and unstructured basins. A pyrex wafer with LPCVD grown silicon is used. In step 2, one side of the substrate is spin coated with photoresist of AZ 9260 of thickness 20 µm. This is followed by exposure at a light density of 365nm for 2 minutes and development in 1% NaOH for 85 seconds. In step 3, the top side of the wafer is made to undergo advanced silicon etching to remove the exposed silicon. The photoresist stripping is done in step 4 using acetone and isopropanol. In Step 5, the top side of the wafer undergoes HF etching of pyrex to etch cavities having a uniform depth. The KOH etching done in step 6 on the top side of the wafer removes the silicon layer. The photoresist AZ 9260 deposited on the substrate in step 7 marks the basin borders. In step 8, the gas phase deposition of the anti-stiction monolayer on the topside is done.

Process description Schematic cross sections 1. Pyrex wafer with LPCVD grown silicon

2. Photolithography on the top side

3. ASE (Advanced Silicon Etching)

4. Stripping of the photoresist

5. HF etching of pyrex

16

6. KOH etching of the silicon

7. Photolithography on the surface with wells

8. Gas phase deposition of anti-stiction monolayer on the topside

4.3. CASTING OF SILICONE STAMPS After the processing described in paragraph 4.2 the resulting basins of the pyrex wafer are lightly overfilled with an experimental amount of silicone. This is followed by vacuum drying of the pyrex wafer for a period for twenty four hours. The photoresist AZ 9260 of thickness 20 µm is deposited on the substrate. The advantage of thicker photoresist is that when silicone is doused in the basins of the pyrex wafer, the resulting silicone stamp can be easily lifted off the substrate. After curing, the silicone stamps are cut out and the shape of the structured wells is measured with the help of a microscope. This gives the actual values of the resulting circumference when the silicone film is wrapped around the metal needle and the area and volume of the structured wells. These values are then used to calculate the volume of silicone in µl to be dribbled into the basins to achieve the required base thickness of 20 µm of the silicone stamps.

Figure 11 shows the microscopy measurement values obtained for the length, the height, and the curvature length of a 20 µm spacing well structure.

FIGURE 11 CROSS SECTION OF THE SILICONE STAMP SHOWING THE CURVATURE LENGTH (IN RED), THE HEIGHT (IN YELLOW) AND THE LENGTH (IN GREEN) OF A 20 µM SPACING WELL STRUCTURE

17

Figure 12 shows the microscopy measurement values obtained for the length, the height and the area of a 20 µm spacing well structure.

FIGURE 12 CROSS SECTION OF THE SILICONE STAMP SHOWING THE AREA (IN YELLOW), THE HEIGHT (IN RED) AND THE LENGTH (IN GREEN) OF A 20 µM SPACING WELL STRUCTURE

The total volume of silicone to be dribbled in the basins is the sum of the volume of the basins and the volume of the four wells.

𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎 𝑏𝑎𝑠𝑖𝑛 = 3184 × 20 (µ𝑚2)

(3.4.1)

𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑎 𝑏𝑎𝑠𝑖𝑛(𝐴) = 3184 × 20 × 15000 (µ𝑚3) (3.4.2)

𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑓𝑜𝑢𝑟 𝑤𝑒𝑙𝑙𝑠(𝐵) = 4 × 𝑚𝑒𝑎𝑛 𝑎𝑟𝑒𝑎 × 15000 (µ𝑚3)

(3.4.3)

𝑣𝑜𝑙𝑢𝑚𝑒 (µ𝑙) = (𝐴 + 𝐵) × 10−9 (3.4.4)

TABLE 6 VALUES OF SILICONE TO BE DRIBBLED IN THE STRUCTURED WELLS

Silicone Volume Area of a basin (µm3)

Mean area of a well (µm2)

Total Volume (µm3)

Volume (µl)

Volume for a 20 µm spacing well structure

63680 28181.41

2646060000

2.646

The value of mean area of a well is calculated in table 6. The silicone to be dribbled in the basins is added using a volume adjustable pipette. Each drop from the pipette measures 1 µl. The volume calculated is added to the 20 µm spacing well structures.

18

FIGURE 13 SIDE VIEW OF THE 20 µM WELL STRUCTURES AND THE TOP VIEW SHOWING THE DIMENSIONS OF THE BASIN

The calculation to find the thickness of the silicone structure 1 is done taking a metal needle of diameter 0.9 mm. The base radius of the metal needle is 450 µm.

Thickness of the silicone structure

2𝜋𝑟 = 2𝜋𝑏 + 4 × 𝐶𝑚𝑒𝑎𝑛 − 4 × 𝑑𝑚𝑒𝑎𝑛

(3.4.5)

t is the thickness of the silicone structures

𝑟 𝑖𝑠 𝑡ℎ𝑒 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑒𝑡𝑎𝑙 𝑛𝑒𝑒𝑑𝑙𝑒 𝑤𝑖𝑡ℎ 𝑡ℎ𝑒 𝑠𝑖𝑙𝑖𝑐𝑜𝑛𝑒 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒 𝑤𝑟𝑎𝑝𝑝𝑒𝑑 𝑎𝑟𝑜𝑢𝑛𝑑 𝑖𝑡

𝑏 𝑖𝑠 𝑡ℎ𝑒 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑠𝑖𝑙𝑖𝑐𝑜𝑛𝑒 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒 1

𝑑 𝑖𝑠 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑡ℎ𝑒 20 µ𝑚 𝑠𝑝𝑎𝑐𝑖𝑛𝑔 𝑤𝑒𝑙𝑙 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒

𝑑𝑚𝑒𝑎𝑛 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑒𝑎𝑛 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑡ℎ𝑒 20 µ𝑚 𝑠𝑝𝑎𝑐𝑖𝑛𝑔 𝑤𝑒𝑙𝑙 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒

𝐶𝑚𝑒𝑎𝑛 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑒𝑎𝑛 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑢𝑟𝑣𝑎𝑡𝑢𝑟𝑒 𝑜𝑓 𝑎 20 µ𝑚 𝑠𝑝𝑎𝑐𝑖𝑛𝑔 𝑤𝑒𝑙𝑙 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒

The values are for finding t –

𝑟 = (𝑡 + 450) µ𝑚 (3.4.6)

𝑏 = 450 + 20 = 470 µ𝑚 (3.4.7)

𝐶𝑚𝑒𝑎𝑛 = 420. 5 µ𝑚 (3.4.8)

𝑑𝑚𝑒𝑎𝑛 = 128.887 µ𝑚 (3.4.9)

20µm

15000 µm

3184µm

19

TABLE 7 VALUES USED FOR CALCULATION OF AMOUNT OF SILICONE TO BE DRIBBLED IN THE WELLS

No. Size (µm)

Length (µm)

Radius d (µm)

dmean (µm)

Area (µm2)

Mean Area (µm2)

Curvature (µm)

Cmean (µm)

t (µm)

1 20 256 121 128.5 25075 28181.41 395 420.5 121 2 20 284 136 30959 446

2𝜋 × (450 + 𝑡) = 2𝜋 × 470 + 4 × 420.5 − 4 × 128.5 (3.4.10)

𝒕 = 𝟐𝟎𝟓.𝟖𝟗 µ𝒎 (3.4.11)

The thickness of the silicone structure 1 as discussed in chapter 3.3 is 205.89 µm. This is the required thickness for the surface area of both silicone structures to be the same.

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5.0. PREPARATION OF SILICONE STAMPS This chapter deals with the preparation of the silicone stamps to be wrapped around the metal needle. The section deals with the description of the setup used to glue the silicone stamps on the metal needle.

5.1. SETUP FOR PREPARING SILICONE LAYERS The silicone stamps are made by dousing calculated amount of silicone in the wells. They are cut out using a knife with a 100 µm thick tip. This is accounted for in the mask for the silicone stamps. The silicone stamps have a thickness of 205.89 µm. For this reason, a setup was erected to allow for the wrapping of the silicone stamps around the needle without causing them to undergo any tearing.

The metal needle is placed on a stage which can move along the Z-axis. The setup allows the metal needle to be placed into position and it is fixed using a screw from the top. The substrate holding the silicone stamp is placed on another movable stage which can be moved along the X and Y axes. This ensures no movement of the needle when it comes into contact with the silicone stamp. The portion of the needle to be in contact with the stamp is covered with silicone glue and the stage holding the substrate with the silicone stamp is moved upwards to bring it in contact with the metal needle. Once brought into contact, the metal needle is left into position for 24 hours to ensure the complete gluing of the silicone stamp to the needle. The rest of the metal needle is then covered with glue. The screw is then loosened and the needle is turned to wrap the silicone stamp around it.

FIGURE 14 SIDE VIEW OF SETUP FOR GLUING SILICONE LAYERS AROUND THE NEEDLE

Y

X

Z

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FIGURE 15 TOP VIEW OF SETUP FOR GLUING SILICONE LAYERS AROUND THE NEEDLE

FIGURE 16 TOP VIEW OF ANTI STICTION LAYER COATED WAFER WITH SILICONE IN THE WELLS. BUBBLES ARE FORMED DUE TO INSUFFICIENT VACUUM WHILE DRYING OF THE SILICONE FOR TWENTY FOUR HOURS

22

FIGURE 17 TOP VIEW OF METAL NEEDLE OF DIAMETER OF 0.9 MM

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6.0. SETUP FOR THE FRICTION MEASUREMENT This chapter deals with experimental setup used to determine the optimum structured layer to be used for the metal needle. The next section deals with the friction measurement curve. This is followed by a section giving the description of the pressure sensor. The last section deals with the LabVIEW program used to move the stage with the metal needle is described. A subsection deals with the working of the program and the second subsection deals with the calculation of the linear displacement of the actuator used to convert the encoder counts received from the transducer to measurable voltage values.

The metal needle is of a diameter of 0.9 mm. The setup places the cylinder with the human nose tissue on a movable stage which can move along the X, Y and Z-axes. The needle is mounted on a stand placed on a movable stage which is driven by a servo motor and can move a maximum distance of 24 mm in the X direction. The metal needle is screwed to the tip of the sensor which is also mounted on the same stand.

The metal needle is directed to the opening of the cylinder lined with the human nose tissue. The cylinder has human ear tissue wrapped in an aluminum foil inside. The aim of the experiment is to push in and pull out the metal needle from the cylinder and measure the change in voltage values recorded by the pressure sensor during the movement. The recorded values are indicative of the frictional force due to the dynamic viscosity when the needle glides on the human tissue. It is a measure of the resistance the endoscope coating offers to the human nose and Eustachian tube tissue. This will help find the endoscope coating with the minimum measured voltage values i.e. minimum friction.

FIGURE 18 SIDE VIEW OF THE FINAL SETUP TO TEST SILICONE STAMPS ON NEEDLE

Y

X

Z

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FIGURE 19 VIEW OF THE NEEDLE SCREWED ON THE FORCE TRANSDUCER

6.1. FRICTION MEASUREMENT CURVE The frictional force is measured using the ultra low force transducer described in chapter 6.2. The measured values are plotted on the XY graph. In this chapter the shape of the measurement curve for the friction experiment is analyzed. The friction measurements take into account both forward and backward movement of the metal needle so as to explain the cause of the friction forces [13].

The friction arises due to the uneven constitution of the nasal tissue. The analysis for the two homogeneous surfaces for friction forces arising due to any variation in topography was conducted by Bhushan [13]. The trace for the forward movement of the needle through a homogeneous silicon surface having 5 µm square pits of a depth of 0.18 µm and with 10 µm pitch is used. The movement of the needle from the left to right is taken as forward movement trace and the movement from right to left is taken as the retrace in the reverse direction. The sign of the friction signal is reversed in the retrace as the torque is reversed at the end of the tip when the scanning direction is changed. The friction force microscopy (FFM) measures the surface height and the friction force scan in forward direction indicates the variation in friction is point of change in the topography of the silicon surface. The same is in the reverse direction movement of the scanning tip. The last scan is the subtracted friction data of the forward and reverse scans [13].

The analysis has been used to explain the friction curve arising due to the movement of solid metal needle through the nasal tissue as shown in figure 20. The trace for the forward movement of the metal needle has an initial increase in friction as the medium of movement changes from air to the nasal tissue. The curve will show variations in the friction whenever there is any protrusion along the passage way of the metal needle. As the metal traces its path around the changes in topography, there is a change in the friction value recorded. Also, as the metal needle enters into the cylinder, the area of contact increases. This will result in the friction curve increasing as the metal needle moves forward. At the end of the forward movement of the metal needle, there is a stop followed by the change in the direction of the

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metal needle. This will cause an increase in friction and reversal in sign of the friction signal. There will be an increase in friction in the reverse direction as the needle starts to move again due to the moment of inertia of the needle. This is followed by a decrease in friction as the area of contact decreases as the metal needle leaves the cylinder with the nose tissue. There will also be a variation in the frictional value when the metal needle leaves the nasal tissue and the medium changes to air.

FIGURE 20 EXPECTED FRICTION MEASUREMENT CURVE

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6.2. PRESSURE SENSOR The semiconductor strain gauge full bridge sensor SS2 by Althen GmbH is used for friction detection. It is a highly sensitive sensor with a detection range of forces between ± 30g and a sensitivity of ±6.5 𝑚𝑉/𝑉 for tension and compression and a combined error of maximum 0.17% 𝑟𝑚𝑠 and is relatively insensitive to non-centered load, such as side load and bending moments which makes it ideal for the current application. The sensor tip provides a 2-64 UNF thread screw on which the metal needle can be mounted, after tapping a suitable thread hole into the center of the needle holder. It is connected in series with an amplifier and a data acquisition system which allows for the measurement data to be viewed and analyzed by a computer. Together they form the pressure sensor used to measure the strain. The pressure variations are read in volts on the computer. The pressure sensor is used to measure the frictional force between the outer layer of the metal needle and the nose tissue.

FIGURE 21 DIAGRAM SHOWING THE PARTS OF THE PRESSURE SENSOR

The ultra-low force transducer is based on the semiconductor strain gauge technology and is used for measuring small variations in strain in the range ±30𝑔. It is a strain gauge load cell.

The voltage output from the force transducer needs to be amplified before being fed to the Data Acquisition system. This is done by a differential amplifier [17]. The amplified output voltage is sent in digital format to the computer for further analysis. The Data Acquisition device used for the experiment is DI718-U by DATAQ Instruments.

Ultra low Force

Transducer

Amplifier

Data

Acquisition

System

Computer

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6.3. LabVIEW PROGRAM TO MOVE THE STAGE The movable stage mounts the metal needle to be driven into and out of the cylinder with the human ear tissue. The software LabVIEW is used to control the movement of the stage by a computer and to store the recorded values simultaneously. On running the program it first creates a folder with a unique name and saves the data of the sensor recorded by the LabVIEW Data Acquisition system and the position of the stage provided by the servo motor and stored in text files. The recorded data is then plotted to give the graph of the voltage values (resistance) detected by the pressure sensor versus the movement of the stage in x-direction.

6.3.1. WORKING OF THE LABVIEW PROGRAM The LabVIEW program described below has been modified to work with the friction measurement setup [15]. The file generator section has also been modified to work with the entire program [14]. The control for the movable stage has been added to the program.

As shown in figure 22, the frontpanel of the LabVIEW program displays the MG17Motor ActiveX control. This panel is used to view and vary the movement of the DC servo motor which controls the position of the stage.

FIGURE 22 FRONTPANEL OF THE LABVIEW PROGRAM SHOWING THE MG17MOTOR ACTIVEX CONTROL

The frontpanel of the LabVIEW program also has two path display elements. The first is the path display element for the DC servo motor. The second is the path display element for the data acquisition device. The program controls the functioning of the data acquisition device by DATAQ instruments. The voltage values displayed by the DAQ during the forward movement of the metal needle through the cylinder and in the reverse direction on leaving the cylinder are recorded and stored in a text file. In the same way, the program also records the position of the movable stage and stores the values in another text file.

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As shown in figure 23, the path for the text file is generated by the program every time the program is executed. The frontpanel also has two numerical display elements which display the reverse stage position and forward stage position. It also has a stop button which is the exit control for the program.

FIGURE 23 FRONTPANEL OF THE LABVIEW PROGRAM SHOWING THE FOLDER PATHS GENERATED BY THE PROGRAM WHERE THE RECORDED VALUES ARE STORED

FIGURE 24 FRONTPANEL OF THE LABVIEW PROGRAM SHOWING THE ACTIVEX CONTROL DATAQSDK FOR THE DATA ACQUISITION DEVICE, THE STOP BUTTON FOR THE PROGRAM, THE NUMERICAL VALUES OF THE REVERSE POSITION AND FORWARD POSITION OF THE MOVABLE STAGE

The DataqSdk ActiveX control is present on the frontpanel along with the DQChart. The chart displays the variation in the voltage values of the Data Acquisition device during program execution. This is shown in figure 25.

FIGURE 25 FRONTPANEL OF THE LABVIEW PROGRAM SHOWING THE DQCHART USED TO DISPLAY THE MOVEMENT OF THE SENSOR

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The blockdiagram of the program has a stacked sequence structure to execute sub-diagrams sequentially. The sequence has six sub-diagrams. As shown in figure 26, the first sub-diagram has the automation open function in which the ActiveX references to the DataqSdk ActiveX control. The ActiveX control for the DQChart is also connected to the sequence.

FIGURE 26 BLOCKDIAGRAM SHOWING THE OPENNING OF ACTIVEX CONTROL

As shown in figure 27, the first sub-diagram also includes the file generator section. A unique folder is generated at each execution and the both the text files are stored in it. At every execution of the program, the text files which store the numerical data values for the data acquisition device and the position of the movable stage are generated. The file generator generates unique folder names by concatenating the date and time with the folder name. As the date generated is in the mm/dd/yyyy format. The timestamp generated is in the hh:mm:ss format. Any folder name in WINDOWS cannot have the forward slash or the colon character present in the name of the folder.

For this reason, the search and replace string function is used to identify the forward slash in the string and replace it with an underscore character. The same function is used again to identify the colon character and replace it with the underscore character. Then a random number function is used to generate a numerical value between 0 and 1. The numerical value is multiplied by 1000 and converted to a string value using the number to decimal string function. Then, the values of the timestamp, date and the random number are concatenated with a string using the concatenate strings function. The string consists of the path where the folder containing the files will be stored on the computer. The output of the concatenate strings function is passed through the string to path function to generate a path. The path is the input to the create folder function which then creates a folder at the location specified. This create folder function is connected to the folder display element. A new folder is created with unique name – Drive:\path\Namedatetime_number.

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FIGURE 27 BLOCKDIAGRAM SHOWING THE UNIQUE FILENAME GENERATOR CODE

As shown in figure 28, the second sub-diagram is used to initialize the properties of the DataqSdk ActiveX control. The properties used for the data acquisition device are Eventpoint, Device Driver, Device ID and the ADChannel Count. In the same sub-diagram, the MG17Motor property HW Serial number is used. These values helps LabVIEW identify the devices connected to the computer.

The Eventpoint property is set to specify a number of data points after which a newdata event is fired by the data acquisition device. This property is set to 1.

The Device Driver property defines the data acquisition device. The Device ID is also used to identify the device. It defines the computer port to which the device is connected, the device name and the bitrate.

The ADChannel Count sets the number of channels of the data acquisition device is to be enabled. The value to set to 0 as only one channel is used.

The HW Serial Number property is set to the serial number of the DC servo motor.

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FIGURE 28 BLOCKDIAGRAM FOR PANEL 1 SHOWING THE INITIALISATION SECTION

As shown in figure 29, the third sub-diagram starts the operation of the data acquisition device. In this sub-diagram the initial position of the movable stage and the various parameters required for the movement of the stage are defined. This is shown in figure 30. This is controlled by varying the velocity of the movement of the stage. The motor control is first initialized using the property StartCtrl.

The SetJogMode property is used to set the jogging mode for the motor. The IchanID is the channel identifier which is set to 0. The IMode sets the jogging mode to continuous which is denoted by value 1. The IStopMode sets the stopmode to stop immediately which is denoted by value 1.

The SetJogStepSize property is used to define the size of each step of the motor in mm. The same IChanID is passed and fStepSize is set to 0.05 mm.

The SetJogVelParams property is used to specify the velocity parameters for the motor. The IChanID is kept the same. The fMinVel sets the minimum velocity for all the jog moves. The fAccn sets the rate at which the velocity increases to the maximum velocity. The fMaxVel sets the maximum for the jogging.

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FIGURE 29 BLOCKDIAGRAM SHOWING THE STARTING OF OPERATION OF THE DATA ACQUISITION DEVICE

FIGURE 30 BLOCKDIAGRAM SHOWING THE INITIAL MOVEMENT FOR THE MOVABLE STAGE

As shown in figure 31, in the fourth sub-diagram, the data acquisition device (DAQ) and the motor are looped together in a while loop till they are stopped. The DAQ checks first if data is available. If data is available the inner sequence is set to true. The DataqSdk has a property

GetData which acquires the available data from the enabled channel and returns as a variant data type. This data type is converted to data using a Variant to Data function. The data is stored in 2-dimensional array whose size is calculated using array size function. The elements are stored in an index array. The first element in the index array is used to convert counts to volts as per the formula below.

The data is returned from the DAQ instrument in form of A/D converter counts for a 14-bit integer.

𝑉𝑜𝑙𝑡𝑠 = �𝐴𝐷𝑅𝑒𝑎𝑑𝑖𝑛𝑔 ×(𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛)

65536 � +

(𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛)2

𝐴𝐷𝑅 𝑅𝑒𝑎𝑑𝑖𝑛𝑔 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑖𝑔𝑛𝑒𝑑 𝑖𝑛𝑡𝑒𝑔𝑒𝑟 𝑣𝑎𝑙𝑢𝑒 𝑟𝑒𝑡𝑢𝑟𝑛𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝐷𝐴𝑄

𝑉𝑚𝑎𝑥 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑖𝑛𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑎𝑡 𝑠𝑒𝑙𝑒𝑐𝑡𝑒𝑑 𝑔𝑎𝑖𝑛

𝑉𝑚𝑖𝑛 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑖𝑛𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑎𝑡 𝑠𝑒𝑙𝑒𝑐𝑡𝑒𝑑 𝑔𝑎𝑖𝑛

(6.3.1)

𝑉𝑚𝑎𝑥 = 5𝑉 𝑉𝑚𝑖𝑛 = −5𝑉

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The value generated by the DAQ device is to be stored in a text file in the folder which is created at the execution of the program. The path for the text file is specified and combined with the generated folder name by using the build path function. The numerical from the DAQ is stored in a one dimension array and it is then converted to fractional string using the number to fractional string function. The write to spreadsheet file function is used to save the data values in a tabular format in a text file in the location specified.

If the data is not available i.e. data available property gives a value zero then the sequence passes through the false loop where the control goes out of the sequence structure. This is shown in figure 32.

FIGURE 31 BLOCKDIAGRAM SHOWING THE DAQ SECTION

FIGURE 32 BLOCKDIAGRAM SHOWING THE FALSE PATH IN DAQ SECTION

As shown in figure 33, the fourth sub-diagram also includes the motor section. A delay element is placed inside the while loop for the motor section as it operates slower than the data acquisition device for the same data. The motor section is placed in a case structure. Also in the while loop an iteration value 𝒊 is used to ensure the motor section of the structure executes at least once before the data acquisition device. This is to ensure the movable stage is already moving into position before the data acquisition device starts sending the values as it has a higher bit-rate. The motor execution consists of two operations – the forward movement of the stage and the backward movement of the stage. Both operations are performed in a case structure with the forward operation when case value is true and the backward operation when it is false. Each operation of the motor causes the stage to move by 0.05 mm so the loop is executes six times to ensure movement by at least 0.3 mm. When the case value is true, the MoveJog property has the IJogDir set to 1 to ensure forward movement. The final position of

34

the stage is retrieved using the GetPosition property and the value is stored in tabular format in a text file. The delay of 2000ms ensures the loop does not iterate before saving the value to file.

FIGURE 33 BLOCKDIAGRAM SHOWING THE FORWARD PATH

When the case value is false, the MoveJog property has the IJogDir set to 2 to ensure backward movement. This is shown in figure 34.

FIGURE 34 BLOCKDIAGRAM SHOWING THE FALSE PATH

As shown in figure 35, the fifth sub-diagram stops the scanning of values by the DAQ device.

FIGURE 35 SUB-DIAGRAM SHOWING THE STOPPING OF THE DAQ

35

The sixth sub-diagram as shown in figure 36 closes the ActiveX reference of the program.

FIGURE 36 SUB-DIAGRAM SHOWING THE CLOSING OF THE ACTIVEX CONTROL OF THE DAQ SECTION

6.3.2. CALCULATION OF LINEAR DISPLACEMENT PER ENCODER COUNT The motor actuator model for the servo motor is Z825 from THORLABS. This actuator has 512 encoder counts per revolution of the motor. The motor has 67:1 gear head. This means the motor rotates 67 times to rotate the 1mm pitch lead screw one revolution [16].

The linear displacement of the actuator per encoder count is given by [16]–

512 × 67 = 34, 304 𝑒𝑛𝑐𝑜𝑑𝑒𝑟 𝑐𝑜𝑢𝑛𝑡𝑠 𝑝𝑒𝑟 𝑟𝑒𝑣𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑒𝑎𝑑 𝑠𝑐𝑟𝑒𝑤

Linear displacement of the lead screw per encoder count is given by -

1.0𝑚𝑚 ÷ 34304 𝑐𝑜𝑢𝑛𝑡𝑠 = 2.9 × 10−5 𝑚𝑚

The above values are used in the program to make the DAQ calculations.

The values of the voltage values recorded versus movable stage position at that instant are then plotted on an XY graph. The block diagram for that is given below in figure 37.

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FIGURE 37 BLOCKDIAGRAM OF LABVIEW PROGRAM TO DISPLAY THE VALUES ON XY GRAPH

The values are read from the text file and indexed in an array and stored in a 2-dimensional array. It is then converted to dynamic data type to be plotted on the XY graph. The motor values are plotted on X-axis and the values from the data acquisition device are plotted on the Y-axis.

37

7.0. MAKING OF THE SILICONE ENDOSCOPE COATINGS In the initial methods silicone is deposited on the wafer and an apparatus is used to transfer it to the needle. Next, the needle was coated with glue, in order to stick the capillaries; the needle is covered with silicone. The needle used for the experiment is of diameter 0.9 mm.

7.1. METHOD 1 Silicone is deposited in the wells on the wafer. A thin layer of Uhu (glue) is painted on the needle. The glue has a curing time of one minute. The needle is left in contact with the wafer for 1 hour on the apparatus.

Observations:

• The metal needle is stuck with the layer on the wafer at the point of contact. However, on slow wrapping of the remaining part of the needle with the silicone layer on the wafer, the layer does not transfer itself successfully to the needle. There are tears in the silicone layer formed due to the force applied.

• The silicone deposited in the structured wells has the formation of bubbles in some of the wells due to inconsistent spreading of the silicone in the well.

FIGURE 38 SILICONE COATING ON THE WAFER

The same test is repeated with glue having curing time of 1 second. However the result was unsuccessful.

• The silicone forms tangled structures when wrapping around the needle. • The silicone is not transferred to the tip of the needle.

The test is further repeated using silicone as the glue and the apparatus is left for 12 hours to allow the silicone to cure.

• The silicone is cured and the needle is stuck with the silicone layer on the wafer. • On turning the needle to wrap it around the silicone, the layer tears off and the silicone

forms tangled structures on the needle.

38

FIGURE 39 THE INCOMPLETE TRANSFER OF THE SILICONE STAMP ON TO THE SURFACE OF THE NEEDLE

7.2. METHOD 2 The capillaries are separately stuck by Uhu (glue) with curing time of 1 second. Then the needles are painted with a thin layer of silicone and hung tip facing downwards to dry.

Observations:

• The silicone moves downwards to the tip and is cured in the form of a globule at the tip of the needle.

• The capillaries stay in position.

FIGURE 40 SIDE VIEW OF THE NEEDLE OF DIAMETER 0. 53 µM WITH THE SILICONE GLOBULE AT THE TIP

39

7.3. METHOD 3 The capillaries are made to stick to the needle with Uhu (glue) having one second curing time. The needle is then painted with a thin layer of silicone and left to cure in the oven at 90°C for one hour.

Observations:

• The capillaries stay in position in which they are glued, however were at different distances from the tip of the needle.

• The silicone completely covers the capillaries and no structures are visible above the silicone layer on the needle.

• There are positions on the needle with uneven spots of the glue on the capillary.

FIGURE 41 TOP VIEW SHOWING THE TIP OF THE NEEDLE OF DIAMETER 0.53 µM SHOWING THE UNEQUAL SPACING OF THE CAPILLARIES AND ALSO THE EXCESS SILICONE COVERING THE CAPILLARIES

FIGURE 42 SIDE VIEW OF THE ABOVE NEEDLE WITH SILICONE COVERING THE CAPILLARIES AND THE GLUE LEAVING SPOTS OF UNEVEN THICKNESS

40

7.4. METHOD 4 The needle is thinly coated with silicone and all four capillaries are equally spaced on the needle. Then, the needle is hung tip downwards on the setup and the setup is placed in the oven at 90°C for one hour. This is to cure the silicone and at the same time hold the capillaries in position.

Observations:

• The capillaries change position due to cohesion and come close together on the surface of the needle.

• There is uniform deposition of silicone all over the needle on the capillaries.

FIGURE 43 TOP VIEW OF THE NEEDLE SHOWING THE CAPILLARIES WHICH HAVING CHANGED POSITION AND ARE BUNDLED TOGETHER ON THE NEEDLE

41

7.5. METHOD 5 The needle is thinly coated with silicone and one capillary is painted with silicone and is placed into position. Then the needle is placed on the setup and left in the oven to cure at 90°C for one hour. This is done for all four capillaries.

Observations:

• The capillaries are cured into position. • There is uniform distribution of the silicone all over the needle. • The layer of silicone is too thick as the structures of the capillaries were not visible

above the silicone layer on the needle.

FIGURE 44 SIDE VIEW OF THE NEEDLE WITH DIAMETER 0.53 µM WITH CAPILLARIES COATED WITH A THIN FILM OF SILICONE

FIGURE 45 TOP VIEW OF THE SAME NEEDLE SHOWING THE TIP OF THE NEEDLE COATED WITH A THICK LAYER OF SILICONE COVERING THE CAPILLARIES COMPLETELY

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8.0. CONCLUSION AND OUTLOOK

The silicone stamps were developed for the endoscope coating to be encapsulated around the metal needle. The optimum parameters for the structured silicon substrate were found experimentally. The unstructured silicone stamps for lower friction were also realized. To glue the stamps to the metal needles a setup to hold the silicone stamp and the metal needle was assembled.

The setup for the experimental determination of the frictional values was assembled. The pressure sensor, the data acquisition device was connected with the movable stage controlled by a dc servo motor holding the pressure sensor on a holder.

The LabVIEW program for movement of the movable stage to move the metal needle forward and backward was developed. The LabVIEW program was tested successfully with the experimental setup for the forward movement and backward movement of the metal needle on the movable stage.

Further work can be done to realize structured flexible silicone stamps to be encapsulated on the metal needle. Then, the experimental determination of frictional values can be carried out. The experiment also has to be carried out for metal needles encapsulated with sub-µm thick layer of Teflon.

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9.0. REFERENCES [1] Y. Majima, M.Inagaki, K. Hirata, K.Takeuchi, A. Morishita, Y. Sakakura: “The effect of an orally administered proteolytic enzyme on the elasticity and viscosity of nasal mucus.” In: Archives of Oto-Rhino-Laryngology 244 (1988), pp. 355-359

[2] M. S. Quraishi, N. S. Jones, J. Mason: “The rheology of nasal mucus - a review.” In: Clin. Oto-Laryngology 23 (1998), pp. 403-413

[3] Majima, T. Harada, T. Shimizu, K. Takeuchi, Y. Sakakura: “Effect of biochemical components on rheological properties of nasal mucus in chronic sinusitis” In: American Journal of Respiratory and Critical Care Medicine 160 (1999)

[4] M. KING, P. S. P. Silveira, G. M. Böhm et al.: “Computer-assisted rheological evaluation of microsamples of mucus”, In: Computer Methods and Programs in Biomedicine, 39 (1992), pp. 52-53 [5] S Saida: “Ciliary Activity of nasal and maxillary epithelia in man” In: Mie Med J 36 (1986), pp. 9-18

[6] Product Information Dow Corning-184 Silicone Elastomer (2010): pp. 1-3

[7] Bharat Bhushan. “Nanotribiology, Nanomechanics and Materials Characterization, Tribology Letters”. Berlin; Heidelberg: Springer 2007. In: pp. 847-849

[8] Michael Nosonovsky, Bharat Bhushan. “Multiscale Dissipative Mechanisms and Hierarchical Surfaces - Friction, Superhydrophobicity and Biomimetrics”. Berlin; Heidelberg: Springer 2008. In: pp. 34, 43-44, 81-86, 100-107

[9] Le xuan Anh. “Dynamics of Mechanical systems with Coulomb Friction”. Berlin: Springer 2003. In: pp. 11-12

[10] D. S. Viswanath, T. K. Ghosh, D L Prasad, N. V. K Dutt, K. Rani: “Viscosity of liquids – Theory, Estimation, Experiment and Data” (2007). In: Introduction, pp. 1-2 [11] H. A. Barnes. “Viscosity”. Aberystwyth: University of Wales 2002. In: Chapter 1: pp 9

[12] B. Bhushan. “Introduction to Tribology”. New York: Wiley, 2002. In: Adhesion, pp. 185-190, Friction, pp. 247-248

[13] S. Sundararajan, B. Bhushan: “Topography-induced contribution to friction forces measured using an atomic force/friction force microscope” (2000). In: Journal of Applied Physics Vol. 88 No. 8, pp. 4825-4828

[14] Jtagg, Filename Generator for LabVIEW, https://decibel.ni.com/content/docs/DOC-14791, Accessed 4 April 2012.

[15] Anonymous, http://ultimaserial.com/lvtutor.html, Accessed 24 February, 2012

[16] Thorlabs: “DC Servo Motor Driver User Guide” In: HA0142T Rev 14 November 2011

[17] Althen, SS2 Kraftaufnehmer user guide, Version 1.01

[18] N. B. Vargaftik, B. N. Volkov, L. D. Voljak, “International tables of the surface tension of water” In: Journal of Physical and Chemical Reference Data, Volume. 12, No. 3 (1983)

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10.0. APPENDIX

10.1. MASKS

FIGURE 46 MASK FOR THE MAKING OF THE STRUCTURED WELLS ON THE SILICON-PYREX WAFER

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FIGURE 47 MASK FOR THE SECOND PHOTOLITHOGRAPHY PROCESS