Micro-Scale Bio-Inspired Structured Polymer Surfaces for Tissue Adhesion

233 Pages·2016·6.17 MB·English

by  Ashley Bell

Micro-Scale Bio-Inspired Structured Polymer Surfaces for
Tissue Adhesion

Ashley Marie Bell

Submitted in accordance with the requirements for the degree of
Doctor of Philosophy

The University of Leeds

School of Mechanical Engineering

September 2015
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The candidate confirms that the work submitted is his/her own and that
appropriate credit has been given where reference has been made to the work
of others.

This copy has been supplied on the understanding that it is copyright material
and that no quotation from the thesis may be published without proper
acknowledgement.

The right of Ashley Marie Bell to be identified as Author of this work has been
asserted by her in accordance with the Copyright, Designs and Patents Act
1988.

© 2015 The University of Leeds and Ashley Marie Bell
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Acknowledgements
Firstly, I would like to extend my greatest gratitude to Professor Anne Neville, for
her continuous support, knowledge, and guidance throughout the duration of this
work. Your patience, advice, and enthusiasm have made this whole process
enjoyable and a pleasure from start to finish, and for that I am forever grateful.
Also I would like to thank Professor Phillip Gaskell, for co-supervising the project,
providing knowledgeable insights and for all of your support throughout this
project.
I would like to acknowledge the iFS staff for their help and support throughout the
past four years, without their technical support and help this thesis would not have
been possible. I would also like to extend this thanks to Jackie, Fiona and Alice,
who have helped with so many issues over these four years.
I would also like to show appreciation to the cleanroom and CBS staff. Specifically
Dr Li Chen, your valuable knowledge and expertise in micro-fabrication
techniques have been invaluable. I would also like to thank Dr Chris Russell, for
his help and support with everything cleanroom related from day one, and also
Adrian Eagles for guidance with white light interferometry.
I would like to thank the surgical technologies research group, in particular Adrian
and Jen for helping with in-vivo testing and tissue knowledge, and William for
your LabVIEW and Matlab help.
I would like to thank all of my friends and colleagues; it has been a pleasure to
work with such a lovely group of people, particularly office 247 for the tea breaks,
lunches and cakes. I couldn’t have asked for a better group of people to work
with. And to everyone in office 132 who have put up with me during the write up,
listened to my moaning and ranting and supported me throughout the last stages
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of this work. And also to Louise, thank you for being a brilliant friend and keeping
me sane from day one, it would not have been the same without you.
I would also like to thank my close friends and family, especially to Chris, for
motivating me and being so understanding throughout this write up.
Finally, to my Mam, Dad and Matthew, for the endless encouragement, support,
patience and believing in me during not only this thesis but in everything, for this
I am forever thankful.
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Abstract
There are many cases when it would be beneficial to provide reliable, reversible
adhesion at a tissue-device interface. There are currently many adhesion
mechanisms used during surgical procedures such as graspers, vacuum cups
and hooks. However, these are known to cause some scale of tissue trauma.
This thesis investigates the viability of using a bio-inspired, structured polymer
surfaces to provide wet adhesion forces through the formation of liquid bridges
on the tip of discrete pillars. The mechanism described involves a contribution
from both Stefan forces and capillary forces to provide the total adhesion force.
A main factor to this work is the ability to successfully, repeatedly and reliably
fabricate polymer surfaces at a micron scale with varying geometry, specifically
in terms of pillar spacing. The substrate should be flexible and have the ability to
tune the wettability. A number of lithography techniques have been explored for
a range of polymers, finally settling on a nano-imprint lithography technique with
Poly (dimethylsiloxane) (PDMS) as the polymer.
Experimental adhesion tests have been performed and it has been found, that for
such an adhesion mechanism to be successful, an optimal contact angle must be
reached. If the contact angle is too high, repulsive forces in convex menisci will
form and the adhesion will be low as a result. If the contact angle is too low, the
capillary length : pillar height ratio results in the surface acting super-hydrophobic
and completely wetting the pillars, preventing the formation of liquid bridges, and
again resulting in low adhesion, it is proposed that such a mechanism would
occur at contact angles lower than 50°. A mathematical model has been
investigated, encompassing both the Stefan and adhesion forces and the
limitations of this have been discussed in relation to the complexity of this system.
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Table of Contents
Acknowledgements ............................................................................................. ii
Abstract .............................................................................................................. iv
Table of Contents ................................................................................................ v
List of Tables...................................................................................................... xi
List of Figures ................................................................................................... xii
Publication List and Conference Proceedings ................................................ xxv
Chapter 1. Introduction .................................................................................. 1
1.1 Background .......................................................................................... 1
1.2 Motivation ............................................................................................. 2
1.3 Aims and Objectives ............................................................................. 4
1.4 Contributions and Novelty ..................................................................... 4
1.5 Thesis Outline ....................................................................................... 5
Chapter 2. Literature Review ......................................................................... 7
2.1 Introduction ........................................................................................... 7
2.2 The Effect of Technology on Laparoscopic Surgery ............................. 8
2.2.1 Background ..................................................................................... 8
2.2.2 Development of Laparoscopic Surgery ........................................... 9
2.2.3 Single Incision Surgery and NOTES ............................................. 10
2.2.4 Robotics in Surgery ....................................................................... 11
2.2.5 Section Summary .......................................................................... 15
2.3 Mechanisms of Adhesion .................................................................... 16
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2.3.1 Mechanical Interactions ................................................................. 18
2.3.2 Chemical Interactions .................................................................... 18
2.3.3 Magnetic Attachment Systems ...................................................... 18
2.3.4 Passive Adhesion .......................................................................... 20
2.3.5 Wet Adhesion ................................................................................ 21
2.3.5.1 Capillary Adhesion ............................................................... 21
2.3.5.2 Surface Wettability ............................................................... 22
2.3.6 Biomimetic Wet Adhesion ............................................................. 24
2.3.7 Biomimetic Dry Adhesion .............................................................. 27
2.4 Fabrication Techniques and Surface Optimisation ............................. 29
2.4.1 Introduction ................................................................................... 29
2.4.2 Surface Optimisation ..................................................................... 29
2.4.2.1 Surface Geometry ................................................................ 30
2.4.3 Photolithography ........................................................................... 34
2.4.3.1 Soft Lithography ................................................................... 37
2.4.3.2 Nano-Imprint Lithography ..................................................... 38
2.4.4 Surface Chemistry ......................................................................... 40
2.4.4.1 Polyurethane (PU) ................................................................ 40
2.4.4.2 Polymethylmethacrylate (PMMA) ......................................... 41
2.4.4.3 Polydimethylsiloxane (PDMS) .............................................. 41
2.4.4.4 SU8 ...................................................................................... 43
2.4.5 Summary ....................................................................................... 44
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Chapter 3. Experimental Procedures and Methodology .............................. 46
3.1 Introduction ......................................................................................... 46
3.2 Experimental Conditions ..................................................................... 46
3.3 Biological Specimens.......................................................................... 47
3.4 Surface Preparation Equipment .......................................................... 47
3.4.1 Wafer Saw ..................................................................................... 47
3.4.2 Reactive Ion Etcher ....................................................................... 48
3.4.3 Plasma Prep 2 - Plasma Chamber ................................................ 50
3.5 Surface Analysis Equipment ............................................................... 50
3.5.1 Nano-Indentation ........................................................................... 50
3.5.2 Scanning Electron Microscope (SEM) ........................................... 51
3.5.3 White Light Interferometry - Bruker Npflex™ ................................. 54
3.5.4 Contact Angle Goniometry - FTA4000™ ....................................... 55
3.6 Adhesion Test Equipment ................................................................... 56
3.6.1 Modular Universal Surface Test (MUST) Instrument ..................... 56
3.6.2 Tissue Adhesion Testing Rig ......................................................... 61
3.6.3 Modular Mechanical Characterisation (MMC) Rig ......................... 63
3.7 Summary ............................................................................................ 65
Chapter 4. Polymer Selection and Fabrication Techniques ......................... 66
4.1 Introduction ......................................................................................... 66
4.2 Polymer Selection ............................................................................... 67
4.2.1 Polyurethane Acrylate (PU) ........................................................... 68
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4.2.2 Poly (Methyl Methacrylate) (PMMA) .............................................. 69
4.2.3 SU8 ............................................................................................... 70
4.2.4 Poly (dimethylsiloxane) (PDMS) .................................................... 72
4.2.5 Silicon Template ............................................................................ 75
4.3 Polymer Fabrication Techniques ........................................................ 79
4.3.1 Autotex© ........................................................................................ 79
4.3.1.1 Method 1: Autotex© .............................................................. 79
4.3.2 SU8 ............................................................................................... 82
4.3.2.1 Method 2: SU8 2002 as a Base for Autotex©........................ 82
4.3.2.3 Method 4: SU8 2010 as a Mould for PDMS ......................... 86
4.3.3 Poly (Dimethylsiloxane) (PDMS) ................................................... 89
4.3.3.1 Method 5: Direct Patterning of PDMS .................................. 89
4.4 Summary ............................................................................................ 94
Chapter 5. Experimental Results ................................................................. 96
5.1 Introduction ......................................................................................... 96
5.2 Sample Preparation ............................................................................ 96
5.2.1 Polymer Properties ........................................................................ 96
5.2.1.1 Mechanical Properties .......................................................... 97
5.2.1.2 Wettability ........................................................................... 101
5.2.2 Fluid Characterisation ................................................................. 107
5.3 Adhesion Results .............................................................................. 109
5.3.1 Effect of Polymer Selection ......................................................... 110
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5.3.2 Effect of Geometry on Hydrophobic PDMS Samples .................. 113
5.3.3 Effect of Fluid Viscosity on Hydrophobic PDMS Samples ........... 116
5.3.4 Hydrophilic Adhesion against Water ............................................ 118
5.3.4.1 Effect of Wettability on PDMS Samples ............................. 118
5.3.4.2 Effect of Surface Geometry on PDMS Samples ................. 121
5.3.5 Adhesion against Glycerol - Water Mix ....................................... 123
5.3.5.1 Effect of Wettability on PDMS Samples ............................. 123
5.3.5.2 Effect of Surface Geometry on PDMS Samples ................. 125
5.4 Adhesion against Tissue ................................................................... 127
5.4.1.1 Effect of Wettability on PDMS Samples ............................. 128
5.4.1.2 Effect of Surface Geometry on PDMS Samples ................. 129
5.4.2 Effect of Fluid Properties on Adhesion ........................................ 131
5.4.2.1 Initial Traction Testing ........................................................ 135
5.5 Summary .......................................................................................... 137
Chapter 6. Results - Mathematical Wet Adhesion Model ........................... 139
6.1 Introduction ....................................................................................... 139
6.2 Capillarity .......................................................................................... 141
6.3 Tissue Characterisation .................................................................... 144
6.4 Proposed Mechanism ....................................................................... 148
6.5 Theoretical Results ........................................................................... 153
6.5.1 Effect of Separation on the Adhesion Mechanism Occurring ...... 154
6.5.2 Effect of Surface Wettability (Pillared Polymer Surface) .............. 157