Adhesion in Pharmaceutical, Biomedical, and Dental Fields E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Adhesion and Adhesives - Fundamental and Applied Aspects
• Sprache: Englisch

Seitenzahl: 665

Veröffentlichungsjahr: 2017

Beschreibung

The only book to cover adhesion in pharmaceutical, biomedical and dental fields The phenomenon of adhesion is of cardinal importance in the pharmaceutical, biomedical and dental fields. A few eclectic examples will suffice to underscore the importance/relevance of adhesion in these three areas. For example, the adhesion between powdered solids is of crucial importance in tablet manufacture. The interaction between biodevices (e.g., stents, bio-implants) and body environment dictates the performance of such devices, and there is burgeoning research activity in modifying the surfaces of such implements to render them compatible with bodily components. In the field of dentistry, the modern trend is to shift from retaining of restorative materials by mechanical interlocking to adhesive bonding. The book contains 15 chapters written by internationally-renowned subject matter experts and is divided into four parts: Part 1: General Topics; Part 2: Adhesion in Pharmaceutical Field; Part 3: Adhesion in Biomedical Field; and Part 4: Adhesion in Dental Field. The topics covered include: Theories or mechanisms of adhesion; wettability of powders; role of surface free energy in tablet strength and powder flow behavior; mucoadhesive polymers for drug delivery systems; transdermal patches; skin adhesion in long-wear cosmetics; factors affecting microbial adhesion; biofouling and ways to mitigate it; adhesion of coatings on surgical tools and bio-implants; adhesion in fabrication of microarrays in clinical diagnostics; antibacterial polymers for dental adhesives and composites; evolution of dental adhesives; and testing of dental adhesive joints.

Leseprobe

Contents

Cover

Title page

Copyright page

Preface

Part 1: General Topics

Chapter 1: Theories and Mechanisms of Adhesion in the Pharmaceutical, Biomedical and Dental Fields

1.1 Introduction

1.2 Mechanisms of Adhesion

1.3 Summary

References

Chapter 2: Wettability of Powders

2.1 Introduction

2.2 Different Forms of Wetting

2.3 Hydrophilic and Hydrophobic Surfaces

2.4 Contact Angle Measurement in Wettability Studies of Powdered Materials

2.5 Contact Angle and Surface Free Energy

2.6 Surface Free Energy Determination of Powdered Solids by Thin Layer Wicking Method

2.7 Surface Free Energy Determination of Powdered Solids by Imbibition Drainage Method

2.8 Summary

Acknowledgement

References

Part 2: Adhesion in the Pharmaceutical Field

Chapter 3: Tablet Tensile Strength: Role of Surface Free Energy

3.1 Introduction

3.2 Applicability of the Proposed Model to Pharmaceutical Materials

3.3 Discussion

3.4 Summary

3.5 Acknowledgements

References

Chapter 4: Role of Surface Free Energy in Powder Behavior and Tablet Strength

4.1 Introduction

4.2 Surface Free Energy

4.3 Role of Surface Free Energy in Solid Wetting

4.4 Role of Surface Free Energy in Powder Flow

4.5 Role of Surface Free Energy in Powder Tableting

4.6 Concluding Remarks

References

Chapter 5: Mucoadhesive Polymers for Drug Delivery Systems

5.1 Introduction

5.2 Mucoadhesive Drug Delivery Systems

5.3 Mucoadhesive Polymers

5.4 Summary

References

Chapter 6: Transdermal Patches: An Overview

6.1 Introduction

6.2 Factors Affecting Skin Absorption

6.3 Passive Transdermal Drug Delivery Systems

6.4 Types, Structural Components and Materials Used to Design Passive TDDS

6.5 Active Transdermal Drug Delivery Systems

6.6 Production of Transdermal Patches

6.7 Biopharmaceutical Concerns

6.8 Pharmacokinetics of Transdermal Absorption

6.9 Manufacture, Design and Quality Control

6.10 Commercialized Patches

6.11 Regulatory Aspects

6.12 Summary and Future Prospects

Acknowledgement

References

Chapter 7: Film-Forming Technology and Skin Adhesion in Long-Wear Cosmetics

7.1 Introduction

7.2 Long-Wear Foundation: An overview

7.3 Effect of Skin Substrate on Adhesion

7.4 Long-Wear Technologies in Cosmetic Applications

7.5 Summary and Prospects

Acknowledgements

References

Part 3: Adhesion in the Biomedical Field

Chapter 8: Factors Affecting Microbial Adhesion

8.1 Introduction

8.2 Surface Characterization

8.3 Bacterial Adhesion to Material Surfaces

8.4 Summary

Acknowledgments

References

Chapter 9: Factors Influencing Biofouling and Use of Polymeric Materials to Mitigate It

9.1 Introduction

9.2 Origin of Biofouling

9.3 Prevention of Micro-Organisms Adhesion

9.4 Influence of Mechanical Properties

9.5 Influence of Surface Topography

9.6 Concluding Remarks

References

Chapter 10: Coatings on Surgical Tools and How to Promote Adhesion of Bio-Friendly Coatings on Their Surfaces

10.1 Introduction

10.2 Coatings on Various Surgical Tools and Implants in Different Fields of Operative Care to Patients

10.3 Promotion of Adhesion of Bio-Friendly Coatings on Surfaces of Tools and Implants

10.4 Summary

References

Chapter 11: Techniques for Deposition of Coatings with Enhanced Adhesion to Bio-Implants

11.1 Bio-Implants: An Introduction

11.2 Deposition Methods for Enhanced Adhesion of Coatings on Implants

11.3 Summary

References

Chapter 12: Relevance of Adhesion in Fabrication of Microarrays in Clinical Diagnostics

12.1 Introduction

12.2 Protein Microarrays

12.3 DNA Microarrays

12.4 Antibody Microarrays

12.5 Summary

References

Part 4: Adhesion in the Dental Field

Chapter 13: Antibacterial Polymers for Dental Adhesives and Composites

13.1 Introduction

13.2 Major Damage from Oral Biofilm Formed: The Acid Production

13.3 The Chemistry of Current Dental Adhesives and Composites

13.4 The Need for Treatments Targeting Oral Cariogenic Biofilms

13.5 Classification of Antibacterial Polymers for Dental Materials

13.6 Mechanisms of Action of Antibacterial Monomers

13.7 Antibacterial Properties of Dental Adhesives and Composites Containing Antibacterial Monomers

13.8 Considerations of Mechanical Properties

13.9 Summary and Prospects

Acknowledgments

Summary

Chapter 14: Dental Adhesives: From Earlier Products to Bioactive and Smart Materials

14.1 Introduction

14.2 Adhesion to Dental Substrates

14.3 Adhesive Strategies

14.4 Limitations in Bonding to Dental Substrates

14.5 Strategies to Reduce Bond Strength Degradation – Current Advances

14.6 Summary and Prospects

Acknowledgement

References

Chapter 15: Testing of Dental Adhesive Joints

15.1 Introduction

15.2 Various Bond Strength Tests

15.3 Summary

References

Index

End User License Agreement

Guide

Cover

Copyright

Contents

Begin Reading

List of Tables

Chapter 1

Table 1.1: Examples of adherend and adhesive types in the human body.

Table 1.2: General materials related to adhesion and their assessment methods.

Table 1.3: Comparison of adhesion interactions relative to length scale.

Table 1.4: Orders of scale for adherend-adhesive interactions in the pharmaceutical, biomedical and dental fields.

Table 1.5: Concepts and quantities important in electrostatic adhesion.

Table 1.6: Bond strengths of various types of chemical bonds and intermolecular forces.

Chapter 2

Table 2.1: Parameters for hydrophilicity/hydrophobicity measure of solid surfaces [11].

Chapter 5

Table 5.1: Summary of various techniques used for the assessment of mucoadhesive interactions at the macroscopic level.

Table 5.2: Summary of various techniques for the assessment of mucoadhesive interactions at the molecular level.

Table 5.3: Mucoadhesive dosage forms and polymers used in the formulation [10].

Table 5.4: Classification, categories and examples of different mucoadhesive polymers.

Table 5.5: Lectin modified polymers for the development of mucoadhesive drug delivery systems.

Table 5.6: Thiolated polymers investigated for developing mucoadhesive drug delivery systems.

Chapter 6

Table 6.1: Materials commonly used for the design of transdermal patches.

Table 6.2: Selected marketed transdermal patches along with their therapeutic applications.

Table 6.3: Regulatory aspects for approval of transdermal drug products.

Chapter 7

Table 7.1: Summary of skin surface free energy before and after degreasing on volar forearm and forehead.

Table 7.2: Young’s modulus determined

in vivo

using various methods for different skin regions and ages.

Chapter 8

Table 8.1: Roughness (R

q

) and water contact angles on five different glass surfaces, five different stainless steel surfaces (AISI 304) and polystyrene surface. N means untreated glass surface. 3C is the untreated stainless steel surface.

Chapter 11

Table 11.1: Comparison between different methods used for coating of implants.

Table 11.2: Adhesion strengths (in MPa) of different coatings to Ti and CoCrMo substrates [57].

Chapter 12

Table 12.1: Fabrication parameters with respect to the different fabrication techniques. (adapted from Schena, 2002 [5])

Table 12.2: Fabrication techniques for protein and DNA microarrays. (Reprinted from [4] with kind permission from Royal Society of Chemistry)

Table 12.3: Various methods for coupling proteins on a microarray platform. (adapted from Kambhampati, 2006 [36])

Table 12.4: Examples of commonly available glass-based microarray platforms. (adapted from Kambhampati, 2006 [36])

Table 12.5: Examples of common polymer and hydrogel substrates.

Table 12.6: Examples of substrate modifications with modified chemical groups

Table 12.7: Light induced attachment of ligands

Table 12.8: Tagged proteins and their corresponding immobilization systems which are commercially available

Table 12.9: Various immobilization methods for DNA probes.

Table 12.10: The favorable and non-favorable aspects of functionalized DNA chip surfaces [88].

Table 12.11: Solid phase support materials for coupling of biochemical species [101].

Chapter 15

Table 15.1: Mean bond strength values for ten different dentin adhesives used by private practitioners participating in continuous educational courses.

Pages

ii

iii

iv

xv

xvi

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

308

309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359

360

361

362

363

364

365

366

367

368

369

370

371

372

373

374

375

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

396

397

398

399

400

401

402

403

404

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106

Adhesion and Adhesives: Fundamental and Applied Aspects

The topics to be covered include, but not limited to, basic and theoretical aspects of adhesion; modeling of adhesion phenomena; mechanisms of adhesion; surface and interfacial analysis and characterization; unraveling of events at interfaces; characterization of interphases; adhesion of thin films and coatings; adhesion aspects in reinforced composites; formation, characterization and durability of adhesive joints; surface preparation methods; polymer surface modification; biological adhesion; particle adhesion; adhesion of metallized plastics; adhesion of diamond-like films; adhesion promoters; contact angle, wettability and adhesion; superhydrophobicity and superhydrophilicity. With regards to adhesives, the Series will include, but not limited to, green adhesives; novel and high-performance adhesives; and medical adhesive applications.

Series Editor: Dr. K.L. Mittal 1983 Route 52, P.O. Box 1280, Hopewell Junction, NY 12533, USA Email: [email protected]

Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Adhesion in Pharmaceutical, Biomedical and Dental Fields

Edited by

K.L. Mittal

F.M. Etzler

This edition first published 2017 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2017 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.

Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA

For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com.

Limit of Liability/Disclaimer of WarrantyWhile the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read.

Library of Congress Cataloging-in-Publication Data Names: Mittal, K. L., 1945- editor. | Etzler, F. M. (Frank M.), editor. Title: Adhesion in pharmaceutical, biomedical and dental fields / edited by K. L. Mittal and F. M. Etzler. Description: Beverly, MA : Scrivener Publishing ; Hoboken, NJ : John Wiley & Sons, 2017. | Series: Adhesion and adhesives : fundamental and applied aspects | Includes index. | Identifiers: LCCN 2017015278 (print) | LCCN 2017018821 (ebook) | ISBN 9781119323785 (pdf) | ISBN 9781119323792 (epub) | ISBN 9781119323501 (cloth) Subjects: LCSH: Adhesives. | Biomedical materials. | Pharmaceutical industry–Technological innovations. | Dental bonding. Classification: LCC TA455.A34 (ebook) | LCC TA455.A34 A33 2017 (print) | DDC 610.28/4–dc23 LC record available at https://lccn.loc.gov/2017015278

Preface

The phenomenon of adhesion is of cardinal importance in the pharmaceutical, biomedical, and dental fields. A few eclectic examples will suffice to underscore the importance/relevance of adhesion in these three areas. For example, the adhesion between powdered solids is of crucial importance in tablet manufacture. A mundane example is the bandage where the role of adhesion in its performance (sticking and peeling) is all too familiar. The interaction between biodevices (e.g., stents, bio-implants) and body environment governs the performance of such devices, and there is burgeoning research activity in modifying the surfaces of such implements to render them compatible with bodily components. Essentially, there are two distinct approaches which are exploited in this vein: one is to modify the surface chemistry of biodevices by a host of techniques ranging from simple to very sophisticated (e.g., laser treatment) and the second is to deposit suitable coatings. Irrespective of the intended function of the coating, it must adhere to the substrate; so many schemes have been devised to obtain adequate adhesion. The topic of microbial adhesion and biofouling is of grave concern with wide-ranging implications. In the field of dentistry, there has been paradigm shift from retaining of restorative materials by mechanical interlocking to adhesive bonding; so the mantra “adhesive bonding dentistry” has gained much popularity. Those who wear dentures very well know the importance of adhesion. If dentures come out in a social setting, it can be very embarrassing. In the recent past, there has been much activity in ameliorating the existing adhesives or in formulating stronger and better adhesives.

This unique book addresses all these three areas in an easily accessible single source. The impetus for bringing out this compilation stemmed from the lack of a book dealing primarily and specifically with the adhesion aspects in these three areas. So this book was conceived with the express intention to fill this lacuna in the literature.

The book contains 15 chapters written by internationally-renowned subject matter experts and is divided into four parts: Part 1: General Topics; Part 2: Adhesion in Pharmaceutical Field; Part 3: Adhesion in Biomedical Field; and Part 4: Adhesion in Dental Field. The topics covered include: theories or mechanisms of adhesion; wettability of powders; role of surface free energy in tablet strength and powder flow behavior; mucoadhesive polymers for drug delivery systems; transdermal patches; skin adhesion in long-wear cosmetics; factors affecting microbial adhesion; biofouling and ways to mitigate it; adhesion of coatings on surgical tools and bio-implants; adhesion in fabrication of microarrays in clinical diagnostics; antibacterial polymers for dental adhesives and composites; evolution of dental adhesives; and testing of dental adhesive joints.

This book covering many subtopics highlighting the importance/relevance of adhesion should be of great interest and considerable importance to R&D personnel in pharmaceutical, biomedical and dental industries as well as to researchers in academia and other research labs. Also, advanced graduate students carrying out research in these three disciplines will find it very instructive and beneficial. We sincerely hope this book will spur cross-pollination of ideas in these three seemingly different fields and thus new research vistas will emerge.

Now it is our great pleasure to express our thanks to those who were instrumental in materializing this book. First and foremost, we would like to profusely thank the authors of these chapters for sharing their knowledge and experience and for their interest, enthusiasm and cooperation, without which this book would not have seen the light of day. We would like to extend our appreciation to Martin Scrivener (publisher) for his unwavering interest in and steadfast support for this book project.

Kash Mittal P.O. Box 1280 Hopewell Jct., NY 12533 E-mail: [email protected]

Frank M. Etzler LECOM Erie, PA 16509 E-mail: [email protected] April 2017

Part 1GENERAL TOPICS

Chapter 1Theories and Mechanisms of Adhesion in the Pharmaceutical, Biomedical and Dental Fields

Douglas J. Gardner

University of Maine, Advanced Structures and Composites Center, Orono, ME., U.S.A

Corresponding author:[email protected]

Abstract

Adhesion is an important attribute of material behavior in the pharmaceutical, biomedical, and dental fields that influences the interactions among different substances in the human body, and it is also important as it plays an important role in various processes, including, but not limited to, the manufacture of drugs, medical devices and dental care. Adhesive bonding is an important area focusing on the creation of joined substrates and composite materials. Based on the wide variety of adhesive bonding situations, the concept of adhesion can be broadly applied across different material types and interactions. Mechanisms of adhesion fall into two broad areas: those that rely on mechanical interlocking or entanglement and those that rely on charge interactions. There are seven accepted theories of adhesion. These are: mechanical interlocking; electrostatic theory; adsorption (thermodynamic) or wetting theory; diffusion theory; chemical bonding theory; acid-base theory; and theory of weak boundary layers. In addition, elastomeric-based adhesives exhibit a characteristic adhesion behavior described as tackiness or stickiness that aids in the creation of an almost instantaneous adhesive bond. This chapter provides an overview of adhesion theories and mechanisms relative to applications in the pharmaceutical, biomedical and dental fields.

Keywords: Adhesion, mechanisms, theories, adhesives, bonding, mechanical interlocking, electrostatic, adsorption, wetting, diffusion, chemical, acid-base, weak boundary layers, tackiness

1.1 Introduction

Adhesion mechanisms in the pharmaceutical, biomedical, and dental fields are similar to those encountered in other fields of materials science. However, the biggest challenge is that the adhesion mechanisms will typically occur in or will be influenced by the environment of the human body. The primary challenges facing adhesion in the environment of the human body include: creation of an adhesive bond in contact with various bodily fluids, blood, saliva, etc.; durability of an adhesive bond when exposed to various bodily fluids; the biochemical onslaught related to the body’s immune response and cellular regeneration; and exposure to inherent bodily microorganisms such as bacteria and fungi. Common examples of adhesion in the pharmaceutical, biomedical, and dental fields include the manufacture of respiratory inhalants such as albuterol; the application of medical bandages such as Band-aids® used to cover wounds; and the use of denture adhesives to secure false teeth. It is the goal of this Chapter to provide an overview of the current theories and mechanisms of adhesion with reference to applications in the pharmaceutical, biomedical, and dental fields.

1.1.1 Adherend Material Properties Relevant to Adhesion

In the adhesion science and technology community, most materials to be adhesively bonded or glued are referred to as adherends. Adherends in the human body being bonded are usually in a solid form while adhesives are typically in the liquid form (Table 1.1).

Table 1.1 Examples of adherend and adhesive types in the human body.

Adherend type

Examples

Adhesive type

Examples

Dense Solid

Teeth

Low and medium viscosity liquid

Acrylate adhesives

Porous Solid

Bone

Low viscosity liquid or viscous filled-adhesive

Poly(methyl methacrylate)

Soft Solid

Skin

Low viscosity liquid

Cyanoacrylate adhesives for surgical sutures

The processes of joining materials through adhesive bonding to form a bonded assembly in the pharmaceutical, biomedical, and dental fields are quite variable in terms of adherend types and bonding processes including the strength and durability requirements of the resulting adhesive bond. To better understand adhesive bonding processes, adhesion scientists have characterized adhesion mechanisms or theories based on the fundamental behavior of materials being bonded (adherends) as well as the adhesives used to bond the materials. Understanding adhesion requires a close familiarity with the bulk and surface material properties of the adherend and the material property characteristics of the adhesive being used. A list of general material property features to be considered in studying or assessing adhesion is shown in Table 1.2. Surface properties of interest related to adhesion include topography, surface thermodynamics, chemical functionality, hardness, and surface charge. Adhesive features to be considered include: molecular weight, rheology, curing characteristics, thermal transition of polymers, and viscoelasticity. For the bonded assembly, the ultimate mechanical properties, durability, and biological compatibility characteristics are of major importance. In addition, when considering adhesion in the pharmaceutical, biomedical, and dental fields, one also needs to consider cell adhesion. Cellular adhesion is involved with the bonding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules [1]. Cell adhesion continues to receive considerable attention in the adhesion field.

Table 1.2 General materials related to adhesion and their assessment methods.

Material

Assessment methods

Adherend

• Topography, wettability, chemical functionality, hardness, surface charge

Adhesive

• Molecular weight, rheology, curing characteristics, thermal transitions, viscoelasticity

Bonded Assembly

• Mechanical properties, durability, creep behavior, biological compatibility

1.1.2 Length Scale of Adherend-Adhesive Interactions

The prevailing adhesion theories can be assembled into two types of interactions: 1) those that rely on interlocking or entanglement; and 2) those that rely on charge interactions. Furthermore, it is beneficial to know the length scale(s) over which the adhesion interactions occur. The comparisons of adhesion interactions relative to length scale are listed in Table 1.3. It is obvious that the adhesion interactions relying on interlocking or entanglement, mechanical and diffusion, can occur over larger length scales than the adhesion interactions relying on charge interactions. Most charge interactions involve interactions on the molecular level or nano length scale.

Table 1.3 Comparison of adhesion interactions relative to length scale.

Category of adhesion mechanism

Type of interaction

Length scale

Mechanical

Interlocking or entanglement

0.01–1000 μm

Diffusion

Interlocking or entanglement

10 nm–2 μm

Electrostatic

Charge

0.1–1 μm

Covalent Bonding

Charge

0.1–0.2 nm

Acid-Base interaction

Charge

0.1–0.4 nm

Hydrogen Bonding

Charge

0.235–0.27 nm

Lifshitz-van der Waals

Charge

0.5–1 nm

The length scale of adherend-adhesive interactions is also of importance in understanding adhesion mechanisms because although many practical aspects of adhesion occur on the macroscopic length scale (millimeter to centimeter), many of the basic adhesion interactions occur on a much smaller length scale (nanometer to micrometer) (Table 1.4). Wound protection using a Band-Aid® typically occurs on the cm length scale. Interactions between inhaler droplets in the lung occur on the millimeter length scale, and typical microscopic evaluation of the adherend-adhesive bondlines is performed at the 100 μm length scale. Bacteria are on the order of 0.5 to 5 μm in diameter. Nanoparticles are generally in the scale of 10 to 100 nm in diameter.

Table 1.4 Orders of scale for adherend-adhesive interactions in the pharmaceutical, biomedical and dental fields.*

Scale

Test specimen or material characteristics for determining adherend-adhesive interactions

1 cm, 10 mm

Wound protection using a Band-Aid

®

10

–3

meter, 1 mm

Inhaler droplet interactions in the lung

10

–4

meter, 100 μm

Microscopic evaluation of adherend-adhesive bondline

10

–6

meter, 1–4 μm

Size of bacteria

10

–7

meter, 100 nm

Scale of nanoparticles

*Adapted from Gardner et al. [2].

1.2 Mechanisms of Adhesion

There are seven mechanisms or theories of adhesion [3–5]. These are:

Mechanical interlocking or hooking

Electronic, electrostatic or electrical double layer

Adsorption (thermodynamic) or wetting

Diffusion

Chemical (covalent) bonding

Acid-base

Weak boundary layers

It should be mentioned that these adhesion mechanisms are not self-excluding, and several may occur simultaneously in a specific adhesive bonding situation. The concept of stickiness or tack that occurs in rubber-based or elastomeric adhesives will be discussed in more detail later [6].

1.2.1 Mechanical Interlocking Theory

In the field of adhesion, mechanical interlocking was proposed early in the last century [7, 8]. There have been changing views on the importance of mechanical interlocking in adhesion as analytical methods to study adhesion and our fundamental understanding have improved [9]. Mechanical interlocking of adhesives occurs in porous materials like bone through anchoring within the cellular substrate (Figure 1.1). For mechanically interlocked adherends, there are irregularities, pores, or crevices where adhesives penetrate or absorb into and the mechanical properties of the adherends are involved [10]. Using adhesives in an attempt to repair damaged joints in hip or knee replacement surgery is a good example of bonding a porous structure. In addition to geometry factors, surface roughness has a considerable influence on adhesion. Rougher adherend surfaces produce better adhesion than smooth surfaces. High-level adhesion can be attained by improving the adherend surface properties and mechanical keying can be enhanced by increasing the surface area [11].

Figure 1.1 Micrograph of the porous structure of bone (Courtesy of Michael Mason, University of Maine).

Absorption is an important factor in mechanical interlocking, because absorption affects penetration of adhesives into pores or irregularities on adherend surfaces. Greater absorption produces better adhesion in mechanical interlocking systems [12]. The length scale, which changes according to type of interaction, is another factor that affects adhesion. Mechanical interlocking is strongly dependent on the surface properties. When studying mechanical interlocking, the adherend surface properties including the presence of crevices, pores, roughness, and irregularities should be well characterized. Optimizing the surface properties, for instance, increasing the roughness, of the surface will produce stronger or enhanced mechanical interlocking. A primary limitation of the mechanical interlocking theory is that it does not inherently take into account charge interactions that may also occur in the creation of an adhesive bond.

1.2.2 Electrostatic Theory

The electrostatic mechanism of adhesion was proposed in 1948 [13]. The primary tenet of the electrostatic mechanism is that the two adhering materials are viewed as analogous to the plates of an electrical condenser across which charge transfer takes place and adhesion strength is attributed to electrostatic forces (Figure 1.2) [4]. The concepts and quantities important in electrostatic adhesion are listed in Table 1.5.