Nanomaterials in Drug Delivery, Imaging, and Tissue Engineering E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 841

Veröffentlichungsjahr: 2013

Beschreibung

This comprehensive volume provides the reader valuable insight into the major areas of biomedical nanomaterials, advanced nanomedicine, nanotheragnostics, and cutting-edge nanoscaffolds. The ability to control the structure of materials allows scientists to accomplish what once appeared impossible before the advent of nanotechnology. It is now possible to generate nanoscopic self-assembled and self-destructive robots for effective utilization in therapeutics, diagnostics, and biomedical implants. Nanoscopic therapeutic systems incorporate therapeutic agents, molecular targeting, and diagnostic imaging capabilities and they have emerged as the next generation of multifarious nanomedicine to improve the therapeutic outcome including chemo and translational therapy. Nanomaterials in Drug Delivery, Imaging, and Tissue Engineering comprises fifteen chapters authored by senior scientists, and is one of the first books to cover nanotheragnostics, which is the new developmental edge of nanomedicine combining both diagnostic and therapeutic elements at the nano level. This large multidisciplinary reference work has four main parts: biomedical nanomaterials; advanced nanomedicine; nanotheragnostics; and nanoscaffolds technology. This groundbreaking volume also covers: * Multifunctional polymeric nanostructures for therapy and diagnosis * Metalla-assemblies acting as drug carriers * Nanomaterials for management of lung disorders and drug delivery * Responsive polymer-inorganic hybrid nanogels for optical sensing, imaging, and drug delivery * Core/shell nanoparticles for drug delivery and diagnosis * Theranostic nanoparticles for cancer imaging and therapy * Magnetic nanoparticles in tissue regeneration * Core-sheath fibers for regenerative medicine

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

List of Contributors

Part I: Biomedical Nanomaterials

Chapter 1: Nanoemulsions: Preparation, Stability and Application in Biosciences

1.1 Introduction

1.2 Nanoemulsion: A Thermodynamic Definition and Its Practical Implications

1.3 Stable Nanoemulsion Formulation

1.4 Nanoencapsulation in Lipid Nanoparticles

1.5 Interactions between Nanoemulsions and the Biological Medium: Applications in Biosciences

1.6 General Conclusion

References

Chapter 2: Multifunctional Polymeric Nanostructures for Therapy and Diagnosis

2.1 Introduction

2.2 Polymeric-based Core-shell Colloid

2.3 Proteins and Peptides

2.4 Drug Conjugates and Complexes with Synthetic Polymers

2.5 Dendrimers, Vesicles, and Micelles

2.6 Smart Nanopolymers

2.7 Stimuli Responsive Polymer-metal Nanocomposites

2.8 Enzyme-responsive Nanoparticles

2.9 Carbon Nanotubes

2.10 Hybrid Polymeric Nanomaterials

Acknowledgements

References

Chapter 3: Carbon Nanotubes: Nanotoxicity Testing and Bioapplications

3.1 Introduction

3.2 Cytotoxicity Measurement and Mechanisms of CNT Toxicity

3.3 MSCs Differentiation and Proliferation on Different Types of Scaffolds

3.4 Conclusions

Acknowledgements

References

Part II: Advanced Nanomedicine

Chapter 4: Discrete Metalla-Assemblies as Drug Delivery Vectors

4.1 Introduction

4.2 Complex-in-a-Complex Systems

4.3 Encapsulation of Pyrenyl-functionalized Derivatives

4.4 Exploiting the Enhanced Permeability and Retention Effect

4.5 Incorporation of Photosensitizers in Metalla-assemblies

4.6 Conclusion

Acknowledgments

References

Chapter 5: Nanomaterials for Management of Lung Disorders and Drug Delivery

5.1 Lung Structure and Physiology

5.2 Common Lung Diseases and Treatment Methods

5.3 Types of Nanoparticles (NPs)

5.4 Methods for Pulmonary Delivery

5.5 Targeting Mechanisms

5.6 Therapeutic Agents Used for Delivery

5.7 Applications

5.8 Design Considerations of NPs

5.9 Current Challenges and Future Outlook

References

Chapter 6: Nano-Sized Calcium Phosphate (CaP) Carriers for Non-Viral Gene/Drug Delivery

6.1 Introduction

6.2 Vectors for Gene Delivery

6.3 Modulation of Protection and Release Characteristics of Calcium Phosphate Vector

6.4 Calcium Phosphate Carriers for Drug Delivery Systems

6.5 Variants of Nano-calcium Phosphates: Future Trends of the CaP Delivery Systems

Acknowledgements

References

Part III: Nanotheragnostics

Chapter 7: Organics Modified Mesoporous Silica for Controlled Drug Delivery Systems

7.1 Introduction

7.2 Controlled Drug Delivery Systems Based on Organics Modified Mesoporous Silica

7.3 Conclusions

References

Chapter 8: Responsive Polymer-Inorganic Hybrid Nanogels for Optical Sensing, Imaging, and Drug Delivery

8.1 Introduction

8.2 Mechanisms of Response

8.3 Synthesis of Responsive Polymer-inorganic Hybrid Nanogels

8.4 Applications

References

Chapter 9: Core/Shell Nanoparticles for Drug Delivery and Diagnosis

9.1 Introduction

9.2 Core/Shell NPs from Polymeric Micelles

9.3 Phospholipid-based Core/Shell Nanoparticles

9.4 Layer-by-Layer-Assembled Core/Shell Nanoparticles

9.5 Core/Shell NPs for Diagnosis

9.6 Conclusions

Acknowledgments

References

Chapter 10: Dendrimer Nanoparticles and Their Applications in Biomedicine

10.1 Introduction

10.2 Dendrimers and Their Characteristics

10.3 Biomolecular Interactions of Dendrimer Nanocomplexes

10.4 Potential Applications of Dendrimer in Nanomedicine

10.5 Conclusion

Acknowledgements

Indexing Words

References

Chapter 11: Theranostic Nanoparticles for Cancer Imaging and Therapy

11.1 Introduction

11.2 Multifunctional Nanoparticles for Noninvasive Monitoring of Biodistribution

11.3 Multifunctional Nanoparticles for Monitoring Drug Release

11.4 Theranostics to Image Therapeutic Response

11.5 Conclusion and Future Directions

Acknowledgement

References

Part IV: Nanoscaffolds Technology

Chapter 12: Nanostructure Polymers in Function Generating Substitute and Organ Transplants

12.1 Introduction

12.2 Important Nanopolymers

12.3 Medical Applications

12.4 Conclusion

Acknowledgement

References

Chapter 13: Electrospun Nanofiber for Three Dimensional Cell Culture

13.1 Introduction

13.2 Nanofiber Scaffolds Fabrication Techniques

13.3 Parameters of Electrospinning Process

13.4 Electrospun Nanofibers for Three-dimensional Cell Culture

13.5 Conclusions

References

Chapter 14: Magnetic Nanoparticles in Tissue Regeneration

14.1 Introduction

14.2 Magnetic Nanoparticles: Physical Properties

14.3 Synthesis of Magnetic Nanoparticles

14.4 Design and Structure of Magnetic Nanoparticles

14.5 Stability and Functionalization of Magnetic Nanoparticles

14.6 Cellular Toxicity of Magnetic Nanoparticles

14.7 Tissue Engineering Applications of Magnetic Nanoparticles

14.8 Challenges and Future Prospects

Acknowledgement

References

Chapter 15: Core-sheath Fibers for Regenerative Medicine

15.1 Introduction

15.2 Core-sheath Nanofiber Technology

15.3 Application of Core-sheath Nanofibers

15.4 Conclusions

References

Index

Nanomaterials in Drug Delivery, Imaging, and Tissue Engineering

Scrivener Publishing

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Publishers at Scrivener

Martin Scrivener ([email protected])

Phillip Carmical ([email protected])

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Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts.Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-118-29032-3

Preface

The ability to control the structure of materials has scientists geared up to accomplish what once appeared impossible before the advent of nanotechnology. Now is an auspicious time to generate nanoscopic self-assembled and self-destructive robots for effective utilization in therapeutics, diagnostics and biomedical implants.

Therapeutic drugs encapsulated in precisely fabricated lipid-based nanocarriers can be used in imaging and drug delivery. Chapter 1, “Nanoemulsions: Preparation, Stability and Application in Biosciences,” describes the application of such techniques in the pharmaceutical and cosmetic industries. Available synthetic routes help in the development of polymeric materials for potential use in biomedical applications. Chapter 2, “Multifunctional Polymeric Nanostructures for Therapy and Diagnosis,” describes the fabrication of tuneable polymers by both conventional chemical methods and sophisticated processes (i.e., surface modification) using ionizing radiation. Because synthetic materials are often associated with toxic side effects they need to be regulated. The cytoxicity and inflammatory responses of such engineered nanomaterials are investigated in Chapter 3, “Carbon Nanotubes: Nanotoxicity Testing and Bioapplications.” An overview of novel metalla-assemblies as drug carriers is presented in Chapter 4, “Discrete Metalla-Assemblies as Drug Delivery Vectors.” Notable advantages and limitations of metalla-assemblies are assessed from a biological point of view.

Chapter 5, “Nanomaterials for Management of Lung Disorders and Drug Delivery” summarizes different types of nanoparticles currently available for pulmonary drug delivery. Various mechanisms and challenges in inhalational drug delivery technologies are also discussed in this chapter. A thrust towards the development of novel nanoparticles paved the way for successful cancer diagnosis and treatment. Gene therapy for curing multiple inherited and/or acquired diseases has become an active area of research and development. The use of bioceramics as vehicular media for gene delivery is investigated in Chapter 6, “Nano-sized Calcium Phosphate Carriers for Non-viral Gene/Drug Deilvery.” Chapter 7, “Organics Modified Mesoporous Silica for Controlled Drug Delivery Systems,” details the use of mesoporpous silica as a stimuli-responsive drug delivery vehicle. Chapter 8, “Responsive Polymer-Inorganic Hybrid Nanogels for Optical Sensing, Imaging, and Drug Delivery,” analyzes the recent advancements in responsive polymer-inorganic hybrid nanogels.

Nanoparticle fabrication techniques that produce hydrophilic core/shell and incorporate hydrophobic drugs offer considerable advantages for diagnosis and therapy. Chapter 9, “Core/Shell Nanoparticles for Drug Delivery and Diagnosis,” focuses on nanomedicine for tumor-targeting stimulated release of proteins and their cancer imaging capabilities. On the other hand, dendrimetric network polymer nanoparticles offer certain advantages not available in other materials. Their high surface functionalities provide an opportunity to modify an outer surface and achieve multivalent effects. Chapter 10, “Dendrimer Nanoparticles and Their Applications in Biomedicine,” explores the unique features of this nanomaterial for their successful future applications as biotherapeutics. Chapter 11, “Theranostic Nanoparticles for Cancer Imaging and Therapy,” presents advantages and limitations in the development of nanoparticles for cancer theranostics along with recent progress in the field.

Biocompatible polymeric architecture has gained significant attention in scaffolds for their use in tissue regeneration, tissue adhesives, hemostats, and transient barriers for tissue adhesion. Chapter 12, “Nanostructure Polymers in Function Generating Substitute and Organ Transplants,” highlights various nanoengineerd polymeric materials that are being utilized in function generating substitutes and organ transplants. Similarly, nanofibrous scaffolds are widely studied for tissue engineering. Chapter 13, “Electrospun Nanofiber for Three Dimensional Cell Culture,” describes the fabrication and interesting properties of electrospun nanofiber matrices. An overview on the progress of magnetic nanoparticles for cell-directed tissue engineering and regenerative medicine is presented in Chapter 14, “Magnetic Nanoparticles In Tissue Regeneration.” The fabrication of core-sheath nanofibers and its application in regenerative medicine is discussed in Chapter 15, “Core-Sheath Fibers for Regenerative Medicine.” Magnetic particles are gaining momentum for their use in three-dimensional tissue generation.

This book is written for a large readership including university students and researchers from diverse backgrounds such as chemistry, materials science, physics, pharmacy, medical science, and biomedical engineering. It can be used not only as a textbook for both undergraduate and graduate students, but also as a review and reference book for researchers in materials science, bioengineering, biotechnology, nanotechnology and the medical and pharmaceutical fields. We hope this book provides the reader valuable insight into the major areas of biomedical nanomaterials, advanced nanomedicine, nanotheragnostics and cutting-edge nanoscaffolds. The interdisciplinary nature of the topics in this book will help both young researchers and senior academicians. We are grateful to Martin Scrivener for giving us the opportunity to publish a book on a subject of such high scientific curiosity and importance.

EditorsAshutosh Tiwari, PhDAtul Tiwari, PhDJanuary 1, 2013

List of Contributors

Genseon Ahn received his BS degree in mechanical engineering at Soongsil University, Republic of Korea, in 2008, and MS degree in mechanical engineering at Pohang University of Science and Technology (POSTECH) in 2010. Currently, Mr. Ahn is a researcher at Chung-Ang has University studying tissue engineered scaffold fabrication using natural and synthetic polymers. Mr. Ahn published has 5 articles, participated in 15 conference presentations, and co-authored as 1 patent application.

Nicolas Atrux-Tallau is postdoctoral researcher at the ESPCI Paris Tech. He obtained his doctorate in skin biology and physiology with a pharmaceutical technologies background from the School of Pharmacy, University of Lyon, France. Dr Atrux-Tallau first skin research experience was in Dr. Howard I. Maibach team at UCSF and continues his research in the fields of dermatology, cosmetic and dermo-pharmacy.

Jerome Bibette is professor and director of the LCMD Laboratory at Ecole Supérieure de Physique et Chimie Industrielles (ESPCI), Paris. His research interest lies in colloids: their preparation, stability, phase transition properties and use in biotechnologies. He is also founder of several companies like Ademtech, Raindance Technologies and Capsum. He has published about 120 papers, owns 45 patents, has been a member of the Institut Universitaire de France since 1994 and was awarded Silver Medal of CNRS in 2000.

Emilio Bucio graduated in chemical physics at the National University of Mexico in 1999 where he now works in the Nuclear Science Institute. His fields of expertise are radiation chemistry, fluorinated polymers, and synthesis of smart polymers. Awards Medal “Gustavo Baz Prada” in 1993, Medal “Alfonso Caso” in 1999, and LAS/ANS award for best publication of the year in 2010 by Nuclear American Society Latinoamerican Section. He has published 71 articles in international journals.

Tom J Burdon is an undergraduate student working on stem cell therapy, regenerative medicine and nanodelivery systems as a part of his co-op program in Biomedical Engineering Department, McGill University. His research interest includes stem cells, polymer chemistry and biomedicine.

Fang Chen received a BS in materials science and engineering from The Southeast University in 2008 and an MS in materials science from Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS) in 2011. Her research focused on synthesizing of pH-sensitive controlled-release drug delivery systems.

Angel Contreras-García earned his PhD in 2010 at the National University of Mexico is currently a postdoctoral researcher at Ecole Polytechnique, Montreal. He has published 11 articles and 4 chapters in books related to biomaterials to be applied in the medical field, using non-conventional chemical methods to synthesize polymers.

S.F. D’Souza is currently the associate director of the Biomedical Group and also Head, Nuclear Agriculture and Biotechnology Division, at Bhabha Atomic Research Centre, Mumbai, India, where he coordinates institutional programs on food, agriculture and biotechnology. He is also senior professor at the Homi Bhabha National Institute. He has a PhD in Biochemistry and his major research interest has been in the field of enzymes and microbial technology with special reference to immobilized cells for use in bioprocessing, biosensors, bioremediation and nanotechnology. He has to his credit more than 200 scientific papers and invited reviews in reputed international journals.

Thomas Delmas is a PhD R&D project manager at Capsum, in charge of 2 innovative technological platforms based on nanoemulsions and milifluidics for encapsulation and drug delivery. His research interest lies in research at the interfaces between chemical physics and biology, from cancer diagnosis and therapy, to cosmetics and dermo-pharmacy, having published about 10 publications and 6 patents in these fields.

Mark Ernsting received his PhD from the University of Toronto and is a senior biomedical engineer at the Ontario Institute for Cancer Research. Mark specializes in the design and preclinical evaluation of drug delivery systems, with an emphasis on polymeric materials, and has published 13 papers in this field.

Jingke Fu is currently a PhD student at the Key Laboratory of Inorganic Coating Materials at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, P.R. China. Her research focuses on mesoporous silica nanomaterials based stimuli-responsive delivery systems.

Mathieu Goutayer completed a PhD in chemical physics at the University of Paris VI, France in 2008. He joined Capsum in 2009 where he is responsible for a group focused on the design and development of new encapsulation systems, using microfluidics for perfume and cosmetic products. Dr Goutayer owns more than 15 patents in the fields of encapsulations.

Sang-Hoon Han is the director of Cosmetic & Personal Care Research Institute of Amore-Pacific R&D Center. He received his PhD in polymer science and engineering at Sungkyunkwan University in 2010. His research interest lies in developing novel cosmetic formulations from emulsions, gels, and vesicles. He has published 22 international papers about cosmetics.

Jae Yeon Kim is a graduate student at the College of Pharmacy at Korea University. He received his BS degree from the Department of Biology Education at Korea National University of Education in 2007. His work includes the design and development of sustained delivery system for protein-based drug using nanoparticles.

Jin Woong Kim is an associate professor of applied chemistry at Hanyang University. He earned his PhD in 2000 in industrial chemistry from the Hanyang University. His current research involves developing new techniques to fabricate functional soft materials with novel structures. He has more than 130 publications in his research field.

Kwangmeyung Kim is a principle research scientist at the Center for Theragnosis in Korea Institute of Science and Technology (KIST). He received his PhD degree in 2003 from the Department of Materials Science and Engineering at Gwangju Institute of Science and Technology (GIST), Korea. He joined KIST and developed cancer-specific optical imaging systems. His research focuses on noninvasive cancer-specific molecular imaging and therapeutic/diagnostic nanoprobes; he develops smart nanoplatform technology for future diagnosis and therapy of various diseases.

Hisatoshi Kobayashi is group leader of Biofunctional Materials at Biomaterials Centre, National Institute for Materials Science, Japan. He has published more than 150 publications, books and patents in the field of biomaterials science and technology as well as edited/authored three books on the advanced state-of-the-art of biomaterials.

Prashant Kumta is Edward R. Weidlein Chair Professor at University of Pittsburgh; was Professor at Carnegie Mellon University for 17 years; and is Editor in Chief of Materials Science and Engineering, B, Advanced Functional Solid-State Materials. His interests are in electrochemical, electronic, bone tissue engineering, biomineralization, and non-viral gene delivery applications.

Ick Chan Kwon is the Head of the Center for Theragnosis in Korea Institute of Science and Technology (KIST). He received his PhD in 1993 in pharmaceutics and pharmaceutical chemistry from the University of Utah in 1993. He serves as the president of the Korean Society of Molecular Imaging, as an Associate Editor of the Journal of Controlled Release, as an Asian Editor of the Journal of Biomedical Nanotechnology, and as a member of several journal editorial boards. His current research interests are targeted drug delivery with polymeric nanoparticles and the development of smart nanoplatforms for theranosis.

Soonjo Kwon is MS and PhD in biological engineering from the University of California-Irvine and is serving as faculty at Utah State University, Logan, USA. He is decorated with university academic honors and serves as the Director of the Institute of Biological Engineering, Utah.

Donghyun Lee obtained his Bachelor of Engineering in materials science and engineering from Korea University, Seoul, Korea in 2002 and his MS degree materials science and engineering and PhD in biomedical Engineering from Carnegie Mellon University in 2004 and 2009 respectively. He recently joined the Department of Biomedical Engineering at Chung-Ang University as an assistant professor, after serving as a post-doctoral research associate at the University of Pittsburgh. His main research interests are in the synthesis, structure and properties of nanostructured materials for tissue engineering and non-viral gene delivery applications. He has given 12 invited and poster presentations and is the author and co-author of 11 refereed publications.

Eun Hee Lee is a professor at the College of Pharmacy, Korea University. She received her PhD from the Department of Industrial and Physical Pharmacy at Purdue University in 2007 where she also worked as a postdoctoral fellow from 2007 to 2010. Her current research interests are the solid state chemistry and crystallization.

Hwanbum Lee is a graduate student at the College of Pharmacy, Korea University. He received his BS degree from the Department of Advanced Material Chemistry at Korea University in 2010. His work includes the design and development of molecular imaging probes and therapeutic agents using nanoparticles.

Shyh-Dar Li is a principal investigator at the Ontario Institute for Cancer Research focusing on nanomedicine research. He has published 22 peer-reviewed articles over the last five years and received research funding from major agencies, including Prostate Cancer Foundation, Canadian Institutes of Health Research and National Cancer Institute.

J.S. Melo obtained his PhD in biochemistry in 1990 from Mumbai University, India. Currently, he is a senior scientific officer of the Nuclear Agriculture & Biotechnology Division at Bhabha Atomic Research Centre, Mumbai, India, and is also an associate professor at the Homi Bhabha National Institute. He has developed a number of novel techniques for immobilization of enzymes and cells. His current field of interest is bioremediation, nanoscience and sensors. He has to his credit 30 publications in international journals.

Jyothi Menon is a PhD student in bioengineering, the University of Texas at Arlington. She received her MS in bioengineering, UT Arlington in 2010. Jyothi’s research interests include nanoparticulate drug delivery systems, biomaterials and tissue engineering. She has authored 4 journal papers and is the recipient of William L. and Martha Hughes award’09 and Provost’s level Enhanced Graduate Teaching Assistantship and Fellowship at UT Arlington.

Mami Murakami received her PhD under the supervision of Prof. Kataoka from the University of Tokyo in 2009. Currently, she is working with Dr. Li at Ontario Institute for Cancer Research with a postdoctoral fellowship. Her main interest concerns the nanodevices for cancer therapy.

Kytai T. Nguyen is an associate professor in the Department of Bioengineering, the University of Texas at Arlington and The University of Texas Southwestern Medical Center, Dallas. She received her PhD in chemical engineering with an emphasis in bioengineering at Rice University in 2000. Dr. Nguyen’s research interests are in the field of nanotechnology for drug and/or gene delivery systems, cellular, and tissue engineering. Her group has produced more than 40 peer-reviewed manuscripts, 3 patent applications, 5 book chapters, and numerous conference abstracts and papers

Keun Sang Oh is a research professor at the College of Pharmacy, Korea University. He received his PhD from the Department of Advanced Materials at Hannam University in 2009. He then joined the Center for Theragnosis in Korea Institute of Science and Technology (KIST) as a postdoctoral fellow from 2009 to 2012. His current research interest is nanomedicine for effective diagnosis and therapy.

Young In Park is a professor at the College of Pharmacy, Korea University. He received his PhD in biochemistry at Indiana University in 1987. After the postdoctoral research at Indiana University, he joined the Department of Genetic Engineering as an assistant professor in 1988. His current research interest lies on the development of antiallegic drug.

Arghya Paul received his MSc (A) in biotechnology and PhD in biomedical engineering (Faculty of Medicine) from McGill University, Canada. His thesis work was based on developing new biotherapeutic devices using stem cells and nanobiohybrid gene delivery vector systems for cardiovascular applications. Dr. Paul is currently a postdoctoral fellow in Prof. Ali Khademhosseini’s laboratory at Harvard-MIT Division of Health Sciences and Technology, Harvard Medical School. His research areas broadly include nanobiotechnology, stem cell based bioengineering and biotherapeutic device for medical applications.

Satya Prakash is a Professor of Biomedical Engineering and Artificial Cells and Organs Research in the Faculty of Medicine at McGill University and works as director of McGill’s Biomedical Technology and Cell Therapy Research Laboratory and member of physiology and experimental medicine department. He has published more than 225 peer reviewed research articles and abstracts, been listed as an inventor in 52 approved/pending patents. His primary research interest is developing several innovative research areas based of nanomedicine, artificial cells, probiotics, microencapsulation, cell therapy, tissue engineering, medical device engineering, and other biomedical technology developments.

Wei Shao obtained her MSc in biochemistry from the Biochemistry department, McGill University. Currently, she is a PhD student in Prof. Prakash’s Lab in Biomedical Engineering (Faculty of Medicine) in McGill University, Canada. Her thesis focuses on development targeted nano-delivery systems for genes and chemotherapy drugs for cancer treatment using nanomaterials including polymers, virus-like particles, and carbon nanotubes.

Rakesh Sharma has a PhD in biochemistry and MR imaging and teaches nanotechnology at Florida State University. Dr. Sharma’s main research interest is in nanomaterial toxicity and microimaging and has 89 publications. He has received the highest awards by AACR, ISG and is the editor of three journals.

Yashpal Sharma is working as assistant professor in a Government Degree College in India. He has been awarded NIMS Internship fellowship 2011 to carry out research in the Biofunctional Materials Group, National Institute for Materials Science, Tsukuba, Japan. He is a recipient of the prestigious Young Scientist Award presented by International Association of Advanced Materials (IAAM). He has more than 15 research publications in national and international journals and conference proceedings. He is currently working in the area of biological applications of smart polymers and tissue engineering scaffolds.

Dominique Shum-Tim is a cardiac surgeon at McGill University Health Centre and an associate professor at Cardiothoracic Surgery research division. Over the past several years he has been working on different cardiovascular research projects such as myocardial infarction and restenosis using stem cell and biomedical stents. Prior to this, Dr. Shum-Tim had been as a research fellow in Boston Children’s Hospital working in collaboration with the Massachusetts Institute of Technology polymer scientists to create cardiovascular structures through tissue engineering principles.

Ashutosh Tiwari is an assistant professor of nanobioelectronics at Biosensors and Bioelectronics Centre, IFM-Linkoping University, Sweden as well as Editor-in-Chief of Advanced Materials Letters. He has published more than 125 articles and patents as well as authoring/editing in the field of materials science and technology.

Bruno Therrien completed his undergraduate study at the University of Montreal, and obtained his PhD (1998) at the University of Berne under the supervision of Prof. Thomas Ward. He currently holds a position of associate professor at the University of Neuchatel, Switzerland. With more than 200 publications, his main research interests are supramolecular and bioorganometallic chemistry.

A. Tripathi is currently associated with Bhabha Atomic research Center, Mumbai, India. He received his PhD in biotechnology in 2011 and his research interests are in the area of advanced nanobiomaterials, stem cell research, regenerative medicine, tissue engineering, bioprocess engineering and environmental biotechnology. He received the prestigious postdoctoral K.S. Krishnan Award from the President of India. He conducted his postdoctoral research project collectively at Kyushu University, Japan and IIT-K. He has published 15 peer-reviewed articles and has one patent. He is also an executive board member of the ‘Clini-india’ Clinical Institute, India.

Aniket Wadajkar received his PhD in biomedical engineering from the University of Texas at Arlington in 2012. Currently, he is working as research scientist at Applied DNA Sciences Incorporation, Stony Brook, NY. His research interests include biomaterials and nanoscopic drug carriers. Aniket has authored 18 journal papers and 2 book chapters. He is a recipient of predoctoral fellowship from American Heart Association; Alfred and Janet Potvin Outstanding Bioengineering Student award; and Who’s Who Among American Universities and Colleges award.

Weitai Wu received a BS (2003) and a PhD (2008) from the University of Science and Technology of China. He is currently a full professor of chemistry at Xiamen University, China. His research group is currently working in the smart biomaterials, energy and environmental materials, and supramolecular assembled nanomaterials.

Zhiwei Xie is a postdoctoral fellow at the Department of Bioengineering, the University of Texas at Arlington. He received his PhD degree in polymer engineering in 2010 from Auburn University. He has four peer-reviewed publications and an award of INTC Graduate Research Competition 2009. His research interests involve development of polymeric biomaterials and nanomaterials.

Soon Hong Yuk is a professor at College of Pharmacy, Korea University. He received his PhD from the Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) in 1987. He then joined the Department of Pharmaceutics and Pharmaceutical Chemistry at the University of Utah as a postdoctoral fellow from 1987 to 1989. Also, he worked in Korea Research Institute of Chemical Technology (KRICT) to 1999 and Hannam University to 2010. His current research interest is nanomedicine for effective diagnosis and therapy.

Yang Zhao received a BE in materials from Harbin University of Science and Technology in 2008. He has been a PhD candidate in materials physics and chemistry at the Shanghai Institute of Ceramics, Chinese Academy of Sciences since 2009, working on the development of porous materials for biomedical applications.

Shuiqin Zhou is a professor of chemistry at College of Staten Island and Graduate Center, The City University of New York. Her research interest is focused on smart polymers and hybrid nanomaterials for sensing, imaging, and drug delivery. She has over 90 peer-reviewed journal publications and book chapters.

Yingchun Zhu is a professor of the Shanghai Institute of Ceramics, Chinese Academy of Sciences. Currently, his research areas include drug-delivery system, biosensors, and antibacterial biomaterials, published over 100 scientific papers which have been cited more than 2000 times by other scientists.

Rajesh Vasita obtained his PhD in bioengineering from the Indian Institute of Technology Kanpur, India. Currently he is working as a postdoctoral fellow at the University of Milan, Italy while also serving as an assistant professor at the School of Life Sciences, Central University of Gujarat, India.

Fabrizio Gelain is the scientific vice-director of the Center for Nanomedicine and Tissue Engineering at Niguarda Ca’ Granda Hospital in Milan and head of the Nanomedicine Unit at the “CSS-MENDEL” IRCCS Institute in Rome. He was awarded a PhD in bioengineering by the Polytechnic of Milan in 2005 and has worked at the MIT and at The Lawrence Berkeley National Lab.

Part I

BIOMEDICAL NANOMATERIALS

Chapter 1

Nanoemulsions: Preparation, Stability and Application in Biosciences

Thomas Delmas1 Nicolas Atrux-Tallau1,4 Mathieu Goutayer1 Sang Hoon Han2 Jin Woong Kim3 Jérôme Bibette4

1Capsum, Marseille, France

2Amore-Pacific Co. R&D Center, Bora-dong, Giheung-gu, Yongin-si, Gyeonggi-do, South Korea

3Department of Applied Chemistry, Hanyang University, Gyeonggi-do, South Korea

4ESPCI ParisTech, Lab Colloides and Mat Divises, Paris, France

Abstract

Nanoencapsulation is being thoroughly investigated for the encapsulation and delivery of actives and/or contrast agents. Our approach allows for better solubilization, protection, transportation, and delivery of encapsulated molecules to their biological site of action. This is expected to increase treatment efficiency while reducing possible side effects through dose reduction and/or targeted delivery. Among other nanocarriers, lipid nanoparticles are biocompatible, biodegradable, can be easily produced by up-scalable processes and, depending on the lipid physical state, may allow control of the release of encapsulated molecules.

This chapter will explain how nanoemulsions can be efficiently used as nanocarriers for drug delivery and imaging. We will first emphasize the importance of specific formulation to reach long-term physical stability of the nanoparticles in simple and more complex formula. We will then highlight how the lipids’ physical state dramatically impacts the actives encapsulation and release behaviors. Finally, we will explore the interaction of nanoemulsion with biological media in terms of biocompatibility and targeting possibilities. Differences between the two main application domains envisaged, namely pharmaceutics and cosmetics, are detailed, and implications for nanoemulsion preparation are discussed.

Keywords: Nanoemulsion lipid nanoparticles entropic stabilization trapped species active encapsulation release kinetics passive targeting active targeting

1.1 Introduction

The use of nanostructured materials is envisioned to revolutionize biosciences and biomedical applications through earlier and more acute diagnosis, or personalized and controlled therapy [1, 2]. For this purpose, numerous nanocarrier types have been proposed as delivery vehicles for contrast agents or drug molecules for all possible administration routes [3]. Nanocarriers can indeed significantly improve therapeutic efficiency while limiting possible undesirable effects, through specific delivery to the pathological zone and control over active molecules release [4]. One of the first requirements for these systems is to present an absolute harmless-ness and biocompatibility. It is for this reason that the scientific community has principally focused its work on the development of organic particles.

Among a wide variety of nanocarriers, lipid-based systems have aroused interest because of their composition which is based on natural lipids already widely present in the organism, and therefore confers such carriers with high biocompatibility and biodegradability [5, 6]. As previously shown, these lipid nanocarriers can furthermore be easily produced by versatile and up-scalable processes [7-9], and can provide the possibility to control actives encapsulation and release [10].

Hydrophobic molecules encapsulation favors the use of lipid nanospheres, instead of the originally developed nanocapsule initiated by the liposomes discovery in 1964 [11]. The typical structure of a lipid nanosphere is directly derived from nanoemulsion structure, typically relying on a lipid core surrounded by a membrane of diverse surfactants. The history of lipid nanospheres development has followed the understanding of the importance of lipids physical state:

Classical nanoemulsions

were the first lipid nanospheres to be introduced decades ago. These systems were composed of a liquid lipid core, stabilized by a membrane of surfactants. Despite the interest of these initially developed systems for solubilization of lipophilic actives in aqueous phases, few finally reached the market due to formulation issues. Indeed, these systems used to suffer from low colloidal stability (even though this has been dramatically improved as will be shown here), and sustained release of encapsulated actives is difficult to achieve due to the low viscosity of the dispersed phase [12], high surface/volume ratio, and low rigidity of the surfactants membrane. This generally leads to the rapid diffusion of the drugs out of the droplets.

Solid Lipid Nanoparticles (SLN)

have thus been proposed to overcome these limitations. These systems present a structure identical to nanoemulsions. However, the internal lipids forming the nanoparticle core are crystalline lipids here, conferring a solid nature to the particle’s core (

Figure 1.1

). SLN are thus generally composed of pure long chain triglycerides, wax or long chain carboxylic acids [8]. They can be stabilized by all types of surfactants; their choice being principally dictated by the administration route envisaged [8]. SLN fabrication processes are similar to nanoemulsions, but the lipid phase is generally heated above the lipids fusion temperature to favor droplet size reduction. Lipid crystallization then occurs following cooling and storage. However, despite high expectations of such systems for prolonged release of hydrophobic molecules, SLN have shown limited controllability. As will be further explained, crystallization of the lipid phase generally leads to active/lipid phase separation and subsequent expulsion, providing high burst release [13, 14].

Nanostructured Lipid Carriers (NLC)

were introduced as a compromise. Composed of a mixture of liquid and solid lipids, the NLC core presents an imperfect crystallization which favors better encapsulation ratio thanks to lower crystallinity, while allowing control over release kinetics through the solid character of the lipid phase [15]. Three different types of NLC have been proposed (

Figure 1.1

): 1) the imperfect type, whose crystallinity is lowered by creating imperfections in the crystal lattices; 2) the structureless type, which is solid but amorphous; and 3) the multiple O/F/W type, in which small droplets of liquid lipids are phase separated in the solid matrix [16]. Although these 3 types can theoretically be obtained, the mixture of spatially incompatible lipids generally leads to the obtainment of the first type NLC [17, 18]. Several studies have reported the obtainment of the forms II and III, however, there is discrepancy in the conclusions. Some authors thus account for supercooled melt rather than amorphous solid particles concerning the structureless type [15, 19, 20]. Similarly, the same system was described as a typical multiple oil in fat in water type (O/F/W) [21], while a complete demixing of oil from wax occurs in the so-called “nanospoon structure” [22–24].

Figure 1.1 Different types of solid lipid nanospheres.

The further development of these nanoemulsion-based systems for application in biosciences should rely on the complete understanding of nanoemulsion physicochemical properties, production procedures, stability rules, and on control over the internal lipids physical state. We will present here a general approach that can be followed to formulate such systems aiming for biomedical applications. After giving a thermodynamic definition of nanoemulsions, we will first describe possible production procedures and explain the general rules that need to be followed to formulate stable nanoemulsions. We will then investigate the role of the lipids physical state over particle stability, actives encapsulation and release, along with the biocompatibility of the particles. Next we will show how this understanding allows for finely tuning nanoemulsions properties in order to have control over the applicative properties of biodistribution and actives encapsulation/release. Two main domains of application are then detailed through examples of nanoemulsion-based systems for biomedical imaging and drug delivery in the pharmaceutical field, and topical delivery for cosmetics.

1.2 Nanoemulsion: A Thermodynamic Definition and Its Practical Implications

1.2.1 Generalities on Emulsions

An emulsion is a mixture of two immiscible liquids, one liquid being dispersed in the other as droplets, stabilized by surfactants. The size of the droplets may vary from the characteristic size of micelles (10–20 nm) to diameters larger than the micrometer. This mainly depends on the surface tension of the system and the energy provided by the production process [25]. Although direct (oil in water) and inverse (water in oil) similarly exist, we will focus on direct emulsions.

1.2.2 Nanoemulsion vs. Microemulsion, a Thermodynamic Definition

Among emulsions, nanoemulsions differ from micro- or macroscopic emulsions because of their size, ranging from 20–200 nm, which confers upon them unique visual and rheological properties. Because such nanometric size is well below the visible spectrum wavelength (400–800 nm), light is not significantly diffused, even for large refractive index differences between dispersed and continuous phases, making the dispersion transparent, or at least translucent. These unique optical and rheological properties make nanoemulsions of primary interest in such domains as cosmetics and pharmaceutics for the encapsulation and delivery of actives.

Nanoemulsions are thus specific emulsions of nanometric size. Here we need to clearly differentiate them from microemulsions. Indeed, although the two systems present similitudes in terms of composition (ternary system of oil/water/surfactants), droplets size (20–100 nm), and therefore optical appearance and rheological properties, and may finally present similar apparent stability (system stable at one day, one month or even one year), nanoemulsions and microemulsions are in essence completely different systems.

Nanoemulsions

are emulsions, and consequently they are out-of-equilibrium systems. The resulting system is not at the equilibrium because the two phases present a surface tension (>0) which will lead the system to minimize its interfacial area until the obtainment of the thermodynamic equilibrium: two separated phases of oil and water [25, 27].

Microemulsions

(also referred to as “swollen micelles”) are thermodynamically stable systems [31–33]. Microemulsions are also formed by two immiscible liquids (oil and water), one surfactant, and possibly one co-surfactant. Nonetheless, the dissolution of amphiphilic molecules and oil in water leads to the obtainment of a unique phase, optically isotropic, of swollen micelles defining the thermodynamically stable state of the system [34]. It means that, from a thermodynamic point of view, the system is stable, and the phase separation of oil and water will not decrease the free energy of the system. Microemulsions are therefore not emulsions in the thermodynamic sense.

These differences can be highlighted by looking at energy landscape diagrams. Microemulsions, as micelles, are thermodynamically-stable systems, being formed by spontaneous self-assembly (ΔGA→B, < 0) (Figure 1.2a) [33]. Conversely, nanoemulsions are metastable systems that require high energy input to be formed (ΔGA→B” > 0) (Figure 1.2b) [29, 30]. They do not possess a thermodynamic stability, but may sometimes present a kinetic stability ranging from a few hours to a few days, or even to several years [27, 29, 35]. This kinetic stability will, or will not, be observed depending on the height of the energetical barrier ΔGbarrier which separates this nanoemulsion state (B”) from the thermodynamically favored state of phase separation (A). If this barrier is lower, or close to the thermal energy kBT, the system will be unstable; conversely, the system may be blocked in this nanoemulsion state for a long period of time if the barrier is higher. These differences have still been shown to be maintained whatever the close proximity between a nanoemulsion state and a microemulsion state in the pseudoternary (oil/water/surfactant) phase diagram [36].

Figure 1.2 Thermodynamical differences between the microemulsion state (a) and nanoemulsions; (b): Energy diagrams.

Like common emulsions, nanoemulsions are mainly used to solubilize hydrophobic species in a water continuous phase. Nanoemulsions differ from common macro- or microscopic sized emulsions because of their size ranging from 10–200 nm that confers them with unique optical and rheological properties. They are nonetheless true emulsions, in the thermodynamic sense, and dramatically differ from microemulsion, being metastable systems. This implies that they generally require high energy input for their formation and a specific strategy for their stabilization [36].

1.3 Stable Nanoemulsion Formulation

After introducing the specificity of nanoemulsions compared to classical emulsions and microemulsions, the aim of this section is to define possible production processes, discuss ways of stabilizing these metastable systems, and describe the accessible formulation domain giving nanoemulsions possessing long-term physical stability.

1.3.1 Nanoemulsion Production

1.3.1.1 Fabrication Procedures

As previously shown, the formation of nanoemulsions from two separated phases of oil and water generally requires energy input. Considering the aimed small droplets sizes, and thus the large quantity of interface to generate, a huge amount of energy needs, in fact, to be provided to the system. Droplets are split when the applied shear rate is larger than the Laplace pressure. In the case of very diluted dispersion, the Taylor equation obtains the order of magnitude of the shear rate required to reach droplets of a certain size dp (Eq. 1.1) [37]:

(1.1)

with γ, the surface tension (N·m−1); ηc, the continuous phase viscosity (Pa·s) and , the shear rate (s−1).

For instance, the obtainment of 20 nm diameter particles requires the use of shear rates close to 109s−1 [30, 37, 38]. Few devices allow the obtainment of such high shear rates. The most commonly used are the high frequency ultrasonic devices, the high pressure homogeneizers (HPH), and the microfluidizers [30, 39]. Microfluidizers seem to be more efficient than sonication and HPH when aiming for sample homogeneity; nonetheless, production costs are far more important, lowering the viability of its industrial use [40]. The use of ultrasounds for emulsification is increasingly studied for its lower energy consumption, its use of fewer amounts of surfactants, and the obtainment of smaller sizes for more homogeneous products than classical mechanical processes [29, 39, 40]. In addition, sonication is a very flexible procedure that allows working with smaller or more viscous samples rather than with high-pressure homogeneizers [40]. Yet, HPH is an interesting technique for industrial production, as it may allow the easy production of large amounts of nanoemulsion.

It has nonetheless to be noted that nanoemulsions can also be obtained thanks to low energy processes that rely on passages through microemulsion states. The PIT (for Phase Inversion Temperature) approach is among one of the most employed. Its principle relies on the modification of PEG surfactant solubility along a temperature change [41]. It is possible to force the system to continuously pass from a direct microemulsion state to an inverse microemulsion state by a simple temperature increase [42]. Doing a cycle around the HLB temperature, corresponding to the bicontinuous phase of zero spontaneous curvature, it is possible to obtain direct microemulsions that will ultimately lead to nanoemulsion by further decreasing the temperature [35, 43]. A similar principle is used by the CPI (Catastophic Phase Inversion) techniques. In this case, formulations parameters, such as salt concentration or dispersed phase concentration, are used to change the sign of the spontaneous curvature, thus blocking the microemulsion state formed (most of the time through an important dilution) [35]. These low energy methods therefore allow for the obtainment of nanoemulsions. However, the final nanoemulsion properties are dictated by the initial microemulsion, which dramatically restrains the accessible formulation domain [29, 35, 44].

Among all these preparation procedures, we have focused our work on ultrasonic emulsification at laboratory scale, while exploring High Pressure Homogeneization at a larger industrial scale, in order to ensure the preparation of small droplets with easy to implement procedures and low production costs.

1.3.1.2 Nanoemulsion Preparation by Ultrasonication

By using ultrasonication it is also possible to model the size decrease along the preparation procedure. Once a pre-emulsion is formed, the droplet size follows a first-order exponential decay of the sonication time (Figure 1.3a) [36]. The exponential decay suggests that the droplets may reach their final size y0 in a single step during sonication, and do not significantly undergo coalescence as in the “surfactant-rich regime” defined by Taisne et al. [45].

Figure 1.3 Nanoemulsion preparation by ultrasonication: (a) typical droplet size evolution along sonication; (b) droplet size evolution as a function of the energy input.

The characteristic decay time is mainly governed by the energy input, while the saturated size y0 is determined by the characteristic surface tension of the system. The predominant factor affecting the kinetics of droplet size reduction is the energy density provided to the system [36]. It is thus possible to rescale all obtained kinetics to a unique master curve when plotting the size reduction as a function of the total energy provided to the system along sonication (Figure 1.3b). This concept should be generalizable to all preparation procedures, as long as the energy provided is sufficient to counteract the Laplace pressure. The obtained saturated size y0 is then mainly governed by the types and concentrations of the introduced surfactants. Increasing the PEG chain length of PEG-stearate surfactants leads, for instance, to larger particles, as such a change increases the corresponding characteristic surface tension.

As detailed, nanoemulsions being true emulsions, in a thermodynamic sense, they require large energy input for their formation. In addition, this metastable state also additionally needs to be stabilized to prevent it from reaching its favored thermodynamic state of phase separation.

1.3.2 Nanoemulsion Stability Rules

1.3.2.1 Nanoemulsions Destabilization Mechanisms

As previously introduced, nanoemulsions are a thermodynamically unstable system, even though a kinetic stability may sometimes be observed [27, 44, 46]. The small sizes of nanoemulsion droplets prevent them from undergoing reversible destabilization mechanisms linked to gravity. Indeed, the migration velocity of the droplets is governed by the Stockes law (Eq. 1.2):

(1.2)

Equation 1.2 Stockes Law: v, droplet speed of migration (m·s−1); g, gravity acceleration (m·s−2); Δρ, volumic density difference between continuous and dispersed phases (kg·m−3); r, droplet radius (m); ηc, dynamic viscosity of the continuous phase (Pa·s).

The rate of creaming or sedimentation is thus proportional to the radius square, and is thus dramatically reduced in case of nanometric droplets. In addition, flocculation, which may lead to the obtainment of particle aggregates of sizes favorable to this force, are not favored for such small sizes as adhesion decreases with the droplet radius [47, 48]. The Brownian motion therefore becomes predominant, favoring the dispersion homogeneity. The destabilization of such a system thus mainly relies on the irreversible mechanisms of Ostwald ripening and coalescence [29, 30, 35].

Ostwald ripening is defined by the growth of the largest droplets at the expense of the smallest ones. This phenomenon is due to the oil chemical potential difference between droplets of different sizes and thus different curvature radius. As the chemical potential increases when the radius decreases, because of the Laplace pressure (PLaplace ~ 1/r), the smallest droplets tend to give material to the largest ones by diffusion through the continuous phase. The time evolution of a droplets population undergoing Ostwald ripening is described by the LSW theory (Lifshitz-Slyozov-Wagner) [49, 50]. The droplet size increase is in this case proportional to the cube root of time and the size distribution is autosimilar (Eq. 1.3).

(1.3)

Equation 1.3 Time evolution of nanoemulsions size due to Ostwald ripening (LSW theory) with ω, the Ostwald ripening; and r, the droplets radius; C(∞) and Vm, being respectively the infinite solubility in the continuous phase and the molar volume of the component forming the dispersed phase; D and ρ, respectively being its diffusion coefficient and its density; finally R and T, respectively being the ideal gas sonstant and the temperature.

Coalescence occurs when two droplets collide and finally merge to become a larger droplet. In case of coalescence being the main destabilization phenomenon, the time evolution of the average droplet size can follow very different behaviors from perfectly homogeneous growth (monodal distribution whose average size increases with time) to strongly heterogeneous growth (plurimodal distribution with the possibility of very early phase separation). In the case of homogeneous growth, a mean field law can be used to describe the growth of the droplets [51–53] (Eq. 1.4). By integrating this equation, a decreasing linear variation of 1/r2 as a function of time is obtained.

(1.4)

Equation 1.4: Time evolution of nanoemulsions size due to coalescence (homogeneous case). With r, the droplet radius, α, a geometric parameter; ω, the frequency of canal opening between 2 adjacent droplets per surface unit (fusion phenomenon at the origin of coalescence) et dr, the droplet size increment during the time interval dt.

In a nutshell, the small size of nanoemulsions protects them from undergoing reversible phenomena such as creaming and sedimentation. This small size also has a deep implication on the irreversible mechanisms of Ostwald ripening and coalescence. Indeed, the adhesion between droplets significantly decreases with their diameter [47, 48], therefore decreasing the time of contact when it occurs. The probability of coalescence is thus dramatically reduced for small droplets, especially when the interface is charged, favoring electrostatic repulsion, or covered by long hydrophilic polymer chains, favoring steric repulsion. Conversely, Ostwald ripening is favored by elevated Laplace pressure, which can reach several atmospheres for droplets of a few decades of nanometer (PLaplace ~ 1/r) [27, 29].

1.3.2.2 Ostwald Ripening as Main Destabilization Mechanism

An example can be given using a simple system composed by short triglycerides (hydrocarbon chains C8–C10) stabilized by a PEG40-stearate surfactant. Once formed, the system quickly undergoes destabilization which leads to an average droplet size increase. As shown in Figure 1.4a, the cube of the radius follows a linear increase over time, thus highlighting the predominant role of Ostwald ripening in the destabilization of this system. As previously demonstrated, Ostwald ripening highly depends on the temperature, following an Arrhenius law (Figure 1.4b) [36].

Figure 1.4 Classical nanoemulsion destabilization by Ostwald ripening: (a) size evolution along time; (b) Ostwald ripening rate as a function of temperature.

Therefore, for nanoemulsions, the most important destabilization phenomenon to overcome is related to Ostwald ripening. We will now describe a strategy that can be used to limit its practical implications on nanoemulsion stability thanks to the “trapped species” approach.

Ostwald ripening relies on material exchange between droplets through the continuous phase. The dispersed phase solubility in the continuous phase is thus of high importance. Reducing oil solubility in water can thus be envisaged to significantly lower Ostwald ripening. It can be achieved by using apolar oil and/or adding salt to the continuous phase. Another approach can nonetheless be dramatically more efficient while giving access to a larger stable formulation domain: entropic stabilization through trapped species [54–56]. This approach consists of adding insoluble species to the dispersed phase. Emulsions formed by a unique component A will evolve to a larger size via Ostwald ripening as long as A possesses even a very low solubility in the continuous phase. This is due to chemical potential differences between droplets of different sizes arising from Laplace pressure differences (Eq. 1.5a). When a second component B, insoluble in the continuous phase, is added to the dispersed phase, a new term appears in the difference of chemical potential (Eq. 1.5b) [54, 56–58]. This term is an entropic contribution of the A/B mixture favoring their mixture. As B is blocked inside the droplets, it will limit A transfer among droplets in order to preserve the A/B mixture. The previously unstable system can consequently acquire a true thermodynamic stability. This approach can be efficiently implemented by addition of a multitude of insoluble species in both the core (Fig. 1.5a) and at the droplet membrane (Fig. 1.5b) (Eq. 1.5c) [36]. In this case, numerous entropic terms are added to the equation, further improving the droplets stability (Eq. 1.5c). The example described in Figure 1.5 uses a complex wax (containing insoluble species such as long chain triglycerides) and lecithin as trapped species in the core and at the membrane respectively.

Figure 1.5 Nanoemulsion stabilization against Ostwald ripening: trapped species in core (a); and at the membrane (b) through addition of wax and lecithin, respectively.

(1.5)

Equation 1.5 Chemical potential difference of an encapsulated component: case of a unique component (a), and cases of the addition of one (b) or multiple (c) insoluble species.

It is possible to obtain an order of magnitude of the required amount of insoluble species for droplet stabilization against Ostwald ripening by equaling the osmotic pressure to the Laplace pressure (Eq. 1.6).

(1.6)

Nanoemulsions are thus true emulsions of nanometric size, and are therefore thermodynamically metastable systems. This means that they require energy input for their formation and a specific strategy for their stabilization. We have shown that it is possible to: 1) produce nanoemulsions with a simple and easily up-scalable procedure relying on high energy processes such as ultrasonication or HPH, and 2) entropically stabilize the formed droplets through the addition of trapped species in both the core and at the membrane. Stabilization through specific membrane composition opens up new formulation opportunities by allowing fine tuning of the core composition for solubilization/release properties of the encapsulated active, but also for modulating the sensory profile of the final product. It is indeed well known that low molecular oils provide, for instance, a light and dry touch, whereas high molecular weight oils are usually experienced as heavy and greasy [59]. We will now investigate the accessible formulation domain of a nanoemulsion system designed with the aforementioned rules and explore its abilities for active encapsulation and release.

1.3.3 Nanoemulsion Formulation Domain

Understanding both nanoemulsion production and stabilization, we can now exploit the defined rules to evaluate the largest formulation domain giving small nanoemulsions presenting long-term stability.

1.3.3.1 Components Choice

The choice of the components used for nanoemulsion formulation has a deep influence on nanoemulsion stability, and active encapsulation and release, but also on its biocompatibility [60, 61]. The description of the investigated system is given in Figure 1.6.

Figure 1.6 Description of the model system (from ref [60]).

Aiming at biomedical applications, the component choice has been restrained to components already widely used and accepted for human application in both pharmaceutics and cosmetics [60]. A vegetable oil and a phospholipid surfactant have been chosen for their recognized biocompatibility. The addition of a semisynthetic wax allows for the obtainment of a core mixture of oil and wax, composed of saturated and unsaturated long chains of triglycerides, which ensures a limited solubility in the continuous water phase and therefore limits the degradation by Ostwald ripening. As will be shown, this liquid and solid lipids mixture also allows for fine-tuning the lipid core physical state, which provides an opportunity to modulate the actives encapsulation and release properties. Finally a hydrophilic co-surfactant is chosen among the PEG-stearate, aiming for a steric barrier preventing coalescence and favoring small particle diameters. The combination of PEG stearate and phospholipids surfactants at the membrane further improves stabilization of the droplets against Ostwald ripening.

1.3.3.2 Nanoemulsions Formulation

The most important parameters for controlling applications are the size and the stability of the formed nanoemulsions. All formulation parameters can in theory influence both parameters. A “one-variable-at-a-time approach” may be used to explore the formulation domain. Nonetheless, this empirical approach can be very time-consuming and can lead to misinterpretations. The methodology based on the design of experiments overcomes these issues and presents many more advantages [62, 63]. This approach maximizes the number and quality of obtained information, while minimizing the required experimental effort, explaining its growing use for both academic and industrial purposes. In addition, the obtained experimental design further allows particle optimization through questioning with specific requirements linked to size, stability, or others evaluated by physicochemical parameters.

That is why we exploited a specific design of experiment to investigate this system formulation domain [60]. Frontiers have been chosen to restrain the study to systems possessing a mixture of both surfactants (for stability purposes), over a domain allowing the highest encapsulation possibility (the biggest core), while keeping small nanoemulsions sizes. Formulation parameters (weight fractions of core, hydrophilic surfactant and lipophilic surfactants, and continuous phase) have consequently been simultaneously changed, and the effect on particle size, polydispersity, and also on the dispersion appearance (homogeneity, transparency) and viscosity have been followed (Figure 1.7a). At this stage the lipid core mixture has been fixed to 75% wax/25% oil. The Figure 1.7b details the results obtained for particle size. It highlights the accuracy of the model to a priori determine the size of the droplets formed on the size range 20–200 nm.

Figure 1.7 Formulation domain definition through an experimental design: (a). design of experiment definition; (b). correlation plot between experimental and predicted particle diameters (from ref [60]).

Important formulation parameters governing the droplet size have been shown to be: i) the relative viscosities of dispersed and continuous phase (here it mainly depends on the oil/wax mixture of the core), ii) the types and relative concentrations of both surfactants, and iii) the relative proportion of surfactants over the dispersed phase.

This model has finally been used for formulation optimization in order to obtain reproducible droplets population of a specific size and presenting long-term stability. Figure 1.8 presents the obtained monodisperse populations of standard size ranging from 30–120 nm (FXX, with XX defining the average particle size) and their subsequent stability. The use of the aforementioned rules for stability has allowed for the obtainment of very stable formulations, as these nanoemulsions present more than 18 months stability at 4°C, room temperature and 40°C.

Figure 1.8 Standard formulations: (a) size distribution (FXX, with XX the average size of the particles); and (b) accelerated stability (40°C).

Further formulation studies have additionally proven the long-term stability of nanoemulsions possessing different core composition. Nanoemulsions were thus seen to be stable over the 4–40°C range, whether their core ranged from pure oil (NCO) to pure wax (NC100) (Figure 1.9) [60]. Finally, increasing the dispersed phase weight fraction Φ allows for the modification of the dispersion viscosity from highly liquid to gel-like suspensions, while maintaining nanoemulsion stability (Figure 1.10) [60]. Similarly, nanoemulsions can be dispersed in aqueous gels or even biphasic systems, such as cream or macroscopic emulsion, without significant destabilization of the system.

Figure 1.9 Core composition modification: F50 standard formulation with varying wax/oil ratio (NCXX, with XX defines the %w/w of wax in the oil/wax mixture) and stability.

Figure 1.10