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While the first use of a gel probably dates back to the time of the ancient Egyptians,1  the first mention of a “hydrogel” appeared in the scientific literature in 1894.2  While the gel described in this early study was a colloidal gel of copper oxide rather than what is considered today as a hydrogel (in which a polymeric structure holds a solvent), interestingly, it is the first reference to a soft matter system consisting of two phases that exhibits macroscopic properties typical for a gel. The science of modern gels really took off in the sixties when different applications of gel systems were found for controlled drug delivery, food products, adhesives, toothpastes and beauty products. More recently, gels have been applied in tissue engineering and nanoscience.

With such increasing interest in the use of gels, an exponentially growing active field of research has emerged that studies the physical and chemical mechanisms of gels. Indeed, a fundamental understanding of gelation mechanisms, transport within gels and the microstructure of gels enables tailoring gels for specific applications. Analytical physical chemistry methods such as rheology, optical spectroscopy, differential scanning calorimetry, light and X-ray scattering and electron microscopy have been used extensively in studying gel systems, in particular the sol–gel transition. More recently, the versatility of nuclear magnetic resonance (NMR) and NMR imaging (MRI) has opened up an entire new range of analytical methods to study gels, both in terms of their structure and their dynamics such as the sol–gel transition, swelling, transport and binding of other substances within the gel. Not only does NMR help in studying gels, gels can also be employed in several studies that involve NMR or MRI.

This book is composed of two parts: In the first part, the fundamental physical concepts of gels and NMR techniques to study gel systems are reviewed. The second part is dedicated to the application of gels in food technology, in the life sciences and in medical practice to validate radiotherapy and new MRI techniques.

The first chapter, written by Madeleine Djabourov, a leading scientist in gel chemistry, provides a brief general overview of gels, gel chemistry and their physical properties. Different types of gels and their characteristics are discussed and some novel gel applications are mentioned.

The second chapter, written by Yury Shapiro, a pioneer in NMR of gel systems, reviews the different analytical NMR spectroscopic methods that are applied to study gels. Where conventional 1D NMR spectroscopy can help in determining the chemical composition and stereoregularity, several 2D NMR spectroscopic techniques and multiple-quantum NMR spectroscopy provide detailed information on gel morphology, internal chain mobility and intermolecular interactions. The potential of magic angle spinning (MAS) solid state NMR in the determination of the structure of gels is discussed.

The third chapter, written by Faith Descallar and Shingo Matsukawa, reviews the use of pulsed field gradient (PFG) NMR in studying the diffusion of the solvent within the gel matrix as well as tertiary macromolecular components. In 2D NMR diffusion experiments, polymers can be used as a probe to derive microstructural properties of the gel such as the hydrodynamic mesh size.

In the fourth chapter, Mick Mantle and Daan de Kort provide a comprehensive overview of the use of micro-imaging to study transport of substances in hydrogels as occurs in drug release systems. Firstly, NMR imaging principles and quantitative MRI parameters are discussed with inclusion of state-of-the-art fast imaging techniques. Secondly, several time-resolved applications of micro-imaging in hydrogels are discussed. Examples of MRI studies are given of drying and swelling of hydrogels, tablet hydration and dissolution, drug release kinetics and the in vivo application of an ocular ion-sensitive gel.

There is growing interest in “molecular gels” that are self-assembled from low-molecular-weight building blocks. These gel systems are found in future high-tech applications such as in solar cells, fuel cells, catalysts, sensors and lubricants. Chapters five and six review the use of NMR spectroscopy in studying molecular gels. In chapter five, Shingo Tamesue gives an overview of NMR techniques of supramolecular gels. While solution state NMR has been applied successfully to hydrogels, it is severely compromised when the water solvent fraction is reduced or absent. Solid state NMR is gaining interest as a complementary technique to other methods because of its unique ability to provide structural information on low-molecular-weight gels at different stages of their self-assembly process. In chapter six, Nonappa and Erkki Kolehmainen discuss solid state NMR in studying xerogels, aerogels and native gels. Several examples of multi-nuclear solid state NMR of molecular gels are discussed.

In chapter seven, Philip Kuchel and Dmitry Shishmarev describe how gels can be used as an artificial extracellular matrix for in vitro NMR studies on viable biological cells. Methodological approaches of cell matrices are discussed and examples of metabolic NMR studies are given. An exciting application of gels in deforming red blood cells in an NMR spectrometer is proposed and it is shown how 13C NMR and 133Cs NMR reveal that the metabolism is dependent on the deformation of the red blood cells.

The specific use of quantitative NMR methods to study gels in food science is reviewed in chapter eight by Lester Geonzon and others.

In the clinical practice of cancer treatment with radiotherapy, hydrogels in combination with quantitative MRI have been successfully applied in mapping radiation dose distributions. Two different kinds of 3D radiation dosimeters are reviewed in chapter nine. A detailed overview is given of the physical and chemical properties of these radiation dosimeters in terms of their spatial integrity and temporal stability and their interaction with ionizing radiation. Different quantitative MRI techniques to map the 3D dose distributions are discussed.

Hydrogels are also extensively used as test objects in the validation and optimization of clinical MRI sequences and protocols. Examples are given in chapter ten of gel phantoms used to validate MRI thermometry, in vivo MR spectroscopy and functional MRI and in testing MRI compatibility of prostheses and implantable devices. Here it is important that the properties of the hydrogels are closely matched to the human tissue both in terms of MRI contrast and electrical and magnetic interactions. Many human tissue compartments have characteristics similar to hydrogels. The extracellular matrix is basically a hydrogel consisting of a protein and glycoprotein backbone and the high concentration of proteins and cytoskeleton of the intracellular matrix give gel-like properties to the interior of biological cells. Therefore, to mimic the self-diffusion of water in human tissue, hydrogel phantoms are an obvious choice. Chapter 11 is dedicated to a discussion of gel phantoms in diffusion MRI. Different gel phantoms are discussed for isotropic and anisotropic diffusion in vivo. To mimic blood perfusion, hydrogel phantoms can be provided with semi-permeable hollow fibres. In chapter 12, Steven Baete discusses the important aspects of perfusion phantoms that can be applied in dynamic contrast enhanced (DCE) MRI and MRI oximetry.

The topics for this book were selected to provide you with an overview of state-of-the art NMR techniques in the study of gel systems and conversely demonstrate that gels can be used to advance the field of quantitative MRI. I hope that you will enjoy this book and that you will appreciate the enormous potential of gels in the emerging field of quantitative MRI. It is our believe that, with the advancement of new scientific disciplines such as nanotechnology and tissue engineering, gels will play an ever increasing role, where NMR is an invaluable analytical tool to characterize and optimize new gel systems.

Yves De Deene

Sydney, Australia

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