Functional Molecular Gels, Edited by Beatriu Escuder and Juan F. Miravet, RSC Soft Matter Series, 2014, 317 pp, £149.99 (Hardback), ISBN: 978-1-84973-665-7
The intention of the book, Functional Molecular Gels, is to provide a global view of the current state-of-the-art in the field of Molecular Gels. Molecular gels are physical gels within which low molecular weight compounds self-assemble and are held together by non-covalent bonds such as van der Waals interactions, p-interactions, dipolar interactions, hydrogen bonding, coulomb interactions and solvophobic effects. To create a molecular gel, the self-assembly process must be inhibited in at least one direction so that growth extends faster in one or two directions to build a network structure. This network provides the scaffold that imparts elastic properties to create a gel.
Chapter 1 introduces the reader to the field of molecular gels. Although the opening line, ‘Supramolecular gels are hot!’, may irk those with thermodynamic sensibilities the introductory chapter gains momentum in the second paragraph, first defining a gel and then describing molecular gels in context with other types of gel. Some examples of the molecular structure of low molecular weight (LMW) gelators are given alongside reasons for their tendency to aggregate to form networks. A rheological definition of the sol-gel transition is followed by a description of the process of molecular gel formation. The last part of this chapter considers routes for discovering new LMW gelators, one approach is to screen libraries for new candidates and the other is structure based design for which basic design rules are described. Routes for controlling structure formation are considered such as selecting the geometry of self-assembling units, inducing chirality and control of branching and reversible changes in structure.
Chapter 2 covers range of techniques for characterizing molecular gels. Techniques such as Rheology, Differential Scanning Calorimetry, Electron Microscopy, Atomic Force Microscopy, X-ray/Neutron Scattering Methods, Light Scattering, Nuclear Magnetic Resonance, Infrared Spectroscopy, Optical Spectroscopy and Fluorescence and Circular Dichroism Spectroscopy are all covered and the limitations and advantages of each technique are discussed. This would be a very useful chapter for anyone wanting to assess which technique would be the most appropriate for the gathering the information they would like to learn about the self-assembly processes in a molecular gel.
Chapter 3 is an overview of molecular gels developed over the past decade that are responsive to physical and chemical stimuli. Heat, mechanical action, ultrasound and light are all examples of physical stimuli while the gels can be sensitive to pH, the presence of ions, redox reactions or neutral chemical species as examples of chemical stimuli. There are interesting examples of unexpected of gels that have been developed with unexpected properties, e.g. a gel to sol transition on cooling and another which forms a gel on shaking. It is not clear whether any of the molecular gel complexes reported are commercially available or whether they have only been produced within the laboratory.
Chapter 4 reports on progress in the field of enzyme-responsive molecular gels. Enzymes offer unique advantages in that they typically operate in mild and constant conditions and are selective for specific triggers meaning that a range of triggers (corresponding to different enzymes) can be applied in a single system. An enzyme responsive molecular gel will have an assembly directing unit, an enzyme recognition site and a molecular switch which initiates self-assembly upon enzyme action. An interesting application is the absorption of precursor molecules through cell membranes which, once inside the cell, self-assemble under the action of enzymes within the cell to form a gel. This mechanism has been used successfully to target cancer cells and inhibit bacteria growth. Additionally, enzyme responsive gels have been used to deliver drugs to a target location whereupon enzymes present at the target destroy the gel structure to release drugs trapped within the gel structure.
Chapter 5 is an overview of progress made in the field of using molecular gels as containers for molecular recognition, reactivity and catalysis. Functional sites can be incorporated within the fibre surface to generate features such as multivalent interactions, neighbouring effects and cooperativity. One aim of this area of research is to develop smart gellosomes that mimic the cell architecture and the dynamic biological processes working within.
Chapter 6 discusses the biomedical applications of molecular gels as vehicles for therapeutic delivery, a substrate for cell culture and as conduits for tissue regeneration with some ideas presented following on from Chapter 4. One interesting example is the injection of a peptide amphiphile into the extracellular spaces of spinal cord of a mouse model with a spinal cord injury. The ionic strength within the in-vivo environment resulted in gel-like structures which promoted regeneration of motor and sensory fibres. The conclusion points out that all current biomedical applications are based on peptide based molecular gels and that there are opportunities to explore molecular gels based on urea compounds, carbohydrates and steroids. Additionally the authors suggest that more work needs to be carried out on understanding how molecular gels form and what controls their properties so that we can fully capitalize on the potential of molecular gels.
Chapter 7 reports on optic and electronic applications of molecular gels. In terms of optical applications there are discussions on organogelators within liquid crystals that align with a liquid crystalline phase. Alternatively, if organogelators are formed within the isotropic phase, random networks form which can be used to create bistable displays with a transparent ordered state when the electric fields is on and an opaque state with disordered orientation when the electric field is off. Chromophores can be aligned along gel fibres in molecular gels to create fluorescent gels that emit at different wavelengths in response to physical and chemical treatment. Electronic applications include the creation of conducting networks from dried gels, encapsulation of a semiconductor molecular wire in organogels and the decoration of molecular gels with conductive nanoparticles.
Chapter 8 summarizes recent developments in the use of molecular gels as templates for nanostructured materials. Tubular fibres of various metal oxides can be created by depositing inorganic material onto the surface of the organogel network and then the organic template is removed. Interesting biomimetic work has been carried out a hydrogel of a peptide amphiphile which is mineralised to recreate the structural orientation of hydroxyapatite observed on collagen in the formation of bone. It is not only possible to decorate gels with nanoparticles but now in-situ synthesis is possible with gelator molecules capable of reducing metal ions to nanoparticles within the gel matrix.
Ideally, the reader will have a background in advanced physical and organic chemistry. Although the introductory section covers the definition of molecular gels and design principles for creating molecular gels the book lacks depth in defining the physical interactions that lead to self-assembly. Someone new to this field may need to read another text in tandem to appreciate the fundamental mechanisms behind the self-assembly processes occurring in molecular gels.
As the editors state in the preface, the intention of this book is to provide an overview of the current state-of-the-art in the field of Molecular Gels. This is successfully achieved with an impressive range of work reviewed without overbearing detail on any one system. There is some fascinating science and exciting developments to be found between the pages of this book but in some areas the authors risk a tendency to catalogue latest developments in the chemistry without relating clearly the relative advantages of each new system. Since this book sells the potential of molecular gels it would be more satisfying if there was clarity on the extent of commercialisation to-date for this class of gels beyond the biomedical arena.
Dr. Tiffany A. Wood
Edinburgh Complex Fluids Partnership
School of Physics and Astronomy
University of Edinburgh
EH9 3JZ
Ó 2014, T. A. Wood.