What is your “sol-gel” background?
I joined Professor Hench’s group at Imperial College London in 1999. Larry is best known for his invention of melt-derived bioactive glass and the original Bioglass® composition. At Imperial, Larry’s group’s work was divided between investigating the biology behind why bioactive glasses stimulate bone cells and developing sol-gel derived bioactive glasses. With Pilar Sepulveda, a researcher in his group, we developed the first porous bioactive glass scaffold suitable for bone regeneration. Sol-gel provided us the unique opportunity of being able to produce a glass foam with large interconnected pores suitable for 3D bone ingrowth. This was not possible with melt-derived bioactive glasses at the time as they would crystallise during sintering. The scaffolds were synthesised by our sol-gel foaming process, where the sol is foamed by agitation in air with the aid of a surfactant. Due to the inherent mesoporosity of the sol-gel glasses, the scaffolds had a hierarchical porosity of open macropores with nanoscale porosity within the struts. Optimising the process was the focus of my PhD and I then went on to look at bone cell responses to the scaffolds. I started my own research group at Imperial in 2005.
What are the main topics you are working on today ?
My research is focused on creating scaffolds for bone and cartilage regeneration, with a focus on sol-gel chemistry. The main thrust of my work is now the development of inorganic/ organic sol-gel hybrid scaffolds. We believe they have the potential to have all the beneficial properties of a bioactive glass scaffold (bioactivity, pore structure, compressive strength) but also be tough. Surgeons have challenging demands for an ideal scaffold for bone and cartilage regeneration and conventional composites do not seem to be able to cover them, mainly due to the polymer masking the bioactive ceramic or glass and differential degradation rates between the polymer and the glass. By creating polypeptide/ silica sol-gel hybrids, by introducing the polymer into the sol prior to gelation so that the polymer and silica interact at the molecular level, we have produced hybrid scaffolds with controlled mechanical properties and degradation rates. A key step is covalently coupling the silica to the polymer. The amount of coupling has a massive effect on the final properties. In this way porous scaffolds can be synthesised with the flexibility of a polymer foam, the stiffness of a glass, or anywhere in between. My group are also working on taking the bioactive glass foams to preclinical trials by investigating stem cell response and how stem cells react to sub-micron bioactive glass particles that may be released from the scaffolds.
The collaborations you have initiated ?
Interdisciplinary approaches are vital in biomaterials and regenerative medicine. Very important collaborations have been focused on understanding the assembly of sol-gel materials and in characterising their properties. At the atomic scale, Professor Mark Smith’s team at the University of Warwick and Professor Bob Newport’s team at the University of Kent have carried out extensive studies on formation of the atomic structure of bioactive sol-gel glasses and hybrids using solid state NMR and diffraction techniques respectively. Dr Christopher Mitchell at the University of Ulster is carrying out in vivo studies. At Imperial, I work closely with Professor Molly Stevens in terms of investigating cellular responses to our materials, and also with Professor Peter Lee on 3D imaging (e.g. X-ray microtomography), image analysis and macromodelling of properties. With Dr Jonathan Weaver, we are also synthesising new polymers with specific functions for incorporation into hybrids. I also work closely with surgeons in the Imperial College Medical School. I am also chair of the International Commission on Glass’ (ICG) Technical Committee on biomedical glasses (TC4) and we have several international projects underway.
Could you let us know the main challenges you foresee for the sol- gel process and the sol-gel materials in the next future ?
The challenges as I see them are divided between science and technology transfer. The scientific challenged I face are in the biomaterials field, particularly in hybrids. The sol-gel process is great because we can form silica networks at low temperature, but for bone applications we want calcium in the network, as we do in the glasses. However, at low temperature, when calcium salts are used the calcium does not enter the network, so new calcium sources are needed.
The greatest challenge for technology transfer is up-scaling complex processes with many variables, such as foamed scaffolds produced from sol-gel hybrids.
What is your wish for the sol-gel community?
To be more visible in the wider scientific and engineering community at conferences such as Hybrid Materials, Society for Biomaterials (USA or European) and other application focused conferences. The sol-gel is a process that can produce unique materials with properties and functions superior to materials synthesised with other processes. I agree with Vadim Kessler that pesenters are often shy of saying “sol-gel”. Instead they refer to the “Stober process”, for nanoparticle synthesis, or “ordered mesoporous material synthesis”. We should celebrate the process more and the improved functionality and even structural properties it can bring. I look forward to the meeting in China in August.
