Glass microspheres are used in many ways. The microspheres are usually 10 to 100 micrometers in diameter with varying densities, properties, and capabilities. Special morphologies can also be achieved by a variety of techniques in the laboratory. One of the most interesting morphologies of these tiny bubbles, are porous wall hollow glass microspheres (PWHGMs) with a unique porosity that can be used to fill the microspheres with a variety of cargos of various contents that can later be released on demand.
Near-permanent, Inert, Biocompatible
First-In-Class Novel Soft Tissue Filler Material
Although the PWHGMs are revolutionary in themselves, the truly exciting potential for these tiny bubbles is realized when they are combined with various biocompatible materials and matrices. An analogy can be made to fiberglass; epoxies are good for making various glues and adhesives, and E-glass fibers are good for insulation, but when combined, they form an entirely new material (fiberglass), which literally and figuratively can hold water. The resulting composite possesses new properties, characteristics, and, most importantly, can be used for new applications.
As one of many examples, consider the use of glass as a filler material for laryngeal surgery, before now seemingly nonsensical. Now, however, think about the combination of microspheres and a hydrogel made of fibrinogen. The material can be fine-tuned to have exactly the desired mechanical and biological properties for the intended task; the fibrinogen gel is biologically compatible and even active, depending on the cytokines and proteins attached to the fibrinogen. It can hold the microspheres in place until collagen and elastin deposition occurred to anchor them long-term. The glass microspheres are too large to be engulfed by macrophages, and the smooth surface elicits little immune response. Sizing and composition of microspheres (and matrix) can also be altered to achieve desired rheological properties. Many current soft-tissue surgical fillers are extremely difficult to use because of their high viscosity. Because of their size and shape, microspheres have unusual physical characteristics; they can be pipetted like a liquid and can actually reduce the viscosity of the final material (depending on the viscosity of the biocompatible matrix). Finally, the microspheres could even be filled with cargo such as an antibiotic, anesthetic, or stem cell chemoattractant, as well as coated to achieve specific interactions with the biological environment.
Similarly, by combining microspheres of various sizes, composition, and porosity (or nonporous microspheres) with and without tailored coatings, and in biocompatible matrices, the resulting materials and composites can have new and sometimes unexpected physical and biochemical properties. These properties can be exploited for medical applications.
Long-Term Drug Delivery System
Science Fact... not Science Fiction
In the biomedical arena, one of the most critical needs is for improved drug delivery devices. What is exciting about glass microspheres and microsphere composites is the flexibility they can provide in terms of a platform for medical cargos. It is possible to modify the basic PWHGMs physically in terms of the size distribution of the microspheres, the size, complexity, and morphology of the pores and the ability for imparting desired characteristics to the surfaces (e.g., electrostatic charges to regulate the type and size of the molecule that can be loaded).
Additionally, PWHGMs can have different types of “gating” applied to the pores. This can further enhance the type of payload that is inserted and control the rate and delivery of those molecules in a continuous fashion, once implanted or in a triggered inducible manner (e.g., photo-enhanced diffusion effects). This permits the tight control of the spatial– temporal localization of the drug, or biomedical molecule, as it is being released into a tissue, or to control its release systemically into the body.