Rationale of Using Conventional Sol-Gel Derived SiO₂ for Delivery of Biologically Active Agents

Abstract

Progress in the research of mesoporous materials, hierarchical pore structures, chemical modification of surfaces, nanoparticle processing and hybrid materials is important and it provides new and interesting functional properties for silica structures. However, this has also left the conventional, alkoxy-based sol-gel derived silica in the shadow, although it has a lot of non-utilized potential, especially in the delivery and/or encapsulation of sensitive biologically active agents like viral vectors, proteins, nucleic acids and cells. The potential lies in the versatile possibilities to adjust the structure by using alkoxides as precursors and in the proper use of water in different steps of the processing. The conventional, alkoxy-based sol-gel silica structure can be processed so that it results in largely variable biodegradation rates, biodegradation-controlled release of encapsulated agents and beneficial environment even for highly sensitive agents. These kinds of silica structures contain more or less water and hence, they are more or less labile from the traditional viewpoint of materials science. In extreme case they could be called “unfinished silica”. The aim of this paper is to discuss how the biodegradation rate of these kinds of silica materials can be adjusted on a large scale and how this is related to a rather narrow scale adjustment of in vitro dissolution rate of silica, how the unfinished silica structures can be controlled and their properties adjusted, how they can be utilized in the delivery of biologically active agents, and what the potential problems to be solved are.

Silicon oxide based materials for controlled release in orthopedic procedures

“…The sol-gel silica degradation rate depends extensively on the structure of the silica. In fact, under body fluid conditions (pH 7.4, temperature = 37°C), the solubility of amorphous silica is 120-150 ppm, while crystalline silica is ten times less soluble [44,45]. Not only is the solubility, also the dissolution rate is dependent on the structure.…”

“…Thus the extent of silanol groups is related to the degradation rate; and the degradation rate is higher when more silanol groups are present. When the degree of condensation is low, there is an abundance of free silanol groups in the silica surface [44,46].…”

Sol–Gel Processed Oxide Controlled Release Materials

“…61,62 Not only is the solubility, but also the dissolution rate is dependent on the structure. 61,62 Not only is the solubility, but also the dissolution rate is dependent on the structure.…”

“…45,61 A well-hydrolyzed sol of alkoxides gelled to a low degree of condensation can result in a high number of free silanol groups per surface area. Therefore, the amount of silanol groups is related to the degradation rate.…”

“…61,69 The difference in the behavior between in vitro and in vivo data likely stem from two origins: difference in fluid flow in vitro and in vivo, and tissue -sol-gel carrier interaction. The in vivo degradation rate of silica structures is much slower than in vitro degradation rates.…”