A new class of “bioactive” glass can act like an intelligent system-it not only senses its environment but reacts chemically and physically to it.
At Southern Illinois University, Bakul C. Dave (pronounced Da-vey) has formed a transparent glassy material from an organically modified silica gel solution. Mixed with water at room temperature, the sol-gel solution hardens in about five minutes into a solid yet elastic transparent gel.
The sol-gel glass is honeycombed with microscopic pores that can absorb biomolecules such as proteins or enzymes and release them in response to environmental stimuli. This property could bring closer a smart drug-delivery vehicle able to regulate dosages to maintain proper therapeutic levels.
Conventional glass, created by melting silica at high temperature, contains very little water. Sol-gel glass is 20 percent water by weight. The combination of porosity and high water content makes the sol-gel glass a flexible sensor and reactor.
In lab experiments the sol-gel glass swells, shrinks or bends under conditions of changing temperature or acidity, yet reverts back to its original shape when the environment moderates. It also responds to changes in salt or electric field.
Additionally, the glass can react to a variety of chemical stimuli, including proteins and other biomolecules. The glass’s sensitivity, including the choice of target molecule, is controlled by modifications to the silica gel solution.
As the sol-gel glass changes shape, it can absorb or expel biomolecules through its interconnected system of micropores, which also enlarge or shrink. The glass itself is bio-inert, or biocompatible, meaning that it preserves the integrity of biomolecules built into or absorbed into the sol-gel pores. This biocompatibility also means that its use as a pill or implant should not trigger infection or rejection by the body.
Pump or Sponge
Dave’s research team is investigating the sol-gel’s potential as an environmentally regulated drug delivery vehicle. Unlike time-release capsules, whose action is continuous, a sol-gel insulin pump, for example, would mimic the natural response of the pancreas, modulating the amount of insulin it releases in proportion to the fluctuating concentration of glucose in the blood.
Dave’s team has begun to test this approach with sol-gel implants in mice. At expected levels of dosage, the insulin in the one-millimeter implant could last for several years, he says.
In a lab, the bioactive glass could find further uses as a protein sponge or separator, soaking up a specific target molecule from a biological soup. Later the process could be reversed to wring out the separated protein.
Other potential applications include microfluidics and thin films. It might be possible, says Dave, to shape the porous material into tiny channels, valves and reservoirs that open and close by changing shape rather than mechanical action.
Or, as a thin film or coating, the sol-gel glass could function as a sensor, communicating environmental conditions to an underlying material or substrate, allowing pressure within a pipe, or the speed of an engine, to be adjusted as needed.
To Dave, the chief aim of his sol-gel research is the breadth of applications it could open up. “We can effectively convert any chemical or physical signal into a carefully modulated response,” he says.
Honey, I Shrunk the Sample
Like so many scientific developments, Dave’s sensitive glass was discovered almost by accident. Sol-gels have been around for many years and have even been blended with organic and biomolecular materials. In 1997, Dave’s group was working on organic sol-gels for photochemistry applications when a graduate student accidentally left a bright light on a sample. To everyone’s surprise, the sample shrank under the light.
Today, Dave is refining the silica solution’s ability to precisely sense and react to specific environmental stimuli after it solidifies into sol-gel glass. He is also beginning to investigate applications on the nano frontier, where particles of the sol-gel glass could be used for molecule-specific delivery or absorption of drugs and other therapeutic substances.
If he’s successful, we might eventually see sol-gel nanoparticles being used to clean organs or tissues by removing or neutralizing harmful proteins or bacteria.
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