Medical science is always hard at work developing the next miraculous medication, but some of them will be of limited use without an equally miraculous delivery system. Advanced materials science may hold the key to delivering drugs with high precision when an injection simply won’t do.
Biomedical scientists and engineers at the University of North Carolina at Chapel Hill have joined forces looking for a breakthrough. What they’ve come up with is a so-called “smart sponge” that can be loaded up with medication, then release it only when and where it’s needed.
The spherical sponges are microscopic — just 250 micrometers across, and could be made as small as 0.1 micrometers. They are mainly composed of a polymer called chitosan. This material is not naturally occurring, but can be produced easily from the chitin in crustacean shells. In the past chitosan has been used to reduce bleeding, as a source of dietary fiber, and as a fringe theory weight loss supplement.
The long polysaccharide chains of chitosan form a matrix, in which tiny porous nanocapsules are embedded. The nanocapsules can be designed to respond to the presence of some external compound. The researchers tested the smart sponges with insulin, so the nanocapsules in this case contained glucose oxidase.
As the level of glucose in a diabetic patient’s blood increases, it triggers the nanocapsules in the smart sponge. This causes them to release hydrogen ions, which bind to the chitosan strands of the matrix and impart a positive charge. All these positively charged strands next to each other begin pushing apart, just like putting the positive ends of two magnets too close together. The strands continue moving until the openings in the matrix are large enough for the insulin to leak out into the blood.
The process is also self-limiting. As glucose levels in the blood come down after the release of insulin, the nanocapsules deactivate and the positive charge dissipates. Without all those hydrogen ions in the way, the chitosan can come back together to keep the remaining insulin inside. The chitosan is eventually degraded and absorbed by the body, so there are no long-term health effects.
The best way to treat diabetes, they realized, was with a system that administers insulin when it’s needed, not on a set schedule. The smart sponges closely recreate the functionality of insulin-producing cells in a healthy person.
Diabetes is just the first application, though. Smart sponges based on this technique could be used to treat all manner of disease. If scientists can find a suitable enzyme that responds to a disease marker, they should be able to more efficiently deliver medication. In fact, the University of North Carolina team believes cancer could be treated without an enzyme activation system of any sort.
Tumors are naturally highly acidic environments, which means a lot of free hydrogen ions. That’s what the oxidase enzyme in the diabetic smart sponge was producing. Thus, a smart sponge filled with small amounts of chemotherapy drugs could automatically release the compounds in areas with cancer cells.
The team has seen success treating diabetes in rats, so this is more than a hypothetical treatment method. The question to ask next is, can it work well enough to displace standard injections? The jury’s still out on that one.
Research paper: DOI: 10.1021/nn401617u - “Glucose-Responsive Microgels Integrated with Enzyme Nanocapsules for Closed-Loop Insulin Delivery”
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