The sponge is said to expand and contract in response to blood sugar levels to release insulin as needed. The technique could also be used for targeted drug delivery to cancer cells.
‘We wanted to mimic the function of health beta-cells, which produce insulin and control its release in a healthy body,’ said Dr Zhen Gu, lead author of a paper describing the work and an assistant professor in the joint biomedical engineering program at North Carolina State University and the University of North Carolina at Chapel Hill. ‘But what we’ve found also holds promise for smart drug delivery targeting cancer or other diseases.’
The research team includes Daniel Anderson, the senior author and an associate professor of chemical engineering and member of the Koch Institute for Integrative Cancer Research at MIT, and researchers from the Department of Anaesthesiology at Boston Children’s Hospital.
The researchers created a spherical, sponge-like matrix out of chitosan, a material found in shrimp and crab shells. Scattered throughout this matrix are smaller nanocapsules made of a porous polymer that contain glucose oxidase or catalase enzymes. The sponge-like matrix surrounds a reservoir that contains insulin. The entire matrix sphere is approximately 250 micrometers in diameter and can be injected into a patient.
When a diabetic patient’s blood sugar rises, the glucose triggers a reaction that causes the nanocapsules’ enzymes to release hydrogen ions. Those ions bind to the molecular strands of the chitosan sponge, giving them a positive charge. The positively charged chitosan strands then push away from each other, creating larger gaps in the sponge’s pores that allow the insulin to escape into the bloodstream. In type 1 and advanced type 2 diabetes, the body needs injections of insulin, a hormone that transports glucose – or blood sugar – from the bloodstream into the body’s cells.
As the insulin is released, the body’s glucose levels begin to drop. This causes the chitosan to lose its positive charge, and the strands begin to come back together. This shrinks the size of the pores in the sponge, trapping the remaining insulin.
While this work created hydrogen ions by using enzymes that are responsive to glucose, the technique could be simplified to target cancers by eliminating the enzymes altogether. Tumours are acidic environments that have high concentrations of hydrogen ions. If the sponge reservoir were filled with anti-cancer drugs, the drugs would be released when the chitosan came into contact with the hydrogen ions in tumour tissues or cancer cells.
‘We can also adjust the size of the overall ‘sponge’ matrix as needed, as small as 100 nanometres,’ Gu said in a statement. ‘And the chitosan itself can be absorbed by the body, so there are no long term health effects.’
In tests using diabetic laboratory mice, the researchers found the sponge matrix was effective at reducing blood sugar for up to 48 hours. However, the researchers published a separate ‘smart system’ for insulin delivery in May that maintained normal blood sugar levels for 10 days.
‘But we learned a lot from the promising ‘sponge’ research and will further optimize it. Meanwhile, we are already exploring applications to combat cancer,’ Gu said.
The paper, Glucose-Responsive Microgels Integrated with Enzyme Nanocapsules for Closed-Loop Insulin Delivery, is published online in ACS Nano.