Insulin pumps improve the lives of many of the 415 million people
with diabetes around the world by monitoring blood sugar, delivering
insulin, and preventing the need for constant
finger-pricking and blood testing. These small machines include a pump and
a needle, which can sense glucose levels, feed back to the pump, and then calculate how much insulin
to deliver through the needle. But they have a catch: they’re temporary. Within a few days, glucose sensors have to
be moved and replaced. And it’s not just glucose monitors and
insulin pumps that have this problem, but all bodily implants,
at different time scales. Plastic prosthetic knees have to be
replaced after about 20 years. Other implants, such as those used for
cosmetic reasons, can meet the same fate in about 10. That isn’t just a nuisance: it
can be expensive and risky. This inconvenience happens because of
our bodies’ immune systems. Honed by several hundred million
years of evolution, these defensive fronts have become exceptionally good
at identifying foreign objects. Our immune systems boast an impressive arsenal of tools to tackle,
intercept, and destroy anything they believe shouldn’t be there. But the consequence of this constant
surveillance is that our bodies treat helpful
implants, like insulin pumps, with the same suspicion as they would
a harmful virus or bacteria. As soon as the insulin pump has been
implanted in the skin, its presence triggers what’s known as a
“foreign body response.” This starts with free-floating proteins that stick themselves to the surface
of the implant. Those proteins include antibodies, which attempt to neutralize the new object and send out a signal that calls other
immune cells to the site to strengthen the attack. Early-responding inflammatory cells, like neutrophils and macrophages, respond to the emergency call. Neutrophils release little granules filled
with enzymes that try to break down the surface
of the insulin pump’s needle. Macrophages secrete enzymes too, together with nitric oxide radicals, which create a chemical reaction that
degrades the object over time. If the macrophages are unable to dispatch
the foreign body rapidly, they fuse together, forming a mass of
cells called a “giant cell.” At the same time, cells called fibroblasts travel to the site and begin to deposit
layers of dense connective tissue. Those enclose the needle that the pump
uses to deliver insulin and test for glucose levels. Over time this scaffolding builds up, forming a scar around the implant. The scar functions as an almost
impenetrable wall that might start to block vital
interactions between the body and the implant. For example, scarring around pacemakers
can interrupt the electrical transmission that’s
crucial for their functioning. Synthetic knee joints may give off
particles as they’re worn down, causing immune cells to inflame
around these fragments. Tragically, the immune system’s attack
can even be life-threatening. However, researchers are finding ways
to trick the immune system into accepting the new devices we
introduce into our bodily tissues. We’ve discovered that coating implants
with certain chemicals and drugs can dampen the immune response. Those basically make the implants
invisible to the immune system. We’re also making more implants
out of natural materials and in forms that directly mimic tissues, so that the body launches a weaker attack than it would if it came across a
completely artificial implant. Some medical treatments involve implants designed to regenerate lost
or damaged tissues. In those cases, we can design the implants
to contain ingredients that will release specific signals, and carefully tailor our bodies’
immune reactions. In the future, this way of working
alongside the immune system could help us develop completely
artificial organs, totally integrative prostheses, and self-healing wound therapies. These treatments might one day
revolutionize medicine– and transform, forever,
the bodies we live in.
