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.