23-07-2021 | | By Sam Brown
Recently, researchers from the University of Wisconsin have created an implantable device that can speed up bone repair while being entirely dissolvable in the human body. What challenges do implantable electronic devices face, what have the researchers accomplished, and how will it help with medical care in the future.
Implantable medical devices come in all shapes, sizes, and functions, with some being responsible for ensuring a steady heartbeat (pacemakers) while others provide that the human body has sufficient insulin (insulin pumps). Such devices have been responsible for saving countless numbers of lives, but designing and implanting them comes with a whole range of challenges.
Generally speaking, the human body likes to be left alone and rejects anything it doesn’t recognise. A typical example of this is with wood splinters; if not removed, the human body attempts to push them out while also surrounding it in pus. Such objects can either get infected or cause a nasty reaction from the human body, which can cause complications. This rejection of foreign objects is one of the biggest challenges faced with implantable medical devices. Anything that gets placed inside the human body must be known as biologically inert, meaning that the human body ignores whatever is inside. One example of biologically inert material is stainless steel; if a clean piece of stainless steel is left in the body, it is ignored by the immune system.
Therefore, when engineers create implantable devices, they must ensure that the device is biologically inert. However, the second challenge faced with implantable devices is that they may require removal in the future, whether because they are no longer needed or because their power source has run out. This creates an additional risk of infection and damage during removal surgery, as well as causing discomfort to the user.
A group of researchers from the University of Wisconsin has recently demonstrated a device that can be implanted into a living organism, provide a proper medical function (in this case, bone repair), and then safely dissolve inside the body without the need for removal via surgery.
The specific challenge that the researchers tackled is the need for decreasing bone repair time. Bones are remarkable as, when damaged, they can self-heal, and the mechanism by which this healing process takes place is actually partly electrical. Bones are piezoelectric materials that generate electricity when placed under deformation, and it is these electrical signals that encourage bones to strengthen. This is why impacts are essential for improving bone strength; as bones are impacted, they generate the needed electrical signals for bone growth.
It also turns out that applying an electrical signal to broken bones encourages them to repair faster. However, trying to implant electrodes onto a broken bone is invasive and requires removal after the bone has healed. This means that the use of electricity to stimulate bone repair is rarely done (if at all).
However, Professor Xudon Wang (materials science and engineering) decided to tackle this challenge by developing a fully implantable device that can stimulate bone growth using electrical current. The device itself is a thin-film triboelectric nanogenerator that generates electricity when subjected to movement, and small electrodes on the device transfer the generated electricity across a broken bone. To ensure that the device is biologically inert, it was constructed onto a substrate of poly (lactic-co-glycolic acid), and the time which the device takes to dissolve can be adjusted by making minor adjustments in the substrate.
The researchers tested their device in rats subjected to a broken tibia, and the experiment results showed a significant improvement in bone repair time. Furthermore, once the device had healed the broken bone, it successfully dissolved inside the body without causing complications.
While the device demonstrated by the researchers shows great promise in accelerating bone repair, it does have one disadvantage; it requires movement to generate electricity. While rats can move around even with a broken bone, humans put into casts generally have very limited movement. Since the device requires mechanical movement to generate electricity, a patient with such a device may not be able to trigger the device into generating electricity.
However, this has not deterred the researchers as they have recognised a wide range of different mechanical movements that the implanted device could take advantage of. One example given by the researchers is blood pressure change; such changes can be used to flex a triboelectric generator.
The device demonstrated by the researchers will unlikely be used to heal broken arms and legs due to the invasive surgery needed to implant such devices. However, more severe damage that requires surgery would be a prime application for an implantable medical device. As a surgeon would already be trying to set the bone, they would have access to the broken area, and thus a tiny implantable device could be integrated with ease.