Electronic devices for constant monitoring of the patient's health are in great demand in modern medicine. These implants can be made from completely safe materials and signal surges in blood sugar levels, blood pressure, or the appearance of an immune response to medications.
Despite the long-term performance, such devices will need to be disposed of someday. The obvious solution to the problem - surgical removal of the implant - is clearly not the best, since any such intervention will be painful and sometimes dangerous.
Therefore, many groups of bioengineers around the world are developing devices that are built into the body, which could independently dissolve and be excreted from the body after the expiration date.
“The creation of such implants is a big step forward. Until recently, there has been no progress in the development of soluble biomedical devices,”says co-author Jeffrey Borenstein of the Draper Laboratory in Massachusetts, USA.
In 2012, Borenstein's colleague materials scientist John Rogers of the University of Illinois and his group presented a series of biodegradable silicon chips capable of controlling temperature or mechanical deformation, transmitting information to devices outside the body (to a computer or smartphone, for example), and even heating body tissues. to prevent infection. Some of these chips were powered by induction coils to provide wireless power from external sources.
But wireless transmission of energy is not very suitable for subcutaneous implants, which sometimes need to be placed in deep layers of tissue or even under the bone. In addition, the components for such devices are very complex and cumbersome. After investigating these issues, Rogers and his team have created optimized fully biodegradable batteries to complement existing devices.
The engineers used magnesium foil as anodes, and a plate of iron, molybdenum, or tungsten for cathodes. All these metals will slowly dissolve in the body, and their ions in low concentrations are biocompatible.
The electrolyte between the two electrodes is a sodium phosphate buffer. All of these components are also packaged in a biodegradable polymer, polyanhydride.
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As reported in an article published in the journal Advanced Materials, the amperage of the device can vary depending on the metal used in the cathode. For example, a cell with an area of one square centimeter with a 50 micrometer thick magnesium anode and an 8 micrometer thick molybdenum cathode gives 2.4 milliamperes.
Once dissolved, the battery releases less than 9 milligrams of magnesium. (Photo by University of Illinois)
Once dissolved, the battery releases less than 9 milligrams of magnesium, which is about double the magnesium coronary artery stent that has been successfully tested in clinical trials. Such concentrations may not cause problems, Rogers said.
So far, all versions of the biodegradable device are capable of functioning in the body for 24 hours, but engineers are already working to increase the potential life of productivity. They also hope to increase the energy density by modifying the surface of the magnesium foil. The large surface area will increase the reactivity of the material. According to preliminary estimates of the authors of the study, a battery of 0.25 square centimeters and only one micrometer thick is quite capable of powering a subcutaneous sensor during the day.
Note that Rogers' development is a potential competitor to Christopher Bettinger's project: the latter used the skin pigment melanin to create the anodes for maximum safety of the bioaccumulator. Nonetheless, comparative analysis showed that Rogers' magnesium anode batteries are just as safe, but have a higher energy density and a longer life, which means they win.
Borenstein adds that any such devices can be used not only for biomedical monitoring and drug delivery, but also, for example, as sensors to continuously assess the state of the environment. Degradable sensors can be placed in the ocean, where they monitor the degree of contamination, and at the end of their life they will dissolve almost without a trace.