A four-cell battery made of biodegradable materials completely dissolves after three weeks in water. Click here to view the stages of bi...
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| A four-cell battery made of biodegradable materials completely dissolves after three weeks in water. Click here to view the stages of biodegradation |
Medical implants would monitor vital signs or dispense therapies before vanishing
A
biodegradable, implantable battery could help in the development of
biomedical devices that monitor tissue or deliver treatments before
being reabsorbed by the body after use.
“This
is a really major advance,” says Jeffrey Borenstein, a biomedical
engineer at Draper Laboratory, a non-profit research and development
center in Cambridge, Massachusetts. “Until recently, there has not been a
lot of progress in this area.”
In
2012, materials scientist John Rogers at the University of Illinois at
Urbana-Champaign unveiled a range of biodegradable silicon chips that
could monitor temperature or mechanical strain, radio the results to
external devices, and even heat up tissue to prevent infection (see ‘Biodegradable electronics here today, gone tomorrow’). Some of those chips relied on induction coils to draw wireless power from an external source.
But
wireless power transfer is problematic for devices that need to go deep
within tissue or under bone, says Borenstein. The components that
receive the power are also quite complex, he says: “anything you put in
there is going to take up space”. To provide a tidier solution, Rogers
and his collaborators have now created a fully biodegradable battery.
Dissolvable devices
Their devices, described last week in Advanced Materials, use anodes of magnesium foil and cathodes of iron, molybdenum or tungsten. All these metals will slowly dissolve in the body, and their ions are biocompatible in low concentrations. The electrolyte between the two electrodes is a phosphate-buffered saline solution, and the whole system is packed up in a biodegradable polymer known as a polyanhydride.
Their devices, described last week in Advanced Materials, use anodes of magnesium foil and cathodes of iron, molybdenum or tungsten. All these metals will slowly dissolve in the body, and their ions are biocompatible in low concentrations. The electrolyte between the two electrodes is a phosphate-buffered saline solution, and the whole system is packed up in a biodegradable polymer known as a polyanhydride.
Currents
and voltages vary depending on the metal used in the cathode. A
one-square-centimeter cell with a 50-micrometer-thick magnesium anode
and an 8-micrometer-thick molybdenum cathode produces a steady 2.4
milliamps of current, for example. Once dissolved, the battery releases
less than 9 milligrams of magnesium — roughly twice as much as a
magnesium coronary artery stent that has been successfully tested in
clinical trials, and a concentration that is unlikely to cause problems
in the body, says Rogers. “Almost all of the key building blocks are now
available” to produce self-powered, biodegradable implants, he says.
All
versions can maintain a steady output for more than a day, but not much
longer. The team hopes to improve the batteries' power per unit weight —
known as power density — by patterning the surface of the magnesium
foil to increase its surface area, which should enhance its reactivity.
The authors estimate that a battery measuring 0.25 cm2 and just one micrometer thick could realistically power a wireless implantable sensor for a day.
In the field
The devices could also find environmental applications, says Borenstein. For example, to help remediation efforts during an oil spill, environmental officials could drop hundreds of thousands of tiny wireless chemical sensors across the slick. These would later simply dissolve in the ocean. Space is less of a constraint in these applications: a stack of several cells, for instance, can produce up to 1.6 volts — enough to power a light-emitting diode or generate a radio signal.
The devices could also find environmental applications, says Borenstein. For example, to help remediation efforts during an oil spill, environmental officials could drop hundreds of thousands of tiny wireless chemical sensors across the slick. These would later simply dissolve in the ocean. Space is less of a constraint in these applications: a stack of several cells, for instance, can produce up to 1.6 volts — enough to power a light-emitting diode or generate a radio signal.
Magnesium
batteries are not the only solution. Last year, biomaterials scientist
Christopher Bettinger of Carnegie Mellon University in Pittsburgh,
Pennsylvania, unveiled an edible sodium-ion battery with electrodes made
from melanin pigments. But Rogers’ team report that their magnesium
batteries have a relatively higher current and power density, and last
for longer.
Borenstein
hopes that further research into both types of batteries could
eventually yield implantable drug-delivery devices that are controlled
by radio signals, or that dispense pharmaceuticals in response to a
specific acute problem, such as an epileptic seizure.
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