Scientists from Swiss Federal Laboratories for Material Studies and Technology and Federal Polytechnic School of Lausanne developed a prototype of super-condenser to be implanted into a human body for decades. The created device combines high energy density, chemical stability and complete bio-compatibility, which turns it into potential substitute of lithium-ion batteries in medical implants.
Modern implants (from cardio-stimulators and hearing kits to neural interfaces and mini-sensors) more and more often need off-line flexible power source. Today, lithium-ion batteries are used for these purposes. However, they have critical deficiencies: limited service-life, capacitance losses in case of repeated cycles, rigid case and, much more importantly, toxicity in case of damage. For implants being in direct contact with living tissues such risks cannot be allowed. That is why engineers all over the world are searching for an alternative – reliable accumulators, which could stay operational for decades, do not require replacement and do not interact with human organism.
Instead of traditional metallic casing and electrolyte solutions the new appliance uses porous carbon fibers impregnated with active substances – titanium carbide with atomic layer (MXene), graphene and conductive polymer polyaniline boosting the capacitance due to fast oxidation-reduction. Such materials feature high electrical conductivity, corrosion resistance and proved bio-compatibility.
The aqueous solution of phosphoric acid is used as electrolyte solution safe for living tissues in small concentrations, which is widely used in food industry. To stabilize the composition and to protect electrodes, polyvinyl alcohol forming gel-like structure was added to the solution. It makes electrolyte solution viscous and stable, prevents titanium wash-out of MXene, impedes the formation of side compounds and thus increases the service-life if the device.
The developed super-condenser is capable of accumulating up to 6.07 watt-hours of energy per one kilogram of the mass and of preserving up to 86% of capacitance even after 60 thousand charging-discharging cycles. To compare: most of the lithium-ion micro-batteries start losing their capacitance after 1,000-2,000 cycles already. On top of that, high charging speed allows for contactless feeding of the device – e.g., using the induction field, as is already done in modern implants.
The biological safety of all the components was confirmed during a series of trials according to ISO and OECD international standards. The cell tests showed high viability (over 90%) and lack of toxicity or genetic defects. Even the completely assembled device did not cause any inflammatory responses, which confirms that it is fit for implanting.
Polyvinyl alcohol played a special role: not only does it stabilize electrolyte solution, but also prevents deterioration of electrodes. After some time, titanium phosphate crystals impeding the conductivity appeared in the samples without this polymer. Adding alcohol excluded this effect completely by protecting active layers from oxidation and prolonging the service life.
The Swiss researchers are convinced that their technology will help to create “perpetual” micro-sensors, cardio-stimulators and neural interfaces relieving patients from the need to replace batteries. Now the scientists are focused on clinical trials of the device.
