A biodegradable implant powered by a new single-atom iron electrode delivered acupuncture-style pain relief in mice, then dissolved harmlessly in the body.
(Nanowerk Spotlight) Supercapacitors sit between batteries and ordinary capacitors in the energy storage landscape. They charge in seconds rather than hours, survive hundreds of thousands of cycles without wearing out, and deliver bursts of power on demand. What they don’t do well is hold much energy for their size. A battery-sized supercapacitor runs out long before a battery-sized battery would, and the usual strategy for closing that gap is to add chemistry to the electrodes.
A plain supercapacitor stores charge physically, by pulling ions from a liquid electrolyte onto the surface of a porous electrode and holding them there like iron filings on a magnet. Nothing reacts. The ions sit, and when the circuit closes, they leave. Adding metal atoms to the electrode changes that. The metal can accept and release electrons through small chemical reactions at its surface, storing extra charge through fast surface redox reactions on top of the physical kind. More charge per unit area means more energy in the same device, which is exactly what supercapacitors need.
The catch is that the metal sites don’t just store charge. They also pull on the ions that balance the electrons they trade, and an ion held tightly is an ion that moves slowly. Sluggish ions mean sluggish charging, which is the one thing supercapacitors are supposed to be good at. Electrode designers have generally had to choose between energy and speed.
Schematic Illustration of the bioinspired self-assembled, biodegradable implantable Fe SA/cBSAs supercapacitor for acupoint electrostimulation. (A) The preparation process for atomic-scale dispersed Fe-O4 site catalysts from collagen, iron ions, and tannic acid via bioinspired co-assembly and high-temperature pyrolysis. (B) The structure and in vivo operating-mode schematic of the biodegradable supercapacitor for acupoint electrical stimulation with Fe single-atom material electrodes. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The electrode starts as a bioinspired self-assembly. Collagen, iron ions, and tannic acid are combined in water, and the phenolic hydroxyl groups on the tannic acid grab the iron and the collagen at once, pulling the three components into a porous three-dimensional network. Heating the scaffold in the absence of oxygen converts the organic material to carbon while the iron atoms, pinned by their oxygen neighbors, stay in place. The pore network survives the process.
The critical question is whether the iron really ends up as isolated atoms or whether it clumps into particles, because clumps would defeat the mechanism the paper proposes. Direct imaging at atomic resolution showed bright points scattered evenly across the carbon with no visible clusters, and X-ray absorption measurements confirmed that each iron atom had four oxygen neighbors and no iron neighbors. The oxidation state sat between +2 and +3, consistent with an Fe-O₄ site embedded in carbon rather than an oxide particle sitting on top of it.
Three versions of the electrode then went into a standard electrochemical cell: the single-atom material, the bare carbon scaffold, and a control in which iron chloride had simply been mixed with the carbon without the self-assembly step. The single-atom electrode stored 279.5 mF cm⁻² of charge. The bare carbon managed 245.5 mF cm⁻².
The mixed control came in worst at 207 mF cm⁻², because the iron residues settled into the pores and blocked them without contributing any chemistry. Only iron placed atom by atom into its oxygen pocket added capacity rather than subtracting it.
Calculations explained why. The iron sites add electronic states where electrons can move freely, speeding the surface reactions that store charge, and they redistribute surface charge in a way that weakens the electrostatic attraction between sodium ions and nearby oxygen atoms. On bare carbon, a sodium ion sits in an adsorption well of −4.1 eV, deep enough to make escape slow. On the iron-modified surface, the well shallows to −0.43 eV. Ions that were stuck become mobile, at the same places where the new redox chemistry is happening.
The full device pairs two of these electrodes with a hydrogel electrolyte of polyvinyl alcohol mixed with phosphate-buffered saline, sealed in a polylactic acid casing. The hydrogel matches the ionic composition of the fluid that bathes cells in the body, and the polylactic acid breaks down on a timescale compatible with short-term therapy, addressing the persistent challenge of powering implantable biomedical electronics.
The assembled supercapacitor delivered capacitance, energy density, and power density competitive with other reported biodegradable designs, kept 86.1 % of its capacity after 10 000 charge-discharge cycles, and tolerated flexing without significant loss.
Biological testing moved from components to whole device. The electrode materials, the hydrogel, and the casing all proved biocompatible in contact with cultured cells and blood. Whole devices implanted under the skin of mice degraded progressively and had essentially disappeared by day 120, and throughout the implantation period, liver and kidney biomarkers, blood counts, and organ histology were indistinguishable from those of untreated controls.
For the therapeutic test, mice received injections of Complete Freund’s Adjuvant, a compound that produces swelling and hypersensitivity in the injected paw, and then received daily electrical stimulation at the ST36 acupoint on the hind leg, delivered through a molybdenum wire connecting the implanted supercapacitor to a fine needle.
Treated animals regained body weight faster than untreated controls, tolerated more pressure on the inflamed paw before withdrawing it, and had lower tissue levels of the inflammatory signaling molecules TNF-α, IL-6, and IL-1β. Footprint analysis showed stride length recovering toward normal.
The pain-relief result is the visible payoff, but the durable contribution is the one made in the electrochemical cell and the calculations that explain it. An active site that adds energy density usually costs rate capability, and the Fe-O₄ motif sidesteps that exchange by doing both jobs at the same place on the surface. For biodegradable implants specifically, the usual assumption has been that safety and performance pull in opposite directions, and work on other biodegradable supercapacitor designs for medical devices has reinforced how difficult the tradeoff can be. This result is evidence that it is softer than it looked.
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