How Does the Body Remove Graphene Oxide?

The body removes graphene oxide through several overlapping systems: immune cells digest it with enzymes, the kidneys filter smaller sheets into urine, the liver processes and excretes it through bile, and even gut bacteria can break it down. No single pathway handles the job alone. Which route dominates, and how quickly the material clears, depends heavily on the size of the graphene oxide sheets and what coatings or chemical groups sit on their surface. The process can take anywhere from hours to many months, and some material lingers in organs long after initial exposure.

Enzymatic Breakdown by Immune Cells

The most direct form of removal is enzymatic degradation, where immune cells chemically dismantle graphene oxide sheets. Neutrophils, the white blood cells that arrive first at sites of inflammation, produce an enzyme called myeloperoxidase (MPO). This enzyme was originally shown to digest oxidized carbon nanotubes, and researchers confirmed that graphene oxide also breaks down when exposed to purified MPO.¹ The enzyme works by generating highly reactive molecules, particularly hypochlorous acid, that attack oxygen-containing groups on the graphene oxide surface. These reactive species punch holes in the carbon lattice, gradually fragmenting the sheet until it loses its structure entirely.

Neutrophils are not the only immune cells with this capability. Eosinophils, another type of white blood cell commonly associated with allergic responses and parasite defense, produce their own peroxidase called eosinophil peroxidase (EPO). In laboratory experiments, recombinant EPO successfully degraded three different graphene oxide samples within about 90 hours when given hydrogen peroxide and bromide ions to work with. Electron microscopy and spectroscopic analysis confirmed that the graphene oxide structure was genuinely dismantled, not just altered on the surface.²

The immune response to graphene oxide goes beyond quiet enzymatic digestion. Neutrophils confronted with graphene oxide generate bursts of reactive oxygen species and can deploy neutrophil extracellular traps (NETs), web-like structures made of DNA and antimicrobial proteins that neutrophils release to ensnare pathogens.³ The size of the graphene oxide particles influences which defensive strategy neutrophils choose: larger sheets tend to trigger NETosis (the release of those extracellular traps), while smaller, nanometer-scale particles prompt degranulation, where the neutrophil dumps its enzyme contents directly.⁴

Where Graphene Oxide Accumulates in the Body

Before it can be cleared, graphene oxide has to go somewhere. In studies using intravenously injected graphene oxide in mice, the material disappeared from the bloodstream quickly and accumulated primarily in the liver and lungs, regardless of the particle size used.⁵ The spleen is another major collection site. These three organs, the liver, spleen, and lungs, make up the reticuloendothelial system, a network of immune cells whose job is to filter foreign particles from the blood. PEGylated graphene oxide (coated with polyethylene glycol, a common biocompatible polymer) follows the same general pattern, accumulating in the liver and spleen before being gradually cleared through a combination of urine and fecal excretion.⁶

One early biodistribution study in mice found that graphene oxide was concentrated in the lung, liver, and spleen, with very little reaching the kidneys and none detected in the brain, suggesting it does not cross the blood-brain barrier.⁷ The large sheet dimensions of some preparations made the researchers speculate that graphene oxide is primarily expelled through liver secretion into the bile tract rather than through kidney filtration. Later research has complicated that picture somewhat, showing that kidney excretion is real and significant for smaller sheets, but the liver-bile pathway remains important for larger ones.

Size also affects lung accumulation specifically. Imaging studies comparing three size ranges of graphene oxide found that all three accumulated in the spleen and liver, but only the largest sheets piled up substantially in the lungs.⁸ All three sizes showed extensive urinary excretion, which means that even when the material first lands in an organ, a portion eventually makes its way out.

Kidney Filtration and Urinary Excretion

The kidneys are a surprisingly active clearance route for graphene oxide, given that the material’s sheet-like shape might seem too large to pass through the kidney’s ultrafiltration barrier. The key is that graphene oxide sheets are extremely thin, essentially flat, and when they are small enough in lateral dimensions they can slip through the glomerular filter the way a letter slides through a mail slot even though the envelope is wider than the opening.

Smaller graphene oxide sheets pass through the glomerulus, the kidney’s primary filtration unit, while larger sheets take a different route: they are excreted through a process called proximal tubular secretion, and they actually leave the body faster than the smaller ones do.⁹ This is a somewhat counterintuitive finding, since you might expect bigger particles to be harder to excrete. The catch is that the larger sheets were associated with greater kidney injury, meaning faster clearance came at a cost.

For very thin, functionalized graphene oxide sheets, renal excretion can account for a large fraction of the injected dose. One PET/CT imaging study found that roughly three-quarters of the injected dose of thinner sheets was excreted through the glomerular filter into urine.¹⁰ Graphene quantum dots, the smallest category of graphene-based particles, pass through the kidney’s filtration barrier efficiently and are excreted in urine without apparent kidney damage.¹¹

Processing Through the Liver and Bile

The liver handles graphene oxide in two main ways, and which cells do the work depends on particle size. Smaller graphene oxide sheets end up inside hepatocytes (the liver’s main functional cells) and are spotted in bile ducts, appearing as dot-like particles that suggest they had already begun degrading inside the hepatocyte before being secreted into bile.¹² Larger sheets, by contrast, were found inside Kupffer cells, the liver’s resident macrophages, suggesting they get taken up by the immune component of the liver rather than processed through the bile route. Smaller sheets also caused some structural changes in the liver, including widening of blood-filled spaces and swelling of fat-storing cells.

Material that reaches the bile tract eventually empties into the intestine and leaves the body in feces. Together, the kidney-urine and liver-bile-feces pathways account for the bulk of whole-body clearance of graphene oxide that makes it into the bloodstream. Which pathway dominates for a given particle depends, as noted, on sheet dimensions and surface chemistry.

Clearance from the Airways

When graphene oxide is inhaled rather than injected, the clearance situation looks different. The airways have their own defense: a mucus layer that traps particles, moved along by the beating of cilia until it reaches the throat and is swallowed. A study using a three-dimensional human airway model exposed repeatedly to aerosolized graphene oxide over 30 days found signs of this mucus-based clearance at work, with an overall decrease in graphene oxide-loaded vesicles per cell by the end of the exposure period.¹³ But the researchers also noticed that remaining vesicles had migrated toward the basolateral side of the tissue, the bottom layer facing the bloodstream, hinting that some graphene oxide was translocating through the airway wall rather than being cleared upward. This means that mucociliary clearance handles part of the inhaled dose, but not all of it.

In animal studies, graphene nanoplatelets instilled into the lungs were found to translocate over time into mediastinal lymph nodes, the immune tissue clusters near the center of the chest.¹⁴ Acute lung inflammation resolved within weeks, but the progressive accumulation in lymph nodes suggests that immune cells in the lungs were slowly ferrying particles out of the airways and depositing them elsewhere, a pattern seen with other inhaled nanoparticles as well.

Why Size and Surface Chemistry Steer the Outcome

A recurring theme across all these pathways is that the physical and chemical characteristics of graphene oxide sheets determine almost everything about how the body handles them. Size dictates which organs accumulate the material, which kidney pathway filters it, and which neutrophil defense mechanism gets activated. But surface chemistry is equally important for enzymatic breakdown specifically.

Myeloperoxidase, the primary enzyme that immune cells use to digest graphene oxide, does not work equally well on all preparations. Research comparing graphene oxide samples with different degrees of oxidation found that MPO completely degraded highly dispersible (well-oxidized, water-loving) sheets but failed to break down sheets that tended to clump together in solution.¹⁵ The key factors were the material’s water-friendliness, negative surface charge, and stability in solution. Graphene oxide that aggregated into clumps essentially hid reactive sites from the enzyme, making enzymatic attack much harder.

Coating graphene oxide with polyethylene glycol (PEG) changes the clearance dynamics as well. PEGylation reduces retention in the liver, lungs, and spleen and promotes faster clearance from those organs compared to uncoated graphene oxide. But even with a PEG coating, graphene oxide material was still detectable in mice three months after injection.¹⁶ PEGylation helps, but it does not make graphene oxide disappear overnight.

Once graphene oxide enters the bloodstream, proteins from blood plasma rapidly coat its surface, forming what researchers call a protein corona. This corona changes how the immune system recognizes the material and which cells take it up, effectively giving the graphene oxide a new biological identity. The composition of this protein layer depends on the original surface chemistry, charge, and size of the sheets, which is part of why small changes in manufacturing can produce dramatically different clearance profiles.

How Long Clearance Takes

The timeline for graphene oxide removal varies enormously. At one extreme, small functionalized sheets passing through the kidneys can show up in urine within hours. At the other extreme, graphene oxide captured by spleen macrophages can take the better part of a year to degrade.

The most detailed timeline comes from a study tracking biological-grade graphene oxide sheets in the spleens of mice. Using electron microscopy and Raman spectroscopy, the researchers watched macrophages in the spleen’s marginal zone slowly dismantle trapped graphene oxide over nine months. By day 270, the crystalline structure of the accumulated material had been largely lost, and the sheets had become increasingly defective, riddled with holes, and difficult to distinguish from cellular debris.¹⁷ The proposed mechanism involves macrophages generating a cocktail of reactive species, including peroxynitrite, hypochlorite, and hydroxyl radicals, that attack the oxygen-containing groups scattered across the graphene surface. Iron ions already present inside the macrophages may also contribute by catalyzing the breakdown of hydrogen peroxide into additional radicals.

A three-month study of few-layer graphene sheets (a close relative of graphene oxide) found persistence in the lung, liver, spleen, and kidney throughout the study period, with pathological changes including chronic inflammation, fluid buildup in the lungs, and structural damage in the liver and spleen at the doses tested.¹⁸ Carboxylation and PEGylation of the sheets altered the degree of these effects, consistent with the general principle that surface chemistry modulates both clearance speed and tissue damage.

The overall picture is that a single dose of graphene oxide does not linger forever, but it is far from a quick exit. Some fraction leaves through the urine and feces within days; the rest gets progressively dismantled by immune cells over weeks to months, with complete clearance from organs like the spleen potentially taking nine months or longer.

Gut Bacteria as a Surprising Clearance Pathway

One of the more unexpected findings in this field is that gut bacteria can break down graphene oxide and actually incorporate the carbon into their own metabolic products. A study using isotope-labeled carbon nanomaterials, including both single-walled carbon nanotubes and graphene oxide, found that the gut microbiota of mice degraded and fermented these materials.¹⁹ Through isotope tracing, the researchers showed that inorganic carbon from the graphene oxide was being incorporated into butyrate, a short-chain fatty acid normally produced by gut bacteria during the fermentation of dietary fiber. The carbon atoms from the nanomaterial were entering the bacteria’s pyruvate pathway and coming out the other side as an organic molecule the body actually uses.

This pathway is relevant for any graphene oxide that reaches the intestine, whether it arrives via bile excretion from the liver, mucociliary clearance from the lungs (swallowed with mucus), or direct oral ingestion. Rather than simply passing through the gut as inert waste, a portion of the material gets co-opted into the gut’s existing biochemistry. The practical significance of this pathway in terms of how much graphene oxide it accounts for is still being worked out, but it represents a genuinely different kind of clearance: not excretion or enzymatic destruction, but metabolic recycling. Your gut bacteria, in other words, can eat the stuff and turn it into something your colon cells normally feed on.

Why Reduced Graphene Oxide Behaves Differently

Most of this research focuses on graphene oxide in its oxidized form, which is studded with oxygen-containing groups that make it water-dispersible and relatively accessible to enzymes. Reduced graphene oxide (rGO), which has had many of those oxygen groups stripped away, behaves more like pristine graphene and is harder for the body to process. It is less water-soluble, more prone to aggregation, and offers fewer chemical handholds for peroxidase enzymes to grab onto. The enzymatic degradation that works reasonably well on well-oxidized graphene oxide slows down or fails entirely when the material is more reduced and hydrophobic, following the same logic as the dispersibility findings for MPO.

Reduced graphene oxide does still interact with the immune system and can accumulate in lymph nodes when PEGylated and injected, where it has been explored as a potential vaccine delivery platform.²⁰ But the body’s ability to fully break down and clear reduced forms is slower and less certain than for graphene oxide, which is one reason that many biomedical applications prefer the oxidized version when the material needs to eventually disappear from the body.