The technique at the heart of the work is called layer-by-layer assembly, a method that builds films one molecular layer at a time by alternately dipping a charged substrate into solutions of positively and negatively charged polymers. In this case, the substrate was a commercially available nylon microfiltration membrane with a positively charged surface. Each dipping cycle deposits a nanometer-thin pairing, or bilayer, of negatively charged humic acid followed by positively charged chitosan, and repeating the cycle builds up a controlled nano-composite coating whose thickness and composition can be tuned with molecular precision.

Humic acid, a major component of the natural organic matter found in soils and waterways, is rich in oxygen-containing functional groups such as carboxyl, carbonyl, and hydroxyl moieties that can complex with dissolved solutes. Chitosan, a linear polysaccharide derived from chitin, carries protonated amino groups under acidic conditions that form strong electrostatic salt bridges with the carboxylate groups of humic acid. Together, the two biopolymers create a dense, interactive separation skin on top of a membrane whose pores would otherwise be far too large to catch small organic molecules like herbicides.

Characterization of the coatings confirmed the assembly proceeded as designed. Ultraviolet-visible spectroscopy showed the absorbance at 256 nanometers rising linearly with each deposited bilayer, indicating uniform growth. Infrared spectroscopy revealed peak shifts consistent with electrostatic bonding between the carboxylate groups of humic acid and the ammonium groups of chitosan. Spectroscopic ellipsometry measured bilayer stacks growing from about 7 nanometers at three bilayers to roughly 19 nanometers at nine bilayers. Atomic force microscopy showed surface roughness increasing from 163 to 321 nanometers after modification, while the effective pore diameter shrank from 0.448 micrometers to 0.2 micrometers, and thermogravimetric analysis confirmed the modified membranes remained thermally stable up to around 450 degrees Celsius.

The filtration tests focused on four herbicides representing distinct chemical classes: the chlorophenoxy compounds 2,4-D and 2,4,5-T, the phenyl urea herbicide buturon, and the neutral amide herbicide diphenamid. All were tested at concentrations of 10 to the minus 4 moles per liter in a dead-end ultrafiltration cell operating at just 20 pounds per square inch and 500 revolutions per minute. The bare nylon membrane barely rejected any of the compounds, with removal rates between roughly 7 and 16 percent. Once coated, performance improved steadily with each added bilayer, and the nine-bilayer membrane delivered the best results of all.

The standout result came from 2,4-D, one of the most widely used weed killers in the world and a suspected human carcinogen, which was rejected at approximately 99 percent. Buturon followed at around 97 percent, 2,4,5-T at about 85 percent, and diphenamid at roughly 33 percent. The differences among these compounds reveal the physics of the separation. The two chlorophenoxy acids are anionic at neutral pH and hydrophobic, so they are retained through a combination of electrostatic repulsion from like charges in the polyelectrolyte matrix, hydrophobic adsorption, and steric blocking. The more polar 2,4-D experienced stronger repulsion than its less polar cousin, explaining its superior rejection.

Buturon, though non-ionic, carries a high dipole moment of 5.44 debyes and a log octanol-water partition coefficient near 3, indicating substantial hydrophobicity. Its rejection appears to arise mainly from hydrophobic adsorption onto humic acid sites combined with steric hindrance from the highly charged bilayer stack. Diphenamid fared worst because this neutral molecule has low polarizability, a modest dipole moment of 3.60 debyes, and limited hydrophobicity, leaving steric effects as its only barrier. Infrared spectra taken after filtration showed a new carbonyl peak at 1717 wavenumbers on the used membranes, direct evidence that herbicide molecules had been adsorbed within the bilayer architecture rather than simply screened by pore size.

The study also mapped how preparation and operating conditions shape performance. The pH of the chitosan deposition bath proved critical: at pH 1.7, chitosan is fully protonated and forms well-fabricated bilayers densely populated with solute-accessible interactive sites, delivering maximum rejection, while higher deposition pH values produced weaker coatings and lower efficiency. Adding salt to the deposition medium screened the charges on the polyelectrolytes, causing them to coil and thicken the multilayer while weakening electrostatic rejection of anionic herbicides. Similarly, anions such as phosphate, sulfate, nitrate, chloride, and acetate in the feed water reduced the rejection of the negatively charged herbicides but left the non-ionic compounds largely unaffected. Flipping the membrane so humic acid formed the exposed outer layer instead of chitosan slightly altered performance for several compounds, underscoring that solute interactions with the outermost layer matter.

Practical durability is where the results become genuinely compelling. Nine-bilayer membranes stored for six months retained nearly all of their original rejection efficiency, and repeated filtration cycles over the same membrane showed only a slight, gradual decline attributed to a reversible fouling layer and the progressive occupation of active sites rather than any mechanical failure of the coating. Because the system operates at low pressure, it consumes far less energy than reverse osmosis or nanofiltration, produces less waste brine, and avoids the aggressive chemical cleaning cycles that shorten the life of high-pressure membranes. The entire separating layer is made from natural, biodegradable materials, giving the approach an environmental profile that synthetic polyelectrolyte coatings struggle to match.

The implications reach well beyond the four herbicides tested. Layer-by-layer coatings of humic acid and chitosan have previously been adapted to capture pesticides as diverse as atrazine, picloram, and metolachlor, and the present work extends that toolbox to chlorophenoxy, phenyl urea, and amide chemistries under a single platform. Because the assembly process works on substrates of varying geometry and can be scaled with straightforward dipping procedures, the author suggests the system could inform the design of pilot plants for membrane-based removal of chemical contaminants from drinking water, bringing affordable, low-energy herbicide filtration closer to real-world deployment for communities whose water supplies carry agricultural residues.

Subject of Research: Nano-composite biopolymer membranes for herbicide removal from water via layer-by-layer assembly under ultrafiltration

Article Title: Low pressure nano-composite biopolymer membranes for the removal of herbicides from water under ultrafiltration conditions

Article References: P., N. C. (2026). Low pressure nano-composite biopolymer membranes for the removal of herbicides from water under ultrafiltration conditions. Journal of Materials Science: Polymers, 1(1), Article 7. https://doi.org/10.1007/s44493-026-00007-4

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00007-4

Keywords: ultrafiltration, layer-by-layer assembly, chitosan, humic acid, herbicides, water purification, nanocomposite membranes, biopolymers, 2,4-D, membrane filtration, micropollutants, low-pressure membranes

Heparin is a sulfated polysaccharide, a sugar molecule studded with negatively charged sulfate groups that give it its signature anticoagulant power. Nearly all clinical heparin is extracted from porcine intestinal tissue, which ties the world’s supply of this critical drug and coating material to the health of pig herds. That dependency has produced real crises. In 2008, contamination of heparin with oversulfated chondroitin sulfate, a by-product of production, caused numerous fatalities and exposed the risks inherent in animal-sourced supply chains. A decade later, the outbreak of African swine fever in China raised the specter of a global heparin shortage, prompting researchers worldwide to search for substitutes that do not depend on livestock at all.

The Colorado State-led team turned to kappa-carrageenan, a polysaccharide harvested from red seaweeds and already widely used as a gelling and thickening agent in food and pharmaceuticals. Kappa-carrageenan carries a single sulfate group per disaccharide unit, giving it a structural echo of heparin, but the researchers went further. Through a chemical modification known as carboxymethylation, they added carboxylic acid groups to the polymer backbone, producing carboxymethyl kappa-carrageenan, or CMKC. The reaction is comparatively gentle: monochloroacetic acid is activated in basic solution and added to the polymer in a propanol-water mixture at 40 to 60 degrees Celsius for a few hours. Crucially, because kappa-carrageenan already contains sulfate groups, the approach sidesteps the harsh and hazardous sulfation chemistry, involving agents like chlorosulfonic acid in pyridine, that would otherwise be needed to build heparin-mimicking molecules from scratch.

The resulting polymer combines two negatively charged functional groups that matter for blood compatibility. Sulfate groups provide the strong negative charge that allows electrostatic interactions with antithrombin III, the natural inhibitor of coagulation that heparin exploits. Carboxyl groups boost hydrophilicity and water retention at the material interface, which helps suppress the nonspecific protein adsorption and cell adhesion that typically trigger clotting on foreign surfaces. Previous work by the same group had shown that CMKC is biocompatible, antioxidant and antibacterial, and that it can be spun into wound-dressing nanofibers or immobilized on 3D-printed bone scaffolds. The new study asked a more demanding question: could CMKC, assembled into nanometer-scale coatings, reproduce the blood-repelling performance of heparin itself?

To build the coatings, the team used the layer-by-layer technique, a method in which alternating layers of positively and negatively charged polymers are deposited onto a surface, driven purely by electrostatic attraction. Chitosan, a positively charged polysaccharide derived from crustacean shells, served as the polycation, while CMKC or, for comparison, heparin served as the polyanion. The researchers grew multilayers of ten, eleven, sixteen and seventeen layers on oxidized glass, monitoring the assembly in real time with Fourier-transform surface plasmon resonance, which confirmed steady, controlled film growth. X-ray photoelectron spectroscopy verified that sulfate groups from the polyanion and amine groups from chitosan were present in every film, and that the surface chemistry could be tuned simply by choosing which polymer formed the final layer.

Physical characterization reinforced the resemblance between the two coating systems. Water contact angle measurements showed that all the multilayers dramatically improved the wettability of glass, a property associated with reduced clotting. Atomic force microscopy revealed nanoscale roughness of roughly 40 nanometers for the sixteen-layer CMKC-chitosan films, considerably rougher than the corresponding heparin films at about 11 nanometers. That difference in topography, the authors suggest, turned out to be consequential, shaping how blood proteins arranged themselves on the surface and, ultimately, how platelets responded.

Before any blood tests, the coatings had to prove they were safe for the cells they might touch. Adipose-derived stem cells cultured on the CMKC-chitosan multilayers showed cytotoxicity values between roughly 5 and 8 percent, comfortably below the 10 percent threshold set by the ISO 10993-5 standard, and metabolic viability comparable to tissue-culture polystyrene controls. Cells actually adhered and proliferated better on the CMKC-terminated films than on untreated glass, with the sixteen-layer coating supporting 28 percent more cells than its chitosan-terminated counterpart after four days. The coatings also displayed striking antibacterial behavior: the sixteen-layer film reduced live Staphylococcus aureus coverage by about 85 percent relative to bare glass after 24 hours, while both film thicknesses held live Pseudomonas aeruginosa coverage below half a percent, with evidence of bacterial membrane damage and no biofilm formation.

The hemocompatibility experiments then pitted the seaweed coating directly against heparin. Measuring protein adsorption by X-ray photoelectron spectroscopy, the team found that the CMKC surfaces adsorbed more fibrinogen and less albumin than the heparin surfaces. That might sound like bad news, since fibrinogen is the protein that seeds clot formation, but the story is more subtle. Fibrinogen’s ability to promote clotting depends on its conformation after adsorption, and the researchers argue that the combination of surface roughness and high negative charge density on the CMKC films binds fibrinogen in a configuration that is unfavorable for platelet attachment. The functional outcome supported this interpretation: platelet adhesion on the CMKC coatings dropped by about 86 percent compared with tissue-culture polystyrene, outperforming the heparin coatings, and scanning electron microscopy showed that the few platelets that did adhere remained largely round and unactivated rather than spreading into their clot-promoting shapes.

The whole-blood clotting assay delivered the headline result. When drops of fresh human blood, collected from healthy donors with institutional review board approval, were placed on the coated surfaces, the blood clotting index, a measure of free hemoglobin released from unclotted red cells, remained roughly 55 percent higher on both the CMKC and heparin coatings than on bare glass after 15 minutes. By 30 minutes, clotting on glass had progressed further, while on both coated surfaces the index actually rose, a sign that any clotting that had begun was being reversed. Statistically, the anticoagulant effect of the seaweed coating was indistinguishable from that of the heparin coating. Notably, the CMKC surfaces also attracted fewer white blood cells than the heparin surfaces, hinting at a reduced likelihood of triggering an inflammatory immune response at the material interface.

The implications extend beyond a single laboratory result. Because layer-by-layer coatings can be deposited on polymers, metals, ceramics and nanostructured materials, and because the process avoids hazardous solvents and waste, the approach could in principle be translated to cardiovascular stents, vascular grafts, catheters and blood-contacting sensors. The authors caution that animal studies and long-term device integration tests will be needed before CMKC coatings reach the clinic, and the in vitro results reported here represent an early but rigorous validation step. Still, the prospect of a blood-compatible surface built from farmed seaweed, free of the contamination risks, ethical concerns and supply shocks that shadow animal-derived heparin, gives the biomaterials community a concrete target. If subsequent studies confirm these findings, the humble red algae that thickens puddings and ice cream may find its most consequential application inside the human bloodstream.

Subject of Research: Carboxymethyl kappa-carrageenan-chitosan polyelectrolyte multilayers as sustainable, heparin-free anticoagulant coatings for blood-contacting medical devices.

Article Title: Carboxymethyl kappa carrageenan polyelectrolyte multilayers as blood contacting surfaces

Article References: Madruga, L. Y. C., Baghersad, S., Câmara, P. C. F., Sabino, R. M., Kipper, M. J., & Popat, K. C. (2025). Carboxymethyl kappa carrageenan polyelectrolyte multilayers as blood contacting surfaces. Discover Biotechnology, 2(1), Article 37. https://doi.org/10.1007/s44340-025-00043-w

Image Credits: AI Generated

DOI: 10.1007/s44340-025-00043-w

Keywords: carboxymethyl kappa-carrageenan, heparin alternative, polyelectrolyte multilayers, blood compatibility, chitosan, anticoagulant coatings, biomaterials, red seaweed polysaccharides, platelet adhesion, antibacterial coatings, layer-by-layer assembly, medical device surfaces