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

The work, published in Biomedical Microdevices, extends a long-running effort to build what the group calls an artificial placenta: a lung assist device assembled from arrays of single oxygenator units, each a microfluidic chip fabricated from polydimethylsiloxane, the transparent silicone elastomer beloved by microfluidics engineers. PDMS is easy to mold, gas-permeable and optically clear, but in contact with blood it is profoundly thrombogenic. When plasma proteins adsorb onto its hydrophobic surface, they undergo conformational changes that activate the coagulation factors, platelets and complement proteins that normally patrol the vasculature. In a device whose channels are measured in tens or hundreds of micrometers, even a small clot can occlude flow, degrade gas exchange and shed emboli into the patient’s circulation.

The McMaster strategy centers on a molecule with an unusual pedigree. Antithrombin is the body’s natural brake on coagulation, a serine protease inhibitor that neutralizes thrombin and factor Xa. Heparin accelerates this inhibition dramatically, but heparin immobilized on biomaterial surfaces has historically underperformed, because the pentasaccharide sequence that activates antithrombin must be presented in a specific orientation and because bound heparin alone cannot catalyze inhibition without recruiting antithrombin from plasma. To sidestep these limitations, Anthony Chan, John Brash and colleagues developed a covalent antithrombin-heparin complex in which the two molecules are permanently linked, preserving the catalytic machinery in a single, surface-tethered unit. Earlier studies showed that such complexes, when coated onto flat PDMS using polydopamine as an adhesive layer, could render the material blood-compatible.

Polydopamine itself is a piece of bioinspired chemistry borrowed from marine mussels, which anchor themselves to rocks in churning surf using adhesive proteins rich in the amino acid DOPA. When dopamine is oxidized under mildly alkaline conditions, it polymerizes into a thin, conformal film that adheres tenaciously to virtually any surface, from metals to polymers, through a combination of covalent and noncovalent interactions. In the new study, the team flowed a polydopamine solution through the microchannels of the oxygenator units, then introduced the antithrombin-heparin complex under flow as well, allowing the coating to build up uniformly inside the tortuous three-dimensional geometry that flat-surface experiments cannot fully replicate.

Quantifying what sticks to the inside of a sealed microfluidic device is a technical challenge in its own right. The researchers solved it by radiolabelling the antithrombin-heparin complex, which allowed them to measure surface density directly: the coated units carried 0.21 plus or minus 0.05 micrograms of the complex per square centimeter. More importantly, the coating proved durable. When the modified devices were perfused with flowing blood for two days, 76 percent of the bound complex remained on the surface, a stability figure that matters enormously for any device intended to support a neonate for days or weeks. A coating that leaches away within hours would offer only fleeting protection and might itself become a source of embolic debris.

Surface density alone does not guarantee function, so the team also measured whether the immobilized heparin retained its biological activity. Their assay exploited the fact that active heparin binds antithrombin from plasma with high affinity. Devices coated with the antithrombin-heparin complex captured 47.78 plus or minus 10.63 nanograms of antithrombin per square centimeter from plasma, roughly four times the 11.56 plus or minus 4.58 nanograms per square centimeter measured on devices coated with polydopamine alone. That fourfold difference demonstrates that the covalent complex presents heparin in a catalytically competent configuration, effectively turning the entire blood-contacting surface of the device into an anticoagulant reactor that continuously neutralizes thrombin as blood passes through.

The functional consequences were visible at the macroscopic scale. When plasma was perfused through the modified units for one hour, the devices resisted clotting, whereas unmodified or polydopamine-only controls showed the fibrin deposition and flow obstruction characteristic of biomaterial-triggered coagulation. Just as critically, the researchers verified that the coating did not compromise the device’s primary job. Oxygen permeability, the property that allows the thin PDMS membranes to transfer gas between an oxygen supply and the blood, was unchanged by the surface treatment. That dual requirement, anticoagulant function without degraded gas exchange, has been the stumbling block for many previous hemocompatibility strategies, including polyethylene glycol layers and zwitterionic coatings, which can delaminate or alter transport properties over time.

The clinical context gives the work its urgency. Neonatal extracorporeal membrane oxygenation, or ECMO, remains an anticoagulation enigma, as pediatric intensivists have described it, because the systemic heparin required to keep circuits patent exposes infants, whose hemostatic systems are immature, to serious bleeding risks including intracranial hemorrhage. Ventilator-induced lung injury adds another layer of harm for preterm babies, whose alveoli can be damaged by the very pressures meant to keep them alive. A lung assist device whose internal surfaces actively inhibit clot formation could reduce the systemic anticoagulation burden, and the artificial placenta concept envisions pumpless microfluidic oxygenator arrays that could support preterm neonates with far less trauma than conventional extracorporeal circuits.

What distinguishes the new study is the translation from flat substrates to functional devices under realistic flow conditions. Coating chemistry that works on a flat coupon frequently fails inside a microchannel, where flow profiles, channel aspect ratios and surface-to-volume ratios conspire to produce uneven films. By performing both the polydopamine deposition and the complex immobilization under flow, the team showed that the strategy scales to the device level, a prerequisite for assembling the single oxygenator units into the integrated arrays that would constitute a clinical lung assist device. The authors note that the results demonstrate a previously developed modification strategy can be translated from flat PDMS substrates to microfluidic units, providing device-level anticoagulant function without measurably compromising membrane oxygen permeability under the conditions tested.

Challenges remain before the technology reaches the neonatal intensive care unit. The one-hour plasma clotting resistance and two-day stability experiments, while encouraging, must be extended to longer durations, whole blood and ultimately animal models, and the group has already explored pairing the antithrombin-heparin coating with immobilized tissue plasminogen activator to add fibrinolytic activity to the anticoagulant function. Regulatory pathways for combination products that blend a device with a pharmacologically active surface will also demand careful scrutiny. Still, the study offers a compelling proof of concept: by borrowing the adhesive tenacity of a mussel and the catalytic elegance of a natural anticoagulant complex, the researchers have shown that the surfaces of life-supporting microdevices can be engineered to fight the clotting response they provoke, bringing the artificial placenta one step closer to the bedside of the tiniest patients.

Subject of Research: Covalent antithrombin-heparin surface modification of PDMS microfluidic oxygenator units for anticoagulant function in neonatal lung assist devices

Article Title: Surface modification of microfluidic oxygenator units with an antithrombin-heparin (ATH) covalent complex for enhanced anticoagulant function

Article References: Li, S., Sandejas, D., Saraei, N., Dabaghi, M., Atkinson, H. M., Fusch, G., Rochow, N., Fusch, C., Selvaganapathy, P. R., Chan, A. K. C., Brash, J. L., & Sask, K. N. (2026). Surface modification of microfluidic oxygenator units with an antithrombin-heparin (ATH) covalent complex for enhanced anticoagulant function. Biomedical Microdevices, 28(3), Article 60. https://doi.org/10.1007/s10544-026-00842-w

Image Credits: AI Generated

DOI: 10.1007/s10544-026-00842-w

Keywords: antithrombin-heparin complex, microfluidic oxygenator, polydopamine, PDMS surface modification, blood compatibility, neonatal respiratory distress, artificial placenta, lung assist device, anticoagulant coating, hemocompatibility, Biomedical Microdevices, ECMO