Coatings & surface treatment
Hydrophilic zwitterionic surfaces Zwitterionic polymer molecules contain both positive and negative charges, so while they are electrically balanced overall, they have locally charged regions that can affect how they interact with other molecules. For example, one of the main benefits of zwitterionic molecules is that they favourably interact with water to form a tightly bound hydration layer that creates a hydrophilic surface. “To adhere to a medical device surface, a protein or polysaccharide has to physically displace the water. This is challenging in the presence of densely packed zwitterions,” states Cook.
The hydrophilic surface resists interaction with biological matter, reducing protein adsorption and helping to limit subsequent biofilm formation and bacterial colonisation. Zwitterionic coatings can be applied to medical device surfaces via methods such as Surface-Initiated Atom Transfer Radical Polymerisation (SI-ATRP), photo-polymerisation, plasma deposition, dip coating, layer-by-layer (LbL) assembly or covalent grafting.
“I think one of the biggest misconceptions is that zwitterionic coatings are not antimicrobial coatings in the traditional sense. They don’t kill bacteria like silver or antibiotics do. Instead, they work by preventing the very first step that allows bacteria to colonise a medical device,” explains Gregory Teh, an expert in hydrophilic zwitterionic surface coatings for medical devices. “Materials such as phosphorylcholine, sulfobetaines and carboxybetaines form a highly stable hydration layer on the surface, with phosphorylcholine being one of the most clinically established zwitterionic materials. That hydrated interface is energetically unfavourable for proteins, platelets and bacteria to attach.” Zwitterionic surfaces can strongly resist protein adsorption and biofouling, although performance depends on the specific chemistry and application. For example, the hydration layer, or hydration shell, generated by the zwitterionic surface acts as both a physical and energetic barrier to protein adsorption. Overall charge neutrality limits electrostatic attraction to charged biomolecules. In contrast, the nature and arrangement of the charged groups can be tailored to alter surface properties for particular medical devices and the biofluids they encounter. The strongly hydrated interfaces can also remain effective under physiological ionic conditions, although performance varies with the chemistry and environment.
Zwitterionic surfaces are versatile and can be used with many types of medical devices. These range from heart valves and stents to bone screws and even medical tubing and biosensors. “Zwitterionic polymers can be beneficial to any implantable medical device, as they can resist both capsule formation and bacterial colonisation, but one good example is indwelling sensors needed for monitoring biological molecules, such as glucose,” Cook confirms.
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Teh expands the list by stating: “Zwitterionic coatings are particularly attractive for implantable devices where long-term blood or tissue contact is unavoidable, including neurovascular flow diverters and intracranial stents, coronary and peripheral vascular stents, heart valves, vascular grafts, haemodialysis catheters, central venous catheters, long-term urinary catheters, membrane oxygenators (ECMO), ventricular assist devices (VADs) and drug- delivery implants.”
Medical devices that regularly contact blood are particularly susceptible to biofouling, and zwitterionic surfaces can therefore serve as an advanced barrier coating. “For blood-contacting devices, there’s an additional benefit. Platelet adhesion is likewise significantly reduced, meaning thrombosis and inflammation are also minimised. Since inflammation itself promotes bacterial colonisation, reducing thrombogenicity provides another indirect mechanism for lowering infection risk,” says Teh. “This is why phosphorylcholine coatings have become particularly attractive. They simultaneously address haemocompatibility, biofouling and infection risk through one surface engineering strategy rather than requiring multiple coatings.”
Although zwitterionic coatings are among the promising advanced technologies for reducing bacterial colonisation and biofilm formation, challenges remain. “One important challenge for permanent or very long-term implants is whether passive anti-fouling alone can maintain its effectiveness over many years. Over time, this biological conditioning layer may partially mask the zwitterionic surface, reducing its anti-fouling effectiveness,” says Teh. “For devices intended to remain implanted for many years, maintaining coating integrity, durability and resistance to biological ageing becomes just as important as the initial anti-fouling performance. Long-term stability therefore depends not only on the zwitterionic chemistry itself but also on robust covalent surface attachment, coating durability, resistance to hydrolysis, and mechanical wear under physiological conditions.”
A recurring challenge raised by experts in both academia and industry is ensuring that the coating remains attached to the medical device. “The next challenge we face is making the coatings durable,” states Cook, and Teh expands on this. “The biggest challenge is making sure the coating remains attached and functional throughout manufacturing, sterilisation, storage and clinical use. Medical devices undergo numerous processing steps, including laser cutting, crimping, balloon expansion, catheter assembly, sterilisation and packaging. A coating must survive all of these without cracking, delaminating or changing performance.” On the subject of manufacturing, Teh also explains that: “Manufacturability is yet another challenge. It’s relatively easy to coat a flat coupon in a laboratory. It’s
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