Materials
couple of decades as the next generation of non-fouling materials for biomedical applications”.
What are zwitterionic polymers? Zwitterionic polymers are a specialist type of polymer which have oppositely charged groups within the repeating unit of polymer. These oppositely charged groups are equally distributed at the molecular level along the chain of the polymer, meaning that zwitterionic polymers still have an overall neutral charge (i.e. a balanced molecular charge). Plus, because they readily interact with water, this “forms the foundation for a range of exceptional properties, such as resistance to protein adsorption, promoting the stability of proteins, microbial resistance, high ionic conductivity, pH-responsiveness, and lubrication properties,” says Zhang. “Because of these properties, zwitterionic polymers are mainly used as surface coatings for medical devices. They are used for anticoagulant, anti-infection, and lubrication purposes”. There are also several types of zwitterionic polymers, such as polybetaines, amino acid-derived zwitterionic polymers, and mixed charge pseudo-zwitterionic polymers. The types most used in medical devices are polysulfobetaine (PSB), polycarboxybetaine (PCB), and phosphorylcholine (PC)-based zwitterionic polymers (such as PMPC) because they all exhibit a good biocompatibility, strong hydrophilicity, strong resistance to non-specific adsorption, and anti-fouling capabilities. While the common polymers share key properties, they all also bring their own specific properties to a medical device (these vary from polymer to polymer and device to device) – which is why many different materials are used today. Thanks to the range of zwitterionic polymers available, they can be integrated into a wide range of medical devices to reduce the reliance on PEG.
A superior surface
The reason for using zwitterionic polymers so widely is directly related to their fundamental properties. And it’s these which set them apart from conventional PEG materials. For one, the regularity of cationic (positively charged) and anionic (negatively charged) groups along the polymer chain means that zwitterionic polymers retain water via ionic solubilisation (an ability that allows the polymer to absorb and ‘trap’ water in its polymer network using the groups on its surface). PEG binds to water via hydrogen bonding methods, but the interactions in zwitterionic polymers provide a much better resistance to non-specific adsorption from proteins and bacteria. “Protein adsorption is the first step of many biofouling events, such as thrombosis, biofilm formation and foreign body reaction,” says Zhang. The anti-protein and anti-bacteria properties in zwitterionic polymers stem from their surface properties, he explains. “Zwitterionic polymers are
super hydrophilic and super-biocompatible. They’re the best materials known to resist the nonspecific protein adsorptions at the material-tissue interface and have better surface hydration properties than PEG.” Zwitterionic polymers have good anti-fouling properties because their enhanced surface hydration properties lead to superhydrophilicity. With water molecules held closer to the surface of the polymer, it makes it more difficult for proteins and bacteria to stick to that surface. This is why fouling occurs more in materials that have hydrophobic properties, as there is a drier solid interface that can easily be adhered to. Due to their positive and negatively charged regions, zwitterionic polymers have an enhanced electrostatic interaction, Zhang explains. “This improves the hydrogen bonding between water molecules and hydrophilic charged groups in zwitterionic materials, allowing water to be held more strongly…this strong hydration is key to their non-fouling properties”. The overall neutral charge of zwitterionic polymers plays a role here, too, as adhesion is favoured in materials with a net charge. This is because there are more active sites to form electrostatic interactions, hydrophobic interactions, and Van der Waals interactions (intermolecular forces that are based on attraction and repulsion between molecules and atoms).
Improving medical devices The versatility of zwitterionic polymer structures, coupled with their excellent superhydrophilicity, are useful for a number of medical devices used both inside and outside the body – and particularly for devices that directly interface with blood and other biofluids. “All devices in contact with blood can use zwitterionic polymer surface coatings,” says Zhang. This is because these devices – like stents, catheters, and ventricular assist devices (VADs) – need to interact with blood without causing toxicity, a property called haemocompatibility. Zwitterionic polymers have been shown to inhibit thrombosis, control the degradation
Medical Device Developments /
www.medicaldevice-developments.com 75
Zwitterionic polymer coatings can enhance haemocompatibility in blood-contact devices such as stents, preventing thrombosis, controlling degradation, and reducing corrosion.
Photo Oz/
Shutterstock.com
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