Surface coating could prevent blood clots in medical implants

Zwitterions — a common macromolecule found in human cells — are being used at The University of Sydney to create materials that could stop blood clots from forming in medical devices like heart valves and stents. Such devices play a crucial role in saving lives, yet proteins in blood can cling to the sides of the medical implants, building up over time and forming a blood clot, which often requires invasive surgery to remove or replace the implant.

Zwitterions are remarkable molecules because they are positive and negative at the same time, making them neutral; the word ‘Zwitter’ means ‘hybrid’ in German. They are especially effective at forming bonds with water molecules and are already in our cells as part of the cell membrane. They create a thin layer of water and make sure blood and other proteins travel through the heart and other organs without sticking to other surfaces.

Now, inspired by the cell membrane, Dr Sina Naficy is leading a research team developing heart valves that are more resistant to blood clots — homing in on the zwitterion’s chemically neutral but water-loving ability. The team recently published a review in Cell Biomaterials on the potential of zwitterions in biomedicine, providing a blueprint for the design of surface-coating technologies. 

“Medical implants are constantly under pressure to perform in the human body. A heart valve is constantly under high pressure to pump blood, opening and closing half a billion times over 10 years,” Naficy said.

“The current average lifespan of existing heart valve implants is less than 10 years and there is always a risk of them degrading or complications occurring. By using zwitterion-coated materials, we aim to decrease the risk of blood clots and increase the lifespan of heart valves and other medical implants.”

A zwitterionic coating has been created by the team, and it has been found that on areas of the material ‘painted’ with the coating — only a few nanometres thick — it successfully created a layer and bubble of water, like a ‘watery armour’. On material without the coating, it repelled and spread water beyond the material’s boundaries.

“We are currently exploring new formulations capable of being chemically attached to the surface of any type of implant (made from tissues, metals or plastics/rubbers) with the aim of reducing their interactions with blood,” said team member Dr Sepehr Talebian. The biggest challenge is to determine how many zwitterions are ‘just right’, in something of a biomedical goldilocks problem.

“What is the best way to use zwitterions?” Talebian asked. “What is the ideal thickness of the coating? What concentration should we use? We cannot just dip an artificial heart valve in the zwitterionic substance without investigating the best conditions. Too much, and it could make the clotting worse; too little, and the risk of blood clots remains.

“We also need to investigate the best way to ‘anchor’ zwitterions to the surface of a material, and the best environment for zwitterions. This includes finding the best concentration of salt in a solution with the zwitterions. Too much salt makes Zwitterionic brushes clump together. We want them to spread evenly across surfaces.

“The curious case of zwitterions means researchers like us are working hard to find the optimal conditions for this macromolecule to realise their full potential.”

Image caption: A spiral painted in a zwitterionic coating is revealed after it is dipped in water with food dye. Image ©Ivy Shih/University of Sydney.