From joint replacements to cardiac implants and dialysis machines, medical devices enhance or save lives on a daily basis. However, any device implanted in the body or in contact with flowing blood faces two critical challenges that can threaten the life of the patient the device is meant to help: blood clotting and bacterial infection.
A team of Harvard engineers and scientists may have a solution. They have developed a new surface coating for medical devices using materials already approved by the US Food and Drug Administration (FDA). The coating repelled blood from more than 20 medically relevant substrates the team tested – made of plastic to glass and metal – and also suppressed biofilm formation in a study reported in Nature Biotechnology. But that is not all.
The team implanted medical-grade tubing and catheters coated with the material in large blood vessels in pigs, and it prevented blood from clotting for at least eight hours without the use of blood thinners such as heparin. Heparin is notorious for causing potentially lethal side-effects like excessive bleeding, but is often a necessary evil in medical treatments where clotting is a risk.
"Devising a way to prevent blood clotting without using anticoagulants is one of the holy grails in medicine," said Don Ingber MD PhD, founding director of Harvard's Wyss Institute for Biologically Inspired Engineering and senior author of the study. Ingber is also the Judah Folkman professor of vascular biology at Harvard Medical School and Boston Children's Hospital, as well as professor of bioengineering at Harvard School of Engineering and Applied Sciences (SEAS).
The idea for the coating evolved from SLIPS, a pioneering surface technology developed by co-author Joanna Aizenberg PhD, who is a Wyss Institute core faculty member and the Amy Smith Berylson professor of materials science at Harvard SEAS. SLIPS stands for slippery liquid-infused porous surfaces. Inspired by the slippery surface of the carnivorous pitcher plant, which enables the plant to capture insects, SLIPS repels nearly any material it contacts. The liquid layer on the surface provides a barrier to everything from ice to crude oil and blood.
"Traditional SLIPS uses porous, textured surface substrates to immobilise the liquid layer whereas medical surfaces are mostly flat and smooth – so we further adapted our approach by capitalising on the natural roughness of chemically modified surfaces of medical devices," said Aizenberg, who leads the Wyss Institute's Adaptive Materials platform. "This is yet another incarnation of the highly customisable SLIPS platform that can be designed to create slippery, non-adhesive surfaces on any material."
Super-repellant coating for devices
[caption id="attachment_18379" align="alignright" width="500"] This scanning electron microscope image shows how red blood cells coagulate to form a blood clot, which is a common and life-threatening risk associated with the use of implanted medical devices (credit: James Weaver, Harvard’s Wyss Institute)[/caption]
The Wyss team developed a super-repellent coating that can be adhered to existing, approved medical devices. In a two-step surface-coating process, they chemically attached a monolayer of perfluorocarbon, which is similar to Teflon. Then they added a layer of liquid perfluorocarbon, which is widely used in medicine for applications such as liquid ventilation for infants with breathing challenges, blood substitution, eye surgery and more. The team calls the tethered perfluorocarbon plus the liquid layer a tethered-liquid perfluorocarbon surface, or TLP for short.
In addition to working seamlessly when coated on more than 20 different medical surfaces, and lasting for more than eight hours to prevent clots in a pig under relatively high blood flow rates without the use of heparin, the TLP coating achieved the following results: