Tribology of Cardiac Implants

Introduction

According to American Heart Association’s 2017 Heart Disease and Stroke Statistics Update, nearly 801,000 deaths in the US had cardiovascular disease as the underlying cause of death. That’s about 1 of every 3 deaths in the US [1]. Cardiovascular devices are used to diagnose and treat heart disease and related health problems [2]. The global cardiovascular devices market was valued at $40.05bn in 2016 and is expected to grow at a high rate in the coming years. These implants comes with a hefty price tag and adding surgery and other hospital costs, can rip apart anybody’s savings hence many patients are unable to afford these implants [3][4]. If someone is paying a large sum of money for these implants, the quality and performance of these devices should be impeccable. These devices are tested both for biological and mechanical problems.

Cardiovascular implants are susceptible to tribological problems both friction and wear related. A summary of the mechanical issue that can hamper their performance is shown below:

Ventricular Assist Device (VAD)

These are mechanical pumps that are used to direct blood away from a damaged ventricle and provides a proper blood flow maintaining the blood circulation. These mechanical pumps contain metal bearings. Friction and wear of these bearings is common can result in overheating and a reduced lifetime. VAD bearings faliure can lead to ailments like Thrombosis (formation of a blood clot inside a blood vessel) and Haemolysis (rupture or destruction of red blood cells). [5][6]

As a solution to that problem are VAD equipped with magnetic bearings, by removing the mechanical contact, friction, wear, and overheating are reduced. In one of the research work carried out by Qian KX and co-workers [8], it has been found that the use of rolling bearing instead of sliding bearings in rotary pumps reduces friction to about 1/15 of the sliding bearing. Also rollers made of ultra-high-molecular weight polythene have anti-wear property 8 times better than the metal ones.

Mechanical seals used in Ventricular Assist Devices also experience friction and wear realted issues. The VAD mechanical seals maintain the blood pressure while acodigin the leakage of blood. Koki Kanda and co-workers [9] studied the effect of surface texture on frictional properties of mechanical seals. They found that frictional properties of the mechanical seals were stabilized by creating small, dispersed concave features with wet blast fabrication, followed by coating with diamond-like carbon.

Stents and grafts

Stents are cylindrical metallic frames which are expanded at high pressure inside a blood vessel to keep the passageway open (see animation below). On the other hand, Grafts are tubes that are shaped like blood vessels. These you used to bypass a blockage or defect entirely by creating an alternate vessel for your blood to flow through, rather than try and fix it [10]. The material used for stents and grafts should have low friction, wear and low corrosion characteristics as they are in continuous contact with the soft tissues in the region and the endothelial cell layer.

Nitinol which is a Nickel Titanium alloys are generally used to manufacture stents. Characteristics such as super-elasticity, shape memory effect, corrosion resistant, low friction and wear makes it a prime choice. Stents can also be coated with biocompatible coatings or polymer-free coatings which can also help achieve low friction and wear. UHMWPE (Ultra high molecular weight polyethylene) and ePTFE (Expanded PTFE) are the most preferred graft materials. Both UHMWPE and ePTFE have low coefficient of friction and anti-wear capabilities. [11][12][13]

Artificial heart valves

An artificial heart valve is a device implanted in the heart of a patient to replace a malfunctioning natural valve. Mechanical heart valves (MHV) are commonly used as artificial heart valves. The material used for manufacturing of MHVs should be strong, durable, low friction and wear resistant. In the absence of these qualities, the MHV’s material can give rise to several other diseases. [14]

Pyrolytic Carbon or PyC is a fatigue resistant, biocompatible, durable material which is normally used for manufacturing MHVs. Diamond-like carbon (DLC) is an attractive wear-resistant coatings for MHVs. Biocompatibility and excellent tribological properties of DLC coating provide long durability to MHV’s material. [15]

References

  1. Heart Disease and Stroke Statistics 2017 At-a-Glance, https://www.heart.org/idc/groups/ahamah-public/@wcm/@sop/@smd/documents/downloadable/ucm_491265.pdf
  2. Cardiovascular Devices https://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/CardiovascularDevices/default.htm
  3. Report details, Global cardiovascular devices market forecast 2017-2027, https://www.visiongain.com/Report/1768/Global-Cardiovascular-Devices-Market-Forecast-2017-2027
  4. India-made heart pump at 1/3rd cost, http://timesofindia.indiatimes.com/life-style/health-fitness/health-news/India-made-heart-pump-at-1/3rd-cost/articleshow/52667592.cms
  5. Left ventricular assist devices PPT on slideshare.net by patacsi https://www.slideshare.net/patacsi/left-ventricular-assist-devices
  6. The Future is Here: Ventricular Assist Devices for the Failing Heart, http://www.medscape.com/viewarticle/709956_2
  7. Wang, Zhi-Qiang, Hua-Chun Wu, Pu Chen, Zheng-Yuan Zhang, and Yong-Wu Ren. “Numerical Simulation of Performance for Axial Flow Maglev Blood Pump.” Medicine Sciences and Bioengineering (2015): 3-6
  8. X. Qian, P. Zeng, W. M. Ru, H. Y. “Axial Reciprocation of Rotating Impeller: A New Concept of Antithrombogenecity in Centrifugal Pump.” Journal of Medical Engineering & Technology 25.1 (2001): 25-27.
  9. Kanda, Koki, Hirotsuna Sato, Hisa Kinoshita, Takayuki Miyakoshi, Hideki Kanebako, and Koshi Adachi. “Influence of Surface Texture on Friction Properties of Mechanical Seals for Blood -Design Concept of Sealing Surface of Mechanical Seal for Ventricular Assist Device-.” Tribology Online 11.2 (2016): 366-75
  10. Difference between stent and graft, https://www.quora.com/What-is-the-difference-between-Stent-and-Graft
  11. Dunn, Alison C., Toral D. Zaveri, Benjamin G. Keselowsky, and W. Gregory Sawyer. “Macroscopic Friction Coefficient Measurements on Living Endothelial Cells.” Tribology Letters 27.2 (2007): 233-38
  12. Isa CT Santos, Alexandra Rodrigues, Lígia Figueiredo, Luís A Rocha, João Manuel RS Tavares. “Mechanical properties of stent–graft materials” Mechanical properties of stent-graft materials. Proceedings of the Institution of Mechanical Engineers, Part L – Journal of Materials Design and Applications, Vol 226, Issue 4, 2012
  13. Drug-eluting Stent Coatings, http://www.ozee.com/shows/sa-re-ga-ma-pa-lil-champs-2017
  14. Artificial heart valve https://en.wikipedia.org/wiki/Artificial_heart_valve
  15. Hauert, R. “A Review of Modified DLC Coatings for Biological Applications.” Diamond and Related Materials 12.3-7 (2003): 583-89.

Harshvardhan Singh works as a Senior Service Engineer at a mining firm in India. He is currently working into oil analysis field. Has worked in the filed of tribology and lubrication and loves to write about the same.

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