Tribology of cardiovascular devices: A brief review

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 come 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.

Talking about mechanical problems, these devices are susceptible to tribological problems like friction and wear. Below we will have a look on various friction and wear related issues related to these devices that can hamper their performance.

Ventricular assist devices: These are mechanical pumps that are used to direct blood away from damaged ventricle and provide proper blood flow to maintain blood circulation. These mechanical pumps contain metal bearings. Friction and wear of these bearings is common which results in reduced lifetime and overheating. This kind of failure 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 above problem, modern day pumps are now using magnetic bearings which do not have any mechanical contact, thereby reducing friction, wear, overheating, Thromobosis and Haemolysis [7]. In one of the research work carried out by Qian KX and co-workers [8], it has been found that 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 has anti-wear property 8 times better than metal.

Apart from bearing, mechanical seals used in ventricular assist devices also suffers friction and wear. A mechanical seal in ventricular assist devices help in containing blood pressure and avoids leakage of blood. Koki Kanda and co-workers [9] studied the effect of surface texture on frictional properties of mechanical seals. Through their research 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. 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 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 allow is 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 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:[email protected][email protected][email protected]/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 Abstract only
  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. Abstract only
  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 Abstract only
  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 Abstract only
  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 Abstract only
  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. Abstract only
  16. Image used is free for commercial use and is downloaded from pexels.com

HARSHVARDHAN SINGH
About HARSHVARDHAN SINGH 19 Articles
Harshvardhan Singh is an Automotive Engineer and has good experience in lubrication science and experimental tribology. He loves to write about tribology and related fields such as coating technology, surface engineering and others.

3 Comments

  1. Harsh, this is a very interesting and complicated topic. Here, tribology is not the only consideration, but it has overwhelmed the discussion to the detriment of many patients. This engineering/tribological tunnel-vision has led to many deaths. I’ll explain…

    With regard to LVAD (Left Ventricular Assist Devices, a/k/a heart pumps, or generically – MCS (Mechanical Circulatory Support) devices), yes, maglev (magnetic levitation) bearings have been suggested and used. The real issue is that blood is not meant to spin at 3,250 RPM. Using axial rotation pumping methods with blood has been implicated in lethal “platelet activation” (clotting), causing thromboembolic events (PE – pulmonary embolism, stroke, kidney failure, etc.). It’s not always just a question of friction and wear. Sometimes, the cure is just a lethal as the disease.

    As for cardiovascular stent technology, it is a shinning example of this engineering/tribological tunnel-vision. Your article mentions “Nitinol” (nickel titanium alloy) and plastic polymers. There have been numerous documented adverse reactions (allergy) to both the polymers used to manufacture these stents and to nickel. Many believe the nickel toxicity to be lethal. Many responsible cardiothoracic surgeons around the world now do nickel sensitivity testing prior to implantation.

    With regard to artificial heart valves, the new technology is in biological (human, pig, cow, horse) valves; see also the Ross procedure (harvesting the patient’s own valves and tissue). Mechanical valves are associated with clotting and resulting morbidity/mortality, if not accompanied by lifelong anticoagulant therapy.

    So, it’s not all anti-friction and wear properties. Sometimes, engineers need to look beyond the tribophysics and see all the complicated interdisciplinary science involved.

    • Yes I know its a very complicated topic and requires detailed writing and knowledge. I read some papers and even for me the medical terms and other things mentioned in many related papers were difficult to understand so I decided to give a short intro to our technical and non-technical readers about usage of tribology in this field, in plain simple writing. For further detailed insight you can go through this paper which is available for download http://www.sciencedirect.com/science/article/pii/S2405451815000628

      • Harsh, an interesting tribological note to the Chinese paper you cite above…

        Section 3.1.2 thereof (Surface Modification) discusses typical surface profiles (roughness) for the surfaces of these devices being measured in submicron values. There is discussion of heparin (an anticoagulant) coated DLC (H-DLC) surfaces (attributed to Dr. Said Jahanmir – MIT) and molybdenum coated DLC (Mo-DLC, attributed to X.S. Tang, et al.) as being an answer to many of the problems identified. These surface coatings will not change the fact that the surface roughness is measured in submicron units, and that those surface imperfections are the avenue for bacteria and platelet attachment to the prostheses, as well as for the generation of local heat and wear.

        Now, consider the implications of ex-situ nanopolishing of these device surfaces to the subnanometer range.

        Interesting how there is no discussion in the literature of superpolishing the surfaces to resolve the common (sometimes lethal) issues with these devices. Could it be that there was previously no means to reduce surface roughness to near atomic-level smoothness?

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