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Surface Coatings for Rotary Ventricular Assist Devices: A Systematic Review

Zhang, Meili*,†; Tansley, Geoffrey D.*,‡; Dargusch, Matthew S.; Fraser, John F.*,§,¶; Pauls, Jo P.*,‡

Author Information
doi: 10.1097/MAT.0000000000001534

Abstract

Heart failure is a global pandemic affecting approximately 2% of the adult population worldwide. Its prevalence is age-dependent, ranging from less than 2% of people younger than 60 years to more than 10% of those older than 75 years.1 The gold standard treatment of patients suffering from end-stage heart failure is still a heart transplant. However, there is a severe shortage of donor hearts, and ventricular assist devices (VADs) are used more and more frequently to provide short-term or long-term mechanical circulatory support to those patients.2 There are three generations of VADs with the first generation of VADs being pulsatile volume displacement pumps. However, these devices were large and not fully implantable and had many moving parts that were prone to wear and other forms of mechanical damage that caused device failure.3 The second and third generations of devices are developed as small (some are fully implantable) rotary pumps which generate continuous flow. These rotary VADs use either mechanical bearings (in second-generation pumps) or hydrodynamic/magnetic levitation systems (in third-generation pumps) to suspend impellers within their pump housings.3

As the key components of rotary VADs, impellers and housings have stringent requirements on wear resistance and hemocompatibility of their construction materials and surfaces. The interaction between blood and the blood-contacting surfaces of pumps like impellers and housing walls can cause complications like thrombosis and hemolysis.4 Coating the blood-contacting surfaces with more hemocompatible materials has been shown to be an effective way to reduce such adverse effects.5,6 Besides these complications, mechanical durability of long-term implantable devices, especially their moving parts, still needs to be improved.7 Wear-resistant coatings have been pursued to tackle the problem of bearing wear in second-generation pumps.7,8 To achieve minimal mechanical wear, the third-generation pumps are operated with hydrodynamically or magnetically levitated impellers. However, hydrodynamically and magnetically levitated pumps are vulnerable to impeller-housing contact, especially at the site of hydrodynamic bearings.9 Such contact can cause damage to the surface of VADs commonly made of poorly wear-resistant titanium (Ti) and its alloys Ti6Al4V,10 which may further result in thrombosis and hemolysis because of the change of surface roughness.11,12 Thus, wear-resistant coatings are needed in this situation.

A review of surface coatings for VADs by Sin et al.13 was published in 2009, which focused on coatings and their impact on improved hemocompatibility; however, wear resistance and durability were not investigated. In the present work, a systematic review on the role of surface coatings for second- and third-generation of VADs on blood-contacting surfaces especially on the impeller and housing wall has been performed. Particular attention has been paid to coating preparation (especially temperature), coating properties (wear resistance, durability, and hemocompatibility), and coating evaluation methods. Overall, the review seeks to present a comprehensive understanding of surface coatings for rotary VADs and provide potential guidance for future research directions.

Methods

We followed the preferred reporting items for systematic reviews and meta-analyses (PRISMA) protocols14,15 when carrying out this systematic review.

Inclusion Criteria

Types of documents

This review considered peer-reviewed journal and conference articles that described the coatings for rotary VADs and were written in English.

Types of data

Articles must discuss the coating applied to blood-contacting surfaces of rotary VADs, especially impellers and housings, or describe the coating of materials which are intended for rotary VADs impeller or housing manufacturing.

Exclusion Criteria

This review excluded the following:

  • nonrelated to coating for VADs,
  • the first-generation of VADs,
  • all editorials, patents, and abstracts with no full text available,
  • literature that referred to coatings on cannula/graft and antimicrobial coatings,
  • non-English articles.

Search Strategy

The present systematic review included relevant publications that were published at the time of the literature search completed by April 3, 2020. A Boolean search with the search terms ([coating OR “surface modification*” OR “surface engineering” OR texture*] AND [“rotary blood pump*” OR “centrifugal blood pump*” OR “axial flow blood pump*” OR “*ventricular assist” OR “cardiac assist” OR “heart assist”]) in title/abstract/keywords was conducted in four databases: Web of Science, Scopus, PubMed, and ScienceDirect. Relevant articles that did not appear in the database search but were known to the reviewers were handpicked and extracted into EndNote (EndNote X9; Clarivate Analytics, Philadelphia, PA).

Process of Selecting and Evaluating Articles

Based on the inclusion and exclusion criteria established by all authors on types of documents and language, search returns from the previously mentioned databases were screened and extracted into EndNote by the first author. Duplicates were removed. Publications were selected through reading the titles and abstracts, and final eligibility was assessed by reading each full text by the first author.

Process of Extracting Relevant Information

Eligible documents were read, and information on coating materials, coating preparation methods (temperature being a focus), wear property, durability, and hemocompatibility as well as evaluation methods was extracted. “Wear property” straightforwardly applied to coatings for improving wear resistance. “Durability” referred to the retention ability of the coating on the substrate in simulated or real usage. “Hemocompatibility” included in vitro and in vivo situations; in vitro mainly involved platelet (adhesion/activation) and hemolysis and in vivo mainly referred to hemolysis and thrombus formation.

Data Synthesis

The extracted information was synthesized in the following aspects: first, an overview of the types of coatings, application, and popularity was given; second, preparation methods of coatings were analyzed; third, wear properties and durability of coatings together with evaluation methods were identified; finally, hemocompatibility of coatings as well as evaluation methods was illustrated.

Results and Discussion

The PRISMA flow chart outlining the journey of the articles from identification to inclusion is shown in Figure 1. A total of 527 search publications were extracted into Endnote. After removing duplicates, 288 publications remained. Through reading titles and abstracts, 62 publications remained for further screening. These articles were then read in full, and 45 of them were included for final synthesis.

F1
Figure 1.:
The systematic reviews and meta-analyses (PRISMA) flow diagram. PRISMA, preferred reporting items for systematic reviews and meta-analyses; VAD, ventricular assist device.

Overview of Types of Coatings

Eighteen coatings were presented in the literature that improved wear resistance or hemocompatibility of rotary VADs (Table 1). For ease of comparison, the coatings were divided into two groups: 1) wear-resistant and hemocompatible coatings and 2 hemocompatible (only) coatings.

Table 1. - Overview of the Coatings for Rotary Ventricular Assist Devices
Classification Coatings Application Percent of Publications
Wear-resistant and hemocompatible coatings Diamond-like carbon5,16–22 EvaHeart (Sun Medical Technology Research Corporation, Suwa, Nagano, Japan) LVAD, VentrAssist (Ventracor Limited) LVAD, MiTiHeart (Mohawk Innovative Technology, Inc., Albany, NY) LVAD 17.8
Amorphous carbon23 Arrow CorAide LVAD 2.2
BioMedFlex8,24 Cleveland Heart’s LVAD/RVAD, Cleveland Clinic PediPump LVAD 4.4
TiN25,26 ReligaHeart ROT LVAD 4.4
Ultrananocrystalline diamond7 Jarvik 2000 2.2
Plasma electrolytic oxidation27,28 In in vitro studies 4.4
Hemocompatible (only) coatings 2-methacryloyloxyethyl phosphorylcholine5,17,18,29–36 EvaHeart (Sun Medical Technology Research Corporation) LVAD, TinyPump (Department of Artificial Organs, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University), MedTech Dispo 24.4
Heparin6,37–44 Biomedicus (Medtronic Inc.), Sarns Delphin (Sarns 3M, Ann Arbor, MI), Gyro C1E3 (Department of Surgery, Baylor College of Medicine, Houston, TX), MicroMed DeBakey (DeBakey Heart Center, Department of Surgery, Baylor College of Medicine, Houston, TX) VAD, Berlin Incor, The Terumo (Terumo Corporation, Japan) implantable left ventricular assist device system 20.0
Glycoprotein IIb/IIIa receptor inhibitor45 Baylor Gyro 710 (Department of Surgery, Baylor College of Medicine, Houston, TX) VAD 2.2
Silicone46 A magnetically suspended centrifugal pump 2.2
Polyurethane47 An implantable centrifugal blood pump 2.2
Ti, Ti6Al7Nb48 MedTech Heart 2.2
Apatite, apatite-albumin, apatite-laminin49 In in vitro study 2.2
Sintered Ti microspheres50–53 HeartMate (Abbott) III, HeartMate (Abbott) II 8.9
Endothelial cells54–56 In in vitro studies 6.7
The percentage (%) of publications was calculated as the number of publications reporting work on specific coating divided by 45 (the total number of included publications for data synthesis). There were three publications working on both diamond-like carbon and 2-methacryloyloxyethyl phosphorylcholine coatings.
LVAD, left ventricular assist device; ROT, Artificial Heart Laboratory of Zbigniew Religa Foundation for Cardiac Surgery Development, Zabrze, Poland; RVAD, right ventricular assist device; Ti, titanium; TiN, Ti nitride; VAD, ventricular assist device.

Wear-resistant and hemocompatible coatings

Ceramic coatings of diamond-like carbon (DLC),5,16–18 amorphous carbon,23 BioMedFlex (BMF) (Denver, NC),8,24 Ti nitride (TiN),25 and ultrananocrystalline diamond (UNCD)7 are both wear-resistant and hemocompatible and have been applied in current clinically available devices or discontinued devices. Among them, DLC, essentially amorphous hydrogenated carbon (a-C:H), was the most popular with 17.8% of publications because of its unique properties of high strength, low frictional coefficient, chemical inertness, high thermal conductivity, and excellent biocompatibility.19 BMF is a resilient, hard-carbon, thin-film coating of 2–4 µm thickness and created from layers of nanocrystalline diamond and nanocrystalline silicon carbide (SiC).8 The unique combination of diamond and SiC gives BMF high flexural strength and wear resistance. The UNCD films consist of 2–5 nm grains of crystalline diamond separated by atomically abrupt grain boundaries that contain a mixture of diamond and graphitic carbon.7 UNCD possesses bioinertness, extreme resistance to wear, and very low friction coefficient, and specifically, it has much lower surface roughness as compared to conventional microcrystalline diamond as shown in Figure 2. TiN is an extremely hard ceramic material, often used as a coating on Ti alloys, steel, etc. to improve their surface properties.26 Its desirable mechanical properties and biocompatibility enable it a good coating material for VADs. Figure 3 showed the application of TiN coating in ReligaHeart ROT (Artificial Heart Laboratory of Zbigniew Religa Foundation for Cardiac Surgery Development, Zabrze, Poland).25 Plasma electrolytic oxidation (PEO), also known as microarc oxidation, is a well-established way to improve wear resistance of Ti and its alloys.57 It can produce thick ceramic Ti oxide films on Ti (as anode) surface through electrode reactions in electrolytes initiated by potentials. The work of Klein et al.27 first studied PEO coating as wear-resistant coating aiming for rotary VADs run with passively suspended impellers. In addition, the textured layer of Ti oxide generated by PEO was aimed at increasing endothelialization.28

F2
Figure 2.:
Atomic force microscope images (top) and 1D morphology (bottom) of (A) a fine-polished SiC coupon, (B) UNCD deposited on the SiC coupon and (C) conventional nanocrystalline diamond deposited on SiC coupon. Lateral scan range = 5 μm, and vertical scan range is with a unit of nm. Scale bar of all images represents 1 μm.7 SiC, silicon carbide; UNCD, ultrananocrystalline diamond.
F3
Figure 3.:
ReligaHeart ROT device elements with TiN layer modified surface: (A) pump house; (B) half of impeller core with blood flow channel; (C) impeller cover with blades of hydraulic bearing.25 ROT, Artificial Heart Laboratory of Zbigniew Religa Foundation for Cardiac Surgery Development, Zabrze, Poland; TiN, titanium nitride.

Hemocompatible (only) coatings

There were 12 coatings designed to improve hemocompatibility. 2-methacryloyloxyethyl phosphorylcholine (MPC) was the most widely studied for antithrombogenesis with 24.4% of publications because of its unique capability to suppress protein adsorption and platelet adhesion and activation.29 These properties are because of the extreme hydrophilicity and electrically neutral nature of the polymers, as well as to the ability of phosphorylcholine to induce bulk-like water covered on its surface.58 Heparin is well-known for its anticoagulation function via binding of antithrombin (AT) to catalyze the rate of AT-mediated inhibition of the various clotting factors such as thrombin and factor Xa, which has been employed as surface coating on a variety of medical devices.59 Here, it also saw the widest application in multiple VADs (e.g., Biomedicus, Medtronic Inc., Sarns 3M [Ann Arbor, MI], Berlin Incor).6,37–44 Linneweber et al.45 applied platelet glycoprotein (GP) IIb/IIIa receptor inhibitors to the blood-contacting surfaces of Baylor centrifugal blood pumps because they were effective in preventing shear-induced platelet activation through competing with other proteins like fibrinogen to occupy the GP IIb/IIIa receptors on platelets. Sintered Ti microspheres were used by HeartMate (Abbott) III and II because the textured surfaces could reduce the risk of thromboembolism and anticoagulation requirements by promoting the formation of neointima.50,51 Endothelial cell seeding is a more advanced way for endothelialization of the blood-contacting surfaces to improve hemocompatibility. The effectiveness of this approach is still being evaluated with in vitro studies.54–56

Coating Preparation

Overall, 62% of publications included information regarding the coating preparation. However, only 31% of publications provided relatively detailed description of their preparation methods, whereas other publications simply mentioned the types of method that were used. Furthermore, 24% of publications provided information regarding process temperature. Table 2 summarizes the information that could be extracted about coating preparation.

Table 2. - Summary of Coating Preparation Methods
Coating Substrate Preparation Temperature References
DLC Ti6Al4V or Ti Two stage ion-beam sputtering Unknown 5,17,19
Ti6Al4V Plasma enhanced CVD Unknown 21
Polycarbonate Radio frequency plasma CVD Room temperature 22
BMF Ti Plasma-assisted CVD Below 200°C 8,24
TiN Ti grade 2 Glow discharge plasma CVD 830°C 25
UNCD SiC and Ti6Al4V Tungsten hot-filament CVD Unknown 7
PEO Ti grade 4 PEO Unknown 27
MPC Ti, Ti6Al4V, polycarbonate Dip coating Unknown 5,17,35
Ti6Al4V Plasma-induced graft polymerization Silanization at 90°C 29
Ti6Al4V Condensation reaction Silanization at 90°C 30
Heparin Blood-contacting surfaces Carmeda Unknown 37–40,42
Blood-contacting surfaces Duraflo II Unknown 41
Blood-contacting surfaces Ionic bond via the trichlorodecylmethylammonium chloride Unknown 43
Blood-contacting surfaces Ionic immobilization via plasma glow discharge treatment and acrylic acid grafting Low temperature plasma 6
Glycoprotein IIb/IIIa receptor inhibitor Blood-contacting surfaces Dip coating Unknown 45
Apatite, apatite-albumin, apatite-laminin Ti Sodium hydroxide and heat-treated Ti soaked in supersaturated calcium phosphate solution supplemented with albumin or laminin Sodium hydroxide treatment 60°C, heat treatment 600°C 49
Endothelial cells Smooth Ti, sintered Ti Cell seeding 37°C 54–56
BMF, BioMedFlex; CVD, chemical vapor deposition; DLC, diamond-like carbon; MPC, 2-methacryloyloxyethyl phosphorylcholine; PEO, plasma electrolytic oxidation; SiC, silicon carbide; Ti, titanium; TiN: Ti nitride; UNCD, ultrananocrystalline diamond.

The review has shown that chemical vapor deposition (CVD) demonstrated the versatility to prepare various ceramic coatings including DLC, BMF, TiN, and UNCD. The process temperature could be varied from hundreds of degrees centigrade to room temperature.22,25 Dip coating is a simple and cost-effective way to prepare polymer coatings regardless of the shape of the coated parts. MPC was coated on the blood-contacting surfaces of EvaHeart (Sun Medical Technology Research Corporation, Suwa, Nagano, Japan) by dip coating.5 The major drawback was the loss of 20–30% of the coating within a month, primarily on those areas facing a high bloodstream velocity. Only 5% of MPC remained after 91 days operation from explant investigation. In comparison, Ye et al.29 used a plasma-induced graft polymerization method to make MPC covalently attach onto a presilanated Ti6Al4V surface. Their in vitro durability studies suggested no change in surface modification more than a 1 month period. Regarding the coatings of amorphous carbon, polyurethane, Ti, Ti6Al7Nb, and sintered Ti microspheres, their preparation was not described in the included publications. We found in an early study of the Thermedics model pneumatic VAD, the preparation of housing wall with sintered Ti microspheres was described.60 Ti6Al4V spheres sieved to 75–100 µm in diameter were attached to the static blood-contacting device surfaces to form a continuous layer 3–4 spheres in thickness. The device was then heated under vacuum to sinter the spheres to each other as well as to the housing.

In third-generation VADs, permanent magnets are embedded within the impellers. The commonly used magnets are sintered neodymium-iron boron (NdFeB) of various grades (i.e., N44) with a default maximum working temperature of around 80°C.61 High temperatures can deteriorate their magnetic strength.62 The choice of coating procedure and temperature is dependent on the manufacturing process of the impeller. If the impeller is assembled after the coating procedure, higher temperatures may be used; however, if a fully assembled impeller is being coated, lower temperatures need to be considered to avoid deterioration of magnetic strength. However, none of the publications have related the temperature of the coating process to impeller magnets. Some reasons could be that they did not have the concern of temperature for their cases or they have not considered it. Here, the attention to temperature is raised for future impeller coating preparation. DLC coating was prepared on polycarbonate (one pump impeller material) at room temperature via radio frequency plasma CVD,22 which could be a choice for preparing wear-resistant and hemocompatible coatings at low temperatures on fully assembled impellers. PEO could also be a technical route because the temperature of the electrolyte could be controlled by a water cooling system.63,64

Evaluation and Performance of the Wear Properties and Durability of Coatings

There were 22% of publications which mentioned wear, but only 4% did wear tests. As shown in Table 3, DLC, BMF, TiN, UNCD, and PEO were chosen as wear-resistant coatings mainly because of their well-known wear-resistant properties. There were two studies about DLC investigations of wear. In the work by Grenadyorov et al.,20 the coefficient of friction and wear rate of DLC coating on Ti6Al4V were measured on a ball-on-disk tribometer. A ball made of tungsten carbide-cobalt alloy was used as a counterbody. The test could differentiate DLC-coated and noncoated Ti6Al4V but did not simulate the real wear pair in the pump. As well as the characteristics of the mating surfaces undergoing wear, the rotation speed, the load, and the medium (or lubricant) should also correlate with real service situation. An interesting study by Jahanmir et al.21 investigated improving the wettability of DLC-coated Ti6Al4V hydrodynamic thrust bearings by further heparin treatment. A thrust bearing tribometer was used, and a mixture of water and glycerol simulating the rheology of blood worked as the lubricant. The experimental results showed that the fully wetted bearing combination provided the highest load support and torque and reduced friction. Overall, the tribometer was a well-recognized tool to test wear properties.65 The materials associated with the surfaces subjected to wear, and the lubricant should also be chosen appropriately in future wear testing.

Table 3. - Wear Resistance and Durability of Coatings
Coating Wear Resistance Durability References
DLC Mentioned “high strength and low friction coefficient.” Unknown 5,18,19
Increased hardness, reduced coefficient of friction and wear rate. Unknown 20
Incompatible with aqueous lubricants because of hydrophobic nature and heparin treatment used to improve it. Stable (endured >50 start/stop cycles in comparison with 10 cycles of uncoated parts). 21
Amorphous carbon Unknown Stable (no delamination after first three times of ethylene oxide sterilization and further 100 start/stop tests). 23
BMF Mentioned “high flexural strength and wear resistance.” Good durability (no surface damage or defects after 29 days in vivo test). 8
TiN Mentioned “needs to create efficient tribological condition.” Mentioned “needs to be stable during long-term work.” 25
UNCD Mentioned “extreme resistance to wear and very low friction coefficients.” Durable (unchanged after 3 weeks of artificial plasma circulation). 7
PEO Mentioned “extremely hard and wear-resistant.” Unknown 27
MPC NA Undurable (20–30% loss after 1 month, >90% loss after 3 months in vivo test). 5
NA Durable (remained under continuously mixed deionized water for 1 month). 29
Heparin NA Durable (remained after 2 weeks under 6 L/min dynamic flow of saline at 37°C). 6
BMF, BioMedFlex; DLC, diamond-like carbon; MPC, 2-methacryloyloxyethyl phosphorylcholine; NA, not available; PEO, plasma electrolytic oxidation; TiN, titanium nitride; UNCD, ultrananocrystalline diamond.

Eighteen percent of publications mentioned durability, and 13% had supporting data. The extracted information on durability is summarized in Table 3. In vitro start/stop testing was reported for ceramic coatings like DLC21 and amorphous carbon coatings.23 In a study of UNCD, artificial plasma was circulated through the devices for a few weeks. Posttest evaluations were performed to investigate changes.7 Similarly, one in vitro study of MPC coating on Ti6Al4V sheet tested durability by incubating the materials in continuously mixed deionized water for 1 month.29 Another durability study on heparin was performed under 6 L/min dynamic flow of saline at 37°C for 2 weeks.6 Overall, the principle was similar, but the test details need to be standardized for better comparison among different studies. Compared with ceramic coatings, polymer coatings like MPC and heparin showed a greater propensity for degradation.5,6 The improvement of coating preparation could ameliorate this situation as shown in the work of Ye et al.29 and Muramatsu et al.6 Cleaned Ti6Al4V surfaces were pretreated with H2O vapor-plasma and silanated with 3 methacryloylpropyltrimethoxysilane (MPS). The silanated Ti6Al4V samples were treated by Argon plasma and then immersed in MPC solution under ultraviolet irradiation for a plasma-induced graft polymerization.29 This covalent bond enabled MPC 1 month durability under continuous mixed water. The blood-contacting surface, mainly composed of polycarbonate, was first treated by plasma for grafting acrylic acid, then coated with polyethyleneimine as a linker, and finally coupled with heparin by multiionic binding.6 Such heparin coating maintained its chemical composition and bioactivity for 2 weeks under 6 L/min dynamic flow of saline at 37°C. Polymer coating delamination was another issue as stated by Takaseya et al.8 and Saeed et al.23 The fluorinated ethylene propylene (FEP) coating on Ti bearing of CorAide LVD-4000 Assist System (Arrow International, Reading, PA) was shown to delaminate at varying degrees in postexplant analysis. Therefore, the FEP coating was replaced by more resilient BMF coating.8 Similarly, polymer coating delamination from the internal surface of the pump (Figure 4) occurred and caused hemolysis and pump stoppage in patients, and the polymer coating was replaced by amorphous carbon coating.23 Inspired by the work of Ye et al.29 and Muramatsu et al.6 as previously mentioned, it is potential to address the issue of delamination by building covalent or ionic bond between coating and substrate for coating preparation.

F4
Figure 4.:
Delamination and blistering of the polymer coating on the internal surface of the CorAide pump that was detected in a patient with hemolysis and sudden pump stoppage after implantation.23

Evaluation and Performance of Hemocompatibility of Coatings

Good hemocompatibility is an essential property that a blood pump coating should possess. Ninety-three percent of publications investigated hemocompatibility of their coatings in either in vitro or in vivo studies or both. Among 42 publications with hemocompatibility data, there were 15 in vitro studies, 23 in vivo studies (including five clinical studies), and four studies with both in vitro and in vivo data. Platelet adhesion and hemolysis (plasma-free hemoglobin) were the most widely adopted measures in in vitro tests for indicating thrombogenesis and blood damage, respectively. Specifically, platelet adhesion was investigated in more than 50% of studies, and the figure for hemolysis test was more than 40%. Besides, bulk phase platelet activation,29,30 protein (fibrinogen) adsorption,30,45 and plasmatic coagulation (AT III and fibrinogen)27 were also applied but in less than 10% of studies. For in vivo conditions, hemolysis and thrombus formation were the most common assessments for hemocompatibility. Here, a potential issue was that the in vivo studies run on pumps reflected the performance of the whole pump with part of the contribution from the coating. In the condition of no head-to-head comparison of coating versus noncoating, it was difficult to make a valued judgment on the coating. Most of the in vivo studies aimed to evaluate the pumps which combined all the advantages of design, materials, and manufacturing, while coatings were not the only focus. There were only several in vivo studies on pumps with such head-to-head comparison.24,41,42,46

Based on the evaluation methods previously mentioned, the obtained information on hemocompatibility of these coatings is summarized in Supplementary Table 1 (Supplemental Digital Content 1, https://links.lww.com/ASAIO/A694). All the listed coatings were hemocompatible based on the reported in vitro or in vivo data. Among them, DLC, MPC, heparin, and sintered Ti microspheres showed more data support compared with other coatings to demonstrate their good hemocompatibility, although there were two39,43 of eight publications reporting that heparin coating could not entirely avoid thrombus. DLC-5,18,19 and MPC-5,18,31–35 coated blood-contacting surfaces always showed no thrombus formation exampled by Figure 5 (DLC coating)19 and Figure 6 (MPC coating).5 The surface of sintered Ti microspheres in HeartMate (Abbott) III induced unremarkable pannus formation and collagen islands typical of historical textured surface experience (Figure 7).50 Regarding BMF coating, the two in vivo studies reviewed showed that it was hemocompatible with no thrombus formation or biological deposition on its surface (Figure 8).8,24 A preclinical evaluation of TiN-coated Ti showed that it was nontoxic, nonirritant, nonhemolytic, and biocompatible.26 TiN coating on ReligaHeart ROT left VAD did not cause erythrocyte damage and platelet adhesion in the dynamic high shear stress conditions, suggesting good hemocompatibility.25 However, there was a lack of in vivo data. Endothelial cell coating has been pursued as an ideal antithrombogenic coating that mimics the native lining of blood vessels and the heart. Here, the included publications all showed it could significantly reduce platelet adhesion as compared with noncoated counterparts in in vitro studies.54–56 For other coatings like amorphous carbon,23 PEO,27 GP IIb/IIIa receptor inhibitor,45 silicone,46 polyurethane,47 Ti,48 Ti6Al7Nb,48 and apatite,49 their hemocompatibility was investigated in a very limited number of publications. In the publication of UNCD,7 the author did not investigate the hemocompatibility but gave the information by citing other publications which showed UNCD was antithrombogenic.

F5
Figure 5.:
The Sun Medical centrifugal pump Prototype 1 after 200 days of support is shown: the inside of the pump casing (right) and the impeller and the pump base (left).19 POD, postoperative day.
F6
Figure 6.:
The inspection of MPC polymer-coated EvaHeart (Sun Medical Technology Research Corporation, Suwa, Nagano, Japan) left ventricular assist system at explant after 34 day operation without anticoagulation.5 MPC, 2-methacryloyloxyethyl phosphorylcholine.
F7
Figure 7.:
Representative photos of HeartMate (Abbott) III components after 60 days of support: inflow conduit within the left ventricle (A) and the outflow connector (B).50
F8
Figure 8.:
A: BMF-coated rotating assembly in case 1 RVAD showed no surface damage, defects, or depositions at autopsy; (B) BMF-coated stator housing in case 3 LVAD showed no surface damage, defects, or depositions at autopsy.8 BMF, BioMedFlex; LVAD, left ventricular assist device; RVAD, right ventricular assist device.

Conclusions

This review provides readers with a systematic collection and catalogue of the coatings reportedly used/explored for rotary VADs. The findings show that 18 coatings have been investigated with DLC, MPC, and heparin being the most popular. Reports of these coatings mainly focused on hemocompatibility with very limited data on wear resistance and durability. Overall, all coatings showed good in vitro or in vivo hemocompatibility based on the reported data. Ceramic coatings were more wear-resistant and durable than polymer coatings. The preparation of ceramic coatings was more complex and demanding on the equipment and experiment conditions (e.g., temperature and vacuum control), whereas polymer coating preparation was relatively simple such as dip coating. However, the review identified that complex chemical reactions were needed to achieve strong binding between the polymer coating and the substrate. The review did not identify any publications that investigated the roles of temperature of coating preparation on impeller/magnets assembly. Another finding was that there was lack of comprehensive studies which worked on the entire spectrum from coating preparation to evaluation of properties using both in vitro and in vivo testing.

Recommendations for Future Research

Some recommendations for future studies are given as follows: 1) for coating on magnetically levitated impellers, the temperature of coating preparation should be an area of significant focus; 2) more comprehensive and robust studies are needed, which cover the entire spectrum from coating preparation to evaluation of properties (mechanical and biological, in vitro and in vivo) with the noncoated counterpart as an essential control; and 3) combinations of ceramic coatings and polymer coatings can be a direction for future surface modification of rotary VADs. Polymer coatings can provide effective antithrombogenesis in the short term, which can reduce anticoagulation therapy during the first months after postsurgery. Then, ceramic coatings can provide the next long-term service.

Acknowledgment

The authors would like to recognize the financial assistance provided by The Prince Charles Hospital Foundation (TM2017-04 and RF2018-04) and Australian Research Council through the ARC Research Hub for Advanced Manufacturing of Medical Devices (IH150100024).

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    Keywords:

    coating; surface modification; rotary ventricular assist device; hemocompatibility; wear resistance; durability

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