In vitro comparative evaluation under static conditions of the hemocompatibility of four types of tubing for cardiopulmonary bypass

Hemocompatibility challenge of membrane oxygenator for artificial lung technology

The artificial lung (AL) technology is one of the membrane-based artificial organs that partly augments lung functions, i.e. blood oxygenation and CO₂ removal. It is generally employed as an extracorporeal membrane oxygenation (ECMO) device to treat acute and chronic lung-failure patients, and the recent outbreak of the COVID-19 pandemic has re-emphasized the importance of this technology. The principal component in AL is the polymeric membrane oxygenator that facilitates the O₂/CO₂ exchange with the blood. Despite the considerable improvement in anti-thrombogenic biomaterials in other applications (e.g., stents), AL research has not advanced at the same rate. This is partly because AL research requires interdisciplinary knowledge in biomaterials and membrane technology. Some of the promising biomaterials with reasonable hemocompatibility - such as emerging fluoropolymers of extremely low surface energy - must first be fabricated into membranes to exhibit effective gas exchange performance. As AL membranes must also demonstrate high hemocompatibility in tandem, it is essential to test the membranes using in-vitro hemocompatibility experiments before in-vivo test. Hence, it is vital to have a reliable in-vitro experimental protocol that can be reasonably correlated with the in-vivo results. However, current in-vitro AL studies are unsystematic to allow a consistent comparison with in-vivo results. More specifically, current literature on AL biomaterial in-vitro hemocompatibility data are not quantitatively comparable due to the use of unstandardized and unreliable protocols. Such a wide gap has been the main bottleneck in the improvement of AL research, preventing promising biomaterials from reaching clinical trials. This review summarizes the current state-of-the-art and status of AL technology from membrane researcher perspectives. Particularly, most of the reported in-vitro experiments to assess AL membrane hemocompatibility are compiled and critically compared to suggest the most reliable method suitable for AL biomaterial research. Also, a brief review of current approaches to improve AL hemocompatibility is summarized. STATEMENT OF SIGNIFICANCE: The importance of Artificial Lung (AL) technology has been re-emphasized in the time of the COVID-19 pandemic. The utmost bottleneck in the current AL technology is the poor hemocompatibility of the polymer membrane used for O₂/CO₂ gas exchange, limiting its use in the long-term. Unfortunately, most of the in-vitro AL experiments are unsystematic, irreproducible, and unreliable. There are no standardized in-vitro hemocompatibility characterization protocols for quantitative comparison between AL biomaterials. In this review, we tackled this bottleneck by compiling the scattered in-vitro data and suggesting the most suitable experimental protocol to obtain reliable and comparable hemocompatibility results. To the best of our knowledge, this is the first review paper focusing on the hemocompatibility challenge of AL technology.

Red-Colored Urine in the Cardiac Surgical Patient-Diagnosis, Causes, and Management

Red-colored urine occurring in the intraoperative and early postoperative periods after cardiac surgery is often a cause for concern. This observation may be a result of hematuria from pathology within the urinary tract, anticoagulant-related nephropathy, drug-induced acute interstitial nephropathy, excretion of heme pigment-containing proteins, such as myoglobin and hemoglobin, and hemolysis occurring during extracorporeal circulation. Within the kidneys, heme-containing compounds result in pigment nephropathy, which is a significant contributory factor to cardiac surgery-associated acute kidney injury. Concerted efforts to reduce red blood cell damage during cardiopulmonary bypass, together with early recognition of the at-risk patient and the institution of prompt therapeutic intervention, may improve outcomes. This review addresses the diagnosis, causes, and management of red-discolored urine occurring during and after cardiac surgery.

3D Direct Printing of Silicone Meniscus Implant Using a Novel Heat-Cured Extrusion-Based Printer

The first successful direct 3D printing, or additive manufacturing (AM), of heat-cured silicone meniscal implants, using biocompatible and bio-implantable silicone resins is reported. Silicone implants have conventionally been manufactured by indirect silicone casting and molding methods which are expensive and time-consuming. A novel custom-made heat-curing extrusion-based silicone 3D printer which is capable of directly 3D printing medical silicone implants is introduced. The rheological study of silicone resins and the optimization of critical process parameters are described in detail. The surface and cross-sectional morphologies of the printed silicone meniscus implant were also included. A time-lapsed simulation study of the heated silicone resin within the nozzle using computational fluid dynamics (CFD) was done and the results obtained closely resembled real time 3D printing. Solidworks one-convection model simulation, when compared to the on-off model, more closely correlated with the actual probed temperature. Finally, comparative mechanical study between 3D printed and heat-molded meniscus is conducted. The novel 3D printing process opens up the opportunities for rapid 3D printing of various customizable medical silicone implants and devices for patients and fills the current gap in the additive manufacturing industry.

In Vitro models for thrombogenicity testing of blood-recirculating medical devices

INTRODUCTION

Blood-recirculating medical devices, such as mechanical circulatory support (MCS), extracorporeal membrane oxygenators (ECMO), and hemodialyzers, are commonly used to treat or improve quality of life in patients with cardiac, pulmonary, and renal failure, respectively. As part of their regulatory approval, guidelines for thrombosis evaluation in pre-clinical development have been established. In vitro testing evaluates a device's potential to produce thrombosis markers in static and dynamic flow loops.

AREAS COVERED

This review focuses on in vitro static and dynamic models to assess thrombosis in blood-recirculating medical devices. A summary of key devices is followed by a review of molecular markers of contact activation. Current thrombosis testing guidance documents, ISO 10993-4, ASTM F-2888, and F-2382 will be discussed, followed by analysis of their application to in vitro testing models.

EXPERT OPINION

In general, researchers have favored in vivo models to thoroughly evaluate thrombosis, limiting in vitro evaluation to hemolysis. In vitro studies are not standardized and it is often difficult to compare studies on similar devices. As blood-recirculating devices have advanced to include wearable and implantable artificial organs, expanded guidelines standardizing in vitro testing are needed to identify the thrombotic potential without excessive use of in vivo resources during pre-clinical development.

Sticky or Slippery Wetting: Network Formation Conditions Can Provide a One-Way Street for Water Flow on Platinum-cured Silicone

In the course of studies on Sylgard 184 (S-PDMS), we discovered strong effects on receding contact angles (CAs), θrec, while cure conditions have little effect on advancing CAs. Network formation at high temperatures resulted in high θadv of 115-120° and high θrec ≥ 80°. After network formation at low temperatures (≤25 °C), θadv was still high but θrec was 30-50°. Uncertainty about compositional effects on wetting behavior resulted in similar experiments with a model D(V)D(H) silicone elastomer (Pt-PDMS) composed of a vinyl-terminated poly(dimethylsiloxane) (PDMS) base and a polymeric hydromethylsilane cross-linker. Again, network formation at high temperature (∼100 °C) resulted in high CAs, while low-temperature curing retained high advancing CAs but gave low receding CAs (θrec 30-50°). These changes in receding CAs translate to strong effects on water adhesion, wp, which is the actual work required to separate a liquid (water) from a surface: wp ∝ (1 + θrec). When the values θrec 84° for high-temperature and θrec 50° for low-temperature network formation are used, wp is ∼1.5 times higher for curing at low temperature. The origin of low receding contact angles was investigated by attenuated total reflectance IR spectroscopy. Absorptions for Si-OH hydrogen-bonded to water (3350 cm(-1)) were stronger for low- versus high-temperature curing. This result is attributed to faster hydrosilylation during curing at higher temperatures that consumes Si-H before autoxidation to Si-OH. Sharp bands at 3750 and 3690 cm(-1) due to isolated -Si-OH are more prominent for Pt-PDMS than those for S-PDMS, which may be due to an effect of functionalized nanofiller. To explore the impact of wp on water droplet flow, gradient coatings of S-PDMS and Pt-PDMS elastomers were prepared by coating a slide, maintaining opposite ends at high and low temperatures and thus forming a thermal gradient. When the slide was tilted, a droplet moved easily on the high-temperature end (slippery surface) but became pinned at the low-temperature end (sticky surface) and did not move when the slide was rotated 180°. The surface was therefore a "one-way street" for water droplet flow. Theory provides fundamental understanding for slippery/sticky behavior for gradient S-PDMS and Pt-PDMS coatings. A model for network formation is based on hydrosilylation at high temperature and condensation curing of Si-OH from autoxidation of Si-H at low temperatures. In summary, network formation conditions strongly affect receding contact angles and water adhesion for Sylgard 184 and the filler-free mimic Pt-PDMS. These findings suggest careful control of curing conditions is important to silicones used in microfluidic devices or as biomedical materials. Network-forming conditions also impact bulk mechanical properties for Sylgard 184, but the range that can be obtained has not been critically examined for specific applications.

Factor XII activation markers do not reflect FXII dependence of thrombin generation induced by polyvinylchloride

The role of factor XII (FXII) as the main trigger of the coagulation cascade during haemodialysis has been recently challenged. Polyvinylchloride (PVC) is the standard polymer for haemodialysis circuit tubings, but its interaction with FXII has not been extensively characterized. In a modified Chandler tubing loop model using heparinized fresh human whole blood we selectively inhibited coagulation factors VII, X or XII with monospecific antibodies. Contact of whole blood with PVC induced a strong thrombin generation [thrombin-antithrombin complexes (TAT) 64 ± 24 μg/l, before <1 μg/l]. Despite this, levels of FXII coagulation activity, free FXIIa or FXIIa-C1 inhibitor complexes remained unchanged. The anti-FXII antibody abolished thrombin generation (TAT 8 ± 5 μg/l, P < 0.05) and made the free FXIIa undetectable. Inhibition of FVII did not affect coagulation activation (TAT 68 ± 26 μg/l). Our data provide definitive evidence that PVC triggers the coagulation system via FXII. However, all FXII activation markers in plasma failed to detect contact activation.

Obstacles in haemocompatibility testing

ISO 10993-4 is an international standard describing the methods of testing of medical devices for interactions with blood for regulatory purpose. The complexity of blood responses to biomaterial surfaces and the variability of blood functions in different individuals and species pose difficulties in standardisation. Moreover, in vivo or in vitro testing, as well as the clinical relevance of certain findings, is still matter of debate. This review deals with the major remaining problems, including a brief explanation of surface interactions with blood, the current ISO 10993 requirements for testing, and the role of in vitro test models. The literature is reviewed on anticoagulation, shear rate, blood-air interfaces, incubation time, and the importance of evaluation of the surface area after blood contact. Two test categories deserve further attention: complement and platelet function, including the effects on platelets from adhesion proteins, venipuncture, and animal derived- blood. The material properties, hydrophilicity, and roughness, as well as reference materials, are discussed. Finally this review calls for completing the acceptance criteria in the ISO standard based on a panel of test results.

Biocompatibility: meeting a key functional requirement of next-generation medical devices

The array of polymeric, biologic, metallic, and ceramic biomaterials will be reviewed with respect to their biocompatibility, which has traditionally been viewed as a requirement to develop a safe medical device. With the emergence of combination products, a paradigm shift is occurring that now requires biocompatibility to be designed into the device. In fact, next-generation medical devices will require enhanced biocompatibility by using, for example, pharmacological agents, bioactive coatings, nano-textures, or hybrid systems containing cells that control biologic interactions to have desirable biologic outcomes. The concept of biocompatibility is moving from a "do no harm" mission (i.e., nontoxic, nonantigenic, nonmutagenic, etc.) to one of doing "good," that is, encouraging positive healing responses. These new devices will promote the formation of normal healthy tissue as well as the integration of the device into adjacent tissue. In some contexts, biocompatibility can become a disruptive technology that can change therapeutic paradigms (e.g., drug-coated stents). New database tools to access biocompatibility data of the materials of construction in existing medical devices will facilitate the use of existing and new biomaterials for new medical device designs.

Activation of Neutrophil Granulocytes in an In Vitro Model of a Cardiopulmonary Bypass

Activated neutrophils play a central role in the pathogenesis of postoperative organ dysfunction after surgery with cardiopulmonary bypass. The researchers used an in vitro roller pump model to investigate the relative importance of the biomaterial, platelets, plasma proteins including activated complement, and flow mode on neutrophil activation as shown by the adhesion, degranulation, and increased the surface expression of CD11b. Neutrophil adhesion to the biomaterial increased with platelet addition, but not with plasma. Biomaterial contact activated neutrophils in a serum-free buffer, but was significantly increased by activated complement. Platelets increased neutrophil degranulation in a serum-free buffer but tended to reduce it in plasma. CD11b expression increased in both media. Complement activation was higher with neutrophils alone than with neutrophils and platelets combined. The roller pump reduced neutrophil adhesion and increased degranulation compared to passive rotation. Neutrophil interaction with platelets and complement were more important for activation than biomaterial contact and use of the roller pump. Improvement of biocompatibility is dependent on modifying complement activation and platelet interaction with neutrophils.

Design and evaluation of novel blood incubation systems for in vitro hemocompatibility assessment of planar solid surfaces

Success in the development of hemocompatible biomaterials depends on adequate equipment and procedures for standardized analysis of blood-materials interactions in vitro. In view of the limited standard of knowledge on that important aspect, two novel incubation systems were designed, built, and evaluated for the in vitro assessment of the hemocompatibility of planar solid surfaces: A screening setup was introduced for the comparison of up to 12 different samples. A perfusion setup was developed to model the directed blood flow in the vascular system during incubation by a recirculation circuit, allowing the variation of the wall shear rate at the sample surface. The incubation procedures utilized freshly drawn, heparinized whole human blood. Hemocompatibility in terms of selected aspects of coagulation, thrombogenicity, and immune responses was quantified through plasma levels of characteristic molecules (immunoassays), cell counting, and analysis of adherent cells and fibrin formation (scanning electron microscopy), respectively. Prevention of blood-air contact and mechanical stress, constant temperature and blood pH during incubation, and the suitable choice of reference materials were found to be crucial for reliable testing. Considering those requirements, screening and perfusion system both provided sensitive discrimination between a given set of planar solid surfaces. In conclusion, the suggested methods for an in vitro hemocompatibility assessment permit versatile, sensitive, and efficient analysis of important blood-material interactions despite the unavoidable variability of blood characteristics in different experiments.

Influence of abciximab on the adhesion of platelets on a shielded plasma gradient prepared on polyethylene

INTRODUCTION

Thrombotic effects of biomaterial implants are mediated merely through activation of the platelet glycoprotein IIb-IIIa (GpIIb-IIIa) receptor. Consequently, platelet GpIIb-IIIa receptor inhibitors are successfully used during stent implantation procedures to prevent thrombosis. However, currently a new generation of stents contains surface coating, which changes the surface to more hydrophobic or hydrophilic. This change markedly affects the interaction of platelets and may influence the efficiency of GpIIb-IIIa inhibitors.

MATERIALS AND METHODS

To study the influence of the wettability of biomaterials on the effectiveness of abciximab, 5-cm polyethylene gradients with contact angles of 100 degrees to 40 degrees were made by means of glow discharge. Fresh whole blood with or without abciximab was recirculated over this gradient.

RESULTS

Inhibition of platelet adhesion by abciximab was maximal, but not complete, on the hydrophobic and moderate hydrophobic part of the gradient, with contact angles of 55 degrees to 90 degrees. Percentage inhibition by abciximab was maximal around 60 degrees.

CONCLUSIONS

Intermediate hydrophobicity of currently applied stent materials, such as stainless steel, seems optimal in combination with abciximab. However, on hydrophobic and particularly on hydrophilic materials, abciximab is less effective.

Platelet adhesion and activation on a shielded plasma gradient prepared on polyethylene

Contact of blood with foreign materials evokes thrombogenic effects to an extent determined partly by the wettability of the biomaterials surface. Tools to study blood response towards a variation in materials wettability with minimal variation in chemistry are "gradient surfaces". However, most gradients have been prepared by diffusion or density immersion techniques, which results in a limited gradient range. Through glow discharge with partial shielding, gradients on polymers were prepared over a length of 5 cm, which facilitated studies to platelet adhesion on separate gradient sections. On polyethylene, advancing water contact angles varied from 90 degrees to 40 degrees, with a hysteresis of 30 degrees. ESCA indicated an increasing incorporation of oxygen towards the hydrophilic end. To examine the role of materials wettability on the activation of adhering platelets, sections of shielded plasma gradients were incubated in anticoagulated whole human blood. Fewer platelets adhered to the hydrophobic end, but those platelets were more activated than those on the hydrophilic end, as judged from their morphology and exposure of GpIIb-IIIa complex. However, partly related to the increased binding of platelets, the clotting activation after platelet deposition was highest on the hydrophilic end. Concluding, this new technique results in a large gradient range, which facilitates studies of formed blood elements in relation to the wettability. Platelets are more activated on hydrophobic polyethylene, while on moderate hydrophilic polyethylene more platelet adhesion and activation of the clotting system occurs.