Artificial lungs: a new inspiration

Numerical modeling of pulsatile blood flow through a mini-oxygenator in artificial lungs

While previous in vitro studies showed divergent results concerning the influence of pulsatile blood flow on oxygen advection in oxygenators, no study was done to investigate the uncertainty affected by blood flow dynamics. The aim of this study is to utilize a computational fluid dynamics model to clarify the debate concerning the influence of pulsatile blood flow on the oxygen transport. The computer model is based on a validated 2D finite volume approach that predicts oxygen transfer in pulsatile blood flow passing through a 300-micron hollow-fiber membrane bundle with a length of 254 mm, a building block for an artificial lung device. In this study, the flow parameters include the steady Reynolds number (Re = 2, 5, 10 and 20), Womersley parameter (Wo = 0.29, 0.38 and 0.53) and sinusoidal amplitude (A = 0.25, 0.5 and 0.75). Specifically, the computer model is extended to verify, for the first time, the previously measured O₂ transport that was observed to be hindered by pulsating flow in the Biolung, developed by Michigan Critical Care Consultants. A comprehensive analysis is carried out on computed profiles and fields of oxygen partial pressure (P_O2) and oxygen saturation (S_O2) as a function of Re, Wo and A. Based on the present results, we observe the positive and negative effects of pulsatile flow on P_O2 at different blood flow rates. Besides, the S_O2 variation is not much influenced by the pulsatile flow conditions investigated. While being consistent with a recent experimental study, the computed O₂ volume flow rate is found to be increased at high blood flow rates operated with low frequency and high amplitude. Furthermore, the present study qualitatively explains that divergent outcomes reported in previous in vitro experimental studies could be owing to the different blood flow rates adopted. Finally, the contour analysis reveals how the spatial distributions of P_O2 and S_O2 vary over time.

Bioengineering Progress in Lung Assist Devices

Artificial lung technology is advancing at a startling rate raising hopes that it would better serve the needs of those requiring respiratory support. Whether to assist the healing of an injured lung, support patients to lung transplantation, or to entirely replace native lung function, safe and effective artificial lungs are sought. After 200 years of bioengineering progress, artificial lungs are closer than ever before to meet this demand which has risen exponentially due to the COVID-19 crisis. In this review, the critical advances in the historical development of artificial lungs are detailed. The current state of affairs regarding extracorporeal membrane oxygenation, intravascular lung assists, pump-less extracorporeal lung assists, total artificial lungs, and microfluidic oxygenators are outlined.

Bioengineering lungs - current status and future prospects

INTRODUCTION

Once pulmonary disease progresses to end-stage pulmonary disease, treatment options are very limited. An important advance in the field is the development of a bioartificial lung derived from a generic matrix scaffold populated with patients' own cells. Significant progress has already been made in the engineering of bioartificial lungs.

AREAS COVERED

This review explains how previous and current research contributes to the goal of creating a successful bioartificial lung, and the barriers faced in doing so. We will also highlight some of the design considerations being explored to optimize bioartificial lungs and considerations for clinical translation.

EXPERT OPINION

While current bioartificial lungs are able to provide short-term gas exchange in large-animal studies, much work is still required to combine the disciplines of cell biology, materials science, and tissue engineering to create such clinically useful and functioning artificial lungs.

De novo lung biofabrication: clinical need, construction methods, and design strategy

Chronic lung disease is the 4th leading cause of death in the United States. Due to a shortage of donor lungs, alternative approaches to support failing, native lungs have been attempted, including mechanical ventilation and various forms of artificial lungs. However, each of these support methods causes significant complications when used for longer than a few days and are thus not capable of long-term support. For artificial lungs, complications arise due to interactions between the artificial materials of the device and the blood of the recipient. A potential new approach is the fabrication of lungs from biological materials, such that the gas exchange membranes provide a more biomimetic blood-contacting interface. Recent advancements with three-dimensional, soft-tissue biofabrication methods and the engineering of thin, basement membranes demonstrate the potential of fabricating a lung scaffold from extracellular matrix materials. This scaffold could then be seeded with endothelial and epithelial cells, matured within a bioreactor, and transplanted. In theory, this fully biological lung could provide improved, long-term biocompatibility relative to artificial lungs, but significant work is needed to perfect the organ design and construction methods. Like artificial lungs, biofabricated lungs do not need to follow the shape and structure of a native lung, allowing for simpler manufacture. However, various functional requirements must still be met, including stable, efficient gas exchange for a period of years. Design decisions depend on the disease state, how the organ is implanted, and the latest biofabrication methods available in a rapidly evolving field.

Ex Vivo Evaluation of a New Extracorporeal Lung Assist Device: NovaLung® Membrane Oxygenator

When lung function is compromised, alternative devices need to be deployed in order to maintain blood oxygenation. A new device, NovaLung®, has been designed for acute lung failure. We went about evaluating its gas exchange capability.

Three calves (79.5±7.8 kg) were connected to the NovaLung® System with a priming volume of 240 mL, gas exchange surface area of 1.3 m2 and exhibiting a biologically coated surface. A standard battery of blood samples were taken before implantation and over a six hour period.

Hematocrit remained stable ranging from 27±4% (baseline) to 29±5% (6 hrs). Platelets were preserved ranging from 882±27.4 U/L (baseline) to 734±147 (6 hrs). LDH remained stable at 719±85 U/L (baseline) vs 686±190 U/L (6 hrs) and the pressure drop was maintained below 20 mmHg. Minimal hemolysis was observed. Oxygen transfer peaked at two hours acute extracorporeal lung support (ECLS) with a mean value of 130±50 ml/min.

In conclusion, the device is easy to use, provides adequate O2 and CO2 transfer for partial lung support in an acute setting. Shows minimal signs of hemolysis and platelets levels are maintained throughout the six hour ECLS period.

The evolution of patient selection criteria and indications for extracorporeal life support in pediatric cardiopulmonary failure: next time, let's not eat the bones

Bill James, baseball statistician and author, tells the story of hungry cavemen sitting about a campfire, waiting for tomatoes to ripen. One has the inspiration to throw an ox on the fire, and the first barbecue ensued and was endured. After eating, the conversation goes something like this. "There were some good parts." "Yeah, but there were some bad parts." And the smart one says, "This time, let's not eat the bones." The evolution of patient selection criteria for the use of extracorporeal support (ECLS) is a bit like those cavemen and their first barbecued ox. Extracorporeal life support technology and application to patient care is the unique result of a long standing history of ambitious attempt, evaluation, debate, collaboration and extension.

Artificial lung basics: fundamental challenges, alternative designs and future innovations

There exists a growing demand for new technology that can take over the function of the human lung, from assisting an injured or recently transplanted lung to completely replacing the native organ. Many obstacles must be overcome to achieve the lofty goals and expectations of such a device. An artificial lung must be able to sustain the gas exchange requirements of a normal functioning lung. Pursuant to this purpose, the device must maintain appropriate blood pressure, decrease injury to blood cells and minimize clotting and immunologic response. Attachment methods vary, and ideally researchers want to find a way that minimizes bodily trauma, maximizes gas exchange and utilizes the inherent properties of the native lung. The currently proposed methods include the parallel, in-series and venous double-lumen cannula configurations. For the time being, current research focuses on the extracorporeal (i.e., outside the body) placement, but ultimate long-term goals look toward total implantation.

A biohybrid artificial lung prototype with active mixing of endothelialized microporous hollow fibers

Acute respiratory distress syndrome (ARDS) affects nearly 150,000 patients per year in the US, and is associated with high mortality ( approximately 40%) and suboptimal options for patient care. Mechanical ventilation and extracorporeal membrane oxygenation are limited to short-term use due to ventilator-induced lung injury and poor biocompatibility, respectively. In this report, we describe the development of a biohybrid lung prototype, employing a rotating endothelialized microporous hollow fiber (MHF) bundle to improve blood biocompatibility while MHF mixing could contribute to gas transfer efficiency. MHFs were surface modified with radio frequency glow discharge (RFGD) and protein adsorption to promote endothelial cell (EC) attachment and growth. The MHF bundles were placed in the biohybrid lung prototype and rotated up to 1,500 revolutions per minute (rpm) using speed ramping protocols to condition ECs to remain adherent on the fibers. Oxygen transfer, thrombotic deposition, and EC p-selectin expression were evaluated as indicators of biohybrid lung functionality and biocompatibility. A fixed aliquot of blood in contact with MHF bundles rotated at either 250 or 750 rpm reached saturating pO(2) levels more quickly with increased rpm, supporting the concept that fiber rotation would positively contribute to oxygen transfer. The presence of ECs had no effect on the rate of oxygen transfer at lower fiber rpm, but did provide some resistance with increased rpm when the overall rate of mass transfer was higher due to active mixing. RFGD followed by fibronectin adsorption on MHFs facilitated near confluent EC coverage with minimal p-selectin expression under both normoxic and hyperoxic conditions. Indeed, even subconfluent EC coverage on MHFs significantly reduced thrombotic deposition adding further support that endothelialization enhances, blood biocompatibility. Overall these findings demonstrate a proof-of-concept that a rotating endothelialized MHF bundle enhances gas transfer and biocompatibility, potentially producing safer, more efficient artificial lungs.

[Extracorporeal lung assist in severe respiratory failure and ARDS. Current situation and clinical applications]

Despite improvements in ventilation support techniques, lung protection strategies, and the application of new support treatment, acute respiratory distress syndrome continues to have a high mortality rate. Many strategies and treatments for this syndrome have been investigated over the last few year. However, the only therapeutic measure that has systematically shown to be able to improve survival is that of low volume lung protective ventilation. Thus, using a low tidal volume prevents added lung damage by the same mechanical ventilation that is essential for life support. In this context, the use of extracorporeal lung assist systems is considered an exceptional use rescue treatment in extreme cases. On the other hand, it could be a potentially useful complementary method for an ultra-protective ventilation strategy, that is, by using even lower tidal volumes. The currently available extracorporeal lung assist systems are described in this article, including high flow systems such as traditional extracorporeal membrane oxygenation, CO₂ removal systems (interventional lung assist or iLA, with or without associated centrifugal pumps), and the new low flow and less invasive systems under development. The aim of this review is to update the latest available clinical and experimental data, the indications for these devices in adult respiratory distress syndrome (ARDS), and their potential indications in other clinical situations, such as the bridge to lung transplantation, multiple organ dysfunction syndrome, or COPD.

[Extracorporeal lung assist in severe respiratory failure and ARDS. Current situation and clinical applications]

Despite improvements in ventilation support techniques, lung protection strategies, and the application of new support treatment, acute respiratory distress syndrome continues to have a high mortality rate. Many strategies and treatments for this syndrome have been investigated over the last few year. However, the only therapeutic measure that has systematically shown to be able to improve survival is that of low volume lung protective ventilation. Thus, using a low tidal volume prevents added lung damage by the same mechanical ventilation that is essential for life support. In this context, the use of extracorporeal lung assist systems is considered an exceptional use rescue treatment in extreme cases. On the other hand, it could be a potentially useful complementary method for an ultra-protective ventilation strategy, that is, by using even lower tidal volumes. The currently available extracorporeal lung assist systems are described in this article, including high flow systems such as traditional extracorporeal membrane oxygenation, CO₂ removal systems (interventional lung assist or iLA, with or without associated centrifugal pumps), and the new low flow and less invasive systems under development. The aim of this review is to update the latest available clinical and experimental data, the indications for these devices in adult respiratory distress syndrome (ARDS), and their potential indications in other clinical situations, such as the bridge to lung transplantation, multiple organ dysfunction syndrome, or COPD.

Pulsatile Flow and Oxygen Transport Past Cylindrical Fiber Arrays for an Artificial Lung: Computational and Experimental Studies

The influence of time-dependent flows on oxygen transport from hollow fibers was computationally and experimentally investigated. The fluid average pressure drop, a measure of resistance, and the work required by the heart to drive fluid past the hollow fibers were also computationally explored. This study has particular relevance to the development of an artificial lung, which is perfused by blood leaving the right ventricle and in some cases passing through a compliance chamber before entering the device. Computational studies modeled the fiber bundle using cylindrical fiber arrays arranged in in-line and staggered rectangular configurations. The flow leaving the compliance chamber was modeled as dampened pulsatile and consisted of a sinusoidal perturbation superimposed on a steady flow. The right ventricular flow was modeled to depict the period of rapid flow acceleration and then deceleration during systole followed by zero flow during diastole. Experimental studies examined oxygen transfer across a fiber bundle with either steady, dampened pulsatile, or right ventricular flow. It was observed that the dampened pulsatile flow yielded similar oxygen transport efficiency to the steady flow, while the right ventricular flow resulted in smaller oxygen transport efficiency, with the decrease increasing with Re. Both computations and experiments yielded qualitatively similar results. In the computational modeling, the average pressure drop was similar for steady and dampened pulsatile flows and larger for right ventricular flow while the pump work required of the heart was greatest for right ventricular flow followed by dampened pulsatile flow and then steady flow. In conclusion, dampening the artificial lung inlet flow would be expected to maximize oxygen transport, minimize work, and thus improve performance.

Bridge to lung transplantation through a pulmonary artery to left atrial oxygenator circuit

BACKGROUND

There is no mechanical device available to support patients with end-stage lung failure for weeks and months until appropriate donor organs for lung transplantation are available.

METHODS

In a 38-year-old female patient with primary pulmonary hypertension a paracorporeal artificial lung (PAL) system was placed parallel to the pulmonary circulation with connections to the pulmonary artery and to the left atrium. The key component of the PAL was a low-resistance membrane oxygenator.

RESULTS

After institution, the PAL had a blood flow of 3.5 L/min and created a PaO(2)/fraction of inspired oxygen ratio of 270, while the oxygenator was provided with oxygen 3 L/min. The pulmonary artery pressure declined by almost 50%. The PAL worked well over 62 days until appropriate donor lungs were available. With resuming more physical activity, an increased flow through the native lung augmented the fraction of unsaturated blood arriving at the left atrium, which mandated increasing oxygen flow to the PAL.

CONCLUSIONS

The data obtained with this case encourage further research into PAL systems, which may hopefully serve as a bridge to lung transplant device in appropriate patients in the future.

Oxygenation of Human Blood Using Photocatalytic Reaction

This report, as a proof of concept, presents the results of oxygenation of human blood using photocatalytic reaction (at lambda = 352 nm) involving the semiconductor thin film junction: tin doped indium oxide (ITO)/nano titanium oxide (TiO2 in anatase structure). These thin films were prepared at room temperature (300 K) on quartz, using reactive DC Magnetron sputtering techniques from pure metallic targets. The results indicate that when 10.0 mL of blood was exposed to ITO/nano TiO2 (14.32 cm surface area) at a wavelength of 352 nm for 120 minutes, the absolute increase in the whole blood oxygen content was 10.13 mL of oxygen per 100.0 mL of blood. This experiment, to our knowledge, is the first of its kind on human blood.

Pulsatile Blood Flow and Oxygen Transport Past a Circular Cylinder

The fundamental study of blood flow past a circular cylinder filled with an oxygen source is investigated as a building block for an artificial lung. The Casson constitutive equation is used to describe the shear-thinning and yield stress properties of blood. The presence of hemoglobin is also considered. Far from the cylinder, a pulsatile blood flow in the x direction is prescribed, represented by a time periodic (sinusoidal) component superimposed on a steady velocity. The dimensionless parameters of interest for the characterization of the flow and transport are the steady Reynolds number (Re), Womersley parameter (α), pulsation amplitude (A), and the Schmidt number (Sc). The Hill equation is used to describe the saturation curve of hemoglobin with oxygen. Two different feed-gas mixtures were considered: pure O2 and air. The flow and concentration fields were computed for Re=5, 10, and 40, 0≤A≤0.75, α=0.25, 0.4, and Schmidt number, Sc=1000. The Casson fluid properties result in reduced recirculations (when present) downstream of the cylinder as compared to a Newtonian fluid. These vortices oscillate in size and strength as A and α are varied. Hemoglobin enhances mass transport and is especially important for an air feed which is dominated by oxyhemoglobin dispersion near the cylinder. For a pure O2 feed, oxygen transport in the plasma dominates near the cylinder. Maximum oxygen transport is achieved by operating near steady flow (small A) for both feed-gas mixtures. The time averaged Sherwood number, Sh̿, is found to be largely influenced by the steady Reynolds number, increasing as Re increases and decreasing with A. Little change is observed with varying α for the ranges investigated. The effect of pulsatility on Sh̿ is greater at larger Re. Increasing Re aids transport, but yields a higher cylinder drag force and shear stresses on the cylinder surface which are potentially undesirable.

Pulsatile Blood Flow and Gas Exchange Across a Cylindrical Fiber Array

The pulsatile blood flow and gas transport of oxygen and carbon dioxide through a cylindrical array of microfibers are numerically simulated. Blood is modeled as a homogeneous Casson fluid, and hemoglobin molecules in blood are assumed to be in local equilibrium with oxygen and carbon dioxide. It is shown that flow pulsatility enhances gas transport and the amount of gas exchange is sensitive to the blood flow field across the fibers. The steady Sherwood number dependence on Reynolds number was shown to have a linear relation consistent with experimental findings. For most cases, an enhancement in gas transport is accompanied with an increase in flow resistance. Maximum local shear stress is provided as a possible indicator of thrombosis, and the computed shear stress is shown to be below the threshold value for thrombosis formation for all cases evaluated.

The history of subcutaneous oxygen therapy

Soon after the discovery of oxygen, experiments began on the use of oxygen for therapeutic purposes, including subcutaneous administration of oxygen, on humans and animals. The history of subcutaneous oxygen therapy (SQOT) is examined in the context of the growing understanding of the use and methods of oxygen administration. Little was written about this therapy until the 19th century, despite an advocacy for its use in some circles. There was resurgence in the use of SQOT in the early 20th century. Investigators in the field of anesthesia, including such notable figures as Paul M. Wood, Ralph M. Waters, and John Henry Evans, contributed to the growth in popularity of the therapy and to the literature on the subject. Although SQOT has been supplanted by other means of administration, it may have a role in management of some inflammatory or pain conditions.

A new pumpless extracorporeal interventional lung assist in critical hypoxemia/hypercapnia*

OBJECTIVE

Pump-driven extracorporeal gas exchange systems have been advocated in patients suffering from severe acute respiratory distress syndrome who are at risk for life-threatening hypoxemia and/or hypercapnia. This requires extended technical and staff support.

DESIGN

We report retrospectively our experience with a new pumpless extracorporeal interventional lung assist (iLA) establishing an arteriovenous shunt as the driving pressure.

SETTING

University hospital.

PATIENTS

Ninety patients with acute respiratory distress syndrome.

INTERVENTIONS

Interventional lung assist was inserted in 90 patients with acute respiratory distress syndrome.

MEASUREMENTS AND MAIN RESULTS

Oxygenation improvement, carbon dioxide elimination, hemodynamic variables, and the amount of vasopressor substitution were reported before, 2 hrs after, and 24 hrs after implementation of the system. Interventional lung assist led to an acute and moderate increase in arterial oxygenation (Pao2/Fio2 ratio 2 hrs after initiation of iLA [median and interquartile range], 82 mm Hg [64-103]) compared with pre-iLA (58 mm Hg [47-78], p < .05). Oxygenation continued to improve for 24 hrs after implementation (101 mm Hg [74-142], p < .05). Hypercapnia was promptly and markedly reversed by iLA within 2 hrs (Paco2, 36 mm Hg [30-44]) in comparison with before (60 mm Hg [48-80], p < .05], which allowed a less aggressive ventilation. For hemodynamic stability, all patients received continuous norepinephrine infusion. The incidence of complications was 24.4%, mostly due to ischemia in a lower limb. Thirty-seven of 90 patients survived, creating a lower mortality rate than expected from the Sequential Organ Failure Assessment score.

CONCLUSIONS

Interventional lung assist might provide a sufficient rescue measure with easy handling properties and low cost in patients with severe acute respiratory distress syndrome and persistent hypoxia/hypercapnia.

Transintestinal Systemic Oxygenation Using Perfluorocarbon

PURPOSE

Perfluorocarbons have an excellent oxygen- and carbon dioxide-carrying capacity. This prompted us to investigate the feasibility of transintestinal systemic oxygenation using perfluorocarbon.

METHODS

A rat hypoventilation model (room air, 20 breaths/min and a tidal volume of 10 ml/kg) was thus established, and FC-77 (Sumitomo-3M, Osaka, Japan) was used as a perfusate. Oxygenated FC-77 was perfused through the small intestine for 4 h. The rats were allocated into three groups as follows. Group 1 (n = 6): hypoventilation only; Group 2 (n = 6): saline was perfused instead of FC-77; Group 3 (n = 6): FC-77 was perfused. Arterial blood samples were drawn from the common iliac artery every 30 min until the end of perfusion. A standard blood gas analysis was performed.

RESULTS

The PaO2 level in Group 3 was significantly higher than in Groups 1 or 2 (P = 0.006: at the end of perfusion, Group 1 = 58.6 +/- 14.5 mmHg, Group 2 = 65.2 +/- 29.4 mmHg, Group 3 = 84.0 +/- 35.5 mmHg). The PaCO2 level in Group 3 was significantly lower than that in Groups 1 or 2 (P = 0.014: at the end of perfusion, Group 1 = 56.8 +/- 8.5 mmHg, Group 2 = 52.6 +/- 5.7 mmHg, Group 3 = 44.4 +/- 11.1 mmHg).

CONCLUSION

Our findings indicate that transintestinal systemic oxygenation is indeed possible and could therefore become a useful new modality for respiratory assist.

A novel method for selective delivery of drugs to the pulmonary arteries

Drug treatment of pulmonary hypertension may be limited by systemic hypotension. Selective action of a vasodilator drug in pulmonary arteries could be achieved by administering a vasodilator gas into systemic venous blood so that it dilates pulmonary arteries before immediate first-pass elimination via exhalation. This article presents in vivo data to show that a pharmacologically active gas can be delivered safely into systemic venous blood where it has a distribution pattern and physiologic effects similar to those observed when the gas is inhaled into pulmonary venous (systemic arterial) blood. This is a first step toward development of first-pass pulmonary clearance as a mechanism to concentrate drugs in pulmonary arteries.

Pulsatile Flow and Mass Transport Over an Array of Cylinders: Gas Transfer in a Cardiac-Driven Artificial Lung

The pulsatile flow and gas transport of a Newtonian passive fluid across an array of cylindrical microfibers are numerically investigated. It is related to an implantable, artificial lung where the blood flow is driven by the right heart. The fibers are modeled as either squared or staggered arrays. The pulsatile flow inputs considered in this study are a steady flow with a sinusoidal perturbation and a cardiac flow. The aims of this study are twofold: identifying favorable array geometry/spacing and system conditions that enhance gas transport; and providing pressure drop data that indicate the degree of flow resistance or the demand on the right heart in driving the flow through the fiber bundle. The results show that pulsatile flow improves the gas transfer to the fluid compared to steady flow. The degree of enhancement is found to be significant when the oscillation frequency is large, when the void fraction of the fiber bundle is decreased, and when the Reynolds number is increased; the use of a cardiac flow input can also improve gas transfer. In terms of array geometry, the staggered array gives both a better gas transfer per fiber (for relatively large void fraction) and a smaller pressure drop (for all cases). For most cases shown, an increase in gas transfer is accompanied by a higher pressure drop required to power the flow through the device.