7.Design of Artificial Lung- Membrane Oxygenator

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Artificial Lung

B i o c o m p a t i b i l i t y

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Biocompatibility

• The biocompatibility of any artificial organ must be considered in its design

• Both material-specific and device-specific factors should be accounted for

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Biocompatibility

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Biocompatibility

• Silicone rubber and microporous polypropylene have proven to be the best materials for artificial lungs because of their high diffusivities for CO2 and O2

• Hollow fibers made of polypropylene are currently the industry standard for manufacturing membrane oxygenators because of the small diameter and compactness that can be achieved, thus this material has emerged as the choice for artificial lung development, as well

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Biocompatibility

• Moreover, a higher gas transfer rate per unit of surface area may be achieved with microporous polypropylene than with silicone rubber

• A disadvantage of the microporous polypropylene is that plasma leakage into the gas phase may occur over time

• This results in impaired gas exchange and device failure

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Biocompatibility

• In order to prevent plasma leakage, chemical modification or surface coating of the fibers is recommended

• Surface coating with a thin silicone-based membrane was used for the intravascular oxygenator (IVOX)– Is being actively investigated for application to the implantable

artificial lung

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Biocompatibility

• Although polypropylene is relatively inert, because of the large surface area of the material exposed to blood, – There is enormous potential for blood-surface interactions

• Heparin bonding of the fibers is one tactic that is being taken in order to ameliorate blood-surface interactions and possibly avoid systemic anticoagulation with the use of the artificial lung

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Current Designs

• Current designs of artificial lungs have taken two general approaches

• The first, being pursued by Tatsumi et al. in Japan, is essentially a variant of extracorporeal membrane oxygenation (ECMO)

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Current Designs –1st Approach• An integrated artificial heart-lung implanted in the

paracorporeal position

• Tatsumi’s device utilizes a polyolefin hollow fiber membrane lung that has been– surface-treated

• to reduce plasma leakage

– heparin-bonded• to reduce anticoagulation requirements

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Current Designs –1st Approach• The artificial lung is integrated with inlet and outlet

pusher-plate type pneumatic blood pumps

• Venoarterial connections to the blood pumps complete the circuit

• Venoarterial = Artery + Vein

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Current Designs –2nd Approach• The second approach, adopted by investigators in the

United States

• A fully implantable, pumpless artificial lung that derives its blood flow from right ventricular output

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Current Designs –2nd Approach• There are currently two different designs of pumpless

artificial lungs that are being developed, both of which employ the cross-flow configuration to enhance convective mixing of the blood

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Current Designs –2nd Approach

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Side and top views of theartificial lung being developedby Cook et al.

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Current Designs –2nd Approach

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Side and top views of the artificiallung being developed by Cook et al.

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2nd Approach –First Design• Consists of

– A microporous hollow fiber polypropylene fiber bundle that is potted at each end

– Enclosed in a compliant, elastomeric housing

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2nd Approach –First Design• The fiber bundle is potted at each end with a polyurethane

compound that keeps the bundle intact and prevents gas and blood side mixing

• Blood enters on one side of the fiber bundle à passes cross-flow to the fibers to become oxygenated à purged of carbon dioxide à exits on the opposite side of the fiber bundle

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2nd Approach –First Design• The oxygenating gas flows through the fibers via connecting

tubing attached to manifolds enclosing the potted ends of the fiber bundle

• Design improvements over earlier prototypes have included the use of a compliant housing for the fiber bundle, reduction of fiber bundle resistance and reduction of inlet and outlet connection resistances

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2nd Approach –First Design• The device utilizes 209 μm outer diameter microporous

polypropylene hollow fibers that are woven into mats and wrapped around a flat, polycarbonate frame

• The void fraction is approximately 74%

• The total fiber surface area is approximately 1.83 m2

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2nd Approach –First Design• The total device volume is only 800 cm3 with a priming

volume of 350 ml

• The connections to the blood inlet and outlet – Are via 18 mm inner diameter Dacron grafts

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2nd Approach –Second Design• The second implantable lung design is being developed

by Montoya et al. at Michigan Critical Care Consultants, Inc., in collaboration with Bartlett and his colleagues at the University of Michigan

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2nd Approach –Second Design• The design

– Consists of 300 μm outer diameter matted microporouspolypropylene hollow fibers

• Are wrapped around a mandrel• Housed in a cylindrical shell with two circular endcaps

– Blood flow enters axially through the center of the core à proceeds in a radial direction, in a cross-flow fashion à across the fiber bundle to become oxygenated à purged of carbon dioxide

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2nd Approach –Second Design• Once the blood reaches the periphery of the fiber bundle à It is directed by a gutter to exit the device

• The housing enclosing the fiber bundle currently is made of a rigid, noncompliant plastic

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2nd Approach –Second Design• Total fiber surface area is 2.25 m2

• The priming volume is 240 ml

• Inlet and outlet gas lines are attached to ports in the endcaps

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2nd Approach –Second Design

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Schematic cross-sectional view ofthe artificial lungbeing developed byCook et al.

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2nd Approach –Second Design

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End and side views of the artificial lungbeing developed by Montoya et al.

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2nd Approach –Second Design

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Schematic cross-sectional view of the Michigan artificial lung, showing the blood andgas paths

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Artificial Lung

I n V i t r o E v a l u a t i o n

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In Vitro Evaluation

• A mixture of O2, CO2, and N2 is used to adjust fresh anticoagulated bovine blood to venous conditions

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In Vitro Evaluation

• In vitro evaluation of prototype artificial lung devices allows for the tight control of operating conditions

• Precise measurement of pressure losses and oxygen and carbon dioxide transfer rates

• In vitro tests have been conducted in both water and in animal blood• Testing the artificial lungs in water permits multiple evaluations to be

performed on a single device and is much easier to accomplish than testing the devices in blood

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