Blood-Cleansing Device for Sepsis Therapy

Introduction

Sepsis is a life-threatening complication of an infection that occurs when chemicals released in the bloodstream to fight the infection trigger inflammatory responses throughout the body. This inflammation can start a cascade of changes that can cause multiple organs to fail.

Sepsis afflicts ~18 million people worldwide every year with a 30-50% mortality rate even in state-of-the-art hospital intensive care units.

Typical treatment includes intravenous broad-spectrum antibiotics because it takes days to identify the source of the infection and blood cultures are often negative. But these broad-spectrum agents are not as effective as therapeutics targeted against specific microbes, and they can produce severe side effects. As a result, mortality rates increase as much as 9% for every hour before the correct antibiotic therapy is administered. This situation is even more devastating in patients with antibiotic-resistant pathogens because of the lack of effective drugs, and in immunosuppressed patients and neonates with fungal infections because of the high toxicity of antifungal agents.

Treating sepsis with an extracorporeal blood-cleansing device

One way around the problem of slow identification of the source of infection is to rapidly and broadly remove microorganisms and endotoxins from blood without the need to first identify the source of the infection and without altering blood contents.

First step in this process is to engineer a broadly binding agent, such as a version of mannose binding lectin (MBL) that binds to a wide variety of pathogens. Next step is to bring the pathogen-binding agent in contact with blood, followed by pathogen capture and removal from the bloodstream.

A method of implementing this approach relies in applying MBL molecules to surfaces of magnetic nanobeads and using magnetic forces for removing captured pathogens from the bloodstream.

Functional design requirements of the device

One set of design requirements pertains to engineering an MBL molecule capable of effectively binding a broad spectrum of pathogens while the other set of requirements deals with designing a flow-through device that extracts magnetic nanobeads with captured pathogens from blood at high efficiency and high blood flow to minimize the patient treatment time.

To maximize pathogen capture, magnetic nanobeads need to be well-dispersed within the blood. To minimize patient treatment time, the device must allow for high flowrates of blood. However, to use magnetic forces to extracting magnetic nanobeads from the bloodstream, beads cannot be positioned too far from the source of magnetic field due to the steep decay of magnetic force  with distance.

Because it is desirable not only capture pathogens but also to remove them from the bloodstream, the device must contain means for flushing magnetic nanobeads with captured pathogens from the bloodstream. This points to using parallel network of shallow channels with one network containg blood while the other a suitable flushing fluid. [pdf]

Concept

To maximize separation efficiency:

• increase magnetic force

• decrease effective particle diameter

• increase particle residence time

  • by increasing extraction zone length

  • by decreasing flow velocity

Some other things to consider:

• Magnetic force decays rapidly with distance

• Blood is heavier than saline

Design

Device fabrication

• Device was fabricated from polycarbonate sheets ~1.28 mm in thickness

• Network of the blood channels at the device bottom side was ~0.6 mm deep while network of the saline channels at the device top side was ~0.34 mm deep

• Slits spanning the channel width in the extraction zone and fluidically connecting the blood and saline channels in the extraction zone were ~2 x 0.34 x 0.34 mm

• Channels on both sides of the device as well as through slits were formed by hot embossing using elastomeric masters (performed at EdgeEmbossing)

• To fabricate the elastomeric masters, an acrylic mold was CNC machined and the master were cast from the mold

• Networks of channels on both sides of the device were sealed with a thin film 

Device performance

For device extraction efficiencies of C. albicans, S. Aureus, E. coli and other pathogens from fresh blood, see [pdf].

Device optimization by manifold branching

Biomimetic optimum branching described by Hess-Murray law for minimum work requirement:

d03 = d13 + d23

where d0, d1, and d2 are diameters of circular cross-section conduits

and lengths of segments Ln are proportional to diameters dn.

In this type of branching, wall shear stress is constant

τn = τ0

and hydraulic resistance of every generaration remains constant.

To achieve high extraction efficiencies, it is desirable to perform manifold branching in a way that leads to reduce linear velocity in the extraction zones. Disadvantage of this non-biomimetic branching is high pressure drop across the entrance channels. Solution to this problem is introducing variable blood channel depth across branching generations.

• variable depth of blood channel depth to reduce shear stress at blood entrance channels

• compact branching to minimize branching area and maximize size of extraction zones

  • blood and saline channels overlap only in the extraction zones, keeping device thickness at 1.28 mm

• non-branched saline channel used to facilitate more efficient purge of captured pathogens

• 127.8  x 85.5 x 1.2 mm plate size (multiwell tissue culture plate footprint format)

With magnets

Compared to the published device, the optimized device allows increasing flowrate from ~19 liters/hour to ~ 43 liters/hour for the same pressure drop across the device.