Artificial Organs 26(2):84–90, Blackwell Publishing, Inc. © 2002 International Society for Artificial Organs
Patient Safety Technology for Microadsorbent Systems in Extracorporeal Blood Purification *D. Falkenhagen, *W. Strobl, †J. Hartmann, *A. Schrefl, *I. Linsberger, *K.-H. Kellner, ‡F. Aussenegg, and ‡A. Leitner *C. Doppler Institute for Adsorption Technology in Medicine, and †Center of Biomedical Technology, Danube University, Krems; and ‡Institute for Experimental Physics, Karl-Franzens University, Graz, Austria
Abstract: Alternative technologies for extracorporeal blood purification systems based on microadsorbents in suspension are discussed. Principally, microadsorbents offer higher efficiency and flexibility when compared to conventional column-based adsorption systems. Systems already clinically employed (e.g., BioLogic DT) or close to clinical application (e.g., the microspheres-based detoxification system, MDS) are described. The MDS technology, in particular, is characterized by efficiency and a high degree of flexibility with respect to both the use of different adsorbents as well as the combination with hemodialysis/
hemofiltration therapy. It was designed for continuous use in intensive-care units, but enables also the removal of low-density lipoprotein, fibrinogen, autoimmune antibodies, immune complexes, and other pathophysiologically relevant substances. Alternative anticoagulation regimes and safety systems on fluorescence sensor technology have recently been developed for the MDS and are presented in this paper. Key Words: Adsorption—Blood purification—Liver failure—Liver support—Microspheres— Multiorgan failure.
Blood purification treatment is mainly based on membrane permeation processes such as hemodialysis, hemofiltration, or various combinations of these procedures. Due to modern membrane and electronic technology, the procedures are biocompatible, efficient, and safe. However, neither diffusive nor convective membrane separation permits specific removal of substances. Instead, solutes are separated by molecular size regardless of their chemical nature and physicochemical properties. Membrane separation is efficient only for the elimination of low-molecular-weight solutes without significant affinity to serum albumin such as in hemodialysis. Larger solutes, on the other hand, can only be separated from cellular entities and removed wholesale by plasmapheresis as performed with plasma filtration membranes. Variations of pore diameter enable a rough fractionation of plasma, how-
ever, without discrimation of individual solutes. Rather, selective or specific removal of substances requires the use of surface modification either of membranes or particles. ADSORPTION: A QUESTION OF SURFACE AREA Various physicochemical forces, such as hydrophobic interactions (van der Waals forces), hydrogen bonds, electrostatic interactions, or even covalent binding, are involved in adsorption of substances onto solid surfaces (1). Regardless of the various mechanisms, there are two important parameters for all adsorption processes; namely, surface area and accessibility (2). Figure 1 demonstrates the impact of size on surface area by comparing large- and small-sized spheric particles. Spheres of 1 cm in diameter display an outer surface area of 3.14 cm2 whereas spheres of only 1 m in diameter carry about 31,000 cm2 at the same volume. Based on external surface, it is easily conceivable that smaller particles, microparticles, may offer considerable benefits with respect to adsorptive procedures. Apart from outer surface, how-
Received June 2001. revised July 2001. Presented in part on the 65th birthday of Prof. Dr. Horst Klinkmann, on May 7, 2000, in Rostock, Germany. Address correspondence and reprint requests to Dr. Dieter Falkenhagen, Center of Biomedical Technology, Department of Environmental and Biomedical Studies, Dr. Karl Dorrek-Strasse 30, Krems A-3500, Austria. E-mail:
[email protected]
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MICROADSORBENT SYSTEMS
FIG. 1. Strategies are shown to increase the surface area of adsorption particles.
ever, one has to consider the internal surface, which contributes substantially to overall surface area of porous materials such as activated charcoal and resins. Activated charcoals of spherical structure have been characterized with 1,000–2,000 m2/g, mainly due to a complex porous structure in the nanometer range (3). Likewise, porous microparticles of 1–5 m in size have been recently described (4). The diffusion distance of adsorbates (substances to be adsorbed) is much shorter for small particles than for large particles. As a result, substances are bound on microadsorbents much quicker than on largeparticle adsorbents. Figure 2 shows the different endotoxin adsorption kinetics for small- and largeparticle adsorbents as measured by endotoxin activity (limulus amebocyte lysate test reactivity) in plasma supernatants. Adsorbents are characterized by three major parameters; namely, adsorption rate (kinetics), capacity (expressed as equilibrium isotherms), and selectivity. Functionalization of the suface improves the latter by optimizing the interaction to a group of substances (selective adsorbents) or even to a specific solute (specific adsorbents). Microparticles have been functionalized with immobilized antibodies and
FIG. 2. Comparison of small (2–5 µm) and large (150–300 µm) particles for endotoxin adsorption in vitro are shown. Endotoxins were exposed to 10% (vol/vol) of diethylaminoethyl (DEAE) cellulose of identical nature but different size. No adsorbent was added in control experiments.
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specific peptides (5) as well as spacer-linked aromatic groups (6) for the efficient removal of immunoglobulin G. Likewise, microspheric adsorbents with cationic amino/imino groups such as diethylaminoethyl or polyethylenimine groups showed considerable adsorption rate and capacity for bacterial endotoxins (7). Smaller diffusion distances in microparticles may result in greater accessibility and higher adsorption capacity. However, the higher the porosity of an adsorbent, the less pronounced the impact of particle size on adsorption capacity. As long as active surface is entirely accessible, it is just a matter of time until adsorption capacity is exhausted and all substances are bound onto the inner surface. It is conceivable that the time required to achieve an adsorption equilibrium is much shorter in smaller than in larger particles even for adsorbents showing no size effect on adsorption capacity. In fact, adsorption capacity could be reached after 6–24 h using adsorbent particles in the range of 150–600 m in comparison to 5–30 min for microparticles in the range of 1–5 m in diameter. THE BIOLOGIC DT SYSTEM The first clinically used microadsorbent system was designed by S. Ash and coworkers for temporary extracorporeal liver support. Based on pulverized activated charcoal and a flat dialyzer membrane, the system was meanwhile commercialized as BioLogic DT (8,9). The filtrate compartment of this dialyzer contains pulverized adsorbent material suspended by the pump-dialyzer (Fig. 3). An infrared sensor is integrated in the efferent bloodstream to detect microparticles in case of membrane rupture and to avoid their infiltration into the patient. The detection of the microparticles is based on the fact that activated charcoal absorbs light in the infrared range at much higher levels than blood. As an additional safety system, a hemolysis detector is integrated in
FIG. 3. Shown is a schematic representation of the Biologic DT system. Artif Organs, Vol. 26, No. 2, 2002
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the adsorbent circuit and is separated from the microadsorbents by a specific membrane. FPSA AND MDS: TWO SYSTEMS FOR EXTRACORPOREAL BLOOD PURIFICATION Two novel blood purification systems have been developed by the authors; namely, one microadsorbent system and one column-based system. In both systems, hollow fiber filters with plasma fractionation membranes of various sieving properties are employed. Plasma fraction is countercurrently recirculated in a closed circuit, thereby inducing internal filtration (Starling) flow along the whole fiber length with plasma filtration at blood inlet area and plasma backfiltration at blood outlet. Before leaving the closed circuit by backfiltration, plasma is exposed to adsorbents, either in columns (fractionated plasma separation and absorption [FPSA] system) or as microparticles in suspension (MDS). The fractionated plasma separation and adsorption system The FPSA system was designed as an extracorporeal liver support device (15). Blood is filtered by a novel polysulfone membrane, which is permeable to albumin but effectively prevents fibrinogen and immunoglobulins from passing. Critical cellular and humoral components are therefore not exposed to adsorbents, and major biocompatibility parameters (e.g., cytokines and coagulation) indicate superior biocompatibility of the system. Contrary to many plasma solutes, albumin is able to cross the membrane and carry all toxic waste—albumin-bound solutes such as unconjugated bilirubin and bile acids— directly to the adsorbents. Once exposed to ion exchangers and neutral resins, toxins are transferred and bound to particles whereas depleted albumin penetrates the membrane into the blood compartment (Fig. 4). Transmembrane passage of albumin, followed by rapid adsorption of solutes from albumin to adsorbents, is a key for effective removal of liver-failurederived toxins. Albumin releases the toxic waste onto the adsorbent particle. In alternative detoxification procedures, albumin unloads its toxic waste by equilibrium and surface diffusion processes such as in the so-called molecular adsorbent recirculation system (MARS). Unconjugated bilirubin, the superior marker for albumin-bound substances, accumulates in plasma when the liver fails. Albumin is depleted of bilirubin and the other hydrophobic solutes in two ways (Fig. 5): bilirubin either travels freely on its albumin shuttle through the membrane, driven by Artif Organs, Vol. 26, No. 2, 2002
FIG. 4. Shown is a schematic representation of the fractionated plasma separation and adsorption (FPSA) system. Recirculated at typically 200–600 ml/min, the plasma filtrate passes through columns containing one or two porous macroadsorbents with a diameter of 150–500 µm.
convective flux, to be bound directly onto the adsorbent (FPSA); or bilirubin is passed on through the small pores of a dialyzer by an array of surfaceimmobilized albumin molecules until it is finally picked up by a recirculating albumin molecule, which carries the toxin to the adsorbent (MARS). Clearly, the direct way is more efficient as is removal of bilirubin in a laboratory setting (Fig. 6). The microspheres-based detoxification system The microspheres-based detoxification system (MDS) (10–14) combines fractionated filtration with microadsorbent technology. Rather than perfusion of plasma in columns, such as in FPSA, adsorbents are recirculated and suspended in plasma (Fig. 7). Flow rates are typically between 4–6 L/min, inducing effective filtration and backfiltration along the hollow fiber membrane. To enable continuous dialysis and ultrafiltration, an adapted peritoneal dialysis (PD) unit is attached to the filtrate circuit (Fig. 8).
FIG. 5. Shown is a schematic representation of fractionated plasma separation and adsorption (FPSA) and molecular adsorbent recirculation system (MARS) liver-support systems.
MICROADSORBENT SYSTEMS
FIG. 6. Shown is the removal of unconjugated bilirubin and tryptophan by fractionated plasma separation and adsorption (FPSA) and the molecular adsorbent recirculation system (MARS). Given are the plasma concentration courses (initial concentration 300 µM bilirubin, 2 mM tryptophan) over 120 min.
High filtration and adsorption rates (through recirculation and microsadsorbents) contribute to great removal efficiency of MDS as demonstrated for unconjugated bilirubin (Fig. 9). Only 5 ml of a novel microadsorbent was needed to decrease the bilirubin concentration in 2 L of blood from 250 mol/L to less than 130 mol/L. This means that 20–25 ml of microadsorbents might be enough in the clinical setting. Another advantage of MDS is the potential to use various microadsorbents, either sequentially or simultaneously, for the removal of different substances. Figure 10 shows the sequential use of microadsorbents for the specific removal of interleukin-1 (IL-1) (injected at 30 min), tumor necrosis factor-␣ (TNF-␣) (injected at 60 min), and endotoxins (lipopolysaccarides, LPS) (injected at 90 min) (7,14). SAFETY ISSUES FOR MICROADSORBENT SYSTEMS Systems operating with microparticles are inherently associated with two possible risks: hemolysis is
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FIG. 8. The microspheres-based detoxification system (MDS) is shown including a module for continuous hemodialysis.
induced but not detectable by the system; and microparticles penetrate the filter and enter the patient. Optical sensors, used for standard hemodialysis to detect hemolysis, are masked by suspended microadsorbents in plasma. Already low concentrations of resins and even lower concentrations of activated charcoal increase the background noise of the sensor to levels incompatible with sensitive hemoglobin measurement. As a result, the sensor has to be separated from the adsorbents by a membrane. Semicontinuous or continuous measurement of adsorbentfree plasma filtrate enable early and safe detection of hemolysis in the system. More sophisticated technology is required to detect microadsorbent particles in the blood. Basically, the sensor has to be able to discriminate between blood cells and adsorbents of the same size even at low concentrations of particles and at blood flow rates of typically more than 350 ml/min. For MDS, a fluorescence technique was developed for the early detection of particle transfer into the blood compartment (16). The fluorescence-based particle detector Principle The new sensor is based on the detection of fluorescence-marked microparticles in blood down-
FIG. 7. Schematic representation of the microspheres-based detoxification system (MDS) is shown.
FIG. 9. Shown is the removal of unconjugated bilirubin by the microspheres-based detoxification system (MDS) (in vitro, n = 3). Artif Organs, Vol. 26, No. 2, 2002
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FIG. 10. Shown is the removal of IL-1, TNF-␣, and LPS by the microspheres-based detoxification system (MDS): In vitro with 1 L of human plasma: blood flow, 200 ml/min; filtrate flow, 6 L/min; initial concentrations of TNF-␣ and IL-1, 500 pg/ml; LPS of Pseudomonas aeruginosa, 1 ng/ml. Plasmaflux P2 (Fresenius Medical Care, Bad Homburg, Germany) adsorbents: microparticles linked with polyclonal antibodies against IL-1 and TNF-␣ (100 mg of each adsorbent), and polyethylene-imine-coated cellulosic microspheres for LPS (40 ml); 37°C.
stream of the filter. Particles (
