DE8233283U1 - DEVICE FOR STATIC MEMBRANE FILTRATION - Google Patents

Description

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Device for static, membrane filtration

The invention relates to a device for static membrane filtration / wherein the medium to be filtered is in contact with the membrane under an overpressure, in particular for the separation of very small amounts of substance from suspension or solution.

Membrane filtration means material separation processes through selectively permeable membranes under the influence of a hydrostatic pressure difference. Depending on the type of membrane used, one speaks of micro-, ultrafiltration or hyperfiltration t when the retained fractions are particles, dissolved high-molecular substances or low-molecular substances.

Membranes for microfiltration include electron microscopy recognizable pores _t ranging in size between 0.01 and 15 may be μ. Application examples are sterile and clarification filtration of solutions. Ultrafiltration membranes generally have no pores that can be recognized by an electron microscope and are characterized by specifying the molecular weight of a protein or peptide that is just completely retained (separation limit). Ty-

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Typical applications are concentration or fractionation of protein solutions. The main area of application of The hyperfiltration membrane is the desalination of water.

The transitions between these areas are fluid. So can microfiltration membranes with a nominal pore size Quantitatively hold back proteins of even higher molecular weight from 0.0 5 μ. The transition from ultrafiltration to hyperfiltration is generally drawn at a cut-off point of 500 Daltons, but ultrafiltration membranes show with a cut-off of 50 0 Daltons i.a. already has a considerable retention capacity for low molecular weight salts on.

Static filtration is the simplest filtration method. The medium to be filtered is without forced flows under an overpressure in contact with the membrane. It can then be used with success, if only very small amounts of substance have to be separated from the suspension or solution, as is the case during extraction of sterile filtrates from weakly contaminated solutions is the case.

However, the higher the concentration of the dissolved or suspended substances to be separated, the more there will be

with static filtration the achievable filtration rates (Filtratmerige per area and time) through the Enrichment on the membrane surface impaired. S Im In the case of microfiltration, this process affects the formation of a filter cake and clogging of the pores, in ultrafiltration on the formation of a so-called secondary membrane, i.e. one more or less thin layer of concentrated protein, which in turn has a separation characteristic that differs from the membrane used. This will not just do the rate of passage of the solvent is reduced, but also the passage of dissolved low molecular weight Substances hindered, which is particularly annoying in fractionation tasks. With hyperfiltration has an additional disadvantage that the low molecular weight substances enriched on the membrane already have a high osmotic pressure at relatively low concentrations, so that the driving force

the filtration (difference between osmotic and hydraulic Pressure difference) is reduced. At ultra

and hyperfiltration we speak of concentration polarization here. Within certain limits, these processes can be delayed by using lower working pressures dispenses with high initial filtration performance. This is used, for example, in microfiltration

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S made use of, where this results in a significant increase in the "Service life" (total amount of filterable liquid per unit area) can be achieved. Obviously plays the lower compression of the resulting filter cake plays a role here. Low working pressures and large filter surfaces also improve the fractionation effect the separation of proteins of different molecular weights.

However, the use of large filter areas is not just that often impracticable due to the high technical effort involved. In particular for analytical filtration tasks that require the most complete recovery possible of concentrate or filtrate and their unadulterated composition is important, must be as small as possible Filter surfaces are applied.

It goes without saying that large filter areas also result in correspondingly large losses of adhering filtrate or concentrate. In addition, however, can also the composition of which can be undesirably changed by the filter material. On the one hand included most of the filter materials, especially ultrafiltration membranes, Washable auxiliaries (wetting agents, plasticizers, glycerine, bactericides, etc.) that are in higher concentrations can interfere, but washing them out would lead to the dilution of the sample. On the other hand, FiI-

I * · <? ^: fii. Termaterialien change the composition of the sample both in its total concentration and in the concentration ratio of the individual components to one another through specific or unspecific ad- * '* sorption effects. Examples of this, which will be discussed later, are the adsorption of unbound pharmaceuticals during the ultrafiltration of serum and the partial protein adsorption \ · during the concentration of proteins in urine, |

The disadvantages of static filtration can be largely eliminated by using the tangential overflow principle However, it cannot be used in the area of small and very small volumes because of the losses that occur.

The invention is now based on the object of avoiding falsifications in the removal of concentrate or filtrate in a simpler manner than before, even with the smallest amounts, and of facilitating ^{the automation of these processes 1.}

In a device of the type mentioned at the outset, this is achieved, surprisingly, in that the membrane is on the outside a hollow body floating in the filtration medium, which is open to the air space, is attached.

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The preferred driving force behind the filtration is the difference in level between the filtration medium and filtrate as well as the centrifugal acceleration resulting hydrostatic pressure difference of at least 0.5 bar used. The device can be designed so that the centrifugal acceleration is not a vector in Has direction of the membrane.

A similar device appears to be described in US Pat. No. 3,960,727. In contrast to the measure after the Invention, however, hydrostatic pressure differences are not only not needed there, they must rather be avoided so that the device can work. There should be a cylindrical hollow body made of plastic with a filter material sink on one side in the blood to be separated until a lower edge with the sedimented solid components comes into contact. The separation into the two phases should be achieved by gravitational forces, supported by centrifugal force. Calculations indicate that either the measures under this US patent what is not effective is that the serum to be filtered does not have time to get through the filter into the interior of the apparatus or that plastics are proposed which already have certain functions in terms of density how weightless rest cannot bring about. In any case, pressure should be prevented from being exerted on the blood

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because it is feared that small particles will be forced through the filters. Incidentally, is the well-known Device unsuitable for the purposes of the invention, since there is only a filter to separate the Soluble from the solid constituents of the blood are suggested, but the separation is essentially up Sedimentation is based.

The objective of the aforementioned US-PS can only be achieved if the wall material of the device has a very low density (0.6 g / cm ³ ) and the wall thickness is also very small, so that before the serum penetrates into the interior the depth of penetration is shallow.

In the case of the measure according to the invention, the simplicity of the apparatus arouses astonishment, especially in the Is important with regard to the manufacture of single-use devices.

The static filtration in the acceleration field of a centrifuge is used in such a way that the direction of migration of the filtrate is different from the direction of centrifugal acceleration.

In contrast to known devices for "die- ätatiöche Filtration, in which the centrifugal "·

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acceleration is used to generate the effective hydrostatic pressure difference (e.g. German Patent 19 06 179, Amicon) has no centrifugal acceleration in the measure according to the invention Vector in the direction of the diaphragm. This avoids existing sedimentable constituents the membrane are deposited or the concentrate formed, which generally has a higher density, than the starting solution, can accumulate on the membrane.

In addition, the measure according to the invention achieves that in the course of the filtration, the movable membrane can follow the falling concentrate level and up to Completion of the concentration process, the entire area is wetted by the Filtratxonsmedium; i.e. the effective filtration area remains unchanged throughout the process.

It is advisable to stop the filtration at a desired, predetermined volume of concentrate Arranged stop for the movable component carrying the membrane.

Due to the simple structure of the device the production costs and with it the commercial risk

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kept low.

The filter insert then expediently shows the glued membrane has a hole from which the filter insert extends inside to the wall thickness of the hollow filtrate insert expanded *

The measure according to the invention can be applied to the

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recovery of protein-free ultrafiltrate s.

The invention finds particular application in the determination of substances that are not bound to proteins, in particular Pharmaceuticals, in patient blood.

However, microfiltration and reverse osmosis are also included. Here the minimum pressure difference can also be 0.5 bar, in exceptional cases up to down to 0.1 bar, but still significantly higher than according to the prior art.

The following example explains the invention without limiting it.

This problem comes up in the biochemical resp.

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medical analysis and pharmaceutical research often before *

A simple example is the separation of proteins before the column chromatographic determination of amino acids (Proteins make the column filling unusable by irreversible binding. Enzymatic protein hydroly- ^ Therefore, proteins and biological fluids must be freed from proteins before the amino acid analysis).

Another application is the determination of free (non-protein-bound substances (e.g. pharmaceuticals) in patient's blood *

When multiple doses of medication is used for maintenance a therapeutic concentration in the blood serum, knowledge of the initial concentration is crucial Meaning. In most cases, this cannot be easily estimated from the previously administered doses because the biological half-life can vary widely between individuals and times. About that Furthermore, in most cases it is not the total concentration in the serum that is decisive, but the one that is not bound to protein Proportion of. As a rule, there is no direct correlation between total salary and free share, so that it is unambiguous Data for post-dosing only from the direct

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Determination of the free fraction can be obtained.

This is particularly important when if the therapeutically effective concentration differs only slightly from the toxic concentration * This case is, for example, in digitoxin therapy for those with kidney failure Patients before (P * Kramer; Medical Research 28: 15-26 (1981)).

In such cases it is desirable to be able to obtain the ultrafiltrate directly from blood, because it not only saves time but also blood. (If in a separate Working process plasma is obtained, more blood must be used because of the losses that occur than with direct ultrafiltration of blood. However, this can be done with more frequent determinations on the often anemic patient anyway mean an additional risk. In addition, the cited study has shown that it is precisely Digitoxin of the Most common membrane materials are adsorbed in a disruptive manner (this is not only due to the membrane materials itself, but also the fleece-like carrier materials often used for reinforcement). It it is therefore important in any case to get by with a minimum of filter surface.

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Blood belongs because of its high content of molded components (Blood cells) and the high paw concentration are among the most difficult ultrafiltrate media. A device according to the invention for the direct extraction of ultrafiltration from blood is shown in the figure, where the state after the start of the filtration is shown. The formed components (blood cells) 2 are already at the bottom of the centrifuge tube I> the filtration insert 4 with the membrane 8 floats on the plasma 3. The lateral distance 14 between the insert 4 and tube * i is exaggerated for clarity shown and is about 0.1 mm. (In the case of the mostly conical tubes, the inner dimension in the lower part is decisive) . The twist 12 prevents one from climbing up capillary liquid film at rest (when the centrifuge is at a standstill). The membrane 8, preferably an asymmetrical one Ultrafiltration membrane with a separation limit of 20,000 Daltons, is with the active (filtration effective) Side to the outside by means of a welding or adhesive edge 9 connected to the insert 4 in such a way that the transverse flow the membrane is blocked in this area. (Asymmetrical ultrafiltration membranes show is known to have a relatively coarse-pored protein-permeable support layer under the filtration-effective layer. So if the punched edge of the membrane remains open, what about

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Use of adhesives that are not in the backing layer penetrate, the active layer is "infiltrated" by the protein solution). With the prefer Process, the thermal welding, the tightness is guaranteed.

After passing through the membrane, the filtrate 6 flows through a known system of fine Channels 10 and the bore 11 and collects in the interior of the insert 4. The lower part of the interior is preferably to facilitate complete removal of the filtrate conical.

The driving force of the filtration is the hydrostatic pressure difference, which results from the level difference 13 between plasma level 7 and filtrate level 5 results under the influence of centrifugal acceleration. With advancement the pressure difference becomes lower and lower during the filtration process, but the rate of filtration becomes lower little affected. (A difference in level of 3mm corresponds to a centrifugal acceleration of 3000 g of a pressure difference of 0.9 bar. An improvement the pressure difference in addition leads to the common asymmetric ultrafiltration membranes with high hydraulic permeability hui? i /. very much an insignificant

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An accumulation of the concentrated protein on the membrane surface is avoided in that the concentrate due to its higher density (an increase in protein concentration of 1 g / 100 ml corresponds to approximately one Density increase by 0.004 g / ml) in the direction of centrifugal acceleration 15 by convection back into the starting solution wanders and mingles with her.

In the application described here, it is desirable to use as much ultrafiltrate as possible from as little as possible To get blood. In the case of blood, the limiting factor here is the oncotic occurring in the concentrated plasma (Colloid osmotic) pressure, because the filtration comes to a standstill when the oncotic pressure of the concentrate and effective hydraulic pressure difference are equal. In practice, the upper limit of the proportion that is ultrafilterable is Liquid here at about 50% of the initial volume, whereby this value is of course dependent on the kematocrit value, Protein concentration, centrifugal acceleration and the geometry of the device depends.

In the case of extremely small blood volumes or the filtration of dilute protein solutions, the filtration can already come to a standstill prematurely if, as shown in Fig. 1

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placed, a tube with a round bottom is used because the insert 4 abuts the curve. As most commercially available centrifuge tubes have a round bottom, a hemispherical displacement body must be used in these cases be placed on the bottom of the tube with a flat top when the filtrate volume is at its maximum achieved v / should be grounded.

An additional advantage of this device results in the determination of the free calcium in blood. The equilibrium between free and bound calcium is strongly pH-dependent, changes in the pH of the blood during filtration must therefore be avoided. This is only possible if contact of the blood with the atmosphere is avoided during all manipulations, because the CO ^ equilibrium partial pressure is above the CO ^ partial pressure of the atmosphere. Direct contact of the blood with the atmosphere would therefore _{lead to an increase in the pH value as a result of the escape of CO 2} and thus to a falsification of the CA values. In a publication (Berghof) a device for anaerobic ultrafiltration of serum is therefore described, which essentially consists of a stirred cell, the construction of which enables the filtration of serum in the absence of air (flask).

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In the device described above, this is achieved in that the blood against the atmosphere through the filtration insert is almost completely covered.

The embodiment described here can equally be used to obtain particle-free and / or sterile filtrates, a microfiltration membrane expediently is used.

To distinguish it from the familiar, the under the designation "Centriflow" or "Mikrotrennsystem MPS 1" commercially available devices are used, which are also based on the principle of static filtration in Working acceleration field of a centrifuge. In the cited patent it is claimed that between the direction of the centrifugal acceleration and the diaphragm, an angle of less than 22.5 ° is maintained. The embodiments described there make it clear that a component of the acceleration vector flows away of the concentrate formed along the membrane to the outside (viewed from the axis of rotation). With In other words, the concentrate does not move away from the membrane as in the process described here, but instead is kept in contact with the membrane by the acceleration. The speed with which this

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The flow of the Köüzeiltrate is dependent on His Π

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centering on the membrane higher than in the remaining part 1 of the solution * This does not only apply to the patent ί script described and sold under the name Centriflow device, but also for the micro-separation system MPS 1, provided that it is used in accordance with the operating instructions, i.e. in a centrifuge with an angle head * If the the latter device, on the other hand, is used in a vibrating head centrifuge, as is customary in most laboratories is used, it is a simple static filtration without reducing the concentration polarization, i.e. the concentrate is without possibility pressed against the membrane during removal.

The following comparative measurements are intended to compare the results of the method according to the invention with the micro-separation system MPS 1 realized state of the art to illustrate the progress achieved. Both devices were made with the same membrane type (an asymmetrical ultrafiltration membrane with a separation limit of 20,000 Daltons, which is sold by Sartorius GmbH under the name SM 145 49). The effective filtration area

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was MPS 1 0/92 cm ^2, wherein the device erfindüngsgemäßen 0,63cm ² · The filtration time in minutes betrüg ällön traps 15th and were identical Filträtionsmedien used. The angular head centrifuge used had 45 ° i. The oscillating head centrifuge worked with 2330 g, with the angular head centrifuge with 3300 g.

Table 1

Filträtvölümen after filtration of 1 ml bovine plasma 15 15 min. Filtration time.

Device according to Fig. 1 (0.63 cm ² filter area)

Angle head oscillating head

Centrifuge 380 μΐ 440 μΐ

MPS 1 (Amicon)
(0.92 cm ² filter area)

Wimkelköpf swinging head

Centrifuge 220 μ! 150 μΐ

Table 2

Filtrate volume after filtration of 1 ml bovine blood 15 min. Filtration time.

Device according to FIG. 1

MPS 1 (Amicon)

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Angle head oscillating head Angle head oscillating head

Centrifuge centrifuge

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From these data it can be seen that the erfindüngsgemäße Device at. Plasma significantly higher filtration speeds despite smaller effective filtration area enables / than according to the state of the art. In addition, it can be seen that in MPS 1 the extraction of ultrafiitrate from whole blood has ended after the first drop, while amounts of filtrate that can be used according to the invention are entirely usable are attainable.

Claims (6)

'' European Patent Attorneys German Patent Attorneys Dr. W. Müller-ΒθΓέ f Dr. Paul Deufel Dipl.-Chem., Dipl.-Wirtscii.-Ing. Dr. Alfred ScMn Dr. MftUer-ΒθΓέ and partner · POB 26 02 47 ■ D-8000 Munich 26 -_. , _, Dipl.-Chem. r άο m oft ·} ς WernerHertel G 82 33 283.5 Dipi.-Phys. Sartorius GmbH Dietridi Lewald S 3343 Lw / Ge "Schwimmer" DiPi.-ing. Dr.-Ing. Dieter Otto Dipl.-Ing. Brit. Chartered Pateai Agent B. David P. Wetters M.A. (Oxon) Ch. Chem. M.R.S.C. CLAIMS 1. Device for static membrane filtration, the to The filtering medium is in contact with the membrane under positive pressure, especially for separation very small amounts of substance from suspension or solution, characterized in that the membrane (8) on the The outside of a hollow body (4) floating in the filtration medium (3), which is open to the air space, is accommodated. 2. Apparatus according to claim 1, characterized in that for a termination of the filtration at a desired one a stop for the movable component (4) carrying the membrane (8) is arranged beforehand is. 3. Device according to one of the preceding claims, characterized in that the lateral gap between the hollow body (4) carrying the membrane (8) and the centrifuge tube (1), in the case of conical centrifuge tubes in the lower part of which is 0.1 mm. D-80GB Munich 2 POB 26 02 47. ". ₍ .'Kabel: '..'« "Eele & ei Telecopier Infotec 6400 B Toleji Isflrtorpiatsi 6 D-8000 Münthetf 20 · »'Müeb'opät ·« ·· * · Ο89? Ζ2ΐ403-7 GII + ΙΙΪ (089) 220643 6-24285 1 · ■ ■ ■ I * ■> - ² - 4. Device according to one of the preceding claims, characterized in that a twist (12) on the opening side to prevent capillary action at rest on the filtration insert (4) is provided. 5. Device according to one of claims 1 and 2, characterized characterized in that the membrane (8) is an asymmetrical ultrafiltration membrane and against the filter insert with the active side on the outside edge is tightly taped (9). 6. Apparatus according to claim 1 to 2 and 3 to 5, characterized characterized in that the filter insert (4) has a bore (11) adjoining the glued-on membrane (8), from the inside of the filter insert (4) down to the wall thickness of the thin, hollow filtrate insert (4) extended. NH «IM 4 * I I · * «·
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