Membrane filter including bile acid and a method of manufacturing the same

Abstract

A membrane filter 26 is disclosed comprising cellulous material 23 allowing the transition of fluid therethrough, and, in a substantially dry state, said membrane comprising also a salt of deoxycholic acid. Optionally, the air side of the membrane (the side facing away from the screen or belt used to manufacture the membrane) faces the sample fluid during use of the membrane. A method of manufacture of the membrane material is disclosed also, employing deoxycholic acid as a surfactant, to improve the recovery rate of the membrane filter in use.

Claims

1. A membrane filter comprising: a membrane material for retaining microorganisms having an air-side surface and a belt-side surface, the air-side surface being rougher than the belt-side surface; and a bile acid or bile acid derivative residual salt dispersed throughout the membrane material and non-covalently associated therewith.

2. The membrane filter of claim 1, wherein a salt residue of 7-deoxycholic acid is dispersed throughout the membrane material.

3. The membrane filter of claim 1, wherein the membrane filter has a recovery rate defined by method 9222D of 90% or better.

4. The membrane filter of claim 1, wherein the membrane filter has a dry thickness of about 130 to about 140 m.

5. The membrane filter of claim 1, wherein the membrane material is composed substantially of a mixture of cellulose acetate and cellulose nitrate.

6. The membrane filter of claim 1, wherein the membrane filter includes a surface pattern.

7. A method of manufacturing a membrane filter material, the method including the steps of: a) preparing a casting mix, including cellulosic polymers, preferably cellulose nitrate and cellulose acetate dissolved in at least one organic solvent; b) introducing a surfactant in the casting mix including a bile acid or bile acid derivative; and c) casting the mix on a carrier belt to form the membrane filter material by controlled evaporation of the solvent(s), leaving the surfactant in the membrane filter material.

8. The method of manufacturing the membrane filter material of claim 7, the method further including the steps of: d) drying the membrane material; and e) brushing the side of the membrane material opposite the belt side of the material.

9. The method of manufacturing the membrane filter material of claim 8, the method further including the steps of: f) printing a grid pattern on the air side; and g) optionally, cutting the material to shape for use.

10. The method of manufacturing the membrane filter material of claim 7, wherein the bile acid or bile acid derivative is deoxycholic acid, in a concentration in the casting liquid is less than 0.06% (w/w).

11. The method of manufacturing the membrane filter of claim 7, wherein the casting liquid is 0.02 to 0.04% (w/w).

12. The method of manufacturing the membrane filter of claim 7, wherein the casting liquid is about 0.03% (w/w).

13. The membrane filter of claim 1, wherein the membrane filter has a dry thickness of about 135 m.

14. The membrane filter of claim 6, wherein the surface pattern is a grid pattern printed on the air-side surface.

Description

(1) The invention can be put into effect in numerous ways, one embodiment only being described and illustrated, with reference to the attached drawings, wherein:

(2) FIG. 1 shows apparatus for manufacturing a membrane filter according to the invention; and

(3) FIG. 2 shows an enlarged sectional view of a membrane filter according to the invention.

(4) Referring to FIG. 1, a schematic representation of apparatus 10 for manufacturing a membrane filter web 20 is shown. A vat 30 includes a so called casting mix 32 of a solvent, cellulose acetate, cellulose nitrate, and deoxycholic acid which acts as a surfactant. In this deoxycholic acid comprises around 0.03% (w/w) of the casting mix.

(5) A continuous belt 40 travels in the direction of arrows D, and picks up the casting mix 32 poured from the vat 30 to form a continuous film. The web material is, heated at station 44, within a ventilated cabinet 42, to produce a porous membrane material resulting from evaporation of the solvent. Brushing of the side of the membrane material which faces away from the belt can be carried out later, to provide a more readily printable surface.

(6) The now dried web 22 is peeled off the belt 40 and rolled onto a storage roll 24. Membrane material on the storage roll 24 can be cut or punched to make individual membranes (26 FIG. 2). The membranes are printed with a grid pattern to aid quantification of microbiological material in use.

(7) Referring to FIG. 2, a section through a membrane 26 is shown comprising a mixture 23 of cellulose acetate and cellulose nitrate formed to be porous membrane. The membrane 26 will contain also salts which are a residue from the deoxycholic acid used in the casting process described above.

(8) The membrane is about 135 m in thickness and has a pore size of about 0.1 to 12 m, preferably 0.1 to 1 m.

(9) One side 21, of the membrane is the side which faced the belt 40 during production, whereas the other side 28, the so-called air side, faced away from the belt 40 during production. Under a microscope it is possible to see that the air side has a rougher surface which appears to catch and retain microorganisms better than the belt side. It was found that using the air side as the fluid facing side improved the recovery rate by around 10% where deoxycholic acid surfactant was employed.

(10) In experiments other surfactants were used but they did not perform as well as deoxycholic acid. For example:

(11) TABLE-US-00001 Retention of E. coli Recovery of E. coli (%) according to (%) according to Standard Method Surfactant % (w/w) in Standard Method DIN 58355, casting mix 9222D (belt side) part 3 (belt side) 0.06% Statexan K1 67 100 0.01% Statexan K1 74 100 0.01% Deoxycholic acid 78 100 *0.03% Deoxycholic acid 82 100 0.01% Tween 20 74 100

(12) For the 9222D method, the table above shows that the recovery rate of membranes using Statexan improved with lower concentrations of that surfactant, as would be expected, because there would be lower concentrations of surfactant residue in the finished membrane and therefore less anti-microbial activity in the membrane in use. Problems with forming the membrane web were encountered with use of Tween 20 as a surfactant in higher concentrations than 0.01%, so that was dismissed. Contrary to logical thoughts, increasing levels of deoxycholic acid improved both the web quality and the recovery rate of membranes produced using that casting mix. It was found that 0.03% was an optimum value, but that 0.02% to 0.04% was considered to give satisfactory results, and less than 0.06% was considered to be workable. Increasing levels of Deoxycholic acid up to around 0.03% improved recovery rates, which is opposite to the results found for increasing Statexan. Thus the recovery rates and ease of manufacture of the membrane web are unpredictable when selecting the surfactant to be used and its concentration.

(13) For method DIN 58355, the table above shows that the retention of bacteria in the membranes was not influenced by the surfactant and there were no bacteria found in the filtrate of bacterial solutions filtered through the membranes. Thus for this test methodology, the choice of surfactant used has no bearing on the retention efficiency of the membrane. Thus, again, the effects of the choice of surfactant employed are not predictable.

(14) Selection of the air side as mentioned above improved the recovery rate of membrane of the invention still further by an additional 10%, bringing the overall recovery rate of the preferred membrane (* in the table above) to around 92%.

(15) The membrane is used in a manner described above which is conventional. In another use, the membrane filter may retain microorganisms on its surface without sample fluids passing completely through the membrane.

(16) Although one embodiment only has been described, it would be readily apparent to the skilled addressee that modifications, additions and omissions are possible within the scope of the invention. For example, cellulosic based membrane filters are discussed, but other base materials could be employed for example other polymers such as PVDF or polysulfones. Other constituents may be included in the casting mix, for example binders, or microbial staining reagents, or dyes.

(17) A printed grid pattern is preferred but that is not essential. The membrane could be unprinted, or printed with another pattern, for example a hexagonal pattern. Other means of marking the membrane besides printing could be employed. The continuous manufacturing technique described could be replaced by manual production where a screen is used to cast the membrane material, and thereby form a matrix of material rather than a continuous web as described.