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A syringe filter (sometimes called a wheel filter if it has a wheel-like shape) is a single-use filter cartridge. It is attached to the end of a syringe for use. Syringe filters may have Luer lock fittings, though not universally so. The use of a needle is optional; where desired it may be fitted to the end of the syringe filter.
A syringe filter generally consists of a plastic housing with a membrane that serves as a filter. The fluid to be purified may be cleaned by drawing it up the syringe through the filter, or by forcing the unfiltered fluid through the filter.
In scientific applications, the most common sizes available are 0.2 or 0.22 µm and 0.45 µm pores. These sizes are sufficient for HPLC use. The smallest known sterile syringe microfilter have pore sizes of 0.02 µm. Membrane diameters of 10 mm, 13 mm, 25 mm are common as well. Some syringe filters for small volumes may not resemble a wheel at all.
The syringe filter body may be made of such materials as polypropylene and nylon. The filter membrane may be of PTFE, nylon, or other treated products for specific purposes. Most manufacturers publish compatibility wallcharts advising users of compatibility between their products and organic solvents or corrosive liquids (e.g. trifluoroacetic acid).
Syringe filters may be used to remove particles from a sample, prior to analysis by HPLC or other techniques involving expensive instruments. Particles easily damage an HPLC due to the narrow bore and high pressures within. Syringe filters are quite suitable for Schlenk line work, which makes extensive use of needles and syringes (see cannula transfer). Being relatively affordable, they may be used for general purpose filtration, especially of smaller volumes where losses by soaking up filter paper are significant.
Syringe filters are also available for the filtration of gases, and for the removal of bacteria from a sample.
Disk filters are frequently used for the onsite manufacture of parenteral drugs and sterile eye drops, in order to remove microbiological contaminations (sterile filtration).
Harm reduction in recreational drug use
Syringe filters may be used to filter solid drugs (powder and pill forms for example) prior to injection. Commonly filtered recreational drugs include heroin, methadone, amphetamines, ecstasy or benzodiazepines. The filter still lets the drug through, but gets rid of many impurities such as fungal spores, bacteria or 'filler' used in the drug. Filters with 0.1 µm compared with 0.2 µm pore size have enhanced bacterial removal according to one study. The use of wheel filters is strongly recommended when illicit drug users seek to inject prescription medications such as morphine and benzodiazepines that come in tablet form. However it should be noted that some benzodiazepines, such as diazepam, have only a very slight solubility in water, so there may potentially be no more benefit gained from injecting a diazepam tablet, even with a filter, than swallowing it. For the best advice on injecting such tablets, information should be sought from a harm reduction service with a good knowledge of the drugs that "wheel" filters are best used for. If, however, you feel that you must inject crushed diazepam tablets, (despite the fact that there is potentially no difference in the effect gained from the drug) as the solubility of diazepam is less than 1 mg/mL at 68 °F (20 °C), it is much safer to use a filter than nothing at all, as the potential for particles to become lodged in veins or arteries carries serious health risks.
Pills like prescription opiates or benzodiazepines which are often converted by users into crude injectables, have ingredients such as chalk and wax as a 'filler', and illicit drugs are often adulterated with fillers to increase profits. Correctly used, wheel filters with at least pore size 0.5 µm may reduce these impurities entering the blood stream and help avoid vein problems such as collapsed veins, abscesses, infections, embolism, 'dirty tastes', disease and septicemia.
While wheel filters are the most effective filter available for injecting drug users along with luer lock syringe tip filters, other more common types of filters used include cotton wool, tampons, and cigarette filters. While these can serve as basic filters, they have a greater risk of bacterial infection or contamination from pieces of the filter itself. The condition known as cotton fever is caused by cotton used as a filter.
- Sterile Filtration of Ophthalmic Solutions MILLIPORE TECHNICAL PUBLICATIONS Lit No: AN1575EN00 retrieved 10-08-2012
- Sundaram, S; Auriemma, M; Howard Jr, G; Brandwein, H; Leo, F (1999). "Application of membrane filtration for removal of diminutive bioburden organisms in pharmaceutical products and processes". PDA journal of pharmaceutical science and technology / PDA. 53 (4): 186–201. PMID 10754712.
- Loxley, W., International Journal of Drug Policy,11 (6),Doing the possible: harm reduction, injecting drug use and blood borne viral infections in Australia pp. 407–416