A diffusion and/or filtration device comprising: a) housing means, the housing means defining a longitudinally extending internal chamber including a first end and a second end; b) a bundle of semi-permeable hollow fiber membranes disposed within the internal chamber, the hollow fiber membranes extending longitudinally from the first end of the housing to the second end of the housing, the hollow fiber membranes having an outer surface, and a first end and a second end corresponding to the first end and the second end of the internal chamber, the lumen of each of the hollow fiber membranes being in fluid connection to both the first end and the second end of the internal chamber; c) end wall means supporting the first and second ends of the hollow fiber membranes within the internal chamber so as to sealingly separate the first and second ends of the hollow fiber membranes from the outer surface of the hollow fiber membranes between the first and second ends thereof; d) first inlet means for the introduction of a fluid into the first end of the housing means, the first inlet means being defined by a first end cap covering the first end of the housing; e) first outlet means for the evacuation of a fluid from the second end of the housing means, the first outlet means being defined by a second end cap covering the second end of the housing, the first and second end caps being applied to the first and second ends of the housing in a fluid-tight manner; f) second outlet means for the evacuation of a fluid from the internal chamber at a location between the first and second end of the housing means; characterized in that the bundle of semi-permeable hollow fiber membranes comprises at least two different types of hollow fiber membranes uniformly distributed throughout the entire bundle. a) housing means, the housing means defining a longitudinally extending internal chamber including a first end and a second end; b) a bundle of semi-permeable hollow fiber membranes disposed within the internal chamber, the hollow fiber membranes extending longitudinally from the first end of the housing to the second end of the housing, the hollow fiber membranes having an outer surface, and a first end and a second end corresponding to the first end and the second end of the internal chamber, the lumen of each of the hollow fiber membranes being in fluid connection to both the first end and the second end of the internal chamber; c) end wall means supporting the first and second ends of the hollow fiber membranes within the internal chamber so as to sealingly separate the first and second ends of the hollow fiber membranes from the outer surface of the hollow fiber membranes between the first and second ends thereof; d) first inlet means for the introduction of a fluid into the first end of the housing means, the first inlet means being defined by a first end cap covering the first end of the housing; e) first outlet means for the evacuation of a fluid from the second end of the housing means, the first outlet means being defined by a second end cap covering the second end of the housing, the first and second end caps being applied to the first and second ends of the housing in a fluid-tight manner; f) second outlet means for the evacuation of a fluid from the internal chamber at a location between the first and second end of the housing means;