Not my field at all, so let's wait and see what the experts think - but I have a feeling it's "hollow".

"MEDICELL offers Spectrum's microfiltration products which are based on hollow fiber filters, bundles of small tubes (hollow fibers) of filter media."
http://www.medicell.co.uk/Hollowover.htm

And this does seem relevant, I have to say...

"The spiral wound membrane format was commercialized in 1967 to resolve the problems associated with previous attempts to make practical use of organic polymeric membranes. The spiral offers a proven combination of superior mechanical, hydraulic and economic performance for water and wastewater applications, compared to predecessors such as the *hollow fiber* format. The reverse osmosis membrane industry generally adopted and standardized on the spiral format 37 years ago for these reasons. However, until now, ultrafiltration and microfiltration membranes have to a large extent been manufactured using the hollow fiber format. This is due to the inability of the spiral and the ability of the hollow fiber, to be backflushed."
http://66.249.93.104/search?q=cache:MGkjjO0Q84wJ:www.trisep....