the present invention relates to a process for producing a bioreactor using viable immobilized biological material and to the bioreactor itself . more particularly , the present invention relates to a process for producing a siliceous layer capable of biomass immobilization and selectively cutting off macromolecules having a molecular weight higher than a selected threshold , comprising the steps of : a ) supplying a gas flow of a gas carrier saturated by a mixture of silicon alkoxides selected from the group comprising si 4 , sih 3 , r ′ si 3 and r ′ sih 2 wherein r and r ′, equal or different each other , are alkyl and / or aryl groups , wherein said gas flow is prepared by bubbling the gas carrier into a liquid mixture of said alkoxides in the ratio of 40 – 85 / 0 – 60 / 0 – 60 / 0 – 60 , preferably in the ratio of 40 – 85 / 0 – 50 / 0 – 50 / 0 – 50 , more preferably in the ratio of 50 – 80 / 0 – 20 / 5 – 30 / 5 – 30 , at a temperature of from 20 to 180 ° c ., preferably of from 20 to 100 ° c ., and b ) exposing a support comprising a biomass to the gas flow of step a ), wherein said selected threshold of molecular weight is chosen in the range of between 10 , 000 dalton and 150 , 000 dalton and wherein the ratio between the , , and si derivatives in step a ) is chosen as a function of the molecular weight of the macromolecules to be cut off .