A membrane for use in an implantable glucose sensor including at least one crosslinked substantially hydrophobic polymer and at least one crosslinked substantially hydrophilic polymer; wherein the first and second polymers are different polymers and substantially form an interpenetrating polymer network, semi-interpenetrating polymer network, polymer blend, or copolymer. The membranes are generally characterized by providing a permeability ratio of oxygen to glucose of about 1 to about 1000 in units of (mg/dl glucose) per (mmHg oxygen). Three methods of making membranes from hydrophobic and hydrophilic monomers formed into polymer networks are provided, wherein according to at least two of the methods, the monomers may be substantially immiscible with one another.