Immunoisolatory Membranes

True immunoisolation requires restricting the membrane transport of all relevant molecules belonging to the host and the encapsulated cells. Whereas a molecular weight cutoff of 50-100 kDa likely prevents infiltration of IgG, greater size restrictions must be employed to achieve a complete barrier. Growth factors, cytokines, complement, and smaller metabolites several logs lower in molecular weight exist within immunoisolated systems that are immunogenic and can lead to graft rejection. Limiting passage of these smaller molecules would retain the cell-secreted molecule of interest, and thus such a system would not be practical. However, this level of protection is not always necessary, particularly in allogeneic or syngeneic transplantation, and immunosuppressants can be administered to overcome this effect.

The polymer membrane of an immunoisolatory device is a thin barrier that restricts transport between adjacent phases.[1-4] Most membranes are produced from thermoplastics such as polysulfone or acrylic copolymers that are cast into hollow fiber membranes less than 1 mm in diameter with a wall thickness of 10-15% of the total fiber diameter.[5-7] The majority of thermoplastic ultrafiltration and microfiltration membranes used to encapsulate cells are manufactured from homogenous polymer solutions by phase inversion. Ultrafiltration membranes have pore sizes ranging from 5 nm to 0.1 mm, while microfiltration (or microporous) membranes have pores ranging from 0.1 to 3 mm. Phase inversion forms membranes with a wide variety of nominal molecular weight cutoffs, permeabilities, and morphologies. The polymer is first dissolved in an appropriate solvent and then cast as a flat sheet or extruded as a hollow fiber. As part of the casting or extrusion procedure, the polymer solution is precipitated by a phase transition brought about by temperature change or solution composition. This process involves the transfer of a single-phase liquid polymer solution into a two-phase system consisting of a polymer-rich phase that forms the membrane structure and a second liquid polymer-poor phase that forms the membrane pores. Any polymer that forms a homogenous solution that will separate into two phases can be used. Thermodynamic and kinetic parameters, such as the chemical potential of the components and the free energy of mixing of the components, determine the manner of the phase separation.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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