Development of light-responsive polymeric membranes for controlled caffeine delivery based on photochromic spiro-compounds

Different approaches to obtain light-responsive membranes with the ability to switch their caffeine permeability resistance were developed in a copolymerization and in a postmodification process.
Synthesis of six photochromic methacrylic and acrylic spiropyrans and spirooxazines was successfully achieved. Copolymerization of these monomeric spiro-compounds resulted in photochromic materials. Additionally, a spiropyran with carboxylic acid functionality was synthesized. This enabled to postmodify non-photochromic polymeric materials with alcohols or amines on their surface to obtain photochromic materials.
The first developed approach was to coat porous polymeric membranes with photochromic materials. Covalently linked coatings were achieved either by surface induced atom transfer random polymerization (ATRP) or by plasma activation of the membrane surface followed by random free radical polymerization. Copolymerization of spiro-compounds during the coating process as well as postmodification of preformed non-photochromic coatings with spiropyran resulted in photochromic membranes. UV-irradiation of such membranes resulted in a strong coloration of the membrane, in a reduction of surface tension and in a decreased caffeine permeability resistance.
Plasma activation resulted in thin coatings with thicknesses below 10 nm. Nevertheless it was possible to produce a membrane that reduced its caffeine permeability resistance of about 97% when the membrane was irradiated with UV-light compared to measurements at daylight.
Thicknesses of ATRP-coatings were found to be depending on the polymerization time and the monomer concentration during the polymerization process. If too much coating was on the membrane, no caffeine permeability was any longer detectable. If there was not enough coating on the membrane, the caffeine permeability resistance was not switching. Therefore, an intermediate coating thickness turned out to be important for the production of useful photochromic membranes. Membranes produced with an ATRP-process lowered in the best case their permeability resistance of about 65% under UV-irradiation compared to measurements at daylight.
Production of nonporous photochromic membranes was achieved by producing amphiphilic conetworks containing spiropyrans. UV-irradiation of such photochromic amphiphilic conetworks resulted in a red coloration and in a decreased caffeine permeability resistance. Changing the composition of the amphiphilic conetwork allowed tuning the basic caffeine permeability resistance. Copolymerization of spiropyran during the production of the amphiphilic conetwork resulted in a membrane that lowered its caffeine permeability resistance under UV-irradiation of around 43% compared to measurements at daylight.
Combination of porous membranes with amphiphilic conetworks was achieved in a grafting through process. The surface of a porous membrane was coated with acrylic units and the amphiphilic network was produced on that surface. This covalently linked amphiphilic conetwork resulted in an inversed permeability switch. The caffeine permeability resistance was about eight times higher under UV-irradiation than at daylight.
Stability measurements under UV-irradiation showed that postmodified membranes had in general lower fading rates than copolymerized membranes.