A new visible light induced graft polymerization method was utilized to prepare pH-sensitive hydrogel layers covalently attached to polymer substrates for drug delivery. In our strategy, isopropyl thioxanthone semi-pinacol (ITXSP) dormant groups were firstly introduced on the surface of a polycaprolactone (PCL) film by a UV-induced abstracting hydrogen-coupling reaction. Then visible light induced graft crosslinking polymerization was performed to initiate polymerization of poly(ethylene glycol) diacrylate (PEGDA) and acrylic acid (AA), resulting in the formation of a hydrogel layer. The thickness of the hydrogel film can be controlled by varying the exposure time and monomer composition. The grafted hydrogel layers showed a flat morphology and dense structure, which is different from the traditional reported porous structure. The water contact angle of the hydrogel layer exhibited a reversible change from 381 to 181 when the film was alternatively treated in buffers of pH 2.0 and 7.4, respectively. Patterned hydrogel layers were prepared as a model to determine the change in the height of the grafted hydrogel layer as a function of pH. As the pH changed from 2.0 to 7.4, the hydrogel pattern showed an increase in height due to the swelling of the hydrogel network, and the hydrogel layer formed by 0.2 wt% PEGDA and 25 wt% AA showed the most increase (30%) in height. Bovine serum albumin (BSA) and lysozyme as models of protein drugs were incorporated in the hydrogel network, and their release also showed obvious pH-sensitivity. At pH 2.0, hydrogels present a faster initial burst release due to the squeezing mechanism. Tertiary structure analysis showed that encapsulation and release did not affect the protein conformation. These findings have improved our understanding of hydrogel thin films, which may be useful as potential vehicles of therapeutic proteins in drug delivery applications. Scheme 1 Schematic illustration of the synthesis of a P(PEGDA/AA)/ protein hydrogel thin layer on the PCL film. (a) ITXSP was immobilized on the surface of the PCL substrate under UV irradiation. (b) Hydrogel layer was formed under visible light irradiation. (a 0 ) and (b 0 ) are the reaction mechanisms for steps (a) and (b), respectively.

Enzyme immobilization is a core technique of enzymatic biochemical engineering because it can remarkably reduce the cost of enzymes and improve the enzyme recovery procedure. The most crucial issues for enzyme immobilization include (1) maintaining its activity, both in the immobilization process and in the batchwise catalyst course; (2) separating the immobilized enzyme from the reaction mixture; and (3) the readiness and cost of the immobilization process. Herein, we report a new strategy to immobilize xylanase within a hydrophilic and nonswelling polyethylene glycol (PEG) netcloth grafted on a polypropylene nonwoven fabric (PP NWF ) membrane by a visible light-induced surface graft cross-linking polymerization. The xylanase was in situ entrapped within the PEG net-cloth. The nonswelling PEG netcloth can effectively maintain the xylanase without leakage in long-term operation. As for the hydrolysis of corncob hemicelluloses, the experimental results showed that the as-formed immobilized xylanase retained 80% of its original activity after being reused for 25 cycles and 60% after 50 cycles, which is far better than that of other immobilization methods by entrapment. Notably, this simple in situ entrapment of enzymes on routine polymeric matrix would lead to an easy industrial production at low cost, while the form of end-products as a sheet can be readily separated from the reaction mixture and reused for batchwise production. After immobilization, the xylanase showed no significant shift in pH or temperature optima as compared with its free form. These results suggest that the immobilization of xylanase within the PEG net-cloth grafted on PP NWF is promising for industrial applications because of its long-term operation stability and convenient recovery for reuse.